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Hindawi Publishing CorporationJournal of Construction
EngineeringVolume 2013, Article ID 293809, 10
pageshttp://dx.doi.org/10.1155/2013/293809
Research ArticleImprovement of Bearing Capacity of Shallow
Foundation onGeogrid Reinforced Silty Clay and Sand
P. K. Kolay, S. Kumar, and D. Tiwari
Department of Civil and Environmental Engineering, Southern
Illinois University Carbondale, 1230 Lincoln Drive, MC
6603,Carbondale, IL 62901, USA
Correspondence should be addressed to P. K. Kolay;
[email protected]
Received 6 December 2012; Revised 12 May 2013; Accepted 26 May
2013
Academic Editor: Mohammed Sonebi
Copyright 2013 P. K. Kolay et al. This is an open access article
distributed under the Creative Commons Attribution License,which
permits unrestricted use, distribution, and reproduction in any
medium, provided the original work is properly cited.
The present study investigates the improvement in the bearing
capacity of silty clay soil with thin sand layer on top and
placinggeogrids at different depths. Model tests were performed for
a rectangular footing resting on top of the soil to establish the
loadversus settlement curves of unreinforced and reinforced soil
system.The test results focus on the improvement in bearing
capacityof silty clay and sand on unreinforced and reinforced soil
system in non-dimensional form, that is, BCR. The results show
thatbearing capacity increases significantly with the increased
number of geogrid layers. The bearing capacity for the soil
increaseswith an average of 16.67% using one geogrid layer at
interface of soils with / equal to 0.667 and the bearing capacity
increaseswith an average of 33.33% while using one geogrid in
middle of sand layer with / equal to 0.33. The improvement in
bearingcapacity for sand underlain silty clay maintaining / and /
equal to 0.33; for two, three and four number geogrid layer
were44.44%, 61.11%, 72.22%, respectively. The finding of this
research work may be useful to improve the bearing capacity of soil
forshallow foundation and pavement design for similar type of soil
available elsewhere.
1. Introduction
The use of geosynthetic materials to improve the bearingcapacity
and settlement performance of shallow foundationhas gained
attention in the field of geotechnical engineering.For the last
three decades, several studies have been con-ducted based on the
laboratory model and field tests, relatedto the beneficial effects
of the geosynthetic materials, on theload bearing capacity of soils
in the road pavements, shallowfoundations, and slope
stabilizations. The first systematicstudy to improve the bearing
capacity of strip footing by usingmetallic strip was by Binquet and
Lee [1, 2]. After Binquetand Lees work, several studies have been
conducted on theimprovement of load bearing capacity of shallow
foundationssupported by sand reinforced with various reinforcing
mate-rials such as geogrids [39], geotextile [1012], fibers [13,
14],metal strips [15, 16], and geocell [17, 18].
Several researches have demonstrated that the ultimatebearing
capacity and the settlement characteristics of thefoundation can be
improved by the inclusion of reinforce-ments in the ground. The
findings from several laboratory
model tests and a limited number of field tests have
beenreported in the literature [1925] which relates the
ultimatebearing capacity of shallow foundations supported by
sandreinforced with multiple layers of geogrid. Recently, Yin
[26]compiled extensive literature in the handbook of
geosyntheticengineering on reinforced soil for shallow foundation.
Forthe design of shallow foundations in the field, the
settlementbecomes the controlling criteria rather than the
bearingcapacity. Hence, it is important to evaluate the
improvementin the bearing capacity of foundations at particular
settlement() level. From the finding of numerous researchers, it
canbe concluded that the bearing capacity of soil also changedwith
various factors like type of reinforcingmaterials, numberof
reinforcement layers, ratios of different parameters ofreinforcing
materials, and foundations such as (footingwidth), / (location of
the 1st layer of reinforcement towidth of footing), / (vertical
spacing between consecutivegeogrid layer to width of footing), /
(width of the geogridlayer to width of footing), / (depth of
footing to widthof footing), type of soil, texture, and unit weight
or density ofsoil, [6, 7].
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2 Journal of Construction Engineering
Table 1: Properties of biaxial geogrid used in the present
study.
