Citation
Effects of subsurface conditions on performance of structures in Tacna during the June 23, 2001 earthquake

Material Information

Title:
Effects of subsurface conditions on performance of structures in Tacna during the June 23, 2001 earthquake
Creator:
Williams, Jennifer Lea
Publication Date:
Language:
English
Physical Description:
187 leaves : illustrations ; 28 cm

Subjects

Subjects / Keywords:
2001 ( fast )
Southern Peru Earthquake, Peru, 2001 ( lcsh )
Earthquake engineering ( lcsh )
Buildings -- Earthquake effects -- Peru -- Tacna ( lcsh )
Soil mechanics -- Peru -- Tacna ( lcsh )
Buildings -- Earthquake effects ( fast )
Earthquake engineering ( fast )
Soil mechanics ( fast )
Peru ( fast )
Peru -- Tacna ( fast )
Genre:
bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

Notes

Bibliography:
Includes bibliographical references (leaves 181-187).
General Note:
Department of Civil Engineering
Statement of Responsibility:
by Jennife Lea Williams.

Record Information

Source Institution:
University of Colorado Denver
Holding Location:
Auraria Library
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
54522578 ( OCLC )
ocm54522578
Classification:
LD1190.E53 2002m W54 ( lcc )

Full Text
EFFECTS OF SUBSURFACE CONDITIONS ON PERFORMANCE OF
STRUCTURES IN TACNA DURING THE JUNE 23, 2001 EARTHQUAKE
by
Jennifer Lea Williams
B.S., Colorado School of Mines, 1997
A thesis submitted to the
University of Colorado at Denver
in partial fulfillment
of the requirements for the degree of
Master of Science
Civil Engineering
2002


This thesis for the Master of Science
degree by
Jennifer Lea Williams
has been approved
by
Nien-Yin Chang
h
Brian Brady

Trever Wang
Date


Williams, Jennifer Lea (M.S., Civil Engineering)
Effects of Subsurface Conditions on Performance of Structures in Tacna During The
June 23, 2001 Earthquake
Thesis directed by Professor Nien-Yin Chang
ABSTRACT
Many researchers have postulated the theory of the effect local site conditions can
have on surface ground motion and resulting building damage intensities. Some of
the earliest papers on geotechnical earthquake engineering (i.e. Seed and Idriss 1969,
Seed et al 1972) presented these ideas. However, due to the relative infrequency of
earthquake occurrences when compared to other civil engineering subjects, a limited
number of earthquakes have been evaluated in detail to help support the site effect
theory with case studies. This thesis presents a case history of the June 23, 2001
southern Peru earthquake, focusing on evaluating the site effects and correlating
building damage intensities to site soil conditions, taking into account construction
practices. At a magnitude of Mw 8.4 (USGS 2001), the southern Peru earthquake was
the largest earthquake to have occurred anywhere in the world in the last 36 years and
the largest in the region in 133 years. This thesis focuses on the study area of the city
of Tacna located in southern Peru. The city has two zones with distinct soil profile
types and provides a good control of building types due to the relatively uniform
construction practices. This thesis presents the seismic characteristics of southern
Peru and the June 23, 2001 earthquake, and presents a damage correlation based on
identified soil types and damage intensity surveys for the city of Tacna. A one-
dimensional (1-D) seismic site response analysis was also performed using the
program SHAKE2000 based on the one-dimensional vertical propagation of shear
waves through a horizontal soil deposit. The methodology used to develop the wave
propagation model and the resulting ground motions, surface amplifications, and its
correlation to the superstructure damage are presented herein.
This abstract accurately represents the content of the candidates thesis. I recomi
its publication.

Signed ///
- ^
Nien-Yin Change /
in


ACKNOWLEDGMENTS
I would like to express my thanks to all those who supported this effort. My thanks
goes to my advisor, Nien-Yin Chang for his guidance and understanding of a
demanding schedule; Pedro Repetto of URS Corporation who has served as a mentor
for this and other endeavors; the National Science Foundation who supported this
research; Adrian Rodriguez-Marek for sharing his experience and knowledge; Adolfo
Gonzales Palma and Daryl Cotrado for sharing their in-depth knowledge of the city of
Tacna; Efrain Rondinel and Jorge Zegarra-Pellanne of Catholic University of Peru for
their assistance; and URS Corporation for supporting my educational endeavors over
the past two years.
IV


CONTENT
Abstract...................................................................iii
Acknowledgements............................................................iv
Figures...................................................................viii
Tables.....................................................................xii
Chapter
1. Introduction.............................................................1
1.1 The City of Tacna, Study Area ...........................................2
1.2 Potential Causes of Selective Structural Damage.........................11
1.3 Research Objectives.....................................................13
1.4 Research Approach.......................................................13
2. Seismological Aspects and Ground Motions................................15
2.1 Historical Seismicity of Region.........................................15
2.2 Seismological Setting..................................................20
2.3 The June 23, 2001 Southern Peru Earthquake.............................21
2.3.1 Seismological Aspects.................................................21
2.3.2 Magnitude and Intensity...............................................29
2.3.3 Ground Motion at Moquegua.............................................33
3. Subsurface Conditions and Distribution .................................37
3.1 Geologic and Geotechnical Setting.......................................37
3.2 Bedrock and Water Table................................................47
3.3 Subsurface Investigations and Soils Characterization ..................50
3.4 Typical Soil Profiles and Their Effects on Ground Motion Characteristics.66
3.5 Microtrepidation Measurements..........................................71
3.6 Seismic Microzonation of Tacna.........................................71
3.7 Shear Wave Velocities..................................................76
v


4. Building Distribution and Damage Survey................................80
4.1 Construction Practice..................................................80
4.2 Overview of Damage....................................................81
4.3 Damage Survey.........................................................84
4.4 Institute Geoftsico del Peru Intensity Survey.........................92
5. Damage Versus Subsurface Conditions....................................95
5.1 Potential Causes of Structural Damage and Failures.....................95
5.2 Spatial Distribution of Observed Damage...............................97
5.2.1 Isolating Subsurface Conditions from other Factors..................97
5.2.2 Identifying Trend of Damage Spatial Distribution....................98
5.3 Establishing a Correlation............................................98
6. Ground Motion Modeling................................................101
6.1 Rock Motions in Tacna.................................................102
6.1.1 Deconvolution of Recorded Moquegua Time History....................102
6.1.2 Selection and Calibration of Attenuation Curve.....................106
6.1.3 Estimated Rock Motions in Tacna....................................109
6.2 Wave Propagation Analysis............................................110
6.2.1 SHAKE2000..........................................................110
6.2.2 Selected Soil Profile for Tacna....................................110
6.3 Results of Wave Propagation Analysis.................................114
6.3.1 Resulting Acceleration.............................................114
6.3.2 Response Spectrum..................................................116
6.3.2.1 Predominant Period................................................120
6.3.2.2 Spectral Shapes...................................................121
6.4 Damage Versus Ground Motion Characteristics..........................124
7. Summary, Conclusions and Recommendations for Further
Study ................................................................126
7.1 Summary...............................................................126
7.2 Conclusions..........................................................127
vi


7.3 Recommendations for Further Study..............................129
7.3.1 Additional Subsurface Data...................................129
7.3.2 Topographic Effects of Recording in Moquegua.................130
Appendix
A. Damage Survey Site Photos......................................131
B. SHAKE2000 Input and Output Files...............................164
References.........................................................181
vii


FIGURES
Figure
1.1 Location Map of Southern Peru.........................................1
1.2 Topographic Map of Tacna and Surrounding Region.......................3
1.3 Aerial Photo of Tacna District........................................5
1.4 Aerial Photo of Gregorio Albarracin District..........................5
1.5 Aerial Photo of Downtown Area.........................................6
1.6 Aerial Photo of Alto de la Alianza District...........................6
1.7 Panoramic Photo Downtown Area.......................................7
1.8 Panoramic Photo Downtown Area.......................................7
1.9 Photo of Adobe House Downtown Area...............................8
1.10 Photo of Adobe House Downtown Area...............................8
1.11 Photo of Five-Story Building Downtown Area..........................9
1.12 Photo of Brick-bearing-wall House....................................10
1.13 Photo of Brick-bearing-wall House....................................10
1.14 Normalized Average Spectra for Typical Soil Profiles.................12
2.1 Distribution of Maximum Intensities Observed in Peru.................16
2.2 Iso-Intensity Map Earthquake of August 12, 1868....................17
2.3 Iso-Intensity Map Earthquake of May 9, 1877........................18
2.4 Iso-Intensity Map Earthquake of January 15, 1958...................19
2.5 Schematic Diagram of Subduction Zone in Southern Peru................20
2.6 Shallow Seismic Sources..............................................22
2.7 Intermediate and Deep Seismic Sources................................23
2.8 Cross-Section of Seismicity Through Southern Peru....................24
2.9 Maximum Accelerations with a 10% Probability of
Exceedance in 50 Years...............................................25
viii


2.10 Maximum Accelerations with a 10% Probability of
Exceedance in 100 Years.............................................26
2.11 Epicenter and Aftershocks of June 23, 2001 Southern Peru
Earthquake Showing Approximated Rupture Plane......................27
2.12 Iso-Intensity Map of the June 23, 2001 Southern Peru
Earthquake..........................................................32
2.13 Acceleration, Velocity, and Displacement Time History -
June 23, 2001 Southern Peru Earthquake..............................35
2.14 Acceleration Response Spectrum June 23, 2001 Southern
Peru Earthquake.....................................................36
3.1 Physiographic Units in the Tacna Region.............................37
3.2 Geologic Map of Tacna and Surrounding Region........................40
3.3 Generalized Stratigraphic Column of Tacna Region....................41
3.4 Geologic Profile Along Tacna Valley.................................42
3.5 Geologic Cross-Sections Across Tacna Valley.........................42
3.6 Typical Conglomerate Profile in Tacna...............................43
3.7 Coastal Cliffs of Conglomerate in Lima, Peru........................45
3.8 Coastal Cliffs of Conglomerate in Lima, Peru........................45
3.9 Conglomerate Deposit in Limas Cliffs...............................46
3.10 Location of Subsurface Investigations and Microtrepidation
Measurements........................................................49
3.11 Plate Load Test Results at Site J-13................................65
3.12 Plate Load Test Results at Site J-13................................65
3.13 Soil Profile Distribution...........................................68
3.14 Test Pit in Conglomerate............................................69
3.15 Test Pit in Volcanic Tuff...........................................69
3.16 Slope in Conglomerate...............................................70
3.17 Slope in Volcanic Tuff..............................................70
3.18 Iso-Period Contour Lines............................................74
IX


3.19 Seismic Microzonation of Tacna and Geotechnical
Microzonation of Ciudad Nueva and Alto de la Alianza
Districts..........................................................75
3.20 Down-Hole Measurements in Limas Conglomerate......................78
3.21 Down-Hole Measurements in Limas Conglomerate......................78
3.22 K2 Relationship for Various Relative Densities.....................79
4.1 Damaged Adobe House................................................81
4.2 Damaged Adobe House................................................82
4.3 School with Severe Structural Damage...............................83
4.4 School with Minor Damage to Non-Bearing Walls......................84
4.5 Damage Survey Sites................................................89
4.6 Distribution of Structural Damage Index............................90
4.7 Damage Survey Site 1 Before the June 23, 2001 Earthquake...........91
4.8 Damage Survey Site 1 After the June 23, 2001 Earthquake............91
4.9 Distribution of MSK Intensities....................................94
6.1 Assumed Profile for Moquegua Recording Station....................103
6.2 Acceleration Time History for Rock Outcrop in Moquegua............104
6.3 Recorded Ground and Deconvoluted Rock Motions for Moquegua........105
6.4 Normalized Ground and Deconvoluted Rock Motions for Moquegua.....105
6.5 Youngs et al. (1997) PGA Prediction for Various
Magnitude Earthquakes at a Distance of 100 km....................107
6.6 Youngs et al. (1997) PGA Attenuation Relationship
For Inter-Plate Subduction Earthquakes (Mw = 8.4, Depth = 30 km).108
6.7 Typical Soil Profiles Used for SHAKE2000 Analysis.................112
6.8 Seed and Idriss (1970) Dynamic Shear Moduli and Damping Ratio.....113
6.9 PGA Versus Depth for 3 m, 6 m and 9 m of Loose
to Medium Dense Silty Sand........................................114
6.10 Acceleration Time Histories for Rock Outcrop Sites (Input
Motion), and 3 m, 6 m and 9-m of Loose to Medium Dense
Silty Sand........................................................115
6.11 Depiction of Typical Acceleration Response Spectrum...............116
x


6.12 Acceleration Response Spectra for Rock Outcrop, 3-, 6- and
9-m Soil Profiles......................................................118
6.13 Normalized Acceleration Response Spectra for 3-, 6- and
9-m Soil Profiles......................................................119
6.14 Resultant Acceleration, Velocity, and Displacement
Spectrums for 9-m Soil Profile.........................................123
xi


TABLES
Table
2.1 Rupture Zone of the June 23, 2001 Southern
Peru Earthquake........................................................28
3.1 Summary of Water Well Profiles from the Southern
Region of the City of Tacna............................................48
3.2 Summary of Test Pits Compiled by Cotrado and Sina
(1994) (C-l through C-55)..............................................51
3.3 Summary of Test Pits and Laboratory Testing Performed
by Cotrado and Sina (1994) (E-l through E-21)..........................56
3.4 Summary of Laboratory Testing Performed by Silva and
Berrios (1998) (S-l through S-19)......................................59
3.5 Test Pit Profiles Compiled From Various Site
Investigations (J-l through J-12)......................................62
3.6 Grain Size Analysis at Site J-l3.......................................64
3.7 Summary of Microtrepidation Measurements Compiled and
Performed by Cotrado and Sina (1994)...................................72
3.8 Geotechnical Zonation of Ciudad Nueva and Alto de la
Alianza Districts......................................................76
4.1 Structural Damage Index Used for Mapping of Damage
Patterns...............................................................85
4.2 Summary of Structural Damage Index Survey..............................85
4.3 Damage Evaluation of Surveyed Buildings................................87
6.1 Material Properties of the Northern Cone Soil
Profile used in Wave Propagation Analysis.............................112
6.2 Summary of Spectral Parameters for 5% Damping.........................120
xii


