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Direct shear tests on geosynthetic-encased granular columns
Geotextiles and Geomembranes 44 (2016) 396e405
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Direct shear tests on geosynthetic-encased granular columns Sunil Ranjan Mohapatra a, K. Rajagopal a, *, Jitendra Sharma b a b
Department of Civil Engineering, Indian Institute of Technology Madras, Chennai, 600 036, India Department of Civil Engineering, York University, Toronto, M3J 1P3, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2015 Received in revised form 17 December 2015 Accepted 14 January 2016 Available online xxx
The behaviour of geosynthetic-encased granular columns (EGC) under vertical loads is reasonably well understood. To date, very little research has been done to understand the behaviour of EGCs subjected to lateral loads. The main objective of this paper is to quantify the effect of encasement on the lateral load capacity of EGCs. Several direct shear tests are performed on granular columns with and without encasement in a shear box having plan dimensions of 305 305 mm. Tests are conducted at different normal pressures varying from 15 kPa to 75 kPa. Two different diameters of columns, three types of encasements and three different plan configurations are studied in this research work. The results from these tests are discussed in terms of the increase in the shear strength due to geosynthetic encasement and the strength envelopes for understanding the influence of the encasement. Different types of failures observed in the granular columns (with and without encasement) subjected to lateral load are also discussed. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Geosynthetics Granular column Geosynthetic encasement Soft clay Lateral loading Direct shear tests
1. Introduction In recent times, granular columns have found wide applications for the construction of various rigid and flexible structures like buildings, embankments and oil storage tanks over soft clay (e.g., Murugesan and Rajagopal, 2006, 2007, 2008; Gniel and Bouazza, 2009, 2010; Ali et al., 2012; Shahu and Reddy, 2014). The ground reinforced with granular columns behaves as a composite with higher strength and stiffness compared to virgin soils (Alamgir et al., 1996; Murugesan and Rajagopal, 2010). In addition to improving the bearing capacity of the foundation soil, granular columns also reduce the time taken to post-construction ultimate settlements by accelerating the rate of consolidation of soft clay. It is well understood that granular columns derive their load carrying
Abbreviation: Ar, Area replacement ratio; E1, Woven geotextile encasement; E2, Socks encasement; E3, paper towel encasement; EGC, Geosynthetic Encased granular Column; LDS, Large direct shear (305 mm 305 mm); sn, Applied normal pressure; OGC, Ordinary granular column (without any geosynthetic encasement); 50C, Single 50 mm diameter granular column at centre of shear box; 100C, Single 100 mm diameter granular column at centre of shear box; 50T, 50 mm diameter granular columns in triangular arrangement (three columns); 50S, 50 mm diameter granular columns in square arrangement (four columns). * Corresponding author. Tel.: þ91 44 2257 4263; fax: þ91 44 2257 4252. E-mail addresses: [email protected] (S.R. Mohapatra), [email protected] (K. Rajagopal), [email protected] (J. Sharma). http://dx.doi.org/10.1016/j.geotexmem.2016.01.002 0266-1144/© 2016 Elsevier Ltd. All rights reserved.
capacity by relying on the lateral confinement provided by the surrounding soil (Hughes and Withers, 1974; Hughes et al., 1975). However, sufficient lateral confinement may not be available in the case of very soft clays having low undrained shear strengths (cu < 15 kPa) (Raithel et al., 2002; Murugesan and Rajagopal, 2007). Due to lateral flow in soft clays (Barksdale and Bachus, 1983), support for the granular column from the surrounding soil reduces, leading to the bulging of granular columns at shallow depth and resulting in higher settlement for overlying structures (Murugesan and Rajagopal, 2006; Black et al., 2007). Lateral flow of the foundation soil leads to shear failure of the columns (Fig. 1). Clogging of the granular column from the surrounding soft clay is also a major issue, which reduces the discharge capacity of the column (Murugesan and Rajagopal, 2008; Weber et al., 2010; Castro and Sagaseta, 2011; Indraratna et al., 2012). The above-mentioned problems with granular columns can be effectively overcome by using a geosynthetic encasement to the granular column, which provides additional confinement, leading to mobilization of higher shear resistance. Although the behaviour of ordinary and encased granular columns under vertical loads is reasonably well understood; (e.g., di Prisco et al., 2006; Murugesan and Rajagopal, 2006, 2007, 2010; Yoo and Kim, 2009; Gniel and Bouazza, 2009, 2010; Lo et al., 2010; Khabbazian et al., 2010; Pulko et al., 2011; Elsawy, 2013; Ali et al., 2012, 2014; Keykhosropur et al., 2012; Dash and Bora, 2013;
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Fig. 1. Embankment on granular column improved soft foundation soil.
Ghazavi and Afshar, 2013; Hosseinpour et al., 2014; Zhang and Zhao, 2015), there have not been many studies on their behaviour under lateral loading. Murugesan and Rajagopal (2008) carried out plane-strain laboratory model tests to understand the behaviour of OGC and EGC subjected to shear loading and reported significant improvement in the shear resistance of granular columns due to encasement. Schnaid et al. (2014) carried out field tests on geosynthetic-encased sand columns and concluded that due to geosynthetic encasement, horizontal earth pressure on adjacent foundation decreases significantly. Chen et al. (2015) carried out physical model test and 3-dimensional numerical modelling to understand the behaviour of embankment loading on geosynthetic-encased stone columns in soft soils and reported that encased stone columns fail in bending instead of shear. However, the effects of variation of column diameter and stiffness of encasement were neglected. Almeida et al. (2015) carried out field study by constructing a 5.35-m-high trial embankment on soft foundation improved by EGCs and observed that the rate of radial strain of geosynthetic encasement reduced progressively with the consolidation of foundation soil. The lateral deformation of soft soil observed at the toe of the embankment for the case of EGCs was observed to be four times lower than that for an unimproved soft ground. It is evident from the above discussion that EGCs provide increased resistance to lateral loading compared to OGCs; however, there have been no previous studies on the effect of size of columns and the stiffness of geosynthetic encasement on the shear resistance of EGCs. Additionally, there have been no previous studies on the group effect of EGCs under lateral loading. These effects should be quantified in order to have a more complete understanding of the behaviour of EGCs. This paper focuses on understanding the behaviour of OGC and EGC under lateral loading by conducting large direct shear tests. Three different types of encasement materials were used. The tests were carried out using single granular columns with two different diameters and also with groups of three or four granular columns in triangular and square arrangements, respectively. From the experimental results, qualitative and quantitative improvement in lateral load capacity of soil due to the inclusion of OGCs and EGCs were observed. 2. Materials and methods The laboratory model studies on lateral load capacity of granular columns were carried out using a large direct shear box having plan
size of 305 mm 305 mm and a depth of 140 mm. The dimensions of the test set-up are fairly small compared with typical dimensions of full-scale granular columns; however, the tests have been conducted at normal stress levels that are typical of full-scale embankments. As such, issues, such as too much dilation of the granular material in small-scale laboratory tests, have been avoided and the stress-strain behaviour of both the sand and the granular column has been simulated appropriately in the test set-up. All the tests were performed in dry conditions. The details of laboratory tests are given in the subsequent sections. 2.1. Material properties 2.1.1. Sand and aggregates Poorly-graded fine sand (effective particle size D10 of 0.24 mm) was used for the laboratory tests. The peak and critical state friction angles of the sand measured from large direct shear tests are 36 and 29 , respectively. Two types of crushed granular aggregates were used for forming the granular columns. Two different diameters of granular columns e 50 mm and 100 mm e were used for the laboratory tests. The 50-mm-diameter granular columns were formed using aggregate passing through a 4.75 mm sieve and retained on a 2 mm sieve. The 100-mm-diameter granular columns were formed using aggregate passing through a 9.5 mm sieve and retained on a 2 mm sieve. Smaller aggregates were used for the 50-mm-diameter stone columns and larger aggregates were used for 100-mm-diameter stone columns to achieve the same diameter to aggregate size ratio of nearly 10. A value of 10 for this ratio was considered adequate based on the works of Fox (2011) and Stoeber (2012) wherein a ratio of around 6 for the triaxial specimen diameter to maximum particle size was found to be satisfactory for mine waste rock, which is similar in mechanical behaviour to the aggregates used in granular columns. Table 1 shows the different properties of sand and aggregates used in large-scale direct shear tests. 2.1.2. Geosynthetic encasement Three different types of encasement materials - a woven geotextile (E1), cotton socks (E2) and paper towel (E3) - were used to study the effect of variation of encasement properties (ultimate tensile strength, initial and secant modulus and failure strain). All the properties of the three different encasement materials are listed in Table 2. The encasement material E1 had the highest ultimate tensile strength and modulus value compared to the other two materials. The initial modulus of E3 is higher than that of E2 but its
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Table 1 Properties of coarse-grained materials. Property
Sand
Smaller aggregate (used for 50-mm-dia columns)
Larger aggregate (used for 100-mm-dia columns)
Specific gravity Dry density (g/cm3) USCS classification Effective particle size (D10; in mm) Average particle size (D50; in mm)
2.65 1.66 ± 0.03 SP 0.24 0.35
2.70 1.65 ± 0.03 GP 2.2 3.2
2.70 1.75 ± 0.05 GP 2.6 4.5
Table 2 Properties of encasement materials (ASTM D4595-05, 1986). Strength properties
Unit
Woven geotextile (E1)
Socks (E2)
Paper (E3)
Ultimate tensile strength Strain at ultimate strength Initial modulus of parent material 5% secant modulus of parent material Ultimate strength (seam) Initial modulus (seam) 5% secant modulus (seam)
kN/m % kN/m kN/m kN/m kN/m kN/m
34 37 175 153 2.2 66 29
4.28 498 0.2 0.06
0.28 14 3.5 1.7 0.20 4 0.45
a
a a a
Socks are seamless.
