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Dynamic behavior of a geotextile reinforced sand

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Dynamic behavior of a geotextile reinforced sand
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Liu, Hsing-Cheng
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78 leaves : illustrations ; 29 cm

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Subjects / Keywords:
Geotextiles -- Testing ( lcsh )
Sand -- Testing ( lcsh )
Geotextiles -- Testing ( fast )
Sand -- Testing ( fast )
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bibliography ( marcgt )
theses ( marcgt )
non-fiction ( marcgt )

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Bibliography:
Includes bibliographical references (leaves 76-78).
General Note:
Submitted in partial fulfillment of the requirements for the degree, Master of Science, Department of Civil Engineering.
Statement of Responsibility:
by Hsing-Cheng Liu.

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University of Colorado Denver
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Auraria Library
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Full Text
DYNAMIC BEHAVIOR OF A GEOTEXTILE REINFORCED SAND
by
Hsing-Cheng Liu
B.S., National Cheng-Kung University, 1979
A thesis submitted to the
Faculty of the Graduate School of the
University of Colorado in partial fulfillment
of the requirements for the degree of
Master of Science
Department of Civil Engineering
1987


This thesis for the Master of Science degree by
Hsing-Cheng Liu
has been approved for the
Department of
Civil Engineering
by
Date


Liu, Hsing-Cheng (M.S., Civil Engineering)
Dynamic Behavior of a Geotextile Reinforced Sand
Thesis directed by Professor Nien-Yin Chang
Soil is weak in tension as compared to its compressive
strength. As the most abundant and economical engineering material,
the attempt of soil improvement has been one of the oldest
engineering concerns. Both mechanical and chemical methods have
been attempted to upgrade the soil performance under different
conditions. The method of geotextile reinforced earth incorporates
the flexible but tension resistant synthetic material with soil to
increase the tensile resistance of soil-reinforcement composite.
In this study, the dynamic response of an Ottawa 30-40 sand
reinforced with a nonwoven geotextile, Bidim C-34, was
investigated. Both reinforced and nonreinforced samples were tested
using triaxial test device and resonant column test device. All
samples were prepared at 70% relative density and two confining
pressures, 15 psi and 45 psi, were used throughout the study.
Static triaxial tests were conducted to examine the static
effectiveness of the reinforcement and to determine the equivalent
Poission's ratio of samples for use in the shear strain control of
cyclic triaxial tests. Cyclic triaxial tests and resonant column
tests were conducted to examine the variation of shear modulus and
damping ratio at different strain amplitudes. The effect of.
reinforcing pattern on both the static and dynamic performance of
reinforced sample was also investigated by varying the number of
reinforcing layers.


iv
Under static loading, the test results indicate that the
ultimate strength increases as the number of reinforcing layer
increases. Also, for the type of reinforcing material used, a
significant increase in strain is required before the ultimate
strength is.exhibited. When under dynamic loading, depending upon
the magnitude of induced strain amplitudes, the reinforced sample
may behave with lower or higher stiffness as compare to that of
nonreinforced sample.


ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to Professor
Nien-Yin Chang for his constant encouragement, guidance and
financial support throughout the preparation of this
thesis. Gratitude is extended to Assistant Professor Joseph
F. Labuz and Associate Professor Tzong H. Wu for serving on the
final exam committee and providing constructive suggestions.
Special thanks are extended to Mr. H.-H. Chiang and.Mr. J.-W. Chen
for their kind help in performing tests and analyzing test results,
and to Ms. Pauline LeBlanc for typing the thesis. Special
appreciation is also extended to the University of Colorado at
Denver for providing the wonderful research environment in
geotechnical engineering and also to Mr. Glen Wittstock of the
Quline Corporation for providing Bidim geotextiles.
Finally, I would like to thank my wife, Shu-Wen Liu and my
daughter Emily Y. Liu for their love and patience, and my parents
for their encouragement.


CONTENTS
ACKNOWLEDGEMENT.................................................... v
LIST OF TABLES.................................................. viii
LIST OF FIGURES................................................... ix
CHAPTER
I. INTRODUCTION................................................ 1
1.1 Geotextile Reinforced Earth........................... 1
1.2 Objective of Study.................................... 3
1.3 Scope of Study........................................ 3
II. PRINCIPLE OF EARTH STRUCTURE REINFORCEMENT.................. 5
2.1 Origin and Recent Development......................... 5
2.2 Mechanism and Concept................................. 8
2.2.1 Enhanced Confining Pressure Concept................ 11
2.2.2 Anisotropic Cohesion Concept....................... 11
2.3 Dynamic Behavior of Geotextile Reinforced Earth 12
2.3.1 Dynamic Behavior of Soils.................... 12
2.3.2 Factors Affecting Dynamic Behavior of Soils.. 13
2.3.3 Dynamic Behavior of Soils with
Geotextile Reinforcement........................... 17
III. TEST ON GEOTEXTILE REINFORCED SAMPLE....................... 19
3.1 Test Materials....................................... 19
3.1.1 Ottawa 30-40 Sand.................................. 19
3.1.2 Bidim C-34 Nonwoven Geotextile..................... 21


vii
CHAPTER
3.2 Test Equipment and Test Program...................... 26
3.2.1 Static and Cyclic Triaxial Test Equipment ... 26
3.2.2 Resonant Column Test Equipment...................... 29
3.2.3 Test Program........................................ 32
3.3 Sample Preparation and Test Procedures.............. 33
3.3.1 Static Triaxial Test................-......... 33
3.3.1.1 Determination of The Amount of Soil
for a Sample..................................... 33
3.3.1.2 Placement of a Sample...................... 36
3.3.1.3 Test Procedures.................................. 43
3.3.2 Cyclic Triaxial Test................................ 44
3.3.2.1 Sample Preparation............................... 44
3.3.2.2 Test Procedures............................ 44
3.3.3 Resonant Column Test................................ 46
3.3.3.1 Sample Preparation............................... 46
3.3.3.2 Test Procedures............................ 47
IV. ANALYSIS AND DISCUSSION OF TEST RESULTS.................... 49
4.1 Static Triaxial Test Results......................... 49
4.2 Cyclic Triaxial Test Results......................... 63
4.3 Resonant Column Test Results......................... 67
V. SUMMARY, CONCLUSION AND RECOMMENDATION FOR FUTURE
STUDY...................................................... 74
5.1 Summary.............................................. 74
5.2 Conclusion........................................... 74
5.3 Recommendation for Future Study...................... 75
BIBLIOGRAPHY
76


LIST OF TABLES
Table
2.1 Parameters Affecting Shear Modulus and Damping
ratio for Complete Stress Reversal..................... 15
3.1 Typical Physical Properties of Bidim Engineering
Geotextile.............................................. 24
3.2 Summary of Test Program................................. 34


LIST OF FIGURES
Figure
1.1a Deformation of a Nonreinforced Soil Mass Under
Loading.................................--......... 2
1.1b Deformation of a Reinforced Soil Mass Under
Loading.................................................... 2
2.1 Example of Application of Reinforced Earth
Structure in Animal Kingdom................................ 6
2.2 Typical Stress-Strain Relationship of Reinforced
and Nonreinforced Soil.................................... 10
2.3 Hysteric Stress-Strain Relationship....................... 14
2.4 Shear Modulus and Damping Ratio versus Shear Strain
Relationship.............................................. 16
3.1 Gradation Curve of Ottawa 30-40 Sand...................... 20
3.2 Mirafi-600X Woven Geotextile.............................. 22
3.3 Bidim C-34 Needle Punched Geotextile...................... 23
3.4 Thickness versus Pressure Relationship of Bidim
C-34 Geotextile........................................... 25
3.5 Reinforcing Patterns...................................... 27
3.6 Triaxial Cell for 6-in. Diameter Sample................... 28
3.7 Drnevich Free-Free Torsional Resonant Column
Apparatus.......................................... 30
3.8 Schematic Diagram of Resonant Column Device............... 31
3.9 Split Mold................................................ 35
3.10 Vernier for Sample Height Measurement..................... 38
3.11 Zero Height Raining Device................................ 39


