Research Article Improvement of Bearing Capacity …downloads.hindawi.com/archive/2013/293809.pdfgiven rectangular footing, silty clay soil, sand, and geogrid will depend on di erent

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 𝑢/𝐵 equal to 0.667 and the bearing capacity increaseswith an average of 33.33% while using one geogrid in middle of sand layer with 𝑢/𝐵 equal to 0.33. The improvement in bearingcapacity for sand underlain silty clay maintaining 𝑢/𝐵 and ℎ/𝐵 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 Lee’s 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 [3–9], geotextile [10–12], 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 [19–25] 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(𝑠) 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 𝐵 (footingwidth), 𝑢/𝐵 (location of the 1st layer of reinforcement towidth of footing), ℎ/𝐵 (vertical spacing between consecutivegeogrid layer to width of footing), 𝑏/𝐵 (width of the geogridlayer to width of footing), 𝐷𝑓/𝐵 (depth of footing to widthof footing), type of soil, texture, and unit weight or density ofsoil, [6, 7].

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., 𝑢/𝐵, ℎ/𝐵, 𝑏/𝐵, and 𝐷𝑓/𝐵), types ofgeosyntheticmaterials (e.g., geogrid, geotextiles, and geocell),effects of footing width (𝐵), 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 (𝐿 𝑡) 762.0mm, width (𝐵𝑡) 304.8mm, anddepth (𝐷𝑡) 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 (𝐿) equal to 284.48mm, width (𝐵) equal to 114.3mm,and thickness (𝐷) equal to 48.26mm, was used in theexperimental study. The footing dimensions were selectedbased on the model tank’s 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].

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 (𝑢), the vertical spacingbetween consecutive reinforcement layers (ℎ), and the lengthof each reinforcement layer (𝑏), 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 (𝐵) is supported by sand at the top layer andsilty clay soil at the bottom layer reinforced with 𝑁 numberof geogrid layers having a width “𝑏”. The vertical spacingbetween consecutive geogrid layers is “ℎ”. The top layer ofgeogrid is located at a depth “𝑢” measured from the baseof the model footing. The depth of reinforcement, 𝑑, belowthe bottom of the foundation can be calculated by using thefollowing:

𝑑 = 𝑢 + (𝑁 − 1) × ℎ. (1)

The magnitude of the bearing capacity ratios (BCR) for agiven rectangular footing, silty clay soil, sand, and geogridwill depend on different parameters like 𝑏/𝐵, ℎ/𝐵, 𝑢/𝐵, and𝑑/𝐵 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 𝑢/𝐵and ℎ/𝐵 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., 𝐷𝑓 = 0)having multiple layers of reinforcements for given values of𝑏/𝐵, ℎ/𝐵, and 𝑑/𝐵, the magnitude of BCRu (for unreinforcedcase) increases with 𝑢/𝐵 and attains a maximum value at(𝑢/𝐵)cr. If 𝑢/𝐵 is greater than (𝑢/𝐵)cr, themagnitude of BCRudecreases. By analyzing several test results, Shin et al. [6]determined that (𝑢/𝐵)cr for strip footing can vary between0.25 and 0.5. Similarly, for given ℎ/𝐵, 𝑢/𝐵, and 𝑑/𝐵 values,the optimum value of 𝑏/𝐵 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:

𝑢/𝐵 = 0.33, 0.67; ℎ/𝐵 = 0.33; 𝑏/𝐵 = 6.444,number of geogrid layers (𝑁): 0, 1, 2, 3, 4,length of each reinforcement layer (𝑏): 73.66 cm.

3. Methodology

The specific gravity (𝐺𝑠) 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 (𝑐) 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 (𝜙) 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, 𝐷𝑅,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.


Table 2: Different tests and parameters used in the experimental investigation.

Test no. Test types 𝑁 𝑢/𝐵 ℎ/𝐵 𝑏/𝐵1 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 (𝐺𝑠) 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 (<2𝜇m), 67.9% silt-sized particles(2𝜇m to 75𝜇m), and 2.1% sand-sized particles (75𝜇m to2mm).

