In-Ground Dynamic Stress Measurements for Geosynthetic Reinforced Subgrade/Subbase

Abstract

This study documents field measurements from controlled test sections with in-ground piezoelectric earth pressure cells embedded above and below different polymer geosynthetic materials placed at the interface of soft clay subgrade and crushed limestone subbase. A control section with no geosynthetic layer was monitored for comparison. In-ground stresses were monitored during vibratory roller compaction and trafficking under a heavy loaded truck. One of the geogrids is a relatively new product for which there is less field performance data than the more traditional biaxial geogrids. Results from this study demonstrate better performance of the new geogrid test section compared to other test sections as verified using compaction and rut depth measurements. Improved performance is attributed to higher lateral restraint of the subbase layer and reduced horizontal stresses in the soft subgrade layer under loading.

Full text

In-Ground Dynamic Stress Measurements for Geosynthetic Reinforced
Subgrade/Subbase
D. J. White1 Ph.D., A.M. ASCE, P. K. R. Vennapusa1 Ph.D., A.M. ASCE, H. H.
Gieselman1, S. C. Douglas1, M. ASCE, P.E., Jiake Zhang1, and M. H. Wayne2 Ph.D.,
P.E., A.M. ASCE,
1Department of Civil, Construction and Environmental Engineering, Iowa State
University, 2711 South Loop Drive, Suite 4600, Ames, IA 50010-8664; PH (515)
294-1463; FAX (515) 294-8216; email: djwhite@iastate.edu, pavanv@iastate.edu,
giese@iastate.edu, calebd@iastate.edu, zhjiake@iastate.edu
2Tensar International Corporation, 5883 Glenridge Drive, Suite 200, Atlanta, GA,
30328; PH (404) 214-5373; FAX (404) 345-9103; email: mwayne@tensarcorp.com
ABSTRACT
This study documents field measurements from controlled test sections with
in-ground piezoelectric earth pressure cells embedded above and below different
polymer geosynthetic materials placed at the interface of soft clay subgrade and
crushed limestone subbase. A control section with no geosynthetic layer was
monitored for comparison. In-ground stresses were monitored during vibratory roller
compaction and trafficking under a heavy loaded truck. One of the geogrids is a
relatively new product for which there is less field performance data than the more
traditional biaxial geogrids. Results from this study demonstrate better performance
of the new geogrid test section compared to other test sections as verified using
compaction and rut depth measurements. Improved performance is attributed to
higher lateral restraint of the subbase layer and reduced horizontal stresses in the soft
subgrade layer under loading.
INTRODUCTION
Use of geosynthetics in pavement layers serve at least one of five functions:
separation, reinforcement, filtration, drainage, and containment (U.S. Army Corps of
Engineers 2003). This paper focuses on the reinforcement application. Geogrids are a
type of geosynthetic material commonly used for reinforcement applications, which
consists of connected sets of tensile ribs with apertures. Most commonly, the geogrid
aperture shape is square. A new geogrid is evaluated in this study which has
equilateral triangular shaped apertures and is reported to provide better confinement
than a square aperture shape (Tensar 2009). Woven geotextiles is another type of
geosynthetic material which is commonly used for separation applications, although
they are sometimes used in reinforcement applications. The reinforcement
mechanism in geosynthetic reinforced pavement layers has been studied by several
researchers. Perkins and Ismeik (1997) identified three fundamental reinforcement
mechanisms in geogrid reinforced granular layers: (a) lateral restraint, (b) improved
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bearing capacity, and (c) tensioned membrane effect. Lateral restraint refers to the
lateral “locked-in” stresses or confinement developed in the granular material during
loading which restricts lateral movement of the material under load (U.S. Army
Corps of Engineers 2003). According to FHWA (2008), lateral restraint of roadway
aggregate placed directly above the geosynthetic can reduce stresses in the underlying
subgrade. Since granular materials are generally stress-dependent, increasing
confinement causes an increase in shear strength and stiffness of the material. Recent
field studies by Kwon and Tutumluer (2009) demonstrated a “stiffening” effect
around a biaxial geogrid placed in a granular layer and its contribution to improved
pavement response and performance. Kwon and Tutumluer (2009) attributed the
“stiffening” effect to the interlock of aggregate particles in the apertures of the
geogrid and the related interactions.
