Model tests on geotextile-encased granular columns under 1-g and undrained conditions

Geotextiles and Geomembranes 44 (2016) 13e27

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Model tests on geotextile-encased granular columns under 1-g and undrained conditions Yung-Shan Hong a, 1, Cho-Sen Wu a, *, Yi-Sheng Yu b a b

Department of Civil Engineering, Tamkang University, Tamsui, Taipei 25137, Taiwan Debao Group, Taipei, Taiwan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 September 2014 Received in revised form 8 June 2015 Accepted 21 June 2015 Available online xxx

This paper investigates the effects of encasement stiffness and strength on the response of individual geotextile encased granular columns embedded in soft soil through model tests. Similarity analysis was first executed to determine the suitable properties of the constituents used in the model tests to ensure that the prototype-scale and model-scale geotextile encased granular columns exhibit comparable behaviour. Experimental results show that encasement improves the bearing capability of all modelled sand columns, even when encasement rupture occurs; marked improvement is achieved for sand columns encased with geotextiles of relatively medium to high stiffness. Encasement also restrains the radial strain of the columns significantly. Predominate bulging of the encased sand column occurs in the top 2.5D depth of sand columns encased with low stiffness geotextile, however, sand columns encased with relatively high stiffness geotextile exhibit roughly uniform lateral deformation along the height of the column. The bearing stresses of the encased columns modelled in this study agree well with values predicted by an analytical solution using cavity expansion theory. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Geotextile Model test Encased sand column Soft clay Experimental test Similarity analysis

1. Introduction Encasing granular columns with tensile resistant materials is considered an effective technique for improving granular column performance (Kempfert et al., 1997; Raithel and Kempfert, 2000; Alexiew et al., 2005; Raithel et al., 2005; de Mello et al., 2008; Araujo et al., 2009). A number of laboratory tests and numerical analyses have been performed over the years for encasement applications (Murugesan and Rajagopal, 2006, 2010; Wu and Hong, 2009; Wu et al., 2009; Yoo and Kim, 2009; Lo et al., 2010; Khabbazian et al., 2010; Ali et al., 2012; Hong, 2012; Dash and Bora, 2013a). Previous studies have reported a significant increase in bearing capacity and reduction in settlement (Murugesan and Rajagopal, 2007; Gniel and Bouazza, 2009; Yoo and Lee, 2012). More attention has been given to the responses of encased columns (model-scale as well as prototype scale), increase in loading capacity, reduction in settlement and lateral expansion; however, less

* Corresponding author. Tel.: þ886 2 26222796; fax: þ886 2 26209747. E-mail addresses: [email protected] (Y.-S. Hong), [email protected] (C.-S. Wu). 1 Tel.: þ886 2 26260433; fax: þ886 2 26209747. http://dx.doi.org/10.1016/j.geotexmem.2015.06.006 0266-1144/© 2015 Elsevier Ltd. All rights reserved.

attention has been paid to the properties of encasements, in this case geotextiles, and surrounding soil that lead result in comparable behaviour in prototype-scale and model-scale encased granular columns. In an experimental study of granular column behaviour, Hughes and Withers (1974) reported that bulging took place within the upper part of the column, at approximately a four diameter distance from the top of the column. Many works have used this bulging length to evaluate the reinforcing effect of encasements, while ignoring the material properties of the constituents. However, experimental tests and numerical analyses of model encased granular columns have reported predominate bulging of various lengths (Malarvizhi and Ilamparuthi, 2007; Murugesan and Rajagopal, 2007; Gniel and Bouazza, 2009; Yoo and Kim, 2009; Yoo and Lee, 2012; Dash and Bora, 2013b). Model tests provide a relatively viable technique for examining the performance of reinforced soil structures. However, problems of similarity between reduced-scale models and equivalent field-scale prototypes lead to uncertainty about whether the behaviour and mechanisms observed in reducedscale models typical of the field-scale prototype. This study reports some small-scale laboratory experiments that address the

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nature of column responses upon using comparable materials. The influences of the mechanical properties of encasement on the bearing capability and deformation of encased columns are examined in this study. Accordingly, similarity analysis was first executed to determine the suitable properties of the constituents used in the model tests to ensure comparable behaviour between the prototype-scale and model-scale geotextile encased granular columns. A series of model tests was conducted on encased sand columns embedded in soft clay. In these tests, the columns were encased with geotextiles of different stiffness and strengths. Finally, comparisons were made between the bearing stresses of the experimental measurements and the estimated values of analytical methods. 2. Testing program and procedure 2.1. Similarity analysis In the present study, a testing program was implemented to evaluate the effects of geotextile stiffness and strength on the mechanical responses of prototype encased granular columns embedded in soft soil using reduced-scale model tests. To ensure comparable behaviour between the prototype-scale and modelscale encased sand columns (ESCs), similarity analysis was conducted to select the characteristics of the materials employed in the model tests. Because the corresponding systems are homologous in similarity analysis, a model system can predict prototype response. Similarity analysis is extensively described in Baker et al. (1991). The physical characteristics of the model and prototype satisfy the following relationship (Baker et al., 1991):

Ip ¼ Sf Im

(1)

where Ip and Im ¼ physical quantities that pertain to the prototype and the model, respectively, and Sf ¼ a scaling factor. The quantities that affect the behaviour of an ESC embedded in soft clay soil satisfy the following relationship:


f r; g; Tg ; Jg ; f; H; D; cu ; Gc ¼ 0

(2)

where r ¼ the density of the sandy soil filled in the column or the soft clay, g ¼ the acceleration due to gravity, Tg ¼ the tensile strength of the encasement, Jg ¼ the tensile stiffness of the encasement, f ¼ the internal frictional angle of the sandy soil filled in the column, H ¼ the height of the sand column, D ¼ the diameter of the sand column, cu ¼ the undrained shear strength of the soft clay, and Gc ¼ the shear modulus of the soft clay. Table 1 shows the scaling factors for the model and prototype with the same sandy soil density. A model sand column of 0.05 m in diameter was chosen to simulate a prototype column of 0.5 m in diameter, the ratio of the prototype column diameter to the model column diameter (l) is 10. Therefore, stresses on a full-scale sand column and surrounding clay are ten times those measured and

