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Laboratory model studies on unreinforced and geogrid-reinforced sand bed over stone column-improved soft clay
Geotextiles and Geomembranes 29 (2011) 190e196
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Technical note
Laboratory model studies on unreinforced and geogrid-reinforced sand bed over stone column-improved soft clay Kousik Deb a, *, Narendra Kumar Samadhiya b,1, Jagtap Babasaheb Namdeo b a b
Department of Civil Engineering, Indian Institute of Technology Kharagpur, Kharagpur e 721302, India Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee e 247667, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 July 2009 Received in revised form 6 May 2010 Accepted 12 June 2010
Results from a series of laboratory model tests on unreinforced and geogrid-reinforced sand bed resting on stone column-improved soft clay have been presented. The diameter of stone column and footing has been taken as 50 mm and 100 mm, respectively for all the model tests carried out. Load was applied to the soil bed through the footing until the total settlement reached at least 20% of footing diameter. As compared to unimproved soft clay, the increase in load-carrying capacity under different improved ground conditions has been observed. Influences of the thickness of unreinforced as well as geogrid-reinforced sand bed and the size of geogrid reinforcement on the performance of stone column-improved soft clay bed have also been investigated. Significant improvement in load-carrying capacity of soft soil is observed due to the placement of sand bed over stone column-improved soft clay. The inclusion of geogrid layer within sand bed further increases the load-carrying capacity and decreases the settlement of the soil. Due to the placement of sand bed, the bulge diameter of stone column reduces while the depth of bulge increases. Further reduction in the bulge diameter and increase in bulge depth are observed due to application of geogrid layer. The optimum thickness of unreinforced sand bed is twice the optimum thickness of geogridreinforced sand bed. Under specific material properties and test conditions, it is further observed that the optimum diameter of geogrid layer is thrice the diameter of footing. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Geogrid reinforcement Bulging Laboratory model tests Soft clay Stone column Sand bed
1. Introduction Stone column, one of the most commonly used soil improvement technique, has been utilized worldwide to increase the bearing capacity of soft soils and reduce the settlement of superstructures constructed on them. Several researches have been carried out to study the behaviour of stone column-reinforced ground over the past three decades (Madhav and Vitkar, 1978; Balaam and Booker, 1981; Alamgir et al., 1996; Poorooshasb and Meyerhof, 1997; Lee and Pande, 1998; Muir-Wood et al., 2000; Ambily and Gandhi, 2007; Elshazly et al., 2007; Krishna et al., 2007; Black et al., 2007; Madhav et al., 2008; Bouassida et al., 2009). Horizontal geosynthetic reinforcement sheets can be used in the granular columns to increase the load-carrying capacity as well as decrease the bulging of the columns (Madhav et al., 1994; Sharma et al., 2004; Wu and Hong, 2008). Geosynthetic
* Corresponding author. Tel.: þ91 3222 283434. E-mail addresses: [email protected], [email protected] (K. Deb), [email protected] (N.K. Samadhiya), [email protected] (J.B. Namdeo). 1 Tel.: þ91 1332 285467. 0266-1144/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2010.06.004
encasement can also be used to extend the use of stone columns for extremely soft soil condition (Murugesan and Rajagopal, 2006; Murugesan and Rajagopal, 2007; Gniel and Bouazza, 2009; Wu and Hong, 2009; Lo et al., 2010). A granular layer of sand or gravel, 0.3 m or more in thickness, is usually placed over the top of the stone columns to provide a drainage path and distribute the stresses coming from the superstructures (Mitchell, 1981). Shahu et al. (2000) developed a simple theoretical approach to analyze the granular pile-reinforced soft ground with granular mat placed on the top. Deb (2008) developed a mechanical model for predicting the behaviour of stone column-improved soft ground with granular bed placed over the stone columns. It has been observed that the presence of granular bed on top of stone column-reinforced ground reduces the stress concentration near top of the columns. The granular bed also helps to reduce the maximum as well as differential settlement and increase the load-carrying capacity of the stone column-improved soft soil. The granular bed can be further reinforced with geogrid to enhance the load-carrying capacity and reduce the settlement of the stone column-improved soft clay. Han and Gabr (2002) performed a numerical analysis of geosynthetic-reinforced and pile-supported
K. Deb et al. / Geotextiles and Geomembranes 29 (2011) 190e196
350 Unconfined compressive strength (kPa)
earth platforms over soft soil. Based on lumped parameter modelling approach, models have been developed for single layer (Deb et al., 2007) and multilayer (Deb et al., 2008) geosynthetic-reinforced granular bed resting on stone column-improved soft soil. It has been observed that many analytical or numerical studies have been carried out to study the effect of unreinforced and geogrid-reinforced granular bed on settlement and bearing capacity of stone column-improved soft soil. Very limited experimental investigations have been conducted on this topic. In the present study, laboratory model tests have been conducted on single-stone column to study the effect of reinforcement diameter and thickness of reinforced as well as unreinforced sand bed on settlement response, bearing capacity and bulging of the stone column. The optimum thickness of the reinforced and unreinforced sand bed has also been determined.