This study provides experimental test results comparing the performance of
two different types of polymer geogrids and a woven geotextile placed at the
interface of a subgrade and subbase layers, in comparison with a control section with
no geosynthetic treatment. Controlled field test sections were constructed on soft
subgrade (California bearing ratio, CBR = 2 to 3) with two overlying (300 mm thick)
layers of crushed limestone subbase. Field measurements included: (1) piezoelectric
earth pressure cells (EPCs) for evaluation of in-ground vertical and horizontal
stresses; (2) compaction density/moisture content, and (3) rut depth after repeated
trafficking with a heavy loaded truck. New insights into performance differences
between the test sections in relationship with horizontal stress development in
subbase and subgrade layers are provided in this paper.
MATERIALS
The test sections constructed in this study included soft clay subgrade and
crushed limestone subbase materials. Index properties of the materials are
summarized in Table 1. Three geosynthetic materials were used in this study as
summarized in Table 2. The BX1200 geogrid is manufactured of a stress resistant
polypropylene material with square openings. The TX160 geogrid is a newly
developed product, which is manufactured from a punched polypropylene sheet and
oriented in three equilateral directions. The woven polypropylene (W-PP-GT) is a
geotextile composed of high-tenacity polypropylene yarns woven into a network. A
summary of the geosynthetic material mechanical properties is provided in Table 2.
Table 1. Summary of material properties
Parameter Subgrade Subbase
Standard Proctor maximum dry unit weight, γdmax (kN/m3) 17.4 —
Optimum moisture content, wo
p
t (%) 17.2 —
Maximum and minimum index density (kN/m3) — 22.4/16.7
Liquid Limit (LL), Plasticity Index (PI) 45, 21 Non-plastic
AASHTO A-7-6(16) A-1-a
USCS CL GP-GM
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Table 2. Summary of geosynthetic material mechanical properties
Designation Type Mechanical Properties
BX1200 Square aperture
geogrid
Tensile Strength: 6.0 kN/m at 2% strain and 19.2
kN/m ultimate
TX160 Triangular aperture
geogrid Radial Stiffness: 300 kN/m at 0.5% strain
W-PP-GT 14.5 oz. woven
geotextile
Tensile Strength: 14.0kN/m at 2% strain and 70.0
kN/m ultimate
EXPERIMENTAL TESTING
Construction of Test Sections
Four test sections were constructed as part of this study (Figure 1) with two
nominal 300 mm thick subbase layers placed over the compacted subgrade. The W-
PP-GT, BX1200, and TX160 test sections consisted of respective geosynthetic
treatments placed on the subgrade layer prior to placement of subbase lifts. A control
section with no geosynthetic treatment was constructed for comparison.
The subgrade layer was prepared targeting a California bearing ratio (CBR) in
the range of 2 to 3 (resilient modulus, Mr, between 3000 to 4500 psi). Subgrade
construction process involved scarifying to a depth of about 300 mm with a larger
tiller, moisture conditioning to about 1.5% to 3.5% wet of the standard Proctor
optimum moisture content, and compacting using two smooth drum roller passes.
CBR of the subgrade was measured by conducting one dynamic cone penetrometer
(DCP) test in accordance with ASTM D6951-03 (ASTM 2003) in each test section.
CBR of the subgrade ranged from about 2.0 to 2.4. Following subgrade compaction,
EPCs were installed at a depth of about 200 mm below the top of the subgrade to
monitor horizontal stresses in the subgrade and vertical stresses on top the subgrade
(Figures 1 and 2). More details on EPC installation and data acquisition are provided
in the following section of this paper. A subbase layer of nominal 300 mm thickness
was placed on top of the subgrade. A piezoelectric EPC was installed in the subbase
lift 1 at a depth of about 150 mm above the subgrade to measure horizontal stresses in
the subbase (Figures 1 and 2). The subbase lift 1 was compacted using 13 roller
passes with a 10 ton vibratory smooth drum roller and tested using two trafficking
(described in detail in the following sections of the paper) passes. Another subbase
layer of approximately 300 mm in thickness was similarly placed, compacted using
22 to 24 vibratory roller passes, and tested using 150 trafficking passes.
