ARTICLE

Influence of geosynthetic encasement on the performance of stonecolumns floating in soft claySujit Kumar Dash and Mukul Chandra Bora

Abstract: This paper investigates the influence of geosynthetic encasement on the performance of stone columns floating in softclay. It was found that with unencased columns the bearing capacity improvement is about 3.5 fold, but with geogrid encase-ment the improvement increases to 5 fold, where 60% of the column length is encased. With full-length encasement (i.e., 100%),the improvement is only about 3 fold. It is therefore evident that partially encased floating columns are superior to the fullyencased ones. In contrast, with end-bearing stone columns, full-length encasement is reported to have exhibited better perfor-mance improvement than the partially encased ones. In the former case (floating columns), it is the bulge formation at a deeperdepth that enhances the bearing capacity, while in the latter case (end-bearing columns), it is the stiffening effect of theencasement that enables the column to transmit the surcharge pressure onto the competent strata below.

Key words: foundations in clay, floating stone columns, geogrid encasement, model tests.

Résumé : Cet article étudie l’influence d’un revêtement de géosynthétique sur la performance de colonnes de pierre flottantesdans l’argile molle. Il a été déterminé que pour des colonnes sans revêtement, l’amélioration de la capacité portante estd’environ 3,5 fois, mais avec un revêtement fait de géogrille l’amélioration augmente jusqu’a 5 fois, lorsque 60 % de la longueurde la colonne est recouverte. Lorsque la longueur totale est recouverte (c’est-a-dire 100 %), l’amélioration est de seulementenviron 3 fois. Ainsi, il est évident que les colonnes flottantes partiellement recouvertes sont supérieures aux colonnes com-plètement recouvertes. Par contre, pour les colonnes de pierre a pointe portante, le revêtement complet offre une meilleureamélioration de la performance que lorsque le revêtement est partiel. Dans le premier cas (colonnes flottantes), c’est laformation d’un renflement en profondeur qui augmente la capacité portante, tandis que dans le deuxième cas (colonnes apointes portantes), c’est l’effet de rigidité offert par le revêtement qui permet a la colonne de transmettre la pression desurcharge sur la couche de sol sous-jacente. [Traduit par la Rédaction]

Mots-clés : fondations dans l’argile, colonnes de pierre flottantes, revêtement en géogrille, essais modèles.

IntroductionSoft clay deposits with low strength and high compressibility

often pose serious problems to geotechnical structures. Installingstone columns can significantly improve the performance of suchweak soils (Greenwood 1970; Hughes et al. 1975; Charles andWatts1983; Mitchell and Huber 1985; Rao et al. 1997; McKelvey et al.2004; Ambily and Gandhi 2007; Black et al. 2011). Formed throughcompacting aggregates into the soft soil, stone columns are ofteneconomically viable and environmentally acceptable.

Typical stone column lengths range from 3 to 15 m (McKelveyet al. 2004) although, with state-of-the-art equipment availablethese days, they can go up to 30 m (Black et al. 2007). A very longstone column generally fails through excessive bulging, right un-der the point of loading, leaving much of its bearing capacityimmobilized (Hughes and Withers 1974; Hughes et al. 1975;McKelvey et al. 2004). This problem is more acute when the col-umns are installed in soft clay that offers very low lateral confine-ment. To improve capacity against bulging, Van Impe (1989)proposed the concept of encasing the columns with geotextile.Subsequently, several studies were reported highlighting the ben-eficial effect of this technology. Raithel et al. (2002) successfullyused geosynthetic-encased granular columns for founding a dykeover soft soil. A similar approach was used for the construction ofa high-speed railway embankment in the Netherlands and an air-craft factory in Germany (Black et al. 2007). Ayadat and Hanna

(2005) reported that encasement of stone columns is highly ben-eficial in collapsible soils. Malarvizhi and Ilamparuthi (2007) andMurugesan and Rajagopal (2007, 2010) observed that an increasein stiffness from the encasement results in a noticeable increasein load capacity of stone columns. Therefore, use of stiffer geogridsfor encasement would be advantageous as compared to that offlexible geotextiles.

