lable at ScienceDirect

Geotextiles and Geomembranes 41 (2013) 26e35

Contents lists avai

Geotextiles and Geomembranes

journal homepage: www.elsevier .com/locate/geotexmem

Improved performance of soft clay foundations using stone columnsand geocell-sand mattress

Sujit Kumar Dash a, *, Mukul Chandra Bora b

a Department of Civil Engineering, Indian Institute of Technology Kharagpur, Kharagpur 721 302, Indiab Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati 781 039, India

a r t i c l e i n f o

Article history:Received 23 November 2012Received in revised form23 August 2013Accepted 31 August 2013Available online

Keywords:FoundationGeosyntheticsStone columnsGround improvement

* Corresponding author. Tel.: þ91 3222 282418; faxE-mail address: sujit@civil.iitkgp.ernet.in (S.K. Das

0266-1144/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.geotexmem.2013.09.001

a b s t r a c t

A series of experiments have been carried out to develop an understanding of the performanceimprovement of soft clay foundation beds using stone column-geocell sand mattress as reinforcement. Itis found that with the provision of stone columns, of adequate length and spacing, about three foldincreases in bearing capacity can be achieved. While with geocell-sand mattress it is about seven timesthat of the unreinforced clay. But if combined together, the stone column-geocell mattress compositereinforcement, can improve the bearing capacity of soft clay bed as high as by ten fold. The optimumlength and spacing of stone columns giving maximum performance improvement are, respectively, 5times and 2.5 times of their diameter. The critical height of geocell mattress can be taken equal to thediameter of the footing, beyond which, further increase in bearing capacity of the composite foundationbed is marginal.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Rapid urbanisation and growth of infrastructure, in the presentdays, has resulted in dramatically increased demand for land space.This has compelled the building industry to improve the soft soilgrounds which otherwise are unsuitable for construction activities.Amongst the various ground improvement techniques used, stonecolumns and geosynthetic reinforcement are probably the mostpopular ones. This is primarily due to their simplicity, ease ofconstruction and overall economy that finds favour with thepracticing engineers.

A stone column is a column of stones, made through opening upa vertical cylindrical hole in the soft clay bed and subsequentlyfilling it up with compacted stone aggregates. Due to higherstrength and stiffness, the stone columns sustain larger proportionof the applied load, than their soft soil counterpart, leading tosignificant performance improvement of foundation beds (Hughesand Withers, 1974; Juran and Guermazi, 1988; Christoulas et al.,2000, Wood et al., 2000, McKelvey et al., 2004, Ambily andGandhi, 2007; Black et al., 2007, Cimentada et al., 2011, Dash andBora, 2013). Moreover, being highly permeable the stone columnsact as vertical drains facilitating consolidation of the soft clayaround and thereby improving the long term performance of thefoundation system.

: þ91 3222 282254.h).

All rights reserved.

Geocell reinforcement is a latest development in the avenues ofgeosynthetics. It is a three dimensional, polymeric, honeycomb likestructure of cells interconnected at joints that the reinforcingmechanism is primarily through all-round confinement of soils.Besides, geocells intercept the potential failure planes and theirrigidity forces them deeper into the foundation soil. This induces ahigher surcharge loading on the failure plane, giving rise toincreased load carrying capacity (Webster and Watkins, 1977; Bushet al., 1990; Cowland and Wong, 1993; Dash et al., 2001, 2003a,2003b, 2004; Zhou and Wen, 2008; Sireesh et al., 2009;Leshchinsky and Ling, 2013; Tanyu et al., 2013).

Review of literature shows that geocell-sand mattress and stonecolumns are effective means of performance improvement of softclay foundations. Their individual applications though have beenintensely studied, but combined application of both has remainedunexplored. It is expected that the geocell-sandmattress with stonecolumns underneath shall further enhance the load carrying ca-pacity of the foundation system. Moreover, a cushion of sand isgenerally provided over the stone columns for the purpose ofdrainage. Limited research reported in the literature indicates thatthis sand layer when reinforced with planar geosynthetics cannoticeably improve the bearing capacity of the foundation system(Deb et al., 2007, Abdullah and Edil, 2007; Deb et al., 2011). Arulrajahet al. (2009) have reported the use of a geogrid-soil platform overstone columns in the construction of high speed railway embank-ments in Malaysia. In this arrangement the reinforced soil cushionserves as a flexible raft over the stone columns, similar to that of a

LStone column

Clay

Sand h

u

Geocell layer

DT DTDT DTDT DTDTDT

Footing

D D D

(0,0) x/Dx/D

Fig. 1. Schematic diagram of test configuration.transverse member diagonal member

bodkin joint

b

b

Fig. 3. Geocell system in chevron pattern: plan view.

Table 1Details of model tests.

Testseries

Typeof reinforcement

Details of parameters investigated

1 Unreinforced clay bed with cu of 5 kPa2 SC Variable parameter: L/dsc ¼ 1, 3, 5, 7

Constant parameter: S/dsc ¼ 2.53 SC Variable parameter: S/dsc ¼ 1.5, 2.5, 3.5

Constant parameter: L/dsc ¼ 54 GC Variable parameter: h/D ¼ 0.53, 0.9, 1.1, 1.6

Constant parameter: dgc/D ¼ 0.8, b/D ¼ 65 GC þ SC Variable parameter: L/dsc ¼ 1, 3, 5, 7

Constant parameter:h/D ¼ 0.53, dgc/D ¼ 0.8, b/D ¼ 6, S/dsc ¼ 2.5

6 GC þ SC Variable parameter: L/dsc ¼ 1, 3, 5, 7Constant parameter:h/D ¼ 0.9, d /D ¼ 0.8, b/D ¼ 6, S/d ¼ 2.5

S.K. Dash, M.C. Bora / Geotextiles and Geomembranes 41 (2013) 26e35 27

piled raft system leading to improved load capacity. However, geo-cell is a superior form of reinforcement over the planar one (Dashet al., 2004; Madhavi and Vidya, 2007; Latha and Somwanshi,2009). This is mostly due to its three dimensional confining struc-ture that prevents lateral spreading of soil. Therefore, the sandcushion over the stone columns if reinforced with geocells is ex-pected to produce enhanced performance improvement. These as-pects are studied herein through a series of laboratory-scale modeltests. The results have been analysed in developing an under-standing of the behaviour of clay foundations reinforced with thestone column-geocell mattress composite system.

2. Details of model tests

2.1. Planning of experiments

Schematic sketch of a typical test configuration is shown inFig. 1. The stone columns were left floating in the clay bed. This wasto simulate the situation commonly encountered in coastal areaswherein soft clay deposits extend over very large depths that thestone columns are generally terminated in the clay itself. The col-umns were placed in triangular pattern at a regular spacing, S(Fig. 2). In all the tests, diameter of stone columns (dsc) was keptconstant as 100 mm.

Geocells were formed in chevron pattern (Fig. 3) as it givesbetter performance improvement over the diamond pattern (Dashet al., 2001). Diameter of geocells (dgc), taken as equivalent diam-eter of geocell pocket opening, was kept constant as 0.8Dthroughout (D, diameter of footing). The geocell mattresses wereplaced at a constant depth (u) of 0.1D from the base of the footing,which was found to be the optimum location giving maximumperformance improvement (Dash et al., 2008).