Table 1 Characteristic values used for model to prototype scaling. Dimensionless factor

Characteristic

Scaling factor (prototype/model)

Prototype value

Model value

J

Jg (kN/m)

l2 (¼100)

50 ~ 2000

0.5 ~ 20

Tg rgH2 cu rgH D H

Tg (kN/m)

l2 (¼100)

10 ~ 1000

0.1 ~ 10

g p1 ¼ rgH 2

p2 ¼ p3 ¼ p4 ¼

cu (kPa)

l(¼10)

<20

<2

D (m)

l(¼10)

0.2 ~ 1.2

0.02 ~ 0.12

Fig. 1. Grain size distribution of the test sand.

evaluated on the model sand column and surrounding clay deposit. 2.2. Materials used for testing In the model tests, uniform silica sand with sub-angular particles was used as the fill material to form the sand columns. The properties of the sand are as follows: median grain size D50 ¼ 0.87 mm, specific gravity Gs ¼ 2.65, maximum dry unit weight gd(max) ¼ 16.6 kN/m3, and minimum dry unit weight gd(min) ¼ 13.7 kN/m3. The model tests were carried out on a sand column with 60% relative density. The sand with 60% relative density was subjected to confining pressures ranging between 20 kPa and 200 kPa. Under these conditions, the peak friction angle obtained from the triaxial compression test was fpeak ¼ 36.6 e38.6 . Fig. 1 is a plot of the particle-size distribution of the test sand used in this study. The test sand columns were embedded in moist clay stratum. The properties of the clay are as follows: specific gravity Gs ¼ 2.72, liquid limit LL ¼ 38, plastic limit PL ¼ 21, and activity A ¼ 0.65. Activity is the slope of the line correlating PI and % finer than 2mm for the clay (Skempton, 1953). With the target of 40% moisture content, the preparation work achieved clay bed moisture contents that ranged between 39.3% and 40.1%. The undrained shear strength cu of the clay under the operated moisture contents ranged between 1.25 kPa and 1.36 kPa. To fabricate the encased sand columns, one of the three nonwoven geotextiles, namely GT1, GT2 and GT3, was used to encase each sand column. The chosen geotextiles have anisotropy characteristics of tensile stiffness and strength, which provide favourable arrangements for studying the effects of encasement stiffness and strength on the behaviour of an encased sand column. The tensile force-strain relations of the test geotextiles, under wide width tensile testing, are presented as solid circles in Fig. 2; the letters M and X refer to machine and cross machine directions, respectively. Since the encasement was formed by stitching a piece of geotextile into a sleeve, the tensile force-strain relations of seamed pieces of test geotextiles were also acquired using a widewidth test and the results are presented as blank circles in Fig. 2. The results in Fig. 2 show that the un-seamed and the seamed

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Fig. 2. Tensile force-strain relations of the test geotextiles: (a) GT1; (b) GT2; (c) GT3.

geotextiles achieve almost identical tensile force-strain curves up to a considerable strain. Table 2 presents the mechanical properties of the nonwoven geotextiles tested in this study. As shown in Table 2 and Fig. 2, the geotextiles GT1M and GT2X have similar tensile strength values (0.26 kN/m vs. 0.25 kN/m), but different stiffnesses (3.64 kN/m vs. 0.48 kN/m). The geotextile GT1M reaches its peak and ultimate tensile strengths at relatively low strains as compared to the other test geotextiles. The geotextiles GT1X and GT2X have similar tensile stiffness values (0.34 kN/m vs. 0.48 kN/m), but different tensile strengths (0.05 kN/m vs. 0.25 kN/m). Among the test geotextiles, GT2M has relatively mid-range values in tensile stiffness and strength, and GT3M has the highest values in tensile stiffness and strength. Table 2 Mechanical properties of the test geotextiles. Type of geotextile

GT1

GT2

GT3

Thickness (mm) Direction

0.284 Machine

1.218 Machine

Tensile stiffness (kN/m) Tensile strength (kN/m) Strain at peak strength (%) Rupture strain (%)

3.64

Cross machine 0.34

0.235 Machine

0.26

6.06

Cross machine 0.48

0.05

0.75

0.25

7.42

13

25

45

92

49

20

43

54

103

74

15.13

2.3. Test arrangement and procedure The following tests were arranged to examine the effectiveness of encasement: on a clay bed without a sand column, with an ordinary sand column (OSC), and with sand columns encased with different geotextile stiffnesses and strengths (encased sand column, ESC). All of the sand columns were 50 mm in diameter and penetrated the full depth of the clay bed (i.e. 250 mm). Prior to running the model tests, the diameter of the test tank was checked to confirm that there was no boundary effect on the column behaviour. The load tests for a 70 mm diameter plate resting on a clay bed were performed using different diameter tanks, viz. 200 mm, 300 mm, 400 mm and 500 mm. The results showed that test tanks having diameters larger than 300 mm produce almost identical loadedisplacement curves. The distance from the footing centre to the boundary is larger than the failure wedge observed in the foundation bed (i.e. 2e2.5 times the footing width; Chummar, 1972). Numerically analyzed results also indicate that a 500 mm diameter test tank is appropriate for performing a load test with a footing of 70 mm in diameter. Therefore, a cylindrical tank of 500 mm in diameter and 250 mm in depth was chosen for the present study. The distance from the tank wall to the centre of the column was 5 times the column diameter. In a numerical study of the behaviour of encased granular columns, Khabbazian et al. (2010) suggested that a numerical model using 4 times the radius of the column as the lateral extent of the soft soil around the column minimizes the

