Use of MSE technology to stabilize highway embankments and slopes in Oklahoma.

              T 1300.8 U84m 2011 c.1 CE THROUGH RESEARCH - OKLAHOMA
FINAL REPORT - FHWA-OK-11-04
USE OF MSE TECHNOLOGY TO
STABILIZE HIGHWAY
EMBANKMENTS AND SLOPES IN
OKLAHOMA
KIANOOSH HATAMI, ASSISTANT PROFESSOR
GERALD A. MILLER, PROFESSOR
DANIAL ESMAILI, PH.D. STUDENT
SCHOOL OF CIVIL ENGINEERING AND ENVIRONMENTAL
SCIENCE
UNIVERSITY OF OKLAHOMA
NORMAN, OKLAHOMA
PLANNING & RESEARCH DIVISION
ENGINEERING SERVICES BRANCH
RESEARCH SECTION
spr@odot.org, office: (405)522-3795
OKLAHOMA DEPARTMENT OF TRANSPORTATION
200 NE 21ST STREET, OKLAHOMA CITY, OK 73105-3204
TECHNICAL REPORT DOCUMENTATION PAGE
1. REPORTNO. 2. GOVERNMENTACCESSIONNO. 3. RECIPlENT=SCATALOGNO.
FHWA - OK-11- 04
4. TITLEANDSUBTITLE: 5. REPORTDATE
March 2011
Use of MSE Technology to Stabilize Highway Embankments and Slopes in
Oklahoma
6. PERFORMINGORGANIZATIONCODE
7. AUTHOR(S):Kianoosh Hatami, Gerald A. Miller and Danial Esmaili 8. PERFORMINGORGANIZATIONREPORT
9. PERFORMINGORGANIZATIONNAMEANDADDRESS 10. WORKUNITNO.
School of Civil Engineeri"g and Environmental Science, University of
Oklahoma, Norman, OK
11. CONTRACTORGRANTNO.
ODOT SPR Item Number 2214
12.SPONSORINGAGENCYNAMEANDADDRESS 13. TYPEOF REPORTANDPERIODCOVERED
Oklahoma Department of Transportation, Planning and Research Division, Final Report
200 N.E. 21st Street, Room 3A7, Oklahoma City, OK 73105
10/01/2009 - 09/30/2010
14. SPONSORINGAGENCYCODE
15. SUPPLEMENTARYNOTES
16.ABSTRACT:Departments of transportation across the U.S. are faced with the persistent problem of landslides and slope
failures along roads and highways. Repairs and maintenance work associated with these failures cost these agencies
millions of dollars annually. Over the past few decades, Mechanically Stabilized Earth (MSE) technology has been
successfully used as a cost-effective solution for the construction and repair of slopes and retaining structures in
transportation applications. Significant cost-savings in the re-construction and repair of highway slopes and embankments
could be achieved by using locally available soils and reinforcing them with geosynthetics. However, locally available soils
in many locations are of marginal quality and their shear strength and interaction with the geosynthetic reinforcement can
be significantly dependent on their moisture content. As a result, the influence of soil moisture content and suction on the
soil-reinforcement interaction needs to be properly accounted for in the design of reinforced soil slopes and
embankments. Provisions related to the influence of soil suction on the shear strength of soil-reinforcement interfaces are
currently lacking in the existing design guidelines for these structures. In this stUdy, a moisture reduction factor was
developed for the pullout resistance of a geotextile reinforcement material in an Oklahoma soil (termed here as Chickasha
soil) that could be used for the design of reinforced soil structures with marginal soils.
17. KEYWORDS: Mechanically stabilized earth, 18. DISTRIBUTIONSTATEMENT
Slope stability, Marginal soil, Geosynthetics,
Moisture reduction factor No restrictions. This publication is available from the planning and
research Div., Oklahoma DOT.
19. SECURITYCLASSIF.(OFTHISREPORT) 20. SECURITYCLASSIF. 21. NO.OFPAGES 22. PRICE
(OFTHISPAGE)
Unclassified rt N/A
Unclassified
ii
* (MODERN METRIC) CONVERSION FACTORS
Il
-in-=2---lsquare inches
Iff Isquarefeet
lyd2 Isquareyard
lac , lacres
Imi2 Isquaremiles
AREA
1645.2 IsquaremillimetersImm2
Isquaremeters 1m2
Isquaremeters l-m"-2--I
Ihectares Iha
Isquarekilometers l-km~--2I
10.093
10.836
100405
12.59
I NOTE: volumesgreaterthan 1000 L shall be shownin m3
1-----------M-A-S-S-----------1
loz lounces 128.35 Igrams
lib l-po-u-n-ds------rI0-A-5-4----rlk-il-og-r-am-s---r---I
rr-IShort tons (2000 Ib) 10.907 megagrams(or
I "metricton")
.~ - ~-'
iii
I AREA
Imm2 Isquaremillimeters 10.0016 Isquareinches lin2
1m2 Isquaremeters 110.764 Isquarefeet Iff
1m2 Isquaremeters 11.195 Isquareyards lyd2
lha Ihectares 12.47 lacres lac
Ikm2 Isquarekilometers 10.386 Isquaremiles Imi2
.-'
MASS
Igrams 10.035 lounces loz
---I-ki-lo-g-ra-m-s-----1r-2-.2-0-2----lpounds lib
r------rm-e-g-ag-r-a-m-s-(-0-r-"m-e-tr-ic-lr--1-.1-0-3-----i-s-h-o-rt-t-o-n-s-(2-0-0-0-r=r--
ton") Ib) I
I ILLUMINATION
I-Ix---llux 10.0929 Ifoot-candles Ifc
ICd/m2 '-ca-n-d-e-la-'m--;2.------'10-.2-9-1-9----I-fo-o-t--La-m-b-e-rt-s--Ifl
¥;-
*SI is the symbol for the InternationalSystemof Units.Appropriateroundingshould be madeto
comply with Section 4 of ASTM E380.
(RevisedMarch 2003)
iv
The contents of this report reflect the views of the author(s) who is responsible for the
facts and accuracy of the data presented herein. The contents do not necessarily reflect
the views of the Oklahoma Department of transportation or the federal Highway
Administration. This report does not constitute a standard, specification, or regulation.