rupture strain is very low. The material E2 has very low initial modulus and hence undergoes large elongation before rupture. Interestingly, the ultimate tensile strength of E2 is higher than that of E3, which was mobilized at nearly 500% strain, whereas ultimate tensile strengths for E1 and E3 are mobilized at 37% and 14% strains respectively. Quick setting adhesive was used for fabrication of the encasement tubes for all types of encasement materials. The seam width was maintained constant at 15e20 mm for all the test specimens. The E2, being a tubular product, had no seam. 2.2. Test set-up 2.2.1. Large direct shear box (LDS) The large direct shear box (LDS) consists of upper and lower boxes. The bottom box moves in horizontal direction on smooth rollers. The top box is fully constrained from lateral movement. The shear forces developed are continuously recorded by an S-type load cell of 44 kN capacity. The plan area of LDS was 93,025 mm2 (305 mm 305 mm) as shown in Fig. 2. The maximum available depth inside the shear box is 203.2 mm. The actual sample height inside the shear box was maintained at 140 mm for the tests. The
Fig. 2. Open-ended steel tubes in triangular pattern with geosynthetic encasements on the outside of the tubes.
maximum allowable shear displacement of the shear box was 96 mm. The shear displacement was measured using a LVDT attached to the bottom box. The soil samples were sheared at a uniform strain rate of 1 mm/min. Uniform normal pressure was applied on the soil through a pressurized flexible bladder placed inside the stationary upper box. A pressure regulator and gauge was used to control the applied pressure. 2.2.2. Sample preparation In the case of tests with OGCs, open-ended steel tubes with inner diameter corresponding the desired diameter of the granular column were erected inside the empty shear box at the desired locations using a triangular or a square configuration (Fig. 2). For each experiment, a pre-weighed amount of dry sand was poured into the shear box around the steel tubes and densified in three layers of roughly equal thickness using a needle vibrator so as to achieve a dry density of 1.66 ± 0.03 g/cm3 (corresponding to 72% relative density). The required amount of stone aggregate was placed inside the steel tubes in three equal layers and compacted by applying 30 blows to each layer using an 8-mm-diameter steel tamping rod dropped from a height of 200 mm. After pouring and compacting the aggregate to the full height of the shear box, the steel tubes were gradually withdrawn from sand bed by pulling them up vertically. Using steel tubes with smooth outer and inner surfaces minimized the disturbance of the granular column and the surrounding sand. Upon complete removal of the steel tube, 5 blows were applied to compact the top portion of the aggregate columns using a 3.16-kg, 140-mm-diameter circular steel plate that was dropped from a 100 mm height. It is acknowledged that the sample preparation method does not fully simulate the enhanced confinement of the surrounding soil produced by the displacement method of granular column installation; however, it does offer excellent reproducibility in terms of respective dry densities of the granular columns and the surrounding sand layer. For the case of encased granular columns (EGC), the open-ended steel tubes were first lined on the outside with geosynthetic encasements. The steel tubes were then erected inside the empty shear box at the desired locations using a triangular or square configuration (Fig. 2). A pre-weighted quantity of sand was then poured around the steel tubes and compacted using a needle vibrator. Then, as with the installation of OGCs, the required quantity of aggregates was placed and compacted inside the tubes. After the formation of granular columns, the steel tubes were
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withdrawn gradually in the vertical direction as described earlier. The 100-mm- and 50-mm-diameter granular columns were constructed by compacting the aggregates to a dry density of 1.75 g/ cm3 and 1.65 g/cm3, respectively. Sand was used to prepare the soil bed in place of normally consolidated clay soil due to the ease of placement, ease of achieving consistency between the tests and similarity of strength behaviour (di Prisco et al., 2006). The objective of the test program was to study the improvement in lateral resistance of virgin soil due to the installation of OGC and EGC. As such, whether the lateral resistance is provided by a sandy or a clayey soil is not important as long as that lateral resistance can be estimated with reasonable accuracy. As long as the baseline shear strength of the surrounding soil can be estimated and deducted from the shear stress mobilized by the sand-OGC or sand-EGC system, it should be possible to isolate the behaviour of OGCs and EGCs from the test results. It is acknowledged that the use of dry sand in place of saturated soft clay does not allow the simulation of localized drainage effects around the stone columns, which can potentially increase the undrained shear strength of the surrounding clay. 2.3. Test procedure The granular columns were installed in different plan arrangements to study the response with single and group of columns. A single granular column of either 50 mm or 100 mm diameter was installed at the centre of the shear box to analyze the effect of single granular columns subjected to lateral deformations. The 50-mmdiameter columns were also installed in triangular and square arrangements at 100 mm centre-to- centre spacing to study the group effects. The tests were conducted at different normal pressures ranging from 15 kPa to 75 kPa in order to develop the strength envelopes. The details of the different types of encasements and the plan configurations are given in Tables 2 and 3, respectively. As the normal pressure is applied through a flexible air pressure bag, it is reasonable to assume that uniform pressure acts all over the plan area of the box. The laboratory tests were carried out at different area replacement ratios (Ar), where Ar is the ratio of the plan area of the granular columns to the plan area of the shear box (Table 3). Fig. 3 shows the plan view of the large direct shear box with granular column at the centre of the box. Figs. 4 and 5 show the granular columns arranged in triangular and square patterns to analyze the behaviour of groups of granular columns subjected to lateral loading. The arrow direction in Fig. 4 illustrates the direction of movement of the bottom shear box. Tests on OGCs were terminated upon reaching 40 mm horizontal displacement of the bottom shear box because at this displacement both the peak and the critical state shear resistance were mobilized. Tests in which a single EGC was installed at the centre of the shear box were terminated at 80 mm horizontal displacement so as to mobilize tensile resistance of the geosynthetic encasement. Tests involving groups of EGCs were terminated at 60 mm horizontal displacement in order to avoid boundary effects.
Fig. 3. Plan view of large direct shear box with granular column at centre of box.
Fig. 4. 50 mm diameter granular columns in triangular arrangement at 100 mm centre-to-centre.
3. Results and discussion 3.1. Failure mechanism of granular columns After the completion of a shear test, sand was removed carefully around the columns to observe the failure mechanism of the granular column. In tests involving a single OGC at the centre,
Table 3 Details of the large direct shear testing program. Test arrangement
Test identifier
Area replacement ratio Ar (%)
Dense sand 50 mm diameter granular column at centre of box 100 mm diameter granular columns at centre of box 50 mm diameter granular columns in triangular pattern 50 mm diameter granular columns in square pattern Dense aggregate
Sand 50C 100C 50T 50S Aggregate
0 2.11 8.44 6.33 8.44 100
400
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100
Shear stress (kPa)
80 60 40 50S OGC 100C OGC Sand
20
50T OGC 50C OGC
0 0
5
10 15 20 25 30 Horizontal displacement (mm)
35
40
Fig. 7. Shear stress vs. horizontal displacement for OGC (sn ¼ 75 kPa).
Fig. 5. 50 mm diameter granular column in square arrangement at 100 mm centre-tocentre.
complete shearing of the granular column was observed along a predefined shear plane. Two types of shear failures were observed in tests involving EGCs depending upon the modulus and the strength of the geosynthetic encasement. In the case of the E3 encasement (high initial modulus but low tensile strength), the encasement was completely sheared along with the granular column. This is illustrated as Type1 failure in Fig. 6. In the case of the E1 (high initial modulus and high tensile strength) and the E2 (low initial modulus and high rupture strain), the encasement did not undergo any rupture. As such, the EGC did not shear but underwent excessive bending around the pre-defined shear plane. This is illustrated as Type-2 failure in Fig. 6. Since the EGCs did not shear completely in case of a Type-2 failure, they would have continued to act as vertical drains because of the continuity of flow path at large deformation. The ability of a geosynthetic encasement to prevent shear failure of granular columns is important in case of earthquake-induced liquefaction during which the foundation soil may undergo large deformation. In such cases, EGCs will continue to drain the liquefied ground and restore effective stresses rapidly after the earthquake, thereby preventing complete failure of the structure. 3.2. Effect of the OGCs The shear stresses are found to increase upon reinforcing the sand with OGCs due to the higher shear resistance of the combined soil-granular column system (Fig. 7). The granular column and the
surrounding soil act as a composite material that mobilizes higher shear resistance. On increasing the area ratio of the granular columns, higher shear resistances are mobilized because the percent area occupied by the granular column is higher in the plane of shear. This behaviour can be clearly observed for both 50 mm and 100 mm diameter columns. The ordinary granular columns of 50 mm diameter arranged in triangular and square patterns mobilized higher shear resistance compared to a single 50 mm diameter at the centre of the box, Table 4. 3.3. Effect of geosynthetic encasement Fig. 8 shows the results of woven geotextile (E1) encased granular column at a normal pressure of 75 kPa. On encasing the column, higher shear stresses are mobilized as compared to the corresponding OGC. In the case of OGC, the soil specimen had undergone considerable strain softening (Fig. 7) whereas for EGC minimal strain softening occurred and in a few cases strain hardening was pronounced at higher strain levels. The peak and the large-strain shear stresses observed for different encasements are reported in Table 5. An increase in shear strength due to encased granular columns can be described by the apparent cohesion due to the effects of geosynthetic confinement. The geosynthetic straining during the shear displacement produces higher confining pressures within the granular columns leading to this additional shear strength. The effect of geosynthetic confinement was found to be very high at low normal pressures which gradually reduced with an increase in normal pressures. The highly non-linear strength behaviour of the reinforced soil can be approximated by a linear strength envelope drawn with significant vertical intercept. This vertical intercept is referred to as “apparent
Fig. 6. Different types of failure of encased granular column.