191
300 250 200 150 100 50 0 10
15
20
25
30
35
Water content (%)
2. Experimental investigation
Fig. 1. Variation of unconfined compressive strength of clay with water content.
2.1. Material used Clay, sand, stone and geogrid were used for the experimental investigations. The properties of clay have been presented in Table 1. Unconfined compressive strength (UCS) tests were carried out on clay samples at different water content and the variation of UCS of the clay with water content has been presented in Fig. 1. Water content of the clay was maintained at 30% throughout the series of tests and the corresponding UCS value of the clay (19 kPa) has been determined from Fig. 1. The bulk unit weight of the clay at 30% water content was determined to maintain identical unit weight in all the tests. Sand particles passing through 4.75 mm sieve were used to prepare the sand bed placed over the stone columnimproved soft clay. Crushed stone materials of size 2 mme6 mm were chosen to prepare the stone column. The properties of sand and crushed stone materials have also been presented in Table 2. From the particle size distribution curves of sand and stone column materials (as shown in Fig. 2), the uniformity coefficient, Cu and the coefficient of curvature, Cc values have been determined and presented in Table 2. Biaxial geogrid, made of high-density polyethylene, was used as a reinforcement layer. The properties of geogrid reinforcement have been presented in Table 3. 2.2. Experimental setup To prepare the soft soil bed, a square tank of 525 mm 525 mm size and 400 mm high was used in all the tests. A 50 mm diameter auger was used to dig the circular hole for preparing the stone column. Steel pipe of diameter 50 mm was used to finish the internal surface of the hole made by auger before filling it with stones. The stone column was installed up to the end of clay bed. Compactors with different sizes and weights were used to compact the clay, stones and sand to achieve the required density of the Table 1 Properties of clay.
materials. Drilling guide was used to support the auger and place it vertical during drilling of hole in clay bed. Steel circular plate of diameter 100 mm and thickness 12.5 mm was used as footing to apply the load. Three arms were attached with the footing to fix up the dial gauges for measuring the settlement of footing during the application of load. Mechanical jack-frame arrangement was used to apply load on the soil stratum through the footing plate (as shown in Fig. 3). The load was applied through plunger and proving ring of 7.5 kN capacity. Three dial gauges were fixed at 120 angles to each other. The diameter of stone column was chosen to be 50 mm in all the tests and the depth of clay bed was maintained at 300 mm. The first test was carried out on clay bed without any improvement techniques and the load-settlement behaviour was investigated. Thereafter, other tests were carried out on soft soil improved by stone column alone and on soft soil improved by stone column along with unreinforced and geogrid-reinforced sand bed. Summary of the tests conducted has been presented in Table 4. Fig. 3 shows the schematic diagram of the experimental setup. 2.3. Preparation of clay bed In all the tests, identical technique was adopted to prepare the clay bed. To maintain similar properties throughout the tests, clay bed was prepared at 30% water content in all the cases. The bulk unit weight at 30% water content was found as 19.8 kN/m3. Before filling the tank with clay, polythene sheet was laid on internal walls of the tank to avoid any friction between clay and walls of tank and to prevent loss of water. To maintain same unit weight of clay in each test, the tank was filled in six equal layers of 50 mm thickness and the required weight of clay in each layer was calculated based on bulk unit weight of 19.8 kN/m3. Each layer was compacted with steel rammers of diameter 45 mm, 70 mm, and square hammer of 150 mm 150 mm to achieve the required thickness. Smaller Table 2 Properties of sand and stone.
Parameters
Value
Specific gravity Liquid limit (%) Plastic limit (%) Plasticity index Optimum moisture content (%) Maximum dry unit weight Bulk unit weight at 30% water content Undrained cohesion Compression Index Classification based on plasticity characteristics (USCS)
2.57 43.3 19.5 23.8 18.3 16.7 kN/m3 19.8 kN/m3 9.5 kPa 0.244 CL
Parameters
Specific gravity Maximum dry unit weight Minimum dry unit weight Internal friction angle (f) at 70% relative density Bulk unit weight at 70% relative density Uniformity coefficient (Cu) Coefficient of curvature (Cc)
Values Sand
Stone
2.75 19.8 kN/m3 16.17 kN/m3 42
2.70 17.2 kN/m3 15.1 kN/m3 45
18.55 kN/m3 3.6 0.63
16.5 kN/m3 2.1 0.96
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2.6. Test procedure
100 90
diameter hammer was used to remove the air bubbles induced at the time of filling of tank.
