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Mechanically reinforced granular shoulders on soft subgrade: Laboratory and full
scale studies
Mohamed M. Mekkawy a,*, David J. White b,1, Muhannad T. Suleiman c,2, Charles T. Jahren d,3
a Fugro Atlantic Inc., 350 World Trade Center, Norfolk, VA 23510, United Statesb Department of Civil, Construction and Environmental Engineering, Iowa State University, 476 Town Engineering, 50011-3232, United Statesc Department of Civil and Environmental Engineering, Lehigh University, 326 STEPs Building, 1 W Packer Avenue, Bethlehem, PA 18015, United Statesd Department of Civil, Construction and Environmental Engineering, Iowa State University, 458 Town Engineering, 50011 3232, United States
a r t i c l e i n f o
Article history:
Received 13 March 2010
Received in revised form
24 August 2010
Accepted 16 October 2010
Available online xxx
Keywords:
Geogrids
Geosynthetics
Granular shoulders
Rutting
Soil stabilization
Mechanical reinforcement
a b s t r a c t
A recently completed eld study in Iowa showed that many granular shoulders overlie clayey subgrade
layer with California Bearing Ratio (CBR) value of 10 or less. When subjected to repeated trafc loads,
some of these sections develop considerable rutting. Due to costly recurring maintenance and safety
concerns, the authors evaluated the use of biaxial geogrids in stabilizing a severely rutted 310 m tests
section supported on soft subgrade soils. Monitoring the test section for about one year, demonstrated
the application of geogrid as a relatively simple method for improving the shoulder performance. The
eld test was supplemented with a laboratory testing program, where cyclic loading was used to study
the performance of nine granular shoulder models. Each laboratory model simulated a granular shoulder
supported on soft subgrade with geogrid reinforcement at the interface between both layers. Based on
the research ndings, a design chart correlating rut depth and number of load cycles to subgrade CBR
was developed. The chart was veried by eld and laboratory measurements and used to optimize the
granular shoulder design parameters and better predict the performance of granular shoulders.
2010 Elsevier Ltd. All rights reserved.
1. Introduction
Shoulder rutting performance problems are common in areas
where the granular shoulder material is supported by weak
subgrade. With repeated trafc loads,bearing capacity failure in the
subgrade occurs leading to progressive rutting. In addition to being
hazardous to drivers, severely rutted shoulders are expensive to
maintain. Ruts are commonly maintained by shoulder blading and
as necessary adding granular material. These maintenance prac-
tices, however, are considered temporary solutions as they neither
address the problem nor prevent it from reoccurring.
The performance of 25 granular shoulder sections in Iowa was
recently evaluated using various in situ tests with the objective
improving shoulder performance while keeping ownership costslow (Mekkawy et al., 2010). At about 40% of the inspected sections,
the subgrade layer had a California Bearing Ratio(CBR) of 10 or less.
As a result, bearing capacity failure of the subgrade as well as lateral
displacement of the granular and subgrade materials with repeated
trafc loads were frequently observed. Based on the ndings of the
eld study and on a pilot study basis, a granular shoulder test
section overlying a soft subgrade was constructed and monitored.
The test section involved three sections with different geogrids at
the interface between the subgrade and the granular layer. Moni-
toring the test section for a period of about one year demonstrated
the success of geogrid stabilization in eliminating rutting. To
supplement theeld study, a laboratory box model was designed to
evaluate several stabilized models, which were subjected to cyclic
loading with three incremental loading stages. The soil properties
and displacement before and after each test were recorded and
compared. The laboratory box model comprised of a loading frame,
reaction beam, hydraulic actuator, and a steel box to contain the
soil. The overall scope of the eld and laboratory experimentations,which is the focus of this paper, was:
Evaluate the use of geogrid reinforcement to eliminate rutting.
Compare and contrast, through laboratory testing, selected
geogrid stabilizers.
Develop simple designs tools, which will result in more stable
shoulder sections and better prediction of performance in
terms of rut depth and number of loading cycles.
To help design stable granular shoulders, a design chart was
developed from the semi-empirical method proposed by Giroud
* Corresponding author. Tel.: 1 510 267 4436; fax: 1 510 268 0545.
E-mail addresses: [email protected] (M.M. Mekkawy), djwhite@iastate.
edu (D.J. White), [email protected] (M.T. Suleiman), [email protected] (C.T.
Jahren).1 Tel.: 1 515 294 1463; fax: 1 515 294 8216.2 Tel.: 1 610 330 5413.3 Tel.: 1 515 294 3829; fax: 1 515 294 3845.
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Geotextiles and Geomembranes
j o u r n a l h o m e p a g e : w w w . e l s e v i e r .c o m / l o c a t e / g e o t e xm e m
0266-1144/$e see front matter 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.geotexmem.2010.10.006
Geotextiles and Geomembranes 29 (2010) 149e160
Please cite this article in press as: Mekkawy, M.M., et al., Mechanically reinforced granular shoulders on soft subgrade: Laboratory and full scalestudies, Geotextiles and Geomembranes (2010), doi:10.1016/j.geotexmem.2010.10.006
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and Han (2004a). The chart correlates rut depth and subgrade CBR
(CBRSG). The reliability of the chart was conrmed by comparing it
to eld and laboratory rut depth measurements. Field measure-
ments that were visibly due to subgrade deformation only were
used in this comparison since rutting can sometimes be due to the
cumulative deformation of both the granular and subgrade layers.The chart was developed for a specic axle load, granular layer
thickness, tire pressure, and granular layer CBR (CBRGL). Similar
charts can be generated for a selected set of CBR proles and
loading parameters.
2. Background
According to Fannin and Sigurdsson (1996), where trafc is
channelized, rut is dened as the distance between the initial
elevation of the surface before trafcking and the lower point in the
rut beneath the wheel. Where trafc is not channelized, an erratic
pattern of ruts develop, which can be dened as the distance
between adjacent high and low spots of the base course thickness(Giroud and Han, 2004a). According to Giroud and Han (2004a),
surface rutting occurs by one or more of the following mechanisms:
Compaction of the base course aggregate and/or subgrade soil
under repeated trafc loading.
Bearing capacity failure of the base course or subgrade due to
normal and shear stresses induced by initial trafc.