41
42
45
50
51
52
53
55
57
59
60
61
62
64
65
66
68
Small Level
7r-Tape for Sample Diameter Measurement............
Use of Guide Tube for Pressure Chamber Assembly....
Stress-Strain Relationship for Samples Tested at
15 psi. Confining Pressure.........................
Stress-Strain Relationship for Samples Tested at
45 psi. Confining Pressure.........................
Peak Stress Difference versus Number of Reinforcing
Layer Relationship.................................
Initial Young's Modulus versus Number of
Reinforcing Layer Relationship.....................
An Idealized Stiffness Model for a Reinforced
Sample.............................................
A General Model for Stiffness Analysis of
Reinforced Sample..................................
Thickness versus Pressure Relationship of Bidim
C-34 Geotextile in Normal Scale....................
Analytical Initial Young's Modulus versus Number of
Reinforcing Layer Relationship Based on Fig.4.6....
Lateral Strain versus Axial Strain Relationship
at 15 psi. Confining Pressure......................
Lateral Strain versus Axial Strain Relationship
at 45 psi. Confining Pressure.....;................
Approximate Lateral Strain versus Axial Strain
Relationship at 15 psi. Confining Pressure.........
Approximate Lateral Strain versus Axial Strain
Relationship at 45 psi. Confining Pressure.........
Mohr-Coulomb Envelopes for Samples with Different
Reinforcing Patterns...............................
Shear Modulus and Damping Ratio versus Shear Strain
Relationship for Cyclic Triaxial Test at 15 psi.
Confining Pressure.................................


xi
Figure
4.15 Shear Modulus and Damping Ratio versus Shear Strain
Relationship for Cyclic Triaxial Test at 45 psi.
Confining Pressure........................................ 69
4.16 Shear Modulus and Damping Ratio versus Shear Strain
Relationship for Resonant Column Test at 15 psi.
Confining Pressure........................................ 71
4.17 Shear Modulus and Damping Ratio versus Shear Strain
Relationship for Resonant Column Test at 45 psi.
Confining Pressure...................................... 72
4.18 Maximum Shear Modulus versus Number of Reinforcing
Layer Relationship........................................ 73


CHAPTER I
INTRODUCTION
1.1 Geotextile Reinforced Earth
Geotextile Reinforced earth is a composite of soil and
geotextile reinforcement which improves the strength property of
soil by introducing tensile strength into the soil-reinforcement
composite. When a soil mass fails, a large strain is bound to occur
as shown in Fig.1.1a. If a flexible but tension resistant material
could be embedded inside the soil mass to reduce this lateral
deformation, the soil mass can then maintain a close-to-K0 condition
as long as the strength of the reinforcing geotextile is mobilized
and adequate as shown in Fig.1.1b! In order to mobilize the tensile
strength of embedded reinforcement, a bonding force between soil and
reinforcing inclusion is required. The mechanism of geotextile
reinforced earth lies in the induced bonding force at soil-
reinforcement interfaces which provides the tensile strength to the
composite. This bonding force is attributed to, to a great.extend,
the soil-geotextile interface friction.
With the increasing number of success of its application,
ease and low cost, the geotextile reinforced earth construction has
become popular throughout the world. Although design criteria were
developed for a number of applications, such as retaining walls,
foundations and embankments, a better understanding of the behavior


2
Fig. 1.1a Deformation of a Nonreinforced Soil Mass Under
Loading
Fig. 1.1b Deformation of a Reinforced Soil Mass Under
Loading


3
of geotextile reinforced earth is still needed for a sound design of
so reinforced earth structures, especially to resist dynamic
forces. Further studies, are therefore, needed to update the design
criterion for geotextile reinforced earth structures.
1.2 Objective of Study
With reinforcing inclusion, the conventional assumptions of
isotropy and homogeneity on soils no more apply to the geotextile
reinforced earth. The study of geotextile reinforced earth, there-
fore, experiences the difficulty of finding proper test methods and
devices. As scaled and full size model tests are usually very
costly, some common soil strength test devices, such as direct
shear test device and triaxial test device were first used in
studying the effectiveness of geotextile reinforcing.
While most studies have been conducted under static loading,
it is important to study the response of geotextile reinforced earth
to dynamic loading. The main objective of this study was to
investigate the dynamic property and behavior of a geotextile
reinforced sand by using conventional cyclic triaxial test apparatus
and resonant column test device. By varying cyclic strain levels
and reinforcing patterns, it was hoped that the effectiveness of
geotextile in strengthening and stiffening granular soils under
dynamic loading can be better understood.
1.3 Scope of Study
In this study, triaxial tests were conducted to invest-
igate both the static strength and the dynamic properties of a


4
horizontally reinforced soil subjected to vertical cyclic loading.
Resonant column tests however, were specifically intended to
investigate the damping characteristic and the small-strain dynamic
properties, of the reinforced soil under torsional vibration due to
the introduction of flexible reinforcing inclusion. A cyclic
triaxial test device, which is capable of determining both static
soil properties and dynamic soil properties at shear strain levels
between 10~4 to 10"2 together with a resonant column test device for
dynamic properties at shear strain levels smaller than 10*4 were
used in this study.
In the static triaxial test, three different reinforcing
patterns plus one nonreinforced pattern were used to investigate the
effectiveness of reinforcement with different number of reinforcing
layer. In the cyclic triaxial test and the resonant column test,
however, only two different reinforcing patterns plus one nonrein-
forced pattern were used.
An Ottawa 30-40 sand prepared at a relative density of 70%
and a nonwoven geotextile with commercial name, Bidim C-34, were
used in this study. Static triaxial tests were first conducted to
find the stress-strain relationships and the equivalent Poission's
ratios of the reinforced composite at different combinations of
confining stress and reinforcing pattern. Cyclic triaxial tests
were then conducted at four different shear strain levels between
10*4 and 10*2. Resonant column tests were conducted at shear strain
levels below 10*4 at last.


CHAPTER II
PRINCIPLE OF EARTH STRUCTURE REINFORCEMENT
2.1 Origin and Recent Development
No one can really tell when and who started the application
of reinforced earth structure. It is said that human learned the
technique from animal kingdom. Fig. 2.1 shows such an example of
reinforced earth structure in which the swallow nest is built of mud
with straw as reinforcement. Earliest human application of earth
reinforcement dates back to the ancient Roman Empire when ancient
Romans used reed mat in road construction over soft ground. In the
Middle East and the Far East, the reinforcing of large earth
structures using reeds, rushes or bamboo was reported to have lasted
for millennia (Rankilor, 1981).
However, the recent systematic study of earth structure
reinforcement which led to increasing application in modern con-
struction was due to the French architect and engineer, Henri
Vidal. In the following story, Vidal revealed the incident that
directed him to the study of earth reinforcing:


6
Fig. 2.1 Example of Application of Reinforced
Earth Structure in Animal Kingdom


7
"One day, while on vacation in the Baleares Island in
the Mediterranean sea, I was on a beach and mechanically
I was playing with the sand, making a pile of sand and
looking at, how it's normal slope, always the same
occurred.
There were plenty of pine needles around me, so
I mechanically began to lay out rows of flexible
pine needles in the sand: a layer of needles, a
layer of sand, and so on.
It seemed that the slope of the pile of sand
became somewhat steeper, and naturally I asked
myself this question: Is it useful or not to bury a
flexible string in the sand (Symposium on Earth
Reinforcement, Pittsburgh, 1978)?"
That was how the reappreciation of an ancient technique
started in the early 1960's. After years of study on metal strip
earth reinforcing, Vidal was able to convince the French Highway
Administration to apply metal strip reinforced earth on highway
retaining wall successfully in 1967 and 1968. In 1968, Oleg Wager
of the Swedish Geotechnical Institute described in a paper about
using sheet piling reinforced earth for embankment stabilization.
Based on the theoretical analysis and experimental observation, a
schematic application of steel rod reinforced earth on foundation
was developed by Bassett, et al. (1978).
An empirical procedure for the seismic design of reinforced
earth walls using laboratory shake table test results was developed
by Lee, et al. (1975). Following study by testing a prototype metal
strip reinforced earth wall and four other existing commercial walls
showed a reasonable agreement between the measured forces and the
predicted forces based upon the above mentioned empirical procedure
(Lee, et al., 1977) .