In all the cases, the prepared test bed in tank along with stone column and overlying sand bed was placed under a loading frame. Loading was applied through a footing resting on the prepared soil bed and resistance offered by test bed with or without stone column was measured with the help of proving ring. Short-term loading test was conducted in all the cases. Load was applied in equal increments and each increment of the load was maintained until negligible change in the settlement was observed. The settlement due to increment of each equal interval of loading step was observed through three mechanical dial gauges having least count of 0.01 mm fixed on the footing at 120 angle to each other. Loading was applied until the total settlement of the footing attained was at least 20% of footing diameter. To observe the bulging of the stone column after testing, thin cement slurry was added in the stone column without disturbing the column and allowed to get sufficient strength so that even after removal of clay surrounding the stone column, the shape of the tested stone column remained unaffected. After removal of the stone columns from the clay, the bulge depth and bulge diameter of the columns were measured. Fig. 4 shows the photograph of a stone column after testing.
2.4. Preparation of stone column
3. Results and discussion
After preparing the clay bed of 300 mm, a cylindrical hole of diameter 50 mm was made at the centre of the clay bed by an auger of 50 mm diameter. The hole inside the clay bed was smoothly finished from inside with the help of 50 mm diameter steel pipe. The relative density of stones was maintained as 70%. The unit weight of stones was determined at 70% relative density and using the known volume of the hole, the total weight of stone required to fill up the hole was determined. Total weight of stone material was divided into six equal batches to fill up the hole. Each batch of stone was poured and compacted with steel-tamping bars of diameter 10 mm and 30 mm in such a manner that the finished height of each layer of stone column was 50 mm.
3.1. Load-settlement characteristics
Sand Percentage finer
80 70 60
Stone column material
50 40 30 20 10 0 0.01
0.1
1
10
Particle size (mm) Fig. 2. Particle size distribution curves for sand and stone column material.
2.5. Preparation of sand bed The weight of sand required to form a certain thickness of the bed was determined with the known unit weight of sand (at 70% relative density). For different thicknesses of sand, the required weight of sand was calculated and preparation of bed was carried out in layers. Each layer was compacted with a wooden square hammer with equal efforts of compaction to achieve the required relative density of sand bed. In case of geogrid-reinforced sand bed, initially a sand layer of 5 mm thickness was placed in between geogrid layer and clay bed as it has been reported in literature (Han and Gabr, 2002) that in field, a thin granular layer is generally placed over the soft soil before placing the geosynthetic reinforcement layer. Thereafter, the circular geogrid layer was placed in such a way that the centre of geogrid coincides with the centre of stone column. Same procedure was followed to prepare the sand bed over the geogrid layer upto required thickness.
Fig. 5 shows the load-settlement characteristics of the unimproved clay bed, clay bed improved by stone column alone and clay bed improved by stone column along with 30 mm thick unreinforced and geogrid-reinforced sand bed. The diameter of the geogrid layer has been taken as 500 mm. The improvement in load-carrying capacities under different conditions has been computed at 20 mm settlement. From Fig. 5, it has been observed that the placement of sand bed over stone column-improved soft clay increases the loadcarrying capacity of the improved soil and the use of geogrid layer within the sand bed is effective in further increment of the same. As compared to unimproved clay bed, an improvement of 69% in loadcarrying capacity has been observed when the clay bed is improved with stone column only. As compared to unimproved clay bed, 141% improvement in load-carrying capacity has been observed when unreinforced sand bed is placed over stone column-improved soft clay and for reinforced sand bed the improvement is 233%. For a loading intensity of 0.5 kN, as compared to unimproved soil, the settlement has been reduced by 67%, 91% and 94% when the soil is improved by only stone column, by stone column along with unreinforced and geogrid-reinforced sand bed, respectively. For a loading intensity of 1.0 kN, as compared to unreinforced sand bed, 44% reduction in settlement has been observed when geogrid-reinforced sand bed is used; whereas for a loading intensity of 1.3 kN, the reduction in settlement is 55%. Thus, it can be said that the geogrid reinforcement is more effective for higher loading intensity than for lower loading intensity. Similar behaviour has been observed by Deb et al. (2007) in the developed model for geosynthetic-reinforced granular fill-soft soil system with stone column. 3.2. Optimal thickness of the unreinforced and geogrid-reinforced sand bed
Table 3 Properties of geogrid. Parameters
Value
Mesh aperture size Thickness Weight Ultimate tensile strength Strain at maximum load
1 mm 1 mm (square) 1.0 mm 261 gm/m2 1.84 kN/m 15%
To determine the optimum thickness of unreinforced sand bed, the thickness of sand bed was varied from 0.2 to 0.8 times the diameter of the footing (as shown in Table 4). Fig. 6 shows the loadsettlement characteristics of the unreinforced sand bed of different thicknesses placed over stone column-improved clay. The loadcarrying capacity at 20 mm settlement has been calculated and
K. Deb et al. / Geotextiles and Geomembranes 29 (2011) 190e196
193
Fig. 3. Schematic diagram of the test setup.