The liquid limit and the plastic limit for the silty clay soilsample were measured to be 42% and 19%, respectively. Theparticle size distribution of sand sample used in the presentstudy was also presented in Figure 3. Uniformity coefficient(𝐶𝑢) and coefficient of curvature (𝐶𝑐) calculated to be 1.83and 1.89, respectively, and the effective particle size (𝐷10)is calculated to be 0.18mm. Hence, the sand is classified aspoorly graded sand (SP) according to unified soil classifiedsystem (USCS).

The results of standard Proctor compaction test for siltyclay soil is presented in Figure 3. From Figure 3 it is observedthat the maximum dry unit weight (𝛾𝑑max) and optimummoisture content (OMC) of the silty clay soil are 16.73 kN/m3and 19%, respectively.

The properties of silty clay soil used in the presentstudy are summarized in the Table 3. The results of uncon-fined compressive strength (UCS) test are also presented inTable 3.

Based on the two UCS tests, the average value of uncon-fined compressive strength is equal to 90.32 kN/m2, andundrained cohesion (𝑐) is calculated to be 45.16 kN/m2. Thephysical and engineering properties of the sand tested arepresented in Table 4.

0

10

20

30

40

50

60

70

8090

100

0.001 0.01 0.1 1 10

Fin

er (%

)

Particle size (mm)

Silty claySand

Figure 2: Particle size distribution curve for the silty clay soil andsand used in the study.

12

13

14

15

16

17

18

5 10 15 20 25 30Moisture content, w (%)

Dry

uni

t wei

ght (

kN/m

3)

Figure 3: Standard Proctor compaction curve for the silty clay soil.

4.2. Determination of Ultimate Bearing Capacity. Figure 4shows the bearing pressure versus settlement curves obtainedfrom all the tests conducted in this study. From Figure 4, itis noticed that no distinctive failure point has been observedin bearing capacity tests. Several methods are available toestimate the ultimate bearing capacity (UBC, i.e., 𝑞𝑢) from


Table 3: Properties of silty clay soil used in the present study.

Property ValuesSpecific gravity (𝐺𝑠) 2.67Liquid limit (LL), % 42.00Plastic limit (PL), % 19.00Plasticity index (PI), % 23.00Maximum dry unit weight (𝛾dmax ), kN/m

3 16.73Optimum moisture content (OMC), % 19.00Undrained cohesion (𝑐) from UCS test, kN/m2 45.16USCS classification CL

Table 4: Properties of sand used in the present study.

Property ValuesSpecific gravity (𝐺𝑠) 2.64Liquid limit (LL), % N/APlastic limit (PL), % NonplasticPlasticity index (PI), % N/AMaximum void ratio (𝑒max) 0.675Minimum void ratio (𝑒min) 0.466Relative density (𝐷𝑅) of sand, % 96.00Angle of internal friction (𝜙), (∘) 35.40Coefficient of uniformity (𝐶𝑢) 1.83Coefficient of curvature (𝐶𝑐) 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 <hyperbolic method. Out of all available methods, we used the10%width of footingmethod (i.e., 0.1 Bmethod), and tangentintersection method (TIM) to find the ultimate bearingcapacity for each case in our experimental study.

4.3. Ultimate Bearing Capacity of the Silty Clay Soil. At first,the bearing capacity test was performed on the silty clay soiland the settlement (𝑠) is expressed into a nondimensionalform by dividing the width of footing (𝐵). The bearingpressure (𝑞) versus settlement/width ratios (i.e., 𝑠/𝐵) is shownin Figure 5. By analyzing the load settlement curve, nodistinct failure point has been observed for a rectangularfooting in the silty clay soil. FromFigure 5, it can be estimatedthat the ultimate bearing capacity (𝑞𝑢) for silty clay soil isabout 172.37 kN/m2.

The bearing capacity test conducted on only sand layercompacted at 97% of its maximum density is presented inFigure 6. From Figure 6, it can be calculated that the averageultimate bearing capacity (𝑞𝑢) of sand is about 174.76 kN/m

2.