S

dsc

S

Stone column

Fig. 2. Layout of stone columns.

In total twelve series of model load tests were conducted thedetails of which are presented in Table 1. Within each series, onlyone parameter was varied. This was to understand the influence ofthis particular parameter on the overall behaviour of the founda-tion system, while the others were kept constant. Tests in series 1were performed on unreinforced clay beds. Series 2 and 3 consistedof testing the stone column reinforced clay beds, wherein, the in-fluence of length (L) and spacing (S) of the columnswere studied. Inall these tests there was no sand cushion over the clay beds. Theeffect of height of geocell-sand mattress (h) was studied underseries 4. Subsequently, tests in series 5e12 were designed to

gc sc

7 GC þ SC Variable parameter: L/dsc ¼ 1, 3, 5, 7Constant parameter:h/D ¼ 1.1, dgc/D ¼ 0.8, b/D ¼ 6, S/dsc ¼ 2.5

8 GC þ SC Variable parameter: L/dsc ¼ 1, 3, 5, 7Constant parameter:h/D ¼ 1.6, dgc/D ¼ 0.8, b/D ¼ 6, S/dsc ¼ 2.5

9 GC þ SC Variable parameter: S/dsc ¼ 1.5, 2.5, 3.5Constant parameter:h/D ¼ 0.53, dgc/D ¼ 0.8, b/D ¼ 6, L/dsc ¼ 5

10 GC þ SC Variable parameter: S/dsc ¼ 1.5, 2.5, 3.5Constant parameter:h/D ¼ 0.9, dgc/D ¼ 0.8, b/D ¼ 6, L/dsc ¼ 5

11 GC þ SC Variable parameter: S/dsc ¼ 1.5, 2.5, 3.5Constant parameter:h/D ¼ 1.1, dgc/D ¼ 0.8, b/D ¼ 6, L/dsc ¼ 5

12 GC þ SC Variable parameter: S/dsc ¼ 1.5, 2.5, 3.5Constant parameter:h/D ¼ 1.6, dgc/D ¼ 0.8, b/D ¼ 6, L/dsc ¼ 5

Note: SC: Stone columns, GC: Geocell-sand mattress.

0.001 0.010 0.100 1.000 10.000 100.000

Particle size (mm)

0

20

40

60

80

100

Per

cent

fine

rby

wei

ght

stone aggregate

sand

clay

Fig. 4. Particle size distribution of stone aggregate, sand and clay soil.

Fig. 5. Test set-up.

S.K. Dash, M.C. Bora / Geotextiles and Geomembranes 41 (2013) 26e3528

investigate the combined application of both the reinforcements,i.e. stone columns and geocell-sand mattress.

2.2. Materials used

The model clay beds were prepared using a locally available soilthat had 70% fractions finer than 75 mm (Fig. 4). Its liquid limit,plastic limit and plasticity index were found to be 40%, 21% and 19%respectively (ASTM D4318, 2005). As per the Unified Soil Classifi-cation System (USCS, ASTM D2487, 2006) the soil can be classifiedas clay with low plasticity (CL).

The stone columns were formed out of a poorly graded crushedgranite aggregate, with particle sizes in the range of 2e10 mm (d10,d30, d50, of 2.20, 3.15, 4.90 mm respectively, Fig. 4) and uniformitycoefficient of 2.32. Its direct shear friction angle (peak) at place-ment density of 15.3 kN/m3 was found to be 48�. The diameter ofmodel stone columns (100 mm) and size of aggregates used(d50 ¼ 4.9 mm) were approximately in 1:7 scale representation ofprototype stone columns of 700 mm diameter with averageaggregate size of 35 mm.

The geocell reinforcement used was fabricated from a biaxialgeogrid having aperture size of 36 mm � 36 mm, ultimate tensilestrength of 19.3 kN/m and 5% strain secant modulus of 135 kN/m(ASTM D6637, 2001). The joints of the geocells, formed out of 6 mmwide and 3mm thick polypropylene strips, had a tensile strength of4.75 kN/m (ASTM D4884, 2009). Such low strength of joints wasadopted to scale down the overall strength of the geocells that it issuitable for the model tests. The geocells were filled with a poorlygraded sand that had average particle size of 0.44 mm and uni-formity coefficient of 2.52 (Fig. 4). In all the tests the sand wasplaced at 80% of relative density. Its peak friction angle obtainedthrough triaxial compression tests, in the pressure range of 100e200 kPa, was 44�.

2.3. Test setup

The model tests were carried out in a laboratory scale test bed-cum-loading frame assembly (Fig. 5). The test beds were preparedin a steel tank of 1000 mm long, 1000 mmwide and 1300 mm high.To avoid yielding during tests, the four sides of the tank werebraced laterally on their outer surfaces with steel channels. Thefooting used was made of a rigid steel plate and measured 150 mmin diameter (D). In order to create a rough base condition a thinlayer of sand was glued onto its bottom. In all the tests the footingwas placed at the centre of the test tank. Loading was appliedthrough an automated hydraulic jack system supported against areaction frame fixed onto the ground. The load transmitted to the

footing was recorded through an electronic load cell of 20 kN ca-pacity with an accuracy of 0.01 N. The settlements of the footingwere measured by two linear variable differential transducers(LVDTs), placed at diametrically opposite ends (DT1, DT2; Fig. 1).Deformations (heave/settlement) on foundation bed too weremeasured by LVDTs, placed through small plastic strips on the soilsurface (DT3eDT8; Fig. 1). The LVDTs were of 50 mm travel with anaccuracy of 3 microns. The load cell and the LVDTs were connectedto a computer controlled data acquisition system.

Selig and McKee (1961) and Chummer (1972) have observedthat the failure wedge in the foundation bed extends over a dis-tance of about 2e2.5 times the footing width, away from its centre.In the present tests the distance of tank walls from centre of footingbeing more than 3.3D, the slip planes are not likely to be interferedwith. Besides, the geocell mattress being flexible deforms down-ward under the footing loading and thereby gets pulled away fromthe tank side walls reducing the boundary effect to a practicallynegligible value. Indeed, Dash et al. (2003a) through instrumentedmodel tests have observed that the footing loading, in nearlysimilar test conditions, did not induce any additional pressure onthe tank walls.

The stone columns used had a maximum length of 700 mm (i.e.L ¼ 7dsc) and the geocell mattresses used had maximum height of255 mm (i.e. h ¼ 1.7D). Therefore, the minimum clear spacing be-tween the stone column base and the tank bottom, maintained inthe tests, was 345 mm [i.e., 1300 � (700 þ 255)]. This is about 3.45times the diameter of the stone column (dsc). Mayerhof and Sastry(1978) have observed that the failure zone below a rigid pile ex-tends over a depth of about 2 times its diameter. The stone columnsbeing flexible, this depth would further be less. A stress analysisconsidering the dispersion in geocell mattress (Dash et al., 2007)and group action of stone columns (similar to that of rigid piles inclay, Bowel,1988)was carried out. It shows that for height of geocellmattress of 1.7D and stone column length of 7dsc, the stress inducedat the bottom of the test-tank was less than 2% of the appliedpressure. In view of this it can be said that the test-tank used in thepresent investigation was considerably large enough and not likelyto interfere with the failure zones and hence the experimental re-sults. Besides, the confinement due to the tank walls simulated theactual field conditions for the interior columns in a large group(Ambily and Gandhi, 2007).