8
It seems that the concept of earth reinforcement could be
applied to almost all the earth related structures safely and
economically (Holtz, 1978).
The ancient applications of reinforced earth structure
involved natural reinforcing materials such as bamboo, reed, wood
etc. Galvanized steel strip was considered an optimal
reinforcing material by Vidal after an extensive study (Vidal,
1978). Although metal-is highly susceptible to corrosion, it was
found that even in an aggressive soil environment, the type of
galvanized steel strip available for earth reinforcing has a useful
life of more than 120 years (Darbin, et al., 1978).
In the early 1960's, a synthetic polymer material was
introduced into civil engineering construction (Rankilor,1981). The
material, now called geotextile in permeable form, or, geomembrane
in nonpermeable form, serves the functions of filtration, drainage,
separation and reinforcement in ground engineering. Being a
relatively new application, the durability of synthetic polymer
embedded in different soil environment is still questionable. How-
ever, its advantage for being versatile in material composition,
fabrication pattern and thickness variation has brought itself into
an overwhelming popularity in the earth reinforcing industry. The
prosperity of synthetic polymer as earth reinforcement is expected
in the future.
2.2 Mechanism and Concept
Similar to concrete, soil is relatively weak in tension as
compare to its strength in compression. As the most abundant and


9
economical engineering material, the attempt of soil improvement has
been one of the oldest engineering concerns. Both mechanical and
chemical methods have been attempted to upgrade the soil performance
under different conditions.
The method of geotextile reinforced earth incorporates the
flexible but tension resistant material with soil to increase the
tensile strength of soil-reinforcement composite through the
mobilization of tensile strength in the reinforcing inclusion.
In order to define the basic mechanism of earth reinforce-
ment, a series of triaxial tests were conducted on a dry sand
reinforced with discs of aluminum foil by Long, et al. in 1972.
The strength of the reinforced soil was found to be as that
of nonreinforced soil, very much confining pressure dependent.
Below a certain critical confining pressure, a sliding between soil
and reinforcement accounted for the failure mechanism while above
the critical confining pressure, the failure was accompanied by the
rupture of reinforcement and therefore was brittle in nature.
Fig. 2.2 shows the typical stress-strain curves for the reinforced
and nonreinforced soil from Long's study. In the figure, the larger
strain was also found necessary to reach peak load for the rein-
forced soil.
At about the same time, a series of triaxial tests using
orthogonal woven fiberglass as reinforcing material was conducted by
Yang (1972) and similar reinforcing effect was found for the rein-
forced sand. A later study on the stress-strain behavior of a


Stress
10
Axial Strain, e
Fig.
2.2
Typical Stress-strain Relationship of
Reinforced and Nonreinforced Soil


geotextile reinforced sand using several different kind of geo-
textiles also concluded that the inclusion of reinforcement
increases the ultimate strength of sand under triaxial compression
(Holtz, et al., 1982).
2.2.1 Enhanced Confining Pressure Concept
According to Yang (1972), the effect of soil reinforcing was
due to the fact that the tensile stresses built up in the horizontal
reinforcing layers were transferred to the soil through sliding
friction and caused an increase in the confining pressure. The
concept, called Enhanced Confining Pressure Concept, then expressed
in terms of Mohr-Coulomb formulation as:
a1f = (cr3 + Acr)tan2(45 + 0/2)
where is the vertical stress at failure.
ct3 is the applied confining stress.
0 is the frictional angle of the unreinforced sand.
2.2.2 Anisotropic Cohesion Concent
Sharing the similar test results ; Long however,
interpreted the effect of earth reinforcing as that of a
cohesive-frictional Mohr-Coulomb material with strength defined as
follows:
aif = <^3 tan2 (45 + 0/2) + 2Ctan(45 + 0/2)
where a1f is the vertical stress at failure.
ct3 is the applied confining stress.
0 is the frictional angle of the unreinforced sand.


12
C is termed Anisotropic Cohesion which accounts for the
strength increase due to the reinforcing inclusion.
Although the effect of earth reinforcing could be inter-
preted with the C and Ao3 hypotheses as described above, it however
should be noted that these interpretation are both limited to
condition where failure occurs by breaking the reinforcement rather
than by soil-reinforcement sliding which is much more complicated
condition. Besides, enough amount of strain needs to be developed
to reach the peak strength which is used in both interpretations.
2.3 Dynamic Behavior of Geotextile Reinforced Earth
The satisfactory performance of a structure requires the
stability under both static and dynamic loading. In the field of
geotechnical engineering, the dynamic stability of structures
involves not only the characteristics of applied dynamic loading but
also the change in soil properties due to the dynamic loading. The
dynamic behavior of soil and the influence of dynamic loading on the
behavior of geotextile reinforced soil are discussed as follows.
2.3.1 Dynamic Behavior of Soils
To perform the dynamic analysis of an earth structure,
requires dynamic properties of soils. The importance of dynamic
properties of soils lies in the fact that the dynamic properties of
a soil are dependent on the dynamic strain level which the soil is
subjected to.
Under a compression-extension cycle, the shear stress versus
shear strain relation of a soil takes the shape of a hysteresis loop


13
which implies an energy loss. Although the stress strain relation-
ship of a soil is'generally nonlinear and nonelastic, an idealized
hysteresis loop is normally used to define the equivalent dynamic
elastic properties of a soil under that particular strain level.
Shown in Fig. 2.3 is an idealized hysteresis loop in which the
equivalent shear modulus, G, is defined as the slope of the secant
BB' and shear damping ratio, D, is defined as the ratio of the
absorbed energy (the area of the loop) to the applied energy (the
area OAB+OA'B') times a constant, tt/2 .
2.3.2 Factors Affecting Dynamic Behavior of Soils
Based on the studies of several researchers, Hardin, et al.
(1972) summarized the parameters affecting shear modulus and damping
ratio for complete stress reversal as shown in Table 2.1. For both
the sandy and cohesive soils, the table shows that strain amplitude,
effective mean principal stress and void ratio are three very
important affecting factors to both the shear modulus and damping
ratio. Shear modulus of soils decreases rapidly with increasing
shear strain while the damping ratio of soils increases rapidly as
the strain amplitude increases. Fig. 2.4 shows the typical varia-
tion of shear modulus and damping ratio with shear strain ampli-
tudes. An increase in effective mean principal stress causes an
increase in the shear modulus of soils and a decrease in the damping
ratio of soils. An increase in void ratio results in a decreased


14
Shear Stress, t
Fig. 2.3 Hysteric Stress-strain Relationship


Table 2.1
Parameters Affecting Shear Modulus and Damping
Ratio for Complete Stress Reversal
Source: Hardin, et al., "Shear modulus and damping
in soils: measurement and parameter effects," J. o
the Soil Mech. and Found. Div., ASCE, Vol. 98,
No. SM6, June 1972.
IMPORTANCE TOa
Parameter Modulus Damping
(1) Clean sands (2) Cohesive soils (3) Clean sands H> Cohesive soils (5)
Strain Amplitude V V V V
Effective Mean Principal Stress V V V V
Void Ratio V V V V
Number of Cvcles of Loading Rb R V V
Degree of Saturation R V L u
Overconsolidation Ratio R L R L
Effective Strength Envelope L L L L
Octahedral Shear Stress L L L L
Frequency of Loading (above 0.1 Hz) R R R L
Other Time Effects (Thixotropy) R L R L
Grain Characteristics, Size, Shape, Gradation, Mineralogy R R R R .
Soil Structure R R R R
Volume Change Due to Shear Srain (for strains le6S than 0.S %) U R U R
a V' means Very Important, L means Less Important, and R means Relatively Unim-
portant except as it may affect another parameter; U means relative importance is not
clearly known at this time.
bExcept for saturated clean sand where the number of cycles of loading is a Less
Important Parameter.