1.
Introduction
On June 23, 2001 at 3:33 p.m. (local time), an M8 class earthquake struck near the
coast of southern Peru. The earthquake caused extensive damage in the areas around
the cities of Arequipa, Camana, Moquegua, and Tacna (Figure 1.1). Most of the
damage in Camana was due to a tsunami wave. As of July 20, 2001, the Civil
Defense Institute in Peru (INDECI) had reported 80 casualties and 64 missing
persons, with an estimated 36,000 homes damaged affecting a total of approximately
223,000 people.
' v
^Expedia
! Huanuhuanu
o
Chaparra
achaca
Andris
deWaohaoj
Jm-------^
.. 0*ntl1<|'\
'CfT?
,t,c. ! !Lrip.uo
Neva Sajama .'
,*.. .......................- 18*
Tacna ; /
, jy
Vi- W *
..ORURO
CHILE ^ A^coWsi .
Turco
Amazaca
; ] Huachacalla
A-rmcnrMTS
DESERT _y
^Cerro
eg. Caharaya
Figure 1.1. Location Map of Southern Peru (Expedia.com 2001)
A reconnaissance team sponsored by the U.S. National Science Foundation (NSF)
was assembled to observe and report the geotechnical earthquake engineering aspects
of the damage. The NSF team included the following members:
1


Dale Baures, URS Corporation
Pedro Repetto, URS Corporation
Adrian Rodriguez-Marek, Washington State University
Efrain Rondinel, Catholic University of Peru
Joseph Wartman, Drexel University
Jennifer Williams, URS Corporation and University of Colorado at
Denver
Jorge Zegarra-Pellanne, Catholic University of Peru
The team visited and documented sites including major cities, mining facilities, and
transportation routes. The major cities visited included Tacna, Moquegua, Ilo,
Arequipa, Camana, and Ocona. Mining facilities included the Cerro Verde and
Cuajone copper mines, and the Quebrada Honda tailings impoundment.
Transportation routes consisted mainly of the roads connecting these sites. Figure 1.1
shows the main impacted cities. A preliminary report that summarized observations
performed between July 8 and July 12, 2001 (Rodriguez-Marek et al. 2001) was
published on the Berkeley web page http://peer.berkeley.edu/peru_earthquake/.
The results of this reconnaissance indicated that site effects had played a significant
role in the distribution of damage in Tacna. Therefore, a second reconnaissance visit
was conducted on August 24 to 28, 2001 by Jennifer Williams and Pedro Repetto to
observe and document damage to schools and other structures in Tacna, and to
compile information about subsurface conditions.
1.1 The City of Tacna, Study Area
The city of Tacna is located at the southern end of Peru, near the border with Chile,
approximately 38 km north-east of the Pacific coastline (Figure 1.1) on an arid strip
of land bounded to the east by the steep chain of the Andean Mountains. Elevation
ranges between 560 and 650 meters above sea level (m.a.s.l.). Precipitation is
infrequent, with an annual average of about 20 mm. Population at the July 1993
census was about 179,000 people. Current population is estimated at about 250,000.
The city is divided into five districts: Tacna, Pocollay, Gregorio Albarracin, Alto de
la Alianza, and Ciudad Nueva. Old and new construction exists in the district of
Tacna, that includes the downtown area. Construction in the rest of the city is mostly
new. A topographic map of Tacna and the surrounding region, compiled from the
Instituto Geografico Nacional (IGN) maps, at a 1:100,000 scale with 50 m contours is
presented in Figure 1.2 (IGN 1995a and 1995b). This map shows the location of the
city in the valley of Tacna, between the Intiorko hill to the northwest and the Arunta
hill to the southeast.
2




Figures 1.3 through 1.6 show aerial views of various areas of the city of Tacna taken
on July 8, 2001 during the first reconnaissance visit.
Most of the construction in the city consists of 1 to 2 story high brick-bearing-wall
buildings, except in the downtown area where there are also old adobe houses and
some modem buildings up to 5 to 6 stories high. Figures 1.7 and 1.8 show panoramic
photos of the downtown area; Figures 1.9 and 1.10 show photos of old adobe houses
in the downtown area; Figure 1.11 shows a 5-story building, also in the downtown
area; and Figures 1.12 and 1.13 show typical brick-bearing-wall houses common
throughout the city.
Significant damage to 1 to 2 story brick-bearing-wall buildings occurred in Alto de la
Alianza and Ciudad Nueva districts. Damage to similar buildings was imperceptible
to light in other parts of the city. Various levels of damage to the older, adobe
buildings were also observed in the downtown area.
A seismic microzonation of the city of Tacna was prepared in 1993 as a civil
engineering thesis (Cotrado and Sina 1994). This document has been a valuable
source of information about subsurface conditions throughout the city due to the
otherwise limited amount of geotechnical information on subsurface conditions. The
seismic microzonation of Tacna included in that thesis is discussed in Section 3.6. A
summary of this thesis was presented at the VI International Course on Microzonation
and Safety of Lifeline Public Systems (Cotrado and Sina 1995). Additional
subsurface information for the Ciudad Nueva and Alto de la Alianza districts was
obtained from another civil engineering thesis (Silva and Berrios 1998).
4


Figures 1.4. Aerial Photo of Gregorio Albarracin District
5


Figure 1.6. Aerial Photo of Alto de la Alianza District
6


Figures 1.7 and 1.8. Panoramic Photos Downtown Area
7


Figures 1.9 and 1.10. Photos of Adobe Houses Downtown Area
8


Figure 1.11. Photo of Five-Story Building Downtown Area
9


Figures 1.12 and 1.13. Photos of Brick-bearing-wall Houses
10


1.2 Potential Causes of Selective Structural Damage
At the time of the second reconnaissance visit, a preliminary hypothesis was
formulated to explain the observed selective damage. In general, the city of Tacna
can be divided into two main areas based on subsurface conditions, as discussed
further in Chapter 3. The Tacna, Pocollay, and Gregorio Albarracin districts are
underlain at shallow depth by a dense, coarse gravel deposit locally called
conglomerate, while most of the Alto de la Alianza and Ciudad Nueva districts are
underlain by loose to medium dense sandy and silty soils of variable thickness.
As indicated above, significant damage occurred in the Alto de la Alianza and Ciudad
Nueva districts that are underlain by the silty and sandy soils. Similar buildings
performed well in the rest of the city underlain by the gravel deposit and damage was
limited to old adobe construction. No signs of differential settlement or bearing
capacity failures were observed in either area, which indicated that seismically
induced settlements or soil collapse was not the main cause of failure.
Due to the different performance of similar structures founded on different soil
formations, it appeared that damage was due mainly to site effects. The preliminary
hypothesis was that site effects which influence levels of damage could consist of: (1)
amplification of seismic waves through the sandy and silty soils, resulting in a higher
peak ground acceleration (PGA) at the site; (2) modification of the predominant
period of vibration of the seismic waves through the sandy and silty soils, resulting in
a predominant period similar to the fundamental period of the damaged buildings; or
(3) a combination of (1) and (2).
Hypothesis (1) is based on acceleration-time histories from previous earthquakes
recorded on various types of soil deposits. The amplification of ground motion is
usually defined as the intensifying effects of ground motion as it propagates from the
soil-bedrock interface through a soil deposit (Chiang and Chang 1995). The effects
of soil profile type on spectral amplification was demonstrated by Seed et al. (1976).
Seed et al. (1976) characterized amplification by normalizing the spectral
amplification (maximum acceleration at various periods) by dividing it by the peak
ground acceleration of the corresponding acceleration time history, as shown on
Figure 1.14. The maximum ordinate of this normalized spectrum is termed herein as
spectral amplification. Figure 1.14 shows average normalized spectra
corresponding to 28 acceleration-time histories recorded on rock, 31 on stiff soil
conditions, 30 on deep cohesionless soils, and 15 on soft to medium stiff clay and
sand. These normalized spectra clearly illustrate that spectral amplifications are the
lowest in rock outcrops, and that spectral amplification increases for weaker and
deeper soil deposits. More recently, Chiang and Chang (1995) have characterized
11


ground motion amplification using the ratio between peak acceleration of a surface
ground motion and the peak acceleration of a nearby rock outcrop motion, which is
termed herein as soil amplification. By running numerous analytical models of a
clay and sand layered profile, Chiang and Chang (1995) demonstrated that surface
ground motions vary with the total depth of soil deposit and with the percentage of
the total depth comprised of clay.
Period seconds
Figure 1.14. Normalized Average Spectra for Typical Soil Profiles (Seed et al. 1976)
Hypothesis (2) is related to the very short fundamental period of vibration of typical 1
to 2-story brick-bearing-wall structures in Tacna. Studies of Peruvian buildings
(Delpiano 1977; Ottazi and Valeriano 1980) have shown that the fundamental period
of those buildings is in the range of 0.05 to 0.15 sec. As shown on Figure 1.14,
average spectra for stiff soil deposits (such as the conglomerate) exhibit a peak also at
a low period of about 0.2 sec. Therefore, after initial cracking, the brick-bearing-wall
structures would become more flexible and move toward the right on the response
spectrum, that would normally be the descending branch, thus decreasing the seismic
force acting on the structure. On the other hand, the opposite would occur for similar
structures founded on medium to loose soil deposits. Since the spectral peak for these
soils occurs at a longer period than the fundamental period of the structures, after
12


initial cracking these structures would move on the ascending branch of the spectrum,
thus increasing the seismic force acting on them.
1.3 Research Objectives
As indicated above, the initial reconnaissance indicated that site effects had played a
significant role in the distribution of damage in Tacna. Therefore, the main objectives
of this research are to verify that a correlation exists between subsurface conditions,
ground motion, and damage distribution, and to evaluate structure performance as a
function of the resulting ground motion characteristics.
In order to achieve the indicated objectives, this thesis has been organized in the
following subsequent chapters:
Seismological Aspects and Ground Motions. This chapter presents general
background information on the seismo-tectonic setting and factual data about the
recorded ground motions.
Subsurface Conditions and Distribution. This chapter comprises a description and
evaluation of subsurface conditions in the city of Tacna, including the geologic
and geotechnical setting, and a compilation of available subsurface investigations
and related studies.
Building Distribution and Damage Survey. This chapter provides a discussion of
building types throughout the city, a general evaluation of damage distribution,
and the results of a damage survey of selected representative structures.
Damage Distribution versus Subsurface Conditions. This chapter presents a
correlation between subsurface conditions and observed building damage that
demonstrates building damage is strongly influenced by subsurface conditions,
which addresses the main research objectives indicated above.
Ground Motion Modeling. This chapter comprises (1) the estimate of ground
motion characteristics on bedrock in Tacna based on attenuation functions; (2)
wave propagation analyses performed using the Program SHAKE2000 (Ordonez
2000) to incorporate the effects of local subsurface conditions; and (3) evaluation
of structure performance as a function of the ground motion characteristics.
1.4 Research Approach
The research approach comprised three phases: compilation of available data, field
evaluation, and analysis.
The compilation of available data comprised historical seismicity of the region,
seismological setting, factual data on the June 23, 2001 earthquake, and geologic and
13


subsurface conditions throughout the city of Tacna. This information was compiled
to serve as a basis for correlation of damage with subsurface conditions and to
estimate site-specific ground motion characteristics in the city. The sources used for
this compilation and the relevant data obtained are presented and discussed in
Chapters 2 and 3.
The field evaluation included meetings with local engineers to discuss subsurface
conditions, construction practices, and damage pattern; limited observation of
subsurface conditions at locations where rock formations and soil deposits were
exposed; general observation of building distribution throughout the city; and detailed
damage survey. The methodology and results of the building and damage distribution
evaluations are described in Chapter 4.
The analysis comprised: (1) correlation between subsurface conditions and observed
building damage; (2) estimation of ground motion characteristics in outcropping
bedrock in Tacna; (3) shear wave propagation modeling to study ground motion
characteristics at locations underlain by soil profiles of various depths; and (4)
correlation of observed damage pattern with subsurface conditions and ground motion
characteristics. These analyses and their results are presented in Chapters 5 and 6.
14


2. Seismological Aspects and Ground Motions
2.1 Historical Seismicity of Region
The continental margin along the west coast of South America, including Peru,
experiences some of the greatest tectonic activity in the world. The oceanic Nazca
plate and the South American continental plate converge along this margin, resulting
in the subduction of the Nazca plate. Consequently, the oceanic and coastal areas of
Peru are frequently subject to severe earthquakes, with magnitudes in excess of Mw 8.
The most notable physiographic features that result from convergence of the two
plates are the oceanic trench located at the surface contact of the two plates and the
orogenetic process that formed the Andes mountains.
Historical seismicity has been recorded in Peru since arrival of the Spaniards to
America in the late 15th century. A compilation of maximum Modified Mercalli
(MM) intensities observed throughout Peru is presented in Figure 2.1 (Alva et al.
1984). This figure indicates that the maximum estimated historical Modified Mercalli
intensity reported in Tacna is IX. Other locations in Peru show intensities of up to X
MM.
Some of the major documented earthquakes that affected the Tacna region occurred
on August 13, 1868, May 9, 1877, and January 15, 1958. Figures 2.2 through 2.4
present iso-intensity maps for these three events (Cotrado and Sina 1994). As shown
in these figures, average Modified Mercalli intensities in Tacna were estimated as
VIII, VII, and IV, respectively. Magnitude of the 1958 earthquake was reported as
7.3 (Silgado 1978). Magnitudes for the 1868 and 1877 events are unknown because
seismographs did not exist at those times. However, recent approximations have
estimated the magnitudes of these earthquakes to be Mw = 9.1 and Mw = 8.9
respectively (Kausel 1986).
15


16


70 W
Figure 2.2. Iso-Intensity Map Earthquake of August 12, 1868
(Kausel 1986, Cotrado and Sina 1994)
17


70W
65W
1 5S
Potosi
* t- 20S
25S
-4- 30S
Figure 2.3. Iso-Intensity Map Earthquake of May 9, 1877
(Kausel 1986, Cotrado and Sina 1994)
18


Figure 2.4. Iso-Intensity Map Earthquake of January 15, 1958
(Cotrado and Sina 1994)
19