0102030405060708090

0 200 400 600 800 1000 1200

Settl

emen

t, s (

mm

)

Silty claySilty clay and sand layer

Bearing pressure (kN/m2)

1 geogrid at junction of two soil (u/B = 0.667)1 geogrid at the middle of sand layer (u/B = 0.333)2 geogrid at double layer soil system (u/B = 0.33 & h/B = 0.33)3 geogrids at double layer soil system (u/B = 0.33 & h/B = 0.33)4 geogrids at double layer soil system (u/B = 0.33 & h/B = 0.33)

Figure 4: Bearing pressure versus settlement curves for all theexperimental tests.

01020304050607080

0 200 400 600 800 1000

Settl

emen

t , s/

wid

th, B

(%)

Silty clay and sand layerSilty clay


1 geogrid at junction of two soils (u/B = 0.667)1 geogrid at middle of sand layer (u/B = 0.33)

2 geogrid at double layer soil system (u/B = 0.33 & h/B = 0.33)3 geogrid at double layer soil system (u/B = 0.33 & h/B = 0.33)4 geogrid at double layer soil system (u/B = 0.33 & h/B = 0.33)

Figure 5: Bearing pressure versus 𝑠/𝐵 (%) curves for all theexperimental tests.

4.4. Theoretical Ultimate Bearing Capacity. The theoreticalultimate bearing capacity for double-layer soil system iscalculated by using Meyerhof and Hanna’s [29] equation asfollows. They assumed that the top layer is strong sand andbottom layer is saturated soft clay.

The ultimate bearing capacity for top layer can be calcu-lated by using (2)

𝑞𝑡 = 𝛾1𝐷𝑓𝑁𝑞1𝐹𝑞𝑠1 + 0.5𝛾1𝐵𝑁𝛾1𝐹𝛾𝑠1. (2)

The ultimate bearing capacity for bottom layer can be calcu-lated by using the following:

𝑞𝑏 = (1 +0.2𝐵

𝐿

) 5.14𝑐2 + 𝛾1 (𝐷𝑓 + 𝐻) . (3)


0

5

10

15

20

25

0 40 80 120 160 200

Test 1Test 2


Settl

emen

t, s (

mm

)

Figure 6: Bearing pressure versus settlement curves for the sand.

Hence, the ultimate bearing capacity, 𝑞𝑢, for the double layersystem can be calculated by using the following:

𝑞𝑢 = (1 +0.2𝐵

𝐿

) 5.14𝑐2

+ 𝛾1𝐻2(1 +

𝐵

𝐿

)(1 +

2𝐷𝑓

𝐻

)𝐾𝑠

tan𝜙𝐵

+ 𝛾1𝐷𝑓

≤ 𝛾1𝐷𝑓𝑁𝑞1𝐹𝑞𝑠1 + 0.5𝛾1𝐵𝑁𝛾1𝐹𝛾𝑠1,

(4)

where c2 is the undrained cohesion for silty clay soil and 𝐾𝑠is the punching shear coefficient which depends on ratio of𝑞2/𝑞1 where 𝑞2/𝑞1 = 𝑐2𝑁𝑐2/0.5𝛾1𝐵𝑁𝛾1.

In the present study, the top layer is poorly graded sand(SP) with an effective particle size (𝐷10) equal to 0.18mm.With an angle of internal friction, 𝜙 = 35∘, the bearingcapacity factor, 𝑁𝑐, 𝑁𝑞, 𝑁𝛾, can be obtained as 46.12, 33.30,and 48.03, respectively. The bottom layer is local silty clay(CL)with 19%water content and the angle of internal friction,𝜙 = 0∘. For𝜙 = 0∘ the bearing capacity factors can be obtained

as𝑁𝑐 = 5.14, and𝑁𝑞 = 1,𝑁𝛾 = 0.From (4), the ultimate bearing capacity (𝑞𝑢) for double-

layer soil system can be obtained as 250.59 kN/m2. Also from(4), the bearing capacity of the top layer can be calculatedas 43.31 kN/m2, which is quite low because the model widthof the footing is only 114.3mm as compared to the realfoundation size.

4.5. Ultimate Bearing Capacity of Two-Layered Soil SystemUsing Geogrid. Five tests were conducted on double- or two-layer soil system by placing geogrid at different depths fromthe base of the footing and also varying the number of geogridlayers. Figure 4 shows the bearing pressure versus settlementcurves for all tests. There is no distinct failure point observedon bearing capacity versus settlement curve.