Bearing capacity failure of the base course or subgrade after
repeated trafc loads which can result in progressive deterio-
ration of the base course, reduction in effective base course
thickness due to contamination by the subgrade soil, a reduc-
tion in the ability of the base course to distribute trafc loads to
the subgrade, or a decrease in the subgrade strength due to
pore pressure build up or disturbance. Lateral displacement of base course and subgrade material due
to accumulation of incremental plastic strains induced by each
load cycle.
During recent years the use of geosynthetics to reinforce
unpaved structures has shown a marked increase. Geosynthetics,
which are typically placed at the interface between the base course
and the subgrade, can carry higher trafc volumes, and can prevent
lateral movement of the base aggregate stiffening the layer and
distributing the wheel loads over a greater area of the subgrade
(U.S. Army Corps of Engineers, 2003). The following are benets of
using geosynthetics as mechanical reinforcement (Berg et al., 2000;
Giroud and Han, 2004a; Subaida et al., 2009; Hufenus et al., 2006,
andPowell et al., 1999):
Reduce the stress on the subgrade;
Enhance subgrade connement and reducing heave
Reduce pumping of the subgrade nes into the base layer;
Prevent contamination of the base materials allowing for more
open graded, free-draining aggregates;
Reduce the excavation depth required to remove unsuitablesubgrade materials;
Reduce the aggregate layer thickness required to stabilize the
subgrade;
Minimize subgrade disturbance during construction;
Reduce maintenance and extend the life of the pavement; and
Prevent development and growth of local shear zones and
allow the subgrade to support stresses close to the plastic limit
while acting elastic.
Two types of geosynthetics are typically used: geogrids and
geotextiles. Geogrids and woven geotextiles have been used as
a reinforcement to increase the resistance to trafc loadings (Giroud
and Noiray,1981). Non-woven geotextiles havebeen mainly usedfor
separation of the base course aggregate and the subgrade.The fundamental reinforcement mechanisms involving the use of
geogrids are: lateral restraint; improved bearing capacity; and
tensioned membrane effect (Giroud and Noiray, 1981and Hufenus
et al., 2006). Lateral restraint refers to the interlocking and conne-
ment of aggregate during loading restricting the lateral ow of the
material (Hufenus et al., 2006). This increases the modulus of the
base course material, which subsequently, increases the area on
which the vertical stress is applied on the subgrade. The bearing
capacity of the shoulder system is improved since more shear stress
is transferred to the reinforcement, which would otherwise be
applied to the soft subgrade layer (Hufenus et al., 2006). The
tensioned membrane effect refers to the deformation of a geogrid
under tensile stress, which in turn improves the vertical stress
distribution. In early research stages, the tensioned membraneeffectwas believed to govern the reinforcement mechanism. However,
later research demonstrated that reinforcement benets are
obtained without signicant deformation, and that lateral restraint
can be the primary reinforcement mechanism followed by the
improved bearing capacity concept (U.S. Army Corps of Engineers,
2003).
The design method for geogrid-reinforced unpaved roads pre-
sented byGiroud and Han (2004a,b) was adopted for comparing
measured and predicted soil displacement as well as fordeveloping
a shoulder design chart. The chart, presented later in this paper,
shows the relationship between rut depth, CBRSG, and number of
load cycles. In their design method, Giroud and Han (2004a,b)
account for the inuence of geogrids by the bearing capacity
factor (Nc), which implies interlock between the geogrid and the
Fig. 1. Shoulder section on new Highway 34 bypass (a) visually suitable shoulder (b) 76 mm rut developed with a few truck passes.
M.M. Mekkawy et al. / Geotextiles and Geomembranes 29 (2010) 149e160150
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base course materials, and aperture stability modulus (J), which isdened as the resistance of the geogrid to in-plane rotational
movementor in-plane rigidity, and is linked to the increase in stress
distribution angle. The design method also accounts for the quality
of the base course materials, the variation in the stress distribution
angle with number of load cycles, and the inuence of rut depth.
Only failure of the subgrade, which is assumed to be saturated and
have low permeability (behaves in an undrained manner), is
considered in this method (Giroud and Han, 2004a). Relationships
between rut depth, CBRSG, and the number of load cycles are pre-
sented later in the text for the date collected in this study.
3. Field observations
Encountering weak subgrade conditions was a reoccurring
observation during a eld study aimed to evaluate the performance
of granular shoulders in Iowa (Mekkawy et al., 2010). By conducting
Dynamic Cone Penetration (DCP) tests (ASTM D6951, 2003), it was
revealed that about 40% of the inspected shoulder sections had
a CBRSGof 10 or less. Of those sections, rutting was observed with
varying degrees depending on the level of off-tracking. Initially,
a section with a soft subgrade layer may appear stable immediately
after construction (Fig. 1a). However, when loaded by trafc,
pumping and rutting develops along the wheel path (Fig.1b). In the
example shown, 76 mm of rut developed only after six truck pas-
ses.DCP tests conducted with varying distance from the pavement
edge yielded CBR values of 6e12 in the upper 200 mm of the
crushed limestone layer, 4e10 in the underlying earth shoulder ll
underlying layer, and 2e29 in the subgrade underlying the earth
shoulderll. It should be noted that some of the rut developed in
this section can be caused by lateral deformation of the granular
layer; however, the authors attribute the majority of the developed
rut to subgrade deformation since contamination of the granular
layer by the underlying subgrade material was observed (Fig.1b) as
well as punching and shear failures. Currently, the Iowa Depart-
ment of Transportation (DOT) has no design requirement for theCBRSG, which consequently does not restrict shoulders from being
constructed over soft foundation soils.
4. Geogrid stabilizatione highway 218 Nashua, IA
4.1. Site description
The inside granular shoulder was experiencing severe rutting
(up to 200 mm at some locations) due to soft subgrade conditions.
The problematic section extended a distance of about 9.6 km. Soft
regions that needed repair were identied and isolated by driving
a fully loaded dump truck weighing 21,337 kg over the shoulder
section and measuring the rut depth at pre-identi
ed locationsalong the wheel path, conducting Clegg Impact tests using a 20 kg
hammer (ASTM D5874, 2007), and DCP tests. The prole of rut
depth and Clegg Impact Value (CIV) with distance starting from
milepost 224 is shown in Fig. 2. The region with the highest rut
depth and lowest CIV, indicating soft conditions, extended about
2000 m (from milepost 220.85e219.60). DCP tests conducted
within this region showed a weighted average CBR of 6 in the upper
200 mm and 5 at a depth between 200 and 500 mm.