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Table 4 Comparison of results at normal pressure of 75 kPa for OGC. Mode
Area replacement ratio (%)
Peak shear stress (kPa)
Shear stress (kPa) at 40 mm displacement
Sand 50C OGC 100C OGC 50T OGC 50S OGC Aggregates
0 2.11 8.44 6.33 8.44 100
64 74 77 84 86 173
43 52 55 56 61 110
200
Aggregates 50T EGC 50C EGC
Shear stress (kPa)
160
50S EGC 100C EGC Sand
120
80 40
strength, which could be used in slope stability analyses of stone column treated soils. Comparing the results of single and group EGC in Fig. 8 it can be observed that in the case of a single EGC shear resistance of soil reduces after the peak then begins to increase at larger displacements (100C), whereas in group arrangements a slight reduction occurs in the shear resistance after the peak and at large displacement shear strength increases to the peak shear stress. Table 5 presents a direct comparison of the peak and large strain shear stresses for different cases of encased granular columns. 3.5. Effect of tensile modulus of the geosynthetic encasement
0 0
10
20 30 40 Horizontal displacement (mm)
50
60
Fig. 8. Shear stress vs. horizontal displacement for EGC, E1 (sn ¼ 75 kPa).
cohesion” in this paper (Table 6). The initial increase in apparent cohesion may be due to the increase in the stiffness of the granular columns whereas at higher strains significant apparent cohesion was mobilized due to tensile forces developed in the encasement. The apparent cohesion exhibited by dry granular soils is an effect of the particular soil deformations that takes place in large shear box tests and absent in small shear box tests (Liu, 2006; Bareither et al., 2008). The shear strength parameters for different test configurations are summarized in Table 6. 3.4. Effect of group action of granular columns On arrangement of granular columns in a group, the granular column and the intervening soil between granular columns act as a composite material. Because of the confinement effect in the soil from the neighbouring columns, higher shear resistance is mobilized whereas for the single OGC (50C and 100C) only the shear resistance of sand and column composite controls the shear strength mobilization. From Table 4 it can be seen that due to group arrangement of ordinary granular columns, higher shear strength is mobilized both at peak and critical states as compared to an equivalent single column with similar Ar. Though Ar of 50T is smaller than that for the case of 100C, larger shear strength is developed due to group effect (Table 4). This result shows that the zone of soil between the columns behaves as a soil with higher Table 5 Comparison of results at normal pressure of 75 kPa for EGC. Mode
Area replacement ratio (%)
Sand 50C EGC 100C EGC 50T EGC 50S EGC
0 2.11 8.44 6.33 8.44
3.6. Effect of increase in normal pressure
Peak shear stress (kPa)
Shear stress (kPa) at 40 mm displacement
E1
E3
E1
E2
E3
87 90 90 88
43 75 86 96 102
66 73 77 75
53 56 58 64
64 88 91 102 101
E2 88 85 84 86
In order to study the effect of the tensile modulus of the geosynthetic encasement, tests were conducted on encased granular columns with three different types of encasement materials E1, E2 and E3, as described previously. Fig. 9 shows the increase in the shear strength for 50C due to the variation in encasement materials at a normal pressure of 30 kPa. Fig. 10 shows the increase in the shear strength for 50S due to the variation in encasement materials at the same normal pressure of 30 kPa. It is clear from the above data that encasement with a higher modulus helps in mobilizing higher shear strength. As the modulus of E2 material is low in comparison with E1 and E3, it can undergo large deformation along with the aggregate without rupture. Due to high initial tensile modulus, the peak friction angle of the E3 encased columns is found to be similar to the columns encased in woven geotextiles. However, immediate rupture failure of the E3 encasement after the peak has reduced the strength of these columns to that of unreinforced columns at large strains (Figs. 9 and 10). It can also be seen in Fig. 9 that the mobilized additional shear strength for OGC is insignificant in comparison with sand at both peak and at 40 mm displacement. In contrast, the mobilized shear strength is higher for the group columns (Fig. 10). Encased granular columns mobilize higher apparent cohesion in comparison with OGC at both peak and at large displacement as set out in Table 6. The differential increase in apparent cohesion at different normal pressures is attributed to the different dilatancy effects at different normal pressures. The E1 and E2 type materials did not fail in rupture and developed considerable increase in apparent cohesion at large shear displacements, whereas a smaller increase was observed in E3 type encasement due to the rupture failure of encasement at large shear displacements.
The tests were performed at four different normal pressures. Fig. 11(a) shows that by increasing the normal pressure, higher shear strength is mobilized by soil treated with ordinary granular columns (100C OGC). It can be seen that after the peak, strain softening takes place and shear strength remained constant thereafter. Fig. 11(b) shows the increase in shear resistance due to the inclusion of woven geotextile (E1) encased 100 mm diameter
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Table 6 Shear strength parameters from different tests. Type
Area replacement ratio (Ar %)
Sand OGC
Woven geotextile encased granular column (E1)
Socks (E2)
Paper (E3)
Aggregate
0 2.11 8.44 6.33 8.44 2.11 8.44 6.33 8.44 2.11 8.44 6.33 8.44 2.11 8.44 6.33 8.44 100
Peak shear strength parameters
(50C) (100C) (50T) (50S) (50C) (100C) (50T) (50S) (50C) (100C) (50T) (50S) (50C) (100C) (50T) (50S)
f ( )
c (kPa)
f ( )
10 10 10 10 10 23 25 27 25 17 18 12 11 17 22 15 13 37
36 40 41 44 45 40 41 44 45 40 41 44 45 40 41 44 45 61
5 5 5 5 5 20 28 36 40 16 15 14 13 7 6 8 9 26
29 32 34 34 36 32 34 34 36 32 34 34 36 32 34 34 36 48
50
Shear stress (kPa)
40 30 20 10
E1 OGC
0 0
10
E2 Sand
20 30 40 Horizontal displacement (mm)
E3
50
60
Fig. 9. Shear stress vs. horizontal displacement for 50C (sn ¼ 30 kPa).
granular column at centre. Due to the increase in stiffness of the granular column a higher peak shear strength is mobilized compared to OGC (Fig. 11(a)). After the peak, very little strain softening is observed compared to OGC and shear resistance becomes almost equal to the peak value at 40 mm displacement. It can be seen that the encasement has also increased the shear modulus
Shear stress (kPa)
60
40
20 E1
E2
E3
OGC
Sand
0
0
10
Shear strength parameters at 40 mm displacement
c (kPa)
20 30 40 50 Horizontal displacement (mm)
Fig. 10. Shear stress vs. horizontal displacement for 50S (sn ¼ 30 kPa).
60
of the soil mass as evident from the steeper displacement vs. shear stress graphs, Fig. 11 (b), (c) and (d). Fig. 11(c) shows the influence of normal pressure on the shear resistance of soil treated with sock (E2) encased granular columns. Due to the confinement provided by the encasement, higher shear strength is mobilized both at the peak and large displacement states. As the initial modulus of E2 is smaller than that of E1 and E3, smaller peak shear strength is mobilized as compared with that mobilized in the case of E1 and E3. After the peak, no strain hardening is observed because of the low modulus of the encasement. On the other hand, strain softening was also not seen as there was no rupture of encasement. Fig. 11(d) shows the behaviour of paper (E3) encased 100 mm diameter granular column. From the figure it can be seen that peak shear strength mobilized by the soil sample is nearly equal to that of E1. It is interesting to see that after the peak a sudden failure takes place due to the rupture of the encasement. It can also be seen that after 40 mm displacement shear strength has fallen to that given by similar OGCs. Generally peak shear stress increases with an increase in normal pressure and increases with an increase in Ar. Fig. 12(a) and (b) show the variation of peak shear stress with Ar in case of OGC and EGC at different normal pressures. In the case of OGC peak shear stress almost remains constant at lower normal pressure. In the case of higher normal pressure an increase in peak shear stress is observed with Ar. In the case of EGC an increase in peak shear stress with Ar is observed even at lower normal pressure due to the increase in stiffness of the granular column. Fig. 13(a) and (b) show the variation of shear stress at 40 mm displacement with Ar for both OGC and EGC at different normal pressures. In the case of OGC, the strength behaviour is similar to that observed at peak stress. But in the case of EGC a steady increase in shear stress beyond the peak is observed with an increase in Ar due to mobilization of additional confinement from tensile forces in geosynthetic encasement. From Figs. 12 and 13 it can be observed that a square arrangement gives a higher percentage of improvement for peak stress in case of OGC, whereas triangular arrangement gave higher strength in the case of EGC. At 40 mm displacement a square arrangement mobilizes higher shear stress in both OGC and EGC. Also it can be seen that in the case of OGC, a marginal increase in shear strength is observed with an increase in Ar whereas a substantial increase in the strength is observed for EGC.
