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Evaluation of the Long-Term Performance of Woven Geotextile Used between
Base Course and Subgrade of a Paved Road
Article in Transportation Research Record Journal of the Transportation Research Board · April 2019
DOI: 10.1177/0361198119827567
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Research Article
Evaluation of the Long-Term
Performance of Woven Geotextile Used
between Base Course and Subgrade of a
Paved Road
Transportation Research Record
1–12
Ó National Academy of Sciences:
Transportation Research Board 2019
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DOI: 10.1177/0361198119827567
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Saad Ullah1, Burak F. Tanyu1, Erol F. Guler1, Edward J. Hoppe2,
and Emre Akmaz1
Abstract
The purpose of this research was to investigate the properties of the exhumed geotextile from a low-volume road on the
Virginia Department of Transportation network. The exhumed geotextiles have been in service for 23 years, which provided
an opportunity to evaluate the longevity of the materials as well as to make assessments of how it relates to the changes in
material properties. During this investigation, subgrade and base course materials were also obtained from the same site and
an experimental program was developed to evaluate the effectiveness of the exhumed geotextiles for separation, stabilization,
and filtration for the base course thicknesses of 4, 6, and 8 in. The results from this study combined with the results from the
previous studies conducted at the same site showed that when the geotextile is placed between the subgrade and base
course, the thinner the pavement section, the more evident the effectiveness of the geotextile improvements. One important
finding of this research was that the placement of a geotextile reduced the particle breakage caused by abrasion under the
applied transient loads. This was observed as a stabilization effect of the geotextile inclusion. As a general conclusion, for
low-volume roads with relatively thin pavement sections, properly selected geotextiles provide benefits for separating the
subgrade and base course (minimizing pumping), filtering infiltrated or ground water, and stabilizing the road profile. These
benefits become more apparent when the thickness of the base course is less than 8 in.
In the literature it is often shown that roadway with a geosynthetic undergoes less rutting; however, these works
often do not provide strong evidence to describe in what
mode the geosynthetic was effective in improving the performance (e.g., stabilization, separation, filtration, etc.). In
this research, there was an opportunity to exhume woven
geotextile placed above the subgrade and below the base
course that has been in service for 23 years. The site was
initially constructed as part of research funded by the
Center for Innovative Technology (1). This opportunity
allowed the study described in this manuscript to focus on
evaluating the long-term performance of the exhumed geotextile and its effectiveness in providing efficient filtration,
stabilization, and separation. The findings obtained from
these evaluations are then used to make recommendations
for the best pavement profile scenario for the use of geotextiles in construction of low-volume roads.
AASHTO M-288 (2) defines the function of geotextiles in roadway applications as subsurface drainage (i.e.,
filtration), separation, stabilization, and enhancement
(which combines all three functions). Several field studies
have previously been conducted with woven geotextile
placed in between subgrade and base course in pavement
profiles (3–6). Some of these studies involved exhuming
geotextiles after they had been in service for up to
12 years and others were primarily based on monitoring
performance. The overall results showed that the functionality of the geotextiles depends on their properties as
well as the conditions of the site where they are used.
Over time, the permittivity and tensile strength of the
geotextiles reduce, sometimes pumping of subgrade
through the geotextile is observed, and often results show
a stabilization effect based on modulus values. The general consensus of the findings from these studies is that
the thinner the base course, the more effective the geotextile in providing its intended functions. However,
1
Volgenau School of Engineering, George Mason University, Fairfax, VA
Virginia Transportation Research Council (VTRC), Charlottesville, VA
2
Corresponding Author:
Address correspondence to Burak F. Tanyu: btanyu@gmu.edu
2
Transportation Research Record 00(0)
Figure 1. Layout of the field test site.
these studies do not indicate whether or not there is an
upper threshold for thickness of the base course for the
geotextile to provide effective support as it relates to
separation, filtration, and stabilization, and especially as
it relates to roads where the traffic is considered low volume. The study described in this manuscript makes an
attempt to provide such guidance.
Characterization of the Field Test Site from
Previous Studies
The site where the geotextiles were exhumed in this study
was constructed in 1994 as part of a research study led
by Virginia Polytechnic Institute and State University
(7). The test site is located in Bedford County, Virginia
and is classified as a low-volume road. The average daily
traffic was predicted during the initial phase of the construction as approximately 500 (700 in summer) vehicles,
with approximately 8% trucks (8). The traffic measurements conducted between 2001 and 2016 indicate that
the average daily traffic was 1,300 vehicles per day for
Route 616 and 1,500 for Route 757 (9).
The pavement section is composed of nine individual
sections (;50 ft each). The base courses of the first three
sections were constructed with a 4-in., another three with a
6-in., and the remaining three with an 8-in. thick base
course. For each base course thickness, one control, one
section with geotextile, and another with geogrid was constructed. In all sections, the geosynthetics were placed
between the subgrade and the base course. The layout of
the test site is shown in Figure 1. For the purposes of this
study, only the Sections 1, 4, and 7 (control) and 2, 5, and
8 (all of which were constructed with same geotextile) have
been evaluated. Samples for this study were exhumed only
from the sections located within the southbound lane.