The 10% width of footing method and, tangent inter-section method are used to estimate the ultimate bearingcapacity for shallow foundation which is shown in Figures 7and 8, respectively. From Figure 7, it is clear that the bearing

0

5

10

15

20

25

0 50 100 150 200 250 300 350 400 450 500

Settl

emen

t, s/

wid

th, B

(%)



Figure 7: Estimation for ultimate bearing capacity (𝑞𝑢) frombearingpressure versus 𝑠/𝐵 (%), (10%BM).

0 50 100 150 200 250 300 350 400 450 500Bearing pressure (kN/m2)

0

5

10

15

20

25


Settl

emen

t, s/

wid

th, B

(%)

Figure 8: Estimation for ultimate bearing capacity (𝑞𝑢) frombearing pressure versus 𝑠/𝐵 (%), (TIM).

capacity increases with the increase in the number of geogridlayers. Out of five tests, two tests were conducted by usingone geogrid layer but at various positions, that is, the depthof geogrid from the base of footing is different. This is thecase of varying 𝑢/𝐵 (i.e., depth of first layer of geogrid/widthof footing) ratio by keeping the number of geogrid layerconstant. While in other tests, the 𝑢/𝐵 ratio (depth of firstlayer of geogrid/width of footing) and ℎ/𝐵 (consecutiveheight of two geogrids layers) ratio were kept constant butvarying the number of geogrid layer. 10%𝐵 (width of footing)method is used to find out the ultimate bearing capacity for allthese cases.The ultimate bearing capacity values with geogridlayer can be compared with unreinforced soil condition forsingle layer and also for two-layer system. The results ofvarious tests conducted on two-layer soil system with andwithout geogrids are presented in Table 5.


Table 5: Ultimate bearing capacities for different tests for two layers under different conditions.

Testno. Different conditions of two-layer soil

Ultimate bearingcapacity (kN/m2)

Percentage (%)improvement in BC BCR

10% BM TIM 10% BM TIM 10% BM TIM1 Two-layer soil 184.34 141.25 0.00 0.00 1.00 1.002 1 geogrid in interface of silty clay soil and sand layer 201.10 153.22 9.00 8.49 1.09 1.083 1 geogrid at the sand in two-layer condition 229.83 172.37 24.67 22.03 1.24 1.22

4 1 geogrid in between the sand and 1 geogrid in thejunction of two soils 248.98 201.10 35.06 42.37 1.35 1.42

5 1 geogrid in between the sand, 1 in the junction oftwo soils, and 1 in silty clay soil, respectively 277.71 210.67 50.06 49.15 1.50 1.49

6 1 geogrids in between the sand, 1 in the junction oftwo soils, 2 in silty clay soil, respectively 296.86 215.46 61.03 61.03 1.61 1.52

Table 6: Ultimate bearing capacity comparison for all tests with respect to silty clay soil only (based on 10% footing width method).

Test no. Test types Ultimate bearingcapacity (kN/m2)

Percent (%)improvement in

BCBCR

1 Silty clay soil only 172.37 0.00 1.002 Silty clay soil and the top layer sand 184.34 7.00 1.073 1 geogrid in the interface of silty clay soil and sand layer 201.10 16.67 1.164 1 geogrid at the sand in two-layer condition 229.83 33.33 1.33

5 1 geogrid in between the sand and 1 geogrid in the junction of twosoils 248.98 44.44 1.44

6 1 geogrid in between the sand, 1 in the junction of two soils, and 1 insilty clay soil, respectively 277.71 61.11 1.61

7 1 geogrid in between the sand, 1 in the junction of two soils, 2 in siltyclay soil, respectively 296.86 72.22 1.72