Most previous studies have focused on end-bearing stone col-umns that rest on a strong stratum. However, in the case ofvertically extensive soft clay deposits, a situation commonly en-countered in coastal areas, the stone columns are left floating inthe clay. The work reported herein investigates this issue throughphysical model tests and provides valuable insight into the influ-ence of geogrid encasement on the performance of stone columnsfloating in soft clay.

Experimental programThe experimental program consisted of a series of model plate

load tests with stone columns in a soft clay bed. Tests were alsocarried out on unreinforced clay beds, without stone columns.The details of parameters studied, materials used, experimentalsetup, and test procedure are presented in the following sections.

Details of model testsThe schematic diagram of a typical test configuration is shown

in Fig. 1. The stone columns were installed at uniform spacing,

Received 26 November 2012. Accepted 23 April 2013.

S.K. Dash. Department of Civil Engineering, Indian Institute of Technology Kharagpur, Kharagpur – 721 302, India.M.C. Bora. Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati – 781 039, India.

Corresponding author: Sujit Kumar Dash (e-mail: sujit@civil.iitkgp.ernet.in).

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Can. Geotech. J. 50: 754–765 (2013) dx.doi.org/10.1139/cgj-2012-0437 Published at www.nrcresearchpress.com/cgj on 29 April 2013.

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S, in the arrangement shown in Fig. 2. The diameter of the columns(dsc) was kept constant at 100 mm. Four different series of modeltests were conducted, the details of which are presented inTable 1. In series 1, load–deformation behaviour of unreinforcedclay beds were investigated. In series 2 and 3, stone column rein-forced clay beds were studied wherein influence of length (L) andspacing (S) of columns were investigated. Four different lengths,100, 300, 500, and 700mm lengths, representing the L/dsc ratios of1, 3, 5, and 7 were considered. Depending on the test configura-tion, columns were placed at a spacing (S) of 1.5dsc, 2.5dsc, and3.5dsc that corresponds to area replacement ratios (i.e., area ofstone column over its tributary area; Poorooshasb and Meyerhof1997) of 7%, 14%, and 40%, respectively. Under similar test condi-tions Rao et al. (1997), Ambily and Gandhi (2007) found no signif-icant performance improvement when the spacing was increasedbeyond 3dsc. Therefore, in the present study, column spacingwithin this range (1.5dsc–3.5dsc) has been investigated. Influence ofgeosynthetic encasement on the performance of stone columnswas studied under test series 4. Three different lengths of encase-ment, Lesc equal to 1dsc, 3dsc, and 5dsc, were considered. The Lesc/dscratios of 1 and 3 represent 20% and 60% of partial encasements,respectively, while Lesc/dsc of 5 represents 100%, or full-length en-casement. In all these tests the length and spacing of columnswere kept constant at their critical values, i.e., L = 5dsc and S =2.5dsc, as obtained from tests in series 2 and 3.

Materials usedA locally available soil that had 70% fraction finer than 75 �m

was used for forming the test beds. It had a liquid limit of 40%,plastic limit of 21%, and as per ASTM (2006) D2487-06 was classi-fied as clay with low plasticity, CL.

The aggregates used to form the columns were angular crushedstones with particle sizes in the range of 2–10 mm. The stones hada uniformity coefficient of 2.32, curvature coefficient of 0.88, andwere classified as poorly graded gravel with letter symbol GP(ASTM (2006) D2487-06). The peak friction angle at a placementdensity of 15.3 kN/m3, obtained through direct shear tests, was48°. The diameter of the stone columns and average size of aggre-gates used (dsc = 100 mm, D50 = 4.9 mm) are approximately aone-seventh scale representation of a prototype with diameter of700 mm and average aggregate size of 35 mm.

The geogrid encasement wasmodelled through a commerciallyavailable window-mesh, referred to as “geomesh” in this paper. Itwas made of unoriented polymer and had diamond-shaped aper-ture openings of 2 mm × 2 mm. Its ultimate tensile strength asobtained throughwide width tension tests (ASTM (2001) D6637-01)was 2.9 kN/m. Given its low strength and scaled geometry, it issuitable for model tests. The encasement was formed throughstitching of a piece of geomesh sheet into a sleeve of 100 mmdiameter with a 10 mm of overlap at the joint. The seam tensilestrength was found to be 2.45 kN/m (ASTM (2009) D4884-09).