The properties of the subgrade have been described in
literature (8) as primarily a reddish-brown residual soil,
which was classified as high-plasticity clay with a soaked
California bearing ratio (CBR) of 2% to 3%. Parts of
the test site also included a yellowish-brown residual soil
Figure 2. Grain size distribution of the base course.
that was classified as low-plasticity silt; however, during
this study all of the exhumed geotextiles only had the
remnants of the reddish-brown residual soil. According
to AASHTO M288 (2), the properties of the subgrade at
this site would call for the installation of a woven geotextile for stabilization application. The grain size distribution of the base course aggregate was characterized in
the literature (10) as 51% of the particles passing a #4
sieve, 9% passing a #40 sieve, and 5% passing a #200
sieve (Figure 2). The properties of the woven geotextile
are summarized in Table 1.
Characterization of Exhumed Subgrade
and Base Course Samples
Even though the subgrade and base course was previously characterized by Bhutta (10), during the efforts to
exhume the geotextiles in this study, samples of subgrade
and base course were also obtained and used for characterization. Properties of the subgrade and base course
samples obtained from the field are depicted in Table 2.
The results show that the subgrade is primarily characterized as low plasticity silt (ML) even at the sections
that were previously characterized as high-plasticity clay.
However, when the Atterberg limits of this material are
evaluated closely, the results show that the properties lie
around the borders of the A and B lines defined in
Casagrande’s plasticity chart. Therefore, even though in
this study the subgrade is defined as ML, it is not
believed that overall there is a discrepancy between the
previous literature and this study.
The base course material gradation matches within
the boundaries of what is specified by the Virginia
Department of Transportation (DOT) as 21A aggregate
(13). The grain size distribution obtained from multiple
Ullah et al
3
Table 1. Properties of the Geotextile
Property
Test standard
Value
Grab tensile strength
Grab elongation
Mullen burst strength
Puncture
Trapezoidal tear strength
UV resistance
ASTM D4632
ASTM D4632
ASTM D3786
ASTM D4833
ASTM D4533
ASTM D4355
Apparent opening size
Permittivity
Water flow rate
ASTM D4751
ASTM D4491
ASTM D4491
Tensile strength
Machine direction
Ultimate elongation
Machine direction
Tensile strength
Cross machine direction
Ultimate elongation
Cross machine direction
ASTM D4595
200 lbfa
15%a
406 psia
90a–95b lbf
75 lbfa
70% of tensile
strengtha
40 US sieve sizeb
0.05 sec–1 b
42 US
gallon/min/m2 b
20 kips/ftc
ASTM D4595
14.8%c
ASTM D4595
18 kips/ftc
ASTM D4595
9.9%c
a
Source of information: Bhutta 1998 (10).
Source of information: Manufacturer’s Data (11).
c
Source of information: Al-Qadi and Appea 2003 (12).
b
samples (15 tests) of the base course gave results that are
quite similar to each other. Therefore the average values
have been given in Table 2. When the grain size distribution of the base course provided in the literature (10) is
compared with the average grain size distribution results
presented in Table 2, it is observed that the amount passing the #4 sieve remained almost the same. However, the
percentages passing the #40 and #200 sieves appeared to
be slightly increased over time in 23 years, by about 6%
(from 9% to 15% for the #40 sieve) and 5% (from 5% to
10% for the #200 sieve), respectively. In addition, a comparison was made from the grain size distribution of the
bottom 2 in. of the 4-in. base course taken from the field
with and without geotextile. In that comparison (in which
the age of the samples is the same), the results showed
that for the #40 sieve, the percentages of particles passing
are 13% and 16% with and without geotextiles, respectively. For the #200 sieve, the percentages of the passing
particles are 9% and 11% with and without geotextiles,
respectively. Furthermore to understand the characteristics of the fines content of the base course, Atterberg limit
tests were conducted and it was determined that this content is of non-plastic nature as indicated in Table 2.
Summary of Previous Results Obtained by
Others
The falling weight deflectometer (FWD) tests conducted
directly on the pavement at the site between 1994 and
1997 have been described in the literature (10). The
results showed that the geotextile sections had larger
stiffness values compared with the control sections,
although the effects of the geotextile were more pronounced in Section 2 where the base course thickness
was 4 in.
In 1999, base course samples from the control and
from sections with geosynthetics were exhumed from the
field (8). Their findings show that the fines percentage
(as defined by the #200 sieve) of the base course material
had increased over the first 3 years after construction
from 5% to 17% for the control section and to 12%
for the geotextile stabilized section (Section 1 versus
Section 2).
The dynamic pressure response right at the subgrade–
base course interface has been measured by past researchers, in 1999 (1). When the pressures corresponding to the
vehicle speed of 50 km/h were evaluated, the results
showed that in the 4-in. base course with geotextile section, the measured pressures were 5% to 7% less than the
pressures measured from the control sections. This indicates that the installed geotextile provides lateral constraint to the base course, causing the stress of the tire to
be distributed over a larger area and reducing the maximum pressure transmitted to the subgrade.