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Table 3 Summary of model test arrangements and test results. Model No. (Settlement, mm) OSC (50) ESC1X (50) ESC1M (50) (17.4)a ESC2X (50) ESC2M (30) ESC3M (30) a

Depth of maximum bulging (mm)

Encasement strain (%)

Tensile force of encasement (kN/m)

Lateral pressure provided by clay (kPa) (Eq. (4))

Sleeve- induced confining pressure (kPa) (Eq. (11))

Measured bearing stress (kPa)

Predicted bearing stress (kPa) (Eq. (3) or 10)

Sleeve- induced confining pressure (kPa) (Eq. (13))

71

e

e

4.01

e

22.5

17.3

e

74

42

0.032

3.91

0.90

27.7

20.8

1.82

70 e

e 13

e Max. ¼ 0.26

4.09 2.15

e 9.20

38.3 50.8

e 49.0

e 6.32

62

36

0.152

3.58

4.47

42.2

34.8

2.57

60

18

0.55

2.90

18.64

93.5

93.0

18.9

60

13

1.42

2.58

50.27

222.2

228.2

47.2

is the displacement corresponding to the maximum pressure measured at the top of the column.

boundary effect on column behaviour. Table 3 presents the arrangement and some results of the tested model columns. Fig. 3 presents the schematic diagram of the laboratory test set-up. 2.3.1. Preparation of the encasing sleeve When used to encase a sand column, the geotextile in the designated machine or cross machine direction was oriented along the circumferential direction of the encasing sleeve to produce hoop stress. The 250 mm long encasing sleeve was stitched from a piece of geotextile 260 mm in length. The 10 mm long sleeve end had been cut radially into 2 mm wide fragments, and the fragmented skirts were glued to the bottom of the tank to fix the sleeve. 2.3.2. Preparation of the clay bed and erection of encasement The clay soil to be filled into the tank was thoroughly soaked to a moisture content of 1% over the target moisture content in an airtight container and set aside for 24 h. A casing pipe with an

outer diameter equal to the diameter of the sand column was inserted into the encasing sleeve. Prior to filling the clay mass into the tank, the casing pipe, along with the encasing sleeve, was erected in the centre of the test tank. The encasing sleeve was secured by gluing the sleeve skirts on the bottom of the test tank. A thin circular acrylic plate was also screwed atop the skirts to enhance the firmness (Fig. 4(a)). The erected casing pipe and encasing sleeve combination was bound by a rigid hoop at the top to prevent column movement during placement of the clay bed (Fig. 4(a)). The soft clay placement was divided into 5 layers, and the weight and volume of the deposited clay was checked at each layer. The moisture content and undrained shear strength of the clay was measured upon completion of the loading test. The undrained shear strength measurements were taken at three different depths and locations using an in-situ vane shear test device (Wykeham Farrance Engineering, LTD) with large vane size (d ¼ h ¼ 38.1 mm). The measured results show that this procedure produces clay beds of uniform moisture content and consistency.

Fig. 3. Schematic of the encased sand column model test set-up.

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deposit. Furthermore, the encased sand columns were red-marked on the sleeve surface to facilitate vertical deformation measurements at different heights. 2.3.4. Load test on sand columns A 70 mm diameter circle concentric with the sand column was marked on the clay bed surface to help with the alignment of the loading plate. The loading of the columns was applied through a loading plate of 70 mm in diameter, which was displaced by an electric motor through a reaction frame at a constant rate of 1 mm/ min. Furthermore, a dial gauge was used for displacement measurements. Stresses that acted on the column were recorded using two pressure cells of 24.5 mm in diameter and a load cell. The two pressure cells were mounted respectively on the top of loading plate and the bottom of the test tank to measure pressures at both ends of the sand column. A load cell mounted on the loading rod (Figs. 3 and 4(c)) measured the total load applied on the loading plate. The total load includes the loads from a sand column and the circular area of clay concentric with the sand column. Three encased sand columns (i.e. ESC1X, ESC1M and ESC2X) were loaded to displacement levels of 50 mm and two (i.e. ESC2M and ESC3M) were loaded to displacement levels of 30 mm due to the capacity limit of the load cell. The loadedisplacement data were recorded through a computer controlled system.

Fig. 4. Photographs of the model test preparation: (a) erection of a sleeve; (b) compaction of sand; (c) set up the instrument.

2.3.3. Installation of sand columns The encased sand columns were prepared using the cast-in place method. The sand was poured into the casing pipe and encasing sleeve combination from a tube. Constant height of pluviation was pre-calibrated to produce sand of desired relative density (i.e. 60%). The sand placement was divided into 10 layers, and the uniformity of the deposited sand was checked at each layer. After pluviating each layer of sand, the casing pipe was lifted up to a given height to maintain an overlap of 10 mm between the bottom of the casing pipe and the sand filled into the casing pipe. Each layer of sand was compacted with a tamping rod if necessary to produce a consistent sand column with a relative density of 60% (Fig. 4(b)). The rigid hoop at the top of the casing pipe guided the sand columns, keeping them vertical throughout column installation. Ten dyed sand layers, each about 2 mm thick, and separated by a vertical spacing of 25 mm, facilitated visual observation of the deformed ordinary sand column (OSC) exhumed from the clay