Shear Modulus, G (MPa)
16
Q
C
C3
O'
cn
c
E
n
o
Shear Strain Amplitude, y
Fig. 2.4 Shear Modulus and Damping Ratio
Versus Shear Strain Relationship


17
shear modulus and an increased damping ratio. Other factors listed
as less important or have insignificant influence on the dynamic
properties of sandy soils include number of loading cycles, degree
of saturation, over consolidation ratio, loading frequency etc.
2.3.3 Dynamic Behavior of Soils with Geotextile Reinforcement
To reinforce the tensile strength of a soil, the
reinforcement has to be strong in tension resistance. As soil is
usually adequate in compression strength, the material used for
earth reinforcing can be flexible while subjected to compression.
The advantage of using flexible reinforcement includes ease of
construction, low cost etc.
When using flexible material for reinforcing, the mobiliza-
tion of tensile strength in reinforcement requires a certain amount
of deformation in the reinforced soil. The more rigid is the
reinforcing material, the smaller is the required deformation. It
is only when the amount of deformation in soil is enough to stretch
the reinforcement, the tensile strength of reinforcement will be
mobilized. This implies that depending upon the planar
compressibility of the geotextile, the geotextile reinforced earth
may exhibit insignificant or even negative reinforcing effect when
subjected to the dynamic loading at a low strain amplitude.
A reinforcing material with high compressibility may not be
effective in stiffening soils when subjected to low strain amplitude
dynamic loading. However, the damping characteristic of the
reinforced composite may be effective in reducing the energy carried
by a soil mass and may alleviate adverse effects of dynamic loading.


Therefore, the effectiveness of geotextile reinforced earth under
dynamic loading is an interplay of the increase in both strength and
damping characteristics. It is believed that a better understanding
to the dynamic behavior of geotextile reinforced earth is necessary
for the optimized design of geotextile reinforced earth structure in
resisting dynamic loading.


CHAPTER III
TEST ON GEOTEXTILE REINFORCED SAMPLE
3.1 Test Materials
3.1.1 Ottawa 30-40 Sand
A commercially available white clean sand, Ottawa 30-40 sand
was used throughout the study. Its gradation curve shown in
Fig. 3.1 gives the coefficient of uniformity, Cu, of 1.43 and the
coefficient of curvature, Cc, of 1.21. The sand was classified as
SP, poorly graded sand, based on the Unified Soil Classification
System.
The maximum and minimum unit weights of the sand were deter-
mined at the Soil Laboratory at the Bureau of Reclamation located in
the Denver Federal Center. The procedures given in the Appendix
E-12 on "Relative Density of Cohesionless Soils" of the Earth Manual
(2nd edition, 1974) published by the U.S. Bureau of Reclamation were
followed. The values of the maximum and minimum unit weights of the
sand were determined as 7max = 112.19 pcf and 7min = 97.52 pcf,
respectively.
The unit weight of the sand at the relative density of Dr =
70% was used in this study. The density was determined using the
following equation:


cn IT) o o O * IT) CM -
r- T CM 2 r- o O o
O d o o
d o o o d o
Grain diameter, mm
Fig. 3.1
Gradation Curve of Ottawa 30-40 Sand


21
1 1
7 - 7
Dr = 70% '--nun-----Z_2_
_1_________1_
7 7
min max
and the density was found to be y70 = 107.35 pcf.
3.1.2 Bidim C-34 Nonwoven Geotextile
A woven geotextile, Mirafi-600X, with a piece of sample
shown in Fig. 3.2, was first considered as the reinforcing
material. After a few trial tests, it was found that with more than
one layer of reinforcement the reinforcing condition became compli-
cated due to the plane anisotropic characteristics of the reinforc-
ing material.
Bidim C-34, a needle punched nonwoven geotextile with a
piece of sample shown in Fig. 3.3, was later decided to be used as
reinforcing material. Table 3.1 shows the typical physical
properties of Bidim Engineering geotextile provided by the
manufacturer. The Bidim C-34 geotextile was cut into circular discs
with diameter slightly smaller than the overall sample diameter to
facilitate the sample preparation.
As the thickness of the geotextile changed with the mag-
nitude of the applied pressure, a thickness versus logarithmic
pressure curve was established as shown in Fig. 3.4. Based on the
curve, the thickness of the reinforcement under different confining
pressure was accounted for in determining the sample height for
density control.


22
Fig
3.2
Mirafi-600 Woven Geotextile


23
Fig. 3.3 Bidim C-34 Needle Punched Geotextile


24
Table 3.1 Typical Physical Properties of Bidim
Engineering Fabrics
Source: Product Specification for Bidim
Engineering Fabrics, Quline Corporation
bidim Product Code No. U.S. C-22 C-28 C-34 C-38 C-42
bidim Product Code No. Internat'l U-14 U-24 U-34 U-44 U-64
n oz/yd2 Nominal gm/M2 Nominal 4.5 6 2 ' 8.0 rou- 16.0
150 210 270 340 550
SURFACE Thickness Mils (ASTM 0-1777)
0.005 bar 60 75 90 110 190
2.000 bar 23 31 41 51 82
Milimeters 0.005 bar 1.5 1.9 2.3 2.8 4.4
2.000 bar . . 0.6 0.8 1.05 1.3 2.1
Porosity (calculated) 0.005 bar % 93 92 91 91 91
2.000 bar % 82 82 81 81 81
Nominal Permeability* ioM/sec 3 3 3 3 3
Planar Permeability* 10M/sec .6 .6 .6 .6 .6
Grab Tensile(ASTM 0-1117)lbs/force 115 160 255 300 610
Grab Elongation (ASTM 0-1117) % 85 80 75 65 60
Trapezoid Tear Strength lbs/force 62 93 125 170 250
(ASTM D-2263)
Mullen Burst Strength psi 225 360 400 500 850
Restrained Tensile Test*-lbs force/in 65 95 120 150 270
Elongation % 35 35 35 35 35
E.O.S.** D, 50 50 70 100 100
dm 70 100 100 140 140
Abrasion Resistance 40 120 135 165 295
Heat ResistanceF @ 50 psi Loading 480 480 480 480 480
Puncture Strength 55 95 125 145 255
(ASTM D-751 modified)
Monsanto Test
Corps of Engineers Test


CM
I
O
oO
LO
u
SI
I
_Q
rO
Pmssnrp. P (nsH
Fig. 3.4 Thickness Versus Pressure Re] nr.i.onship
of Bicliin C-34 Goo textile
N>
cn