S.R. Mohapatra et al. / Geotextiles and Geomembranes 44 (2016) 396e405 100
75 kPa 30 kPa
120 100
Shear stress (kPa)
Shear stress (kPa)
80
45 kPa 15 kPa
60
40 20
80 60 40 20
0
0
10 20 30 Horizontal displacement (mm)
75 kPa 30 kPa
0
40
0
75 kPa 30 kPa
40
(b) 100C EGC (E1) 100
45 kPa 15 kPa
Shear stress (kPa)
Shear stress (kPa)
80
45 kPa 15 kPa
10 20 30 Horizontal displacement (mm)
(a) 100C (OGC) 100
403
60 40 20
75 kPa 30 kPa
80
45 kPa 15 kPa
60 40
20 0
0
0
10 20 30 Horizontal displacement (mm)
40
0
10
(c) 100C EGC (E2)
20 30 40 Horizontal displacement (mm)
50
60
(d)100C EGC (E3)
Fig. 11. Shear stress vs. horizontal displacement behaviour of different systems.
120
75 kPa
45 kPa
30 kPa
75 kPa
45 kPa
30 kPa
15 kPa
15 kPa Shear stress (kPa)
Shear stress (kPa)
120
80
40
80
40
0 0
0 0
2
4
Ar (%)
6
8
45 kPa
Ar (%)
6
8
10
8
10
(a) OGC
120
30 kPa
75 kPa
45 kPa
30 kPa
15 kPa
15 kPa Shear stress (kPa)
Shear stress (kPa)
75 kPa
4
10
(a) OGC
120
2
80
40
80
40
0 0
0
0
2
4
Ar (%)
6
8
10
2
4
Ar (%)
6
(b) EGC (E1)
(b) EGC (Reinforcement Type E1 – Woven geotextile) Fig. 12. Variation of peak shear stress mobilized vs. area replacement ratio (Ar).
Fig. 13. Variation of shear stress mobilized at 40 mm shear displacement vs. area replacement ratio (Ar) for normal stress values of 15, 30, 45 and 75 kPa.
404
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90
105
50S50S OGC OGC
60
OGC 50T50T OGC
90
100C OGC 100C OGC
75
Shear stress (kPa)
Shear stress (kPa)
75
OGC 50C50C OGC sandsand
45
30
Series1 50S EGC 50T EGC Series2 100C EGC Series3 50C EGC Series4 sand Series5
60 45 30 15
15
0 0
0
20
40 Normal stress (kPa)
60
20
80
40 Normal stress (kPa)
60
80
60
80
(a) Peak
(a) Peak 105
90 50S50S OGC OGC OGC 50T50T OGC
75
Shear stress (kPa)
Shear stress (kPa)
100C OGC 100C OGC 60
OGC 50C50C OGC
sandsand
45
50S50S EGCEGC 50T50T EGCEGC 100C 100C EGCEGC 50C50C EGCEGC Sandsand
90
30
75 60
45 30 15
15
0 0
0 0
20
40 Normal stress (kPa)
60
80
(b) At 40 mm shear displacement Fig. 14. MohreCoulomb failure envelopes for the OGC tests.
Failure envelopes of sand and OGC at peak and at 40 mm shear displacement are shown in Fig. 14(a) and (b). Fig. 15(a) and (b) show the failure envelopes of EGC at peak and 40 mm shear displacements. In the case of EGC, the failure envelopes were drawn with the same slope as in the corresponding OGC cases, which results in much higher apparent cohesion due to nonlinear effects of dilation, Fig. 15.
Fig. 15. MohreCoulomb failure envelopes for the EGC tests (Reinforcement Type E1 e Woven geotextile).
3. 4.
5.
1. Geosynthetic encasement increases the lateral load capacity of granular columns due to mobilization of tensile forces in the encasement layer. The resistance of EGC increases with increasing shear displacements until complete rupture of encasement material occurs. The encasement with higher modulus mobilises higher lateral resistance forces at smaller displacements. 2. For OGC, a marginal increase in shear strength was observed with an increase in area replacement ratio in contrast to
40 Normal stress (kPa)
(b) At 40 mm shear displacement
4. Conclusions The present work pertains to the development of understanding the behaviour of granular columns subjected to lateral (shear) loading. The behaviour of OGC and EGC was studied using laboratory large direct shear tests and their results are compared to show the improvement of lateral load carrying capacity of granular columns due to encasement. Based on the observed results the following conclusions are made:
20
6.
substantial increase in the strength for EGC with an increase in area replacement ratio. After the rupture of encasement in EGCs, the strength reduces to the levels of OGCs. At smaller shear displacements, the strength increase observed in EGCs is more likely due to the increase in stiffness of the granular columns while at larger displacements, the increase in strength is due to the tensile forces mobilized in the encasement. Shear failure of OGC takes place along the shear plane. EGC undergoes bending type deformation without rupture of the encasement material due to the flexible nature of geosynthetic (E1/E2) encasement. A group arrangement mobilizes higher shear resistance compared to single OGC and EGC (for the same Ar) due to the confinement offered to the intervening soil. This aspect is important and needs to be considered for more rigorous analyses and designs.
Acknowledgements The first author is thankful to the Canadian Commonwealth Scholarship Program and the Natural Science and Engineering Research Council (NSERC) Discover Grant Program for sponsoring his research visit at University of Saskatchewan, Saskatoon, Canada during September 2012 to June 2013. The help and suggestion provided by Mr. Adam Hammerlindl, Mr. Zeeshan Ahmad, Mr.
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Nadeem Khokhar and Mr. Prateek Jindal during the experiments at University of Saskatchewan are highly appreciated. References Alamgir, M., Miura, N., Poorooshasbh, H.B., Madhav, M.R., 1996. Deformation analysis of soft ground columnar inclusions. Comput. Geotech. 18 (4), 261e290. Ali, K., Shahu, J.T., Sharma, K.G., 2012. Model tests on geosynthetic-reinforced stone columns: a comparative study. Geosynth. Int. 19 (4), 292e305. Ali, K., Shahu, J.T., Sharma, K.G., 2014. Model tests on single and groups of stone columns with different geosynthetic reinforcement arrangement. Geosynth. Int. 21 (2), 103e118. Almeida, M.S.S., Hosseinpour, I., Riccio, M., Alexiew, D., 2015. Behaviour of geotextile-encased granular columns supporting test embankment on soft deposit. ASCE J. Geotech. Geoenvironmental Eng. 141 (3), 04014116. ASTM D4595, 1986. Standard Test Method for Tensile Properties of Geotextiles by the Wide-width Strip Method. ASTM International, West Conshohocken, PA, USA. Barksdale, R.D., Bachus, R.C., 1983. Design and Construction of Granular Columns. Report No. FHWA/RD-83/026. Federal Highway Administration Office of Engineering and Highway Operations Research and Development, Washington, D.C. Black, J.A., Sivakumar, V., Madhav, M.R., Hamill, G.A., 2007. Reinforced stone columns in weak deposits: laboratory model study. J. Geotech. Geoenvironmental Eng. 1339, 1154e1161. Bareither, Christopher A., Benson, C.H., Edil, T.B., 2008. Comparison of shear strength of sand backfills measured in small-scale and large-scale direct shear tests. Can. Geotech. J. 45 (9), 1224e1236. Castro, J., Sagaseta, C., 2011. Deformation and consolidation around encased stone columns. Geotext. Geomembr. 29 (3), 268e276. Chen, J.-F., Li, L.-Y., Xue, J.-F., Feng, S.-Z., 2015. Failure mechanism of geosyntheticencased stone columns in soft soils under embankment. Geotext. Geomembr. 43 (5), 424e431. Dash, S.K., Bora, M.C., 2013. Influence of geosynthetic encasement on the performance of stone columns floating in soft clay. Can. Geotech. J. 50 (7), 754e765. di Prisco, C., Galli, A., Cantarelli, E., Bongiorno, D., 2006. Geo-reinforced sand columns: small scale experimental tests and theoretical modeling. In: Proceedings of the 8th International Conference on Geosynthetics, Yokohama, Japan, pp. 1685e1688. Elsawy, M.B.D., 2013. Behaviour of soft ground improved by conventional and geogrid-encased stone columns, based on FEM study. Geosynth. Int. 20 (4), 276e285. Fox, Z.P., 2011. Critical State, Dilatancy and Particle Breakage of Mine Waste Rock. Master's thesis. Colorado State University, Fort Collins, USA. Ghazavi, M., Afshar, J.N., 2013. Bearing capacity of geosynthetic encased stone columns. Geotext. Geomembr. 38, 26e36. Gniel, J., Bouazza, A., 2009. Improvement of soft soils using geogrid encased granular columns. Geotext. Geomembr. 27 (3), 167e175. Gniel, J., Bouazza, A., 2010. Construction of geogrid encased granular columns: a new proposal based on laboratory testing. Geotext. Geomembr. 28 (1), 108e118.