Index property MD values XMD valuesAperture size, mm 25.00 33.00
25.00 33.00Minimum rib thickness, mm 0.76 0.76Tensile strength @ 2%
strain, kN/m 4.10 6.60Tensile strength @ 5% strain, kN/m 8.50
13.40Ultimate tensile strength, kN/m 12.40 19.00Structural
integrity
Junction efficiency, (%) 93.00Flexural stiffness, Mg-cm
250,000Aperture stability, m-N/deg 0.32
DurabilityResistance to installation damage, %SC/%SW/%GP
95/93/90Resistance to long-term degradation, % 100Resistance to UV
degradation, % 100
Out of several studies, very few studies are available onthe
two-layer soils. Generally, all the studies are ultimatelyrelated
to improvement in the bearing capacity of soil usingreinforcing
materials and related to the effect of variousparameters on bearing
capacity. The ratio of improvement inthe bearing capacity can be
expressed in a nondimensionalform as bearing capacity ratio (BCR)
which is the ratio ofbearing capacity of reinforced soil to bearing
capacity ofunreinforced soil. Several studies [5, 6, 26] show the
effect ofvarious parameters (e.g., /, /, /, and /), types
ofgeosyntheticmaterials (e.g., geogrid, geotextiles, and
geocell),effects of footing width (), types of soils, layer of
soils, andso forth. But no studies are available on silty clay soil
ofCarbondale, Illinois, related to the improvement in
bearingcapacity of rectangular footing by placing sand layer on top
ofsilty clay soil (i.e., two-layered soil) and geogrid system.
Mostof the studies either used sand or clay only and used geogridas
the reinforcing material.The present study investigates thebearing
capacity of two layers of soil (i.e., a thin sand layerunderlain by
silty clay) and also of single-layer silty clay soil(for comparison
purpose) with varying the number of biaxialgeogrid at different
layers and by keeping other propertiesconstant.
2. Experimental Study
2.1. Materials Used. Two types of soils were used to conductthe
experimental study, that is, silty clay soil and sand.
2.2. Silty Clay Soil and Sand. The silty clay soil sample
wascollected from New Era Road in Carbondale, Illinois.
Thecollected soil was sun-dried, pulverized, and passed throughUS
sieve # 10 (i.e., 2mm) for different physical,
engineeringproperties and bearing capacity test. The properties of
thesilty clay soil were determined in the laboratory by perform-ing
several tests using respective ASTM standard. A thin layerof sand
was placed on top of silty clay soil (two-layer soilsystem) to
evaluate the improvement on load bearing capacityof the silty clay
soil.
2.3. Geogrids. Biaxial geogrid was used in the present
exper-imental study. Biaxial geogrid has tensile strength in
twomutually perpendicular directions so that it gives morestrength
to the soil. Different properties of the biaxial geogridare
presented in Table 1.
2.4. Model Test Tank. Amodel test tank with the dimensionshaving
length ( ) 762.0mm, width () 304.8mm, anddepth () 749.3mmwas
designed and fabricated to performthe test. The horizontal and
vertical sides of the model tankare stiffened by using steel angle
sections at the top, bottom,and middle of the tank to avoid any
lateral yielding duringsoil compaction in the tank and also while
applying load atmodel footing during the experiment. Two side walls
of thetank were made of 25.4mm thick Plexiglas plates, and theother
two side walls of the tank were made of 12.7mm thickPlexiglas
plates, and these were also supported by 19.05mmwooden plates. The
inside walls of the tank were smooth toreduce the side
friction.
2.5. Model Footing. Amodel footing, with the dimensions oflength
() equal to 284.48mm, width () equal to 114.3mm,and thickness ()
equal to 48.26mm, was used in theexperimental study. The footing
dimensions were selectedbased on the model tanks dimension.Themodel
footing wasdesigned in such a way that its width is less than 6.5
times thedepth of themodel tank so that the effect of the load
could notreach the bottom of tank. The bottom surface of the
modelfooting was made rough by cementing a layer of sand withepoxy
glue to increase the friction between the footing baseand the top
soil layer. Also a 12.7mm thick steel plate wasused at the top of
the model footing to reduce bending whileapplying the load.