26
In order to investigate the variation of reinforcing effect
due to the increase in reinforcing layer, a number of reinforcing
patterns were used in reinforcing the samples. In static triaxial
tests, samples both without and with reinforcement were tested. The
reinforcement was placed at the spacing of 6 in., 3 in. or 2 in. In
the cyclic triaxial test and the resonant column test, the
reinforcing pattern varies as without or with reinforcement placed
at the spacing of 6 in. or 3 in. Fig. 3.5 shows a summary of the
reinforcing patterns used in this study.
3.2 Test Equipment and Test Program
3.2.1 Static and Cyclic Triaxial Test Equipment
A triaxial cell which is capable of testing samples with
6-in. diameter as shown in Fig. 3.6 was used for both static and
cyclic triaxial tests. As the cell was originally designed for
static test only, some minor modification to the loading ram was
made to enable the cyclic test capability of the cell. MTS, a
closed loop servo electrohydraulic system was used for loading
application. The system with 20,000 lb. maximum load capacity and
5.0 in. maximum piston travel can apply both monotonic and cyclic
loads. Some cyclic wave forms such as sine, haversine, haversquare
and various ramp functions including straightline, dualslope, tri-
angle and trapezoid are available. The period for the ramp func-
tions may range from 0.001 second to 11.45 days and the frequency of
cyclic waves ranges from 0.00001 to 990 Hz. The axial load on the
specimen can be measured from a load cell located at the base plate


27
Reinforcing Patterns for Static Triaxial Test
Reinforcing Patterns for Cyclic Triaxial
and Rosonant Column Test
Fig. 3.5 Reinforcing Pattern


28
Fig. 3.6
Triaxial Cell for
6 '
-in. Diameter Sample


29
of the MTS, while the axial deformation of the specimen can be
measured with a linear variable displacement transducer (LVDT)
directly connected to the MTS loading piston. A data logger
connecting to the MTS was used to obtain the digital record of axial
load and axial deformation output during the test while a built-in
X-Y plotter which receives analog signals from the load cell and the
LVDT was used to obtain the load versus deformation plot.
The confining pressure was applied from a pressure control
panel through a graduated burette. The graduated burette was also
used to measure the sample volume change during the test.
3.2.2 Resonant Column Test Equipment
Drnevich Free-Free Torsional Resonant Column Apparatus as
shown in Fig. 3.7, was to obtain the small strain dynamic proper-
ties. Fig. 3.8 shows the schematic diagram of the device. The
device is capable of testing specimens with 2.8, U or 6 in. dia-
meter. The same pressure control panel used in triaxial test was
also used in resonant column test for the application of confining
pressure.
The electronic devices used for the resonant column test
include a function generator, a power amplifier, a digital voltmeter
and two storage oscilloscopes. Sine wave and six other wave func-
tions can be generated in the function generator and amplified at
the signal amplifier before feeding into the driving coils located
at the bottom of the resonant column device. Sine waves were used
in this study. The frequency of the input wave can be adjusted by a


30
Fig. 3.7 Drnevich Free-free Torsional Resonant
Column Apparatus


31
Fig. 3.8 Schematic Diagram of Resonant Column Device


32
frequency dial on the function generator and read by a digital
counter.
The sine wave induces an electro-magnetic field around the
coil which reacts with the permanent magnetic field and produce
torsional wave as shear wave. The wave travels upward through a
sample and the output is picked up by the two velocity transducers
at the top of the specimen.
The voltage of the input wave is read through a digital
voltmeter when the switch on the control panel is on the "power"
position while the voltage of the output wave is read with the
switch on the "passive" position. The resonant frequency is
determined using the Lissajous figure observed on the oscilloscope.
3.2.3 Test Program
Dry Ottawa 30-40 sand was used in this study. Dry samples
were prepared and tested under drained condition. Two different
confining pressure, 15 psi and 45 psi, were used for each
reinforcing pattern in each type of tests.
Static triaxial compression test was first conducted at a
deformation rate of 0.05 in./min. to determine the stress-strain
relationship of both reinforced and nonreinforced samples. The
vertical strain versus lateral strain relation for each static test
was also established through monitoring of sample volume change
during the test. The equivalent Poission's ratio of each sample was
then determined. The equivalent Poission's ratio determined in the
static triaxial compression test was later used to control the shear
strain amplitude in the cyclic triaxial test.


33
All cyclic triaxial tests were conducted using sine wave
function at a frequency of 0.02-Hz. Four different shear strain
levels were used. Two samples were prepared at each reinforcing
pattern and tested under the same confining pressure. Each of the
two samples was tested sequentially at two different strain levels,
one at the strain levels of 5x10"4 and 2.5x10"2, the other one at
the strain levels of 5x103 and 1x10"2.
Resonant column test was conducted on samples prepared and
tested under the same conditions as those used in the cyclic
triaxial test. A summary of the test program is shown in Table 3.2.
3.3 Sample Preparation and Test Procedures
3.3.1 Static Triaxial Test
3.3.1.1 Determination of The Amount of Soil for A Sample
In order to determine the amount of soil for a sample at a
given relative density, the volume of the sample needs to be
estimated in advance. The inner diameter of a split mold and the
double thickness of a rubber membrane were first measured.
The split mold as shown in Fig. 3.9 consists of two iden-
tical parts. When assembled they form a cylindrical space to
support the rubber membrane. Applying vacuum between the mold and
the membrane, the membrane will rest smoothly on the inner wall of
the mold to facilitate the placing of soil inside the rubber
membrane.
The approximate sample diameter, Da, was estimated by
subtracting the double thickness of the rubber membrane from the


34
Table 3.2 Summary of Test Program
Static Triaxial Test Cyclic Triaxial Test Resonant Column Test
Equipment MTS Triaxial Cell MTS Triaxial Cell Free-Free Resonant Column device
Relative Density .70% 70% 70%
Confining Pressure 15 psi, 45 psi 15 psi, 45 psi 15 psi, 45 psi
Reinforcing Spacing None, 6 in., 3 in., 2 in. None, 6 in., 3 in. None, 6 in., 3 in.
Shear Strain Levels/Range NA 5.0 x 10'4 2.5 x 102 5.0 x 103 1.0 x 102 10'7--10'4


35
Fig. 3.9 Split Mold


36
inner diameter of the split mold. The estimated sample volume, Vs,
was obtained using following equation:
Vs = l (Da)2 Hs
where Hs is the desired sample height.
As the desired sample height of sample for this study was
12 in. and the unit weight of the soil used at the relative density
of 70% was known, the amount of soil required for a sample, Ws, was
estimated as follows:
Ws = Vs 770
where 77(j is the unit weight of Ottawa 30-40 sand at 70% relative
density.
For the sample with reinforcement, the total thickness of
the reinforcing material, Hr, needs to be subtracted from the
desired sample height, Hs, before estimating the required volume of
soil. The estimated soil volume for the reinforced sample, Vsr, was
determined as:
VSr = l (Da)2(Hs-Hr)
3.3.1.2 Placement of A Sample
The sample preparation desk must maintain level throughout
the preparation. With the level maintained, the total height of the
top cap, the base pedestal, two porous stones, and two filter paper
discs was measured and recorded as the initial height without
sample, Hw/0. The desired final height with sample was then


37
determined as Hw = Hw/0 + 12 in. The vernier shown in Fig. 3.10 was
used for height measurement.
After the measurement of initial height, the top cap, the
porous stones, and the filter paper discs were removed and a thin
layer of silicon grease was applied around the base pedestal. One
end of the rubber membrane was then placed around the base pedestal
and two 0-rings were used to help the sealing. .The rubber membrane
must be vertical and wrinkle free. The split mold was assembled
around the base pedestal. The top end of the rubber membrane was
then folded back and wrapped around the top end of the split mold.
Vacuum was then applied to the space between the membrane and the
mold to suck the membrane against the inner wall of the split mold.
The base porous stone and the base filter paper disc were placed on
the base pedestal and a zero height raining device as shown in
Fig. 3.11, was placed inside the mold before placing the
sand. Although Ottawa 30-40 sand was classified as a uniform sand,
care was exercised to maintain a level soil surface during soil
placing to ensure a better uniformity. After placing all the soil,
the zero height raining device was lifted up slowly manually to
maintain uniform placement density.
For samples with fabric reinforcement, the process of sample
placement was divided into stages depending upon the number of
reinforcing layers. The soil and reinforcing discs were placed in
alternate layers. The level surface was maintained throughout the
sample preparation process. For each soil layer, zero height
raining device was also-used.