2.3.5. Measurement of encasement hoop strain After the loading plate had been unloaded and removed, Plaster of Paris Paste was injected into the sand column to solidify the deformed sand column. A 0.5 mm diameter tube intruding through the filled sand facilitated the injection of Plaster of Paris Paste into the voids of the filled sand. Injection commenced from the bottom of the column and the tube was gradually lifted up throughout the injection process. This technique induces minimum disturbance to the sand column and applies low pressure to expel water from the bottom up. The subsequent excavation of the clay and exhumation of the solidified column showed that the injection method produces intact sand columns and captures the shape of the deformed columns well. The diameters along the length of the exhumed column were measured at vertical intervals of 25 mm. Fig. 5 shows the graphical representation of the test procedures. Since the deformed sand column was plastered after unloading, part of the hoop stress acting on the column might have been released upon unloading. As a consequence, the release of the lateral thrust of the sand column and the elastic behaviour of the encasement might have recovered part of the extended encasement strain caused by the previous axial load, making the diameter of the plastered column smaller than that under load. Therefore the radial deformations measured along the column are residual hoop strains on the encasement due to plastic deformations rather than the mobilized strains under load. Murugesan and Rajagopal (2010) used strain gauges to measure hoop strain development along the length of the encased columns and observed significant hoop strain recovery in the unloaded geosynthetic-encased stone column. In the above study, the measured residual hoop strain was less than the mobilized hoop strain developed during the loading process. To retrieve encasement hoop strain during loading, a series of loading-unloading tests was performed to obtain the tensile strainresidual strain relationship for each test geotextile. The relationship was established by stretching the geotextile specimen to a target strain and releasing the load. Several targets and the corresponding residual strains for the three test geotextiles in machine and cross machine directions were recorded and the results are plotted in Fig. 6. The results show that geotextile GT1 in machine and cross machine directions (i.e., GT1M and GT1X) and GT2 in the cross machine direction (i.e., GT2X) recover no strain after reaching

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Fig. 5. Graphical representation of the test procedures.

certain tensile strains; whereas geotextiles GT2 and GT3 in the machine direction (i.e., GT2M and GT3M) recover considerable amounts of tensile strain during unloading. Leshchinsky et al. (2010) used a similar technique to estimate the reinforcement tensile forces mobilized within a geosynthetic reinforced soil (GRS) structure. Nguyen et al. (2013) also used this technique to estimate reinforcement tensile strains mobilized within a geosynthetic laminated sand column from residual strains of unloaded reinforcements. In addition, uniaxial compression tests were executed on isolated columns encased with GT2M and GT3M to verify that the extended sleeves can squeeze back the deformed encased sand columns ESC2M and ESC3M, and retrieve their elastic deformations after removal of the loadings. The circumferential lengths of the isolated encased sand columns were measured during loading and

unloading, and the results showed that both sand columns encased by geotextiles GT2M and GT3M did recover the same amount of elastic strain as in the tensile strain-residual strain test performed on plain specimens. 3. Results and discussion Vesic (1963) defined the failure load of a foundation as the point at which the slope of the load-settlement curve first reaches zero or a steady minimum value. For a foundation with an unobvious failure load, the failure stress is usually taken as the applied stress corresponding to settlement equal to a certain fraction of footing width or a constant numerical value. The pressure-displacement curves of sand columns encased with medium and strong geotextiles exhibit continuous increases in applied stress as displacement increases; therefore, vertical pressure-displacement relations are adopted for all tested columns to study the effectiveness of encasement on a sand column. 3.1. Vertical pressure-displacement relations for soft clay and ordinary sand column

Fig. 6. The relations between loading and residual tensile strains for all test geotextiles.

Loading tests performed on soft clay and ordinary sand column (OSC) were employed to explore the effectiveness of encasement; the pressure-displacement curves for both tests are depicted in Fig. 7. The load was applied on a 70 mm diameter loading plate resting either on soft clay, in the clay bearing performance test, or on a 50 mm diameter sand column and a circular area of clay concentric with the sand column. The pressures presented in Fig. 7 and the following relevant figures are the pressures measured on the centre area of both column ends (the diameter of the pressure cells is 24.5 mm), and the total applied load divided by the loading plate area. The latter pressure is referred to subsequently as average applied pressure. The top centre pressure and the average applied pressure values suggest a sharing of load between the column and the surrounding clay. The test results in Fig. 7(a) show that soft clay resists more pressure as the displacement of the loading plate increases. The average applied pressure exerted on the loading plate is higher than the pressure measured at the centre area of the loading plate. This

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to the bottom of the column as compared to the soft clay (i.e. 11.6 kPa vs. 2.2 kPa) (Fig. 7). This is different from the findings of Sivakumar et al. (2004) and Dash and Bora (2013a) in experimental tests of model sand columns floating in clay deposits. They reported that a sand column with a length approximately five times longer than its diameter does not produce a further increase in load-carrying capacity. The difference may be attributed to the full column penetration carried out in the present study. Fig. 8(a) and (b) display the picture and profile of the exhumed sand column. This figure reveals that the predominate bulging of the column occurs in the top portion over a depth of about 1.5D beneath the loading plate or 2.5D below the clay deposit surface. The maximum bulging occurs at approximately 0.25D beneath the loading plate or 1.25D below the clay deposit surface. This is in line with the experimental findings from Greenwood (1970) and Hughes et al. (1975) who showed that the predominate bulging of a granular column occurs in the top portion above a depth of about 4 times the column diameter. Malarvizhi and Ilamparuthi (2007) conducted an experimental test and numerical analysis of a model sand column installed in a clay deposit and observed that the maximum bulging occurs at column depths of about 1.5D and 1.75D, respectively. In Fig. 8(b), the percentages shown for each segment of the deformed column display the volumetric strain of the column segments separated by the red-dyed sand layers. For clear presentation, two dyed sand layers were combined into one calculation segment. The values are calculated based on the superficial observation of the red-dyed sand layers, assuming the sand column has uniform vertical deformation (i.e. all sand particles at a specific vertical level have identical vertical displacement). The percentage values in Fig. 8(b) indicates expansion on the middle three and contraction on both end segments of the sand column. Observations of cross sectional deformations show concave-down deformation for the upper three red-dyed sand layers, especially in the second layer (Fig. 8(c)). It is reasonable to deduce that for the top two segments, the magnitude of contraction in the first segment is less than the values shown in Fig. 8(b), while the expansion in the second segment is more than the values shown in the figure.