In 2001, 8 years after the construction of the road, rut
measurements were performed by researchers (12). These
values show that for a base course thickness of 4 in., the
rutting was significantly less in the section with geotextile
(0.75 in. on average) compared with the control section
(1.8 in. on average). For the 6-in. and 8-in. base course
thicknesses, the geotextile sections did not differ significantly from the control sections.
The information from the past studies of the same site
provides evidence that when the geotextile is placed
between the subgrade and the base course, on lowvolume roads, the thinner the base course, the more
effective geotextile becomes in stabilization. However,
the past studies do not provide evidence or discussion of
the contribution of the specific functions of the geotextile
that results in these improvements. Moreover, none of
the previous results depict performance behavior after
the geotextiles have been in service for 23 years.
Methodology Used in This Study to
Evaluate Exhumed Samples
All geotextiles used in this study were exhumed in the fall
of 2017 with the assistance of the Virginia DOT. As part
of this effort, the Virginia DOT has exposed an area
within the base course approximately 5 ft by 7 ft by
removing the hot-mix asphalt (HMA) layer. After carefully sampling and removing the base course, the perimeter of the geotextile was cut, and geotextiles were
4
Transportation Research Record 00(0)
Table 2. Properties of Subgrade and Base Course as Determined in This Study
Classification
Average values
Base Course (21A Aggregate)
Specific gravity (ASTM C127)
Apparent specific gravity
2.8
Water absorption (%)
0.5
Average grain size distribution (GSD) (ASTM D5444)
Passing 3/4’’ (\19 mm)
95%
Passing 3/8’’ (\9.5 mm)
75%
Passing #4 sieve (\4.75 mm)
54%
Passing #40 sieve (\0.43 mm)
15%
Passing #200 sieve (\0.075 mm) (per ASTM D1140)
10%
GSD from the bottom 2-in. of the 4-in. base course in the field
Section 1
Section 2
(without geotextile)
(with geotextile)
Passing #40 sieve (\0.43 mm)
16%
13%
Passing #200 sieve (\0.075 mm)
11%
9%
Atterberg limits (ASTM D4318)
Liquid limit (%)
14.0
Plastic limit (%)
Non-plastic
USCS classification
Well graded sand with silt (SW-SM)
(ASTM D2487 2017)
California bearing ratio (CBR) (ASTM D4429)
Unsoaked CBR (%)
75
Soaked CBR (%)
55
Field density measurements (ASTM D2937)
Field moisture content (%)
4.0
Field density (pcf)
130
Subgrade
Hydrometer test (ASTM D7928)
Passing #10 sieve (\2.00 mm)
100
Passing #200 sieve (\0.075 mm)
77
Clay size fraction (\0.002 mm)
45
Atterberg limits (ASTM D4318)
Liquid limit (%)
49.0
Plastic limit (%)
29.0
Plasticity index (%)
20.0
USCS Classification (ASTM D2487)
Low-plasticity silt (ML)
California bearing ratio (CBR) (ASTM D4429)
Unsoaked CBR (%)
4
Soaked CBR (%)
2
Field density measurements (ASTM D2937)
Field moisture content (%)
29.0
Field density (pcf)
122
Note: USCS = Unified Soil Classification System; pcf = pounds per cubic foot.
gently removed. Samples of the subgrade were also collected from the same locations.
The experimental methodology of this research was
designed to evaluate the effectiveness of the exhumed
geotextiles to function in separation, filtration, stabilization, and potential enhancement. It is understood that it
is not always necessary for the same geotextile to satisfy
all of these functions; however, specific tests were developed in this study to understand how the exhumed geotextile functioned after being in service for 23 years.
Therefore, the research methodology has been divided
into three phases.
In the first phase, the properties of all of the collected
materials were characterized. In the second phase, the
aspect of movement of subgrade particles (pumping) into
the base course and the effectiveness of the geotextile in
providing stabilization were evaluated. In the third
phase, the properties of the exhumed geotextiles in terms
of water being able to permeate were investigated.
Tensile Strength and Permittivity Tests
Tensile strength of the exhumed geotextiles has been
tested following the ASTM D4595 testing protocols.
Ullah et al
Specimens for the tests were obtained from the geotextiles exhumed from the sections with 4-in., 6-in., and
8-in. thick base courses. All tests were conducted to
determine the ultimate strength along both the machine
direction and the cross machine direction. Permittivity
tests were conducted with geotextile specimens with a
diameter of 6 in.
Repeated Load Triaxial Tests
The repeated load triaxial (RLT) tests conducted in this
study involved essentially the same type of set-up as is
typically seen in permanent deformation tests (14). The
samples were 6 in. in diameter and 12 in. in height.