38
Fig. 3.10 Vernier for Sample Height Measurement


39
Fig. 3.11 Zero Height Raining Device


40
After completing sample placement, the sample surface was
levelled using a small level as shown in Fig. 3.12. The upper
filter paper disc, porous stone and top cap were then put in place.
The sample looser than the desired density was then comp-
acted by hitting the split mold surface with a rubber hammer to
bring the total height into slightly higher than the desired final
height with sample, Hw. Then the upper end of the rubber membrane
was released from the split mold and was wrapped around the top cap
and sealed using two 0-rings. A vacuum was applied to the sample
from its lower end to produce an effective stress of about 10 psi.
This prevents the sample from collapsing after the removal of the
split mold.
Upon the application of vacuum, the sample contracted by an
amount depending upon its placement density and the number of
reinforcing layers used. Many trials were attempted before the
control of sample density was regraded satisfactory, +2% deviation
from the desired relative density. Fifteen minutes after the
application of vacuum, the split mold was removed and the sample
dimensions were measured using the vernier and a 7r-tape as shown in
Fig. 3.13. The relative density of the sample was then checked to
make sure that the value was acceptable. With the sample under
vacuum, the triaxial chamber was put in place and locked to the base
of triaxial cell.
The triaxial cell with sample was then moved to the MTS
machine and the triaxial chamber was filled with confining water.-
Slowly the vacuum was released and the confining pressure was


41
Fig. 3.12 Small Level


42
Fig. 3.13 7r-Tape for Sample Diameter Measurement


43
increased equally and simultaneously to maintain the effective
stress inside the sample at 10 psi, a vacuum gage and a pressure
gage were used to measure the vacuum pressure and the confining
pressure. When the confining stress reached 10 psi and the vacuum
stress approached zero, the vacuum line was unplugged. The confining
stress was then raised to a desired magnitude.
3.3.1.3 Test Procedures
Before testing a sample, the MTS machine was turned on to
warm up for 30 min. The loading ram of the triaxial cell was
brought in contact with the specimen top cap by lowering the MTS
piston. No stress was induced in. the sample by MTS machine during
the process. This is to maintain the sample relative density before
testing.
The valve leading to the specimen base was kept opened. The
confining pressure burette level was checked to record any sample
volume change.
The invert ramp function was used and its period was so
adjusted to produce the vertical deformation rate of 0.05 in./min.
The MTS built-in plotter was scaled to record the axial load and the
vertical deformation corresponding to the 15% sample axial strain.
All channels of the data logger was adjusted to zero.
In addition to the recording of axial load and vertical
deformation, the confining pressure burette level was also recorded
every 10 seconds. The test was terminated after the ultimate
strength or 15% axial strain was reached.


44
3.3.2 Cyclic Triaxial Test
3.3.2.1 Sample Preparation
The sample preparation procedure for cyclic triaxial tests
was quite similar to that for static triaxial tests. Only differ-
ences lie in the procedure for placing pressure chamber and connect-
ing the triaxial loading ram. The loading ram was screwed onto the
loading platen before the placement of pressure chamber. This' is to
allow the application of both extension and compression loads during
a cyclic triaxial test. In order not to disturb the sample, a clear
plastic cylinder was used to guide the lowering of the pressure
chamber as shown in Fig. 3.14. The inner diameter of the clear
plastic guide cylinder was designed to be only slightly larger than
the outer diameter of the cell. This guide cylinder forces the
pressure chamber and the triaxial loading ram to be concentric, and
allows the loading ram to go through the triaxial loading ram
bushing smoothly.
3.3.2.2 Test Procedures
The test set up and the test procedure for the cyclic
triaxial test were also very close to those for the static triaxial
test. Except that in the test set up, three clamps were used to fix
the cell to the MTS loading platform so that the application of
extensional load will not cause any upward cell movement. The
loading piston of the MTS machine was lowered and connected to the
triaxial loading ram through a universal joint. The invert sine


45
Fig. 3.14 Use of Guide Tube for Pressure
Chamber Assembly


46
wave function was used for cyclic loading. The loading frequency
was set at 0.02 Hz', and ten cycles of cyclic loading was also set
for the test at each strain level.
The shear strain level was controlled by the converted axial
strain level through a span control knob. Following is the equation
used for the conversion of shear strain, 7 into axial strain, ea,
7 = (1 + v) ea
where u is the equivalent Poission ratio determined from static
triaxial tests.
A FLUKE-2240C data logger and a MTS built-in plotter were
used to record test results.
3.3.3 Resonant Column Test
3.3.3.1 Sample Preparation
Drnevich free-free resonant column device was used in this
study. The procedure for preparing a resonant column specimen was
identical to that for preparing triaxial specimens. The base
pedestal of the resonant column test device was fixed to the
excitation mechanism. The contact surface of the base pedestal and
top loading cap were roughened to ensure an effective transmission
of torsional wave through a sample. No porous stone or filter paper
was used.
After the completion of sample placement, eight aluminum
rods were screwed onto the base plate. Each rod was tightened to
the base plate by means of a nut and washer. The plastic pressure
cylinder was then put in place and the passive transducers were


47
plugged into their receptacle on the lid of the apparatus before the
lid was placed and tightened on the plastic cylinder using a torque
wrench. A 600 lb.-in. torque was used.
3.3.3.2 Test Procedures
Before sample preparation, the apparatus resonant frequency
was determined. In order to obtain the apparatus resonant fre-
quency, lower platen was attached and chamber, rods together with
lid were put in place and tightened. The wiring system shown in
Fig. 3.8 were followed. The lead from the control panel marked
"voltmeter" must be connected to the Y-axis of the oscilloscope
instead of the voltmeter.
The switch "B" was set in the "active" position and a small
amount of power was applied to the coils by gradually turning up the
volume control knob on the amplifier. The switch was then turned to
"on" position and the frequency was adjusted until the Lissajous
figure becomes close to a straight sloping line. The lowest
frequency at which the straight line was observed was the apparatus
resonant frequency. The device is then ready for a resonant column
test. A test began at a low input power corresponding to a low
strain level. The input, and output voltages were recorded and the
resonant frequency of the apparatus with sample was obtained by
adjusting the frequency of the input power from a value slightly
higher than the apparatus frequency up until the Lissajous figure is
close to a straight sloping line. Then one set of input voltage,
output voltage and resonant frequency was obtained. The test was


repeated by gradually increase the input power until the maximum
input power was reached.