3.2. Vertical pressure-displacement relations of encased sand columns Fig. 7. Vertical pressure-displacement relations of the soft clay and the ordinary sand column: (a) soft clay; (b) ordinary sand column.

conforms to the well established pressure distribution pattern of a rigid plate resting on soft clay; the lowest vertical stress occurs underneath the centre of the plate. During low displacement stages, the ordinary sand column shows that the vertical pressure at the top centre of the column is higher than the average applied pressure, and the trend reverses when the loading plate is displaced further (Fig. 7(b)), indicating a high proportion of the load is taken by the column in the early stages of displacement. At higher levels of displacement, the column yields and transfers more of its load to the clay. This demonstrates that installation of a sand column significantly improves the bearing capability of soft clay. At 50 mm displacement, the sand column increases the bearing pressure of the soft clay at the centre of the loading plate by 118% and the average applied pressure by 95%. The pressure-displacement curve for the bottom of the column shows that a significant amount of loading was transmitted

3.2.1. Column ESC1X Fig. 9 depicts the pressure-displacement curves for the encased sand column ESC1X. The test geotextile GT1X has low strength and low stiffness, but it still provides some bearing improvement to the sand column. At a low level of displacement, the sand column bears higher pressure than the average applied pressure exerted on the loading plate, but the trend reverses for larger displacements. Because GT1X has ductile character, ESC1X exhibits a continuous increase in vertical pressure with increasing displacement. From the very beginning to 10 mm displacement, the geotextile improves the vertical pressure capability of the sand column; at 10 mm displacement, the geotextile increases the vertical pressure over the OSC by 46% (Figs. 7(b) and 9). For displacement ranging between 10 mm and 40 mm, the rate of increase of the vertical pressure decreases, which may be attributed to the reduced stiffness and softening behaviour of GT1X at higher tensile strains (Fig. 2(a)). Finally, the increasing trend of vertical pressure ceases at 40 mm displacement. At 50 mm displacement, ESC1X increases to a maximum pressure of 26% great than that of the OSC.

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Fig. 8. Picture and profile of the post test deformed OSC at 50 mm displacement: (a) deformed column; (b) column profile; (c) top portion of the deformed column.

Fig. 10 displays a picture and profile of the exhumed column. Measurement of the radial deformation of the exhumed column shows that the predominate bulging of the column occurs in the top portion over a depth of about 1.5D beneath the loading plate or 2.5D below the clay deposit surface. The maximum hoop strain occurs at a depth of 1.5D beneath the clay bed surface with a magnitude of 42%. The tensile force-strain relation in Fig. 2(a) shows that GT1X reaches its peak and rupture strengths at 25% and 43% strains, respectively. Although the maximum circumferential strain of the exhumed column is close to the rupture strain of the geotextile, no rupture sign was observed in the encasement of the exhumed column.

Fig. 9. Vertical pressure-displacement relation of the ESC1X.

Since the stiffness of geotextile GT1X is low, encased sand column ESC1X and the OSC have similar deformation patterns and pressureedisplacement relationships: predominate bulging occurs at the top portion of the column, while only minimal pressure is transmitted to the bottom of the column (i.e. 13.7 kPa) (Figs. 9 and 10). The pressure measured at the bottom of encased sand column ESC1X approximates the value of the unreinforced OSC. The gradually change in response corresponding to the column encased by a geotextile with low stiffness and strength indicates uniformity in clay deposit preparation. 3.2.2. Column ESC1M Fig. 11 presents the pressure-displacement curves for encased sand column ESC1M. Geotextile GT1M exhibits the least ductile behaviour of all the tested geotextiles, but it is stiffer than GT1X. Encasement stiffness greatly increases the bearing stress of a sand column when the column is displaced to low displacement (i.e. prior to encasement rupture). At 17.4 mm displacement, the sand pressure at the top of the column reaches 3.6 times that of the OSC. However, geotextile rupture near the top of the column results in a pressure drop within the sand column at the top centre. Nevertheless, an encased sand column with ruptured geotextile still provides higher bearing capability than does an unreinforced sand column. At 50 mm displacement, pressure at the top centre of the sand column is 70% higher than that of the unreinforced sand column. Prior to geotextile rupture, higher geotextile stiffness also causes the loading stress to be transmitted deeper; at 17.4 mm displacement, the sand pressure measured at the bottom of the column is about 3.7 times that of the OSC. Geotextile rupture near the top of the column causes a pressure drop in the sand column at the top centre but does not reduce the pressure previously transmitted to the bottom; instead, nearly constant pressure is taken at the bottom of the sand column.

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Upon geotextile rupture, the release of confining pressure previously exerted by the geotextile induces excessive column bulging. The exhumed column profile shows that predominate bulging of the column occurs in the top portion over a depth of about 1.25D beneath the loading plate or 2.25D below the clay deposit surface. The maximum radial strain of the column occurs at a depth of 1.5D beneath the clay deposit surface with a magnitude of 60% (Fig. 12). The appreciable downward transmission of vertical pressure causes greater response in the bottom segment (Fig. 12(b)). Again, it can be deduced that for the bottom two segments, the magnitude of contraction is less than in the data shown in Fig. 12(b), while the expansion is greater than in the data shown in the figure. 3.2.3. Column ESC2X At low tensile strains, the test geotextiles GT1X and GT2X have close stiffness values (i.e. 0.34 kN/m vs. 0.48 kN/m), but GT2X has much higher tensile strength (i.e. 0.047 kN/m vs. 0.25 kN/m) (Fig. 2). The close stiffness values of these geotextiles produces a similar pressure-displacement curve for both encased sand columns ESC1X and ESC2X when they are subjected to low column displacements (less than 10 mm), whereas both columns exhibit different trends in their pressure-displacement curves when they are subjected to higher displacement (Figs. 9 and 13). The different responses of the two encased columns can be easily recognized by their different tensile forceestrain relationships. Encased column

Fig. 10. Picture and profile of the post test deformed ESC1X at 50 mm displacement: (a) deformed column; (b) column profile.