2.2 Seismological Setting
The dip angle of subduction of the oceanic plate along the southern coast of Peru is
typically 12 to 15 degrees from horizontal as shown in Figure 2.5 (Ness and Johnson
1980). Similar subduction zones can also occur in the Pacific Northwest of the United
States and in the Aleutian-Alaskan subduction zone. As a consequence of the
subduction of the oceanic plate under the continental plate, three main types of
seismic sources occur in Peru:
1. Shallow inter-plate of the subduction zone, that dips between the oceanic
trench and the coastal area (thrust-faulting).
2. The Benioff zone of intermediate and deeper depth of the subduction zone.
3. The shallow continental intra-plate, associated with shallow faulting in the
Andean region.
Sab-Awtrwt Zont
Figure 2.5. Schematic Diagram of Subduction Zone in Southern Peru
(Ness and Johnson 1980)
It should be noted that the oceanic trench where the two plates meet is about 50 km
from the coast of Peru and the oceanic plate dips at about 12 to 15 degrees from
20


horizontal (Figure 2.5). Therefore, seismic source zones along oceanic and coastal
areas of Peru are shallow, with typical hypocentral depths of less than 50 km.
Based on the spatial distribution of seismic activity and tectonic features, 20 seismic
source zones have been identified in Peru, each with specific seismo-tectonic
characteristics. These sources are shown in Figures 2.6 and 2.7 (Castillo and Alva
1983). Source zones 1 through 5 are located along the coast and model the shallow
interplate of the subduction zone (0 to 70 km). Source zones 6 through 12 model
shallow continental intra-plate seismicity in the Andean region. Source zones 13
through 19 represent intermediate depth seismicity (71 to 300 km) in the Benioff
zone. Finally, source zone 20 models deep (500 to 700 km) intraplate seismicity in
the Benioff zone. Figure 2.8 (USGS 2001) is a cross section through southern Peru
showing hypocenters. Seismic hazard studies conducted in Peru during the last
decade provide useful information regarding the probabilistic distribution of PGAs.
Figures 2.9 and 2.10 show PGAs with a 10 percent probability of exceedance in 50
and 100 years, respectively (Castillo and Alva 1993). These figures indicate that
PGAs with a 10 percent probability of being exceeded in 50 and 100 years for Tacna
are 0.43 g and 0.51 g, respectively.
2.3 The June 23, 2001 Southern Peru Earthquake
2.3.1 Seismological Aspects
The June 23, 2001 Southern Peru earthquake is a shallow inter-plate subduction event
(Seismic Type 1 from above). The main shock occurred as thrust-faulting along the
interface between the oceanic Nazca plate and the overriding South American
continental plate as depicted in Figure 2.5. Preliminary seismological observations
indicate that the June 23 earthquake appears to have involved rupture of part of the
plate-boundary segment that produced an earthquake of magnitude approximately Mw
9.1 in 1868 (USGS 2001) (see iso-intensity curves in Figure 2.2).
The epicenter was located about 382 km northwest of Tacna. The epicenter is the
vertically projected location of the hypocenter of an earthquake corresponding to the
initial rupture location. Based on information from the USGS, the earthquake
epicenter is located at 16.14S 73.31W, near the city of Ocona. More than sixty
significant aftershocks (Mw > 5.0) were reported through the thirty days following the
main shock with the largest having a magnitude of Mw 7.6 on July 7 (USGS 2001).
Approximate locations of the epicenters of the June 23 earthquake and aftershocks are
shown on Figure 2.11 (USGS 2001). Figure 2.11 also shows the plate rupture
boundary for the Mw9.1 1868 earthquake. It is interpreted from the locations of the
aftershocks with respect to the main event, that rupture propagated southeastward.
21


Figure 2.6. Shallow Seismic Sources (Castillo and Alva 1993)
22


Figure 2.7. Intermediate and Deep Seismic Sources (Castillo and Alva 1993)
23


Interpreted June 23,
001 Rupture Plane
iI_____tgj___i i____i i____i I____i___i i_
III I I I I | I I I I I T" |" I I I 1 I I I j I 1 I
0 200 400 600
distance along prefile (km)
Figure 2.8. Cross Section of Seismicity Through Southern Peru (USGS 2001)
24


Figure 2.9. Maximum Accelerations with a 10% Probability of Exceedance in 50
Years (Castillo and Alva 1993)
25


Years (Castillo and Alva 1993)
26


Figure 2.11. Epicenter and Aftershocks of June 23, 2001 Southern Peru Earthquake
Showing Approximated Rupture Plane (Map from USGS 2001)
The epicenter of the June 23, 2001 earthquake is located within the seismic source
zone F4. However, based on the distribution of aftershocks (Figure 2.11) the rupture
boundary of the June 23, 2001 earthquake shows greater similarity to the rupture area
of the 1868 earthquake than the F4 boundaries. Both the 1868 and 2001 rupture areas
are approximately centered around the F4 and F5 boundary, but rupture did not
propagate as far south during the more recent event as it did in 1868 (see Figure
2.11).
Previously, estimates of rupture length have been based on published relationships
between rupture length and earthquake magnitude or duration of shaking (Seed et al.
1969). The rate at which rupture propagates along a fault was believed to be
approximately 3.2 km per second (2 miles per second), but depended on the type of
rupture. Based on the recorded motion in Moquegua, this average estimate would
27


lead to a rupture length of approximately 250 km to 350 km, depending on the
amplitude and corresponding duration of motion considered. More recently,
teleseismic analyses of recorded P and S waves have been used to determine the
distribution of the fault slip, and thus the fault mechanisms and area of rupture.
Several investigators have estimated the length, width, depth and dip angle of the
2001 rupture zone as summarized below in Table 2.1.
Table 2.1 Rupture Zone of the June 23, 2001 Southern Peru Earthquake
Length (km) Width (km) Depth (km) Dip () Mw Source
200 100 30 (focal) 21 8.2 Kikuchi and Yamanaka 2001
240 100 26 (average) 13 8.4 Ruegg et al 2001
370 150 29-31 27 8.2 IGP 2002
8 6 8.35 USGS 2001
- - 29.6 (centroid) 18 8.4 Harvard Seismology 2001
The estimated length of the fault rupture can be checked against the distribution of
aftershocks occurring immediately after the main shock, making the reasonable
assumption that the immediate aftershocks occurred on or near the main shock
rupture plane. Within 24 hours after the main shock, aftershocks occurred up to about
300 km southeast of the epicenter. It should be noted that Kikuchi and Yamanaka
(2001) interpreted a rupture length of only about 200 km based on their assumption
that there was enough stored energy to produce the largest aftershock (see Figure
2.11) within the southeastern most 100 km of the aftershock distribution, indicating
that this energy was therefore not released during the initial rupture. Their
interpretation also explains the lower moment magnitude reported by them. For the
analyses presented in this thesis, a rupture area of 350 km long by 150 km wide with
a dip angle of 18 degrees was assumed. This rupture area is shown in plan on Figure
2.11 and in section on Figure 2.8.
Although the earthquake had a shallow focal depth, it is difficult to estimate a single
representative focal depth value because a large portion of the plate interface
ruptured. This is evident by the variability of the estimates presented in Table 2.1. It
is unclear from some of the different sources whether depth refers to the focal depth
to the initial rupture or whether it refers to the depth of the centroid of energy release,
often called the centroid depth. For this particular earthquake, a very large
asperity was derived [to] the southeast of the epicenter.. .about 150 km from the
epicenter based on the spatial distribution of fault slip (Kikuchi and Yamanaka
2001). This location of maximum energy release (i.e. slip) is shown by the cross-
hatching in Figure 2.11. Because the area of maximum energy release is located
28


southeast of the epicenter, although it is generally in line with the epicenter along the
rupture plane, there may be some minor deviation between what is termed the focal
depth and the centroid depth.
For the purposes of this thesis, it is assumed that depth refers to centroid depth, but
that it does not vary greatly from the focal depth. A centroid depth of 30 km was
assumed for the purposes of the analyses presented in Chapter 6. The selected size
and orientation of the rupture zone is used to determine closest distances to the
seismic source, as discussed in Section 6.1.
2.3.2 Magnitude and Intensity
Magnitude of an earthquake is a measurement of its size and indirectly represents the
amount of energy dissipated. As well as in the rest of the world, published
magnitudes of Peruvian earthquakes have been calculated using various magnitude
scales. Although most magnitudes are calculated based on the same concept
[amplitudes and periods of seismographs from the world wide standard seismograph
network (WWSSN)], the various scales use different parts or phases of a
seismograph. This complicates comparison, because the various magnitude scales are
not compatible with each other, and there are not simple and general relationships to
convert from one scale to another. Because the different magnitude scales use
different phases of the recorded motion, some use long-period waves and other use
short-period waves, the different magnitude scales allow for the measurement of
different size earthquakes. Below is a brief discussion of the various magnitude
scales in common use today.
Local magnitude (ML), often referred to as the Richter Scale, was initially defined by
Richter in the 1930s for local earthquakes (i.e. seismic source is within 600 km of the
recording station) of southern California, by the expression:
Ml = log A
Where A is the maximum amplitude of the seismic wave, measured in thousands of
millimeters, recorded in a Wood-Anderson seismometer at 100 km from the
epicenter. Although this scale is most often used to classify Californian earthquakes,
it is sometimes used in other regions as well.
Prior to 1963, when the WWSSN was established, magnitudes of shallow earthquakes
(focal depth less than 70 km) were generally calculated using surface waves, referred
to as Surface Wave Magnitude. This scale was based on the maximum amplitude of
surface waves recorded at an epicentral distance greater than 20 degrees
29


(approximately 2200 km) and period between 17 and 20 seconds. This magnitude
gives similar values to Richters magnitude (Ml) (Bolt 1975).
Deep and intermediate depth earthquakes (focal depth greater than 70 km) do not
generate surface waves as substantial as shallow depth earthquakes; therefore, a
magnitude based on surface wave amplitude can be underestimated. Prior to 1963,
magnitudes of these earthquakes were calculated using body waves (P and S) with
periods in the range from 5 to 12 seconds (Geller and Kanamori 1977), and was
termed the Body Wave Magnitude.
The definition of magnitudes discussed above used by various authors were different.
The definition of magnitude based on surface waves has been standardized since then
to be based on Rayleigh waves of 20 seconds of period (Geller and Kanamori 1977).
Magnitudes calculated based on this definition are called Ms. The definition of
magnitude based on long period body waves has been standardized to be based on P
waves with a period of 1 second and is designated as mb. The mb scale was intended
initially to be equivalent to Ms but it was found that they are not.
A particular seismic phase may include information related only to the dimensions of
the source that are comparable to its average wave length. Since the dimensions (size
of the rupture area) of the source increase with the size of the earthquake, the short
period phases (short length waves) stop containing information about the earthquake
source above a given size. As a result, the ML scale begins to saturate around a
magnitude of 6.5. The mb scale, based on P waves with period of 1 second, becomes
saturated at approximately 6.8 mb and is not a useful measure of the earthquakes
above approximately 6.5 nib Similarly, the Ms scale becomes saturated at about 8.5
because it is based on longer period waves that allow representation of larger source
dimensions.
A new magnitude scale has been developed by Kanamori (1977) to address the above
limitations and is called the Moment Magnitude (Mw). This scale eliminates the
problems of saturation and dependency on specific seismic phases. The moment
magnitude Mw is calculated based on the seismic moment at the source rather than the
amplitude of ground motions at the station, as in the previously discussed scales. The
seismic moment can be determined based on the area of fault rupture, the average
amount of slip, and the rock rigidity (frictional resistance) or can also be determined
from the shape of the acceleration spectra of seismic waves that indicates the total
energy recorded. This scale can be measured using both local and teleseismic
recording stations. Unlike the other scales, the main limitation to this scale is the
minimum magnitude that can be registered. A magnitude of about 3.5 for local
events and 5.5 for teleseismic events are usually necessary to generate enough energy
30


to determine the Mw (UC Berkeley 2002). Because the moment magnitude can be
estimated from seismo-geological information, it can be used to estimate the
magnitude of historic earthquakes that occurred prior to recording stations.
Various magnitude values have been reported for the June 23, 2001 event (Mw 8.4 by
the Harvard CMT, Mw 8.35 by the USGS, Mw 8.2 by the Earthquake Information
Center, Tokyo, and Mw 8.2, mb 6.9 and Ms 7.9 by the Institute Geofisico del Peru
[IGP]). It should be noted that the mb and Ms scales become saturated at about 6.5 and
8.5, respectively. Therefore, the values reported by IGP are already at about the
saturation values. For the purposes of this thesis, the moment magnitude has been
used because it is considered to be the relevant representation of an earthquake of this
size.
An iso-intensity map of the June 23, 2001 earthquake published by the Geophysical
Institute of the National University of San Agustin in Arequipa (UNSA 2001) is
included as Figure 2.12. This map shows that the average Modified Mercalli
intensity assigned to Tacna was VII. It also shows a maximum intensity for the
earthquake of VIII in the region surrounding Moquegua.
31


Figure 2.12. Iso-Intensity Map of the June 23, 2001 Southern Peru Earthquake
(IGP 2002)
32


2.3.3 Ground Motion at Moquegua
The main shock of June 23, 2001 was recorded only by one local ground motion
station, located in the city of Moquegua, about 116 km northwest of Tacna and 276
km southeast of the epicenter. The ground motion station at Moquegua is owned by
CISMID (Peru Japan Center for Seismological Investigations and Disaster
Mitigation) and is located at S17.1868 W70.9287. The accelerometer is located in a
flat alluvial terrace in the south side of an east-west trending river valley. The closest
topographic features are the river, located about 200 m from the accelerometer, and
the valley wall located about 1,000 m from the accelerometer in the opposite
direction. The instrument is housed in a wooden shelter next to a one-story reinforced
concrete building. A concrete block wall located immediately behind the instrument
shelter collapsed during the earthquake without damaging the shelter.
The National University of San Agustin (UNSA) in nearby Arequipa performed a
subsurface investigation of locales in the city of Moquegua (Kosaka et al. 2001).
Based on seismic refraction and electric resistivity soundings at the airport, located on
the same alluvial terrace and across the river from the recording station, and
discussions with engineers local to Moquegua, the strong motion station is located on
top of a coarse, dense alluvial gravel, with possibly a shallow layer (2 to 4 m) of
medium dense to dense sandy overburden. Measured P-wave velocities (Vp) at the
airport were about 250 mps in the silty sand overburden (0-2.6 m) and in the gravel
deposit were about 2,091 meters per second (mps) from 2.6 to 16.2 m of depth, and
2,478 mps below this depth. Corresponding shear wave velocities (Vs) were estimated
to be approximately 133, 1,120 and 1,325 mps using the below equation and
assuming a poissons ratio, v, of 0.3.
V,
/> _
2-2v
ii-
2v
Eqn. 2.1
Based on the above shear wave velocities and regional geology information, the
gravel deposit is interpreted to consist of the dense alluvial conglomerate discussed in
Chapter 3 and is assumed to extend to bedrock.
Acceleration time histories recorded by CISMID (2001) at Moquegua show total
duration of the earthquake was about 120 sec and duration of the strong motion
(a> 0. lg) was about 36 sec. The PGAs were 0.30 g and 0.22 g in the east-west and
north-south directions, respectively. The recorded vertical PGA was 0.16 g. It should
be noted that in Peru, the east-west component generally has the highest PGA because
it coincides with the overall direction of displacement of the subducted oceanic plate.
33