Piezoelectric Earth Pressure Cell Measurements
Geokon 3500 piezoelectric total earth pressure cells (EPC) were installed to
measure in-ground total stresses during and after compaction, and trafficking. The
EPCs were 100 mm diameter and 10 mm thick sensors made of two stainless steel
plates welded together and filled with de-aired hydraulic fluid. As stress is applied to
the cell a pressure transducer measures the change in pressure of the fluid. The EPCs
were carefully calibrated in the laboratory by placing the EPCs in compacted poorly
graded ASTM silica sand in a pressure controlled calibration chamber and applying
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known stresses over the EPC measurement range (0 to 1000 kPa). The calibration
values are affected by variations in the temperature of the hydraulic fluid. Therefore,
correction factors were determined in a temperature controlled room by taking zero
readings from 4.4° C to 37.8° C in 5.6° C increments. A data acquisition system was
used to interface a field laptop computer to record EPC readings. During installation
of EPCs in the subgrade and subbase layers, the EPCs were placed and surrounded by
the ASTM standard silica sand used for calibration (Figure 2). Measurements were
obtained at a frequency of 1613 Hz. Periodic temperature readings were obtained to
correct the EPC measurements.
36.6 m
Subbase Lift 2
Subbase Lift 1
Subgrade
300 mm
300 mm
150 mm
Vertical EPC Horizontal EPC
Transient roller/
traff icking passe s
Trea tme nt La yer
200 mm
Plan Vi ew
Cross-Sectional View
Control Section
W-PP-GT BX1200
TX16 0
3.5 m
3.5 m
Figure 1. Plan view and cross-sectional view of test sections
Silica sand
p laced around
verticalEPC
Card-board box for
silica sand
pl acement aro und
ho ri zontal EPC
Vert ic al
EPC
Horizontal
EPC
Silica sand
bedd ing
Figure 2. Vertical and horizontal EPCs installation on geogrid and subbase
layers
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Trafficking and Rut Depth Measurements
An 18 ton rear single axle test truck was used for trafficking the test sections
(Figure 3). The front and rear tires were inflated to a tire pressure of about 690 kPa.
The truck was operated at 5 km/h nominal speed, following the same wheel path and
in the same direction for each trafficking pass. Rut depths beneath the wheels were
monitored during trafficking passes by measuring the maximum plastic deformation
under the wheel with reference to the undeformed surface of the subbase layer
(Figure 3).
Figure 3. Test truck used for trafficking and rut depth measurements
EXPERIMENTAL TEST RESULTS
Bar charts of average rut depth measurements obtained on subbase lift 2 after
75 and 150 trafficking passes are presented in Figure 4. The average rut depth was
calculated based on measurements obtained from each test section on right and left
wheel paths. The number of test measurement locations for each test section varied
from 66 to 71. Average relative density (Dr) of the subbase material (based on five
measurements per test section) after the final compaction pass on each test section is
also presented on Figure 4. The average rut depth was comparatively higher in the
control section (66 mm at 150 passes) and lower in the TX160 test section (46 mm at
150 passes). Compaction measurements show that the average relative density was
98% for the TX160 section and 90% for the control and BX1200 sections. The
relative density was lowest for the W-PP-GT section at 84%. The higher relative
density achieved in the highly confined aggregate (98% versus 90% for the control
shown in Figure 4) contributed to smaller surface deflections.
Control W-PP-GT BX1200 TX160
Average Rut Depth (mm)
0
20
40
60
80
100
120
75 passes
150 passes
1 * σ
Data obtained from left and right wheel p aths is averaged
Avg. D
r
= 90%
Avg. D
r
= 84%
Avg. D
r
= 90%
Avg. D
r
= 98%
Figure 4. Bar chart comparing average longitudinal rut depth after 75 and 150
truck passes on subbase layer 2
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An example of dynamic vertical and horizontal total stress measurements
obtained from the TX160 test section under a trafficking pass on subbase lift 2 is
presented in Figure 5. The stresses presented in Figure 5 represent the change in
ground stresses minus the initial geostatic (overburden) stresses. Two peaks are
observed under the trafficking pass due to the influence of front and rear tires. As
expected, higher stresses are recorded under the dual rear-tire loaded axle. Stress
measurements in Figure 5 indicate that the stresses before and after the trafficking
pass were in the range of 15 to 20 kPa. These stresses are a result of “locked-in”
stresses and principal stress rotation from prior compaction and trafficking passes.
These “locked-in” stress values and the peak stress (under the rear tire) values were
recorded for each roller and trafficking pass for each test section, and are presented in
Figure 6 and Figure 7, respectively.