Bulk soil samples were obtained from the subgrade and granular
material and classied as SC (A-4) and GW (A-1-a) (Table 1). The
subgrade soil optimum moisture content and maximum dry unit
weight determined using standard Proctor (ASTM D698, 2000) test
were15% and 17.9 kN/m3, respectively. The in situ moisturecontents
and unit weights were measured at 13 locations along the shoulder
section using driven cores and compared to the standard Proctor
curve (Fig. 3). Generally, all in situ dry unit weights were lower than
the standard Proctor curve even in good performing sections. Unit
Distance (m)
0 2000 4000 6000 8000 10000
CIV
0
2
4
6
8
10
12
14
16
18
Ru
tdepth(mm)
0
20
40
60
80
100
120
140
160
180
200
Milepost
218219220221222223224
CIV
Rut depth
Test section
Fig. 2. Rut depth prole measured inside the wheel path and CIV measured at 0.6 m
from the pavement edge.
Table 1
Engineering properties of test section and laboratory box study materials.
Properties of shoulder test section material
Material D10 D30 D60 Cu Cc %P200 %P#4 LL PI USCS AASHTO
Granular 0.75 2.9 6.1 8.1 1.8 4 52 e e GW A-4
Subgrade 0.001 0.008 0.19 190 0.3 49 95 32 23 SC A-1-a
Properties of laboratory box study material
Limestone 0.09 2.0 5.9 66 7.5 10 51 e e S P-SM A-1-a
RAP 0.9 3.1 6.8 7.2 1.5 0.5 42 e e GP A-1-a
Subgrade e 0.006 0.04 e e 78 100 50 32 CH A-7-6
Moisture content (%)
6 8 10 12 14 16 18 20 22 24 26
Dryun
itweight(kN/m3)
13
14
15
16
17
18
19M.P. 223.20
M.P. 224.0
M.P. 223.80
M.P. 223.0
M.P. 223.60
M.P.223.40
M.P. 221.40
M.P. 220.35
M.P. 222.20
M.P. 220.60M.P. 219.20
M.P. 218.40
M.P. 219.80
dmax= 17.9 kN/m
3
wopt= 15%
zav
Fig. 3. Lower eld densities relative to the standard Proctor curve.
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weights measured between milepost 220.60 and 219.80 in partic-
ular were considerably lower than the maximum dry unit weight (%
relative compaction80%). Also, most
eld moisture contents werehigher than the optimum moisture content (8.8%).
4.2. Stabilization of test section
Three biaxial geogrid types, henceforth referred to as BX1, BX2,
and BX3, were used to stabilize the shoulder section. The properties
of the geogrids are presented in Table 2. The purpose of using three
geogrid types was to compare performance as there are mechanical
and cost differences between them. The geogrids were placedat the
interface between the subgrade and a new 200 mm overlying
crushed limestone layer. As shown in Fig. 4, the test section was
approximately 310 m long (from milepost 220.60 up to milepost
220.40) and was about 2.4 m wide. The rst 60 m were left unsta-
bilized as a control section. Following the control section was
a 100 m long section stabilized with BX1. Two sections, each 75 m
long, followed the BX1 section, and were stabilized with BX2 fol-
lowed by BX3.
The shoulder test section was stabilized by rst removing and
discarding the contaminated granular layer (Fig. 5a). This layer was
replaced with about 450 tons of new crushed limestone material.
The subgrade was leveled and compacted using a skid loader fol-
lowed by a pneumatic roller (Fig. 5b). Using a power saw, the
geogrids were cut to 2.4 m wide to match the width of the stabi-
lized area. The geogrids were rolled over the soft subgrade starting
with BX1 followed by BX2 then BX3 (Fig. 5c). Beyond approxi-mately 300 m, the BX3 was damaged, which occurred while
transporting the geogrid to the site. The defective grid, denoted by
BX3*, was placed without alteration to evaluate the effect of
improper geogrid installation. A motor grader followed by a pneu-
matic roller were used to spread and compact the aggregate. The
edges of the geogrid were exposed at about 2.4 m from the pave-
ment edge at the end of construction (Fig. 5d). This was due to
tapering of the granular layer thickness during construction (i.e. the
thickness of the granular layer decreased with increasing lateral
distance from the pavement). The entire process of excavation of
contaminated material, geogrid placement, aggregate placement,
and compaction took approximately 5 h. Upon construction
completion, the section was opened to trafc. Unlike some chem-
ical stabilization, mechanical stabilization using geogrid is fast to
install and does not require curing time, which is a benet for
providing minimum disturbance to trafc.
4.3. Field monitoring
The section was continuously monitored using in situ tests over
a period of about one year. After one month from construction, an
edge rut of about 127 mm was measured at the control section as
Table 2
Geosynthetics engineering properties.
Property Test method Geosynthetic material
BX1 BX2 BX3 Woven geotextile Non-woven geotextile
Polymer type Polypropylene
Grab tensile strength (N) ASTM D4632 e e e 1400 712
Tensile strengtha (5% strain) (kN/m) ASTM D6637 11.8 8.5 8.0 e e
Elongation (%) ASTM D4632 e e e
15 50Flexural stiffness (mg-cm) ASTM D5732 750,000 250,000 250,000
Junction efciency (%) 93 93 93
Aperture stabilityb (kg-cm/deg) e 6.5 3.2 2.8 e e
Minimum rib thickness (mm) 1.27 0.76 0.76
Aperture dimensionc ( mm)/Appar ent openi ng siz e ASTM D4751 25 25 33 40 US S td. sieve ( 0. 425 mm) 70 US S td. sieve ( 0. 212 mm)
Resistance to installation damaged (%SC/%SW/%GP) 95/89/86 90/83/70 90/83/70
Resistance to long term degradation (%) 100 100 100
a Tensile Strength values are measured in the machine direction.b Measured in accordance with U.S. Army Corps of Engineers Methodology for measurement of torsional rigidity.c Reported aperture dimension are measured in the machine direction.d Resistance to loss of load capacity when subjected to mechanical installation stress in clayey sand (SC), well graded sand (SW), and poorly graded crushed stone (GP).