Figure 2.13 shows the acceleration, velocity and displacement time histories for the
Moquegua recorded motion.
Figure 2.14 shows the acceleration response spectra for the east-west, north-south,
and vertical components of the Moquegua record. These spectra show maximum
spectral accelerations of 0.8 g, 0.8 g, and 0.47 g, respectively, which correspond to
spectral amplifications of 2.7, 3.6, and 2.9. The east-west spectrum shows three peaks
of approximately 0.8 g at periods ranging from 0.17 to 0.95 seconds. Predominant
periods of vibration are 0.17 to 0.95,0.65, and 0.6 sec, respectively. (Predominant
frequencies of 5.9 to 1.1, 1.7, and 1.7 cycles/sec). The three peaks of the response
spectra are relatively subtle and form more of a plateau shape. It is noted that this
spectral shape is unusual for a ground motion recorded on a relatively homogenous,
shallow, soil deposit. Ground motions recorded on this type of deposit would
generally be anticipated to show a discrete peak at a relatively short predominant
period.
34


Displacement (cm) Velocity (cm/sec) Acceleration (cm/sec/sec)
300.00
0.0
300.00
0.0
300.00
0.0

N-S 220.040
................
E-W 295.270
VER 160.610
0.0
20.0
40.0
60.0
80.0
100.0
Figure 2.13. Acceleration, Velocity, and Displacement Time History June 23, 2001
Southern Peru Earthquake
35


Acceleration (gal)
Moquegua City Station E-W Component
0 Ot 0 t 1 10 100
Pc riod (sec )
Max Spectral Acceleration: 0.80g
Predominant Period: 0.17,0.45 and 0.95 sec
Moquegua City Station N-S Component
Max Spectral Acceleration: 0.80g
Predominant Period: 0.65 sec
Moquegua City Station U-D Component
Max Spectral Acceleration: 0.47g
Predominant Period: 0.60 sec
Perlod (sec )
Figure 2.14. Acceleration Response Spectrum June 23, 2001 Southern Peru
Earthquake (CISMID 2001)
36


3. Subsurface Conditions and Distribution
3.1 Geologic and Geotechnical Setting
As previously described, Tacna is located approximately 38 km northeast of the
Pacific coastline on a desertic strip of land bounded to the east by the steep chain of
the Andean Mountains. Figure 1.2 shows a regional topographic map (IGN 1995a and
1995b) and Figure 3.1 shows a regional physiographic map (Comision Carta
Geologica Nacional 1963a). As shown in these maps, the city is in the Caplina river
valley, between the Intiorko and Arunta hills to the northwest and the southeast,
respectively.
Figure 3.1. Physiographic Units in the Tacna Region
(Comision Carta Geologica Nacional 1963a)
Figure 3.2 presents a geologic map of Tacna and the surrounding region (Comision
Carta Geologica Nacional 1963a and 1963b). The geology in the valley of Tacna has
37


a number of sinked block-like tectonic depressions that start in Calientes (at elevation
900 m.a.s.l., 20 km upstream of Tacna) and continue to the ocean. These tectonic
depressions are filled with continental deposits of the Moquegua Formation and
alluvial deposits. Figure 3.3 shows a generalized stratigraphic column for Tacna
(Comision Carta Geologica Nacional 1963a). This column shows the following strata
(from top to bottom):
A Quaternary alluvial deposit of the Caplina river, up to 100 m thick. This
deposit is found in the Tacna, Pocollay, and Gregorio Albarracin districts, and
consists mainly of cobbles and boulders, rounded to sub-angular, with diameters
ranging from 1 to 30 cm and average from 10 to 20 cm. Although this is an
alluvial soil deposit, this material is locally called conglomerate.
The Huaylillas Formation, of the upper Tertiary, up to over 300 m thick, formed
by rhyolitic (volcanic) tuffs. This formation outcrops in the northern part of the
city (Ciudad Nueva and Alto de la Alianza districts).
The Moquegua Formation, also of the upper Tertiary, up to over 100 m thick,
formed by sandstones and conglomerates of continental origin.
An undifferentiated sequence that spans from upper Jurassic to lower Tertiary.
The Guaneros Formation, up to over 2,000 m thick, of Callovian age (upper
Jurassic), formed by a volcanic sedimentary sequence of andesites, sandstones,
and shales.
The oldest stratigraphic unit is the Chocolate volcanic of upper Liassic age (lower
Jurassic), consisting mainly of andesites with scarce layers of shale and quartzite.
Figures 3.4 and 3.5 show geologic cross-sections through the city of Tacna (adapted
from Cotrado and Sina 1994). Locations of these cross-sections are shown on Figure
3.2. Cross-section I (Figure 3.4) depicts the geology along the valley of Tacna from
Calientes to the Pacific Ocean. Cross-sections II and III (Figure 3.5) show the
geology across the valley, immediately upstream and downstream of the city
of Tacna, respectively. These cross-sections illustrate the approximate extent of the
alluvial conglomerate in the Caplina valley. It should be noted that volcanic tuffs of
the Huaylillas Formation are encountered exposed or at shallow depths in the
northern part of the city (Ciudad Nueva and Alto de la Alianza). Also, airfall
volcanic ash is found as the uppermost layer at various locations in the area.
Part of the city of Tacna, including the older downtown area and the Pocollay and
Gregorio Albarracin districts, is underlain by the deep alluvial conglomerate deposit
from the Caplina river. The conglomerate is found at shallow depths in some areas
(< 0.5 m) and deeper (0.5 to 3 m) in other areas. When the conglomerate is deeper, it
is overlain by finer sandy, silty, or clayey soils. The rest of the city, consisting of the
north cone (Alto de la Alianza and Ciudad Nueva, and developments on the Intiorko
38


hill slopes) and part of Pocollay district are on volcanic tuffs and silty sands formed
from weathering of the tuffs or airfall volcanic ash. This part of the city is outside of
the area of influence of the Caplina river, so conglomerate or other alluvial soils are
not found.
39


Figure 3.2. Geologic Map of Tacna and Surrounding Region
(Comision Carta Geologica Nacional 1963a&b)


BIST SER1E
QUAT R#c,n*0
o
c
3
CO
Formation
Alluvial Deposit
ZTZJV? ton
Hu o y li II o s
Moquegua
Gu on eros
Chocolate

U 11 A AAA
aaAAAAAA
AAAAAdAA
AAAAAAAA
1111A AAA
a&a&AAAA
AAAAAAAA1
AinA
VVVVVVVV V'
' V V V V V V V V
vvvvvvvvv
VV VVVVV V
VVVV VVVVV;
VVVVVVVVVVV'
1 vv v v vvvvvv*
VVVVVVVVV^

r
Fine to med. grained sand
- Silt, sand, gravel and conglomerate
.Gypsum
Rhyolitic tuffs, compact and
brecciated, pink and brownish red
-Disconformity...... Tuffaceous sandstone,
-Disconformity-
greenish gray, w/
| interbed, brown congl.
nd clays
Andesitic volcanics, gray and reddish brown,
thick layers, porphyritic texture, with variable
amounts of interbedded breccias, reddish
calcareous sandstones, violet sandstones with red
siltstone and shales
Calcareous sandstones, gray and violet,
interbedded with porphyritic volcanic andesite,
greenish gray to reddish brown, gray to reddish
brown and green sandstones, dark gray shales,
and thin layers of gray limestone
-Disconformity-
Andesitic flows and pyroclastics, with scarce
interbedded green shales and gray quartzites
Figure 3.3. Generalized Stratigraphic Column of Tacna Region
(Comision Carta Geologica Nacional 1963a)
41


LEGEND
Vertical
Scale
Rhjrolilk tuff (bedrock) H r
Rite grained sond Ar.f
Coarse grained sand Ar.g
Alluvial conglomerate Csj
White vclconk day tuff Tb.v
Marine conglomerate Cong.m
Pocoilay
PROFILE
Figure 3.4. Geologic Profile Along Tacna Valley (Cotrado and Sina 1994)
Vertical
Scale
Vertical
Scale
Horizontal Scale
i h i i i
OC .
*0
nr.
rr
vn
*cn
C* On Oirzoi
ICO
.TOO
mo

Figure 3.5. Geologic Cross-Sections Across Tacna Valley (Cotrado and Sina 1994)
42


Similar to the Tacna region and the Caplina river, the Andes are very high and are
relatively close to the ocean in central and southern Peru. Therefore, rivers flowing to
the ocean from the Andes are short in length and fairly steep. Because of their short
length, steep gradient and considerable flow, these rivers were able to carry in the
geologic past a considerable amount of large-size material, depositing similar
conglomerate deposits. The conglomerate is geotechnically described as a sandy,
bouldery gravel, poorly graded, usually dense to very dense, with cobbles and
boulders up to 50 cm in diameter. Fines are typically less than 2 percent. These
deposits were formed under very torrential flow regimes, during the Pleistocene
deglaciation (Quaternary). Particle orientation within the conglomerate gives it an
apparent cohesion due to grain interlocking, which allows it to be excavated with near
vertical cuts. These deposits of conglomerate may include some thin layers of hard
sandy clays or dense clayey sands. A typical profile of this soil deposit is shown in
Figure 3.6.
Figure 3.6. Typical Conglomerate Profile in Tacna
43


Evaluation of physical and mechanical properties of alluvial conglomerate deposits is
difficult due to their large particle size. Drilling methods that can be used in this
material, such as rotary or percussion, allow identification of the material but are
destructive. Penetration testing is not representative of their relative density due to the
large particle size in relation to the spoon diameter, even for the large diameter
spoons. Obtaining undisturbed, or even fully representative samples, is difficult to
impossible. Becker rigs are not locally available, therefore Becker testing has not
been performed. Because of these difficulties, geotechnical investigations of this type
of material are frequently conducted in Peru by means of test pits excavated
manually, that allow proper identification of their physical properties, as well as
estimating their relative density by manually evaluating how tight the cobbles and
boulders are within the overall mass.
The alluvial conglomerate deposits encountered in several rivers along the Peruvian
coast have been shown to have similar properties in the different coastal cities,
including Lima and Tacna. Since conglomerate deposits are relatively common along
the coast of Peru, abundant experience is locally available in this type of soil,
including part of Lima. That experience indicates that this type of soil provides
generally very competent foundations. Experience available that provides meaningful
data on the properties and performance of conglomerate deposits includes
measurements of in-situ density, down-hole shear wave velocity, plate load tests, and
large size in-situ direct shear tests (Carrillo-Gil 1979, 1987; Carrillo-Gil and Garcia
1985; Repetto et al 1980; Humala 1982). Also, the observational approach has
provided decades of information related to the performance of structures ranging
from 1-story houses to multi-story buildings founded on conglomerate. Local practice
is to design shallow foundations on these deposits with allowable bearing pressures of
3 to 5 kg/cm2 (300-500 kPa), which has shown to result in minimal settlements and
good performance during several major earthquakes.
Another relevant observation is the performance of cliffs along the coast of Lima that
have remained stable during numerous reported earthquakes since Lima was founded
by the Spainards in 1535. These cliffs are up to 60 meters high and exhibit angles as
steep as 70 degrees from horizontal (See Figures 3.7 and 3.8). Back analysis of those
slopes indicates that for an assumed friction angle of 40 45 degrees, a cohesion of
0.6 to 0.8 kg/cm (60 80 kPa) is required to maintain stability during major
earthquakes. This cohesion is interpreted as an apparent cohesion due to particle
interlock produced by orientation of the large cobbles and boulders (See Figure 3.9)
44


45
JSSiiiL'


Figure 3.9. Conglomerate Deposit in Limas Cliffs
46


As discussed above, a significant part of Lima, the capital of Peru, is founded on a
deep conglomerate deposit that has been studied with much detail (Repetto et al.
1980; Karakouzian et al. 1997; Carrillo-Gil 1979, 1987; Carrillo-Gil and Garcia
1985) . Based on alike formation mechanism, visual observation, and geotechnical
studies reviewed, this conglomerate is inferred to be similar to that found in the study
area of Tacna. Typical conglomerate properties in Lima (located 960 km northwest
from the study area of Tacna) include an effective friction angle (at moderate vertical
loads) greater than 40 degrees; dry unit weights of 2.1 to 2.4 gr/cm3 (21-24 kN/m3);
specific gravity of solids of 2.66; and in-situ void ratios between 0.1 and 0.3. Shear
wave velocities measured in Lima by means of down-hole geophysical surveys
(Repetto et al. 1980) range between 500 and 550 mps at depths of 1 to 7 m, and
between 645 and 800 mps at depths of up to 17 m.
3.2 Bedrock and Water Table
Information about depth to bedrock and to the water table has been obtained from
several water wells (Comision Carta Geologica Nacional 1963a and CORDETACNA
1986) and from previous studies (Cotrado and Sina 1994; Silva and Berrios 1998).
CORDETACNA (1986) reports one well, designated A-209. Comision Carta
Geologica Nacional (1963a) reports three wells, designated T-2, T-3, and H-l,
located approximately 3 to 4 km downstream (southwest) from Tacna.
Well A-209 is located within the city in the Tacna district and provides a very
detailed stratigraphic profile. The location of this well is shown on Figure 3.10. This
well reported a 2-meter thick upper layer of eolian sand with gravel and boulders.
From 2 to 38 m of depth the profile consisted of Quaternary alluvium, mainly
conglomerate with boulders up to 90 cm in diameter. From 38 to 52 m, this well
reported volcanic tuff and tuffaceous sand of the Huaylillas Formation (upper
Tertiary), interpreted to be airfall volcanic ash with variable cementation, that
classifies as a weak rock for civil engineering purposes. Finally, from 52 m to the
bottom of the well (145 m), the profile consists predominantly of gravel and sand
conglomerates of the Moquegua Formation, with two intermediate thin layers of
tuffaceous sand from 120 to 123 m and from 144 to 145 m. These conglomerates are
interpreted to be of Tertiary alluvial origin with variable cementation, that classify as
a weak rock for civil engineering purposes. Wells T-2, T-3, and H-l reported the
same general stratigraphy, except that the upper eolian layer was not present. The
stratigraphy encountered in these four wells is summarized in Table 3.1, and is
considered to represent the southern portion of the city of Tacna.
47