Test setupA schematic diagram of the test setup is shown in Fig. 3. The test

bedswere formed in a steel tankmeasuring 1000mm long, 1000mmwide, and 1300 mm deep. The circular footing used was made ofsteel and measured 150 mm in diameter (D). In all tests it wasplaced over the central stone column coinciding with the centreof the tank. The larger footing (D = 1.5dsc) ensures that the columnis fully loaded, even after bulging.

Load was applied through an automated hydraulic system. Theload transferred to the foundation was recorded through an elec-tronic load cell of 20 kN capacity with an accuracy of 0.01 N leastcount. The resulting settlements of the footing were measuredthrough two linear variable differential transducers (LVDTs),placed at diagonally opposite ends. Deformations of the clay sur-face, at distances (x) of 1D, 2D, and 3D, from the footing centre onboth the sides were also measured by LVDTs. All these sensorswere connected to a data acquisition system.

Fig. 1. Clay bed–stone column foundation system.

Fig. 2. Layout of stone columns.

Table 1. Details of model tests.

Testseries

Reinforcementtype Details of test parameters

1 Unreinforcedclay

Constant parameter: cu = 5 kPa

2 Stone column Constant parameter: cu = 5 kPa, S/dsc = 2.5Variable parameter: L/dsc = 1, 3, 5, 7

3 Stone column Constant parameter: cu = 5 kPa, L/dsc = 5Variable parameter: S/dsc = 1.5, 2.5, 3.5

4 Encased stonecolumn

Constant parameter: cu = 5 kPa, L/dsc = 5,S/dsc = 2.5

Variable parameter: Lesc/dsc = 1, 3, 5

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Fig. 3. Schematic diagram of test setup.

Fig. 4. Bearing pressure versus footing settlement responses: influence of length of stone columns — test series 2. SC, stone column.

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The rupture surface in the clay bed typically spreads over adistance of about 4 to 5 times the footing width (Selig and McKee1961; Chummar 1972). The test tank used was 6.6 times wider thanthe footing, therefore, the slip planes were not likely to be af-fected by the rigid boundary (i.e., tank side wall). Similarly, therupture surface below a pile is limited to a maximum depth ofabout twice the pile diameter (Meyerhof and Sastry 1978). In con-trast, the stone columns used herein had a maximum length of700 mm (L/dsc = 7) so that the minimum clearance at the bottomwas 600mm (i.e., 1300–700mm), equivalent to 6dsc. Therefore, thesize of the test tank was large enough to overcome the boundaryeffects.

Test bed preparationThe soil was mixed with the desired amount of water and was

kept in airtight containers for about 1 week. This was to achievemoisture equilibrium in the soil mass leading to uniform mois-ture content throughout. The clay test bed was prepared in layersof 0.05 m thickness. For each layer, the amount of soil required toproduce the desired bulk density was weighed out and placed inthe test tank, then levelled and compacted. Compaction wasachieved using a wooden board and a drop hammer and markingdepths on the walls of the tank as a guide. Undisturbed soil sam-ples were collected from different locations in the test bed andwere evaluated for their properties. The average moisture con-tent, degree of saturation, bulk unit weight, and vane shear

Fig. 5. Improvement factor versus footing settlement: influence of length of stone columns — test series 2.

Fig. 6. Surface deformation, at x = D, versus footing settlement responses: influence of length of stone columns — test series 2.

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strength (cu) were found to be 36%, 100%, 18.05 kN/m3, and 5 kPa,respectively, with a coefficient of variability less than 1.5%.