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Hosseinpour, I., Riccio, M., Almeida, M.S.S., 2014. Numerical evolution of a granular column reinforced by geosynthetics using encasement and laminated disks. Geotext. Geomembr. 42, 363e373. Hughes, J.M.O., Withers, N.J., 1974. Reinforcing of soft cohesive soils with granular columns. Ground Eng. 7 (3), 42e49. Hughes, J.M.O., Withers, N.J., Greenwood, D.A., 1975. A field trial of the reinforcing effect of a granular column in soil. Geotechnique 25 (1), 31e44. Indraratna, B., Basack, S., Rujikiatkamjorn, C., 2012. Numerical solution of stone column-improved soft soil considering arching, clogging and smear effects. J. Geotech. Geoenvironmental Eng. 139 (3), 377e394. Keykhosropur, L., Soroush, A., Imam, R., 2012. 3D Numerical analyses of geosynthetic encased stone columns. Geotext. Geomembr. 35, 61e68. Khabbazian, M., Kaliakin, V.N., Meehan, C.L., 2010. Numerical study of the effect of geosynthetic encasement on the behaviour of granular columns. Geosynth. Int. 17 (3), 132e143. Liu, S.H., 2006. Simulating a direct shear box test by DEM. Can. Geotech. J. 43 (2), 155e168. Lo, S.R., Zhang, R., Mak, J., 2010. Geosynthetic-encased stone columns in soft clay: a numerical study. Geotext. Geomembr. 28 (3), 292e302. Murugesan, S., Rajagopal, K., 2006. Geosynthetic-encased granular columns: numerical evaluation. Geotext. Geomembr. 24 (6), 349e358. Murugesan, S., Rajagopal, K., 2007. Model tests on geosynthetic encased granular columns. Geosynth. Int. 14 (6), 346e354. Murugesan, S., Rajagopal, K., 2008. Shear load tests on granular columns with and without geosynthetic encasement. Geotech. Test. J. 32 (1), 35e44. Murugesan, S., Rajagopal, K., 2010. Studies on the behavior of single and group of geosynthetic encased granular columns. J. Geotech. Geoenvironmental Eng. 136 (1), 129e139. Pulko, B., Majes, B., Logar, J., 2011. Geosynthetic-encased stone columns: analytical calculation model. Geotext. Geomembr. 29 (1), 29e39. Raithel, M., Kempfert, H.G., Kirchner, A., 2002. Geotextile-encased columns (GEC) for foundation of a dike on very soft soils. In: Proceedings 7th ICG International Conference on Geosynthetics, Nice, France, pp. 1025e1028. Schnaid, F., Winter, D., Silva, A.E.F., Alexiew, D., Kuster, V., Hebmuller, A., 2014. Geotextile encased columns (GEC) under bridge approaches as a pressure-relief system: concept, experience and measurements. In: 10th International Conference on Geosynthetics, Berlin, Germany. Shahu, J.T., Reddy, Y.R., 2014. Estimating long-term settlement of floating stone column groups. Can. Geotech. J. 51 (7), 770e781. Stoeber, J.N., 2012. Effects of Maximum Particle Size and Sample Scaling on the Mechanical Behavior of Mine Waste Rock; a Critical State Approach. Master's Thesis. Colorado State University, Fort Collins, USA. Weber, T.M., Plotze, M., Laue, J., Peschke, G., Springman, S.M., 2010. Smear zone identification and soil properties around stone columns constructed in-flight in centrifuge model tests. Geotechnique 60 (3), 197e206. Yoo, C., Kim, S.B., 2009. Numerical modeling of geosynthetic-encased granular column-reinforced ground. Geosynth. Int. 16 (3), 116e126. Zhang, L., Zhao, M., 2015. Deformation analysis of geotextile-encased stone columns. ASCE Int. J. Geomech. 15 (3), 04014053.
Contents lists available at ScienceDirect
Geotextiles and Geomembranes journal homepage: www.elsevier.com/locate/geotexmem
Direct shear tests on geosynthetic-encased granular columns Sunil Ranjan Mohapatra a, K. Rajagopal a, *, Jitendra Sharma b a b
Department of Civil Engineering, Indian Institute of Technology Madras, Chennai, 600 036, India Department of Civil Engineering, York University, Toronto, M3J 1P3, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 August 2015 Received in revised form 17 December 2015 Accepted 14 January 2016 Available online xxx
The behaviour of geosynthetic-encased granular columns (EGC) under vertical loads is reasonably well understood. To date, very little research has been done to understand the behaviour of EGCs subjected to lateral loads. The main objective of this paper is to quantify the effect of encasement on the lateral load capacity of EGCs. Several direct shear tests are performed on granular columns with and without encasement in a shear box having plan dimensions of 305 305 mm. Tests are conducted at different normal pressures varying from 15 kPa to 75 kPa. Two different diameters of columns, three types of encasements and three different plan configurations are studied in this research work. The results from these tests are discussed in terms of the increase in the shear strength due to geosynthetic encasement and the strength envelopes for understanding the influence of the encasement. Different types of failures observed in the granular columns (with and without encasement) subjected to lateral load are also discussed. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Geosynthetics Granular column Geosynthetic encasement Soft clay Lateral loading Direct shear tests
1. Introduction In recent times, granular columns have found wide applications for the construction of various rigid and flexible structures like buildings, embankments and oil storage tanks over soft clay (e.g., Murugesan and Rajagopal, 2006, 2007, 2008; Gniel and Bouazza, 2009, 2010; Ali et al., 2012; Shahu and Reddy, 2014). The ground reinforced with granular columns behaves as a composite with higher strength and stiffness compared to virgin soils (Alamgir et al., 1996; Murugesan and Rajagopal, 2010). In addition to improving the bearing capacity of the foundation soil, granular columns also reduce the time taken to post-construction ultimate settlements by accelerating the rate of consolidation of soft clay. It is well understood that granular columns derive their load carrying
Abbreviation: Ar, Area replacement ratio; E1, Woven geotextile encasement; E2, Socks encasement; E3, paper towel encasement; EGC, Geosynthetic Encased granular Column; LDS, Large direct shear (305 mm 305 mm); sn, Applied normal pressure; OGC, Ordinary granular column (without any geosynthetic encasement); 50C, Single 50 mm diameter granular column at centre of shear box; 100C, Single 100 mm diameter granular column at centre of shear box; 50T, 50 mm diameter granular columns in triangular arrangement (three columns); 50S, 50 mm diameter granular columns in square arrangement (four columns). * Corresponding author. Tel.: þ91 44 2257 4263; fax: þ91 44 2257 4252. E-mail addresses: [email protected] (S.R. Mohapatra), [email protected] (K. Rajagopal), [email protected] (J. Sharma). http://dx.doi.org/10.1016/j.geotexmem.2016.01.002 0266-1144/© 2016 Elsevier Ltd. All rights reserved.
capacity by relying on the lateral confinement provided by the surrounding soil (Hughes and Withers, 1974; Hughes et al., 1975). However, sufficient lateral confinement may not be available in the case of very soft clays having low undrained shear strengths (cu < 15 kPa) (Raithel et al., 2002; Murugesan and Rajagopal, 2007). Due to lateral flow in soft clays (Barksdale and Bachus, 1983), support for the granular column from the surrounding soil reduces, leading to the bulging of granular columns at shallow depth and resulting in higher settlement for overlying structures (Murugesan and Rajagopal, 2006; Black et al., 2007). Lateral flow of the foundation soil leads to shear failure of the columns (Fig. 1). Clogging of the granular column from the surrounding soft clay is also a major issue, which reduces the discharge capacity of the column (Murugesan and Rajagopal, 2008; Weber et al., 2010; Castro and Sagaseta, 2011; Indraratna et al., 2012). The above-mentioned problems with granular columns can be effectively overcome by using a geosynthetic encasement to the granular column, which provides additional confinement, leading to mobilization of higher shear resistance. Although the behaviour of ordinary and encased granular columns under vertical loads is reasonably well understood; (e.g., di Prisco et al., 2006; Murugesan and Rajagopal, 2006, 2007, 2010; Yoo and Kim, 2009; Gniel and Bouazza, 2009, 2010; Lo et al., 2010; Khabbazian et al., 2010; Pulko et al., 2011; Elsawy, 2013; Ali et al., 2012, 2014; Keykhosropur et al., 2012; Dash and Bora, 2013;
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Fig. 1. Embankment on granular column improved soft foundation soil.