Table 3.1. Summary of Water Well Profiles from the
Southern Region of the City of Tacna
Well Total Depth (m) Eolian Sand (m) Quaternary Alluvial Congl. (m) Volcanic Tuff (upper Tertiary) (m) Conglomerate (upper Tertiary) Water Table (m)
A-209 145.0 0.0 2.0 2.0-38.0 38.0 52.0 52.0-145.0 84.0-89.0
T-2 98.0 - 0.0 24.0 24.0 44.0 44.0- 98.0 78.5
T-3 106.0 - 0.0 32.0 32.0-58.0 58.0- 106.0 64.8
H-l 103.0 - 0.0 27.0 27.0-45.0 45.0- 103.0 47.7
For the northern part of the city, Silva and Berrios (1998) reported bedrock at a depth
of 1.5 m in test pits S-7 and S-8 (see Figure 3.10) excavated at the extreme north of
Ciudad Nueva, in the lower slope of the Intiorko hill. This is an area where the
volcanic Huaylillas Formation outcrops. At these test pits the soil profile consisted of
an upper layer of silty sand, interpreted to be weathered tuff, overlying the less
weathered tuff that was classified as a bedrock due to the impossibility of being
excavated with manual tools.
Based on the results of electrical resistivity soundings performed by the Ministry of
Agriculture in 1974, Cotrado and Sina (1994) describe bedrock as deep, ranging from
about 250 m north of Ciudad Nueva to about 500 m by the airport. In these electrical
soundings, bedrock is identified as hard andesitic rock of the Guaneros or Chocolate
formations (see Figure 3.3), that underlie the weak rock formations encountered in the
water wells.
It can be seen from the water well data and from Silva and Berrios (1998) that for
earthquake engineering purposes, rock is encountered at shallow depths, ranging from
1.5 meters in the northern cone to 38 meters in the southern cone of the city.
However, as discussed later in this chapter, the Quaternary Alluvial Conglomerate
found in the southern cone exhibits rock-like properties. Therefore, for purposes of
estimating wave propagation and site amplification, soil deposits are relatively
shallow throughout the city.
Water table is at depth ranging from 50 m in Magollo; 70 m in the south cone, Para
Chico, and Leguia; 80 m in the downtown area; and 90 m in Pocollay (see
Figure 3.10), based on information from regional water wells.
48


CERRO INTIORKO
CAfLfttltAA
PAMAHf BJCANA NOR It
TERRI NOS Of CUL1IVO
k.M19
.. AT14
*[ ,..'T^ -jlr'i I/.. _ -_______ _ f j
AWSE- :,Vx,C TVft, 3?fc________________,
nrtf&r^
-*=*Bl_f.-sw,i ^,^r- /.;r* i ,;- -I_ C4.C6I0
- /w^(.jfnaTo

-*EjB5?9. -. AAHs,
^ -pAj^PWCO
ms*
AEROPUERTO
- -- n / r~ 71._
-t -*71 r
J13
-*->
^.T12
11
.PLANT* OF
IRATAMCNTO
AGUAS
SERVIOAS
kMH
i *l <.-. '*.-*5^ '*--.';VV£'* t^:**1*
; ".'- . -. -' mi s
C24
ferrocabMl
taccmg^umca
lM13
^ .V : V1: y
* qomo sar;-^
, -- ..-s^w- /--'t*-
c-vt :
'*HM AV V
. ENACt M T?1
: y- .'A.-- '
;c ./. vvai-lv


WELL A 209
S,s>ei;s> ;,** ^/T~ Jc-.-
CV /-'A/ *<.-.' ,\W/--?£>-,
".. '. ^<>-yy)Cy y: iMw
.yv-.. :v: ^ / >
;.;v;-v^ y ^'v>-;^ ^
£v-Tto '.r; '* ,y >;.J
CUARTEL
TARARACA
a. M60 ; >*-
CERRO ARUNTA

W-
T1
A.
r
-yC?.,
mJl'
E19
PARQUE PERU
T3
/-. x
LEGEND
TEST PITS COMPILED BY C0TRAD0 AND SINA (C SITES)
: DEDcnaucn DvrnTDnn anh cima fc citc
________lUMrlLE.L> BY CUIRADU AND biNA
- TEST PITS PERFORMED BY COTRADO AND SINA (E SITES)
* MICROTREPIDATION POINTS COMPILED BY COTRADO AND SINA (M SITES)
MICROTREPIDATION POINTS PERFORMED BY COTRADO AND SINA (T SITES)
TEST PITS PERFORMED RY Sll V4 4Nn RERRIDS fS SITFSl
KU I Ktr ILIAIIU N PUINIS htKEUKIWtU t>T LUIKAU
TEST PITS PERFORMED BY SILVA AND BERRIOS (S SITES)
I TEST PITS COMPILED FOR THIS RESEARCH (J SITES)
TEST PITS COMPILED FOR THIS RESEARCH (J SITES)
Figure 3.10. Location of Subsurface Investigations and Microtrepidation Measurements (adapted from Cotrado and Sina 1994)
49


3.3 Subsurface Investigations and Soils
Characterization
To obtain a more accurate picture of the distribution and properties of the various
subsurface soils in Tacna, several sources of geotechnical site investigations were
researched. The microzonation study conducted by Cotrado and Sina (1994) compiled
subsurface profiles at 55 locations identified as C-l through C-55. Most of these
profiles correspond to test pits to depths ranging from 1 to 6 m. Identification of
these sites and a summary of soil profile and soil classification are presented in Table
3.2. Additionally, 21 test pits were excavated by Cotrado and Sina (1994) to depths
ranging from 1.5 to 3.5 m in areas with data gaps, identified as E-l through E-21.
Identification of these sites and a summary of soil profile, soil classification, and
index properties are presented in Table 3.3. The locations of all these investigations
are shown in Figure 3.10. As explained in Section 3.1, local practice for site
investigations generally consists of manual excavation of test pits to limited depth.
During excavation of test pits E-l through E-21, Cotrado and Sina (1994) performed
measurements of in-situ density at various depths. Maximum and minimum density
laboratory tests were then performed for samples of those soils, as well as index
properties. These data provide 24 values of relative density, as shown on Table 3.3.
This table shows values of relative density ranging between 24 and 64 percent for the
volcanic tuff (silty sand) with an average density of 1.5 gr/cm3 (15 kN/m3). Relative
densities for the gravel deposit were also reported. However, those values are not
considered to be representative, because of the difficulties of performing maximum
and minimum density tests in coarse grained soils.
Silva and Berrios (1998) conducted a microzonation study of Ciudad Nueva and Alto
de la Alianza districts. Nineteen test pits were excavated, generally to a depth of 3 m,
identified as S-l through S-19 and shown on Figure 3.10. In-situ density
measurements were performed and undisturbed block samples from the bottom of
these test pits were collected. Laboratory testing was then performed to determine
index properties, maximum and minimum density, direct shear, and collapsibility. It
should be noted that test pits S-7 and S-8, located at the extreme north of Ciudad
Nueva on the lower slope of the Intiorko hill, encountered bedrock of the Huaylillas
Formation at the depth of 1.50 m. A cohesion of 10 kg/cm2 (1,000 kPa) and friction
angle of 46 degrees was estimated for this rock. All other soils encountered in these
19 pits were classified as silty sand (SM) and are interpreted to be either airfall ash or
weathered tuff of the Huaylillas Formation. It is interpreted that the outcropping
bedrock encountered in the extreme north of Ciudad Nueva is underlying the silty
sand material encountered with variable thickness and relative density elsewhere in
the north eastern section of Tacna (i.e. Ciudad Nueva and Alto de la Alianza
50


districts). A summary of laboratory test results obtained by Silva and Berrios (1998)
is presented in Table 3.4. This table shows values of relative density ranging from 24
to 90 percent, with most values being in the 30 to 50 percent range, with an average
in-situ density of 1.4 gr/cm3 (14 kN/m3).
Table 3.2. Summary of Test Pits Compiled by Cotrado and Sina (1994)
(C-l through C55)
Test Pit Classification Depth to Bottom of Layer
uses (m)
C-l FILL 0.4
GW 1.2
GW 3.2
C-2 SM 1.2
SM 1.3
GP 2.9
C-3 SM 0.8
GP 3.8
C-4 SP 2.9
GP 5.8
C-5 ML 0.5
OL 0.8
SM 1.3
GP 1.9
GW 4.0
C-6 ML 0.7
OL 0.9
SM 1.5
GP/GW 1.9
GP 2.5
GW 4.0
C-l ML 1.5
OL 1.9
SM 2.1
GP 2.5
GW 4.0
C-8 ML 1.0
GP 1.5
GW 4.0
51


Table 3.2. Summary of Test Pits Compiled by Cotrado and Sina (1994)
(C-l through C55) Cont.
Test Pit Classification uses Depth to Bottom of Layer (m)
C-9 ML 1.0
OL 1.2
GP 1.6
GW 4.0
C-10 SP 2.9
GP 6.0
C-ll SC 2.5
GP 4.0
GW 6.0
C-12 FILL/SM 1.2
GW/SP 2.0
GW/SM 5.2
GW/SP 6.0
C-13 FILL 0.6
SP/SM 2.4
SP/GP 3.2
SM/GP 6.0
C-14 CL 0.5
GP 1.6
C-15 FILL 0.6
GW 3.2
C-16 ML 0.5
GP 1.4
GP/GW 1.8
GW 2.8
GW 6.0
C-l 7 OL 1.2
GP 2.8
C-18 FILL 0.4
TUFF 3.2
C-19 FILL 1.2
SM 3.0
GP 4.0
52


Table 3.2. Summary of Test Pits Compiled by Cotrado and Sina (1994)
(C-l through C55) Cont.
Test Pit Classification uses Depth to Bottom of Layer (m)
C-20 FILL 0.8
OL 1.4
SM 2.6
GP 4.0
C-21 FILL 0.8
SM 2.2
GP 4.0
C-22 FILL 0.2
GW 1.0
GW 3.0
C-23 FILL 0.2
SP 1.1
SP 1.8
GP 3.0
C-24 FILL 0.2
SP 0.6
GW 1.2
SC 2.0
GW 3.6
C-25 C-26 FILL/SM 0.3
SW 0.4
CL 1.5
GC 2.0
C-27 FILL/SM 1.2
SP 1.3
CL 2.1
GP 2.3
C-28 SM/SC 2.3
GP 2.7
C-29 C-30 GC 1.5
C-31 SM 1.5
GC 2.0
53


Table 3.2. Summary of Test Pits Compiled by Cotrado and Sina (1994)
(C-l through C55) Cont.
Test Pit Classification uses Depth to Bottom of Layer (m)
C-32 SM 0.9
GC 1.0
SM/SC 1.5
GC 2.1
C-33 GC 1.5
C-34 GC 1.5
C-35 C-37 GC 1.2
C-38 GC 1.2
C-39 FILL/GP 1.7
C-40-C-41 FILL/SM 0.6
GC 1.7
C-42 SM 1.4
GW 1.5
SM/GC 2.3
GC 2.7
C-43 SM 1.0
SP 1.1
SM/SC 1.8
GC 2.2
C-44 FILL 0.5
GC 1.2
C-45 GC 2.0
C-46 GC 2.0
C-47 SM 0.6
GC 1.1
C-48 SC 2.2
GC 2.4
C-49 SM 0.4
GW 0.5
SM/SC 1.5
GC 2.2
54


Table 3.2. Summary of Test Pits Compiled by Cotrado and Sina (1994)
(C-l through C55) Cont.
Test Pit Classification Depth to Bottom of Layer
uses (m)
C-50 FILL 0.3
FILL 0.4
GC 1.1
C-51 GC 1.1
C-52 FILL 0.3
C-53 FILL 2.0
C-54 SM 0.2
C-55 FILL 3.0
SM 1.0
55


Table 3.3 Summary of Test Pits and Laboratory Testing Performed by Cotrado and Sina (1994) (E-l Through E-21)
Test Pit Depth (m) uses w (%) LL PL Unit Natural Weight (gr Max /cm"1) Min Dr (%)
E-l 0.80 SM
1.00 TUFF
2.20 SP 2.20 1.42 1.60 1.37 24.49
E-2 2.50 SM 2.20 23.50 3.56 1.66 1.86 1.55 39.76
E-3 1.40 SP 2.20 15.90 1.38 1.57 1.33 23.70
2.50 SW 1.65 17.90 1.46 1.62 1.40 30.26
E-4 1.40 SM 1.58 1.64 1.55 34.60
1.90 SW 1.65 17.90 1.64 1.70 1.59 47.12
E-5 1.80 SM 2.05 21.50 3.81 1.59 1.66 1.54 43.50
E-6 1.70 SM 2.05 21.50 3.81 1.52 1.62 1.46 39.97
E-7 0.60 SM
1.20 SC
3.00 SM 4.87 22.90 2.18 1.27 1.54 0.97 63.82
E-8 2.60 SM 2.55 22.38 1.90 1.43 1.78 1.10 60.41
E-9 3.00 SM 7.64 15.36 1.10 1.96 2.10 1.75 64.29
E-10 2.00 SW 4.30 2.18 2.35 1.98 58.27
2.70 GW 2.01 2.20 2.50 1.85 61.19
E-l 1 1.00 SW 1.00 1.32 1.52 1.25 29.85
2.00 GP 2.00 2.06 2.16 1.94 57.19