While trade names may be used in this report, it is not intended as an endorsement of
any machine, contractor, process, or product.
v
TABLE OF CONTENT
1. INTRODUCTION 1
2. THEORY 3
2.1. REINFORCEMENT PULLOUT CAPACITY IN MSE STRUCTURES 3
2.2. EXTENDED MOHR-COULOMB FAILURE ENVELOPE 4
3. MATERIAL PROPERTIES AND SUCTION SENSORS 5
3.1. SOIL PROPERTIES 5
3.2. SUCTION SENSORS 8
3.2.1. FREDLUND SENSORS 8
3.2.2. PST 55 PSYCHROMETER 11
3.2.3. FILTER PAPER 12
3.2.4. WP4 POTENTIOMETER 15
3.3. GEOSYNTHETIC REINFORCEMENT 19
4. LARGE-SCALE PULLOUT TESTS 20
4.1. METHODOLOGY 20
4.1.1. TEST EQUIPMENT 20
4.1.2. INSTRUMENTATION 21
4.2. TEST PROCEDURE 25
4.2.1. PROCESSING OF THE SOIL 25
4.2.2. PLACEMENT OF THE SOIL IN THE PULLOUT BOX 27
4.2.3. PULLOUT TEST AND DISMANTLING OF THE TEST SETUP 28
4.2.4. INTERFACE PROPERTIES 29
4.2.5. SOIL MOISTURE CONTENT AND SUCTION 35
4.2.6. PARAMETERS a AND F* 51
5. SMALL-SCALE TESTS 55
5.1. SMALL-SCALE PULLOUT TESTS 56
5.1.1. RESULTS 56
5.2. INTERFACE SHEAR TESTS 61
5.2.1. RESULTS 61
6. MOISTURE REDUCTION FACTOR, ~(w) 63
7. CONCLUSIONS 65
8. REFERENCES 66
vi
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
LIST OF FIGURES
A failed slope of a highway embankment in Chickasha, OK 1
Excavation pit where soil samples were taken from the failed slope in 5
Chickasha, OK
Gradation curve (sieve analysis) of Chickasha soil (The vertical broken line 6
shows the location of #200 sieve)
Compaction test results for Chickasha soil 8
Fredlund sensors placed in a calibration bucket to measure matric suction of 9
the Chickasha soil
Schematic cutaway section indicating the locations of Fredlund sensors in the 10
calibration bucket
Soil suction versus moisture content for Chickasha soil from Fredlund 10
sensors, Note: The vertical lines indicate the mean values of measured
moisture content in each test
(a) A PST55 sensor submerged in NaCI solution; (b) Sensor calibration setup 11
Filter paper calibration curve for Whatman NO. 42 filter paper (from Chao 13
2007)
Variation of moisture content and total suction for Chickasha soil from filter 15
paper test
WP4 water Potentiometer equipment (soil samples in sealed cups are shown 16
in the inset
Soil water characteristic curve for Chickasha soil using WP4 Potentiometer 17
Gravimetric moisture content vs. total suction for Chickasha soil on semi-log 18
plot (pF is the base 10 logarithm of the suction expressed in the cm of water)
Mechanical response of the geotextile used in the pullout tests (Mirafi HP370) 19
as per the ASTM 04595 test protocol and as compared with the manufacturer
data. Note: tow arrows show the ultimate tensile strength and strength of
geotextile reinforcement at 5% strain
One of the pullout test boxes at the OU Fears laboratory 21
(a)Wire-line extensometers attached to the geotextile reinforcement (the 22
numbers in the figure indicate the extensometer number and distance from the
tail end of the geotextile); (b) earth pressure cell placed on the top of the soil
in the pullout test box
Axial strain distribution over the length of geotextile reinforcement from large 25
vii
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
scale pullout test on Chickasha soil at different moisture contents and
overburden pressures
Soil processing equipment at the OU Fears laboratory, (a, b) Soil processors, 26
(c) Soil mixer
(a) Sealed buckets containing processed soil before placing in the pullout box, 27
(b) Soil samples in the oven to determine their moisture content
(a) Sealed compacted soil at the end of large scale pullout box setup, (b) 27
Geotextile specimen at the mid-height of the pullout box
Schematic diagram of the large-scale pullout box test setup 28
Pullout test data and interface strength results for Chickasha soil and 31
comparison of failure envelope for soil-geotextile interface at different
moisture content values (OMC-2%, OMC, OMC+2%)
Pullout test data for Chickasha soil at different overburden pressure values 32
Comparison between the actuator and the geotextile front end displacements 33
for the pullout test at OMC-2% subjected to 50 kPa overburden pressure.
Note: The horizontal dashed line shows the maximum pullout force
Local displacement of the geotextile reinforcement in a large-scale pullout test 35
at OMC subjected to 20 kPa overburden pressure
Distributions of moisture content over the soil depth in the pullout box. Notes: 41
(1) One soil sample was taken from each bucket, (2) The number of soil
samples from each soil lift in the pullout box is given in Table 6 (caption), (3)
The horizontal line indicates the target moisture content for each test series,
(4) The vertical dashed line shows the location the soil-geotextile interfaces, (5)
The mean and COV values reported in the legends are calculated for the fifth
layer dada only
Distributions of total suction over the soil depth in the pullout box from WP4 at 46
different moisture contents. Note: The number of soil samples from each soil
lift in the pullout box is given in Table 6 (caption)
Pullout test data and interface strength results for Chickasha soil at different 48
overburden pressure values and comparison of failure envelopes for soil-geotextile
interface on lateral plane at different moisture contents (OMC-2%,
OMC, OMC+2%)
Extended Mohr-Coulomb failure envelopes for the soil-geotextile interface 50
from large-scale pullout tests
viii
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
Figure 36.