2.4. Test bed preparation

The clay was pulverised, mixed with predetermined amount ofwater and for moisture equilibrium was kept in airtight containers

Fig. 6. Typical geocell layer in the test bed.

0

4

8

12

16

20

24

28

Foo

ting

sett

lem

ent ,

s/D

(%)

0 20 40 60 80 100 120 140 160

Bearing pressure (kPa)

Clay

Clay+GC

Clay+GC+SC (L/dsc= 1)

Clay+GC+SC (L/dsc= 3)

Clay+GC+SC (L/dsc= 5)

Clay+GC+SC (L/dsc= 7)

Fig. 7. Footing pressure-settlement responses: influence of length of stone columns incomposite foundation bed (h/D ¼ 0.53, S/dsc ¼ 2.5) e Test series 5.

S.K. Dash, M.C. Bora / Geotextiles and Geomembranes 41 (2013) 26e35 29

for about a week. The test bed was prepared in lifts of 0.05 mthickness. For each lift the amount of soil required to produce thedesired bulk density was weighted out, placed in the test-tank,levelled and compacted. Compaction was done through a woodenboard and a drop hammer, using depth marking on the sides of thetank as guide. The compaction energy applied was 299 kJ/m3. Un-disturbed soil samples were collected from different locations andtheir properties were evaluated. Sampling was done through a thincylindrical sampler that was pressed into the clay bed and extractedout with the soil within. Apart from the in situ density andmoisturecontent, the specimens were tested for the vane shear strength aswell. The average moisture content, degree of saturation, bulk unitweight and shear strength of the clay, in the test beds, were foundto be 36%, 100%, 18.05 kN/m3 and 5 kPa respectively. Their coeffi-cient of variability was in the range of 1.5%.

The stone columns were constructed by a replacement method.An open-ended stainless steel pipe of 100 mm outer diameter and1.5 mm of wall thickness, smeared with petroleum jelly (to reducefriction), was pushed into the clay bed until it reached the depth ofthe column to be formed. Subsequently, the clay within the pipewas scooped out through a helical auger of 90 mm diameter. Tominimise suction effect, a maximum of 100 mm was removed at atime. On completion, the internal wall of the pipe was cleaned andstone aggregates were charged in. The stones were added inbatches of 0.6 kg and compacted to height of 50 mm, through acircular steel tamper of 0.9 kg with 30 blows of 200 mm drop,leading to a density of 15.3 kN/m3 that corresponds to 65% ofrelative density. The pipe was then slowly raised ensuring a mini-mum of 25 mm overlap within the aggregate that the clay outsidedoesn’t intrude in. This procedure was continued until the stonecolumn was completely formed (i.e., till top of clay bed). The stonecolumn reinforced clay bed thus formed was loaded with a seatingpressure of 2.5 kN/m2, over the entire area for 4 h. This was toachieve uniformity in the test bed (Malarvizhi and Ilamparuthi,2007).

The geocell reinforcement was prepared from geogrid stripsplaced in transverse and diagonal directions and connectedtogether with bodkin joints (Bush et al., 1990). The joint was formedby pulling the ribs of the diagonal geogrid, up through the trans-verse geogrid and slipping a dowel (plastic strip) through the loopcreated (Carroll and Curtis, 1990). Three-dimensional view of atypical geocell structure, placed over the clay bed, is shown in Fig. 6.The geocells were filled with sand through raining. Compared to

unreinforced case, with geocell reinforcement the height of rainingrequired to achieve the target density was relatively more; how-ever, the difference was not much. This was because the geocellsbeing made of geogrids had more than 80% of open area, therefore,did not affect much the free flow of sand during raining. The dif-ference in the placement densities of the sand at various locationsin the test bed, measured through in situ sampling, was found to beless than 1.5%.

2.5. Test procedure

In all the tests loading was applied in strain controlled manner,at the rate of 2 mm/min. This relatively faster rate of loading wasintended to produce undrained response in the saturated clay bed.It is one of the worst field conditions expected as in this case theangle of friction of the soil tends to be zero leading to largereduction in the bearing capacity. Such phenomenon is commonduring rainy seasons, particularly in case of railways and highwayswhere the loading is transient in nature. In all the tests, load wasapplied until the footing settlement reached 40 mm. Through acomputer controlled system the load-deformation data werecontinuously recorded.

On completion of tests the deformed shape of the stone columnswere mapped. This was done through careful removal of stoneaggregates and filling the shaft with Plaster of Paris (CaSO4$0.5H2O)paste. After being hardened, the Plaster of Paris column was takenout and measured for its shape and size. The stone column de-formations thus obtained are presented in terms of radial strain;(rd �ro)/ro, wherein, rd is the deformed radius and ro is the originalradius.

3. Results and discussion

Typical pressure-settlement responses of clay bed alone, thatwith geocell and geocell-stone column composite reinforcement,are presented in Fig. 7. The settlement, s, reported is the average ofthe readings taken at both ends of the footing (DT1 and DT2, Fig. 1).It could be observed that, in case of unreinforced clay, the slope ofthe pressure-settlement response continues to increase until set-tlement (s/D) of about 15%, and thereafter tends to become nearlyvertical. This means that the soil has undergone clear failure andtherefore couldn’t support additional pressure anymore. However,with geocell reinforcement (Clay þ GC) the bearing pressure con-tinues to increase even at settlement (s/D) as high as 25%, althoughthe overall improvement is relatively less. But, with stone columnsin the clay subgrade underneath geocell mattress (Clay þ GC þ SC)

Table 2Bearing capacity improvement factors (IF: IFsc, IFgc, IFgcsc).

Test series Reinforcement Variable parameter Bearing capacity improvement factor (IF)

s/D 1% s/D 5% s/D 9% s/D 13% s/D 17% s/D 21% s/D 25% s/D 27%

2 SC L/dsc ¼ 1 1.69 1.25 1.14 1.19 1.23 1.26 1.28 1.30SC L/dsc ¼ 3 1.81 1.54 1.53 1.60 1.62 1.66 1.73 1.73SC L/dsc ¼ 5 3.59 2.87 2.93 3.12 3.19 3.31 3.40 3.44SC L/dsc ¼ 7 3.69 3.20 3.17 3.37 3.40 3.48 3.56 3.60

3 SC S/dsc ¼ 1.5 3.66 2.90 3.20 3.45 3.49 3.61 3.67 3.70SC S/dsc ¼ 2.5 3.59 2.87 2.93 3.12 3.19 3.31 3.40 3.44SC S/dsc ¼ 3.5 1.97 1.47 1.67 1.84 1.81 1.86 1.90 1.93