presented in Fig. 8. It has been observed that as the ratio of thickness of sand bed to the diameter of the footing (Hs/D ratio) increases from 0.2 to 0.5, the load-carrying capacity increased by 54%, whereas only additional 1.3% increment in load-carrying capacity has been observed when Hs/D ratio increased from 0.5 to 0.6. Thus, the optimum thickness of the unreinforced sand bed is 0.5 times the diameter of the footing. Fig. 7 shows the load-settlement characteristics of the geogridreinforced sand bed of different thicknesses placed over stone column-improved clay. The load-carrying capacity at 20 mm settlement has been calculated and presented in Fig. 8. To determine the optimum thickness of the geogrid-reinforced sand bed, the diameter of geogrid reinforcement was chosen as 5 times the diameter of the footing. It has been observed that as the Hs/D ratio increases the load-carrying capacity also increases up to a value of 0.3, whereas beyond this value as the thickness of the sand bed increases the load-carrying capacity decreases. Thus, the optimum thickness of geogrid-reinforced sand bed is 0.3 times the diameter
of the footing. At low sand bed thickness, large defection has occurred in the geogrid reinforcement directly underneath the footing. The large deflection of the geogrid reinforcement would mobilize the membrane action and induce more mobilized tension in the geogrid layer. The vertical component of the tensile force
Table 4 Summary of experimental programme. Sl. No
Thickness of clay bed (mm)
Diameter of stone column (mm)
Thickness of sand bed (mm)
Diameter of geogrid (mm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
300 300 300 300 300 300 300 300 300 300 300 300 300 300
e 50 50 50 50 50 50 50 50 50 50 50 50 50
e e 20 30 40 60 80 20 30 40 30 30 30 30
e e e e e e e 500 500 500 200 250 300 400 Fig. 4. Stone column after testing.
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Load (kN)
Load (kN) 0
0.5
1
1.5
2
0
2.5
1
1.5
2
0
0 5
5
Settlement (mm)
Settlement (mm)
0.5
10 15 20
15
20mm thick sand bed + 500 mm diameter geogrid
20
Clay bed only Clay bed + stone column Clay bed + stone column + 30 mm thick sand bed Clay bed + stone column + 30 mm thick reinforced sand bed
25
10
30mm thick sand bed + 500mm diameter geogrid 40mm thick sand bed + 500mm diameter geogrid 25
30
Fig. 7. Load-settlement characteristics of reinforced sand bed of different thicknesses.
Fig. 5. Load-settlement characteristics of various reinforced conditions.
acting in the geogrid reinforcement partially counterbalances the superimposed load exerted by the overlying soil. As a result, the vertical stress is reduced in the zone below the reinforcement due to combined action of mobilized tension in the reinforcement and membrane action in its curvature (Burd, 1995; Lee et al., 1999; Basudhar et al., 2008). However, when the sand bed thickness increases such that Hs/D > 0.3, a major portion of the shear failure zone of the soil is developed above the reinforcement layer and the deflection of the reinforcement also decreases. This led to reduction in the utilization of membrane action and less mobilized tension in the geogrid has been induced (Lee et al., 1999). This phenomenon reduces the effectiveness of the geogrid layer causing reduction in bearing capacity. Thus, the stone column under geogrid-reinforced sand bed having Hs/D ¼ 0.4 produces less bearing capacity than that under geogrid-reinforced sand bed having Hs/D ¼ 0.3. Studies show that as the thickness of the reinforced sand bed is equal to or greater than the optimum thickness of the unreinforced sand bed, the bearing capacity of unreinforced and reinforced sand bed is almost same (Lee et al., 1999). This is due to the fact that as the thickness of the reinforced sand bed increases, the deflection of the reinforcement decreases and the effectiveness of the reinforcement also decreases. When the thickness of the reinforced sand bed is equal to or greater than the optimum thickness of the unreinforced sand bed the effectiveness of the reinforcement is almost insignificant. Thus, the geogrid-reinforced sand bed with Hs/D 0.5 will
give almost same bearing capacity as compared to that under an unreinforced sand bed with Hs/D 0.5. The improvement in load-carrying capacity, as compared to unimproved soft clay, at 20 mm settlement is 196% and 233% when unreinforced and geogrid-reinforced sand beds with optimum thickness have been placed over stone column-improved soft clay, respectively. Lee et al. (1999) reported similar observation based on numerical and model studies of strip footing resting on reinforcedgranular fill-soft soil system without stone column inclusions. Due to presence of stiffer stone column in the soft clay, lower optimum thickness of the sand bed has been required as compared to the optimum thickness under without stone column condition to get the maximum improvement in load-carrying capacity of improved ground. However, from the present study and the results reported by Lee et al. (1999), it has been observed that the ratio of optimum thickness of the unreinforced to geogrid-reinforced sand bed is almost similar for both the cases under with and without stone columns. 3.3. Optimal extent of geogrid Fig. 9 shows the load-settlement characteristics of sand bed reinforced by geogrid reinforcement of various diameters. While determining the optimal extent of the geogrid reinforcement, the optimum thickness of the sand bed was considered. From the loadsettlement characteristics, it has been observed that for a particular
2
Load (kN) 0
0.3
0.6
0.9
1.2
1.5
1.75
1.8
0
1.5 5
Load (kN)
Settlement (mm)