Fig. 4. Schematic diagram of the test section (a) plan view (b) cross section.
M.M. Mekkawy et al. / Geotextiles and Geomembranes 29 (2010) 149e160152
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shown inFig. 5e. The stabilized sections showed no signs of rutting
(Fig. 5f).
Part of monitoring the performance of the test section was by
performing plate load tests (PLT). The PLT comprised of a 300 mm
steel plate, hydraulic jack for applying incremental loads, and three
linear variable differential transducers (LVDTs) to measure platedeection. PLT was conducted immediately after construction,
three months, and 10 months from construction at about 1.2 m
from the pavement edge where the granular shoulder is a nominal
200 mm. No tests were conducted near the shoulder edge where
the granular layer started to taper to avoid excessive deformation of
the granular material, which may bias the results by increasing the
magnitude of rutting. Immediately after construction, the highest
modulus of subgrade reaction for both the loading (K) and
reloading (KR) stages of the PLT was measured at the section
stabilized with BX1. K was determined by calculating the secant
modulus between 0 and 10 mm deection. K was slightly higher at
the BX2 section compared to the BX3 section (Fig. 6). This may be
attributed to the small difference in their aperture stability
modulus (the aperture stability modulus of BX2 and BX3 are 3.2
and 2.8 kg-cm/deg, respectively) as well as the small difference in
tensile stiffness (Table 2). As expected, the highest soil deection
was measured at the control section where the average K wasabout
60% less than the BX1K value for the rst loading stage (Table 3).
After three months, a 70% increase in K was measured for all geo-
grid sections compared to values after construction. The increase inK with time can be caused by progressive lateral connement of
aggregate under repetitive trafc loads. It is also possible that as the
section was loaded, the subgrade layer deformed applying tension
forces to the geogrid adding to the stability of the sections. The
increase in K at the control section was due to placing and com-
pacting new crushed limestone material to alleviate the rutting
developed after one month. PLT results obtained after 10 months
showed a reduction in K values for the control section and BX2
section by about 23% and 8%, respectively. The sections stabilized
with BX1 and BX3, however, continued to show increase in K with
time by 5% and 26%, respectively (Table 3).
Using a 20 kg hammer, CIV tests were performed every 15 m.
The tests were performed at different time intervals as shown in
Fig. 7. The CIVs measured after stabilization were about 3 times
Fig. 5. Shoulder reconstruction using geogrids (a) motor grader removing the contaminated granular layer (b) pneumatic roller used to compact the subgrade (c) rolling the geogrid
over the subgrade (d) exposed geogrid at the end of construction (e) rutting developed at the control section after one month (f) no rutting at the geogrid stabilized section after one
month.
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higher than CIVs before stabilization. After three months, addi-
tional increase in surface stiffness was measured at the stabilized
sections evidenced by an average 20% increase in CIVs. No consid-
erable increase in CIVs was measured at the control section. Therewas no clear difference between the CIVs measured at the three
geogrid sections even though their aperture stability modulus
varied. It is possible that the CIV value obtained by dropping a 20 kg
Clegg hammer is inuenced by a relatively shallow depth, and
therefore unable to detect different degrees of connement. At 10
months, CIVs were reduced in all the stabilized sections by an
average of 4%. Relative loosening of the granular layer after the
freeze-thaw period may be a reason behind this reduction.
DCP tests were also used to evaluate and document changes in
the test section performance. DCP testing carried out before and
after stabilization for each geogrid section, and was used to calcu-
late CBRGL(Fig. 8). For the BX1 section, CBRGLincreased from 3 to 18,
whereas for BX2 and BX3 sections, CBRGLincreased from 4 to 19
and 3 to 17, respectively. As anticipated, the strength increase
occurred in the granular layer with no considerable change in CBR
values below the depth of the geogrid (i.e. subgrade layer). The
increase noted at 240 m (Fig. 8c) below about 400 mm is attributedmore to the variations in measurements and repeatability associ-
ated with the DCP test.
Parts of the defective BX3* were exposed after four months from
installation (Fig. 9a). The exposure was a result of severe rutting.
Use of defective geogrid demonstrates no reinforcement benet.
After 10 months, additional geogrid exposure was observed along
section BX3*. The exposed portions were along the edges of the
stabilized section where the grids were originally insufciently
covered with little crushed limestone (Fig. 9b).
In spite of the small reduction in strength parameters and
geogrid exposure, overall, geogrid stabilization improved the
shoulder performance and eliminated shoulder rutting. To prevent
geogrid exposure in future stabilization applications, the thickness
Stre
ss
(kN/m
2)
0
200
400
600
Deflection (mm)
0 10 20 30 40
Stress
(kN/m
2)
0
200
400
600
Deflection (mm)
0 10 20 30 40
Control
Section
BX1
BX2 BX3
K
KR
K
K
K
K
K
KR
KR
KR
KR
15 m, K=12.03 MN/m3, K
R=36.17 MN/m
3
30 m, K=7.18 MN/m3, K
R= 40.83 MN/m
3
322 m, K=12.58 MN/m3, K
R=42.6 MN/m
3
91 m, K=26.28 MN/m3, K
R=80.43 MN/m
3
122 m, K=26.41 MN/m3, K
R=69.67 MN/m
3
183 m, K=18.92 MN/m3, K
R=53.17 MN/m
3
213 m, K=26.23 MN/m3, K
R=79.03 MN/m
3
244 m, K=20.59 MN/m3, K
R=60.65 MN/m
3
274 m, K=15.73 MN/m3, K
R=47.74 MN/m
3
Fig. 6. Plate load test results immediately after construction.
Table 3
Modulus of subgrade reaction determined from plate load testing.
Section Distance (m) After construction 3 months after construction 10 months after construction
K (kN/m3) KR(kN/m3) K (kN/m3) KR(kN/m
3) K (kN/m3) KR(kN/m3)
Control section 15 12030 36170 32050 112260 28100 201470
30 7180 40830 28720 88690 25040 125400
322 12580 42600 7900 34600 10140 54670
BX1 91 26282 80427 28250 107500 40550 136510
122 26400 69670 55090 122430 35650 109470
BX2 183 18900 53170 33670 94340 27380 80680
213 26230 79030 18030 90270 28040 83070
BX3 244 20590 60650 28560 78500 35200 109160
274 15730 45730 25820 60300 25990 111930
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of the overlying granular layer should be uniform. Based on the
experience from this test section, a nominal 150 mme
200 mmgranular layer thickness is recommended.