Fig. 11. Vertical pressure-displacement relation of the ESC1M.

Fig. 12. Picture and profile of the post test deformed ESC1M at 50 mm displacement: (a) deformed column; (b) column profile.

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Fig. 13. Vertical pressure-displacement relation of the ESC2X.

ESC1X experiences strain softening with encasement GT1X, whereas ESCX2 column undergoes an almost linear tensile forcestrain relation with encasement GT2X till the end of column loading. The picture and profile of the exhumed column (Fig. 14) show that predominate bulging of the column occurs in the top portion over a depth of about 1.5D beneath the loading plate or 2.5D below the clay deposit surface. The maximum hoop strain of the encasement is 36% at a depth of 1.25D from the clay deposit surface (i.e. 0.25D beneath loading plate). Since geotextile GT2X undergoes a slight decrease in stiffness and experiences no strength yield for strains less than 36%, hoop strains along the entire length of the ESC2X column are much lower than the strain corresponding to the peak tensile strength of GT2X; the encased sand column exhibits a monotonous increasing pressureedisplacement relationship (Fig. 13). Increments of hoop stress always keep the pressure of the sand column higher than the average applied pressure exerted on the entire loading plate. The test results also show that sand columns encased by low stiffness encasements transmit only minimal loading pressures to the bottom of the columns; ESC1X and ESC2X transmit 13.7 kPa and 14.5 kPa to the bottom of the columns, respectively. The test geotextiles GT1M and GT2X have almost identical peak tensile strength values (i.e. 0.26 kN/m and 0.25 kN/m, Fig. 2), but GT1M is stiffer than GT2X (i.e. 3.64 kN/m vs. 0.48 kN/m). Therefore, encased sand column ESC1M exhibits much stiffer behaviour than does ESC2X during low displacement (for instance, less than 17.4 mm displacement). However, ESC1M experiences a bearing pressure drop due to geotextile rupture whereas column ESC2X exhibits a monotonous increasing pressureedisplacement relationship. 3.2.4. Columns ESC2M and ESC3M The geotextile GT2M has relatively medium stiffness with ductile properties, whereas geotextile GT3M has relatively high stiffness and strength. Sand columns encased with either geotextile GT2M or GT3M exhibit very stiff behaviour like a semi-rigid pile (Figs. 15 and 17). Both columns exhibit significant improvement in bearing capability due to high geotextile stiffness and no strength yielding; the pressures corresponding to 30 mm displacement are 5.4 and 12.9 times those of the OSC for ESC2M

Fig. 14. Picture and profile of the post test deformed ESC2X at 50 mm displacement: (a) deformed column; (b) column profile.

Fig. 15. Vertical pressure-displacement relation of the ESC2M.

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23

Fig. 17. Vertical pressure-displacement relation of the ESC3M.

Fig. 16. Picture and profile of the post test deformed ESC2M at 30 mm displacement: (a) deformed column; (b) column profile.

and ESC3M, respectively. The bi-linear and linear shaped trends of the pressureedisplacement relationships for ESC2M and ESC3M, respectively, correspond to their tensile force-strain curves. This correlation between the pressure-displacement relation of the encased column and the tensile force-strain relation of the geotextile can also be observed for encased columns ESC1X and ESC2X. Since both geotextiles have relatively medium or high stiffness, greater amounts of stress are transmitted from the top to the bottom of the encased sand columns, and consequently, both columns deform in a relatively uniform manner along the height of the columns, as shown in Figs. 16 and 18, which illustrate the profiles of the exhumed sand columns. This finding is similar to that observed for encased end-bearing sand columns reinforced by strong geotextiles (Murugesan and Rajagopal, 2007; Gniel and Bouazza, 2009; Ali et al., 2012, 2014), in which the stressesettlement curves were almost linear when the tests were stopped and failure did not occur for either the geotextile or column. Position of maximum bulge for both columns is indistinct but roughly at 1.2D below the clay deposit surface (i.e. 0.6D beneath the loading plate). Dash and Bora (2013b) observed that the stiffening effect of an encased end-bearing stone column enables the column to transmit the surcharge pressure onto the competent strata below. The hoop strains measured from exhumed columns ESC2M and ESC3M are modified using the loadingresidual tensile strain relations for geotextiles GT2M and GT3M,

as presented in Fig. 6. Because both geotextiles GT2M and GT3M have relatively medium to high stiffness and encounter no yielding for the loads applied to the encased columns, increases in hoop stress always keep the pressure acting on the sand column higher than the average applied pressure exerted on the entire loading plate. By measuring the radial deformation of an exhumed model column, Malarvizhi and Ilamparuthi (2007) found that the maximum bulging was seen at a depth of 1.5D on the column. Numerical analysis indicated that hoop force is mobilized over the entire length of the encasement in a high stiffness geotextile subjected to high pressure (Malarvizhi and Ilamparuthi, 2007). Murugesan and Rajagopal (2007, 2010) performed laboratory model tests on stone columns installed in a unit cell tank. They found that the increase in confining pressure due to encasement can be seen throughout the full length of the encased granular columns. Thus, they concluded that geosynthetic encased sand columns act as semi-rigid piles and do not undergo any catastrophic failures.

3.3. Vertical pressure-displacement relations of all tested sand columns Fig. 19 presents the vertical pressure-displacement relations measured at the top-centre of all columns. The results clearly demonstrate the influence of geotextile stiffness and strength on the pressure-displacement relations of encased sand columns. Prior to geotextile yield, the geotextile stiffness dominates bearing capability of the encased columns. Quantitative evaluations of confining pressure in the geotextile and surrounding soil are presented in the next section.