1.25 10 15
1 0.75
20
0.5
25
0.25
30
20mm thick sand bed 40mm thick sand bed 80mm thick sand bed
30mm thick sand bed 60mm thick sand bed
35 Fig. 6. Load-settlement characteristics of unreinforced sand bed of different thicknesses.
Unreinforced sand bed Geogrid-reinforced sand bed
0 0
0.2
0.4
0.6
0.8
1
Thickness of sand bed / Footing diameter Fig. 8. Variation of load-carrying capacity with thickness of sand bed/footing diameter (at 20 mm settlement).
K. Deb et al. / Geotextiles and Geomembranes 29 (2011) 190e196
Load (kN) 0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
0
Settlement (mm)
5
10
15
500mm diameter geogrid 400mm diameter geogrid 300mm diameter geogrid
20
250mm diameter geogrid 200mm diameter geogrid
25 Fig. 9. Load-settlement characteristics of reinforced sand bed with different diameter of geogrid.
settlement, the load-carrying capacity increases as the diameter of the geogrid increases upto 3 times the diameter of the footing, whereas beyond this value the load-carrying capacity does not change. Thus, the optimal extent of the reinforcement is 3 times the diameter of the footing; and, beyond this diameter any additional reinforcement is ineffective. 3.4. Bulging of stone column It has been observed that, due to application of axial load, a bulge is produced at the top of a single stone column up to a depth of 2 to 3 times the diameter beneath the surface (Barksdale and Bachus, 1983). In the present experiment, the effect of unreinforced and geogrid-reinforced sand bed placed over stone column on bulging behaviour of the column has been studied. Thus, the length-to-diameter ratio was taken greater that 3. In the field, most constructed stone columns have length-to-diameter ratios equal to
a
Bulge diameter (mm) 0
50
b
or greater than 4 to 6 and a bulging failure usually develops depending on whether the tip of the column is floating in soft soil or resting on a firm bearing layer (Barksdale and Bachus, 1983). Thus, the chosen length-to-diameter ratio (equal to 6) of the stone column in the present experimental study (where the tip of the column is resting on firm layer) is relevant to field applications. The bulge of stone column after each test was measured to study the effects of sand bed and reinforcement on bulging of the column. Fig. 10 shows the bulging patterns of the stone column under various test conditions. The vertical axis represents the length of the stone column and horizontal axis represents the bulge diameter of the stone column. In case of only stone column-improved soft clay, a maximum bulge of 12 mm has been observed at a depth of 60 mm from top of stone column. However, in case of stone column along with 30 mm unreinforced and geogrid-reinforced sand bed, the maximum bulge is 10.5 mm at 110 mm depth and 8 mm at 145 mm depth, respectively. Thus, in case of only stone columnimproved clay maximum bulging occurs at a depth of 1.2 times the diameter of column, whereas for unreinforced and geogrid-reinforced sand bed case the depth is 2.2 and 2.9 times the diameter of column, respectively. As compared to only stone column-improved soil, 12.5% reduction in maximum bulge diameter of the stone column has been observed when sand bed is placed over the stone column-improved soft clay. Additional 21% reduction in maximum bulge diameter has been observed when geogrid reinforcement is placed within the sand bed. It can be concluded that the maximum bulge diameter of stone column reduces and the depth of bulge increases with the application of sand bed. Inclusion of geogrid reinforcement further reduces the bulge diameter and increases the bulge depth. Shahu et al. (2000) shows that adequate thickness of granular mat reduces the load carried by granular pile both at the top and bottom and helps to reduce the failure of the granular pile due to bulging of the pile. Very high stress concentration has been observed near the top of the columns (Shahu et al., 2000), which causes high bulging. However, when sand bed is placed over the stone column-improved soft clay significant reduction in stress
Bulge diameter (mm) 0
100
50
c
Bulge diameter (mm) 0
100
50
100
0
0
0
195
1.2 D
2.2 D 100
Depth (mm)
Depth (mm)
100
150
200
250
250
2.9 D
1.21 D
150
200
100 1.16 D 150
200
250 D = 50 mm
D = 50 mm
300
50
50
Depth (mm)
50
1.24 D
300
D = 50 mm
300
Fig. 10. Bulging patterns of the stone column when soft clay has been improved with (a) stone column alone (b) stone column with 30 mm unreinforced sand bed (c) stone column with 30 mm geogrid-reinforced sand bed.