4 GC h/D ¼ 0.53 1.34 1.50 1.64 1.86 1.95 2.11 2.26 2.32GC h/D ¼ 0.9 2.10 2.18 2.83 3.03 3.35 3.59 4.11 4.33GC h/D ¼ 1.1 2.90 4.10 4.77 5.57 6.00 6.60 7.12 7.39GC h/D ¼ 1.6 4.20 4.81 5.43 6.14 6.54 7.08 7.64 7.87

5 GC þ SC L/dsc ¼ 1 1.72 1.98 1.91 2.01 2.15 2.30 2.46 2.53GC þ SC L/dsc ¼ 3 1.87 2.22 2.23 2.36 2.48 2.66 2.88 2.95GC þ SC L/dsc ¼ 5 4.12 4.26 4.29 4.32 4.41 4.73 5.02 5.13GC þ SC L/dsc ¼ 7 4.39 4.79 4.87 4.98 5.02 5.30 5.57 5.69

6 GC þ SC L/dsc ¼ 1 2.24 2.80 2.88 3.12 3.45 3.79 4.17 4.35GC þ SC L/dsc ¼ 3 3.00 3.33 3.28 3.48 3.70 4.08 4.36 4.50GC þ SC L/dsc ¼ 5 4.29 4.63 4.70 5.09 5.45 5.92 6.43 6.73GC þ SC L/dsc ¼ 7 5.36 5.55 5.65 5.89 6.12 6.68 7.20 7.46

7 GC þ SC L/dsc ¼ 1 3.66 5.06 5.18 5.74 6.15 6.86 7.46 7.78GC þ SC L/dsc ¼ 3 5.02 5.98 5.94 6.31 6.69 7.22 7.77 8.01GC þ SC L/dsc ¼ 5 5.66 6.65 6.63 7.00 7.40 8.01 8.50 8.74GC þ SC L/dsc ¼ 7 7.00 7.38 7.24 7.55 7.98 8.50 9.13 9.42

8 GC þ SC L/dsc ¼ 1 3.41 4.80 5.55 6.40 7.20 8.01 8.72 9.01GC þ SC L/dsc ¼ 3 3.88 5.29 6.00 7.00 7.55 8.25 9.02 9.24GC þ SC L/dsc ¼ 5 5.48 6.46 6.84 7.41 7.96 8.59 9.28 9.59GC þ SC L/dsc ¼ 7 6.10 7.00 7.38 7.82 8.32 8.93 9.56 9.88

9 GC þ SC S/dsc ¼ 1.5 4.54 4.82 4.84 4.95 5.16 5.53 5.79 5.88GC þ SC S/dsc ¼ 2.5 4.12 4.26 4.29 4.32 4.41 4.73 5.02 5.13GC þ SC S/dsc ¼ 3.5 3.28 3.44 3.50 3.52 3.54 3.65 3.79 3.89

10 GC þ SC S/dsc ¼ 1.5 5.01 5.46 5.57 5.85 6.24 6.75 7.24 7.47GC þ SC S/dsc ¼ 2.5 4.29 4.63 4.70 5.09 5.45 5.92 6.43 6.73GC þ SC S/dsc ¼ 3.5 3.40 3.54 3.64 4.08 4.16 4.55 4.90 5.06

11 GC þ SC S/dsc ¼ 1.5 6.10 7.23 7.34 7.60 7.97 8.53 9.07 9.37GC þ SC S/dsc ¼ 2.5 5.66 6.65 6.63 7.00 7.40 8.01 8.50 8.74GC þ SC S/dsc ¼ 3.5 5.30 5.84 5.97 6.17 6.65 7.30 7.89 8.22

12 GC þ SC S/dsc ¼ 1.5 6.35 7.13 7.29 7.81 8.40 9.20 9.87 10.2GC þ SC S/dsc ¼ 2.5 5.48 6.46 6.84 7.41 7.96 8.59 9.28 9.59GC þ SC S/dsc ¼ 3.5 5.10 6.29 6.43 7.00 7.50 8.19 8.82 9.06

Note: Boldface values indicate the maximum improvement factors for different reinforcements (i.e. SC, GC and SC þ G).

S.K. Dash, M.C. Bora / Geotextiles and Geomembranes 41 (2013) 26e3530

the bearing capacity has improved significantly, much higher thanthat with geocell mattress alone. Besides, the stiffness of thefoundation bed too has increased substantially, indicated throughreduced slope of the pressure settlement response. Similarbehaviour is noticed in all other cases as well. This is attributed tothe increased resistance against deformation provided by the stonecolumns, through mobilisation of friction and stiffness of the stoneaggregates that provides additional support to the geocell mattress.As a result of which the geocell-sand mattress that behaves as asubgrade supported beam (Dash et al., 2007) stands effectivelyagainst the footing loading leading to improved performance of thefoundation system.

The increase in the bearing capacity due to stone columns,geocell mattress and stone column-geocell mattress compositereinforcement is quantified through nondimensional improvementfactors, IFsc, IFgc, IFgcsc respectively; defined as the ratio of bearingpressure with reinforcement (qsc, qgc, qgcsc) to that of unreinforcedclay bed (qu), both taken at equal settlement of footing (s/D%). Thevalues of these improvement factors for different test cases andfooting settlements are presented in Table 2. It is evident that withstone columns the bearing capacity of soft clay can be enhanced by3.7 fold (IFsc ¼ 3.7, Test series 3), and with geocell reinforcement itcan be up to 7.87 fold (IFgc ¼ 7.87, Test series 4). But, if coupledtogether (i.e. stone columns and geocell mattress) the compositereinforcement can enhance the bearing capacity of the soft clay as

high as by 10 fold (IFgcsc ¼ 10.2, Test series 12). Hence it can be saidthat the stone column-geocell composite is a superior form ofreinforcement that can give better performance improvement overthe conventional ones i.e. stone column, geocell mattress. Influ-ence of different parameters on the overall performance of suchcomposite foundation systems are discussed in the followingsections.

3.1. Influence of length of stone columns

Influence of length of stone columns (L), in the compositefoundation bed, has been studied for four different heights ofgeocells, h ¼ 0.53D, 0.9D, 1.1D and 1.6D (Test series 5e8). It is ofinterest to note that increasing the column length from 3 to 5dsc,leads to sharp increase in performance, both in terms of increasedload carrying capacity (IFgcsc, Table 2) and reduced settlement(Fig. 7). However, with further increase in length (L) to 7dsc, theadditional improvement is much less. Hence it can be said that, inthe composite foundation system, the optimum length of stonecolumns giving maximum performance improvement is about 5times their diameter (i.e. 5dsc). This observation, however, is fromsmall-scale models and needs to be verified through prototypetests. The results are further analysed in terms of the improvementfactor ratios (i.e. IFgcsc/IFgc and IFgcsc/IFsc) that brings out the

0 4 8 12 16 20 24 28

Footing settlement, s/D(%)

0

1

2

3

4

5

Impr

ovem

entf

acto

rra

tio ,

IFgc

sc/I

Fg c

Clay+GC+SC(L/dsc= 1)

Clay+GC+SC(L/dsc= 3)

Clay+GC+SC(L/dsc= 5)

Clay+GC+SC(L/dsc= 7)

Fig. 8. Improvement factor ratio-footing settlement responses: contribution of stonecolumns in composite foundation bed (h/D ¼ 0.53, S/dsc ¼ 2.5) e Test series 5.