Uniquely to this study (as there are no such ASTM standards), all of the samples for these evaluations were created by placing first 6 in. of subgrade and then 6 in. of
base course on top. This set-up was similar to the set-up
used by another piece of research (15); however, in that
research the diameter of the sample was 4 in. The reason
the sample height in this test set-up was chosen as 6 in.
for each layer as opposed to a 4-in., 6-in., and 8-in. base
course was to create a more uniform stress distribution
within the sample. Non-uniform samples result in stress
concentrations differing at the sides of the samples compared with the stresses observed in the middle of the
samples (16, 17). Keeping all sample sizes uniform
allowed the results to be compared with each other. In
the tests in which geotextiles were used, they were placed
in between the subgrade and base course. All samples
and layers within each sample were compacted to 100%
relative compaction at optimum moisture content as
obtained from the literature (10) and field test results.
Exhumed geotextiles from the field from each section
(sections 2, 5, and 8) were cleaned and then used in the
RLT tests that were conducted with the geotextile.
Therefore the geotextiles used in these tests have already
been aged in the field for about 23 years.
To simulate the differences in base course thicknesses
in the field, the samples in the RLT were loaded to simulate the stresses that would be present at the boundary of
subgrade–base course for base course thicknesses of 4, 6,
and 8 in. The magnitude of applied stresses was determined from multilayer and finite element models recommended in the literature based on the 100-psi tire
pressure over the area of 6-in. radius applied on the surface of the pavement (14). The thickness of the asphalt
layer at this site was designed as 3 in (10). The approach
followed in this study by applying different stresses to
simulate different base course profiles was possible
because in a cylindrical sample the stresses applied at the
top of the sample uniformly distribute within the sample.
The load was applied using haversine impulse for 10,000
5
cycles as recommended by prior literature for permanent
deformation tests (14, 15, 18, 19).
All the RLT tests were repeated at least twice. The
results presented in this manuscript are the average of
two test runs. At the end of each test, three different
results were obtained: (i) accumulated permanent deformation with time, (ii) estimated elastic modulus based on
the elastic deformations recorded at the end of the tests,
and (iii) changes in grain size distribution within the base
course after the end of the tests. The grain size distribution was performed on the base course material obtained
only within the 1 in. of the subgrade/base course interface
in the RLT tests. To eliminate the potential errors caused
by blending the subgrade with base course during sample
preparation because of compaction, the samples obtained
from the base course material for the grain size distribution were only taken from the middle of the cylindrical
sample (approximately 0.5 in. away from the edges). In
addition, at the end of each test, the geotextiles exhumed
from the sample were evaluated under the microscope
and traces of the reddish subgrade clay determined to
confirm the changes noted in the grain size distributions
of the base course.
Results were used to evaluate the respective contributions of separation and stabilization functions of the geotextiles. Furthermore, the results from changes in grain
size distribution within the base course were used to evaluate the effectiveness of the filtration function (i.e., minimizing the migration of the fines from subgrade into the
base course).
Hydraulic Conductivity Tests
The hydraulic conductivity tests conducted in this study
had the same type of set-up as is typically seen in the gradient ratio tests per ASTM D5101. The samples were
6 in. in diameter and 4 in. in height. All samples were prepared at 95% relative dry density obtained from prior
studies conducted on the same project (10). The experiments were performed with hydraulic gradient of 1 (the
gradient was gradually increased from 0.25 to 0.5 to 0.75
to 1), which covers the worst-case scenario in pavement
applications (20). The results of the tests were evaluated
in accordance with the interpretations described in literature (21).
The particular intent of conducting these hydraulic
conductivity tests in this study was to understand and
evaluate the filtration performance of the exhumed geotextiles, which were exposed to subgrade and base course
for 23 years. Although permittivity tests were also conducted in this study, during such permittivity tests, the
geotextiles are placed directly under the flow of water.
This can often result in washing out the particles of the
6
Transportation Research Record 00(0)
Table 3. Changes in Tensile Properties of the Exhumed Geotextiles over Time
After 23 years in servicea
Pavement section
Direction
sult. (kip/ft)
eult.
(%)
4-in. base
Machine
X-Mach.
Machine
X-Mach.
Machine
X-Mach.
1.34
1.20
1.31
1.11
1.30
1.00
9.5
14.4
7.8
12.8
9.6
10.8
6-in. base
8-in. base
Manufacturer’s datab
After 5 years in servicec
sult. (kip/ft)
eult.
(%)
sult. (kip/ft)
eult.
(%)
Machine
1.85
14.8
1.23
23.6
X-Machine
1.71
9.9
1.71
12.5
Direction
Note: sult. = Ultimate wide-width tensile strength; eult = Ultimate elongation; X-Mach. = Cross machine.
a
Values obtained as part of this study.
b
Values obtained from the manufacturer’s website; manufacturer data for original samples were minimum average roll values (MARV) (11).
c
Values published in literature (8) (the measured strains at failure are excessive considering the values presented by the manufacturer and measured after
23 years).
specimens and not necessarily depicting the actual permeability/permittivity. To prevent/minimize such effect,
the hydraulic conductivity tests were conducted by placing 4-in. base course aggregate over the top of the
exhumed geotextiles. In addition, one test was conducted
with a cleaned geotextile. The aggregate used for these
tests was obtained from the site and had a hydraulic conductivity of approximately 4 3 10–2 cm/s. Considering
that the manufacturer’s reported permittivity value is
0.05 s–1 and the measured thicknesses of the cleaned samples are approximately 16 mil, the estimated permeability
of the virgin geotextile is expected to be in the order of
2 3 10–2 cm/s (and the permeability of the exhumed
geotextiles is expected to be significantly less than this
value). Therefore, it is assumed that placement of the
base course aggregate not only simulates the actual field
condition but also does not hinder the permeability of
the geotextile that was already in contact with the
subgrade.