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concentration on top of the column has been observed and the variation of stress concentration with depth is more uniform (Shahu et al., 2000). Thus, placement of sand bed on top of the stone column-improved soft clay reduces the bulge diameter and increases the bulge depth of the stone column. The findings reported in this paper are based on small-scale model tests and are subjected to limitations of scale and boundary effects. Thus, the findings cannot be directly applied for field cases. The present study can be extended for prototype-scale model footing resting on unreinforced or geogrid-reinforced sand bed-soft soil system with stone column in single or group to avoid the scaling and boundary effects. However, in the present study, the model tank has been taken as sufficiently large to reduce the boundary effects. To reduce the scaling effects, the dimensions of the various components have been chosen proportionally with the prototype dimensions. In the present experimental study, small aperture size and thin model geogrid with relatively low stiffness has been used to avoid the size effect in the model experimental results. However, in case of field application comparatively large aperture size and thicker geogrids with higher stiffness are usually used. Thus, the chosen model geogrid properties used in the present experiments are suitable to achieve the same performance results as compared to full-scale geogrid. The results of the present laboratory tests show similar trend with the results reported from numerical and analytical studies. Thus, the results of the present laboratory model study are useful to investigate the behaviour of the unreinforced and geogrid-reinforced sand bed resting over stone column-improved soft clay. 4. Conclusions Based on the experimental results the following conclusions can be drawn: 1. The presence of stone column in soft clay improves the loadcarrying capacity and decreases the settlement of the soft soil. The placement of sand bed further increases the load-carrying capacity and decreases the settlement of the stone columnimproved soil. The inclusion of geogrid as reinforcing element in the sand bed significantly improves the load-carrying capacity and reduces the settlement of the soil. As compared to unimproved soft clay, 69%, 141% and 233% improvement in loadcarrying capacity have been observed (at settlement equal to 20% of the footing diameter) when soft clay is improved by stone column alone, by placing of unreinforced and geogrid-reinforced sand bed of optimum thickness over stone column, respectively. 2. The optimum thickness of unreinforced sand bed placed over the stone column-improved soft clay is 1.7 times the optimum thickness of the geogrid-reinforced sand bed. The optimum thickness of unreinforced and geogrid-reinforced sand bed is 0.5 and 0.3 times the diameter of the footing, respectively. Under optimum thickness of geogrid-reinforced sand bed, the optimum diameter of the reinforcement is 3 times the diameter of the footing. 3. Decrease in bulge diameter and increase in depth of bulge have been observed due to placement of sand bed over stone column-improved soft clay. Further decrease in maximum bulge diameter and increase in depth of bulge have been observed due to application of geogrid. The maximum bulge has been observed at a depth of 1.2, 2.2 and 2.9 times the diameter of stone column in case of soil improved by stone column alone and by placing of unreinforced and geogridreinforced sand bed over stone column, respectively. However, the findings of the present experimental study are affected by the various factors, such as, diameter of the stone columns, footing diameter to stone column diameter ratio, length-to-diameter
ratio of the stone column, stiffness of the soft soil, stone column as well as geogrid reinforcement, properties of sand bed and the findings of the present study are observed under specific material properties and test conditions. More tests with various material properties and test conditions have to be conducted to make general conclusions. References Alamgir, M., Miura, N., Poorooshasbh, H.B., Madhav, M.R., 1996. Deformation analysis of soft ground columnar reinforced by columnar inclusions. Computers and Geotechnics 18, 261e290. Ambily, A.P., Gandhi, S.R., 2007. Behavior of stone columns based on experimental and FEM analysis. Journal of Geotechnical and Geoenvironmental Engineering (ASCE) 