The stone columns were formed by replacement technique. Atthe designated location, a thin-walled stainless steel pipe measur-ing 100mm in outer diameter was slowly pushed into the clay beduntil it reached the depth at which the column was to be formed.The pipewas lubricatedwith petroleum jelly so that it could easilybe handled without significant disturbance to the surroundingsoil. Having driven to the target depth, the clay within the pipewas gradually scooped out through a helical auger of 90 mmdiameter. Tominimize suction, amaximumof 100mmof claywasremoved at a time. On completion, the cavity was a charged witha pre-measured quantity of stone aggregate and compacted inlayers of 50 mm height. Each layer of aggregate was compacted

uniformly by a 0.9 kg circular steel tamper with 30 blows from a200 mm drop. Correspondingly, the casing pipe was raised instages ensuring aminimum casing overlap of 25 mm between theaggregate and clay. The procedure was repeated until the columnwas completely formed. The average density of the aggregates inthe stone columns was 15.3 kN/m3. For construction of the en-cased stone columns, the casing pipewith the geomesh sleevewaspushed into the soil bed. Subsequently, the clay within was takenout and the stone aggregate was placed and compacted in thesame way as in the unreinforced case. Simultaneously, the casingpipewas pulled out leaving the geomesh sleeve in place, wrappingaround the aggregates and thus forming the encased stone col-umn. Once all the columns were formed the foundation bed wasloaded with a surcharge of 2.5 kN/m2, for 4 h. This was to neutral-

Fig. 7. Surface deformation, at x = 2D, versus footing settlement responses: influence of length of stone columns — test series 2.

Fig. 8. Surface deformation, at x = 3D, versus footing settlement for different lengths of stone columns — test series 2.

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ize the local disturbances and attain uniformity in the test bed(Malarvizhi and Ilamparuthi 2007).

Test procedureLoad was strain-controlled and applied at a rate of 2 mm/min.

This fast rate of loading was intended to produce an undrainedresponse, thus simulating worst-case conditions where the fric-tion angle tends to zero. The footing was loaded until a settlementof 40 mm was reached. The load and displacement data werecontinuously recorded through the computerized data acquisi-tion system.

After testing, the deformed shapes of the stone columns wererecorded. This was done through carefully removing the aggre-

gates from the column and filling the shaft with a plaster of Parispaste. The plaster of Paris (CaSO4·0.5H2O) is a powder materialthat when mixed with water forms a thick paste. Due to a highviscosity it does not intrude into the surrounding soil and hardenswithin 1 day. Subsequently, the solidified plaster of Paris columnwas exhumed and its exterior dimension, which is the deformedshape of the stone column, was mapped.

Test results and discussionThe test results are presented in form of bearing pressure–

settlement responses, deformation patterns of the clay surface,and bulge characteristics of stone columns. The reported settle-

Fig. 9. Post-test radial strain in central stone columns of different length — test series 2.

Fig. 10. Improvement factor versus footing settlement: influence of spacing of stone columns — test series 3.

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ments of the footing (s) and deformations of the clay surface (�) arethe average of the readings taken at both sides of the footing.Settlements are shown through a (+) sign and heave through a (−)sign. The stone column deformations are presented in the form ofradial strain: (rd − ro)/ro; where rd is the deformed radius and ro isthe original radius. Analysis of test data and discussions are pre-sented in two different sections: (i) behaviour of conventionalstone columns and (ii) behaviour of encased stone columns.

Behaviour of conventional stone columnsTypical bearing pressure–settlement responses depicting the

influence of conventional stone columns (i.e., without encase-ment) on the performance of clay foundations are illustrated inFig. 4. It can be observed that with clay alone (i.e., without stonecolumns) the slope of the pressure–settlement response continuesto increase until it reaches a settlement of about 16%, where ittends to be nearly vertical. This suggests that the soil failed be-cause the footing was unable to sustain additional pressure andpunched down into the soil. With stone columns, however, suchpronounced failure was not observed until large settlements hadoccurred and the bearing capacity had shown a marked increase.Additionally, the pressure–settlement responses with stone col-umns are much stiffer than that of the unreinforced soil, indicat-ing that the stone column reinforcement can reduce the footingsettlement substantially.

The improvement in bearing capacity due to the stone columnsis quantified through a nondimensional factor, IFsc, defined as theratio of bearing pressurewith stone columns at a given settlementto that of unreinforced soil at the same settlement. Variations ofbearing capacity improvement factor, IFsc, with settlement offooting for varied column lengths are shown in Fig. 5. Initially thevalue of IFsc is found to be high, after which it reduces with in-crease in footing settlement. However, for settlement (s/D) beyond5%, the improvement factor continues to increase again. The ini-tially high improvement in bearing capacity is due to the stiffen-ing effect of the stone columnsmobilized through interlocking ofthe compacted aggregates. This effect reduces as the stone col-umn deforms and the aggregates dilate. With dilation takingplace, interlocking of stone particles breaks down giving rise toreduced strength and stiffness of the system. However, after a

threshold limit of deformation, the shear resistance of the soilbegins to mobilize leading to increased load-carrying capacity.