Ghazavi and Afshar, 2013; Hosseinpour et al., 2014; Zhang and Zhao, 2015), there have not been many studies on their behaviour under lateral loading. Murugesan and Rajagopal (2008) carried out plane-strain laboratory model tests to understand the behaviour of OGC and EGC subjected to shear loading and reported significant improvement in the shear resistance of granular columns due to encasement. Schnaid et al. (2014) carried out field tests on geosynthetic-encased sand columns and concluded that due to geosynthetic encasement, horizontal earth pressure on adjacent foundation decreases significantly. Chen et al. (2015) carried out physical model test and 3-dimensional numerical modelling to understand the behaviour of embankment loading on geosynthetic-encased stone columns in soft soils and reported that encased stone columns fail in bending instead of shear. However, the effects of variation of column diameter and stiffness of encasement were neglected. Almeida et al. (2015) carried out field study by constructing a 5.35-m-high trial embankment on soft foundation improved by EGCs and observed that the rate of radial strain of geosynthetic encasement reduced progressively with the consolidation of foundation soil. The lateral deformation of soft soil observed at the toe of the embankment for the case of EGCs was observed to be four times lower than that for an unimproved soft ground. It is evident from the above discussion that EGCs provide increased resistance to lateral loading compared to OGCs; however, there have been no previous studies on the effect of size of columns and the stiffness of geosynthetic encasement on the shear resistance of EGCs. Additionally, there have been no previous studies on the group effect of EGCs under lateral loading. These effects should be quantified in order to have a more complete understanding of the behaviour of EGCs. This paper focuses on understanding the behaviour of OGC and EGC under lateral loading by conducting large direct shear tests. Three different types of encasement materials were used. The tests were carried out using single granular columns with two different diameters and also with groups of three or four granular columns in triangular and square arrangements, respectively. From the experimental results, qualitative and quantitative improvement in lateral load capacity of soil due to the inclusion of OGCs and EGCs were observed. 2. Materials and methods The laboratory model studies on lateral load capacity of granular columns were carried out using a large direct shear box having plan
size of 305 mm 305 mm and a depth of 140 mm. The dimensions of the test set-up are fairly small compared with typical dimensions of full-scale granular columns; however, the tests have been conducted at normal stress levels that are typical of full-scale embankments. As such, issues, such as too much dilation of the granular material in small-scale laboratory tests, have been avoided and the stress-strain behaviour of both the sand and the granular column has been simulated appropriately in the test set-up. All the tests were performed in dry conditions. The details of laboratory tests are given in the subsequent sections. 2.1. Material properties 2.1.1. Sand and aggregates Poorly-graded fine sand (effective particle size D10 of 0.24 mm) was used for the laboratory tests. The peak and critical state friction angles of the sand measured from large direct shear tests are 36 and 29 , respectively. Two types of crushed granular aggregates were used for forming the granular columns. Two different diameters of granular columns e 50 mm and 100 mm e were used for the laboratory tests. The 50-mm-diameter granular columns were formed using aggregate passing through a 4.75 mm sieve and retained on a 2 mm sieve. The 100-mm-diameter granular columns were formed using aggregate passing through a 9.5 mm sieve and retained on a 2 mm sieve. Smaller aggregates were used for the 50-mm-diameter stone columns and larger aggregates were used for 100-mm-diameter stone columns to achieve the same diameter to aggregate size ratio of nearly 10. A value of 10 for this ratio was considered adequate based on the works of Fox (2011) and Stoeber (2012) wherein a ratio of around 6 for the triaxial specimen diameter to maximum particle size was found to be satisfactory for mine waste rock, which is similar in mechanical behaviour to the aggregates used in granular columns. Table 1 shows the different properties of sand and aggregates used in large-scale direct shear tests. 2.1.2. Geosynthetic encasement Three different types of encasement materials - a woven geotextile (E1), cotton socks (E2) and paper towel (E3) - were used to study the effect of variation of encasement properties (ultimate tensile strength, initial and secant modulus and failure strain). All the properties of the three different encasement materials are listed in Table 2. The encasement material E1 had the highest ultimate tensile strength and modulus value compared to the other two materials. The initial modulus of E3 is higher than that of E2 but its
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Table 1 Properties of coarse-grained materials. Property
Sand
Smaller aggregate (used for 50-mm-dia columns)
Larger aggregate (used for 100-mm-dia columns)
Specific gravity Dry density (g/cm3) USCS classification Effective particle size (D10; in mm) Average particle size (D50; in mm)
2.65 1.66 ± 0.03 SP 0.24 0.35
2.70 1.65 ± 0.03 GP 2.2 3.2
2.70 1.75 ± 0.05 GP 2.6 4.5
Table 2 Properties of encasement materials (ASTM D4595-05, 1986). Strength properties
Unit
Woven geotextile (E1)
Socks (E2)
Paper (E3)
Ultimate tensile strength Strain at ultimate strength Initial modulus of parent material 5% secant modulus of parent material Ultimate strength (seam) Initial modulus (seam) 5% secant modulus (seam)
kN/m % kN/m kN/m kN/m kN/m kN/m
34 37 175 153 2.2 66 29
4.28 498 0.2 0.06
0.28 14 3.5 1.7 0.20 4 0.45
a
a a a
Socks are seamless.
rupture strain is very low. The material E2 has very low initial modulus and hence undergoes large elongation before rupture. Interestingly, the ultimate tensile strength of E2 is higher than that of E3, which was mobilized at nearly 500% strain, whereas ultimate tensile strengths for E1 and E3 are mobilized at 37% and 14% strains respectively. Quick setting adhesive was used for fabrication of the encasement tubes for all types of encasement materials. The seam width was maintained constant at 15e20 mm for all the test specimens. The E2, being a tubular product, had no seam. 2.2. Test set-up 2.2.1. Large direct shear box (LDS) The large direct shear box (LDS) consists of upper and lower boxes. The bottom box moves in horizontal direction on smooth rollers. The top box is fully constrained from lateral movement. The shear forces developed are continuously recorded by an S-type load cell of 44 kN capacity. The plan area of LDS was 93,025 mm2 (305 mm 305 mm) as shown in Fig. 2. The maximum available depth inside the shear box is 203.2 mm. The actual sample height inside the shear box was maintained at 140 mm for the tests. The
Fig. 2. Open-ended steel tubes in triangular pattern with geosynthetic encasements on the outside of the tubes.
maximum allowable shear displacement of the shear box was 96 mm. The shear displacement was measured using a LVDT attached to the bottom box. The soil samples were sheared at a uniform strain rate of 1 mm/min. Uniform normal pressure was applied on the soil through a pressurized flexible bladder placed inside the stationary upper box. A pressure regulator and gauge was used to control the applied pressure. 2.2.2. Sample preparation In the case of tests with OGCs, open-ended steel tubes with inner diameter corresponding the desired diameter of the granular column were erected inside the empty shear box at the desired locations using a triangular or a square configuration (Fig. 2). For each experiment, a pre-weighed amount of dry sand was poured into the shear box around the steel tubes and densified in three layers of roughly equal thickness using a needle vibrator so as to achieve a dry density of 1.66 ± 0.03 g/cm3 (corresponding to 72% relative density). The required amount of stone aggregate was placed inside the steel tubes in three equal layers and compacted by applying 30 blows to each layer using an 8-mm-diameter steel tamping rod dropped from a height of 200 mm. After pouring and compacting the aggregate to the full height of the shear box, the steel tubes were gradually withdrawn from sand bed by pulling them up vertically. Using steel tubes with smooth outer and inner surfaces minimized the disturbance of the granular column and the surrounding sand. Upon complete removal of the steel tube, 5 blows were applied to compact the top portion of the aggregate columns using a 3.16-kg, 140-mm-diameter circular steel plate that was dropped from a 100 mm height. It is acknowledged that the sample preparation method does not fully simulate the enhanced confinement of the surrounding soil produced by the displacement method of granular column installation; however, it does offer excellent reproducibility in terms of respective dry densities of the granular columns and the surrounding sand layer. For the case of encased granular columns (EGC), the open-ended steel tubes were first lined on the outside with geosynthetic encasements. The steel tubes were then erected inside the empty shear box at the desired locations using a triangular or square configuration (Fig. 2). A pre-weighted quantity of sand was then poured around the steel tubes and compacted using a needle vibrator. Then, as with the installation of OGCs, the required quantity of aggregates was placed and compacted inside the tubes. After the formation of granular columns, the steel tubes were
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withdrawn gradually in the vertical direction as described earlier. The 100-mm- and 50-mm-diameter granular columns were constructed by compacting the aggregates to a dry density of 1.75 g/ cm3 and 1.65 g/cm3, respectively. Sand was used to prepare the soil bed in place of normally consolidated clay soil due to the ease of placement, ease of achieving consistency between the tests and similarity of strength behaviour (di Prisco et al., 2006). The objective of the test program was to study the improvement in lateral resistance of virgin soil due to the installation of OGC and EGC. As such, whether the lateral resistance is provided by a sandy or a clayey soil is not important as long as that lateral resistance can be estimated with reasonable accuracy. As long as the baseline shear strength of the surrounding soil can be estimated and deducted from the shear stress mobilized by the sand-OGC or sand-EGC system, it should be possible to isolate the behaviour of OGCs and EGCs from the test results. It is acknowledged that the use of dry sand in place of saturated soft clay does not allow the simulation of localized drainage effects around the stone columns, which can potentially increase the undrained shear strength of the surrounding clay. 2.3. Test procedure The granular columns were installed in different plan arrangements to study the response with single and group of columns. A single granular column of either 50 mm or 100 mm diameter was installed at the centre of the shear box to analyze the effect of single granular columns subjected to lateral deformations. The 50-mmdiameter columns were also installed in triangular and square arrangements at 100 mm centre-to- centre spacing to study the group effects. The tests were conducted at different normal pressures ranging from 15 kPa to 75 kPa in order to develop the strength envelopes. The details of the different types of encasements and the plan configurations are given in Tables 2 and 3, respectively. As the normal pressure is applied through a flexible air pressure bag, it is reasonable to assume that uniform pressure acts all over the plan area of the box. The laboratory tests were carried out at different area replacement ratios (Ar), where Ar is the ratio of the plan area of the granular columns to the plan area of the shear box (Table 3). Fig. 3 shows the plan view of the large direct shear box with granular column at the centre of the box. Figs. 4 and 5 show the granular columns arranged in triangular and square patterns to analyze the behaviour of groups of granular columns subjected to lateral loading. The arrow direction in Fig. 4 illustrates the direction of movement of the bottom shear box. Tests on OGCs were terminated upon reaching 40 mm horizontal displacement of the bottom shear box because at this displacement both the peak and the critical state shear resistance were mobilized. Tests in which a single EGC was installed at the centre of the shear box were terminated at 80 mm horizontal displacement so as to mobilize tensile resistance of the geosynthetic encasement. Tests involving groups of EGCs were terminated at 60 mm horizontal displacement in order to avoid boundary effects.