4. Comparison of experimental and analytical results 4.1. Granular columns It is well recognized that the bearing pressure of a granular column depends on the lateral confining pressure. The bearing pressure can be expressed as

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( src ¼ sr0 þ cu

"

Gc 1 þ ln cu

 2 !#) r 1 0 r1

(4)

where, sr0 ¼ initial radial stress, Gc ¼ the shear modulus of the soil surrounding the cavity, and r0 and r1 ¼ the radii of the initial and deformed cavity, respectively. The ultimate cavity expansion pressure src;lim can be obtained by assuming that r0 =r1 approaches zero and is expressed as:

   Gc src;lim ¼ sr0 þ cu 1 þ ln cu

(5)

Eq. (5) is precisely the same as the formula for the limit pressure in a purely cohesive, constant volume material derived independently by Gibson and Anderson (1961). Vesic (1972) called the term Gc =cu a “rigidity-index,Ir ” and reported that this value ranges between 10 and 300 (10  I r 300) for most saturated clays. Thus, the ultimate cavity expansion pressure src;lim is in the range of

ðsr0 þ 3:3cu Þ  src;lim  ðsr0 þ 6:7cu Þ

Fig. 18. Profile of the post test deformed ESC3M at 30 mm displacement.

 sv ¼tan2

 p f þ src 4 2

(6)

A widely used equation proposed by Hughes and Withers (1974) to evaluate ultimate cavity pressure exerted on deformed granular columns is

(3)

where, sv ¼ the ultimate vertical stress acting on the column, f ¼ the friction angle of the material of the column, and src ¼ the lateral pressure provided by the surrounding soil. For a granular column embedded in soil, bulging of the granular column induces expansion of the cylindrical cavity and lateral pressure provided by the surrounding soil. Cavity expansion theory has been applied to characterize the bulging behaviour of granular columns, which has led to evaluation of bearing capacity (Hughes and Withers, 1974; Poorooshasb and Meyerhof, 1997). Yu and Houlsby (1991) developed an analytical solution to analyze the expansion behaviour of a cylindrical cavity in dilatant elasticeplastic soils. The cavity pressure src for cohesive soils (the frictional angle and dilatant angle are zero) is written as (Yu and Houlsby, 1991)

src ¼ ðsr0 þ 4cu Þ

(7)

which falls within the range of Eq. (6). Because the clay employed in the present study is extremely soft, it was difficult to conduct tests on cylindrical specimens to acquire the shear modulus of the clay. A shearing test based on torque-rotation measurements using a vane shear device is adopted to evaluate this value (Biscontin and Pestana, 1999). The relation between shear modulus and applied torque can be expressed as (Biscontin and Pestana, 1999)

G¼

T=ðpDv Hv Þ mwD

(8)

where T ¼ the measured torque, Dv and Hv ¼ the diameter and height of the vane, respectively, w ¼ the angle of rotation in radians, and m ¼ a coefficient which is a function of the vane shape. Fig. 20 presents the relationship between shear modulus and angle of rotation for the test clay, which is obtained from the present test series using an in-situ vane shear test device. Taking the maximum shear modulus of the test clay (i.e. 7 kPa), the ultimate cavity expansion pressure src;lim for the test clay using Eq. (5) is approximately

src;lim ¼ sr0 þ 2:7cu

(9)

Since the present test clay is extremely soft, the ultimate cavity expansion pressure src;lim calculated using Eq. (9) is lower than but close to the low bound of Eq. (6). 4.2. Encased granular columns For an encased granular column embedded in soil, the lateral confining pressure arises from the surrounding soil and the encasement. The bearing pressure of the encased granular column can be expressed as

 sv ¼ tan2 Fig. 19. Vertical pressure-displacement relations of the soft clay and the test sand columns (measured at top-centre).

 p f þ ðsrc þ DsÞ 4 2

(10)

where Ds ¼ the sleeve-induced confining pressure (hoop stress).

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25

assumed to be 38.6 , which was obtained through a triaxial compression test under 20 kPa confining pressure. 4.3. Evaluation of the sleeve-induced confining pressure of granular columns under the assumption of constant volume Because the tensile force Te in Eq. (11) is taken from the tensile force-strain curve of the encasement corresponding to the radial strain of the deformed column, the calculated sleeve-induced confining pressure (hoop stress) Ds is conceivable. However, the radial strain of the deformed column is an indirect measurement in the experimental test and difficult to acquire in practice. The assumption of no volume change has been employed to simplify hoop strain evaluations of encasements (Bathurst and Karpurapu, 1993; Rajagopal et al., 1999; Madhavi Latha et al., 2006, Murugesan and Rajagopal, 2010; Wu and Hong, 2014). When we assume an encased granular column deforms with constant (initial) volume, the encasement circumferential strain (i.e. equal to the radial strain) of the column is Fig. 20. Shear modulus and angle of rotation of the test soft clay.