In general, an increase in length of stone columns leads to anincrease in bearing capacity and reduction in footing settlement.When the length of columns is increased from 3dsc to 5dsc, theperformance improvement increases substantially. This is attrib-uted to adequate mobilization of skin resistance and end-bearingcapacity through an increase in peripheral area and overburdenpressure indicating that the columns have effectively supportedthe surcharge loading, leading to enhanced performance im-provement. Besides, a long column that effectively stands against

Fig. 11. Post-test radial strain in central stone columns with different spacing — test series 3.

Fig. 12. Post-test deformed shape of typical peripheral stonecolumn with S = 1.5dsc — test series 3.

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loading bulges significantly and therebymobilizes additional pas-sive resistance from the surrounding soil that leads to increasedperformance improvement. However, increasing the columnlength beyond 5dsc offers little significant improvement. This isbecause additional resistance due to the increase in length re-mains immobilized due to excessive bulging of the top of thestone column. Due to excessive bulging, the stone aggregates losetheir interlocking properties and continue to deform at their re-sidual strength leading to no significant additional performanceimprovement. It is evident that provision of floating stone col-umns of adequate length (i.e., ≥5dsc) can enhance the bearingcapacity of soft clay by 3.5 fold (IFsc = 3.5).

Typical deformation responses of the fill surface, measured atdistances (x) ofD, 2D, and 3D from center of the footing, are shownin Figs. 6, 7, and 8, respectively. It can be observed that in theregion around the footing (x = D, Fig. 6) the fill surface has under-gone settlement. This is attributed to stress dispersion and to thefact that an additionalmass of soil surrounding the footing settlesas well. In contrast, the adjacent regions show signs of heave(Figs. 7, 8). As the soil is fully saturated and the loading rate is rela-tively fast, in this analysis only the undrained condition is consid-ered. In such a situation, the compressibility of the soil is minimaland is more likely to heave. It can be seen that when the stonecolumn is short (L = dsc), the settlement on thefill surface is large andit reduces as the stone column length increases (Fig. 6). Correspond-ingly, the extent of heave is also reduced (Figs. 7, 8). Hence, it can besaid that relatively long stone columns (i.e., L ≥ 5dsc) can effectivelyreduce the differential settlements in the foundation bed, an addedadvantage against rut formation in railways and highways. This be-haviour can be better explained through bulging in the stone col-umns, depicted in Fig. 9. It can be observed that the short columns(L ≤ 3dsc) have not bulged significantly, indicating that they havepunched down into the soil, leading to large settlements on the claysurface. This is attributed to limited skin resistance owing to lessperipheral area and low end-bearing capacity due to less overburdenpressure from limited depth of surrounding soil. In comparison,relatively large bulging observed in the case of long columns (i.e., L≥3dsc) indicates that instead of punching down, these columns havesustained the footing pressure and hence have undergone radialexpansion. Increase in skin friction and end-bearing capacity due toincreased peripheral area and overburden pressure has provided

higher resistance against footing penetration, leading to reducedsettlement andheaveof the clay surface. Thebulging-inducedexpan-sion of the stone columnsmight have reduced the settlement on thesoil surface, and it is noted that with short columns, (L = dsc, 3dsc) thefill surface around the footing has continued to settle. In contrast,with long columns (L = 5dsc, 7dsc) there is little initial settlement, andsubsequently the surface shows signs of heave (Fig. 6). Settlementtakes place when the stone column readjusts itself through mobili-zation of its strength and stiffness. Once the mobilized strength issufficient, it effectively resists movement. This observation onceagain establishes the proposed load-carrying mechanisms of theshort and long stone columns, floating in soft clay.