4.6. Improvement in Ultimate Bearing Capacity of Silty ClaySoil Using Sand andGeogrid. Thepresent experimental studyinvestigates the effect of reinforcement in the load bearingcapacity of the rectangular footing in silty clay soil. Two testswere performed without using the geogrid for a comparisonpurpose to see the effect of geogrid. The ultimate bearingcapacity obtained from the experimental investigations forreinforced cases was compared with the ultimate bearingcapacity of the unreinforced case, that is, silty clay soil only.The bearing capacity of the silty clay soil only is consideredas the reference value to be compared with the bearingcapacities of all other geogrid reinforced soil system. In allthese investigations, only one type of biaxial geogrid wasused. In these tests the 𝑢/𝐵 ratio is equal to 0.33 (the depthof 1st layer of geogrid from the footing to the width offooting) and ℎ/𝐵 (depth of consecutive layer of geogrid tothe width of footing), ratio remains the same except in onetest where only one geogrid was used at the interface ofsand layer and silty clay soil with 𝑢/𝐵 ratio of 0.667. Theresults of ultimate bearing capacity based on 10%𝐵 method,percentage improvement in bearing capacity with respect tosilty clay soil only, and bearing capacity ratio (BCR) obtainedfrom all test series are summarized in Table 6. The resultsshow that for the same quantity settlement the ultimatecarrying capacity increases with the inclusion of sand andgeogrids layers. Sitharam and Sireesh [30] have conducted

bearing capacity test of circular footing on base geogrid withgeocell-reinforced sand overlying soft clay (CL), and they alsoobserved similar test results. Khing et al. [31] have conductedmodel test to determine the bearing capacity of strip footing,and they found that themaximumbearing capacity increasedwhen geogrid has been placed at the interface between twodifferent layers of soil; the present study also observed similartrend of results. Omar et al. [32] studied the bearing capacityof strip footing with geogrid-reinforced sand with 𝑢/𝐵 andℎ/𝐵 equal to 0.33, and they found that ultimate load per unitarea with 1, 2, 3, and 4 number of geogrid was approximately150, 200, 300, 315 kN/m2, respectively. In the present studywith the same 𝑢/𝐵 and ℎ/𝐵 ratios, the ultimate bearingcapacity varies from 201.10 to 296.86 kN/m2 with the samenumber of geogrids used, when ultimate bearing capacity hasbeen calculated by using 10%BM method. Kumar et al. [33]have studied the bearing capacity of strip footing resting ontwo-layer sand, and they also found similar trend with thepresent study. Demir et al. [34] have conductedmodel studiesof circular footing resting on soft soil, and they also observeda similar trend of 𝑠/𝐷 (settlement/diameter of the footing)versus pressure diagram.

As we can see from the result that, when a small thicknessof sand layer is placed on the top of silty clay soil layer,the bearing capacity increases in small magnitude (i.e., 7%)because the sand has more strength and slightly more unit


Table 7: Comparison of ultimate bearing capacity based on 𝑢/𝐵 ratio.

No. of geogrids Test types 𝑢/𝐵 UBC (kN/m2) % improvement10% BM TIM 10% BM TIM

1 1 geogrid in the interface of silty clay soil and sand layer 0.67 248.98 153.22 N/A N/A1 1 geogrid at the sand in two-layer condition 0.33 229.83 172.37 14.28 12.50

weight as compared to the silty clay soil. After putting thegeogrids in the two-layer system the load-carrying capacityis significantly increased as compared to the bearing capacityof silty clay soil and silty clay soil with the top layer of sand;hence it can be concluded that the bearing capacity mainlyincreased from the geogrid soil interaction.The result provedthat the placement of geogrid also affects the bearing capacityin the two-layer soil system; that is, 𝑢/𝐵 ratio also affects thebearing capacity.

The experimental study was performed in the two layerof soil system; that is, a portion of the silty clay soil wasreplaced by a 76.2mm thick layer of sand at the top. Five testswere performed to evaluate the effect of geogrid layer on thesame kind of soil system. The BCR value is assumed one forthe sand overlying the silty clay soil without using geogrid.It can be taken as a reference value for the comparisonpurpose in the same arrangement; hence, it is possibleto observe the improvement in the bearing capacity afterusing the geogrid. The results are also presented in Table 5.From Table 5 it is concluded that there is a significantincrease in the bearing capacity after increasing the numberof geogrid layers. Therefore, geogrid can be considered as agood reinforcing material.