εq ¼ εr ¼ The sleeve-induced confining pressure (hoop stress) can be expressed as

Ds ¼

Te r1

(11)

where Te ¼ the tensile force per unit length of the encasement corresponding to the circumferential strain of the deformed column radius r1 . In the present study, the tensile force of the encasement is obtained from the tensile force-strain relation obtained from the wide width test performed on the seamed specimen. The radius of the deformed column is measured from the exhumed column. Table 3 presents the calculated lateral pressures provided by clay (Eq. (4)) and the encasement (Eq. (11)), and bearing stress (Eq. (3) or (10)) for each test column. During the loading process, encased sand column ESC1M encountered encasement rupture (at 17.4 mm displacement); therefore, the peak tensile strength and the corresponding strain of the encasement geotextile are taken to evaluate the maximum lateral pressure provided by the encasement geotextile. Thus, bearing stress obtained as such is compared with the maximum bearing stress measured in the test. The calculated results show that the lateral confining pressures offered by the surrounding soil vary column by column; surrounding soil produces less confining pressure in a column confined by a geotextile with higher stiffness. This expected result can be recognized from the reduced radial strain in columns encased by stiffer geotextiles. The calculated lateral pressure provided by geotextiles shows that the stiffness and strength of the geotextile plays an important role in developing the bearing stress of an encased granular column. Comparisons of bearing stresses show reasonably good agreement between the measured and analytical results for unreinforced and encased sand columns tested in this study. Greater discrepancies between the measured and analytical bearing stresses occur in unreinforced column and columns encased with low stiffness geotextiles, where low confining pressures act on the columns. Part of the underestimation of bearing stress may be attributed to the undervalued friction angle in the test sand. The friction angle of granular material generally increases with decreasing confining pressure; however, to calculate bearing pressure, the friction angle of the sand is

pffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pðr1  r0 Þ r1  r0 1  1  ε1 ¼ ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pr0 r0 1  ε1

(12)

where ε1 ¼ the vertical strain of the column, and εq ,εr ¼ the circumferential and radial strain of the column, respectively. Assuming the encasement, a geotextile in this case, obeys a linear tensile force-strain relation, the sleeve-induced confining pressure Ds can be simply expressed as

Ds ¼

pffiffiffiffiffiffiffiffiffiffiffiffiffiffi Jg
Te Jg εq 1  1  ε1 ¼ ¼ r1 r1 r0

(13)

Based on the observed depth of the predominant bulging of stone columns (Hughes and Withers, 1974), the vertical strain in columns has usually been estimated as the measured surface displacement divided by a column height equal to four times the diameter. Values in the last column of Table 3 present the sleeveinduced confining pressures calculated using Eq. (13). The results show that, except for ESC2M and ESC3M, the Ds values evaluated using Eq. (13) deviate from those evaluated using Eq. (11). The deviations can be attributed to two reasons: low geotextile stiffness or excessive geotextile strain. For stone columns confined with low geotextile stiffness or those encountering geotextile rupture, predominate bulging of the encased sand columns takes place in the top portion over a depth less than four times the diameter, thus, Eq. (13) underestimates Ds values (i.e. ESC1X, ESC1M and ESC2X). For columns experiencing excessive geotextile strain, the linear tensile force-strain relation may no longer exist, thus, Eq. (13) overestimates Ds values (i.e. ESC1X and ESC1M). The results for columns ESC1X and ESC1M are influenced by these two factors. The results show that the constant volume assumption enables good prediction of Ds for columns encased with encasements of relatively medium to high stiffness and encasements, geotextiles in this case, conforming to a linear tensile forceestrain relationship (i.e. ESC2M and ESC3M). It also can be concluded that for a sand column reinforced with an encasement with a linear loadeextension relationship, the bearing stress of the column prior to encasement yield can be evaluated using Eqs. (10) and (13). The sleeve-induced confining pressure is proportional to the stiffness of the encasement. 5. Conclusions This paper has examined the responses of geotextile encased sand columns embedded in soft clay using a 1-g model test. The

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properties of the three constituents, namely, sand, geotextile and clay, used in the model tests were chosen or fabricated based on similarity analysis to ensure the prototype-scale and modelscale geotextile encased granular columns exhibited comparable behaviour. The encasements, geotextiles in this case, have a variety of mechanical properties, allowing us to understand a range of conditions likely to be encountered in encased sand column (ESC) applications. The test results provide important insight into the performance of ESCs. The trends obtained in the present study are in good agreement with the results reported in the literature. Furthermore, confining pressures from surrounding soil and encasements are evaluated analytically and compared with those calculated from measured data. The results clearly illustrate the influences of surrounding soil and encasements on the bearing stress of each encased column. The following major conclusions can be drawn from the results of this study. 1. Geotextile encased sand columns exhibit different responses under different geotextile stiffnesses and strengths. The results reveal that in a 1-g model test, in order to obtain the performance features of a field-scale prototype geotextile encased granular column embedded in soft soil, it is important to use similarity analysis to find the suitable materials and suitable dimensions of ESC. 2. Geotextile encased sand columns exhibit different bulging behaviours when using encasements with varying mechanical properties. For unreinforced sand columns and sand columns confined with low stiffness geotextiles, the predominant bulging of sand columns reaches depths of up to 2.5 times the diameter of the sand column from the ground surface; the increase in the stiffness of a geotextile leads to an increase in depth that predominates when the bulging of an encased sand column occurs. An increase in the stiffness of a geotextile also transmits a significant amount of stress deeper into the column. Therefore, the findings suggest that the stiffness of a geotextile is a factor determining the optimum encasing length of a partially encased sand column. 3. For a geotextile encased sand column which experiences no geotextile rupture, the pattern of the vertical pressuredisplacement relation of the column closely follows that of the tensile force-strain relation of the geotextile. 4. For a geotextile encased granular column encountering geotextile rupture, the release of the confining pressure previously exerted by the geotextile induces excessive column bulging. However, a column with ruptured geotextile still provides higher bearing capability than an uncased sand column. 5. Good agreement is achieved between findings of experimental measurements and an analytical method based on cavity expansion theory for the bearing stress of unreinforced and encased sand columns. The simplified approach of assuming constant volume in the granular column also provides good bearing stress evaluation results for columns encased with relatively medium to high geotextile stiffness that encounter no geotextile rupture under the applied load. 6. Results reported in the present study are based on experimental tests conducted on columns penetrated through the entire length of the clay deposit. More studies on geotextile encased sand columns floating in clay deposits are required to better understand the behaviour of geotextile encased granular columns. 7. To further validate the results of this experimental work, fullscale field trials or actual real-life applications would be beneficial.

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