articol

da

72181

havunvisity clay.e th

diameter of the footing, beyond which, further increase in bearing capacity of the composite foundation

f infrasreasedustry tble forementment

construction and overall economy that nds favour with the

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.

2003b, 2004; Zhou and Wen, 2008; Sireesh et al., 2009;

attress and stoneprovement of softthough have beenoth has remainedattress with stoneload carrying ca-ushion of sand is

generally 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 exible raft over the stone columns, similar to that of a

* Corresponding author. Tel.: 91 3222 282418; fax: 91 3222 282254.

Contents lists availab

G

sev

Geotextiles and Geomembranes 41 (2013) 26e35E-mail address: [email protected] (S.K. Dash).practicing engineers.A stone column is a column of stones, made through opening up

a vertical cylindrical hole in the soft clay bed and subsequentlylling 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 tosignicant performance improvement of foundation beds (Hughesand Withers, 1974; Juran and Guermazi, 1988; Christoulas et al.,

Leshchinsky and Ling, 2013; Tanyu et al., 2013).Review of literature shows that geocell-sand m

columns are effective means of performance imclay foundations. Their individual applicationsintensely studied, but combined application of bunexplored. It is expected that the geocell-sandmcolumns underneath shall further enhance thepacity of the foundation system. Moreover, a cpopular ones. This is primarily due to their simplicity, ease of et al., 1990; Cowland and Wong, 1993; Dash et al., 2001, 2003a,Stone columnsGround improvement

1. Introduction

Rapid urbanisation and growth odays, has resulted in dramatically incThis has compelled the building indgrounds which otherwise are unsuitaAmongst the various ground improvcolumns and geosynthetic reinforce0266-1144/$ e see front matter 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.geotexmem.2013.09.001bed is marginal. 2013 Elsevier Ltd. All rights reserved.

tructure, in the presentdemand for land space.o improve the soft soilconstruction activities.techniques used, stoneare probably the most

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 connement 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; BushFoundationGeosyntheticsKeywords:length 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 theImproved performance of soft clay founand geocell-sand mattress

Sujit Kumar Dash a, *, Mukul Chandra Bora b

a Department of Civil Engineering, Indian Institute of Technology Kharagpur, Kharagpurb Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati 7

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 21 September 2013

a b s t r a c t

A series of experimentsimprovement of soft clay fois found that with the proincreases in bearing capacthat of the unreinforced creinforcement, can improv

Geotextiles and

journal homepage: www.elAll rights reserved.tions using stone columns

302, India039, India

e been carried out to develop an understanding of the performancedation beds using stone column-geocell sand mattress as reinforcement. Ition of stone columns, of adequate length and spacing, about three foldan be achieved. While with geocell-sand mattress it is about seven timesBut if combined together, the stone column-geocell mattress compositee bearing capacity of soft clay bed as high as by ten fold. The optimum

le at ScienceDirect

eomembranes

ier .com/locate/geotexmem

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 inuence 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-uence 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

LStone column

Clay

Sand h

u

Geocell layer

DT DT DT DTDT DTDTDT

Footing

D D D

(0,0) x/Dx/Dtransverse member diagonal member

bodkin joint

b

b

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

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

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

6 GC SC Variable parameter: L/dsc 1, 3, 5, 7

S.K. Dash, M.C. Bora / Geotextiles and Geomembranes 41 (2013) 26e35 27piled 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 conning 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 conguration is shown inFig. 1. The stone columns were left oating 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 maximum

Fig. 1. Schematic diagram of test conguration.performance improvement (Dash et al., 2008).

S

dscS

Stone column

Fig. 2. Layout of stone columns.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 1, 3, 5, 7

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

Constant parameter: L/dsc 54 GC Variable parameter: h/D 0.53, 0.9, 1.1, 1.6Constant parameter:h/D 0.9, dgc/D 0.8, b/D 6, S/dsc 2.5

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

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

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




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

times the diameter of the stone column (dsc). Mayerhof and Sastry

andinvestigate 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 ner 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 Unied Soil Classi-cation System (USCS, ASTM D2487, 2006) the soil can be classiedas 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 uniformitycoefcient 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 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 lled with a poorly

0.001 0.010 0.100 1.000 10.000 100.000

Particle size (mm)