Table 3.3 Summary of Test Pits and Laboratory Testing Performed by Cotrado and Sina (1994) (E-l Through E-21)
Cont.
Test Pit Depth uses w LL PL Unit Weight (gi 7cm3) Dr
(m) (%) Natural Max Min (%)
E-l 2 1.00 GP 2.20 1.27 1.32 1.18 66.82
2.00 TUFF
E-l 3 0.40 GP 2.20 1.36 1.45 1.21 66.61
3.00 TUFF
E-14 2.20 GP 2.65 2.16 2.35 1.81 70.52
E-15 0.40 FILL
1.00 GP 2.65 2.10 2.25 1.85 66.96
3.00 GW 2.20 2.19 2.32 1.90 73.15
E-l 6 2.50 GW 2.20 2.31 2.38 2.19 65.07
E-17 3.60 GW 1.95 1.99 2.09 1.85 61.26
E-l 8 0.20 FILL
3.00 GW 2.10 2.10 2.20 1.95 62.86
E-19 0.40 FILL/SM
1.60 SP 2.10
3.60 GP 0.85 2.15 2.25 1.88 76.37
E-20 0.40 ML 1.88
1.60 SP 1.13
3.00 SP 1.13


Table 3.3 Summary of Test Pits and Laboratory Testing Performed by Cotrado and Sina (1994) (E-l Through E-21)
Cont.
Test Pit Depth uses w LL PL Unit Weight (gi /cm3) Dr
(m) (%) Natural Max Min (%)
E-21 0.60 ML 1.62
1.20 SP 1.60
1.60 ML 1.57
2.90 SP 1.56 3.07 2.83 3.31 46.09
U\
00


Table 3.4. Summary of Laboratory Testing Performed by Silva and Berrios (1998) (S-l through S-19)
Test Pit Depth Classification % Finer Than w LL PL Dr c _ A . Yd
(m) uses #4 #200 (%) (%) kg/cm2 degrees kg/cm3
S-l 3.00 SM 100.0 25.6 2.1 23.7 NP 35 0.25*3 27.1*3) 1.54/ 1.683)
S-2 3.00 SM 98.9 38.6 2.9 19.2 NP 34 0.13(,) 27.1(3> 1.54 / 1.63<3)
S-3 3.00 SM 100.0 15.2 0.8 29.2 NP 28 0.20*11 22.0*1' 1.50(l)
S-4 3.00 SM 100.0 27.3 1.0 27.9 NP 48 n o o o 28.8*3) 1.46/ 1.61(3)
S-5 3.00 SM 100.0 17.5 0.8 22.6 NP 46 0.00(l) 28.6(I) 1.54 / 1.55(1>
S-6 3.00 SM 99.3 25.6 0.7 26.7 NP 24 0.30*J) 16.7*1) 1.39/ 1.46*:n
S-9 3.00 SM 100.0 42.2 2.7 36.6 NP 90 0.10*J) 00 1.10/ 1.20 S-10 3.00 SM 98.7 33.9 0.9 36.4 NP 81 0.00*n 44.6(,) 1.07(1)
S-10 3.00 SM 0.22U) 33.9*2) 1.08*2)
S-l 1 3.00 SM 93.8 18.6 0.5 26.2 NP 61 0.05*j) 32.0(l) 1.57/ 1.58(l)
S-12 3.00 SM 93.1 18.9 0.4 25.7 NP 46 0.30*n 29.0*^ "TliTTW17"
S-13 3.00 SM 99.0 24.8 1.0 27.4 NP 27 i o! o'| 19.4U) 1.29/ 1.38(3)
S-14 3.00 SM 100.0 21.6 0.8 27.3 NP 49 0.00*u 29.4(l) 1.49*1 ^
S-15 3.00 SM 100.0 18.8 3.6 28.2 NP 37 0.20*3' 27.1<1) 1.46/ 1.53(3)
S-16 3.00 SM 100.0 35.4 0.8 32.3 NP 36 0.50*j) 26^8(3) 1.05 / 1.15(3)
S-17 3.00 SM 100.0 27.2 2.5 26.7 NP 44 LOO*5' 24.7(3) 1.37 / 1.45(3)
S-18 3.00 SM 98.5 30.9 0.7 36.9 NP 84 o ! 29.20> 1.13/ 1.20(3)
S-19 3.00 SM 99.9 37.2 0.9 37.8 NP 63 0.40*3) 33J0) 1.10/ 1.20,3)
11 Remolded
(2) Remolded partially saturated
(3) Undisturbed


As part of the reconnaissance conducted for the June 23, 2001 earthquake, 16
geotechnical investigation reports were compiled. Three of these contain Cotrado and
Sina (1994) soil profiles C-4 through C-9 (Los Eucaliptos Apartment Complex), C-
16, and C-25 through C-51 (Tacna Housing Complex), and have provided additional
geotechnical information. The remaining 13 have been designated J-l through J-13
and their locations are also shown on Figure 3.10. Locations J-l through J-10
correspond to schools included in the damage survey discussed in Chapter 4. These
site investigations correspond to damage survey sites 9 and 18 through 26 in Table
4.3 described in the next chapter. J-l 1 corresponds to damage survey site 28 in Table
4.3 and J-l2 is a site not included in the damage survey. The soil profiles for these
sites are summarized in Table 3.5.
Additionally, a report of plate load testing on conglomerate was obtained during the
reconnaissance. This site is located in the ZOTAC Industrial Park and is identified as
J-13 in Figure 3.10. Grain size analyses of the conglomerate and overlying soils at
this site are presented in Table 3.6 and the results of the plate load tests are shown in
Figures 3.11 and 3.12. These figures show settlements of 1.3 and 2.4 mm for loads of
10 and 12 kg/cm2 (1,000 and 1,200 kPa), respectively, without approaching failure,
which illustrates the high bearing capacity of the conglomerate deposit. From these
loads and settlements, calculated values of the modulus of vertical subgrade reaction
(Kvi) are 76.9 and 50.0 kg/cm3 (76,900T/m3 and 50,000 T/m3). From NAVFAC
(1982) correlation between relative density of coarse grained soils and modulus of
vertical subgrade reaction, KVi for relative density of 100% is 11.9 kg/cm3 (11,900
t/m3). Because the Kvi values calculated from the plate load test are almost an order
of magnitude higher than the NAVFAC scale, this demonstrates the extremely low
compressibility and high relative density of the conglomerate.
Penetration testing is available only at the Los Eucaliptos site (profiles C-4 through
C-9) and J-2 (damage survey site 18). At the Los Eucaliptos site, located in the Tacna
district, seven test pits were excavated to a depth of 6.50 m. The soil profile consists
of an upper layer of organic, agricultural soil to depths ranging between 0.70 to
1.30 m. This layer overlies a silty sand stratum that reaches depths ranging from 0.80
to 2.33 m. Alluvial conglomerate was encountered underlying the silty sand.
Standard penetration tests (SPT) were performed in the silty sand during excavation
of two of the test pits, at depths of 1.50 and 1.60 m. N-values of 47 and 13 were
obtained in these SPTs, which indicate a medium dense to dense silty sand. It should
be noted that due to the location of this site, the silty sand is interpreted to be of
alluvial origin.
At damage survey site 18 (soil profile site J-2), located in Ciudad Nueva, seven test
pits were excavated to a depth of 3 m and seven dynamic cone soundings were
60


performed at adjacent locations. The soil profile consists exclusively of silty sand
interpreted to be airfall or weathered volcanic tuff. Based on local correlation of the
dynamic cone sounding and SPTs, the silty sand was interpreted to be loose (N< 10)
to depths ranging between 0.80 and 2.00 m, and then transitioning quickly to dense
(or cemented).
61


Table 3.5. Test Pit Profiles Compiled From Various Site Investigations -
(J1 through J12)
Site Depth (m) uses Description References
0-1.5 SM Sand, med. to fine grained, silty, light reddish-brown, w/ lenses of
J-l 1.5-3.0 GP subrounded to rounded gravel Sandy gravel, subrounded to rounded, dark gray, up to 3-in diameter Yacila 1999a
0-0.30 ML Sandy silt, med. stiff, dry reddish- brown Martinez
J-2 0.3-3.2 SM Sand, med. to fine grained, silty, med. dense, w/ interbedded lenses of cemented particles 1999
0-0.5 ML Silt, trace of roots
J-3 0.5-0.9 ML Silt, inorganic, non-plastic Alarcon
0.9-3.0 SM Silty sand, non-plastic 1999 b
0-0.3 SM Silty sand, med. dense
J-4 0.3-3.0 GM Sandy gravel poorly graded, Alarcon
subrounded, dense 1999
0-0.10 PT Organic material, plastic, with slightly gravelly sand
0.10-1.0 SP Gravelly sand, med. dense, reddish-brown, 5-in. max. diameter
J-5 1.0-1.5 GW Sandy gravel, well graded, dense, subrounded particles. Approx. 30% cobble content up to 5-in max. diameter Iruri 1998
1.5-3.0 GW Sandy gravel, well graded, dense, subrounded. Approx. 40% boulder content up to 12-in diameter.
0-1.0 SM Silty sand, reddish-brown
J-6 1.0-2.0 SM Silty sand, dense (Dr=75%), reddish-brown Iruri 1995
62


Table 3.5. Test Pit Profiles Compiled From Various Site Investigations -
(J1 through J12) Cont.
Site Depth (m) uses Description References
J-7 0-1.90 GW Sandy gravel, well graded, loose, w/ up to 8-in cobbles and boulders, light red Orosco 1994
0-0.6 ML Sandy silt, white, dry, non-plastic w/
J-8 0.6-3.0 SM presence of salts and cemented particles Silty sand, med. dense, non-plastic, tan Tello 1997
0-1.0 GP- Sandy gravel, slightly silty, poorly
GM graded, moist, yellowish, up to 3-in. diameter
J-9 1.06-1.6 SP Sand with gravel, moist, reddish-brown, cobbles up to 3-in. diameter Yacila 1996 h
1.6-3.0 GP- Silty gravel, poorly graded, subrounded,
GM moist, yellowish, up to 3-in. diameter
0-1.5 SM Sand, med. to fine grained, silty, moist,
J-10 dark reddish-brown Yacila 1999 c
1.5-3.0 GP Sandy gravel, poorly graded, gray, up to 3-in. diameter
0-2.5 SM, Mix of sand, clay and silt in variable
J-ll SC, CL ratios, dry. Isolated gravel and boulders up to 12-in. Michelena
2.5-4.0 SP- Gravel and sand, subangular, clean, 1970 b
SW, dense, w/ boulders up to 18-in.
GP-
GW
Variable ML Clayey silt, generally reworked by cultivation. Silt-fill in some areas. Reaches depths between 0.8 to 2.7 m.
Variable SM- Silty sand to sandy silt, absent in some
J-12 ML areas. Thickness ranges between 0.15 to 1.2 m where present Michelena 1970 a
Variable GP, Sandy gravel to gravelly sand, with
GW, subangular cobbles and boulders up to
SP, 12-in. diameter
SW
63


Table 3.6. Grain Size Analysis at Site J-13
Test Depth W Percent Passing LL PL PI uses
Pit (m) (%) 3 2 V/i 1 3/4 Vi' 3/a 1/4 N.4 N.10 N.20 N.40 N.60 N.140 N.200 Classification
i 0.2 1.91 100 94.96 86.87 81.71 76.01 71.39 59.44 47.70 36.35 26.93 15.56 12.22 18.21 NP NP SM
i 1.8 2.03 100 86.62 73.87 60.65 52.93 45.62 41.14 36.47 33.42 28.83 24.28 17.34 10.46 3.27 1.95 - - - GP
2 0.2 1.82 100 92.28 87.79 80.29 74.92 68.71 63.36 54.46 43.59 33.07 24.42 14.28 11.23 18.35 NP NP SP-SM
2 1.8 2.49 100 91.86 75.35 57.05 50.27 42.87 38.30 33.99 30.85 26.72 21.37 14.35 7.77 2.08 1.33 - - - GP
3 0.2 1.95 100 92.90 88.17 82.55 78.24 62.34 49.31 37.18 27.51 16.09 12.49 19.00 NP NP SM
3 1.8 2.68 100 85.66 76.95 60.14 52.86 44.46 39.81 34.89 31.47 24.97 19.63 12.20 6.88 1.68 0.77 - - -- GP
4 0.2 1.10 100 93.85 85.31 79.94 73.73 68.41 57.14 46.10 35.29 26.39 15.43 12.11 19.58 NP NP SM
4 1.8 2.74 100 97.81 86.01 71.27 58.86 49.92 43.96 38.50 34.58 28.99 21.64 12.10 5.86 1.79 1.27 - - -- GP
5 0.2 1.18 100 92.59 85.02 79.46 75.25 70.31 65.94 54.31 42.76 32.05 23.65 13.92 11.12 18.64 NP NP SM
5 0.9 1.90 100 99.40 95.80 83.30 65.40 37.30 27.80 21.80 18.86 2.94 SM
5 0.9 2.60 100 96.15 83.28 69.50 59.84 49.68 44.54 38.61 34.43 27.87 21.11 12.85 6.23 1.15 0.45 - - - GP
6 0.2 1.30 100 88.63 83.19 75.70 70.48 64.01 58.47 48.41 37.73 28.07 20.57 12.20 9.57 18.31 NP NP SP-SM
6 1.8 2.61 100 95.09 85.24 74.36 66.50 56.62 50.72 45.32 40.54 29.32 21.07 12.22 5.52 1.00 0.39 - - - GP
7 0.5 1.78 100 99.70 96.60 83.50 66.10 38.20 28.20 21.80 18.26 3.54 SM
7 0.4 1.90 100 98.50 96.80 93.20 75.90 19.70 11.70 NP NP NP SP-SM
7 1.1 2.06 100 81.08 62.42 51.73 44.84 38.01 34.05 30.13 27.32 22.54 18.09 12.82 7.51 2.10 0.99 - - - GP
8 0.2 1.50 100 94.94 85.96 80.12 73.30 67.56 56.10 43.49 32.73 23.97 13.77 11.00 19.30 NP NP SP-SM
8 1.8 2.54 100 90.17 80.80 68.87 61.51 50.15 43.07 36.75 32.02 25.27 19.19 13.03 7.05 1.09 0.10 - - -- GP
9 0.4 1.95 100 92.00 81.40 63.90 47.70 27.70 21.20 21.90 19.10 2.80 SM
9 1.6 2.43 100 95.90 85.48 73.06 66.70 57.91 52.66 47.73 43.47 31.01 29.51 21.86 12.36 3.19 1.41 - - - GP
10 0.5 1.75 100 99.60 92.90 80.10 62.50 34.50 24.80 23.50 19.88 3.62 SM
10 1.5 2.05 100 88.80 74.01 58.26 50.73 43.23 38.66 34.02 30.86 26.68 21.90 15.19 8.72 2.56 1.57 - - -- GP
11 1.2 1.80 100 95.90 89.10 73.80 57.00 33.00 24.80 23.60 19.80 3.80 SM
11 0.3 2.00 100 91.70 81.47 65.69 55.86 47.18 41.88 36.69 33.03 26.95 20.11 12.18 6.41 1.74 1.01 - - -- GP
Notes: (1) GP soils are conglomerate.
(2) SM and SP-SM soils overlie the conglomerate.
64