Properties of the Exhumed Geotextiles
from the Site after 23 Years of Service Life
All of the exhumed geotextiles after 23 years appeared to
be in great shape with no signs of physical degradation.
Tensile strength and permittivity of the exhumed geotextiles have been evaluated to quantitatively determine the
changes in the geotextile properties after being in service
for 23 years.
Table 3 presents the results of the wide-width tensile
tests conducted as part of this study and also compares
these results with the manufacturer’s reported values and
results obtained previously by others on the same geotextile from the same road sections after they had been in
service for 5 years. The results compiled from previous
studies show that by the end of the 5 years of service, the
geotextile retained its tensile strength in the cross
machine direction and that approximately 30% reduction was observed in the machine direction. After
23 years of service, the changes in machine direction
appear to have remained the same as compared with
5 years, but additional changes occurred in the cross
machine direction. The geotextiles exhumed from below
the 4-in., 6-in., and 8-in. thick base courses retained
68%, 64%, and 60% of their tensile strengths, respectively. The strains at ultimate tensile strength values
show that there was not a significant reduction compared with the strains reported by the manufacturer.
Therefore, it is concluded that no significant polymer
aging took place in 23 years. All these results indicate
that the geotextiles retained their original mechanical
properties.
Table 4 summarizes the permittivity values of the geotextiles as reported by the manufacturer and after they
were exhumed at the end of 23 years of service. The socalled ‘‘uncleaned’’ results in Table 4 depict the permittivity values of the exhumed geotextiles and the ‘‘cleaned’’
values show the results after the exhumed geotextiles
were washed and tested. The difference between the permittivity values of the original material and cleaned geotextiles shows reduction in permittivity of about 60% in
the section with 4-in. base course and 72% in sections
with 6-in. and 8-in. base course. The occurrence of these
reductions is interpreted as the geotextiles being compressed under the surcharge pressure for a long time so
that the pores of the geotextile have slightly reduced.
This explanation is also supported by the reduction
being less for the section with the thinnest base course
(Section 2), for which the static overburden pressure was
the smallest. Although there was no filter cake type of
appearances, the surface of all of the exhumed geotextiles
had some of the reddish color subgrade on the base
Ullah et al
7
Table 4. Summary of Hydraulic Properties of the Exhumed
Geotextiles
are interpreted as the effect of the penetrated subgrade
into the pores of the geotextile.
Permittivity test results
Permittivity after
23 years in service (s–1)
Pavement
section
Permittivity of
geotextiles before
installation (s–1)a
4-in. base
0.05
6-in. base
8-in. base
Permeability test results
Sample
Base course (BC)
BC + geotextile from 4-in. base
BC + geotextile from 6-in. base
BC + geotextile from 8-in. base
BC + cleaned geotextile
Uncleaned
Cleaned
0.010
0.011
0.011
0.020
0.014
0.014
System permeability (cm/s)
4.211 3 0–2
8.641 3 0–3
1.011 3 0–2
2.111 3 0–2
3.711 3 0–2
a
Values obtained from manufacturer’s website (11).
course side of the geotextile. Therefore, the difference
between the uncleaned and cleaned permittivity values
Results of Repeated Load Triaxial Tests
Accumulated Permanent Deflections with Time:
Evaluation of Stabilization Function
Figure 3 shows the accumulated permanent deformations from the tests conducted in this study with the
exhumed material after 23 years. For each section, two
sets of tests were conducted: one with the base course–
subgrade combination and the other with a geotextile
between the subgrade and base course. Based on the
applied stress conditions which simulate each base course
thicknesses in the field, the results indicate that the
amount of accumulated permanent deformation is very
sensitive to the magnitude of the applied stress and therefore the pavement section thickness. The following summarizes the observations from these test results.
For a base course thickness of 4 in., the deformations increase very quickly and reach a value of
Figure 3. RLT test on (a) base–subgrade sample and (b) subgrade sample alone, and (c) the schematic showing the RLT test set-up.
8
Transportation Research Record 00(0)
0.57 in. regardless of whether separated by a geotextile or not.
For a base course thickness of 6 in., the accumulated
deformation for the tests conducted without geotextile was 0.08 in. and with geotextile was 0.071 in.