5. Laboratory box study
A laboratory testing program was developed to supplement the
eld study and evaluate the performance ofve geogrids and geo-
textilesand twogranularmaterialsin reducing shoulderrutting.The
selected materials were tested by designing a laboratory box model
to simulate a granular shoulder overlying a soft subgrade. The box
model was then stabilized with each geogrid and geotextile and
subjected to cyclic loading where the surface displacement was
recorded. The cumulative soil displacement and change in soil
properties afterthe test were used toassess theperformanceof each
geogrid and geotextile.
5.1. Test setup
The model setup consisted of a loading frame, reaction beam,
and a hydraulic actuator (Fig. 10a). The actuator had a maximum
force of 250 kN and a dynamic stroke of 150 mm. Twelve 200 mm c-
channels were assembled together to form a 0.2 m3 steel box
(0.6 m 0.6 m 0.6 m), used to contain the soil. The box was
loaded using a 150 mm diameter loading plate (Fig. 10b). To mini-
mize friction and stress concentrations, a compressible 12 mm
thick neoprene pad covered with a Teon sheet was placed at the
interface between the soil and the steel box.
The subgrade soil was placed at the bottom of the box and
compacted in 76 mm lifts by applying a static load to reach a nal
thickness of 300 mm.The reinforcement wasplaced at the interface
between the subgrade and the overlying granular layer. Similar to
the subgrade layer, the granular layer was compacted by applying
a static load to reach a nal thickness of about 150 mm (Fig. 11). To
ensure soft foundation conditions for all tests, the target range of
CBRSGvalues was selected to vary between 3 and 5, whereas the
target range of CBRGLvalues was 4e7.
The hydraulic actuator was used to apply a sinusoidal load pulse
to the 150 mm diameter loading plate. Three incremental cyclic
loads were applied tothe soil each sustained for5000 cycles (totalof
15,000 cycles) at a frequency of 1 Hz. The initial cyclic pressure was
275 kPa,which was then increased to550 kPa and then 827 kPa.The
frequency of one cycle per second was sufcient in sustaining the
applied load despite the large deections observed at some tests.
The hydraulic actuator control system was used to collectdisplacement data at predetermined load cycles. DCP and a 4.5 kg
clegg hammer were used to document the changes in CBR and CIV
for each soil layer before and after each test.
During the test, and for every loading stage, the deection of the
reaction beam was measured using a dial gauge. A linear relation-
ship was developed between the applied load and beam deection.
The measured beam deections were subtracted from the recorded
soil displacement at the corresponding loading stage to calculate
the nal soil displacement.
5.2. Materials
Three soil materials were used throughout the laboratorytesting program. The subgrade soil consisted of Paleosol clay with
a PI of 32 and classied as CH (fat clay; A-7-6). The materials used
for the granular layer were Class A crushed limestone and recycled
asphalt pavement (RAP). The crushed limestone was classied as
SP-SM (A-1-a) with optimum moisture content and maximum dry
unit weight of 6% and 21.9 kN/m3, respectively, whereas the RAP
CBR
1 10 100
Depth(mm)
0
200
400
600
800
CBR
0.1 1 10 100
CBR
1 10 100
Before stabilization After stabilization
180 m 240 m
a b cGranularlayer
Subgradelayer
60 m
Increase inCBRGLafter
stabilization
Fig. 8. DCP results before and after stabilization (a) BX1 (b) BX2 (c) BX3.
Distance (m)
0 50 100 150 200 250 300
CIV
0
2
4
6
8
10
12
14
16
18
20
Before stabilization
After stabilization
3 months
10 months
Controlsection
BX1 BX2 BX3
BX3*
Rutting after1 month.New aggregateadded
D
amage
dgri
d
Fig. 7. CIV prole with time at 1.2 m from the pavement edge.
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material was classied as GP (A-1-a) with a ne content of about
0.5% compared to 10% for the crushed limestone (Table 1).
The laboratory study assessed the performance of three poly-
propylene biaxial geogrid types, polypropylene woven, and non-
woven geotextiles. The biaxial geogrids were the same type as theones used to stabilize the eld test section (i.e. BX1, BX2, and BX3).
The geotextiles were a woven geotextile lm used mainly for soil
separation and stabilization, and a needle-punched non-woven
geotextile ber. The properties of the selected geosynthetics mate-
rials are presented in Table 2.
5.3. Test results
A total of nine tests were executed and compared to determine
the performance differences for the selected materials. During all
tests, the subgrade moisture content for was kept constant at about
25% during placement. The changes in soil properties before and
after each test are presented in Table 4, whereas the soil cumulative
displacements recorded by the hydraulic actuator control systemare presented inFig. 12.
5.3.1. Test no. 1 e control
Therst test performed was a control test, which modeled eld
conditions where a granular shoulder overlies a soft subgrade layer.
At thebeginning of thetest, thedry unit weightfor thesubgradeand
granular layers were 19 kN/m3 and 13.4 kN/m3, respectively. The
CBRSGvalue, calculated from DCP tests, increased from 3 to 9 after
the test was completed as a result of subgrade soil densication. On
the contrary, there was no signicant change in the CBRGLvalues.
The nal soil displacement after 15,000 cycles was 284 mm. Visualinspection revealed about 50 mm of aggregate punching into the
subgrade layer.
5.3.2. Test nos. 2, 3, and 4e BX1
In test Nos. 2, 3, and 4, BX1 was placed at the interface between
the granular and subgrade layers. During test No. 1, the dimensions
of geogrid were 6 mm shorter than the dimensions of the box to
eliminate any interaction between the grid andthe box. The dry unit
weight of the subgrade and granular layers were 18.6 kN/m 3 and
14 kN/m3, respectively. After the test, the CBRSG value increased
from 5 to 7 indicating less soil densication compared to Test No. 1.