From the above findings, it can be concluded that the optimumlength of floating stone columns giving maximum performanceimprovement is about 5 times its diameter (5dsc). Hughes et al.(1975) reported similar observations in the case of end-bearingstone columns where the critical length beyond which the col-umn fails through excessive bulging is about 4dsc. Hence, it can besaid that irrespective of the support condition, i.e., floating orend-bearing, stone columns longer than the critical limit areprone to bulging failure. The present findings on the criticallength of stone columns with 100mmdiameter (i.e., Lcri = 5dsc) arein favorable agreement with that of stone columns with differentdiameters. For example, Rao et al. (1997) observed that with col-umn diameters of 75, 50, and 32 mm, the critical length ratios(Lcri/dsc) are in the range of 5 to 6. Similarly, for a column diameterof 25 mm, McKelvey et al. (2004) observed that a critical lengthratio of 6 gives the maximum bearing capacity improvement.

Variation of bearing-capacity improvement factor (IFsc) withfooting settlement, for spacing ratios (S/dsc) of 1.5, 2.5, and 3.5, areshown in Fig. 10. It is evident that bearing capacity increases as thespacing of stone columns decreases. With reduced spacing, thestone columns surrounding the footing tend to behave like a con-fining ring, inducing enhanced confinement to the central stonecolumn and the soil mass around. Indeed, with reduced spacing,the bulging in the central stone column has been reduced signif-icantly (Fig. 11), caused by establishment of the confining effect ofthe surrounding stone columns. It is of interest to note that therewas a significant increase in bearing capacity when the columnspacing was reduced from 3.5dsc to 2.5dsc. Similar behaviour wasalso observed in the case of end-bearing columns (Ambily and

Fig. 13. Surface deformation, at x = D, versus footing settlement responses: influence of spacing of stone columns — test series 3.

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Gandhi 2007). This indicates that as the spacing reduces from 3dscto 2dsc, the stone columnbehaviour shifts fromnear-isolated to aninteracting mode, where it derives increased passive resistancefrom the adjacent columns, leading to a substantial increase inthe performance improvement. This agrees favourably with thefindings published by Hughes and Withers (1974), which showthat the soil mass beyond a distance of 2.5dsc is not influenced bythe loading on the stone column. When the spacing was reducedbelow 2.5dsc, further increase in the bearing capacity was mar-ginal (Fig. 10). Where spacing is small (i.e., S = 1.5dsc), thearching-induced resistance of the interspersed soil mass in-creases (Terzaghi 1943), which adds to the restraining effect of thestone columns, a mechanism similar to the contiguous pile sys-tem. As the footing continues to settle, lateral pressure on periph-

eral columns increases. When the lateral capacity of the stonecolumns is exceeded, the columns deform and the soil shears,leading to failure. This is evident from the observations that theperipheral stone columns, surrounding the footing, have under-gone visible lateral bending (Fig. 12) and the clay has shown sig-nificant heaving (S = 1.5dsc, Fig. 13). Based on these findings it canbe concluded that the optimum spacing of the floating stone col-umns, to give maximum performance improvement, is about2.5 times the diameter of the column (dsc).

Behaviour of encased stone columnsBearing pressure–settlement responses depicting the influence

of encasement on performance of floating stone columns areshown in Fig. 14. It can be seen that with partial encasement

Fig. 14. Bearing pressure versus footing settlement responses: influence of encasement of stone columns — test series 4.

Fig. 15. Improvement factor versus footing settlement: influence of encasement of stone columns — test series 4.