(approximately 85% improvement compared with
the 4-in. thick base course). Although at the end,
the difference between the tests with geotextile and
without the geotextile was relatively small and therefore negligible, the presence of the geotextile delayed
the deformation build-up significantly. That is,
whereas the sample without the geotextile reached
maximum deformation in only hundreds of cycles,
the sample with geotextile reached a similar amount
of deformation in 10,000 cycles.
For the base course thicknesses of 8 in., the permanent deformation at the end of 10,000 cycles
was 0.04 in. without geotextile and 0.035 in. with
geotextile). However, it can be concluded that the
accumulated deformations were minimal.
The purpose of the RLT tests was not to determine a
value to be used in design but to compare the behaviors
with each other to understand the differences in trends to
potentially evaluate the effects of stabilization provided
by the geotextile to the subgrade/base course system. The
tests representing the 6-in. thick base course clearly show
the effect of the geotextile, but this observation is not as
clear for the tests conducted with the applied stresses
simulating 4-in. and 8-in. thick base course. Prior
researchers also made an attempt to investigate multilayer systems in a similar set-up with and without the
geotextiles (16). Their experiments indicated that the specimen containing the separator experienced significantly
higher deformations across the interface and in the
aggregate. The previous study explained this behavior as
the separator preventing the aggregate from interlocking
with the fine-grained soil and probably providing a lowfriction surface for lateral particle movement (16). The
associated changes in the boundary condition for the
aggregate have been related to the relative increase in
deformation. The results shown in Figure 3 contradict
these conclusions because, in this study, the deformations
observed with and without geotextile have very similar
magnitudes. To understand this behavior, RLT tests were
also conducted with 100% subgrade with applied loads
corresponding to the stresses caused by 4-in., 6-in., and 8in. base courses (Figure 3b). The results show that for the
test simulating the 4-in. base course, the subgrade was
very close to failure and deformations continued to
increase even after 10,000 cycles. Therefore, placement of
the base course and geotextile improved the permanent
deformation behavior. However, the separation of the
contributions of the base course alone versus the base
Table 5. Summary of Elastic Modulus Calculated from the
Dynamic Triaxial Tests
Simulated pavement profiles
4-in. base course over subgrade
4-in. base course over subgrade
with geotextile
6-in. base course over subgrade
6-in. base course over subgrade
with geotextile
8-in. base course over subgrade
8-in. base course over subgrade
with geotextile
Calculated
elastic
modulus (psi)
Percentage
improvement
20,113
21,524
7%
11,019
13,996
27%
10,572
11,620
10%
course with geotextile was not very evident in the 4-in.
base course RLT tests. This is because it is believed that
in a triaxial set-up, the geotextile is not fully engaged (as
is expected to happen in the field) to influence the permanent deformations. Overall, it is believed that even with
these limitations, these test results showed that after being
in service for 23 years, geotextiles were still effective in
proving stabilization as seen in the 6-in. base course test.
Estimated Elastic Modulus: Evaluation of Stabilization
Function
Table 5 shows the summary of the estimated elastic modulus values from each of the RLT tests. These values are
obtained from the elastic strains obtained during the
above described tests, so they are referred to as elastic
modulus (not to be confused with the resilient modulus
values that are determined from the AASHTO TP46-94
testing protocol). The results shown in Table 5 were calculated following the same approach described in the literature (22), whereby the modulus is estimated from the
ratio of the applied deviator stress and corresponding
resilient strain.
The intent of the presented information in Table 5 is
not to provide design values but to show (relative to each
other) that in all cases the exhumed geotextiles were effective in showing an increase in elastic modulus within the
same profiles tested with and without the geotextile. The
magnitude of the values is higher with the tests simulating thinner pavement profiles because in those tests the
applied loads and the corresponding bulk stresses are
also higher. The highest percentage improvement in the
elastic modulus was observed for the 6-in. base course
(27%). This is also reflected in the deformation delay
produced by the geotextile in the representative sample
of the 6-in. base course section.
In addition to the laboratory estimations, in the field,
several light weight deflectometer (LWD) tests were
Ullah et al
9
performed directly over the base course in Sections 1 and
2 to compare the effect of the geotextile in the 4-in. base
course section. The average calculated elastic moduli
from these tests were 7,690 and 9,430 psi for the sections
without and with geotextiles, respectively. These numbers cannot be compared with the values in Table 5 but
when they are compared with each other, considering
that the subgrade is the same, sections with geotextile
show 23% higher elastic modulus. This observation contributes to the discussion that even though the results
from the laboratory do not clearly show the contribution
of the geotextile in 4-in. thick base course, the results
from the field provide clarification.
Changes in Grain Size Distribution within the Base
Course: Evaluation of Separation Function
After 10,000 cycles of repeated loading at the RLT tests,
samples of the base course were collected from the 1 in.
above the geotextile. Location of the geotextile in the
RLT tests is shown in Figure 3c. The collected samples
were evaluated for the changes in grain size distributions
based on the differences at #40 and #200 sieve sizes
(Figure 4). The replicate tests conducted in this study for
each case resulted in very similar gradations, confirming
the strong repeatability of the conducted tests.