Calculation of pullout parameters for Mirafi HP370 geotextile reinforcement 53
from large-scale pullout test data in Chickasha soil
Pullout resistance versus mobilized length at different overburden pressure 54
values
Small-scale pullout tests in Chickasha soil using a DST machine 56
Pullout test data and interface strength results for Chickasha soil and 58
comparison of failure envelopes for soil-geotextile interface at different
moisture contents (OMC-2%, OMC, OMC+2%)
Chickasha soil-geotextile interface strength results from pullout tests: (a) 60
Large-scale tests, (b) Small-scale tests
Mohr-Coulomb envelopes for Chickasha soil-geotextile interface from direct 62
interface shear tests: (a) Envelopes on frontal plane, (b) Envelopes on lateral
plane
Moisture reduction factor, ~(w), for the woven geotextile in Chickasha soil 63
from large-scale and small-scale pullout tests (LP, SP and SI indices stand for
large-scale pullout, small-scale pullout and small-scale shear interface tests,
respectively). The values for Minco silt from a recent study by the authors
(Hatami et al. 2010a) are also shown for comparison purposes
ix
LIST OF TABLES
Table 1. Summaryof Chickasha soil properties 6
Table 2. Summary of PST 55 Psychrometer sensor calibration data using a 1000 12
mmol/kg NaCIsolution (Standard/targetwater potential:2500 kPa)
Table 3. Filter Papertest results for Chickasha soil 14
Table 4. Summaryof McKeen (1992) ExpansiveSoil Classification Methodology 18
Table 5. Large-scalepullout test parameters 20
Table 6. Interface strength propertiesfrom pullout tests in Chickasha soil 34
Table 7. Mean and COV values for the fifth layer (in contact with geotextile) in large 36
scale pullouttests
Table 8. Comparison of total suction values in Chickasha soil as measured using 37
Psychrometer(in-situ)and WP4 (offsite equipment)
Table 9. Interface strength properties from pullout tests in Chickasha soil as a function 49
of the soil total suction
Table 10. Large-scale pullout tests in Chickasha soil to obtain values for a.and F* 52
Table 11. Small-scale pullout test parameters 55
Table 12. Interface strength propertiesfrom small-scale pullout tests 59
x
Gerald A. Miller
USE OF MSE TECHNOLOGY TO STABILIZE HIGHWAY
EMBANKMENTS AND SLOPES IN OKLAHOMA
FINAL REPORT -FHWA-OK-11-04
OOOT SPR ITEM NUMBER 2214
by
Kianoosh Hatami
Assistant Professor
Professor
Danial Esmaili
PhD Student
School of Civil Engineering and Environmental Science
University of Oklahoma
Norman, OK
Research Division
Oklahoma Department of Transportation
200 NE 21sh Street
Oklahoma City, Oklahoma
March 2011
1. INTRODUCTION
Oklahoma Department of Transportation (ODOT) and other departments of transportation
across the U.S. are faced with a persistent problem of landslides and slope failures along the
highways. Repairs and maintenance work due to these failures are extremely costly (i.e. in
millions of dollars annually nationwide). In Oklahoma, many of these failures occur in the
eastern and central parts of the state due to higher topography and poor soil type (Hatami et al.
2010a,b, 2011). A recent example of these failures is the landslide on the US Route 62 in
Chickasha, Oklahoma. (Figure 1.)
(a) (b)
Figure 1. A failed slope of a highway embankment in Chickasha, OK
For proper construction or repair of highway slopes and embankments an ideal solution would
be to work with large quantities of coarse-grained, free-draining soils to stabilize the structures
as recommended by design guidelines and specifications for Mechanically Stabilized Earth
(MSE) structures in North America (e.g. Elias et al. 2001, Berg et al. 2009). However, coarse-grained
soils are not commonly available in Oklahoma and many other parts of the U.S.
Consequently, the costs of the fill material and its transportation can be prohibitive depending
on the location of the high-quality soil.
One possible solution in such cases would be to use locally available soils as construction
materials because they would require significantly less material transportation, fuel consumption
and generated pollution compared to using high-quality offsite soils. It has been estimated the
fuel costs constitute about 20% of the total costs for transportation of high-quality soil (Ou et al.
1
1982). On the other hand, commonly available soils in Oklahoma for the construction of
reinforced slopes are of marginal quality (e.g., soils with more than 15% fines). Geosynthetic
reinforcement can be used to stabilize marginal soils. Using the Mechanically Stabilized Earth
technology (MSE) could help reduce the cost of fill material by up to 60% (Keller 1995).
However, in order to reinforce earthen structures involving marginal soils, it is important to
obtain a satisfactory soil-reinforcement interface performance. The performance of marginal
soils and their interface with geosynthetic reinforcement can be complex under construction or
service loading conditions and may include strain softening behavior, excessive deformation
and loss of strength as a result of wetting.
An important consideration in the design of reinforced soil structures constructed with marginal
soils is the possibility of reduction in interface pullout resistance due to the increase in the soil
moisture content (wetting), loss of suction and possible development of excess pore water
pressure. This can result in excessive deformations and even failure of the reinforced soil
structure. As a result, the design procedures need to take into account the influence of soil
moisture content on soil strength, the strength of soil-geosynthetic interface and the resulting
factor of safety against failure. Such design provisions are currently not available for reinforced
soil structures constructed with marginal soils. Typically, construction materials for reinforced
soil structures are tested at moisture content values near optimum (i.e. Optimum Moisture
Content - OMC). However, in actual construction, several factors could make the fill moisture
content deviate from the design value. Examples include precipitation during construction,
groundwater infiltration and development of excess pore water pressure due to compaction.
These factors, in addition to seasonal variations of soil moisture content, can significantly
reduce the strength of the soil-reinforcement interface and lead to excessive deformations or
failure. A primary objective of this study is to develop a moisture reduction factor (MRF) for the
pullout resistance of soil-geotextile interface for the design of reinforced soil structures with
marginal soils..
It should be noted that this study is not intended to substitute the need for an adequate and
properly located and constructed drainage system in reinforced soil structures and slopes. In
addition, quality control and quality assurance in both design and construction of these
structures are obviously required. In order for reinforced soil structures with marginal soils to be
safe and provide satisfactory performance, a number of crucial factors need to be included in
their design and construction including proper drainage systems, quality control during
compaction (i.e. placement moisture content and density), small spacing between reinforcement
2
(1)
layers and relatively low construction speed to avert the consequences of loss of suction in the
backfill.
2. THEORY
2.1. REINFORCEMENT PULLOUT CAPACITY IN MSE STRUCTURES
For internal stability, the pullout resistance per unit width (Pr) of the reinforcement is determined
using ••••• and it is defined as the ultimate tensile load required to generate outward
sliding of the reinforcement through the reinforced soil mass (Elias et al. 2001, Berg et al. 2009):
Where:
Le: Embedment or adherence length in the resisting zone behind the failure surface
C: Reinforcement effective unit perimeter; e.g., C = 2 for strips, grids, and sheets
L; C: Total surface area per unit weight of reinforcement in the resistive zone behind the
failure surface
F* = tan Opeak : Pullout resistance factor
0peak: Equivalent peak friction angle of the soil-geosynthetic interface
a: A scale effect correction factor to account for a nonlinear stress reduction over the
embedded length of highly extensible reinforcements
()~: Effective vertical stress at the soil-reinforcement interface
Pullout tests are typically used to obtain the parameters a and F* for different reinforcement
materials. The correction factor a depends on the extensibility and the length of the
reinforcement. For extensible sheets (i.e., geotextiles), the recommended value of a is 0.6 (Berg
et al. 2009). The parameter F* (especially in reinforcement types such as geogrids and welded
3
wire mesh) includes both passive and frictional resistance components (e.g., Palmeira 2004,
Abu-Farsakh et al. 2005, Berg et al. 2009).
Routine pullout tests are useful for determining short-term pullout capacity or reinforcement
materials. However, they do not account for soil or reinforcement creep deformations. Tests are
typically performed on samples with a minimum embedded length of 600 mm (24) as
recommended in related guidelines (e.g. ASTM 06706). The pullout resistance (Pr) is taken as
the peak pullout resistance value from the pullout tests.