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(Lesc = dsc, 3dsc) the stone columns have exhibited significantlyincreased bearing capacity. Owing to low strength (cu = 5 kPa), thesupport of the soil around was insufficient to prevent excessivebulging in the unencased columns. However, with geosyntheticencasement an additional confinement was induced that pre-vented the radial column failure, leading to improved perfor-mance. The increase in bearing capacity is quantified through anondimensional factor, IFesc, defined as the ratio of bearing pres-sure with encased stone columns to the bearing pressure of theunreinforced clay bed, both taken at equal settlement. The valuesof this improvement factor for different cases are summarized inFig. 15. It can be observed that while with unencased stone col-umns the bearing capacity improvement was about 3.5 fold (IFsc,Fig. 5 and Fig. 10), with encasement it inreased up to 5 fold (IFesc,Fig. 15). Furthermore, the initial softening behaviour of the unre-inforced stone columns (Figs. 5, 10) is now overcome with theencasement and the improvement factor (IFesc) continues to in-crease with increasing settlement (Fig. 15). However, with partialencasements, a slight reduction in load capacity (IFesc) takes placein the settlement range of 3% to 10%. This is attributed to a dila-tional effect of stone aggregates owing to the bulge formationunderneath. Once the bulge has formed, it mobilizes enhancedbearing capacity, leading to enhanced performance improvement(IFesc). In contrast, with full-length encasement wherein bulgeformation was completely prevented, such a reduction in bearingcapacity was not observed and settlement increased (Fig. 16). Asencasement inhibits shear failure, the stone columns transmit thesurcharge pressure to a deeper depth and redistribute it over awider area. As a result, an increased volume of soil is encompassedunder the stressed zone leading to increased settlement of theclay surface.

It is evident that the performance continues to improve withthe increase in encasement length. However, with full length en-casement, the bearing capacity of the floating columns has re-duced substantially, so much that it is even lower than that withthe unencased ones (Figs. 14, 15). This is contrary to the findingswith end-bearing columns, wherein full-length encasement pro-vided maximum performance improvement (Murugesan andRajagopal 2007; Gniel and Bouazza 2008). This is attributed to thedifference in the support conditions. A fully encased stone col-umn behaves as a semi-rigid pile that, when placed in competent

ground, stands effectively against footing pressure giving rise tohigh performance improvement. However, when placed floatingin soft clay, as in the present case, the end-bearing resistance islow and the column punches down, leaving much of its strengthimmobilized. Also, with geosynthetic wrapping, the shear resis-tance at the column–soil interface reduces, leading to furtherreduction in load-carrying capacity. Similar behaviour is also no-ticed in the surface deformation responses (Fig. 16). With full-length encasement, the fill surface around the footing continuesto settle (�), which indicates punching behaviour. Whereas with

Fig. 16. Surface deformation, at x = D, versus footing settlement responses: influence of encasement of stone columns — test series 4.

Fig. 17. Post-test deformed shape of central stone columns withvaried length of encasement — test series 4: (a) Lesc = dsc; (b) Lesc =3dsc; (c) Lesc = L.

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partial encasement, an initial settlement occurs, after which thefill surface remains almost stable, indicating that the stone col-umns have effectively resisted the footing pressure. No significantheave was observed in the post-settlement stage, which is attrib-uted to the bulging-induced volume expansion of the stone col-umns. In addition, with partially encased columns, the settlementof the clay surface is less than that with fully encased columns.The reduction in settlement indicates that the foundation bed hasexhibited increased resistance against loading. These results canbe further analyzed through the bulging profiles and the radialstrains in the stone columns, depicted in Figs. 17 and 18, respec-tively. It can be seen that with partial encasement, visible bulginghas developed. The encased portion behaves as a stiffened entitythat transmits the surcharge pressure to deeper levels, leading tobulging below the encasement. As the stone column expands inthe region of higher surcharge, the mobilized passive pressureand therefore the confinement from the surrounding soil isgreater. Also, the bulged portion serves as an enlarged bearingthat mobilizes increased resistance from the soil around. Thesetwo factors are believed to have produced the improved perfor-mance of the foundation bed. Furthermore, it was observedthat the foundation bearing capacity with longer encasement(Lesc = 3dsc) is significantly more than that with short encasement(Lesc = dsc). This is because with longer encasement, the bulge ispushed to a deeper depth (Fig. 18), wherein the overburden pres-sure is greater, enabling mobilization of increased resistanceleading to increased load-carrying capacity. In contrast, an end-bearing stone column with increased length of encasement hasshown increased radial strain (Gniel and Bouazza 2008). This isdue to the additional restraint provided by the competent groundbelow, so that the column is restrained against verticalmovementand therefore expands more.