0

20

40

60

80

100

Perc

entf

iner

byw

eigh

t

stone aggregate

sand

clay

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

S.K. Dash, M.C. Bora / Geotextiles28graded sand that had average particle size of 0.44 mm and uni-formity coefcient 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 xed onto the ground. The load transmitted to thefooting 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 exible 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 7dsc) and the geocell mattresses used had maximum height of255 mm (i.e. h 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 (700 255)]. This is about 3.45

Fig. 5. Test set-up.Geomembranes 41 (2013) 26e35(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 exible, 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 connement due to the tank walls simulated theactual eld 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

andfor 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 coef-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 pipe

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

S.K. Dash, M.C. Bora / Geotextileswas 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 outsidedoesnt 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 lled with sand through raining. Compared tounreinforced 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 ow 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 eld 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 lling the shaft with Plaster of Paris (CaSO4$0.5H2O)paste. After being hardened, the Plaster of Paris column was taken

0

4

8

12

16

20

24

28

Foo

ting

settl

emen

t ,s/D

(%)

0 20 40 60 80 100 120 140 160Bearing 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: inuence of length of stone columns incomposite foundation bed (h/D 0.53, S/dsc 2.5) e Test series 5.

Geomembranes 41 (2013) 26e35 29out 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 couldnt support additional pressure anymore. However,with geocell reinforcement (Clay 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 GC SC)

mpr

%

2.23 2.36 2.48 2.66 2.88 2.954.29 4.32 4.41 4.73 5.02 5.13

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

Test series Reinforcement Variable parameter Bearing capacity i

s/D 1% s/D 5

2 SC L/dsc 1 1.69 1.25SC L/dsc 3 1.81 1.54SC L/dsc 5 3.59 2.87SC L/dsc 7 3.69 3.20

3 SC S/dsc 1.5 3.66 2.90SC S/dsc 2.5 3.59 2.87SC S/dsc 3.5 1.97 1.47

4 GC h/D 0.53 1.34 1.50GC h/D 0.9 2.10 2.18GC h/D 1.1 2.90 4.10GC h/D 1.6 4.20 4.81

5 GC SC L/dsc 1 1.72 1.98GC SC L/dsc 3 1.87 2.22GC SC L/dsc 5 4.12 4.26GC SC L/dsc 7 4.39 4.79



S.K. Dash, M.C. Bora / Geotextiles30the bearing capacity has improved signicantly, 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 quantied through nondimensional improvementfactors, IFsc, IFgc, IFgcsc respectively; dened 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 3.7, Test series 3), and with geocell reinforcement itcan be up to 7.87 fold (IFgc 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


9 GC SC S/dsc 1.5 4.54 4.82GC SC S/dsc 2.5 4.12 4.26GC SC S/dsc 3.5 3.28 3.44




Note: Boldface values indicate the maximum improvement factors for different reinforc4.87 4.98 5.02 5.30 5.57 5.692.88 3.12 3.45 3.79 4.17 4.353.28 3.48 3.70 4.08 4.36 4.504.70 5.09 5.45 5.92 6.43 6.735.65 5.89 6.12 6.68 7.20 7.465.18 5.74 6.15 6.86 7.46 7.785.94 6.31 6.69 7.22 7.77 8.016.63 7.00 7.40 8.01 8.50 8.747.24 7.55 7.98 8.50 9.13 9.42ovement factor (IF)

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

1.14 1.19 1.23 1.26 1.28 1.301.53 1.60 1.62 1.66 1.73 1.732.93 3.12 3.19 3.31 3.40 3.443.17 3.37 3.40 3.48 3.56 3.603.20 3.45 3.49 3.61 3.67 3.702.93 3.12 3.19 3.31 3.40 3.441.67 1.84 1.81 1.86 1.90 1.931.64 1.86 1.95 2.11 2.26 2.322.83 3.03 3.35 3.59 4.11 4.334.77 5.57 6.00 6.60 7.12 7.395.43 6.14 6.54 7.08 7.64 7.871.91 2.01 2.15 2.30 2.46 2.53

Geomembranes 41 (2013) 26e35high as by 10 fold (IFgcsc 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. Inu-ence of different parameters on the overall performance of suchcomposite foundation systems are discussed in the followingsections.