Load ( kg/cm2) Load ( kg/cm2)
ZOTAC Industrial Park Tacna
Figures 3.11 and 3.12. Plate Load Test Results at Site J-13 (CISMID 1994)
65


3.4 Typical Soil Profiles and Their Effects on
Ground Motion Characteristics
On the basis of the information discussed above, Cotrado and Sina (1994) concluded
that, from a geotechnical point of view, subsurface conditions throughout the city of
Tacna can be represented by five typical soil profiles. These profiles and the typical
soil characterization are:
Type I. This soil profile consists of an upper layer up to 0.5 m thick of fill, sand,
clay, or a variety of agricultural soils. Underlying the upper layer are gravelly
soils (conglomerate) of undetermined depth. This soil profile presents the best
geotechnical characteristics, with allowable bearing pressures greater than 3.0
kg/cm2 (300 kPa).
Type II. This soil profile comprises much of the city. It consists of an upper layer
from 0.5 to 1.5 m thick of fill, sand, clay, organic silts, or a variety of agricultural
soils. Underlying the upper layer are gravelly soils (conglomerate) of
undetermined depth. Foundations are normally designed on the conglomerate
layer, with allowable bearing pressures in the 2.0 to 3.0 kg/cm (200 300 kPa)
range.
Type III. This consists of an upper layer from 1.5 to 3.0 m thick, formed by one or
more strata of fill, sand, clay, organic silts, or a variety of agricultural soils.
Underlying the upper layer are gravelly soils (conglomerate) of undetermined
depth. Foundations are normally designed on the conglomerate, with allowable
bearing pressures in the 3.0 to 5.0 kg/cm2 (300 500 kPa) range. Some
exceptions are small or private buildings which are often founded within the
upper layer with allowable bearing pressures on the order of 1.5 to 2.0 kg/cm2
(150 200 kPa).
Type IV. This soil profile consists of an upper layer up to 0.5 m thick, formed by
one or more strata of fill, sand, or clay, with high salt content. Underlying the
upper layer is a volcanic tuff (silty fine sand) of undetermined depth. This soil
profile presents fair to poor geotechnical characteristics, with allowable bearing
pressures between 1.0 to 1.5 kg/cm2 (100 150 kPa) when dry.
Type V. This soil profile consists of an upper layer from 0.5 m to 3.0 m thick,
formed by one or more strata of fill, sand, or clay, with high salt content.
Underlying the upper layer is a volcanic tuff (silty fine sand) of undetermined
depth. This soil profile presents poor geotechnical characteristics, with allowable
bearing pressures between 1.0 to 1.5 kg/cm2 (100 150 kPa) when dry.
The distribution of these five typical soil profiles is presented in Figure 3.13 (Cotrado
and Sina 1994).
66


It should be noted that soil profile Types 1, II, and III are similar in that all of them
are underlain by the conglomerate deposit, the difference being the thickness of the
upper layer that is interpreted to generally be also of alluvial origin. Based on other
studies reviewed, allowable bearing pressure for structures founded in the
conglomerate are generally in the range of 3.0 to 5.0 kg/cm2 (300-500 kPa)
irregardless of the thickness of the overlying upper layer. Also, soil profiles IV and V
are similar in that both are underlain by volcanic tuff, the difference being the
thickness of the upper layer, which may consist of airfall ash, tuff of variable
weathering, or fine alluvial soils.
Figures 3.14 shows a test pit excavated in conglomerate (Profile Type I) during
Cotrado and Sinas investigation (1994). The test pit was excavated at site E-16 down
to a depth of 2.5 m and consisted of well graded dense gravel for the entire depth of
the test pit (see Figure 3.10 and Table 3.3). Figure 3.15 shows a test pit excavated in
the extreme northeastern section of the city through the silty sand profile (volcanic
tuff Profile Type V). This test pit was excavated at site J-2 (see Figure 3.10) shortly
after the June 23, 2001 earthquake down to a depth of approximately 2 m and
consisted of SM-SP material classified as volcanic tuff for the entire depth. Figures
3.16 and 3.17 show these two soil profile types exposed in slopes.
As discussed in Section 1.2 and shown on Figure 1.14, weaker and deeper soil
profiles are associated to higher spectral amplifications and longer predominant
periods of vibration. Therefore, 1 to 2 story brick-bearing-wall or framed concrete
structures founded on soil profile Types I and II (dense gravel deposit or
conglomerate) are expected to be subject to lower seismic loading and are more likely
to exhibit good seismic performance. On the other hand, similar structures founded
on soil profile Types IV or V would be subject to higher seismic loading.
67


IERR1N0S Di CULTIVO
CJ>.M. A. 6. LE&AA
. Fl; '"ooucw
W ^ " i! ,
-T- AV. tJfRCHO
RARA RAN PC ' 'j
/ v;. .V ,
-^--V PARA 6hi$3
; .VILLA
AV. PAHAJttJBCAHA SUR
AEROPUERTO
li:1
ijJ-i^'PANAMEpjCANA-.
1
" '
?,
-VANIA Dt
iRATAMtKTO
OC AG4JAS
SfRVTOAS
IE N KE NOS Of CULTIVO
rdfmdd:^
: ; '
lir- I-
COHO
'> P'?.J


LEGEND
Type I
Type II id- ^ Mm
iTypelll Type IV ; rWr
Type V '''
TYPE I
0 < el < 0.5 m
e2 > 0.5 m
_SCALE
Peters
*90 IW
CAft.1T*A
PANAMCfttCANA HOU
Ft .

*0
> ^ *jp\V^ s£zjtts£:;:
/ Z" iyiffi Al'-Jf > *'lc^^sS. ': T?:hJ; il"1::;
AV AUGUSTO S
L£0JJtA
t
ilw 1 -s
-ib
$ }rz''l< j{ A'S; t r : JP" v: *:'
lf F. vd|f:-/ii.fess
jt*WNys/of cuLTtyo Y !<, ,, V'1* / ... --. j;; J: J T : *: :: V: *;;
"' 7~^'- F':".
j rrn *. ?, ? Ji ...n.i ffg
id !*- .^Ji f
tA-NATtVlOAO,;'
/i;
JtHflNO
;, / // /, / y \
k 7.?4-f 1 1 j -r" J. .-/
\ /
rz$l}r

/ i%r v
/>7
X
TYPE II
0.5 < el < 1.5 m
-----1
e2 > 1.5 m
CERRG ARUNTA
TYPE III
-W.YA'
TYPE IV
Ff-C?r i -id
C,£3Sk >/"?'
Vh7; 'Oryrr
nhr-
"/i Tc;!8^*-, ' - tlMENOS 'Dl CULTIVO
zW
AV. CEUSTMO VAAOAS

dyi$
'I

T
< el < 3.0 m
e2 > 3.0 m
0 ||3< 0-5 m
e4 > 0.5 m
0.5
TYPE V
e3 < 3.6 i
e4 > 3.0 m
el: One or more layers of fill, SC, SP, SW, SM, CL, OL (Any)
e2: GP, GW, GC (Any)
e3: One or more layers, including fill, SM, SW, SC, SP, GP
e4: Tuff
Figure 3.13. Soil Profile Distribution (adapted from Cotrado and Sina 1994)
68


69


Figure 3.16. Slope in Conglomerate
Figure 3.17. Slope in Volcanic Tuff
70


3.5 Microtrepidation Measurements
The microtrepidation method has been intensively used in Japan and Peru for seismic
microzonation studies (Akio 1988; Kanai and Tanaka 1961). It consists of a portable
seismograph used to record low intensity vibrations at the ground surface, produced
by ambient sources. At each recording location, the predominant period of vibration
at the ground surface is calculated from the seismograph record, which in turn is a
function of the soil profile. As discussed previously, the longer the period of
vibration at the ground surface at a given location, the weaker and deeper the soil
profile. Although it is recognized that the predominant period of vibration at a
specific location is also a function of the intensity of the motion, this method provides
adequate qualitative information to compare and group many recording locations
within a city.
Cotrado and Sina (1994) compiled microtrepidation measurements performed in 1989
at 85 locations throughout the city, to record vibration periods. Additionally, they
performed microtrepidation measurements at 14 locations not covered in the 1989
surveys. The locations of these measurements are shown in Figure 3.10 and the
summary of measured periods are presented in Table 3.7. Iso-period contour lines are
shown in Figure 3.18.
Based on the measured periods, the city was classified into two zones. Areas with
measured periods from 0.09 to 0.25 s, corresponding to stiff or rock-like soils, were
designated as Zone A, and areas with periods from 0.25 to 0.32 s as Zone B,
corresponding to weaker, deeper soil deposits. These zones are also shown in Figure
3.18.
3.6 Seismic Microzonation of Tacna
In general, seismic microzonation is a process that includes the application of various
disciplines, which vary for each specific setting, and may include geology,
geotechnical characteristics, hydrogeology, seismic damage during previous events,
the measurement of dynamic soil properties, seismic amplifications, topography, etc.,
in order to overlap the effects of each phenomenon and determine relatively safe and
risky areas.
On the basis of local geology, soil profile types, and periods of vibration measured
using the microtrepidation method discussed above, Cotrado and Sina (1994)
proposed three seismic microzones for Tacna, as follows:
71


Seismic Zone III: Union of soil types III, IV, and V, and vibration period Zone B
(soil response)
Seismic Zone II: Intersection of soil type II and vibration period Zone A (stiff,
shallow soil or rock response)
Seismic Zone I: Intersection of soil type I and vibration period Zone A (rock
response)
The microzonation map developed based on this criteria is presented in Figure 3.19.
Zone I exhibits the best seismic performance parameters and Zone III the worst.
Table 3.7. Summary of Microtrepidation Measurements Compiled and Performed by
Cotrado and Sina (1994)
Point Ambient Period (sec)
Horizontal (N-S) Horizontal (E-W) Average Vertical
Ml 0.20 0.19 0.20 0.11
M2 0.18 0.19 0.18 0.13
M3 0.10 0.10 0.10 0.11
M4 0.12 0.15 0.14 0.18
M5 0.17 0.13 0.14 0.17
M6 0.19 0.20 0.20 0.19
M7 0.24 0.23 0.24 0.23
M8 0.10 0.13 0.11 0.32
M9 0.10 0.10 0.10 0.10
M10 0.25 0.17 0.20 0.32
Mil 0.32 0.32 0.32 0.32
M12 0.14 0.14 0.14 0.24
M13 0.24 0.32 0.28 0.10
M14 0.10 0.10 0.10 0.10
M15 0.10 0.10 0.10 0.10
M16 0.25 0.32 0.29 0.25
M17 0.09 0.10 0.09 0.10
M18 0.09 0.09 0.09 0.10
M19 0.11 0.10 0.10 0.25
M20 0.23 0.21 0.22
M21 0.32 0.32 0.32 0.32
72


Table 3.7. Summary of Microtrepidation Measurements Compiled and
Performed by Cotrado and Sina (1994) Cont.
Point Ambient Period (sec)
Horizontal (N-S) Horizontal (E-W) Average Vertical
M22 0.08 0.15 0.11 0.32
M23 0.32 0.32 0.32 0.32
M24 0.23 0.23 0.23 0.19
M25 0.26 0.25 0.25 0.14
M26 0.22 0.21 0.22 0.11
M27 0.14 0.13 0.14 0.11
M28 0.09 0.10 0.10 0.10
M29 0.17 0.16 0.17 0.17
M30
M30-A 0.10 0.32 0.32 0.14
M31 0.10 0.11 0.10 0.32
M32 0.09 0.10 0.11 0.11
M33 0.10 0.09 0.09 0.10
M34 0.09 0.10 0.10 0.08
73


Figure 3.18. Iso-period Contour Lines (Cotrado and Sina 1994)
r
74


CCRRD (NTCORKO
H8RENOS DE CUITIVO
ZONEII
CPA*. A. B IEOU4A
PARA GRANOE
- " i if \ CKISTO ZONE 1
T -. / : if AAffACWCO
ZONE i VILLA ., PANAMERJCAtfA AV SAJH
AEROPUERTO 1
ZONE II
PLAMTAOt
ntATAMCNtO
DC AQUAS
SERVKJAS
TlftftENOS OE CULIIVO
Figure 3.19. Seismic Microzonation of Tacna (Cotrado and Sina 1994) and Geotechnical Microzonation of Ciudad Nueva and Alto de la Alianza Districts (Silva and Berrios 1998)


As discussed previously, Silva and Berrios (1998) conducted a geotechnical study of
the Ciudad Nueva and Alto de la Alianza districts. Based on relative density and
shear strength parameters of the silty sand (airfall ash or weathered tuff), they
subdivided the Ciudad Nueva and Alto de la Alianza districts into five geotechnical
zones, designated A through E, as follows:
Table 3.8. Geotechnical Zonation of Ciudad Nueva
and Alto de la Alianza Districts
Zone Description Soil Type c (kg/cm2) 4> (degrees) Dr (%)
A Shallow Rock Rock 10 46 -
B Very Dense SM 0.00-0.10 44.6-48.4 81 -90
C Medium Dense SM 0.05-0.40 32.0-33.7 61 -63
D Loose SM 0.00 0.30 28.6 29.4 46-49
E Very Loose SM 0.05-1.00 16.7-27.1 24-44
These five zones are also shown on Figure 3.19, superimposed on Cotrado and Sinas
micorzonation.
3.7 Shear Wave Velocities
The maximum shear modulus (Gmax) of soil is required for the wave propagation
analysis presented in Chapter 6. It is defined as the shear modulus at a low strain
level and for granular soils is calculated by the expression (Seed et al. 1986):
Gma* = 1,000*2 (crm) ^ (in psf units) Eqn 3.1
where am is the mean principal effective stress and K2 is an empirical coefficient that
depends on the relative density and gradation of the granular soil.
The maximum shear modulus, in turn, is a function of the shear wave velocity (Vs), as
related by the following expression derived from the Theory of Elasticity:
G_ = Z'/,2 Eqn. 3.2
8
Where: y = unit weight
g = gravity constant
76