Figure 18 shows that the extent of bulging with partial encase-ments reduced to about 2dsc as compared to 4dsc in the case of theunencased stone columns. However, the pattern of bulging isnearly the same and in both the cases the peak bulging happenedat about mid-depth of the bulge and thereafter decreased down-wards. Furthermore, it can be noted that with 20% of lengthencased (Lesc = dsc) the stone column bulged more than thatwith 60% of encasement (Lesc = 3dsc). This is because, in the

former (Lesc/dsc = 1), the geosynthetic encasement being short inlength is subjected to higher intensity of stress and thereforedeforms more, leading to increased bulging. However, with lon-ger encasement, the induced stress is distributed over a largerarea and thereby gets reduced in intensity, leading to reducedbulging. Therefore, it can be said that partial encasement cover-ing about 60% of the full length can provide maximum perfor-mance improvement to the stone columns floating in soft clay.

The post-test exhumed stone columns did not show any visiblefailure in the geosyntetic encasement. This is possibly due to highflexibility of the geomesh and its ability to accommodate thebulging-induced deformations comfortably. However, with shallow-length encasement (Lesc = dsc), a slight rupture was noticed in theseam overlap at the base. This is attributed to local factors such asstress concentration at the discontinuity. It is of interest to notethat the bulging in the floating stone columns was formed over ashort length, right under the encasement, whilst in the case ofend-bearing columns, the entire unencased portion was found tohave bulged (Gniel and Bouazza 2008). This is attributed to thedifferences in the soil support conditions at the bottom of thestone columns.

ConclusionsThe experimental investigation reported in this study shows

that provision of floating stone columns can improve the bearingcapacity of a foundation in soft clay by about 3.5 fold. Relativelyshort stone columns (L ≤ 3dsc) were found to have punched down,whereas the long stone columns (i.e., L ≥ 3dsc) resisted the footingpressure without noticeable movement and underwent large-scalebulging. In addition, the long columns could effectively reduce thedifferential settlements in the foundation beds. However, columnslonger than about 5 times their diameter did not continue to in-crease the bearing capacity. Hence, it can be said that the criticallength of the floating stone columns givingmaximum performanceimprovement is about 5dsc. A significant increase in bearing capacitywas observed when the spacing of the stone columns was reducedfrom 3.5dsc to 2.5dsc, beyond which further improvement was onlymarginal. Therefore, the optimum spacing of the floating stone col-umns can be taken as 2.5 times the column diameter.

Fig. 18. Post-test radial strain in central stone columns with varied length of encasement — test series 4.

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The present study confirms that use of geosynthetic encase-ment in floating stone columns can significantly enhance thebearing capacity. Appropriate length of encasement is a crucialparameter for harnessing the optimal benefit. It is observed thatpartially encased columns are superior to the fully encased ones.With about 60% of the column length being encased, the bearingcapacity of the foundation bed was increased 5 fold. With full-length encasement (i.e., 100%) the increase was only about 3 fold,which is even lower than that with unencased columns. Contraryto this, in the case of end-bearing stone columns, full-length en-casement is reported to have produced maximum performanceimprovement. This is because in the case of floating stone col-umns it is the bulging at deeper depth that simulates behaviour ofan enlarged base and thereby mobilizes enhanced bearing capac-ity. A full-length encasement completely inhibits formation ofsuch deep-seated enlargement, leading to reduced load capacity.With end-bearing, the fully encased stone column behaves as astiffenedmember and thereby effectively transmits the surchargepressure onto the competent strata below, giving rise to a largeperformance improvement.

In the case of soft clay deposits extending over large depths,stone columns will need to be left floating in the weak soil itself.These stone columns will have low end-bearing capacity and willbe vulnerable to excessive bulging, leading to premature failure.In this context, use of geosynthetic encasement is found to be apotential solution for enhanced performance improvement andoptimal design of such foundations.

AcknowledgementThe authors are thankful to the anonymous reviewers for their

meticulous review and valuable suggestions for improving thepresentations in the paper.

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List of symbols

cu undrained shear strength of clayD diameter of footing

D50 average size of aggregatesdsc diameter of stone column

IFesc bearing capacity improvement factor for encased stone col-umns

IFsc bearing capacity improvement factor for conventional stonecolumns

L length of stone columnLcri critical length of stone columnLesc length of encasementro original radius of stone columnrd deformed radius of stone columnS spacing of stone columnss settlement of footingx distance from footing centrez depth� deformation on clay surface

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