2.6. Laboratory Model Tests. In the present study the siltyclay
soil was used at the bottom part of the model tankoverlaid by a
small thickness of sand layer at the top.The criterion of selection
of the thickness of the top sandlayer is based on the studies by
previous researchers [4].
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Journal of Construction Engineering 3
In the geogrid reinforced model tests, the optimum valuesrelated
to the reinforcement arrangement, such as the loca-tion of the
first layer of reinforcement (), the vertical spacingbetween
consecutive reinforcement layers (), and the lengthof each
reinforcement layer (), were adopted based on themodel tank size
and findings from the previous researchers.
Figure 1 shows the cross sectional view of the model tankand the
model footing with two-layer soil system havingdifferent
reinforcement layers.Themodel rectangular footingwith width () is
supported by sand at the top layer andsilty clay soil at the bottom
layer reinforced with numberof geogrid layers having a width . The
vertical spacingbetween consecutive geogrid layers is . The top
layer ofgeogrid is located at a depth measured from the baseof the
model footing. The depth of reinforcement, , belowthe bottom of the
foundation can be calculated by using thefollowing:
= + ( 1) . (1)
The magnitude of the bearing capacity ratios (BCR) for agiven
rectangular footing, silty clay soil, sand, and geogridwill depend
on different parameters like /, /, /, and/ ratios. In order to
conduct model tests with geogridreinforcement in two-layer soil
system, that is, silty clay soiland sand, it is important to decide
the magnitude of /and / to get the improvement of the bearing
capacity fora particular footing. Earlier researchers [10, 13, 14]
foundthat, for a model footing resting on surface (i.e., = 0)having
multiple layers of reinforcements for given values of/, /, and /,
the magnitude of BCRu (for unreinforcedcase) increases with / and
attains a maximum value at(/)cr. If / is greater than (/)cr,
themagnitude of BCRudecreases. By analyzing several test results,
Shin et al. [6]determined that (/)cr for strip footing can vary
between0.25 and 0.5. Similarly, for given /, /, and / values,the
optimum value of / for surface foundation condition toget
themaximum increase in BCRu with using reinforcementcan vary from 6
to 8 for strip foundations [21]. By consideringthe previous
findings, it was decided to adopt the followingparameters for the
present study:
/ = 0.33, 0.67; / = 0.33; / = 6.444,number of geogrid layers ():
0, 1, 2, 3, 4,length of each reinforcement layer (): 73.66 cm.
3. Methodology
The specific gravity () of silty clay soil and sand sample
wasdetermined by using the ASTM D 854 method. For the sakeof
accuracy, the average specific gravity is obtained from theresults
of three tests. The standard Proctor compaction testwas conducted
as per ASTM D 698 method to determinethe maximum dry density and
optimum moisture content(OMC). The particle size distribution of
the silty clay soiland sand samples was obtained by using dry sieve
as wellas hydrometer analyses according to ASTM D 422. ASTMD 4318
method was used to determine the liquid limit andplastic limit of
the silty clay soil, and ASTM D 2166 method
d=152.4
mm
Dt=749.3
mm
Section
b
h
h
h = 38.1mm
u = 38.1mm Sand layer
Silty clay oil
B
1
2
3
N
Figure 1: Layout of geogrid spacing in cross section of model
tankand footing.
has been used for the unconfined compression strength(UCS) test
to determine the cohesion () of the silty clay soil.The maximum
index density (i.e., minimum void ratio) andthe minimum index
density (i.e., maximum void ratio) of thesand samples were obtained
according to ASTM D 4253 andASTMD4254methods, respectively. For
theminimum indexunit weight, a small funnel was used to pour the
sand inmoldfrom a small height (i.e., 25.4mm) and for the
maximumindex unit weight; the sand was vibrated for 10
minutes.Direct shear test has been conducted to determine the
frictionangle () of the sand sample by using the method mentionedin
ASTM D 3080.
The processed silty clay soil sample was kept in a bigcontainer,
and then 19% water (i.e., OMC of the silty claysoil) was added to
the soil and mixed thoroughly to make auniform homogeneous mixture.