0

1

2

3

4

5

6

7

Len

gth

ofst

one

colu

mn,

L/d

sc

0 5 10 15 20 25 30

Radial strain, (rd-ro)/ro (%)

L/dsc= 1

L/dsc= 3

L/dsc= 5

L/dsc= 7

Fig. 10. Post-test deformed profile of central stone column: in composite foundationbeds (h/D ¼ 0.53, S/dsc ¼ 2.5) e Test series 5.

S.K. Dash, M.C. Bora / Geotextiles and Geomembranes 41 (2013) 26e35 31

contribution of individual reinforcement components (i.e. geocellsand mattress, stone columns), as has been explained below.

IFgcscIFsc

¼qgcscquqscqu

¼ qgcscqsc

(1)

Thus the factor (IFgcsc/IFsc) can be taken as the contribution ofgeocell mattress sharing the surcharge loading in the compositefoundation system. Similarly, the factor IFgcsc/IFgc (i.e. qgcsc/qgc)represents the contribution of stone columns. Typical improvementfactor ratios, IFgcsc/IFgc and IFgcsc/IFsc, for the case with h/D ¼ 0.53,are depicted in Fig. 8 and Fig. 9 respectively. It is evident that thecontribution of stone columns, IFgcsc/IFgc, increases with increase inlength and the improvement is the maximum when L/dsc ratiochanges from 3 to 5. This may be because the shorter columns (L/dsc < 3) due to inadequate skin resistance have suffered punchingfailure and therefore didn’t contribute significantly towards theload carrying capacity of the system. In contrast, the long columns(L/dsc � 5) owing to large skin resistance mobilised throughincreased surface area have effectively stood against the footingloading giving rise to visible increase in performance improvement.Further confirmation of the column failure modes was obtainedfrom the post test deformation profiles, a typical of which for thecentral stone columns are shown in Fig. 10. It could be observedthat when short in length (L/dsc ¼ 1 and 3) the bulging in the stone

0 4 8 12 16 20 24 28

Footing settlement, s/D(%)

0

1

2

3

4

5

Impr

ovem

entf

acto

rra

tio,

IFgc

sc/I

Fsc

Clay+GC+SC(L/dsc= 1)

Clay+GC+SC(L/dsc= 3)

Clay+GC+SC(L/dsc= 5)

Clay+GC+SC(L/dsc= 7)

Fig. 9. Improvement factor ratio-footing settlement responses: contribution of geocellmattress in composite foundation bed (h/D ¼ 0.53, S/dsc ¼ 2.5) e Test series 5.

columns is marginal indicating that the column has mostly beenpunched down. But, with increase in length (L/dsc � 5) it haseffectively stood against the footing and therefore has bulgedsignificantly. Furthermore, with column L/dsc ratio of 1, thecontribution of geocell mattress, IFgcsc/IFsc, was the maximumwhich however reduced as the column length increased andremained almost constant for L/dsc � 5 (Fig. 9). This can be analysedthrough the responses of the fill surface depicted in Fig. 11. Thesurface deformations (d), reported herein, are the average of thereadings taken at distance D, on both sides of the footing (DT5 andDT6, Fig. 1). It can be seen that with increase in the length of stonecolumns, settlement (þd) on the fill surface has reduced. Corre-spondingly, heave in the adjacent region (x ¼ 2D and 3D) too wasfound to have reduced. This is attributed to the increased resistanceof stone columns that inhibits settlement and thereby heave in thefoundation bed. With reduced deformations in the soil around, thestrength mobilised in the geocells reduces and so is its contributionto the performance improvement. Stone column length beyond the

0 4 8 12 16 20 24 28

Footing settlement, s/D(%)

0

1

2

3

4

5

Ave

rage

sur

face

def

oram

atio

n,

δ /D

(%)

Clay

Clay+GC

Clay+GC+SC(L/d sc= 1)

Clay+GC+SC(L/d sc= 3)

Clay+GC+SC(L/d sc= 5)

Clay+GC+SC(L/d sc= 7)

Fig. 11. Surface deformation-footing settlement responses: influence of length of stonecolumn in composite foundation bed (x/D ¼ 1, h/D ¼ 0.53, S/dsc ¼ 2.5) e Test series 5.

0

4

8

12

16

20

24

28

Foo

ting

set

tlem

ent,

s/D

(%)

0 20 40 60 80 100 120 140 160

Bearing pressure (kPa)

Clay

Clay+GC

Clay+GC+SC(S/dsc= 1.5)

Clay+GC+SC(S/dsc= 2.5)

Clay+GC+SC(S/dsc= 3.5)

Fig. 12. Footing pressure-settlement responses: influence of spacing of stone columnsin composite foundation bed (h/D ¼ 0.53, L/dsc ¼ 5) e Test series 9.

0 4 8 12 16 20 24 28

Footing settlement, s/D(%)

0

1

2

3

4

Impr

ovem

ent

fact

orra

tio,

I Fgc

sc/I

Fgc

Clay+GC+SC(S/dsc= 1.5)

Clay+GC+SC(S/dsc= 2.5)

Clay+GC+SC(S/dsc= 3.5)

Fig. 13. Improvement factor ratio-footing settlement responses: contribution of stonecolumns in composite foundation bed (h/D ¼ 0.9, L/dsc ¼ 5) e Test series 10.

0 4 8 12 16 20 24 28

Footing settlement, s/D(%)

0

1

2

3

4

Impr

ovem

entf

act o

rra

tio ,

IFgc

sc/I

Fs c

Clay+GC+SC(S/dsc= 1.5)

Clay+GC+SC(S/dsc= 2.5)

Clay+GC+SC(S/dsc= 3.5)

Fig. 14. Improvement factor ratio-footing settlement responses: contribution of geo-cell mattress in composite foundation bed (h/D ¼ 0.9, L/dsc ¼ 5) e Test series 10.

S.K. Dash, M.C. Bora / Geotextiles and Geomembranes 41 (2013) 26e3532

optimum (i.e. 5dsc) though enhances the skin resistance, but itmostly remains unutilised due to excessive bulging at the top. As aresult, the responses of the stone columns and that of the geocellmattress, beyond L/dsc ratio of 5, have almost been stabilised. Thepresent findings are in agreement with the observations of Hughesand Withers (1974), McKelvey et al. (2004) that increasing thelength of stone columns beyond a certain point adds little to theincrease in bearing capacity; however, can help reducing the set-tlement in the foundation bed.

3.2. Influence of spacing of stone columns

Effect of column spacing (S) in the composite foundation bedswas studied under Test series 9e12. Typical responses are shown inFig.12. It can be seen that at low settlements (s/D< 5%), the slope ofthe pressure-settlement plots with stone columns are much lessthan the case without. This indicates that, when intact, the stonecolumns irrespective of their spacing have provided additionalstiffness to the foundation system. This however was not the casewhen the settlement was large, primarily because the columns hadbulged.