Fig. 3. Plan view of large direct shear box with granular column at centre of box.
Fig. 4. 50 mm diameter granular columns in triangular arrangement at 100 mm centre-to-centre.
3. Results and discussion 3.1. Failure mechanism of granular columns After the completion of a shear test, sand was removed carefully around the columns to observe the failure mechanism of the granular column. In tests involving a single OGC at the centre,
Table 3 Details of the large direct shear testing program. Test arrangement
Test identifier
Area replacement ratio Ar (%)
Dense sand 50 mm diameter granular column at centre of box 100 mm diameter granular columns at centre of box 50 mm diameter granular columns in triangular pattern 50 mm diameter granular columns in square pattern Dense aggregate
Sand 50C 100C 50T 50S Aggregate
0 2.11 8.44 6.33 8.44 100
400
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100
Shear stress (kPa)
80 60 40 50S OGC 100C OGC Sand
20
50T OGC 50C OGC
0 0
5
10 15 20 25 30 Horizontal displacement (mm)
35
40
Fig. 7. Shear stress vs. horizontal displacement for OGC (sn ¼ 75 kPa).
Fig. 5. 50 mm diameter granular column in square arrangement at 100 mm centre-tocentre.
complete shearing of the granular column was observed along a predefined shear plane. Two types of shear failures were observed in tests involving EGCs depending upon the modulus and the strength of the geosynthetic encasement. In the case of the E3 encasement (high initial modulus but low tensile strength), the encasement was completely sheared along with the granular column. This is illustrated as Type1 failure in Fig. 6. In the case of the E1 (high initial modulus and high tensile strength) and the E2 (low initial modulus and high rupture strain), the encasement did not undergo any rupture. As such, the EGC did not shear but underwent excessive bending around the pre-defined shear plane. This is illustrated as Type-2 failure in Fig. 6. Since the EGCs did not shear completely in case of a Type-2 failure, they would have continued to act as vertical drains because of the continuity of flow path at large deformation. The ability of a geosynthetic encasement to prevent shear failure of granular columns is important in case of earthquake-induced liquefaction during which the foundation soil may undergo large deformation. In such cases, EGCs will continue to drain the liquefied ground and restore effective stresses rapidly after the earthquake, thereby preventing complete failure of the structure. 3.2. Effect of the OGCs The shear stresses are found to increase upon reinforcing the sand with OGCs due to the higher shear resistance of the combined soil-granular column system (Fig. 7). The granular column and the
surrounding soil act as a composite material that mobilizes higher shear resistance. On increasing the area ratio of the granular columns, higher shear resistances are mobilized because the percent area occupied by the granular column is higher in the plane of shear. This behaviour can be clearly observed for both 50 mm and 100 mm diameter columns. The ordinary granular columns of 50 mm diameter arranged in triangular and square patterns mobilized higher shear resistance compared to a single 50 mm diameter at the centre of the box, Table 4. 3.3. Effect of geosynthetic encasement Fig. 8 shows the results of woven geotextile (E1) encased granular column at a normal pressure of 75 kPa. On encasing the column, higher shear stresses are mobilized as compared to the corresponding OGC. In the case of OGC, the soil specimen had undergone considerable strain softening (Fig. 7) whereas for EGC minimal strain softening occurred and in a few cases strain hardening was pronounced at higher strain levels. The peak and the large-strain shear stresses observed for different encasements are reported in Table 5. An increase in shear strength due to encased granular columns can be described by the apparent cohesion due to the effects of geosynthetic confinement. The geosynthetic straining during the shear displacement produces higher confining pressures within the granular columns leading to this additional shear strength. The effect of geosynthetic confinement was found to be very high at low normal pressures which gradually reduced with an increase in normal pressures. The highly non-linear strength behaviour of the reinforced soil can be approximated by a linear strength envelope drawn with significant vertical intercept. This vertical intercept is referred to as “apparent
Fig. 6. Different types of failure of encased granular column.
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401
Table 4 Comparison of results at normal pressure of 75 kPa for OGC. Mode
Area replacement ratio (%)
Peak shear stress (kPa)
Shear stress (kPa) at 40 mm displacement
Sand 50C OGC 100C OGC 50T OGC 50S OGC Aggregates
0 2.11 8.44 6.33 8.44 100
64 74 77 84 86 173
43 52 55 56 61 110
200
Aggregates 50T EGC 50C EGC
Shear stress (kPa)
160
50S EGC 100C EGC Sand
120
80 40
strength, which could be used in slope stability analyses of stone column treated soils. Comparing the results of single and group EGC in Fig. 8 it can be observed that in the case of a single EGC shear resistance of soil reduces after the peak then begins to increase at larger displacements (100C), whereas in group arrangements a slight reduction occurs in the shear resistance after the peak and at large displacement shear strength increases to the peak shear stress. Table 5 presents a direct comparison of the peak and large strain shear stresses for different cases of encased granular columns. 3.5. Effect of tensile modulus of the geosynthetic encasement
0 0
10
20 30 40 Horizontal displacement (mm)
50
60
Fig. 8. Shear stress vs. horizontal displacement for EGC, E1 (sn ¼ 75 kPa).
cohesion” in this paper (Table 6). The initial increase in apparent cohesion may be due to the increase in the stiffness of the granular columns whereas at higher strains significant apparent cohesion was mobilized due to tensile forces developed in the encasement. The apparent cohesion exhibited by dry granular soils is an effect of the particular soil deformations that takes place in large shear box tests and absent in small shear box tests (Liu, 2006; Bareither et al., 2008). The shear strength parameters for different test configurations are summarized in Table 6. 3.4. Effect of group action of granular columns On arrangement of granular columns in a group, the granular column and the intervening soil between granular columns act as a composite material. Because of the confinement effect in the soil from the neighbouring columns, higher shear resistance is mobilized whereas for the single OGC (50C and 100C) only the shear resistance of sand and column composite controls the shear strength mobilization. From Table 4 it can be seen that due to group arrangement of ordinary granular columns, higher shear strength is mobilized both at peak and critical states as compared to an equivalent single column with similar Ar. Though Ar of 50T is smaller than that for the case of 100C, larger shear strength is developed due to group effect (Table 4). This result shows that the zone of soil between the columns behaves as a soil with higher Table 5 Comparison of results at normal pressure of 75 kPa for EGC. Mode
Area replacement ratio (%)
Sand 50C EGC 100C EGC 50T EGC 50S EGC
0 2.11 8.44 6.33 8.44
3.6. Effect of increase in normal pressure
Peak shear stress (kPa)
Shear stress (kPa) at 40 mm displacement
E1
E3
E1
E2
E3
87 90 90 88
43 75 86 96 102
66 73 77 75
53 56 58 64
64 88 91 102 101
E2 88 85 84 86
In order to study the effect of the tensile modulus of the geosynthetic encasement, tests were conducted on encased granular columns with three different types of encasement materials E1, E2 and E3, as described previously. Fig. 9 shows the increase in the shear strength for 50C due to the variation in encasement materials at a normal pressure of 30 kPa. Fig. 10 shows the increase in the shear strength for 50S due to the variation in encasement materials at the same normal pressure of 30 kPa. It is clear from the above data that encasement with a higher modulus helps in mobilizing higher shear strength. As the modulus of E2 material is low in comparison with E1 and E3, it can undergo large deformation along with the aggregate without rupture. Due to high initial tensile modulus, the peak friction angle of the E3 encased columns is found to be similar to the columns encased in woven geotextiles. However, immediate rupture failure of the E3 encasement after the peak has reduced the strength of these columns to that of unreinforced columns at large strains (Figs. 9 and 10). It can also be seen in Fig. 9 that the mobilized additional shear strength for OGC is insignificant in comparison with sand at both peak and at 40 mm displacement. In contrast, the mobilized shear strength is higher for the group columns (Fig. 10). Encased granular columns mobilize higher apparent cohesion in comparison with OGC at both peak and at large displacement as set out in Table 6. The differential increase in apparent cohesion at different normal pressures is attributed to the different dilatancy effects at different normal pressures. The E1 and E2 type materials did not fail in rupture and developed considerable increase in apparent cohesion at large shear displacements, whereas a smaller increase was observed in E3 type encasement due to the rupture failure of encasement at large shear displacements.