Further, the subgrade modulusdetermined by LWD (ELWD) increased
from 7.0 to 12.0 MPa. Almost no change in properties of the granular
layer was measured. The maximum measured soil displacement at15,000 cycles was 125 mm, which is about 55% less than the control
test. The measured soil displacement was compared to the predicted
soil displacement at the endof each loading stage. The predicted soil
displacement was determined using a semi-empirical method out-
lined by Giroud and Han (2004a,b). It should be noted that this
method does not account for staged loading so rut that developed
Fig. 9. Exposed geogrid after 10 months (a) BX3* (b) BX1.
Fig. 10. Schematic of the laboratory apparatus setup (a) Steel frame and hydraulic actuator used for loading the stabilized soil (b) steel box used to contain the soil.
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during previous stages is not accounted for. Therefore, rut depths
predicted for stages 2 and 3 were based on loading and the number
of cycles only. This may explain some of the discrepancy between
measured and predicted rut depths. The predicted and measured
soil displacements were similar at the end of the rst loading stage
(i.e. at 5000 cycles). However, the predicted soil displacement was
considerably lower than the measured one at the end of the second
and third loading stages (Fig. 13). Examining the unanchored geo-
grid after the test revealed that the geogrid was deformed and
pulled to the center of the box (a phenomenon that would not occur
in the eld) under the effect of repetitive cyclic loading. This
observation may explain the difference between the measured and
predicted soil displacement. Similar to the control test, most of thesoil displacement occurred during the rst 500 cycles of each load
increment. The amount of aggregate punching through the subgrade
layer was reduced compared to Test No. 1 but was not eliminated.
Visual inspection revealed an aggregate punching depth of about
25 mm.
During Test No. 3, the four corners of the geogrid were anchored
using steel rods driven to the bottom of the subgrade layer. The dry
unit weight of the subgrade and granular layers were 19.2 kN/m3
and 14.2 kN/m3, respectively. As a result of soil displacement, the
CBRSGvalue increased from 5 to 8 and the ELWDincreased from 8.0
to 11.0 MPa. The results show no change in the properties of the
granular layer. Partially anchoring the geogrid further decreased
the soil displacement by about 10% compared to Test No. 2 (geogrid
was not anchored). Moreover, partially anchoring the geogrid
decreased the difference between measured and predicted soil
displacement (Fig. 13). At 15,000 cycles the difference between the
measured and predicted soil displacement was 47 mm for Test No.
2 and 35 mm for Test No. 3. Visual inspection showed punching of
aggregate through the subgrade soil to a depth of about 25 mm.
The setup of Test No. 4 was similar to the previous two tests
except that the entire perimeter of the BX1 geogrid was xed to
eliminate any geogrid movement. This was accomplished by xing
the geogrid edges between the c-channels used to assemble the
wallsof thesteelbox.The purpose of eliminating geogrid movement
is to better represent the geogrid eld behavior where movement isprevented yet the geogrid can still deform and put in tension under
repeated load. The dry unit weight of the subgrade and granular
layers were 18.7 kN/m3 and 13.8 kN/m3, respectively. The CBRSGvalue increased from 4 to 7 after the test. Also, the subgrade E LWDincreased from 8.0 to 10.0 MPa. By restricting the geogrid move-
ment, the measured soil displacement was further reduced
compared to Test Nos. 2 and 3. At 15,000 cycles, the soil displace-
ment was 35% lower than that measured in Test No. 3 (75% lower
than the control test). Also, the differences between the measured
and predicted soil displacement at all three loading stages were
reduced (Fig. 13). Since locking the geogrid yielded a more repre-
sentative behavior of a geogrid installed in the eld, other
Fig. 11. Laboratory box setup (a) applying a static load to compact the soil (b) applying cyclic loading through a 150 mm loading plate.
Table 4Change in soil properties before and after each test.
Test No. Test
description
Dry unit weight of
granular layer
(kN/m3)
Dry unit weight of
subgrade layer
(kN/m3)
CBRGL CBRSG CIVGranular CIVSubgrade
Before test After test Before test After test Before test After test Before test After test
1 Control 13.4 19.0 5 6 4 9 2.9 7.1 e e
2 BX1a 14.0 18.6 5 5 4 7 3.7 5.4 3.2 6.2
3 BX1b 14.2 19.2 6 6 5 8 4.8 4.4 4.2 8.1
4 BX1 13.8 18.7 5 6 4 7 4.3 5.3 4.1 7.8
5 BX2 13.5 18.7 4 5 5 8 4.7 3.6 4.0 6.6
6 BX3 14.2 19.2 4 5 5 9 3.7 4.1 3.8 8
7 Woven geotextile 13.4 19.0 5 6 4 10 3.4 6.1 4.1 6.3
8 Non-woven geotextile 14.0 18.9 6 7 4 9 4.9 5.8 3.6 6.5
9 BX1 with RAP 14.0 19.2 5 5 4 8 4.0 4.2 4.0 7.0
a Not anchored.b Partially anchored.
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mechanical reinforcements used later in this laboratory study were
xed to the steel box in a similar manner.
5.3.3. Test nos. 5 and 6e BX2 and BX3
The soft subgrade was stabilized with BX2 in Test No. 5 and BX3
in Test No.6. The engineering properties of both geogrids are
summarized inTable 2. The dry unity weight of the subgrade and
granular layers for Test No. 5 were 18.7 kN/m3 and 13.5 kN/m3,
respectively. Afterthe test,the CBRSG value increasedfrom5 to8 and
ELWD increased from 9.0 to 12.0 MPa. The soil displacement
measuredat 15,000cycleswas about 15%higherthanTest No.4 (BX2
geogrid). However, the soil displacement was still about 70% less
than the control test.
The dry unit weight of thesubgrade and granularsoils forTest No.
6, which was stabilized with BX3, were 19.2 kN/m3 and 14,2 kN/m3,
respectively. Dueto soil densication, the CBRSG increasedfrom5 to9
andELWDincreased from 9.0 to 16.0 MPa. Similarto previous tests,the
properties of the granular layer did not change. Test No. 6 resulted ina similar soil displacement as Test No. 5 (displacements overlap in
Fig.12). This may be attributed to the somewhat similar mechanical
properties of the geogrids. It is apparent from the cumulative
displacement of Test Nos. 4, 5, and 6 that the BX1 showed better
performance; nonetheless, the other geogrid types greatly reduced
the soil displacement when compared to the control test.