With relatively widely spaced stone columns (S ¼ 3.5dsc), stiff-ness of the composite foundation system is almost comparable tothat with geocell reinforcement alone (both the responses arenearly parallel). It could be because, at large spacing the groupaction of the peripheral stone columns diminishes that they behaveas individual entities leading to reduced lateral resistance onto thecentral confined region. In the absence of adequate confinementfrom the surrounding, the central stone column underneath thefooting bulged prematurely and therefore couldn’t enhance thestiffness of the foundation system. Indeed the post test observa-tions have shown that with large spacing the central stone columnhad bulged more. As the spacing reduces, the group action in therings of stone columns builds up inducing confinement in thecentral region that provides increased support against columnbulging leading to enhanced performance improvement.

In general the bearing capacity of the composite foundation bedwas more when the spacing of stone columns was less (Fig. 12).However, the improvement (IFgcsc) with the column spacing (S)reducing from 3.5dsc to 2.5dsc was relatively more than that from2.5dsc to 1.5dsc (Table 2, test series 9e12). Further analysis showsthat the contribution of stone columns in the composite system,IFgcsc/IFgc, was the maximum when they were placed close(S ¼ 1.5dsc) and reduced as the spacing was increased (Fig. 13). Incontrast, when the stone columns underneath were placed wideapart (S ¼ 3.5dsc), the geocell mattress has carried maximum load,

IFgcsc/IFsc, which however reduced significantly as the spacing ofcolumns was reduced to 2.5dsc (Fig. 14). Under footing loading, thestone columns with wider spacing have deformed more. As theunderlying support yields, the geocell mattress through mobi-lisation of anchorage from the adjacent stable region, bridges over,and thereby shares higher proportion of the surcharge pressure.The marginal difference in the load factor ratio IFgcsc/IFsc, with thespacing (S) changing from 2.5dsc to 1.5dsc, indicates that furtherchange in the contribution of geocell mattress is practically negli-gible. With reduced spacing, increased percentage of weak clay getsreplaced by the stiffer stone columns. This gives rise to more uni-formity of stress in the foundation bed that it deforms less. Indeed,reduced settlement and heave on the fill surface, observed withreduced spacing of stone columns, testifies that the deformations inthe foundation bed have reduced down. As a result, the strain in theoverlying geocell reinforcement reduces leading to reduced mobi-lisation of its strength and stiffness. In such case the geocellmattress behaves more of like a pedestal, a load transmittingmember. Whereas, with wider spacing (S ¼ 3.5dsc) of stone col-umns underneath, it behaves as a load sustaining member, like acentrally loaded slab resting over columns. It can therefore be saidthat when the spacing reduces from 3.5dsc to 2.5dsc, there is sig-nificant change in the behaviour of stone columns that it shifts fromnear isolated to an interacting response giving rise to largeimprovement in the performance of the system. Hence the opti-mum spacing of stone columns in the composite foundation bedscan be taken as 2.5dsc.

0 4 8 12 16 20 24 28

Footing settlement, s/D(%)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Impr

ovem

entf

ccto

rra

tio,

IFgc

sc/I

Fgc

Clay+GC+SC(L/dsc= 1)

Clay+GC+SC(L/dsc= 3)

Clay+GC+SC(L/dsc= 5)

Clay+GC+SC(L/dsc= 7)

Fig. 15. Improvement factor ratio-footing settlement responses: contribution of stonecolumns in composite foundation bed (h/D ¼ 1.6, S/dsc ¼ 2.5) e Test series 8.

0

1

2

3

4

5

6

7

Len

gth

ofst

one

colu

mn ,

L/d

sc

0 5 10 15 20 25 30

Radial strain, (rd - ro)/ro (%)

L/dsc= 1

L/dsc= 3

L/dsc= 5

L/dsc= 7

Fig. 17. Post-test deformed profile of central stone column: in composite foundationbeds (h/D ¼ 1.6, S/dsc ¼ 2.5) e Test series 8.

S.K. Dash, M.C. Bora / Geotextiles and Geomembranes 41 (2013) 26e35 33

3.3. Influence of height of geocell mattress

Typical responses of the composite system, for three differentheights of geocell mattress (h ¼ 0.53D, 0.9D, 1.6D), are shown inFigs. 8, 9, 13, 14, 15 and 16, respectively. It could be observed thatwhen shallow in height (h ¼ 0.53D) the geocell mattress has underperformed that the stone columns have shared nearly three timesmore load (IFgcsc/IFgc ¼ 3, Fig. 8). However, with increase in height(h) the contribution of stone columns has reduced and that ofgeocell mattress (IFgcsc/IFsc) has gone up. When geocells are rela-tively deep (h ¼ 1.6D) the improvement factor ratio, IFgcsc/IFgc, isjust in the range of 1e1.2 (Fig. 15) that the load carried by the stonecolumns is at the most 20% that of the geocell mattress. The datapresented in Fig. 16 indeed shows such a response, wherein, thevalue of improvement factor ratio, IFgcsc/IFsc, is as high as 6.5,indicating that most of the footing pressure has been sustained bythe geocell mattress. A deep geocell mattress possesses highmoment of inertia that enables it to stand against the footingpenetration. Besides, with increase in height (h) the geocell areaderiving anchorage from the infill soil increases and so is theanchorage resistance. Therefore, the geocell mattress takes a largeproportion of the footing loading on its own that the stone columnsunderneath mostly remain dormant and thereby contribute less tothe performance improvement. Visibly less bulging observed in thepost-test exhumed stone columns (Fig. 17) establishes that theyindeed had under performed in sharing the surcharge loading.

0 4 8 12 16 20 24 28

Footing settlement, s/D(%)

0

2

4

6

8

10

Impr

ovem

e ntf

acto

rra

tio,

IFgc

sc/ I

Fsc

Clay+GC+SC(L/dsc= 1)

Clay+GC+SC(L/dsc= 3)

Clay+GC+SC(L/dsc= 5)

Clay+GC+SC(L/dsc= 7)

Fig. 16. Improvement factor ratio-footing settlement responses: contribution of geo-cell mattress in composite foundation bed (h/D ¼ 1.6, S/dsc ¼ 2.5) e Test series 8.

The improvement due to the geocell-stone column compositereinforcements are summarised in Table 2 (Test series 5e12). It canbe seen that for height of geocell mattress, h¼ 0.53D, 0.9D, 1.1D and1.6D, the maximum bearing capacity improvement, IFgcsc ¼ 5.69,7.47, 9.42 and 10.2 respectively. This highlights that the increase inperformance improvement with height of geocell mattressincreasing beyond 1.1D is relatively less. A possible reason for thiscould be the stress concentration induced local buckling andyielding of geocells, right under the footing, that the global increasein strength and stiffness of the system due to increase in height ofthe mattress remains immobilised. Indeed, the maximum differ-ence in the values of the factor, IFgcsc/IFsc (i.e. the load shared by thegeocell mattress), for the case with h ¼ 1.6D and 1.1D, was less than5%. Therefore, height of geocell mattress equal to about the diam-eter of the footing (h ¼ D), can be taken as the optimum one, givingmaximum possible performance improvement in the compositefoundation beds. However, full-scale tests are required to verify thisobservation.