Two tests were performed with the same number ofgeogrids to evaluate the effect of the distance (𝑢) between thebase of footing and geogrid, that is, the distance of the 1stgeogrid from the base of the footing. Generally the distance isexpressed in the form of nondimensional unit as 𝑢/𝐵, where𝑢 is depth of first layer of geogrid from the base of footing and𝐵 is width of footing.The ultimate bearing capacity calculatedbased on 𝑢/𝐵 ratio is presented in Table 7. In one test the 𝑢/𝐵was kept 0.33; that is, geogrid was placed at 38.1mm fromthe base of footing and the ultimate bearing capacity of thefooting, supported by the two-layer of soil is measured to be229.83 kN/m2. In the other test the 𝑢/𝐵 was 0.667; that is,the geogrid was placed at 76.2mm from the base of footingin the two-layer system of soil, and the bearing capacity wasmeasured to be 248.98 kN/m2. These results show that, if the𝑢/𝐵 increases, the bearing capacity increases. These resultsare consistent with other studies which show the effect of 𝑢/𝐵ratio on the bearing capacity of various footing supported ondifferent kinds of soils. It has been observed that the bearingcapacity increases as the 𝑢/𝐵 ratio increases, and the presentstudy has also shown the similar trend in case of two-layersoil system.

Figure 9 shows the effect of number of the geogrid layerson the two-layer soil system. The ultimate bearing capacityincreases with the increase in the number of geogrid layers.In the beginning the improvement is more significant ascompared to last stage so that it can be concluded that the top

1

1.2

1.4

1.6

1.8

2

0 1 2 3 4 5

BCR

Number of geogrid (N)

Figure 9: Bearing capacity ratio (BCR) with the number of geogridsin two layer soil systems.

layer of geogrid has a more contribution to the improvementof bearing capacity of silty clay soil. Omar et al. [32] alsoobserved similar trend with BCR approximately equal to 3.8and with 𝑢/𝐵 equal to 0.33, whereas in present study withsame 𝑢/𝐵 ratio BCR is approximately 1.61 with the samenumber of geogrid layers.

5. Conclusions

The present study investigates the effect of geogrids on sandlayer underlain by silty clay soil towards the improvement ofbearing capacity of rectangular footing.The silty clay soil andsand used are classified as CL and SP, respectively, based onthe Unified Soil Classification System (USCS).

A number of model tests were performed to evaluatethe load-carrying capacity of a rectangular model footingsupported on the silty clay soil overlaid with small thicknessof sand and with inclusion of geogrids at different depthsfrom the base of the footing. Based on the model tests, thefollowing conclusions are drawn.

(i) The load-carrying capacity of the silty clay soilobtained from Carbondale, Illinois, increased by 7%when the top of the silty clay soil was replaced with76.2mm thick layer of sand.

(ii) The bearing capacity for the two-layer soil increaseswith an average of 16.67% using one geogrid layer atinterface of soil (i.e., silty clay soil and sand) with the𝑢/𝐵 equal to 0.667. The bearing capacity for the two-layer soil increases with an average of 33.33% whileusing one geogrid in the middle of sand layer havingthe 𝑢/𝐵 equal to 0.33.

(iii) The improvement in bearing capacity for double layersoil maintaining 𝑢/𝐵 and ℎ/𝐵 equal to 0.33; for two,


three and four number geogrid layer were 44.44%,61.11%, 72.22%, respectively.

(iv) Bearing capacity is also dependent on 𝑢/𝐵 ratio; thatis, bearing capacity is higher if the 𝑢/𝐵 is higher.

Based on the results of this study, it is concluded that thebearing capacity of the silty clay soil can be improved by usingthe geogrid. The finding of this research work may be usefulto improve the strength of soil for foundation and pavementdesign for a specific area or similar types of soils availableelsewhere.

Acknowledgments

The authors would like to acknowledge Professor V. K. Purifor his guidance on experimentation and critical commentsthroughout the study. The authors also would like to thankMr. John Hester of Geotech laboratory for fabricating the testtank and instrumentation.

References

[1] J. Binquet and K. L. Lee, “Bearing capacity tests on reinforcedearth slabs,” Journal of Geotechnical Engineering Division, vol.101, no. 12, pp. 1241–1255, 1975.