CHAPTER IV
ANALYSIS AND DISCUSSION OF TEST RESULTS
4.1 Static Triaxial Test Results
The stress-strain relationships for the static triaxial test
under 15 psi. and 45 psi. confining pressures are shown in Fig. 4.1
and Fig. 4.2 respectively. Both figures indicate that under the
same confining pressure, the ultimate strength of the reinforced
samples exceeds that of the nonreinforced samples and the difference
increases with increasing number of layers of reinforcing geo-
textile. The peak stress difference versus the number of layers of
reinforcing geotextile relationship is shown in Fig. 4.3. The figure
gives a clear indication of the increase in ultimate strength due to
the increase in the number of layers of reinforcing geotextile.
In addition to the effect of increase in ultimate strength,
Fig. 4.1 and Fig. 4.2 also show that, for the type of reinforcing
material used, there is a tendency of the decrease in the initial
stiffness of reinforced samples. The amount of this reduction in
stiffness increases with increasing number of reinforcing layers.
The initial Young's modulus versus number of reinforcing layer
curves as shown in Fig. 4.4 summarize this softening tendency.
Fig. 4.4 also indicates that the decrease in initial sample


Stress Difference, oi-c5 (psi)
Fig. 4.1 Scress-Strain Relationship for Samples
Tested at 15-psi. Confining Pressure
tn
o


200
150
100
50
0
6 layers
_ yy' '4 layers 1 layer
0 layer
E______________________I__________:__________I----------------------L
0 5 10 15
Fig. 4.2 Axial Strain, (%) Stress-Strain Relationship for Samples Tested at 45-psi. Confining Pressure
Ln


250
200
150
100
50
0
45psi
15ps i
__i_______i________i_______i_______i-------1---------
1 4 6
Number of Reinforcing Layer, N
Fig. 4.3 Peak Stress Difference Versus Number of
Reinforcing Layer Relationship
Ui
ro


Initial Younq's Modulus, E. (psi)
Number of Reinforcing Layer, N
Fig. 4.4 Initial Young's Modulus Versus Number of
Reinforcing Layer Relationship
ui
LO


54
stiffness is much more drastic for samples tested under 15 psi
confining pressure than those under 45 psi confining pressure. This
is believed to be the result of increased fabric stiffness under a
higher confining pressure.
In order to explain the phenomenon of initial stiffness
reduction in reinforced samples, an idealized model of reinforced
sample is expressed as shown in Fig. 4.5. Ks and Kr represent the
respective stiffness of soil and reinforcement. The equivalent
overall stiffness of the sample, K, can then be expressed in terms
of Ks and Kr:
1/K = 1/KS + 1/Kr (4.1)
or K = Kr/(1+Kr/Ks) = Ks/(1+Ks/Kr) (4.2)
Three different cases, Ks/Kr = 1, Ks/Kr = 10 and Ks/Kr =0.1 can be
discused based upon Equation (4.2):
when Ks/Kr = 1, then K = 1/2KS = l/2Kr, or K < Ks = Kr
when Ks/Kr = 10, then K = 0.91Kr, Ks = 10Kr, so K < Kr < Ks
when Ks/Kr =0.1, then K = 0.09Kr, Ks = 0.1Kr, so K < Kr < Ks
Above discussions show that in all three cases, the overall
equivalent stiffness of the reinforced sample is always less than
that of each of its constituents. Of course this is the case when
soil and reinforcement both contribute equal amount of influence to
the overall stiffness of reinforced sample.


55
Fig. 4.5 An Idealized Stiffness Model for
A .Reinforced Sample


56
Further analysis was conducted based upon a more realistic
representation of the reinforced sample as shown in Fig. A.6. In
Fig. 4.6, L0 represent the total height of sample, Ls, total height
of soil, Lr, the total height of reinforcement. The axial strain,
)
e of the sample is then
e = 6/L0 = (Ss + Sr)/L0 (4.3)
where S Ss and Sr represent the overall deformation of sample,
the component deformations due to soil and reinforcement
respectively. Following derivation is to express the equivalent
Young's modulus of the reinforced sample in terms of the Young's
moduli of soil and reinforcement
e = (6S + 6r)/L0
= ^s/^o ^r/^o
= ^s^s/^o^s + ^r^r/^o^r
= (Ls/L0)x es + (Lr/L0)X£j- (4.4)
where er es are the components of e due to reinforcement and
soil. Since
e = a/E er = a/Er es = a/Es (4.5)
where E, Er and Es are Young's moduli of reinforced sample, of
reinforcement and of soil respectively. Substitution of equation
(4.5) into equation (4.4) results in the relationship


57
AS ^
IZI
T
nJ/i
T
j,
A? SS S SS ~1
2z:

7

rl
r2
r3
/ ^ 'si

\ \ iz

' N dJ3
T
rn-l----7fT
_5Jd.
sn
Lo = Lr Ls
Lr 'rl + Vz +
Ls 'si + 's2 +
+ 1
rn-l
+ 1
sn
Fig. 4.6 A General Model for Stiffness Analysis
of Reinforced Sample


58
1/E = (Lr/L0) x (1/Er) + (Ls/L0) x (1/ES) (4.6)
To estimate the values of Lr, Ls and Er, Fig. 3.4 is redrawn
in normal scale as shown in Fig. 4.7. At two confining pressures,
15 psi. and 45 psi., knowing the number of reinforcing layer, the
height of reinforcement, Lr, and the height of the soil,
Ls = 12 in. Lr, can be estimated using Fig. 4.7. The initial
compressive moduli of reinforcement at two confining pressures are
then determined by dividing the slope of the curve in Fig. 4.7 at
two confining pressures by the thickness of the reinforcement at
corresponding pressure. With Lr, Ls and Er known, equation (4.6)
can be used to estimate the initial Young's modulus of reinforced
samples. The estimated results for reinforced sample are plotted in
combination with the test results as shown in Fig. 4.8. In Fig.
4.8, the analytical results do not match exactly with test results,
however, the general trend of reduction in initial stiffness is
apparent in each case.
The lateral strain versus axial strain curves for the static
triaxial tests under two confining pressures are shown in Fig. 4.9
and Fig. 4.10, respectively. As shown in the figures, the slope of
any single curve, or, the equivalent Poission's ratio for any
sample, varies continuously at low axial strain, and approaches a
constant value as the axial strain increases. To avoid the
difficulty in selecting the equivalent Poission's ratio for the
shear strain amplitude control in cyclic triaxial tests, each of the


Fabric Thickness, T (*10 in.
9
0 10 20 30 40 50 60 70 80
Pressure, P (psi)
Fig. 4.7 Thickness Versus Rre.ssure Relationship of
Bidi.m C-34 Gootoxi ilo in Nonna I Srnlo
Ul
vO


U1
d.
LO
3
"O
O
LTi
OT
C
3
O
>-
12,
10,
Number of Reinforcing Layer, N
Fig. 4.8 Analytical Initial Young's Modulus Versus Number of
Reinforcing Layer Relationship based on Fig. 4.6
CT\
O


Axial Strain, e (.%)
d
Fig. 4.9 Lateral Strain Versus Axial Strain Relationship
at 15-psi. Confining Pressure


Axial Strain, l (%)
d
Fig. it. 10 Lateral Strain Versus Axial Strain Relationship
at 45-psi. Confining Pressure
On
ro


63
curves in Fig. 4.9 and Fig. 4.10 is approximated by two straight
lines as shown in Fig. 4.11 and Fig. 4.12, respectively.
The values of equivalent Poission's ratio as shown in
Fig. 4.11 and Fig. 4.12 indicate that the high compressibility of
the reinforcing material causes a decrease in the equivalent
Poission's ratio. This decrease in the equivalent Poission's ratio
is especially significant at low axial strains when the compress of
reinforcing material accounts for the major part of axial defor-
mation.
Since two different confining pressures were used in the
static triaxial test, Mohr's circles for each test at its peak
vertical stress together with failure envelopes for each of four
different reinforcing patterns were plotted in Fig. 4.13. In
Fig. 4.13, the samples without reinforcement exhibit frictional
resistance with zero cohesion, while the other three sets of
reinforced samples exhibit a near identical friction angle and a
cohesion intercept which increases with increasing number of
reinforcing layers.
4.2 Cyclic Triaxial Test Results
As anticipated from the static triaxial test results, for
the type of reinforcing fabric used, the shear modulus of reinforced
samples decreases with increasing number of reinforcing layers at
strains smaller than 10'3. This tendency of decrease in shear
modulus; however, becomes less significant as the shear strain
increases. When the shear strain increases beyond 5x10'2, the shear
modulus of reinforced samples increases instead of decreases with