5.3.4. Test nos. 7 and 8e woven and non-woven geotextiles
For Test Nos. 7 and 8, the subgrade layer was stabilized with
woven and non-woven geotextiles (Table 2). The geotextiles are
used primarily for soil separation and stabilization. The soil
densication, which occurred during Test No.7, was reected in the
increase in CBRSGfrom 4 to 10 and the increase in E LWDfrom 9.0 to
14.0 MPa. After 15,000 cycles, the soil displacement was about
78 mm equal in magnitude to the displacement measured during
the BX2 and BX3 tests. One advantage of using woven geotextiles is
the complete elimination of aggregate punching by separation of
granular and subgrade materials.
A non-woven geotextile was used to stabilize the soft subgrade
soil during Test No. 8. Similar to previous tests, the CBRSGand ELWDincreased after the test, while there was no considerable change of
the granular soil properties. For the rst and second loading stages
(10,000 cycles), the non-woven geotextile showed better perfor-
mance and managed to reduce soil displacement by up to 25%
compared to the woven geotextile test. Also, the non-woven geo-textile outperformed all geogrids. However, at the third loading
stage, the soil displacement increased rapidly exceeding that
measured during the BX1 test (Test No. 4). The displacement at
15,000 cycles was about 82 mm (70% lower than the control test).
Visual observations showed that the non-woven geotextile elimi-
nated aggregate punching through the subgrade.
Number of cycles
0 2000 4000 6000 8000 10000 12000 14000
Soildispla
cement(mm)
0
50
100
150
200
250
300Control
BX1 not anchored
BX1 partially anchored
BX1
BX2
BX3
Woven geotextile
Nonwoven geotextile
BX1 (RAP)
Fig. 12. Measured soil displacement with increasing load cycles.
Number of cycles
0 2000 4000 6000 8000 10000 12000 14000 16000
Displacement(mm)
0
20
40
60
80
100
120
140 x= Predicted rut value based
on Giroud and Han (2004)
CBRGL= 5
CBRSG= 4
Unanchored
Partiallyanchored
Fully anchored
Fig. 13. (Color) Measured and predicted soil displacement for the BX1 tests.
Measured soil displacement (mm)
0 50 100 150 200 250 300 350
Predictedsoildisplacemen
t(mm)
0
50
100
150
200
250
300
350Control
BX1 not anchored
BX1 partiallyanchored
BX1
BX2
BX3
Woven geotextile
Nonwoven geotextile
BX1 (RAP)
Fig. 14. Measured versus predicted soil displacement.
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5.3.5. Test no. 9e BX1 with recycled asphalt pavement
Due to the increasing use of recycled material in shoulder
applications, this test aimed to document the performance of RAP
material when used as a granular layer. Similar to the setup of Test
No. 4, the underlying subgrade was stabilized by placing the BX1 at
theinterface between both layers. After the test, the CBRSG andELWDvalues increase from 4 to 8 and from 8.0 to 12.0 MPa, respectively.
Compared to Test No. 4, where the granular layer comprised of
crushed limestone, the soil displacement was about 20% higher at
the end of the third loading stage. The authors attribute the differ-
ence in displacement to a less stable granular layer because of the
lower percentage ofnes content in the RAP material.
The measured and predicted soil displacements were compared
forall threeloading stages as shownin Fig.14. Overall,thereis a good
agreement between the measured and predicted soil displacement.
The design method over predictedthe soil displacement for six tests.
The range of over prediction ranged from 8% (control test) to 40%
(woven and non-woven geotextiles tests). The over estimation of
soil displacement was considerably lessfor geogrid tests(an average
of 23%) compared to 40% for the geotextiles. The design method
appears to be more applicable for geogrid reinforcements. The
design method underestimated the soil displacement for the
unanchored BX1, partially anchored BX1, and BX1 with RAP tests.
The range of under estimation ranged from 16% (BX1 with RAP) to
37% (Unanchored BX1). It is believed that the Giroud and Han
(2004a,b) method can, with an acceptable level of accuracy, esti-
mate the magnitude of rutting for granular shoulders.
6. Shoulder design charts
After demonstrated applicable, the Giroud and Han (2004a,b)
method was utilized to develop a shoulder design chart to help
design stable shoulders and mitigate rutting that occurs due to
bearing capacity failure of the subgrade. The magnitude of rutting is
predicted based on deformations in the subgrade layer only. In
other words, rutting that may occur due to degradation of the
granular layer is not accounted for. The chart can be a rapid tool for
designing new granular shoulders and provide basis for QA/QC
specications.Fig. 15shows an example of a design chart that was
developed for an unreinforced granular shoulder with a 150 mm
thick granular layer, 80 kN load, 550 kPa tire pressure, and a CBRGL
of 6, which are common parameters encountered eld values. To
use this chart, an allowable rut depth for a granular shoulder
section is selected and the corresponding CBRSG is computed for
a certain number of load cycles (N). Similar charts can be generated
for any set of shoulder parameters. Field and laboratory rut depth
measurements were compared to the design chart. Even with some
unknown eld parameterssuch as N and trafc loads,the measured
and predicted rut depths are in a relatively good agreement. The
chart was found to be simple yet practical for designing new
unreinforced granular shoulders, QA/QC, and predicting the
performance of existing granular shoulder.
7. Summary and conclusions
A common shoulder performance problem in Iowa is granular
shoulders overlying soft subgrade soils. Once developed, this
problem is both hazardous and difcult to maintain. Field obser-
vations of granular shoulder across the state of Iowa demonstrated
that many existing sections have a soft subgrade layer with a CBRSGof 10 or less. The maintenance alternative using mechanical stabi-
lization was evaluated by constructing a test section, where the soft
subgrade was stabilized using three biaxial geogrids. The shoulder
performance wasevaluated over a periodof 10 month using various
in situ tests. Overall, all three geogrid types managed to eliminate
shoulder rutting and improve the strength properties of the
shoulder section evidenced by the increase in CBR, CIV, and K with
time. Compared to chemical stabilization, geogrid stabilizationallows for a more rapidreconstruction. Further, the repaired section
can be opened immediately for trafc with no curing time required.