2.2. EXTENDED MOHR-COULOMB FAILURE ENVELOPE
The shear strength of an unsaturated soil depends on two stress variables: net normal stress
«(Tn - ua) and soil matric suction (ua - uw) (Fredlund et al. 1978). Net normal stress is the
difference between the total stress and pore air pressure, and the matric suction is the
difference between the pore air and the pore water pressures. This theory is also valid for dry
and saturated soil conditions. Miller and Hamid (2005) proposed the following equation to
determine the shear strength of unsaturated soil-structure interfaces:
(2)
Where:
c~: Adhesion intercept
(Tn: Normal stress on the interface
Ua: Pore air pressure
0': The angle of friction between soil and reinforcement with respect to ((Tn - Ua)
Uw: Pore water pressure
s». The angle of friction between soil and reinforcement with respect to suction
(ua - uw)
In the case of an unsaturated soil, Mohr circles representing failure conditions correspond to a
30 failure envelope, where the shear stress (t) is the ordinate and the two stress variables are
the abscissas «(Tn - ua) and (ua - uw). The locations of the Mohr circles in the third dimension
4
3.1. SOIL PROPERTIES
are functions of matric suction (ua - uw). The planar surface formed by these two stress
variables is called the extended Mohr-Coulomb failure envelope.
3. MATERIAL PROPERTIES AND SUCTION SENSORS
The soil used in the pullout tests for this study is a lean clay found on US Route 62 in
Chickasha, OK (lili(j~i:\").In this report, the soil is referred to as the Chickasha soil. Physical
and mechanical soil property tests were carried out on the soil samples in general accordance
with ASTM 01140 to determine the fines content and ASTM 0422 for sieve analysis and
hydrometer test. The results are given in fflgli!~'@;;'~and t.!lii~j.According to the Unified Soil
Classification System (USCS) and AASHTO, the soil is classified as CL and A-6, respectively.
Figure 2. Excavation pit where soil samples were taken from the failed slope in Chickasha, OK
5
100
,------
----11--------
I
._-+------
..-..
~
80 .:
.s:::
C>
'0)
60 ::
>- .0•...
Q)
40 .§ -c::
Q)o
20 oQ,)
o
0.0001
Figure 3. Gradation curve (sieve analysis) of Chickasha soil (The vertical broken line shows the
location of #200 sieve)
1
i L__
1
1
1
1
1
1 ------.- .. ---'-'---r---
1,
I
10 1 0.1 0.01 0.001
Diameter (mm)
Table 1. Summary of Chickasha soil properties
Property (Lean clay) Value
Liquid Limit (%) 38
Plastic Limit (%) 20
Plasticity Index (%) 18
Specific Gravity 2.75
Gravel (%) 0
Sand (%) 10.6
Silt (%) 49.4
Clay (%) 40
Maximum Dry Unit Weight, kN/m3 (pcf) 17.3 (111)
Optimum Moisture Content (%) 18
6
Four compaction tests (one standard, two Harvard miniature, and one modified proctor test)
were carried out on the Chickasha soil to determine the values of the soil maximum dry unit
weight and optimum moisture content ( ) more accurately. also shows a series
of theoretical curves of the soil dry unit weight versus moisture content for different degrees of
saturation. These curves show different values of degree of saturation at maximum dry unit
weight that were obtained from
(3)
Where:
c; Specific gravity
w: Moisture content
s: Degree of saturation
Yw: Water unit weight
Yd: Soil dry unit weight
The curves corresponding to S = 1, 0.9 and 0.8 are shown as the zero air void line (ZAVL -
representing the minimum void ratio attainable at a given moisture content), 10% AVL and 20%
AVL, respectively (Budhu, 2000). The air void lines in were determined from
'. To plot the ZAVL, the soil saturation was set to unity (S = 1). Then, having specific gravity for
Chickasha soil from (Gs = 2.75) and water unit weight (Yw= 10 kN/m3
), the dry unit
weight (Yd) was calculated at different moisture content (w) values. This procedure was
repeated to obtain the 5%, 10%, 15% and 20% air void lines. shows that the maximum
dry unit weight was attained at S = 0.9 and also, the test results are reliable because the wetting
points are placed below the ZAVL. Based on the results of all compaction tests, the best values
for the maximum dry unit weight and optimum moisture content were chosen as Ydmax = 17.3
kN/m3 (111 Ib/fe) and OMC = 18%, respectively.
7
20 .-----------------------~~----------------------_.
·19
-ME 18 --z~
-.-.r::-.
0> .a> 17 s
:!::: c: ::J
~ 16 0
15
14
• Standard proctor test, maximum.. ,
dry unit weight and optimum , •••• , ,
moisture content ••••' ,
• Harvard miniature test, trial (1), " •••• '- '
maximum dry unit weight and • "."
optimum moisture content • ,..... ,
.A Harvard miniature test, trial (2), , , ••~
maximum dry unit weight and • • ....,
optimum moisture content " ". ~ ,. ,
- ZAVL • •-,,". ~ , "
• • • • •• 10% AVL • .,, '". ...,, '-,'. - ,20% AVL '. ,-,' -, ".'-,
o Modified proctor test ,
o 5 10 15 20 25
Moisture content (%)
Figure 4. Compaction test results for Chickasha soil
3,2. SUCTION SENSORS
In this study, several different methods and sensors were examined to measure the soil suction
and moisture content as described in the following sections:
3.2.1. FREDLUND SENSORS
The thermal conductivity of a porous medium increases with its moisture content. Therefore, the
thermal conductivity of a standard porous (e.g. ceramic) block in equilibrium with the
surrounding soil can be used to measure the moisture content of the ceramic block, which in
turn, is dependent on the matric suction of the surrounding soil (Pereta et al. 2004). The concept
described above makes it possible to calibrate the thermal conductivity of Fredlund sensors
against the matric suction in the surrounding soil.
8
Samples of the Chickasha soil were placed and compacted in a test bucket to examine the
performance of our three available Fredlund thermal sensors in measuring soil suction
(Fredlund et al. 2000, Pereta et al. 2004). Five tests were carried out using these sensors. For
each test, the bucket was filled with a sample of Chickasha soil in three lifts which were
compacted to 95% of its maximum dry unit weight similar to the target compaction level in the
pullout tests. Once each lift was compacted, a cylindrical core was excavated within the soil to
place the Fredlund sensor. The soil was then backfilled around the sensor and compacted
(Figure 5).
