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; pkolay@siu.edu

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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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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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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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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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