133, 405e415. Balaam, N.P., Booker, J.R., 1981. Analysis of rigid raft supported by granular piles. International Journal for Numerical and Analytical Methods in Geomechanics 5, 379e403. Barksdale, R.D., Bachus, R.C., 1983. Design and Construction of Stone Column e Volume I e Final Report, FHWA/RD-83/026. U.S. Department of Transportation, Federal Highway Administration. Basudhar, P.K., Dixit, P.M., Gharpure, A., Deb, K., 2008. Finite element analysis of geotextile-reinforced sand-bed subjected to strip loading. Geotextiles and Geomembranes 26, 91e99. Black, J.A., Sivakumar, V., Madhav, M.R., Hamill, G.A., 2007. Reinforced stone columns in weak deposits: laboratory model study. Journal of Geotechnical and Geoenvironmental Engineering (ASCE) 133, 1154e1161. Burd, H.J., 1995. Analysis of membrane action in reinforced unpaved roads. Canadian Geotechnical Journal 32, 946e956. Bouassida, M., Jellali, B., Porbaha, A., 2009. Limit analysis of rigid foundations on floating columns. International Journal of Geomechanics (ASCE) 9, 89e101. Deb, K., Basudhar, P.K., Chandra, S., 2007. Generalized model for geosyntheticreinforced granular fill-soft soil with stone columns. International Journal of Geomechanics (ASCE) 7, 266e276. Deb, K., Chandra, S., Basudhar, P.K., 2008. Response of multilayer geosyntheticreinforced bed resting on soft soil with stone columns. Computers and Geotechnics 35, 323e330. Deb, K., 2008. Modelling of granular bed-stone column-improved soft soil. International Journal for Numerical and Analytical Methods in Geomechanics 32, 1267e1288. Elshazly, H., Hafez, D., Mossaad, M., 2007. Settlement of circular foundations on stone column reinforced grounds. Ground Improvement 11, 163e170. Gniel, J., Bouazza, A., 2009. Improvement of soft soil using geogrid encased stone columns. Geotextiles and Geomembranes 27, 167e175. Han, J., Gabr, M.A., 2002. Numerical analysis of geosynthetic-reinforced and pilesupported earth platform over soft soil. Journal of Geotechnical and Geoenvironmental Engineering (ASCE) 128, 44e53. Krishna, A.M., Madhav, M.R., Latha, G.M., 2007. Densification effect of granular piles on settlement response of treated ground. Ground Improvement 11, 127e136. Lee, J.S., Pande, G.N., 1998. Analysis of stone-column reinforced foundations. International Journal for Numerical and Analytical Methods in Geomechanics 12, 1001e1020. Lee, K.M., Manjunath, V.R., Dewaikar, D.M., 1999. Numerical and model studies of strip footing supported by a reinforced granular fill-soft soil system. Canadian Geotechnical Journal 36, 793e806. Lo, S.R., Zhang, R., Mak, J., 2010. Geosynthetic-encased stone column in soft clay: a numerical study. Geotextiles and Geomembranes 28, 292e302. Madhav, M.R., Vitkar, P.P., 1978. Strip footing on weak clay stabilized with a granular trench or pile. Canadian Geotechnical Journal 15, 605e609. Madhav, M.R., Alamgir, M., Muira, N., 1994. Improvement granular column capacity by geogrid reinforcement. In: Int. Proceedings of the 5th international Conference on Geotextiles, Geomembranes and Related Products, Singapore, pp. 351e356. Madhav, M.R., Suresh, K., Peter E.C.N, 2008. Effect of creep on response of granular pile reinforced ground. In: Indian Geotechnical Conference, India, pp. 276e280. Mitchell, J.K., 1981. Soil improvement- state of the art report. In: Proc. 10th ICSMFE, Balkema, Rotterdam, The Netherlands. vol. 4 pp. 509e565. Murugesan, S., Rajagopal, K., 2006. Geosynthetic-encased stone columns: numerical evaluation. Geotextiles and Geomembranes 24, 349e358. Murugesan, S., Rajagopal, K., 2007. Model tests on geosynthetics-encased stone column. Geosynthetics International 14, 346e354. Muir-Wood, D., Hu, W., Nash, D.F.T., 2000. Group effects in stone column foundations: model tests. Geotechnique 50, 689e698. Poorooshasb, H.B., Meyerhof, G.G., 1997. Analysis of behaviour of stone columns and lime columns. Computers and Geotechnics 20, 47e70. Shahu, J.T., Madhav, M.R., Hayashi, S., 2000. Analysis of soft ground-granular pilegranular mat system. Computers and Geotechnics 27, 45e62. Sharma, R.S., Phani Kumar, B.R., Nagendra, G., 2004. Compressive load response of granular piles reinforced with geogrids. Canadian Geotechnical Journal 41,187e192. Wu, C.S., Hong, Y.S., 2008. The behaviour of a laminated reinforced granular column. Geotextiles and Geomembranes 26, 302e316. Wu, C.S., Hong, Y.S., 2009. Laboratory tests on geosynthetic-encapsulated sand columns. Geotextiles and Geomembranes 27, 107e120.