Figure 5. Fredlund sensors placed in a calibration bucket to measure matric suction of the
Chickasha soil
The positions of the three sensors in the bucket are schematically shown in Figure 6. After
taking suction readings and finishing each test, soil samples were taken from the areas around
each sensor to measure their moisture content. We waited 24 hours between consecutive
readings for sensors to equilibrate with the surrounding soil. This procedure was repeated on
the soil placed in the bucket at different moisture content values. The resulting Soil Water
Characteristic Curves (SWCC) from Fredlund sensors are plotted in Figure 7.
9
___ 1
,,,,-FN-rOe-d.-3l-un-d--s-e-n-so-r, I'' ,-- ...., '
I ------------~
4.5 •
'---1, i"'-". -=-------, ---1 ---Ft-
1 ( u
. _,- ,~__j-~~.d~~~d;e~~;r-~ 1 I IT ----' ',------------'
n---------- ,------------ ! J- -- -~:Fredlund sensor :
I NO.1 I
.I•..• _----- 1'
4.5·
4.5"
Figure 6. Schematic cutaway section indicating the locations of Fredlund sensors in the
calibration bucket
.--. c(I,J
~-
100
P 80 0
60 -, con
::l en
.-so:: (IJ :2:
40 1oSensor NO.1
I
20 -{ DSensor No.2
I,
~Sensor NO.3
o +- '------'-r- ---'-,----'-- .,.------
o 5 10 15 20 25
Soil Moisture Content (%)
30
Figure 7. Soil suction versus moisture content for Chickasha soil from Fredlund sensors. Note:
The vertical lines indicate the mean values of measured moisture content in each test.
The data in Figure 7 show a reasonable trend of reduction in the soil suction at higher moisture
contents. However, the scatter in data is significant. Moreover, the range of suction values is
significantly lower than what is expected for Chickasha soil (i.e. on the order of 1000 kPa on the
dry side of optimum) as measured using WP4 equipment (See Section 3.2.4). A possible
10
reason for the above shortcomings is that the Fredlund sensors need to be in complete contact
with the backfill soil to function properly. Extra care was taken to compact the soil as best as
possible around the sensors after they were placed in the cavities in the calibration bucket.
However, due to the small amount of soil that needed to be compacted and space limitations
around the sensors, achieving proper compaction without disturbing the intact soil around the
hole proved to be challenging. Results in Figure 7 indicate that readings from these sensors
could be very sensitive to the placement procedure. Therefore, it was decided to search for
other suction/moisture sensors for this study.
3.2.2. PST 55 PSYCHROMETER
PST 55 is an in-situ psychrometer which can measure soil total suctions up to 5000 kPa. Under
vapor equilibrium conditions, water potential of its porous cup is directly related to the vapor
pressure of the surrounding air. This means that the soil water potential is determined by
measuring the relative humidity of the chamber inside the porous cup (Campbell et al. 1971).
PST 55 psychrometers are much smaller than Fredlund sensors and are commonly used in
geotechnical research projects. PST 55 Psychrometer sensors can lose their factory calibration
over time. Therefore, in this study we calibrated them using a 1000 mmol/kg NaCI solution
before we used them in the pullout tests. Figure 8 shows a snapshot of the calibration setup
and procedure for these sensors.
(a) (b)
Figure 8. (a) A PST 55 sensor submerged in NaCI solution; (b) Sensor calibration setup
The data logger shown in Figure 8b was used to read the water potential of the NaCI solution
samples, and the ice chest provided a controlled temperature and moisture environment for the
11
calibration of the sensors. The sensors were submerged in NaCI solutions and kept in the ice
chest for about 2 hours to reach equilibrium (Wescor Inc. 2001). Then, each sensor was
connected to the data logger (one at a time) and the water potential of the control NaCI solution
was read in microvolts (J.lV). Four Psychrometer sensors were calibrated and the results are
given in Table 2.
Table 2. Summary of PST 55 Psychrometer sensor calibration data using a 1000 mmol/kg
NaCI solution (Standard/target water potential: 2500 kPa)
Sensor # Temperature (0C) Sensor output (uv) Water Potential (kPa) Calibration factor
1 23.9 18 2400 1.04
2 23.9 17 2270 1.10
3 23.9 19 2530 0.99
4 23.9 18 2400 1.04
3.2.3. FILTER PAPER
In-contact and non-contact filter paper techniques are used to measure the soil matric and total
suction values, respectively. In the in-contact filter paper technique, water content of the initially
dry filter paper increases due to a flow of water in liquid form from the soil to the filter paper until
the two media come into equilibrium with each other. After equilibrium is established, the water
content of the filter paper is measured. Then, by using the appropriate filter paper calibration
curve, the soil matric suction is estimated. In the non-contact technique, the dry filter paper is
suspended above a soil specimen in a sealed container for water vapor equilibrium between the
filter paper and the soil specimen at a constant temperature. The vapor space above the soil
specimen acts as a true semi-permeable membrane which is only permeable to water vapor but
not to ions from the pore-water. The separation between the filter paper and the soil by a vapor
barrier limits water exchange to the vapor phase only and prevents solute movement.
Therefore, in this technique, the total suction is measured. After equilibrium, the filter paper is
removed and water content of the filter paper determined as quickly as possible. Then, by using
the appropriate filter paper calibration curve, the soil total suction is estimated (Pan et al. 2010).
12
Chickasha soil samples were prepared at OMC-2% and OMC+2% moisture contents to predict
maximum and minimum suction levels in our pullout tests. The filter paper test method was
used as an alternative means to measure the soil matric suction as per the ASTM 05298-10
test standard (ASTM 2010). Each soil sample was cut into two halves with smooth surfaces. A
circular piece of Whatman filter paper with the diameter d = 42 mm was placed between two
larger papers (d = 55 mm). All three papers were sandwiched between the two soil halves which
were then taped together. The entire assembly was placed in a jar. To measure total suction, a
piece of geogrid was placed on the top of the taped soil specimen and two large filter papers
were placed on the top. The geogrid provided a suitable and convenient way to leave a small
gap between the unsaturated soil sample and the filter paper assembly. Next, the jar lid was put
back on and labeled with the information on the soil sample. The jar was placed in a well-insulated
container for suction equilibrium and its temperature was monitored and recorded.
This process was repeated for all other samples. Figure 9 and Table 3 show the calibration
curve for the filter paper used and the test results, respectively .
5 4.0
n:=i
~ 3.0
1)
J0-
g' 2.0
d
•• Data from 3-Week Equflibrafioo Period f----
t<, • Data trom 2-Week EqufObrlliioo Period
<, I I <, !Wllatman No. 42 Filter Paper
-.....::.,.