[2] J. Binquet andK. L. Lee, “Bearing capacity analysis of reinforcedearth slabs,” Journal of Geotechnical Engineering Division, vol.101, no. 12, pp. 1257–1276, 1975.

[3] G. W. E. Milligan, R. J. Fannin, and D. M. Farrar, “Model andfull-scale tests on granular layers reinforced with a geogrid,” inProceedings of the 3rd International Conference on Geotextiles,vol. 1, pp. 61–66, Vienna, Austria, 1986.

[4] K. H. Khing, B. M. Das, V. K. Puri, S. C. Yen, and E. E.Cook, “Foundation on strong sand underlain by weak clay withgeogrid at the interface,”Geotextiles and Geomembranes, vol. 13,no. 3, pp. 199–206, 1994.

[5] B. M. Das, K. H. Khing, and E. C. Shin, “Stabilization ofweak clay with strong sand and geogrid at sand-clay interface,”Transportation Research Record, no. 1611, pp. 55–62, 1998.

[6] E. C. Shin, B. M. Das, V. K. Puri, S. C. Yen, and E. E. Cook,“Bearing capacity of strip foundation on geogrid-reinforcedclay,” Geotechnical Testing Journal, vol. 17, no. 4, pp. 535–541,1993.

[7] C. R. Patra, B. M. Das, and C. Atalar, “Bearing capacityof embedded strip foundation on geogrid-reinforced sand,”Geotextiles andGeomembranes, vol. 23, no. 5, pp. 454–462, 2005.

[8] B. R. Phanikumar, R. Prasad, and A. Singh, “Compressive loadresponse of geogrid-reinforced fine, medium and coarse sands,”Geotextiles and Geomembranes, vol. 27, no. 3, pp. 183–186, 2009.

[9] Y. L.Dong, J.Han, andX.-H. Bai, “Bearing capacities of geogrid-reinforced sand bases under static loading,” in Proceedings ofGeoShanghai International Conference: Ground Improvementand Geosynthetics, pp. 275–281, June 2010.

[10] V. A. Guido, D. K. Chang, and M. A. Sweeney, “Comparisonof geogrid and geotextile reinforced earth slabs,” CanadianGeotechnical Journal, vol. 23, no. 4, pp. 435–440, 1986.

[11] J. P. Sakti and B. M. Das, “Model tests for strip foundation onclay reinforced with geotextile layers,” Transportation ResearchRecord, no. 1153, pp. 40–45, 1987.

[12] P. K. Basudhar, S. Saha, andK.Deb, “Circular footings resting ongeotextile-reinforced sand bed,”Geotextiles andGeomembranes,vol. 25, no. 6, pp. 377–384, 2007.

[13] J. O. Akinmusuru and J. A. Akinbolade, “Stability of loadedfootings on reinforced soil,” Journal of the Geotechnical Engi-neering Division, vol. 107, no. 6, pp. 819–827, 1981.

[14] T. Yetimoglu, M. Inanir, and O. E. Inanir, “A study on bearingcapacity of randomly distributed fiber-reinforced sand fillsoverlying soft clay,” Geotextiles and Geomembranes, vol. 23, no.2, pp. 174–183, 2005.

[15] R. J. Fragaszy and E. Lawton, “Bearing capacity of reinforcedsand subgrades,” Journal of Geotechnical Engineering, vol. 110,no. 10, pp. 1500–1507, 1984.

[16] C.-C. Huang and F. Tatsuoka, “Bearing capacity of reinforcedhorizontal sandy ground,” Geotextiles and Geomembranes, vol.9, no. 1, pp. 51–82, 1990.

[17] S. K. Dash, N. R. Krishnaswamy, and K. Rajagopal, “Bearingcapacity of strip footings supported on geocell-reinforced sand,”Geotextiles and Geomembranes, vol. 19, no. 4, pp. 235–256, 2001.

[18] S. K. Dash, S. Sireesh, and T. G. Sitharam, “Behaviour ofgeocell-reinforced sand beds under circular footing,” GroundImprovement, vol. 7, no. 3, pp. 111–115, 2003.