Other mechanical stabilization alternatives such as woven, non-
woven geotextiles, and the use of alternative shoulder granular
material like RAP were evaluated by conducting a laboratory box
study. Subjected to 15,000 cyclic loads, each stabilizer was evalu-
ated based on the cumulative measured soil displacement. The
following conclusions are withdrawn from the laboratory study:
Thehighest soil displacement was measured during the control
test, and was about 284 mm after 15,000 cycles.
To better represent eld behavior, movement of the mechan-
ical reinforcement was constrained by xing the geogrid
perimeter to the steel box. This reduced the soil displacement
Allowable rut depth (mm)
0 50 100 150 200
CBRSG
0
10
20
30
40
N = 10
N = 100
N = 1,000
N = 10,000
Field measurements
Lab measurements
CBRGL
= 6
Axle load = 80 kNGranular layerthickness = 150 mmRange of CBR values
for chemical stabilization
Range of in situCBR values
Fig. 15. Unreinforced shoulder design chart correlating CBRSG with allowable rut depth.
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and resulted in smaller differences between measured and
predicted soil displacements.
The CBR and ELWD values of the subgrade layer always
increased after the test as a result of soil densication. There
was almost no change in the properties of the granular layer.
BX1 reduced soil displacement by about 75% compared to the
control test, whereas using BX2 and BX3 reduced soil
displacement by about 70%. All geogrids, however, did not
prevent aggregate punching through the subgrade layer.
The woven and non-woven geotextiles reduced the soil
displacement by about 70% compared to the control test. The
performance of the non-woven geotextile started to deterio-
rate after the third loading stage. Both geotextiles were
successful in separating the granular and subgrade layer and
eliminating aggregate punching.
Using RAP as a granular material resulted in a 20% increase in
soil displacement compared to using crushed limestone.
TheGiroud and Han (2004a,b)semi-empirical method slightly
over predicts the soil displacements at the end of each loading
stage, except for Test Nos. 2, 3, and 9 (unanchored, partially
anchored BX1, and BX1 with RAP).
The results of the eld and laboratory studies indicate thatmechanical reinforcement as is an effective method in reducing
rutting and repairing granular shoulder overlying soft foundations.
Additional monitoring of the constructed test section is needed to
verify the long term performance and evaluate the effect of higher
trafc load and freeze-thaw cycles.
To design or repair unreinforced granular shoulders, the
provided design chart can be a simple design tool in selecting
a minimum CBRSGvalue corresponding to an allowable magnitude
of subgrade rutting. The chart may also provide bases for QA/QC.
Acknowledgments
The Iowa Department of Transportation and the Iowa Highway
Research Board sponsored this study under contract TR-531. The
authors appreciate the help of the technical steering committee in
identifying shoulder sections for investigation, and in their assist in
rening the research tasks. The authors would also like to thank
Iowa DOT personnel, Jim Howely for providing geogrid materials,
Heath Gieselman, Mike Kruse, and Amy Heurung, for their assis-
tance with eld and laboratory testing.
References
ASTM D5874, 2007. Standard test method for determination of the impact value (IV)of a soil. American Standard Testing Methods, West Conshohocken, Phila-delphia, pp. 1e9.
ASTM D6951, 2003. Standard test method for use of the dynamic cone penetrom-eter in shallow pavement applications. American Standard Testing Methods,West Conshohocken, Philadelphia, pp. 1e7.
ASTM D698, 2000. Standard test method for laboratory compaction characteristicsof soil using standard effort (12,400 ft-lbf/ft3 (600 kN-m/m3)), West Con-shohocken, Philadelphia, pp. 1e11.
Berg, R.R., Christopher, B.R., Perkins, S., 2000. Geosynthetic Reinforcement of theAggregate Base/subbase Courses of Pavement Structures. Geosynthetics Mate-rials Association, Roseville, MN, pp. 1e176.
Fannin,R.J., Sigurdsson,O., 1996. Field observations on stabilizationof unpavedroadswith geosynthetics. J ournal of Geotechnical Engineering 122 (7), 544e553.
Giroud, J.P., Han, J., 2004a. Design method for geogrid-reinforced unpaved roads. I.Development of design method. Journal of Geotechnical and GeoenvironmentalEngineering 130 (8), 775e786.
Giroud, J.P., Han, J., 2004b. Design method for geogrid-reinforced unpaved roads. II.Calibration and applications. Journal of Geotechnical and GeoenvironmentalEngineering 130 (8), 787e797.
Giroud, J.P., Noiray, L., 1981. Geotextile-reinforced unpaved road design. Journal ofthe Geotechnical Engineering Division 107 (GT9), 1233e1254. Proceedings ofthe American Society of Civil Engineers.
Hufenus, R., Rueegger, R., Banjac, R., Mayor, P., Springman, S., and Brnnimann, R.2006. Full-scale eld tests on geosynthetics reinforced unpaved roads on softsubgrade. 24 1. 21e37.
Mekkawy, M.M., White, D.J., Jahren, C.T., Suleiman, M.T., 2010. Performance prob-lems and stabilization techniques for granular shoulders. Journal of Perfor-mance of Constructed Facilities 24 (2), 159e169.
Powell, W., Keller, G.R., Brunette, B., 1999. Application for geosynthetics on forestservice low-volume roads. Transportation Research Record 1652, 113e120.
Subaida, E.A., Chandrakaran, S., Sankar, N., 2009. Laboratory performance ofunpaved roads reinforced with woven coir geotextiles. Geotextiles and Geo-membranes 27 (3), 204e210.
U.S. Army Corps of Engineers. 2003. Use of geogrids in pavement construction.Technical Letter No. 1110-1-189, Washington, D.C., pp. 1e38.
Notations
The following symbols are used in the paper:
Nc:Bearing capacity factorJ:Aperture stability modulus
CIV:Clegg Impact ValueCBRSG, CBRGL:California Bearing Ratio of the subgrade and granular layersK, KR:Modulus of subgrade reaction during load and reload cyclesELWD:Modulus measured using light weight deectometerD10, D30, and D60:Sieve size through which 10%, 30%, and 60% of the particles would
passCu:Coefcient of uniformityCc:Coefcient of gradation
%P#4: Percent passing the No. 4 sieve%P#200: Percent passing the No. 200 sieve
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