~ ./
I~fO=gIV0.=f}951.4634 - 0.0933 ''''':1 ~ " --~
log IV - 23.012 - 0.6389 'lit I -\ f= 0.712
OJ}
o 10 20 30 40
Rtter Paper Water Cootent, '111(%)
60
Figure 9. Filter paper calibration curve for Whatman No.42 filter paper (from Chao 2007)
13
Table 3. Filter Paper test results for Chickasha soil
Measured OMC-2% OMC+2%
Suction
Top Filter Paper Bottom Filter paper Top Filter Paper Bottom Filter paper
Test (1) (2) Test (1) (2) Test (1) (2) Test (1) (2)
Log kPa 4.251 4.148 4.232 3.989 3.821 3.849 3.775 3.877
Total Suction 17823 14060 17060 9750 6622 7063 5956 7534
(kPa)
Average
Total Suction 14673 6794
(kPa)
Note: Two tests were carried out at each target moisture content (OMC-2% and OMC+2%)
According to Table 3, since the difference between the two suction values in the repeat trials of
nominally identical samples is less than 0.5 Log kPa (ASTM 05298-10), the results are
acceptable and no results should be discarded. Therefore, the mean value of total suction for
each of the OMC-2% and OMC+2% cases for the Chickasha soil is given in the last row of
Table 3. Results of Table 3 are plotted in Figure 10.
14
25000
:::R
~
.- 0 20000 N
n<1,l + .~.....- <I' ~o
c 15000 I 0
0 •• I
:0J c-
(/) I I
<1l 10000 - :::R m
0 0
l- NI
5000
o~ <'OMC-2%
0
I OOMC+2%
0
13 15 17 19 21 23
Soil moisture content (%)
Figure 10. Variation of moisture content and total suction for Chickasha soil from filter paper
tests
Although the results in e!gYif;lI=9. show a reasonable trend of lower total suction at higher
moisture content values, the measured values of total suction for Chickasha soil are significantly
higher than what is expected (see iQ~QJi ,3~~~}4T).here are three critical parameters that must
be considered in order to achieve reasonable results from filter paper tests. First, this test
method requires an extremely clean lab environment. Second, the test should be carried out at
constant temperature and relative humidity. Third, the weights of the filter papers need to be
measured immediately after the samples reach equilibrium. Failure to adhere to anyone of
these requirements could result in significant errors in the measured results.
3.2.4. WP4 POTENTIOMETER
The WP4 equipment measures the soil total suction. It consists of a sealed block chamber
equipped with a sample cup, a mirror, a dew point sensor, a temperature sensor, an infrared
thermometer and a fan (Ji6Yi-"J;1). The soil sample is placed in the sample cup and brought to
vapor equilibrium with the air in the headspace of the sealed block chamber. At equilibrium, the
water potential of the air in the chamber is the same as the water potential or suction of the soil
sample.
15
r-----------
I Preparedsoil :
: sample I
~---- I
: - LCD di;pla; --:
I J
: -Fu~ct~i~;;s-: ~~~ J
r----------- I Disposablecup :
IL -I' :-s~;;~;;;e~-:
~~ J
Figure 11. WP4 Water Potentiometer equipment (soil samples in sealed cups are shown in the
inset)
Seventeen (17) 300-gram samples of Chickasha soil were prepared at different moisture
content values. Approximately 100 grams of each sample was used to measure its moisture
content using the oven-drying method. The rest of the soil was used to make a 1.57 inch
(diameter) by 0.24 inch (height) sample for the WP4 equipment at the same dry unit weight as
was used in the laboratory pullout tests. The WP4 samples were placed in sealed disposable
cups (Figure 11). Before testing each soil sample using WP4, a salt solution of known water
potential (0.5 molal KCI in H20) was used to calibrate the WP4 sensor. For each test, the
sample was placed inside the WP4 sample drawer and was allowed to reach temperature
equilibrium with the equipment internal chamber. Then, the knob on the tray was turned to the
"READ" position to read the water potential of the soil sample. The magnitude of the soil total
suction was recorded once the displayed reading stabilized at a constant value. Figure 12
shows the Soil-Water Characteristic Curve (SWCC) for Chickasha soil that was obtained
through WP4 tests.
16
12000
10500
.-r.o.. 9000
•I~.L..•.• 7500 c0
:;:::; 0 6000 :::l
!/)
(-ij 4500 0
I- 3000
1500
0
6
:o::R
Nc..'>
:o!:
:::R o
N+o
:o!:
9 12 15 18 21 24 27
Gravimetric water content (%)
Figure 12. Soil-Water Characteristic Curve for Chickasha soil using WP4 Potentiometer
Results shown in figuti' .i~indicate that the total suction in Chickasha soil varies between 300
kPa and 1200 kPa for the range of moisture contents between OMC-2% and OMC+2%. This
range of soil suction is consistent with the values that can be found in the literature for lean clay
(Cardoso et al. 2007, Nam et al. 2009).
Analysis of WP4 results also allowed us to determine whether or not Chickasha soil is classified
as an expansive clay. For this purpose, a procedure called McKeen analysis was used. In the
McKeen's classification methodology for expansive soils the slope of the SWCC in a semi-log
plot is used to determine a parameter called the "total suction-water content index". The
swelling potential of expansive soils is qualitatively classified (e.g. "low" or "high") based on the
magnitude of the total suction-water content index (j" lI!'.••.) (McKeen 1992). ~g - f~i't~shows a
plot of the gravimetric moisture content vs. total suction for the Chickasha soil.
17
Table 4. Summary of McKeen (1992) Expansive Soil Classification Methodology
Category Slope Ch He (%) Expansion
> 0.17 -0.027 10 Special case
0.1 - 0.17 -0.227 to -0.12 5.3 High
·0.08 - 0.1 -0.12 to -0.04 1.8 Moderate
0.05 - 0.08 -0.04 to 0 Low
< 0.05 0 Non-Expansion
II
III
IV
V
Note: c, and He are the suction-compression index and vertical movement, respectively as
computed by McKeen (1992).
3 0.25
~c
-(!) 0.2 c0
c
•(.!..). 0.15
:::J_
(;jo> .- -- 0E0'>-' 0.1 o I.-;;: I (!) 0.05 L- E ·s
•C..\.l.
o 0
3 3.5 4
(.0 = - 0.0689 Sl + 0.4402
R2 = 0.9705
4.5 5 5.5
Total suction, s, (pF)
Figure 13. Gravimetric moisture content vs. total suction for Chickasha soil on semi-log plot (pF
is the base 10 logarithm of the suction expressed in cm of water)
According to Table 4 and Figure 13 since the slope of the 'graph is less than 0.08, Chickasha
soil is classified under category IV indicating that its expansive tendency is low.
Based on our experience with different methods of determining the soil suction as described
earlier in this section, we found psychrometers to be the most suitable for in-situ testing and
WP4 as the most suitable laboratory equipment to determine the soil suction in this study.