[19] B. M. Das and M. T. Omar, “The effects of foundation widthon model tests for the bearing capacity of sand with geogridreinforcement,”Geotechnical andGeological Engineering, vol. 12,no. 2, pp. 133–141, 1994.

[20] T. Yetimoglu, J. T. H. Wu, and A. Saglamer, “Bearing capacityof rectangular footings on geogrid-reinforced sand,”Geotextilesand Geomembranes, vol. 23, no. 2, pp. 174–183, 1994.

[21] K. H. Khing, B. M. Das, V. K. Puri, E. E. Cook, and S. C.Yen, “The bearing-capacity of a strip foundation on geogrid-reinforced sand,” Geotextiles and Geomembranes, vol. 12, no. 4,pp. 351–361, 1993.

[22] M. T. Adams and J. G. Collin, “Large model spread footing loadtests on geosynthetic reinforced soil foundations,” Journal ofGeotechnical and Geoenvironmental Engineering, vol. 123, no. 1,pp. 66–72, 1997.

[23] M. T. Omar, B. M. Das, V. K. Puri, and S. C. Yen, “Ultimatebearing capacity of shallow foundations on sand with geogridreinforcement,” Canadian Geotechnical Journal, vol. 30, no. 3,pp. 545–549, 1993.

[24] E. C. Shin, B. M. Das, E. S. Lee, and C. Atalar, “Bearing capacityof strip foundation on geogrid-reinforced sand,” Geotechnicaland Geological Engineering, vol. 20, no. 2, pp. 169–180, 2002.

[25] N. C. Samtani and R. C. Sonpal, “Laboratory tests of stripfooting on reinforced cohesive soil,” Journal of GeotechnicalEngineering, vol. 115, no. 9, pp. 1326–1330, 1989.

[26] J. H. Yin, “Modelling geosynthetic-reinforced granular fills oversoft soil,” Geosynthetics International, vol. 4, no. 2, pp. 165–185,1997.

[27] B. A. Cerato, Scale effects of shallow foundation bearing capacityon granular material [Ph.D. thesis], University of MassachusettsAmherst, Amherst, Mass, USA, 2005.

[28] A. J. Lutenegger andM. T. Adams, “Bearing capacity of footingson compacted sand,” in Proceedings of the 4th InternationalConference on Case Histories in Geotechnical Engineering, pp.1216–1224, 1998.

[29] G. G. Meyerhof and A. M. Hanna, “Ultimate bearing capacityof foundations on layered soils under inclined load,” CanadianGeotechnical Journal, vol. 15, no. 4, pp. 565–572, 1978.


[30] T. G. Sitharam and S. Sireesh, “Effects of geogrid on geocell-reinforced foundation beds,”Geomechanics andGeoengineering:An International Journal, vol. 1, no. 3, pp. 207–216, 2006.

[31] K. H. Khing, B. M. Das, V. K. Puri, S. C. Yen, and E. E. Cook,“Strip foundation on sand underlain by soft clay with geogridreinforcement,” in Proceedings of the 3rd International Offshoreand Polar Engineering Conference, pp. 517–521, Singapore, June1993.

[32] M. T. Omar, B. M. Das, S. C. Yen, V. K. Puri, and E. E.Cook, “Ultimate bearing capacity of rectangular foundations ongeogrid-reinforced sand,” Geotechnical Testing Journal, vol. 16,no. 2, pp. 246–252, 1993.

[33] A. Kumar, M. L. Ohri, and R. K. Bansal, “Bearing capacity testsof strip footings on reinforced layered soil,” Geotechnical andGeological Engineering, vol. 25, no. 2, pp. 139–150, 2007.

[34] A. Demir, M. Ornek,M. Laman, A. Yildiz, and G.Misir, “Modelstudies of circular foundations on soft soils,” in Proceedings ofthe 2nd International Workshop on Geotechnics of Soft Soils, M.Karstunen and M. Leoni, Eds., pp. 219–226, Taylor and Francis,London, UK, September 2008.

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Research Article Improvement of Bearing Capacity …downloads.hindawi.com/archive/2013/293809.pdfgiven rectangular footing, silty clay soil, sand, and geogrid will depend on di erent