3.1. Inuence of length of stone columns

Inuence of length of stone columns (L), in the compositefoundation bed, has been studied for four different heights ofgeocells, h 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 veried through prototypetests. The results are further analysed in terms of the improvementfactor ratios (i.e. IFgcsc/IFgc and IFgcsc/IFsc) that brings out the

5.55 6.40 7.20 8.01 8.72 9.016.00 7.00 7.55 8.25 9.02 9.246.84 7.41 7.96 8.59 9.28 9.597.38 7.82 8.32 8.93 9.56 9.884.84 4.95 5.16 5.53 5.79 5.884.29 4.32 4.41 4.73 5.02 5.133.50 3.52 3.54 3.65 3.79 3.895.57 5.85 6.24 6.75 7.24 7.474.70 5.09 5.45 5.92 6.43 6.733.64 4.08 4.16 4.55 4.90 5.067.34 7.60 7.97 8.53 9.07 9.376.63 7.00 7.40 8.01 8.50 8.745.97 6.17 6.65 7.30 7.89 8.227.29 7.81 8.40 9.20 9.87 10.26.84 7.41 7.96 8.59 9.28 9.596.43 7.00 7.50 8.19 8.82 9.06

ements (i.e. SC, GC and SC GC).

readings 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 (d) on the ll surface has reduced. Corre-spondingly, heave in the adjacent region (x 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 contribution

0 4 8 12 16 20 24 28

Footing settlement, s/D(%)

0

1

2

3

4

5

Impr

ov

emen

tfac

tor

ratio

,IF

gcsc

/IFg 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 0.53, S/d 2.5) e Test series 5.

0

1

2

3

4

5engt

hof

sto

ne

colu

mn

,L/

d sc

0 5 10 15 20 25 30

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

L/dsc= 1

L/dsc= 3

S.K. Dash, M.C. Bora / Geotextiles and Geomembranes 41 (2013) 26e35 31contribution of individual reinforcement components (i.e. geocellsand mattress, stone columns), as has been explained below.

IFgcscIFsc

qgcscquqscqu

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 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 didnt contribute signicantly 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 conrmation of the column failure modes was obtainedfrom the post test deformation proles, a typical of which for thecentral stone columns are shown in Fig. 10. It could be observed

scthat when short in length (L/dsc 1 and 3) the bulging in the stone

0 4 8 12 16 20 24 28


0

1

2

3

4

5

Impr

ov

emen

tfac

tor

ratio

,IF g

csc/

IFsc


Fig. 9. Improvement factor ratio-footing settlement responses: contribution of geocellmattress in composite foundation bed (h/D 0.53, S/dsc 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 bulgedsignicantly. 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 ll surface depicted in Fig. 11. Thesurface deformations (d), reported herein, are the average of the

6

7

L

L/dsc= 5

L/dsc= 7

Fig. 10. Post-test deformed prole of central stone column: in composite foundationbeds (h/D 0.53, S/dsc 2.5) e Test series 5.to the performance improvement. Stone column length beyond the

0 4 8 12 16 20 24 28


0

1

2

3

4

5

Ave

rage

surfa

ce d

efor

amat

ion,

/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: inuence of length of stonecolumn in composite foundation bed (x/D 1, h/D 0.53, S/dsc 2.5) e Test series 5.

0 4 8 12 16 20 24 28


0

1

2

3

4

Impr

ov

emen

tfa

ctor

ratio

,IF g

csc/

IFgc

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 stone

and Geomembranes 41 (2013) 26e35optimum (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 ndings 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. Inuence 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 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 behave

0

4

8

12

16

20

24

28

Foo

ting

settl

emen

t, s/D

(%)

0 20 40 60 80 100 120 140 160Bearing pressure (kPa)

Clay

Clay+GC


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

S.K. Dash, M.C. Bora / Geotextiles32as individual entities leading to reduced lateral resistance onto thecentral conned region. In the absence of adequate connementfrom the surrounding, the central stone column underneath thefooting bulged prematurely and therefore couldnt 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 connement 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 1.5dsc) and reduced as the spacing was increased (Fig. 13). Incontrast, when the stone columns underneath were placed wideapart (S 3.5dsc), the geocell mattress has carried maximum load,IFgcsc/IFsc, which however reduced signicantly 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 ll surface, observed withreduced spacing of stone columns, testies 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 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-nicant 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 beds

columns in composite foundation bed (h/D 0.9, L/dsc 5) e Test series 10.can be taken as 2.5dsc.