18
3.3. GEOSYNTHETIC REINFORCEMENT
A woven polypropylene (PP) geotextile (Mirafi HP370) was used in the pullout tests carried out
in this study. The mechanical response of the geotextile was found as per the ASTM 04595 test
protocol (ASTM 2009) and was compared with the manufacturer's data (Figure 14, Hatami et al.
2010a).
50
.E.- 40
Z.
u-:cxo:-. 30
0 ..J
~ 20 00 c:
Q)
~ 10
• • • • •• • •• • • • • • •• • • • • • • •• • • • • • • • •
_ 1anufacture1" ;. data: !
Ultimate strength ..--/
Strength at :°0 strain
a +--------------,--------------,--------------;
a 5 10
• • • •
Strain (%)
15
Figure 14. Mechanical response of the geotextile used in the pullout tests (Mirafi HP370) as per
the ASTM 04595 test protocol and as compared with the manufacturer's data. Note:
two arrows show the ultimate tensile strength and strength of geotextile reinforcement
at 5% strain.
19
4. LARGE-SCALE PULLOUT TESTS
4.1. METHODOLOGY
A series of large-scale pullout tests were carried out in Chickasha soil and Mirafi HP370 woven
geotextile (S~¢JiQn~tg). These tests were carried out at three different moisture content values
OMC-2%, OMC and OMC+2% (tabl~~).The differences in the magnitude of geotextile pullout
resistance among these cases were used to determine a moisture reduction factor (MRF = ~(ro))
in Eqti~tion1 to account for the loss of reinforcement pullout resistance due to increased
moisture content. The tests for each moisture content value were carried out at three different
overburden pressures as given in T~bJe5.
Overburden pressure, kPa (psf) 10 (207), 20 (417.7),50 (1044.3)
Table 5. Large-scale pullout test parameters
Test information
Soil Chickasha soil
Geosynthetic reinforcement Mirafi HP370, woven PP
Moisture content OMC-2%, OMC, OMC+2%
4.1.1. TEST EQUIPMENT
The nominal dimensions of the large-scale pullout test box used in this study (Figure, 1,5) are
1800 mm (L) x 900 mm (W) x 750 mm (H). The size of the box and its basic components,
including metal sleeves at the front end exceed the minimum requirements of the ASTM 06706
test protocol (ASTM 2010). The boundary effects were further minimized by lining the walls of
the test box with plastic sheets. The large pullout test equipment has a 4" bore, 18" stroke
hydraulic cylinder with a high precision servo-control system. A surcharge assembly including
an airbag and reaction beams on the top of soil surface is used to apply overburden pressures
up to about 50 kPa (i.e. approximately 1050 psf, or equivalent to 2.5 m of overburden soil) on
the soil-reinforcement interface. The pullout load on the reinforcement specimen is applied
20
using a 90 kN (20 kip), servo-controlled hydraulic actuator. In the tests carried out in this study,
only one half of the box length (i.e. 900 mm) was used.
Figure 15. One of the pullout test boxes at the OU Fears laboratory
4.1.2. INSTRUMENTATION
Different instruments were used to measure the movement of geotextile reinforcement and soil
suction near the soil-geotextile interface in the pullout tests. A set of PST 55 Psychrometer
sensors was placed in rows above and below the soil-geotextile interface to measure the soil
suction and moisture content near the soil-reinforcement interface (Section 4.2).
21
The geotextile strains and local displacements were measured using four (4) wire-line
extensometers attached to different locations along the reinforcement length (Figure 16a). A
Geokon Earth Pressure Cell (EPG) was used to verify the magnitude of the overburden
pressure applied on the soil-geotextile interface using the airbag that was placed on the top of
the soil (Figure 16b). Figure 17 shows the strain distributions over the length of geotextile
reinforcement at maximum pullout force at the points to which wire-line extensometers were
attached. The strain near the front end of the geotextile reinforcement was calculated using the
displacements at the front end of the geotextile and at the location of extensometer 1. The
former value was determined by subtracting the calculated elongation of the in-air portion of the
geotextile specimen from the actuator displacement. Results in Figure 17 indicate that strains in
the geotextile reinforcement are greater at higher overburden pressures and lower soil moisture
content values (i.e. higher soil suction).
2.5" t
3" !
2" t
(a) (b)
Figure 16. (a) Wire-line extensometers attached to the geotextile reinforcement (the numbers in
the figure indicate the extensometer number and distance from the tail end of the
geotextile); (b) Earth pressure cell placed on the top of the soil in the pullout test box
28
24
20
.••.....
~0-- 16 c
-'c-o 12 CJ)
8
4
0
-.- OMC-2%, 10 kPa
I Geotextile Reinforcement
-I -h ....
~ : • X
I.
·_·_·_·_·_·e t--------:
-I •. _._._ ...•
_Ii • - - - - - -Jr+_._-_._-_._ ...••.-. --.-- -. --.-...• I £ •
-1,.-------.----'---.----.-----,--e_--,,-----1IIe'---1
8
• o - • - OMC-2%, 20 kPa \ - e- OMC-2%, 50 kPa
-----------------------1 : Location of potentiometers as :
: attached to the geotextile 1 ~----------------------:
o 4 16 20 24
Distance on the geotextile from front end of the soil (in)
(a)
22
cr
o
(7) 12-
28
24
20
:.0~:-:R--. 16
c
•r.o...
- 12 CI)
8
4
0
28
Geotextile Reinforcement -.- OMC, 10 kPa
-. -OMC, 20 kPa
- e- OMC, 50 kPa
20
0_0_0_0_0_0.
24 - ~ ." ...
lri : ~ X \
Location of potentiometers as
attached to the geotextile
8
---------+
4 1I * ------a
0 -! • •
0 4 8 16 20 24
Distance on the geotextile from front end of the soil (in)
(b)
Geotextile Reinforcement -.- OMC+2%, 10 kPa
-. -OMC+2%, 20 kPa
- e- OMC+2%, 50 kPa
j!l'L-==---'- --f&... ex ...::.....a.... • -----" \----11I-.
-I
.0I_0_0_0_0_0. •..-------- .. tr--------.!.0 _ 0 _ 0 _ 0 •
j :~----------•-~.~oLl:...o:-..::0:...-:..::01~•__": ~-:".~
-,- __r-·--~.-------..,-- -_______,----4II.J----1
Location of potentiometers as
attached to the geotextile
o 4 8 20
Distance on the geotextile from front end of the soil (in)
(c)
23
24
24 I
I
20-1+I =:_---'...._.._._....-.--,l.;.....L•.
16 ~ :---" x ""
~ 12 j
~ t------------------..•.•
8 ~.-.-.-.-.- .•
I
4 -I' ------..s
I . - . - . -. i: - - - - -.