The tests were performed at four different normal pressures. Fig. 11(a) shows that by increasing the normal pressure, higher shear strength is mobilized by soil treated with ordinary granular columns (100C OGC). It can be seen that after the peak, strain softening takes place and shear strength remained constant thereafter. Fig. 11(b) shows the increase in shear resistance due to the inclusion of woven geotextile (E1) encased 100 mm diameter
402
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Table 6 Shear strength parameters from different tests. Type
Area replacement ratio (Ar %)
Sand OGC
Woven geotextile encased granular column (E1)
Socks (E2)
Paper (E3)
Aggregate
0 2.11 8.44 6.33 8.44 2.11 8.44 6.33 8.44 2.11 8.44 6.33 8.44 2.11 8.44 6.33 8.44 100
Peak shear strength parameters
(50C) (100C) (50T) (50S) (50C) (100C) (50T) (50S) (50C) (100C) (50T) (50S) (50C) (100C) (50T) (50S)
f ( )
c (kPa)
f ( )
10 10 10 10 10 23 25 27 25 17 18 12 11 17 22 15 13 37
36 40 41 44 45 40 41 44 45 40 41 44 45 40 41 44 45 61
5 5 5 5 5 20 28 36 40 16 15 14 13 7 6 8 9 26
29 32 34 34 36 32 34 34 36 32 34 34 36 32 34 34 36 48
50
Shear stress (kPa)
40 30 20 10
E1 OGC
0 0
10
E2 Sand
20 30 40 Horizontal displacement (mm)
E3
50
60
Fig. 9. Shear stress vs. horizontal displacement for 50C (sn ¼ 30 kPa).
granular column at centre. Due to the increase in stiffness of the granular column a higher peak shear strength is mobilized compared to OGC (Fig. 11(a)). After the peak, very little strain softening is observed compared to OGC and shear resistance becomes almost equal to the peak value at 40 mm displacement. It can be seen that the encasement has also increased the shear modulus
Shear stress (kPa)
60
40
20 E1
E2
E3
OGC
Sand
0
0
10
Shear strength parameters at 40 mm displacement
c (kPa)
20 30 40 50 Horizontal displacement (mm)
Fig. 10. Shear stress vs. horizontal displacement for 50S (sn ¼ 30 kPa).
60
of the soil mass as evident from the steeper displacement vs. shear stress graphs, Fig. 11 (b), (c) and (d). Fig. 11(c) shows the influence of normal pressure on the shear resistance of soil treated with sock (E2) encased granular columns. Due to the confinement provided by the encasement, higher shear strength is mobilized both at the peak and large displacement states. As the initial modulus of E2 is smaller than that of E1 and E3, smaller peak shear strength is mobilized as compared with that mobilized in the case of E1 and E3. After the peak, no strain hardening is observed because of the low modulus of the encasement. On the other hand, strain softening was also not seen as there was no rupture of encasement. Fig. 11(d) shows the behaviour of paper (E3) encased 100 mm diameter granular column. From the figure it can be seen that peak shear strength mobilized by the soil sample is nearly equal to that of E1. It is interesting to see that after the peak a sudden failure takes place due to the rupture of the encasement. It can also be seen that after 40 mm displacement shear strength has fallen to that given by similar OGCs. Generally peak shear stress increases with an increase in normal pressure and increases with an increase in Ar. Fig. 12(a) and (b) show the variation of peak shear stress with Ar in case of OGC and EGC at different normal pressures. In the case of OGC peak shear stress almost remains constant at lower normal pressure. In the case of higher normal pressure an increase in peak shear stress is observed with Ar. In the case of EGC an increase in peak shear stress with Ar is observed even at lower normal pressure due to the increase in stiffness of the granular column. Fig. 13(a) and (b) show the variation of shear stress at 40 mm displacement with Ar for both OGC and EGC at different normal pressures. In the case of OGC, the strength behaviour is similar to that observed at peak stress. But in the case of EGC a steady increase in shear stress beyond the peak is observed with an increase in Ar due to mobilization of additional confinement from tensile forces in geosynthetic encasement. From Figs. 12 and 13 it can be observed that a square arrangement gives a higher percentage of improvement for peak stress in case of OGC, whereas triangular arrangement gave higher strength in the case of EGC. At 40 mm displacement a square arrangement mobilizes higher shear stress in both OGC and EGC. Also it can be seen that in the case of OGC, a marginal increase in shear strength is observed with an increase in Ar whereas a substantial increase in the strength is observed for EGC.
S.R. Mohapatra et al. / Geotextiles and Geomembranes 44 (2016) 396e405 100
75 kPa 30 kPa
120 100
Shear stress (kPa)
Shear stress (kPa)
80
45 kPa 15 kPa
60
40 20
80 60 40 20
0
0
10 20 30 Horizontal displacement (mm)
75 kPa 30 kPa
0
40
0
75 kPa 30 kPa
40
(b) 100C EGC (E1) 100
45 kPa 15 kPa
Shear stress (kPa)
Shear stress (kPa)
80
45 kPa 15 kPa
10 20 30 Horizontal displacement (mm)
(a) 100C (OGC) 100
403
60 40 20
75 kPa 30 kPa
80
45 kPa 15 kPa
60 40
20 0
0
0
10 20 30 Horizontal displacement (mm)
40
0
10
(c) 100C EGC (E2)
20 30 40 Horizontal displacement (mm)
50
60
(d)100C EGC (E3)
Fig. 11. Shear stress vs. horizontal displacement behaviour of different systems.
120
75 kPa
45 kPa
30 kPa
75 kPa
45 kPa
30 kPa
15 kPa
15 kPa Shear stress (kPa)
Shear stress (kPa)
120
80
40
80
40
0 0
0 0
2
4
Ar (%)
6
8
45 kPa
Ar (%)
6
8
10
8
10
(a) OGC
120
30 kPa
75 kPa
45 kPa
30 kPa
15 kPa
15 kPa Shear stress (kPa)
Shear stress (kPa)
75 kPa
4
10
(a) OGC
120
2
80
40
80
40
0 0
0
0
2
4
Ar (%)
6
8
10
2
4
Ar (%)
6
(b) EGC (E1)
(b) EGC (Reinforcement Type E1 – Woven geotextile) Fig. 12. Variation of peak shear stress mobilized vs. area replacement ratio (Ar).
Fig. 13. Variation of shear stress mobilized at 40 mm shear displacement vs. area replacement ratio (Ar) for normal stress values of 15, 30, 45 and 75 kPa.
404
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90
105
50S50S OGC OGC
60
OGC 50T50T OGC
90
100C OGC 100C OGC
75
Shear stress (kPa)
Shear stress (kPa)
75
OGC 50C50C OGC sandsand
45
30
Series1 50S EGC 50T EGC Series2 100C EGC Series3 50C EGC Series4 sand Series5
60 45 30 15
15
0 0
0
20
40 Normal stress (kPa)
60
20
80
40 Normal stress (kPa)
60
80
60
80
(a) Peak
(a) Peak 105
90 50S50S OGC OGC OGC 50T50T OGC
75
Shear stress (kPa)
Shear stress (kPa)
100C OGC 100C OGC 60
OGC 50C50C OGC
sandsand
45
50S50S EGCEGC 50T50T EGCEGC 100C 100C EGCEGC 50C50C EGCEGC Sandsand
90
30
75 60
45 30 15
15
0 0
0 0
20
40 Normal stress (kPa)
60
80
(b) At 40 mm shear displacement Fig. 14. MohreCoulomb failure envelopes for the OGC tests.
Failure envelopes of sand and OGC at peak and at 40 mm shear displacement are shown in Fig. 14(a) and (b). Fig. 15(a) and (b) show the failure envelopes of EGC at peak and 40 mm shear displacements. In the case of EGC, the failure envelopes were drawn with the same slope as in the corresponding OGC cases, which results in much higher apparent cohesion due to nonlinear effects of dilation, Fig. 15.
Fig. 15. MohreCoulomb failure envelopes for the EGC tests (Reinforcement Type E1 e Woven geotextile).
3. 4.
5.
1. Geosynthetic encasement increases the lateral load capacity of granular columns due to mobilization of tensile forces in the encasement layer. The resistance of EGC increases with increasing shear displacements until complete rupture of encasement material occurs. The encasement with higher modulus mobilises higher lateral resistance forces at smaller displacements. 2. For OGC, a marginal increase in shear strength was observed with an increase in area replacement ratio in contrast to
40 Normal stress (kPa)
(b) At 40 mm shear displacement
4. Conclusions The present work pertains to the development of understanding the behaviour of granular columns subjected to lateral (shear) loading. The behaviour of OGC and EGC was studied using laboratory large direct shear tests and their results are compared to show the improvement of lateral load carrying capacity of granular columns due to encasement. Based on the observed results the following conclusions are made:
20
6.
substantial increase in the strength for EGC with an increase in area replacement ratio. After the rupture of encasement in EGCs, the strength reduces to the levels of OGCs. At smaller shear displacements, the strength increase observed in EGCs is more likely due to the increase in stiffness of the granular columns while at larger displacements, the increase in strength is due to the tensile forces mobilized in the encasement. Shear failure of OGC takes place along the shear plane. EGC undergoes bending type deformation without rupture of the encasement material due to the flexible nature of geosynthetic (E1/E2) encasement. A group arrangement mobilizes higher shear resistance compared to single OGC and EGC (for the same Ar) due to the confinement offered to the intervening soil. This aspect is important and needs to be considered for more rigorous analyses and designs.
Acknowledgements The first author is thankful to the Canadian Commonwealth Scholarship Program and the Natural Science and Engineering Research Council (NSERC) Discover Grant Program for sponsoring his research visit at University of Saskatchewan, Saskatoon, Canada during September 2012 to June 2013. The help and suggestion provided by Mr. Adam Hammerlindl, Mr. Zeeshan Ahmad, Mr.
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