0 4 8 12 16 20 24 28


0

1

2

3

4

Impr

ov

emen

tfac

t or

ratio

,IF

gcsc

/IFs c


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

yielding 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 1.6D and 1.1D, was less than5%. Therefore, height of geocell mattress equal to about the diam-eter of the footing (h D), can be taken as the optimum one, givingmaximum possible performance improvement in the composite

0 4 8 12 16 20 24 28


0.0

0.5

1.0

1.5

2.0

2.5

3.0

Impr

ov

emen

tfcc

tor

ratio

,IF

gcsc

/IFgc


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

S.K. Dash, M.C. Bora / Geotextiles and3.3. Inuence of height of geocell mattress

Typical responses of the composite system, for three differentheights of geocell mattress (h 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 0.53D) the geocell mattress has underperformed that the stone columns have shared nearly three timesmore load (IFgcsc/IFgc 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 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 inll 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 the

post-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


0

2

4

6

8

10

Impr

ov

eme n

tfac

tor

rati o

,IF

gcsc

/ IFsc


Fig. 16. Improvement factor ratio-footing settlement responses: contribution of geo-cell mattress in composite foundation bed (h/D 1.6, S/dsc 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, h 0.53D, 0.9D, 1.1D and1.6D, the maximum bearing capacity improvement, IFgcsc 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 and

0

1

2

3

4

5

6

7

Len

gth

ofs

tone

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 prole of central stone column: in composite foundationbeds (h/D 1.6, S/dsc 2.5) e Test series 8.

Geomembranes 41 (2013) 26e35 33foundation beds. However, full-scale tests are required to verify thisobservation.

It is of interest to note that even geocell mattress of mediumheight, h 0.9D, when combined with stone columns can providebearing capacity improvement (IFgcsc 7.46, Test series 6) as highas that of a deep geocell mattress, h 1.6D (IFgcsc 7.87, Test series4). Due to practical constraints, at times, it might be difcult 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 inuencing the response ofgeocell-stone column reinforced foundation systems can be

mensions (i.e. length and force) and therefore can be studied by 13

p m p m m

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; pocket

areas 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

LD

S

d

G

Kg

c

q

andsize 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 conrm 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 withindependent parameters (p1, p2, p3, p4...p13; Buckingham,1914). Hence equation (2) can be written as

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

DpDm

n (4)

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

GpDpgp

GmDmgm

(5)

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

GpGm

DpDm

n (6)

Considering similarity of the p10 terms:

KpgpG2

KmgmG2

:KpG2

KmG2

:KpKm

G2p

G2 n2 (7)

gp1;p2;p3;p4...p13 g s

D

;

hD

;

dgcD

;

hdgc

;

dscD

;summarised as: D, h, dgc, K, dsc, L, d50, S, , cu, g, G; where 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 dened previously. The function (f) thatgoverns the composite foundation system can be written as

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

0 (2)

It comprises of 15 parameters and has two fundamental di-

S.K. Dash, M.C. Bora / Geotextiles34geocell reinforcement it is of the order of 7.8 fold. When coupledThe authors are thankful to the anonymous reviewers for theirvaluable comments and suggestions for improvements of the pre-sentations in the paper.

Notation

Cc coefcient of curvatureCu coefcient of uniformityD diameter of footingdgc diameter of geocellsdsc diameter of stone columnemax maximum void ratioe minimum void ratioreduced. 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 eld trails are required to verifythese observations.

The ndings 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 stabilised

;D

; 50dsc

;Dg

;G2

;u

Dg;

gcsc

qu;f 0 (3)below 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 havetogether, 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 signicantly 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 columns

Geomembranes 41 (2013) 26e35minh height of geocell mattress

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 footing

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Improved performance of soft clay foundations using stone columns and geocell-sand mattress1 Introduction2 Details of model tests2.1 Planning of experiments2.2 Materials used2.3 Test setup2.4 Test bed preparation2.5 Test procedure

3 Results and discussion3.1 Influence of length of stone columns3.2 Influence of spacing of stone columns3.3 Influence of height of geocell mattress

4 Scale effect5 ConclusionsAcknowledgementNotationReferences

articol

Documents

compacted stone aggregates

soft soil counterpart

soft clay bed

soft clayaround

bearing capacity

geocellsand mcolumns

reinforced soil cushionserves

foundation systemdeb