I •. -.-.- ..•
o -1------,-- ..~.i.,•..•.-•--------•'•-.---" -.~----,-·-.~-I
o 4 8 12 16 20
Geotextile Reinforcement -k- OMC-2%, 10 kPa
- • -OMC, 10 kPa
- e- OMC+2%, 10 kPa
Location of potentiometers as
attached to the geotextile
.._.-._.
Distance on the geotextile from front end of the soil (in)
(d)
24 I
20 j+.,_.:..._.;...... '~L.le_•R•':e•L•.fo_rc_l~L..nt
I :---" x ,
16 l ~
~ • '---L-o-c-'at-io-n-o-f-p-ot-e-nt-io-m-e-te-r-s-a-s---'
c t---------. ~ 12 t._._._._._.. attached to the geotextile
en I
8 i
I
4 -I
I •
o -'~----.----~.~r----~~----~~--~~~~
-k- OMC-2%, 20 kPa
-. -OMC, 20 kPa
- e- OMC+2%, 20 kPa
.----.---.-- ...•.-,,'--- .....•
o 4 8 12 16 20
Distance on the geotextile from front end of the soil (in)
(e)
24
24
24
__________________________ 1
24
A • +-------+ .._._._.
Geotextile Reinforcement -a- OMC-2%, 50 kPa
- + -OMC, 50 kPa
-h.1" '" .X .. - ~ OMC+2%, 50 kPa
• • 20 <.
Location of potentiometers as
attached to the geotextile
16
.•.......
~0
---- c
s.••co..o.... 12
8
._._._._.- ...
4
o +-------,,----~-.------~~------.-------.---~--~
o 4 8 12 16 20
Distance on the geotextile from front end of the soil (in)
(f)
24
Figure 17. Axial strain distribution over the length of geotextile reinforcement from large-scale
pullout test on Chickasha soil at different moisture contents and overburden
pressures
4.2. TEST PROCEDURE
4.2.1. PROCESSING OF THE SOIL
After the soil was transported from the borrow site (Figure 2) to the Fears lab, the clayey soil
was air dried and broken into small pieces using two soil processors (Figures 18a,b).
Afterwards, the soil was passed through a #4 sieve in a 7-tray Gilson screen shaker. Next, the
soil was mixed with water to reach the desired moisture content for each test (Figure 18c). This
procedure took approximately 5 to 7 days depending on the initial soil moisture content. The wet
soil was stored in thirty five to forty 25 kg (55 Ib)-buckets and was sealed for more than 24 hours
to reach moisture equilibrium. The soil moisture content in each bucket was measured using the
oven drying method (Figure 19). The above procedure was repeated for every test.
25
(a) (b)
(c)
Figure 18. Soil processing equipment at the au Fears laboratory, (a, b) Soil processors, (c) Soil mixer
26
(a) (b)
Figure 19. (a) Sealed buckets containing processed soil before placing in the pullout box, (b)
Soil samples in the oven to determine their moisture content
4.2.2. PLACEMENT OF THE SOIL IN THE PULLOUT BOX
The pullout box was lined with plastic sheets to preserve the soil moisture content and to
minimize the friction between the soil and the sidewalls during each test. Next, the soil was
placed and compacted in the test box in nine two-inch lifts. The soil was compacted to 95% of
its maximum dry unit weight (i.e. Yd = 16.44 kN/m3 = 104.6 pcf). The compaction for each layer
took approximately 1 hour. The instrumented geotextile was placed at the mid-height of the box.
The pullout box containing compacted soil at its target moisture content was sealed with plastic
sheets on the top (Figure 20).
(a) (b)
Figure 20. (a) Sealed compacted soil at the end of large-scale pullout box setup, (b) Geotextile
specimen at the mid-height of the pullout box
27
The soil was left for at least 24 hours until the Psychrometer sensors reached equilibrium with
their surrounding soil. In all pullout tests, a rectangular block of Styrofoam with dimensions 900
mm (W), 457 mm (H) and 140 mm (T) was used in front of the soil specimen, which in addition
to the 200 mm-wide metal sleeves, helped further minimize the influence of front boundary
condition on the soil-geotextile interface.
4.2.3. PULLOUT TEST AND DISMANTLING OF THE TEST SETUP
The pullout phase of the test usually took between 1 and 2 hours depending on the overburden
pressure and target soil moisture content. The pullout force was applied on the geotextile
reinforcement at a target displacement rate of 1 mm/min according to the ASTM 06706 test
protocol. After the test was completed and the reinforcement underwent pullout failure, the test
assembly was carefully dismantled. First, the surcharge assembly was removed from the top of
the box and the soil was carefully excavated from the box. It usually took about 4 to 5 hours to
carefully dig the entire soil out of the test box. All together, a complete test required 45 to 50
hours of hands-on operation including soil processing, mixing and setting up the box, 24 to 48
hours as equilibrium time for suction sensors and 1 to 2 hours to run the pullout test. Figure 21
shows a schematic diagram of the pullout box. In th•is figure, the white circles represent the
locations of samples that were taken to measure the soil suction with the WP4 Potentiometer
and the black circles show the locations of the in-situ PST 55 Psychrometer sensors.
Air bag~---.-----------~ Soil
Timber
Plastic sheet
Non-Woven geotextile
~
~
,
, '---' >
•...... - r. - c> - <;) Geotextile
Styrofoam
24·
Figure 21. Schematic diagram of the large-scale pullout box test setup
28
4.2.4. INTERFACE PROPERTIES
Figures 22 and 23 show the pullout test data and interface shear strength results for Chickasha
soil for different moisture content and overburden pressure values. The nominal moisture
content values include OMC-2% (16%), OMC (18%) and OMC+2% (20%). The measured
pullout force is plotted as a function of the actuator displacement. Maximum pullout forces on
each graph are indicated using hollow circular markers. Results shown in Figure 22 indicate
that reinforcement pullout resistance increases with overburden pressure. Results in Figure 23
show consistently higher maximum reinforcement pullout resistance in the soil at OMC-2%
compared to the values in the OMC and OMC+2% cases for all overburden pressure
magnitudes tested. As expected, increasing suction led to a higher maximum reinforcement
pullout resistance in otherwise identical test specimens (Figure 22d). The interface strength
parameters shown in Figure 22d are defined in Equation 2 (Section 2.2). Results shown in
Figure 22d indicate that the interface friction angle does not change with moisture content,
whereas the adhesion increases at lower moisture content due to higher suction. This
observation is consistent with those reported by Khoury et al. (2011) from suction-controlled
interface testing of fine-grained soil specimens.
50 I OMC-2%
50 kPa
40
-zE- ~~< 30
<DUL
..
.•0...
....., ::l 20 0-
J 0..
10
0
0 50 100 150 200 250
Actuator displacement (mm)
(a)
29