Before running the tests inthe model tank, the moisture content was
checked for soilwater mixture. To obtain a uniform density, the
silty clay soilwas compacted in 13 layers up to an approximately
673.1mmdepth of the model test tank. An approximately 12.25 kg
flatround hammer was used to compact the silty clay soil in
eachlayer.
In the model test tank, the unit weight of the siltyclay soil
was 86.8% of the maximum dry unit weight atits optimum moisture
content (OMC). After compactionof the silty clay soil in the model
tank up to 673.1mm, a76.2mm thick sand layer was placed above the
compactedsilty clay. For the bearing capacity tests, sand sample
wascompacted in two layers with a thickness of 76.2mm ineach layer.
Biaxial geogrid reinforcements were placed at pre-determined depths
below the base of the model footing. Themodel footing was placed at
the top of sand layer. All testswere conducted at a constant
relative density of sand, ,equal to 96% of sand and relative
compaction of silty clay soil,that is, 86.8% of the maximum dry
unit weight of silty clay.The load was applied to the model footing
by using a manualhydraulic pump system with a capacity of an
approximately44.48 kN. The loading rate was kept constant in every
test.The load and corresponding foundation settlement weremeasured
by using a load cell and a dial gauge, respectively.
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4 Journal of Construction Engineering
Table 2: Different tests and parameters used in the experimental
investigation.
Test no. Test types / / /1 Silty clay soil only 0 0 0 02 Sand
only 0 0 0 03 Local soil and sand layer 0 0 0 04 1 geogrid in the
interface of silty clay soil and sand layer 1 0.67 0 6.445 1
geogrid at the middle of sand layer in two-layer soil 1 0.33 0
6.44
6 1 geogrid at the middle of sand layer and 1 geogrid in the
interface oftwo soils 2 0.33 0.33 6.44
7 1 geogrid at the middle of sand layer, 1 in the interface of
two soils,and 1 in silty clay soil, respectively 3 0.33 0.33
6.44
8 1 geogrid at the middle of sand layer, 1 in the interface of
two soils,and 1 in silty clay soil, respectively 4 0.33 0.33
6.44
In the present study, different tests that were conducted
forsilty clay soil, sand, and two-layer soil system with
varyingnumbers of geogrid layers are presented in Table 2.
4. Results and Discussion
4.1. Physical and Engineering Properties of Silty Clay Soiland
Sand. The results of various physical and engineeringproperties of
silty clay and sand are presented here. Theresults of specific
gravity () test for the silty clay and sandwere measured to be 2.67
and 2.64, respectively.
The particle size distribution curve for the silty claysoil
obtained from sieve analysis and hydrometer tests ispresented in
Figure 2. From Figure 2, it is clear that 97.9% ofthe soil passed
through the US sieve # 200. The soil consistsof 30% clay-sized
particles (
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Journal of Construction Engineering 5
Table 3: Properties of silty clay soil used in the present
study.
Property ValuesSpecific gravity () 2.67Liquid limit (LL), %
42.00Plastic limit (PL), % 19.00Plasticity index (PI), %
23.00Maximum dry unit weight (dmax ), kN/m
3 16.73Optimum moisture content (OMC), % 19.00Undrained cohesion
() from UCS test, kN/m2 45.16USCS classification CL
Table 4: Properties of sand used in the present study.
Property ValuesSpecific gravity () 2.64Liquid limit (LL), %
N/APlastic limit (PL), % NonplasticPlasticity index (PI), %
N/AMaximum void ratio (max) 0.675Minimum void ratio (min)
0.466Relative density () of sand, % 96.00Angle of internal friction
(), () 35.40Coefficient of uniformity () 1.83Coefficient of
curvature () 0.89USCS classification SP
the bearing pressure versus the settlement curve. Eachmethod
gives a different value of the ultimate bearing capacityis and it
hard to decide which method is more accurate.Currently, four
methods are available to estimate the failureof a shallow
foundation, based on load settlement curves, butif there is no
distinct failure pattern of the foundation/soilsystem available,
the values obtained by using differentmethods have the following
order [27, 28]: log-log method< tangent intersection method
(TIM) < 0.1 B method