It is of interest to note that even geocell mattress of mediumheight, h ¼ 0.9D, when combined with stone columns can providebearing capacity improvement (IFgcsc ¼ 7.46, Test series 6) as highas that of a deep geocell mattress, h ¼ 1.6D (IFgcsc ¼ 7.87, Test series4). Due to practical constraints, at times, it might be difficult toaccommodate a relatively large height geocell mattress. In suchsituation provision of stone columns in the underlying subgradewould be a viable alternative to manage with geocell mattress ofrelatively smaller height.

4. Scale effect

Owing to reduced size model tests, the results presented in thispaper are prone to scale effects. Therefore, further studies usingfull-scale tests are required to verify these observations. However,using a suitable scaling law the results from the present study canbe extrapolated to the prototype case (Fakher and Jones, 1996).

The major physical parameters influencing the response ofgeocell-stone column reinforced foundation systems can be

S.K. Dash, M.C. Bora / Geotextiles and Geomembranes 41 (2013) 26e3534

summarised as: D, h, dgc, K, dsc, L, d50, S, ɸ, cu, g, G; where ɸ is theangle of internal friction of soil and stone aggregate, K is the stiff-ness of geocell reinforcement, g is the unit weight of soil andaggregate, G is the shear modulus of the soil and aggregate. Otherparameters have been defined previously. The function (f) thatgoverns the composite foundation system can be written as

f�D; h; dgc;K; dsc; L;d50; S; cu;g;G;f; s; qgcsc; qu

�¼ 0 (2)

It comprises of 15 parameters and has two fundamental di-mensions (i.e. length and force) and therefore can be studied by 13independent parameters (p1, p2, p3, p4...p13; Buckingham,1914). Hence equation (2) can be written as

gðp1;p2;p3;p4...p13Þ ¼ g�� s

D

�;

�hD

�;

�dgcD

�;

�hdgc

�;

�dscD

�;

�LD

�;

�SD

�;

�d50dsc

�;

�GDg

�;

�KgG2

�;

�cuDg

�;

�qgcscqu

�;f

�¼ 0 (3)

For a prototype footing (p) with diameter n times that of themodel (m)

Dp

Dm¼ n (4)

For similarity to be maintained, the p terms, both for model andprototype need to be equal and therefore considering the p9 term:

Gp

Dpgp¼ Gm

Dmgm(5)

Assuming that the soils used in the model and prototype dohave same unit weight (Pinto and Cousens, 1999), equation (5)reduces to

Gp

Gm¼ Dp

Dm¼ n (6)

Considering similarity of the p10 terms:

KpgpG2p

¼ KmgmG2m

:Kp

G2p¼ Km

G2m

:Kp

Km¼ G2

p

G2m

¼ n2 (7)

As can be seen, the strength of prototype geocells should be of n2

times that of the model geocell, where n is the scale factor. Thegeocells used in the present tests have tensile strength of 4.75 kN/m. Therefore, the results from the present study to be applicable inpractice, the prototype geocells should have tensile strength of4.75n2 kN/m. However, the geometric parameters such as; pocketsize and height of geocells, length, diameter and spacing of stonecolumns etc., have shown a linear variationwith the footing size, D.

5. Conclusions

Review of literature shows that both geocell-sand mattress andstone columns are effective means of reinforcing the weak soils.Their individual applications though have been intensely studied,by many researchers, but combined application of both hasremained unexplored. The experimental results obtained in thepresent study confirm that such composite reinforcement is anadded advantage over the conventional ones i.e. stone column orgeocell mattress. With provision of stone columns, the bearingcapacity of soft clay beds can be increased by 3.7 fold and withgeocell reinforcement it is of the order of 7.8 fold. When coupled

together, i.e. stone column-geocell mattress combined, the bearingcapacity was increased by 10.2 fold. Additionally, visible reductionin slope of pressure settlement responses indicates that the stonecolumn-geocell composite reinforcement can increase the stiffnessof the foundation bed significantly leading to large scale reductionin footing settlement.

The load carrying capacity of the geocell-stone column rein-forced foundation bed increases with increase in length of stonecolumns until 5dsc, beyond which further rate of improvement hasreduced down. Similarly, reducing the spacing of stone columnsbelow 2.5dsc does not attractmuch of additional performance in thecomposite system. Besides, with height of geocells increasingbeyond 1.1D the performance improvement is found to have

reduced. This is possibly due to the stress concentration inducedbuckling and yielding of geocells, right under the footing, that theincrease in strength and stiffness of the system due to increase inheight of the mattress remains immobilised. Hence it can be saidthat the critical height of geocell mattress, giving optimum per-formance improvement in the composite foundation bed, is equalto about the diameter of the footing (D).

At times, practical constraints may prevent in going for largeheight geocell mattress or long stone columns, severely compro-mising the performance of the system. In such situations, thegeocell-stone column composite reinforcement provides an effec-tive solution for adequate performance improvement and optimumdesign of foundations on soft clay. This is inferred from the presentstudy that a shallow height geocell mattress along with mediumlength stone columns can provide comparable performance im-provements as that with deep geocells or long stone columns.However, full-scale testing and field trails are required to verifythese observations.

The findings of the present study can be of use in the design andconstruction of structures over soft clay deposits, such as; railways,highways, foundations for liquid storage tanks, large stabilisedareas for parking, platforms for oil exploration etc. The authorshave also conducted tests with basal geogrid underneath the geo-cell mattress and the results shall be reported in a subsequentpaper.

Acknowledgement

The authors are thankful to the anonymous reviewers for theirvaluable comments and suggestions for improvements of the pre-sentations in the paper.

Notation

Cc coefficient of curvatureCu coefficient of uniformityD diameter of footingdgc diameter of geocellsdsc diameter of stone columnemax maximum void ratioemin minimum void ratioh height of geocell mattress

S.K. Dash, M.C. Bora / Geotextiles and Geomembranes 41 (2013) 26e35 35

IFsc bearing capacity improvement factor due to stone columnreinforcement

IFgc bearing capacity improvement factor due to geocellreinforcement

IFgcsc bearing capacity improvement factor due to stonecolumn-geocell, composite reinforcement

L length of stone columnqsc bearing pressure with stone column reinforcementqgc bearing pressure with geocell reinforcementqgcsc bearing pressure with stone column-geocell, composite

reinforcementrd deformed radius of stone columnro original radius of stone columnS spacing of stone columnss settlement of footingu depth of placement of geocell mattress

References

Abdullah, C.H., Edil, T.B., 2007. Behaviour of geogrid-reinforced load transfer plat-forms for embankment on rammed aggregate piers. Geosynth. Int. 14 (3), 141e153.

Ambily, A.P., Gandhi, S.R., 2007. Behaviour of stone columns based on experimentaland FEM analysis. J. Geotech. Geoenviron. Eng., ASCE 133 (4), 405e515.

Arulrajah, A., Abdullah, A., Bouzza, A., 2009. Ground improvement technique forrailway embankments. Ground Improv. 162 (1), 3e14.

ASTM D2487, 2006. Standard Practice for Classification of Soils for EngineeringPurposes (Unified Soil Classification System). ASTM International, West Con-shohocken, PA. http://dx.doi.org/10.1520/D2487-06E01.