The changes in grain size distribution from the tests
simulating each of the base course thicknesses were evaluated. The test results indicate that, for the comparison
of the #40 sieve, the percentages passing with and without geotextiles were 12% and 20% (for 4-in. base course)
and 12% and 19% (for 6-in. base course), respectively.
Similarly, for the #200 sieve, the percentages passing
with and without geotextiles were 10% and 15% (for 4in. base course) and 9% and 12% (for 6-in. base course),
respectively. No changes in gradation were observed
from the tests simulating the 8-in. base course.
These changes in percentage of fines content as
described by the #200 sieve can be interpreted as the
pumping of the subgrade into the base course, as was
also evident from the observations made on the geotextiles exhumed after these tests for which the remnants of
the reddish particles (subgrade) within the base course
were observed. The results show that some pumping
occurs within the thinner base course profiles and
the woven geotextile is effective in reducing (but not
completely eliminating) this effect. As the thickness of
the base course profiles increases, the stresses at the
subgrade–base course reduce and the effectiveness of the
geotextile as a separator diminishes. This interpretation
of pumping is also supported by the differences observed
in the grain size distribution curves presented in the literature (10) (reflecting the original gradation) and
obtained in this study after 23 years of installation,
Figure 4. Evaluation of the changes in grain size distribution of
the base course aggregate from the RLT tests simulating: (a) 4-in.,
(b) 6-in., and (c) 8-in. thick base courses.
whereby the fines content increased by about 5% over
time. Additionally, plastic limit of the base course was
evaluated from the RLT test conducted to simulate the
4-in. base course profile both with and without geotextiles. The results showed that the base course material in
10
the RLT test without the geotextile had a plastic limit of
17 and with the geotextile had eight. When compared
with the properties of base course material before the test
(as depicted in Table 2), the change in the plastic limit is
interpreted to be because of the pumping of the subgrade. Moreover, that the plastic limit measured was significantly lower than the plastic limit of the subgrade
indicates that the amount of clay pumped from the subgrade was minimal.
On the other hand, the changes in percentages passing
the #40 sieve indicate another story because the changes
could not have been caused by pumping from the subgrade as the maximum particle size of the subgrade in
this study was 2 mm (100% passing #10 sieve) (Table 2).
This is interpreted as the changes being partially caused
by pumping and partially caused by the breakage of the
base course particles (which is more evident in the tests
simulating 4-in. base course). Previous literature supports the hypothesis of disintegration of base course
aggregate particles under the action of abrasive forces
(23). Considering that all of the materials used in these
evaluations were exhumed from the site after being in
service for 23 years, these findings lead to a discussion
that some of the base course crushed stone may abrade
under the repeated traffic loading. This assumption of
particle abrasion is also supported by the differences
observed in the grain size distribution curves from the
bottom 2 in. of the 4-in. base course samples obtained
from the field. The effect of potential particle abrasion
was observed to be reduced in the section with the
geotextile.
Transportation Research Record 00(0)
compared against the hydraulic conductivity of the original geotextile values reported by the manufacturer
(Table 4), the results show that over time the hydraulic
ability of the geotextile in all sections has decreased
(perhaps an order of magnitude). However, considering
that 10-2 (or even 10-3) cm/s is within the range of a
typical natural sand (24), the filtration properties of
the exhumed geotextiles are still considered reasonable
and acceptable.
Conclusion
The properties of the exhumed geotextiles showed that
after 23 years, there were differences in tensile strength
and permittivity, but all geotextiles were physically intact.
When all of the observations obtained from this field site
including the ones from the previous studies as well as
the results obtained from this particular investigation are
combined, the following conclusions are drawn related to
the effectiveness of the exhumed geotextiles to function
as a separator, and in filtration, stabilization, and potential enhancement in low-volume roads:
Results of Hydraulic Conductivity Tests
These tests were conducted to evaluate the filtration
effect of the geotextiles after they had been in service for
23 years. The results of these tests are shown in Table 4.
The results of the tests were evaluated, based on the system permeability as defined in the literature (21), as the
overall permeability within the test set-up. The system
permeability is dictated by the lowest permeability of the
material within the system. Because the permeability of
the exhumed base course was slightly higher than any of
the geotextiles, the system permeability values presented
in Table 4 are primarily controlled by the permeability of
the geotextiles. Based on this interpretation, the results
show that in all sections the values are very close to each
other (in the order of 10–2 cm/s). When these results are
compared against the permeability values backcalculated from the permittivity of the ‘‘uncleaned’’ geotextiles (Table 4), it can be seen that they are within the
same order of magnitude.
If the hydraulic conductivity of the cleaned geotextile
(as back-calculated from the permittivity test values) is
Stabilization:
All previous studies on this site indicate an
improvement in elastic modulus and rutting in
sections with geotextiles and they show that the
evidence of improvements is more pronounced in
the sections with thinner base course. These previous observations are also supported from the
results that were obtained from the geotextiles
even after they had been in service for 23 years.