DISCUSSION
The results presented herein demonstrate performance differences between the
four test sections from rut depth and compaction measurements, and that the
inclusion of geosynthetic material at the interface of soft subgrade and subbase layers
affects the development of the “locked-in” horizontal stress following loading.
Development of “locked-in” horizontal stresses in the subgrade and subbase layers
gives a direct indication of the lateral restraint reinforcement mechanism.
Lateral stress ratio (K) (calculated as the ratio of total horizontal and total
vertical stresses) plots for subgrade and subbase layers following roller and
trafficking passes and for peak values under the roller and trafficking passes are
presented in Figure 8 and Figure 9, respectively. The calculated K values from Figure
9 show that during trafficking (i.e., under peak loading), the K values are about 0.3 to
0.7 for the subgrade and 0.5 to 0.7 for the subbase for all test sections. However, the
K values based on the “locked-in” stresses following trafficking passes from Figure 8
vary significantly between the different test sections. Table 3 summarizes the K
values after 75 trafficking passes on subbase lift 2 (i.e., after 112 cumulative roller +
truck passes). Results show buildup of horizontal stresses with relatively high K
values in the control section subgrade layer compared to the geogrid (i.e., BX1200
and TX160) reinforced sections. The K values in the subbase were comparatively
higher in the TX160 section compared to the control and BX1200 sections. The W-
PP-GT section produced the highest K values in the subgrade and subbase layers.
Further examination of horizontal stress measurements and rut depth
measurements show that the ratio of the “locked-in” horizontal stress in the subgrade
to the “locked-in” horizontal stress in the subbase (lateral reinforcement ratio)
provides an indication of the performance of the section. As shown in Table 3, rut
depth measurements generally decreased as the reinforcement ratio increases. This
approach although limited to these site conditions and materials, suggests that the
reinforcement ratio value could be a useful indicator to performance and warrants
further study. For the site and material conditions in this study, the results also
suggest that the TX160 with its unique equilateral triangular shaped apertures
provides comparatively higher reinforcement ratio than the other geosynthetics used
in this study.
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Time (sec)
6 8 10 12 14 16 18 20
Total Vertical Stress (kPa)
0
50
100
150
200
Time (sec)
6 8 10 12 14 16 18 20
Total Horizontal Stress (kPa)
0
50
100
150
200
Subgrade
Subbase
Front Tire
Rear Tire
Front TireRear Tire
Peak value
Figure 5. Example dynamic EPC measurements in TX160 section (under a
trafficking pass on subbase lift 2)
0 20 40 60 80 100 120 140 160 180
Total Vertical Stress (kPa)
0
5
10
15
20
25
W-PP-GT
BX1200
Control Section
TX160
Lift 1 Lift 2
Roller
Truck
Roller Truck Location: Interface between
geosynthetic and subgrade layer
0 20 40 60 80 100 120 140 160 180
Total Horizontal Stress (kPa)
0
5
10
15
20
25
W-PP-GT
BX1200
Control Section
TX160
Lift 1 Lift 2
Roller
Truck
Roller Truck
Location: Subgrade Layer
Roller/Truck Cumulative Pass Count
0 20 40 60 80 100 120 140 160 180
Total Horizontal Stress (kPa)
0
5
10
15
20
25
W-PP-GT
BX1200
Control Section
TX160
Lift 1 Lift 2
Roller
Truck
Roller Truck Location: Subbase Layer
Figure 6. Total vertical and horizontal stresses after roller compaction and test
truck trafficking passes
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0 20 40 60 80 100 120 140 160 180
Total Peak Vertical Stress (kPa)
0
50
100
150
200
250
W-PP-GT
BX1200
Control Section
TX160
Lift 1 Lift 2
Roller Roller Truck Location: Interface between
geosynthetic and subgrade layer
Truck
0 20 40 60 80 100 120 140 160 180
Total Peak Horizontal Stress
0
50
100
150
200
250
W-PP-GT
BX1200
Control Section
TX160
Lift 1 Lift 2
Roller
Truck
Roller Truck Location: Subgrade Layer
Roller/Truck Cumulative Pass Count
0 20 40 60 80 100 120 140 160 180
Total Peak Horizontal Stress
0
50
100
150
200
250
W-PP-GT
BX1200
Control Section
TX160