Axial Strain, e {%)
a
0 1 2 3 4 5 6 7
Fig. 4.11 Approximate Lateral Strain Versus Axial Strain
Relationship at 15-psi. Confining Pressure
CT\


Axial Strain, r. (%)
d
Fig. 4.12 Approximate Lateral Strain Versus Axial Strain
Relationship at 45-psi. Confining Pressure
Ui


Shear Stress,
Normal Stress, o' (psi)
Fig. A.13 Mohr-Coulomb Envelope for Samples with
Different Reinforcing Patterns
ON
ON


67
increasing number of reinforcing layers. Shown in Fig. 4.14 and
Fig. 4.15 are the shear modulus versus shear strain relationships
for the cyclic triaxial test under the respective confining pres-
sures of 15 psi. and 45 psi.
Also shown in Fig. 4.14 and Fig. 4.15 are the shear damping
ratio versus shear strain relations for the two confining pres-
sures. As indicated in both figures, at a low shear strain
level, the shear damping ratio of reinforced sample is greater than
that of nonreinforced sample and increases with increasing number of
reinforcing layers. However, as the shear strain increases, the
shear damping ratio of nonreinforced sample increases significantly
and exceeds that of reinforced samples at high shear strains. This
could be due to the dilation of nonreinforced sample which reduces
the sample density and causes an increase in damping ratio while the
reinforcing material tends to buffer the dilating tendency of
reinforced samples and consequently prevents the damping ratio from
increasing.
The increase of damping ratio due to the reinforcement
implies a possibility that depending upon the type of reinforcing
material used, the advantage of reinforced earth may reach beyond
merely the purpose of strengthening. The reinforcing material may
also function as a filter screening off undesirable dynamic load
disturbance.
4.3 Resonant Column Test Results
In resonant column tests, a cyclic torsional load was
applied instead of an axial compression-extension load as in cyclic


Shear Modulus, G.(psi)
Fig. A. 14 Shear Modulus and Damping Racio Versus Shear Strain
Relationship for Cyclic Triaxiai Test at 15-psi
Confining Pressure
30
25
20
15
10
5
0
cr\
oo
Damping Ratio, D (%)


Shear Modulus, G (psi)
4 layers
1 layer
0 layer
Shear Strain, y
Fig. A.15 Shear Modulus and Damping Ratio Versus Shear Strain
Relationship for Cyclic Triaxinl Test at A5-psi
Confining Pressure
30
25
20
15
10
5
0
cr.
v£>
Damping Ratio, D {%)


70
triaxial tests. The type of reinforcing material used also caused a
reduction in the stiffness of reinforced samples as also indicated
in the cyclic triaxial test results at low strains. Shown in
Fig. 4.16 and Fig. 4.17 are the shear modulus versus shear strain
relationships for both nonreinforced and reinforced samples under
the two confining pressures. Both Fig. 4.16 and Fig. 4.17 indicate
that, at the shear strain of 10"6, the shear modulus of each sample,
reinforced or nonreinforced, approaches a constant maximum value.
Also, under each confining pressure, the nonreinforced sample
exhibits the highest stiffness while the reinforced samples show the
decreasing stiffness with increasing number of reinforcing
layers. Fig. 4.18 shows the maximum shear modulus versus number of
reinforcing layer relation under the two confining pressures.
The shear damping ratio versus shear strain relations for
the samples tested with resonant column test device are also shown
in Fig. 4.16 and Fig. 4.17. For the samples tested under 45
psi. confining pressure, the sample with the most reinforcement
exhibits the highest damping ratio as expected, while the nonrein-
forced sample shows the lowest damping ratio among the three. For
the tests under 15 psi. confining pressure; however, the trend is
not clear and more research is needed. The reinforced samples tend
to have lower damping ratio. This is in contradiction with the
results of the tests under 45 psi confining pressure.


Shear Modulus, G (psi)
- 15
10
Fig. k .16 Shear Modulus and Damping Ratio Versus Shear Strain
Relationship for Resonant Column Test at 15-psi
Confining Pressure
Damping Ratio, D (%)


Shear Modulus, G (psi)
T
24.000
20.000
_G___D_
A A 4 layers
1 layer
O 0 layer
15
16,000
10
12,000
8,000
!
i
/
4,000
//
/
CP
O'
*o
g
10
-6
10'
10
-4
10
-3
10
-2
10
-1
5
Shear Strain y
Fig. 4.17 Shear Modulus and Damping Ratio, Versus Shear Strain
Relationship for Resonant Column Test at 45-psi.
Confining Pressure
'j
to
Damping Ratio ,D (%)


Maximum Shear Modulus, Gmax (psi)

25,000 -
20,000
15,000 k ^ = 45psi
10,000 oc = 15psi
5,000
0 1 1 1 1 1 1 ) 1 4 Number of Reinforcing Layer, N Fig. 4.18 Maximum Shear Modulus Versus Number of Reinforcing Layer Relationship
LO


74
CHAPTER V
SUMMARY, CONCLUSION AND RECOMMENDATION FOR FUTURE STUDY
5.1 Summary
In this study, the dynamic response of an Ottawa 30-40 sand
reinforced with a nonwoven geotextile, Bidim C-34, was investigat-
ed. Both reinforced and nonreinforced samples were tested using
triaxial test device and resonant column test device. All samples
were prepared at 70% relative density and two confining pressures,
15 psi. and 45 psi., were used throughout the study. Static
triaxial tests were conducted to examine the static effectiveness of
reinforcement and to determine the equivalent Poission's ratio of
each test sample. Cyclic triaxial tests and resonant column tests
were conducted to examine the variation of shear modulus and damping
ratio under different strain amplitudes. The effect of reinforcing
pattern on both the static and dynamic performance of reinforced
samples was also investigated by varying the number of reinforcing
layer.
5.2 Conclusion
It is found through this study that, Bidim C-34 geotextile
can be used to increase the ultimate strength of a sandy soil
subjected to triaxial compression. The increase of ultimate
strength in reinforced samples increases with increasing number of


75
reinforcing layers. However, due to the high compressibility of
Bidim C-34 geotextile, the increase of ultimate strength in rein-
forced soil is accompanied by an increase in axial strain. The
amount of axial strain required to achieve ultimate strength
increases with increasing number of reinforcing layers.
In using Bidim C-34 geotextile reinforced soil for dynamic
loading resistance, the high compressibility of.Bidim C-34 geo-
textile makes it impractical if the shear strain amplitude induced
by the dynamic loading is very small. The effectiveness of
reinforcement becomes evident, however, when the induced shear
strain amplitude is increased. Besides, the increase of damping
ratio due to the inclusion of Bidim C-34 geotextile may contribute
to increase of dynamic loading resistance for reinforced samples.
5.3 Recommendation for Future Study
Since the inclusion of a reinforcing material can cause
change in both shear modulus and damping ratio of the reinforced
soil, there exists a possibility that by selecting a proper type and
location of soil reinforcement, an optimum aseismic design of a
reinforced earth structure can be achieved. To do this, laboratory
model test needs to be conducted and mathematical model to describe
the behavior of the model has to be established. Furthermore, field
test is required to justify the validity of the mathematical model.
The justified model can then be used to design a seismic resistant
reinforced earth structure and the dynamic performance of a rein-
forced earth structure may also be predicted based upon the model.


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