The discussions provided from the RLT test
results show that the improved performance of
the sections with geotextiles could be seen more
easily based on modulus comparisons as opposed
to the accumulated permanent deformations.
The grain size distribution of the base course
obtained in the field and the grain size distribution
obtained from the RLT test indicate that particle
breakage may have occurred in the base course.
In both laboratory and field, the amount of
increase in percentages passing the #40 sieve
caused by breakage was reduced by the geotextile.
Therefore it can be concluded that the geotextile
was helpful in reducing the particle breakage and
thereby providing a stabilization function for the
base course. This mechanism helps in keeping the
elastic nature of the base course under repeated
traffic load for a longer duration.
Separation:
Literature (8) indicates that the fines percentage
of the base course in both the sections with and
without the geotextile has increased over time.
Ullah et al
However, this past study did not clearly differentiate the cause of this increase as it relates to distinguishing between the pumping of the subgrade
and potential breakage of the base course aggregate particles.
The results presented in this study showed that
the exhumed geotextiles did not prevent pumping
but have reduced the effect drastically in the sections with the thinner base course.
Filtration:
The selected geotextiles for this study still had the
ability to provide filtration even though over time
the ability of the geotextiles had decreased compared with the original geotextile before it was
placed in the ground. The decrease in the hydraulic ability of the exhumed geotextiles after being in
service for 23 years was considered acceptable in
terms of the ability of the system to convey water.
Enhancement:
The laboratory evaluations in this study showed
the effectiveness of the selected woven geotextile
to provide stabilization, separation, and filtration.
However, there were not enough data from the
field (positive or negative) to also confirm the
effectiveness for enhancement (which refers to the
combined performances of stabilization, separation, and filtration).
The above listed conclusions are based on the specific
woven geotextile exhumed from the Virginia DOT’s test
site. However, the geotextile used in this study is of a
common type that is found in road applications.
Practical Implications
Based on all of the measurements and observations provided in this paper, and considering the general experience accumulated over the years, it is recommended that
a geotextile for stabilization purposes as described by
Clause 8.4 of M-288 (2) should be used between the subgrade and base when all the following circumstances are
met:
The subgrade of the road is considered soft as
defined by AASHTO M-288 (2) (i.e., CBR of subgrade \3). The subgrade in this case study had a
soaked CBR of 2; therefore the use of geotextile
was appropriate for stabilization.
The base course used for construction meets the
typical gradations defined by DOTs (in coarser
aggregates separation between the subgrade and
aggregate may become more important and in
finer aggregates stabilization may become more
important).
11
The thickness of the base course is ł 6 in. This
recommendation is made because the evaluation
at this particular site did not show any evidence
that thicker sections (.6 in.) benefited from the
geotextile as seen in thinner sections. Previous literature also states that geotextile separators placed
underneath relatively thick pavement sections may
not provide significant benefit (25).
The asphalt layer at this site was 3-in. thick. The
combined thickness of the asphalt layer and base
course (i.e., the pavement section) has to be evaluated together for a realistic decision of what to
expect from the roadways with geotextiles.
The selected geotextile meets the property requirements set forth in AASHTO M-288 (2).
In addition, if further improvements are desired (such
as minimizing the potential of the base course aggregates
to break down), consideration may be given to placing
the geosynthetic reinforcement not only at the interface
between the subgrade and base course but also within the
profile of the base course itself (this particular evaluation
has not been conducted in this study but the observations
from the study imply this benefit).
Based on the above list, in low-volume roads, for base
course thicknesses of 8 in. or more and with a slightly
stronger subgrade (i.e., CBR . 2) a special evaluation
must be done and it is possible that no geotextile may
need to be placed between the subgrade and base course.
However, it may still be good practice to place a geotextile primarily for separation purposes because in most
applications the subgrade properties do not always end
up being uniform throughout the project as was also the
case at this site, where there was a small section with fill
material as a subgrade.
The recommendations suggested in this study are based
on the materials, results, and observations of the combined
field and laboratory data generated from this specific site
with the materials tested in this study. Additional evaluations may need to be performed from other sites to
strengthen or revise these recommendations—for example,
comparison of the performance of woven versus nonwoven geotextiles placed in between the subgrade and base
course.
Acknowledgments
The research presented in this paper was funded by the Virginia
Department of Transportation and Geosynthetics Institute
(GSI).
Author Contributions
The authors confirm contribution to the paper as follows:
field work: all authors, laboratory experimentation design:
12
B.F. Tanyu and E. Guler, laboratory testing: S. Ullah and
E. Akmaz, analysis and interpretation of results: B.F. Tanyu,
E. Guler, S. Ullah, and E. Hoppe: manuscript preparation:
B.F. Tanyu, E. Guler, S. Ullah. All authors reviewed the results
and approved the final version of the manuscript.
Transportation Research Record 00(0)
14.
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The Standing Committee on Aggregates (AFP70) peer-reviewed
this paper (19-03677).
The conclusions and recommendations are those of the authors
and may not reflect the opinions and policies of the Virginia
DOT and GSI.