As discussed above, down-hole measurements of shear wave velocities of the
conglomerate are available for Lima (Repetto et al 1980), as shown in Figures 3.20
and 3.21. These figures indicate shear wave velocities values between 500 and 550
mps at depths of 1 to 7 meters, and between 645 and 800 mps at depths of up to 17
meters. Using the two formulas indicated above, and assuming an average density of
2.3 gr/cm3, average values of K/> between 430 and 455 representative of this deposit
were back-calculated. It should be noted that these values are higher than typical K/>
values for gravels from other geologic environments (Seed et al. 1986), which report
K.2 values ranging between 90 and 188. This confirms the high relative density of this
deposit and its rock-like characteristic. Conservatively, a K2 of 250 has been selected
for the conglomerate for the analyses presented in Chapter 6. For the purpose of
seismic wave propagation, the conglomerate exhibits sufficiently rock-like
characteristics (i.e. strength, density, void ratio and shear wave velocity) to assume
relatively little amplification through this material.
Ideally, it would have been desirable to perform a series of deep (20 30 m) test
holes in Tacna and perform down-hole measurements of shear wave velocities,
similar to those performed in Lima. However, due to lack of a specific budget, it was
not possible to carry out geophysical field tests to measure the compression and shear
wave velocities of the soils in Tacna. The shear wave velocity of a material is related
to its modulus of rigidity, or stiffness, and as such, is a valuable indicator of the
behavior of the material under earthquake ground motions. In lieu of those
geophysical measurements, one borehole with penetration tests and a series of relative
density tests were available for the northern area with the silty sand soils and volcanic
tuff. Clearly, there is a pronounced velocity contrast between the areas with finer soils
and the zone underlain by the conglomerate.
For the silty sands encountered in Ciudad Nueva and Alto de la Alianza, relative
densities, generally between 30 and 50 percent have been reported. Based on Seed
and Idriss (1970) correlation between relative density and K2 (Figure 3.22), values of
K2 between 33 and 42 have been selected for this material. Assuming an average unit
weight of 1.5 gr/cm3 (15kN/m3), this indicates average shear wave velocities between
about 160 and 180 mps at a depth of 3 m. The volcanic tuff rock underlying the silty
sands of the northeastern sections of Tacna is considered to be soft to medium hard
with estimated shear wave velocities between 900 and 1200 mps based on average
shear wave velocities for weathered igneous rocks (i.e. cemented ash/tuff) (Hunt
1984).
77


I u) mmq I 0 * $ 1 0 1 Tim* (m sec.) 9 2 O 2 s Vp Im/s ) V* (*/ ) Vp/v. V
426 222 1.915 .31
1 ^ 755 5QO 1509 .11
Vf HAVE M \ \a \ mmt Hl 1.053 706 1.491 .09
Figures 3.20 and 3.21. Down-Hole Measurements in Limas Conglomerate
(Repetto et al. 1980)
78


79


4. Building Distribution and Damage Survey
4.1 Construction Practice
Prevalent construction practice in Tacna has changed throughout the years. Existing
urban construction can be classified within the following three basic types: adobe,
brick-bearing-wall, and reinforced concrete frame with brick in-fill.
Adobe dwellings are typically 1 or 2 stories high with total heights of 4 to 8 m,
respectively, for the old colonial-time dwellings. Roof construction is of wood beams
with a cover of mud, supported on adobe walls (see Figures 1.9 and 1.10). Dwellings
of this kind are typically 75 to more than 100 years old. The structural condition of
these dwellings must have been rather poor by the time of the 2001 event, since
previous earthquakes in 1868, 1877, and 1958 undoubtedly damaged and weakened
these structures. The prevalent construction practice of existing urban structures can
therefore be classified into two basic types: brick-bearing-wall and framed reinforced
concrete.
Brick and concrete construction dates back some 75 years. Typical brick-bearing-
wall dwelling construction in Tacna are typically 1 to 2 stories high. Roofs and
intermediate floors consist of reinforced concrete slabs (with hollow brick infill to
decrease dead weights), supported on 25-cm-wide brick-bearing-walls, with no
structural continuity between slab and wall (see Figures 1.12 and 1.13). Non-bearing
walls are generally 15-cm wide. Within the past 20 to 30 years, in order to provide
confinement to the bearing and non-bearing walls, reinforced concrete columns are
placed at the wall comers. As discussed in Section 1.2, these construction details
make brick dwellings quite rigid, with fundamental periods typically within the range
of 0.05 to 0.15 sec. Institutional buildings (such as schools) and buildings in excess
of three stories are usually of the reinforced concrete frame type (see Figure 1.11).
Design provisions incorporating lateral force effects were not required in Pern until
about 40 years ago, but even when they were introduced, the standard of practice was
considerably below the requirements specified in the current Peruvian Code, and in
many modem building codes. It can perhaps be stated that only in the last 25 years
has the design of structures in Pern been truly earthquake resistant.
80


4.2 Overview of Damage
A preliminary evaluation of damage showed that damage within the Tacna district
was limited mainly to the oldest and weakest adobe construction (see Figures 4.1 and
4.2), which perform differently than brick-bearing-wall or reinforced concrete frame
structures. Therefore, to better isolate site effects from other structural factors that
influence performance of buildings, it was considered necessary to eliminate adobe
construction from the damage evaluation because adobe construction is easily
damaged due to its age, inherent weakness, and damage from previous earthquakes.
The other two general types of buildings, brick-bearing-wall and reinforced concrete
frame are considered to perform relatively similar.
Figure 4.1. Damaged Adobe House
81


I
Figure 4.2. Damaged Adobe House
Since the adobe-type construction was obviously weaker and present within the
Tacna district only, it was clear that this damage could not be taken as an indicator of
the potential effect of local subsurface conditions on the damage itself. Attention was
therefore directed to the performance of brick-bearing-wall and reinforced concrete
framed structures as a means of identifying damage patterns and their distribution
within the study area. The results of this survey are described in the following
section.
Damage to brick and concrete structures was minor in the central and southern
sections of Tacna (Tacna, Gregorio Albarracin, and Pocollay districts, typically
cracks in non-bearing walls), and moderate to severe in the northern part (Ciudad
Nueva and Alto de la Alianza districts).
Since no subsidence or liquefaction were observed, the concentration of damage in
some areas of Tacna suggested the influence of site amplification in the resulting
damage levels. Local engineers indicated the presence of silty sand deposits in areas
of concentrated damage, while areas underlain by stiffer gravel deposits of the
conglomerate suffered less damage. It is noteworthy that public schools in Peru are
constructed to a similar design and using similar construction practices, which
provides a control for comparison. Observations of the performance of school
82


buildings in different locations permit a comparative evaluation of the ground
motions experienced during the earthquake. Figures 4.3 and 4.4 show two reinforced
concrete frame school buildings in Tacna, one with severe structural damage and the
other with minor damage to non-bearing walls. The cause for the difference in
damage is the main topic of this research. It is hypothesized that it is due to a
difference in ground motion amplification due to differences insubsurface conditions.
This is discussed further in Sections 5 through 7.
Figure 4.3. School with Severe Structural Damage
83


Figure 4.4. School with Minor Damage to Non-Bearing Walls
4.3 Damage Survey
A damage survey was conducted with the aim of identifying (1) most heavily
damaged bearing-brick-wall and reinforced concrete frame structures in the city; (2)
damaged and undamaged public schools; and (3) examples of undamaged structures.
As discussed previously, public schools in Peru are built following relatively standard
designs and provide a good comparison basis to evaluate site effects. Also, as
discussed above, the older adobe structures located in the downtown area were not
included in the survey.
In order to follow a uniform procedure to describe and rank the damage level, a
structural damage index scale was developed based on the system described by
Coburn and Spence (1992) and Seed (1974), which consists of assigning to each
building a damage index between DO (no observed damage) and D4 (irreparable
damage or complete collapse of floor or entire structure). The description of these
indices, and the field interpretation of them, is presented in Table 4.1 below.
84


Table 4.1. Structural Damage Index Used for Mapping of Damage Patterns
Index, Id Description Interpretation
DO No Observable Damage No cracking, broken glass, etc.
D1 Light Damage Moderate amounts of cosmetic hairline cracks, no observable distress to load bearing structural elements, broken glass. Habitable.
D2 Moderate Damage Moderate amounts of thin cracks or a few thick cracks. Cracking in load bearing elements, but no significant displacements across the cracks. Habitable with structural repairs.
D3 Severe Damage Large amount of thick cracks. Walls out of plumb. Cracking in load bearing elements, with significant deformations across the cracks. Inhabitable. Major restoration required.
D4 Irreparable Damage (Collapse or Demolition) Walls fallen, roof distorted, column failure. Inhabitable. Total collapse or demolition required.
Structural damage was surveyed at 33 building sites throughout the city. For each
site, the following minimum information was noted: location, number of stories,
structure type, building use, and structural damage index. Other relevant data was
noted, as appropriate. Photos of most sites are presented in Appendix A.
The structural damage index for the building sites surveyed are summarized in Table
4.2. Table 4.3 summarizes the damage intensity for each site. Figure 4.5 shows the
site locations and Figure 4.6 shows the distribution of the structural damage index for
each site.
Table 4.2. Summary of Structural Damage Index Survey
Index, la Number of Sites
DO 9
D1 14
D2 3
D3 1
D4 6
85


It should be noted that Sites 13, 17, and 27 are 3- and 4-story apartment building
complexes (see photos in Appendix A), with several identical buildings at each site.
These three sites are located in the Tacna district and suffered no damage or light
damage. It should also be noted that Site 1 was a 2-story house structured on brick-
bearing-walls, located in Ciudad Nueva. This house suffered total collapse,
apparently due an insufficient shear resistance for the ground motion intensity
experienced. It was possible to obtain a photo of this house prior to the earthquake,
which is shown in Figure 4.7. Figure 4.8 shows the house collapsed after the
earthquake.
86


Table 4.3. Damage Evaluation of Surveyed Buildings
Site No. Site Description District CN = Ciudad Nueva AA = Alto de la Alianza T = Tacna P = Pocollay GA = Gregorio Albarracin No. of Stories Structure Type Building Use Damage Intensity
1 Av. Sol: 2-story house CN 2 Brick (bearing) House D4 Collapse
2 Biblioteca Jose Olaya CN 2 Reinforced concrete frame Library D2 Moderate
3 Municipalidad Distrital CN 2 Reinforced concrete frame Municipality D3 Severe
4 Av. International: Blue House CN 2.5 Brick (bearing) House D4 Irreparable
5 Gray house west of Blue house CN 2 Brick (bearing) House D4- Irreparable
6 Colegio Mariscal Caceres CN 2 Reinforced concrete frame School D3 Severe
7 SENATI CN 2 Reinforced concrete frame School D1 Light
8 Instituto Vigil AA 2 Laminar roof on bearing walls School D2 Moderate
9 CE 42021: Fortunato Zora AA 2 Reinforced concrete frame School D2 Moderate (2-story, older building) DO No Damage (2-story, new building)
10 Arco de Tacna T ~10m Arch Reinforced concrete Monument DO No Damage
11 Av. Circunvalacion Sur: House T 2 Brick (bearing) House D4 Irreparable
12 Gran Hotel Tacna T 3 Reinforced concrete frame Hotel Dl-Light
13 Complejo de Viviendas Jose Rosa Ara (23 de Agosto) T 4 Brick (bearing) Apartment Complex D1 Light
14 Colegio Gregorio Albarracin T 2 Reinforced concrete frame School Dl Light
15 Colegio Haya de la Torre AA Reinforced concrete frame School DO No Damage
16 Several blocks with intense damage. 3-story building with 4th floor half built AA 3 Brick (bearing) House D4 Collapse
87


Table 4.3. Damage Evaluation of Surveyed Buildings Cont.
Site No. Site Description District CN = Ciudad Nueva AA = Alto de la Alianza T = Tacna P = Pocollay GA = Gregorio Albarracin No. of Stories Structure Type Building Use Damage Intensity
17 Agrupamiento 28 de Agosto T 3 Reinforced concrete frame Apartment Complex D1 Light
18 CE 42250: Cesar Cohaila CN 2 Reinforced concrete frame School D1 Light (Older building) DO No Damage (New building)
19 CE 42088: Jose de San Martin AA 1 Reinforced concrete frame School D1 Light (Older building) DO No Damage (New building)
20 CE 42238: Enrique Paillardelle GA 2 Reinforced concrete frame School DO No Damage
21 Instituto Formacion Artistica Francisco Lazo T 3 Reinforced concrete frame School D1 Light
22 CE Guillermo Auza Arce AA 2 Reinforced concrete frame School Dl-Light
23 CE 42020: Rosalina Herazo Morla T 2 Reinforced concrete frame School D1 Light
24 CEI408 (Santisima Trinidad): Comite 24 y 25 CN Reinforced concrete frame School Dl Light
25 CE 42237: Jorge Chavez GA 2 Reinforced concrete frame School Dl Light (Older building) DO No Damage (1995 building)
26 CE 42007: Leoncio Prado T 2 Reinforced concrete frame School Dl Light (Older building) DO No Damage (New Building)
27 Conjunto Habitacional Alfonso Ugarte GA 4 Reinforced concrete frame Apartment Complex DO No Damage
28 Universidad Privada de Tacna Building T 4 Reinforced concrete frame Office Building DO No Damage
29 Five-Story Building T 5 Reinforced concrete frame Office/Apartment Complex Dl Light
30 Two-Story House T 2 Brick (bearing) House Dl Light
31 Four-Story Building T 4 Reinforced concrete frame Apartment Complex Dl Light
32 School T 2 Reinforced concrete frame School DO No Damage
33 General Attorneys Complex T 2-5 Reinforced concrete frame Institutional DO No Damage
88