Lift 1 Lift 2
Roller
Truck
Roller Truck Location: Subbase Layer
Figure 7. Total peak vertical and peak horizontal stresses under roller drum
and test truck rear tire
Table 3. Performance comparison between test sections
Section KSubgrade KSubbase
Reinforcement
Ratio*
Average Rut Depth after 75
trafficking passes on subbase
lift 2 (mm)
Control 3.2 1.2 0.4 50
W-PP-GT 4.3 3.7 0.8 47
BX1200 1.8 1.8 1.0 48
TX160 1 3.2 3.3 32
*calculated as the ratio of horizontal “locked-in” stresses in subbase and subgrade
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0 20 40 60 80 100 120 140 160 180
Lateral Stress Ratio, K
0
2
4
6
8
W-PP-GT
BX1200
Control Section
TX160
Lift 1 Lift 2
Roller Truck Subgrade
Roller/Truck Cumulative Pass Count
0 20 40 60 80 100 120 140 160 180
Lateral Stress Ratio, K
0
2
4
6
8
W-PP-GT
BX1200
Control Section
TX160
Lift 1 Lift 2
Roller Truck Subbase
Figure 8. Lateral stress ratio (ratio of horizontal and vertical stresses) in
subgrade and subbase layers after roller and test truck trafficking passes
0 20 40 60 80 100 120 140 160 180
Lateral Stress Ratio, K
0
2
4
6
8
W-PP-GT
BX1200
Control Section
TX160
Lift 1 Lift 2
Roller
Truck
Roller Truck Subgrade
Roller/Truck Cumulative Pass Count
0 20 40 60 80 100 120 140 160 180
Lateral Stress Ratio, K
0
2
4
6
8
W-PP-GT
BX1200
Control Section
TX160
Lift 1 Lift 2
Roller
Truck
Roller Truck Subbase
Figure 9. Lateral stress ratio (ratio of horizontal and vertical stresses) under
roller drum and test truck wheel loading in subgrade and subbase layers
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SUMMARY AND CONCLUSIONS
Results from a controlled field study comparing performance of three
geosynthetic materials (TX160 geogrid, BX1200 geogrid, and W-PP-GT woven
geotextile) and a control section with no reinforcement are described in this paper.
The geosynthetic materials were placed on top of a soft clay subgrade layer prior to
compacting two overlying layers of crushed limestone. Rut depth measurements
under trafficking from a heavy vehicle and compaction measurements on the subbase
layer, and in-ground stress measurements under compaction and trafficking passes,
provide information on the reinforcing effects provided by the different
geosynthetics. Key findings from this study are as follows:
1. Rut depth and compaction measurements showed better performance of the
TX160 geogrid section compared to other test sections.
2. In-ground stress cell measurements showed that the “locked-in” horizontal
stress in the subgrade after trafficking was lower in the TX160 section
compared to other test sections.
3. Greater “locked-in” horizontal stresses within the subbase following
compaction and trafficking loading were measured in the TX160 section
compared to the other test sections.
4. The reinforcement ratio calculated as the ratio of horizontal “locked-in” stresses
in the subbase and subgrade layers provide an indication of rut performance and
warrants further research.
ACKNOWLEDGMENTS
This study was sponsored by Tensar International Corporation.
REFERENCES
ASTM (2003). “Standard test method for use of the dynamic cone penetrometer in
shallow pavement application – ASTM D6951-03.” American Standards for
Testing Methods (ASTM), West Conshohocken, Pennsylvania.
FHWA (2008). FHWA NHI-07-092. Geosynthetic Design & Construction Guidelines
Reference Manual, U.S. Department of Transportation, Federal Highway
Administration, Washington D.C.
Kwon, J., and Tutumluer, E. (2009). “Geogrid base reinforcement with aggregate
interlock and modeling of the associated stiffness enhancement in mechanistic
pavement layers.” Transportation Research Record, 2079, 85-95.
Perkins, S.W., and Ismeik, M.A. (1997). “Synthesis and evaluation of geosynthetic-
reinforced base layers in flexible pavements: Part I.” Geosynthetics
International, 4(6), 549-604.
Tensar. (2009). The properties and performance advantages of Tensar® TriAx®
Geogrids – Product Brochure . Tensar International, Atlanta, Georgia. S.
Department of the Army, U.S. Army Corps of Engineers (2003). Engineering and
Design. Use of Geogrids in Pavement Construction. Technical Letter ETL
1110-1-189, Washington D.C.
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