ASTM D4318, 2005. Standard Test Methods for Liquid Limit, Plastic Limit, andPlasticity Index of Soils. ASTM International, West Conshohocken, PA. http://dx.doi.org/10.1520/D4318-05.

ASTM D4884, 2009. Standard Test Method for Strength of Strewn Thermally BondedSeams of Geotextiles. ASTM International, West Conshohocken, PA. http://dx.doi.org/10.1520/D4884e09.

ASTM D6637, 2001. Standard Test Methods for Determining Tensile Properties ofGeogrid by Single or Multi-rib Tensile Methods. ASTM International, WestConshohocken, PA. http://dx.doi.org/10.1520/D6637e01R09.

Black, J., Sivakumar, V., MeKinley, J.D., 2007. Performance of clay samples reinforcedwith vertical granular columns. Can. Geotech. J. 44 (1), 89e95.

Bowel, J.E., 1988. Foundation Analysis and Design, fourth ed. McGraw-Hill,Singapore.

Buckingham, E., 1914. On physically similar systems. Phys. Rev. APS 4, 345e376.Bush, D.I., Jenner, C.G., Bassett, R.H., 1990. The design and construction of geocell

foundation mattresses supporting embankments over soft ground. Geotext.Geomembr. 9 (1), 83e98.

Carroll Jr., R.G., Curtis, V.C., 1990. Geogrid connections. Geotext. Geomembr. 9 (4e6),515e530.

Cimentada, A., Costa, A.D., Izal, J.C., Sagaseta, C., 2011. Laboratory study on radialconsolidation and deformation in clay reinforced with stone columns. Can.Geotech. J. 48 (1), 36e52.

Christoulas, S., Bouckvalaas, G., Giannaros, C., 2000. An experimental study on themodel stone columns. Soils Found. 40 (6), 11e22.

Chummer, A.V., 1972. Bearing capacity theory from experimental results. J. SoilMech. Found. Div. ASCE 98 (12), 1311e1324.

Cowland, J.W., Wong, S.C.K., 1993. Performance of a road embankment on soft claysupported on a geocellmattress foundation. Geotext. Geomembr.12 (8), 687e705.

Dash, S.K., Bora, M.C., 2013. 2013. Influence of geosynthetic encasement on theperformance of stone columns floating in soft clay. Can. Geotech. J. 50 (7), 754e765.

Dash, S.K., Krishnaswamy, N.R., Rajagopal, K., 2001. Bearing capacity of strip footingsupported on geocell-reinforced sand. Geotext. Geomembr. 19 (4), 235e256.

Dash, S.K., Sireesh, S., Sitharam, T.G., 2003a. Model studies on circular footingsupported on geocell reinforced sand underlain by soft clay. Geotext. Geo-membr. 21 (4), 197e219.

Dash, S.K., Sireesh, S., Sitharam, T.G., 2003b. Behaviour of geocell reinforced sandbeds under circular footing. Ground Improv. 7 (3), 111e115.

Dash, S.K., Rajagopal, K., Krishnaswamy, N.R., 2004. Performance of different geo-synthetic reinforcement materials in sand foundations. Geosynth. Int. 11 (1),35e42.

Dash, S.K., Rajagopal, K., Krishnaswamy, N.R., 2007. Behaviour of geocell reinforcedsand beds under strip loading. Can. Geotech. J. 44 (7), 905e916.

Dash, S.K., Reddy, P.D.T., Raghukanth, S.T.G., 2008. Subgrade modulus of geocell-reinforced sand foundations. Ground Improv. 12 (2), 79e87.

Deb, K., Chandra, S., Basudhar, P.K., 2007. Generalised model for geosynthetic-reinforced granular fill-soft soil with stone columns. Int. J. Geomech. ASCE 7(4), 266e276.

Deb, K., Samadhiya, N.K., Namdeo, J.B., 2011. Laboratory model studies on unrein-forced and geogrid-reinforced sand bed over stone column-improved soft clay.Geotext. Geomembr. 29 (2), 190e196.

Fakher, A., Jones, C.J.F.P., 1996. Discussion of bearing capacity of rectangular footingson geogrid reinforced sand, by Yetimoglu, T., Wu, J.T.H., Saglamer, A.J. Geotech.Eng. ASCE 122 (4), 326e327.

Hughes, J.M.O., Withers, N.J., 1974. Reinforcing of soft cohesive soils with stonecolumns. Ground Eng. 7 (3), 42e49.

Juran, I., Guermazi, A., 1988. Settlement response of soft soils reinforced by com-pacted sand columns. J. Geotech. Eng. ASCE 114 (8), 930e942.

Latha, G.M., Somwanshi, A., 2009. Effect of reinforcement form on the bearing ca-pacity of square footings on sand. Geotext. Geomembr. 27 (6), 409e422.

Leshchinsky, B., Ling, H., 2013. Effects of geocell confinement on strength anddeformation behaviour of gravel. J. Geotech. Geoenviron. Eng., ASCE 139 (2),340e352.

Madhavi, G.L., Vidya, S.M., 2007. Effects of reinforcement form on the behaviour ofgeosynthetic reinforced sand. Geotext. Geomembr. 25 (1), 23e32.

Malarvizhi, Ilamparuthi, 2007. Comparative study on the behaviour of encasedstone column and conventional stone column. Soils Found. 47 (5), 873e885.

McKelvey, D., Sivakumar, V., Bell, A., Graham, J., 2004. Modeling vibrated Stonecolumns on soft clay. Geotech. Eng. Proc. Inst. Civil Eng. 157 (3), 137e149.

Mayerhof, G.G., Sastry, V.V.R.N., 1978. Bearing capacity of piles in layered soils: part2. Can. Geotech. J. 15 (2), 183e189.

Pinto, M.I.M., Cousens, T.W., 1999. Modelling a geotextile-reinforced, brick-faced soilretaining wall. Geosynth. Int. 6 (5), 417e447.

Selig, E.T., McKee, K.E., 1961. Static and dynamic behaviour of small footings. J. SoilMech. Found. Div. ASCE 87 (6), 29e47.

Sireesh, S., Sitharam, T.G., Dash, S.K., 2009. Bearing capacity of circular footing ongeocell-sand mattress overlying clay bed with void. Geotext. Geomembr. 27 (2),89e98.

Tanyu, B.F., Aydilek, A.H., Lau, A.W., Edil, T.B., Benson, C.H., 2013. Laboratory eval-uation of geocell-reinforced gravel subbase over poor subgrades. Geosynth. Int.20 (2), 47e61.

Webster, S.L., Watkins, J.E., 1977. Investigation of Construction Techniques forTactical Bridge Approach Roads across Soft Ground. Technical Report S-77e1.United States Army Corps of Engineers, Waterways Experiment Station, Mis-sissippi, USA.

Wood, D.M., Hu, W., Nash, D.F.T., 2000. Group effects in stone column foundations:model tests. Geotechnique 50 (6), 689e698.

Zhou, H., Wen, X., 2008. Model studies on geogrid or geocell-reinforced sandcushion on soft soil. Geotext. Geomembr. 26 (3), 231e238.