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Technical note
Laboratory model studies on unreinforced and geogrid-reinforced sand bed over stone column-improved soft clay Kousik Deb a, *, Narendra Kumar Samadhiya b,1, Jagtap Babasaheb Namdeo b a b
Department of Civil Engineering, Indian Institute of Technology Kharagpur, Kharagpur e 721302, India Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee e 247667, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 July 2009 Received in revised form 6 May 2010 Accepted 12 June 2010
Results from a series of laboratory model tests on unreinforced and geogrid-reinforced sand bed resting on stone column-improved soft clay have been presented. The diameter of stone column and footing has been taken as 50 mm and 100 mm, respectively for all the model tests carried out. Load was applied to the soil bed through the footing until the total settlement reached at least 20% of footing diameter. As compared to unimproved soft clay, the increase in load-carrying capacity under different improved ground conditions has been observed. Influences of the thickness of unreinforced as well as geogrid-reinforced sand bed and the size of geogrid reinforcement on the performance of stone column-improved soft clay bed have also been investigated. Significant improvement in load-carrying capacity of soft soil is observed due to the placement of sand bed over stone column-improved soft clay. The inclusion of geogrid layer within sand bed further increases the load-carrying capacity and decreases the settlement of the soil. Due to the placement of sand bed, the bulge diameter of stone column reduces while the depth of bulge increases. Further reduction in the bulge diameter and increase in bulge depth are observed due to application of geogrid layer. The optimum thickness of unreinforced sand bed is twice the optimum thickness of geogridreinforced sand bed. Under specific material properties and test conditions, it is further observed that the optimum diameter of geogrid layer is thrice the diameter of footing. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Geogrid reinforcement Bulging Laboratory model tests Soft clay Stone column Sand bed
1. Introduction Stone column, one of the most commonly used soil improvement technique, has been utilized worldwide to increase the bearing capacity of soft soils and reduce the settlement of superstructures constructed on them. Several researches have been carried out to study the behaviour of stone column-reinforced ground over the past three decades (Madhav and Vitkar, 1978; Balaam and Booker, 1981; Alamgir et al., 1996; Poorooshasb and Meyerhof, 1997; Lee and Pande, 1998; Muir-Wood et al., 2000; Ambily and Gandhi, 2007; Elshazly et al., 2007; Krishna et al., 2007; Black et al., 2007; Madhav et al., 2008; Bouassida et al., 2009). Horizontal geosynthetic reinforcement sheets can be used in the granular columns to increase the load-carrying capacity as well as decrease the bulging of the columns (Madhav et al., 1994; Sharma et al., 2004; Wu and Hong, 2008). Geosynthetic
* Corresponding author. Tel.: þ91 3222 283434. E-mail addresses: [email protected], [email protected] (K. Deb), [email protected] (N.K. Samadhiya), [email protected] (J.B. Namdeo). 1 Tel.: þ91 1332 285467. 0266-1144/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.geotexmem.2010.06.004
encasement can also be used to extend the use of stone columns for extremely soft soil condition (Murugesan and Rajagopal, 2006; Murugesan and Rajagopal, 2007; Gniel and Bouazza, 2009; Wu and Hong, 2009; Lo et al., 2010). A granular layer of sand or gravel, 0.3 m or more in thickness, is usually placed over the top of the stone columns to provide a drainage path and distribute the stresses coming from the superstructures (Mitchell, 1981). Shahu et al. (2000) developed a simple theoretical approach to analyze the granular pile-reinforced soft ground with granular mat placed on the top. Deb (2008) developed a mechanical model for predicting the behaviour of stone column-improved soft ground with granular bed placed over the stone columns. It has been observed that the presence of granular bed on top of stone column-reinforced ground reduces the stress concentration near top of the columns. The granular bed also helps to reduce the maximum as well as differential settlement and increase the load-carrying capacity of the stone column-improved soft soil. The granular bed can be further reinforced with geogrid to enhance the load-carrying capacity and reduce the settlement of the stone column-improved soft clay. Han and Gabr (2002) performed a numerical analysis of geosynthetic-reinforced and pile-supported
K. Deb et al. / Geotextiles and Geomembranes 29 (2011) 190e196
350 Unconfined compressive strength (kPa)
earth platforms over soft soil. Based on lumped parameter modelling approach, models have been developed for single layer (Deb et al., 2007) and multilayer (Deb et al., 2008) geosynthetic-reinforced granular bed resting on stone column-improved soft soil. It has been observed that many analytical or numerical studies have been carried out to study the effect of unreinforced and geogrid-reinforced granular bed on settlement and bearing capacity of stone column-improved soft soil. Very limited experimental investigations have been conducted on this topic. In the present study, laboratory model tests have been conducted on single-stone column to study the effect of reinforcement diameter and thickness of reinforced as well as unreinforced sand bed on settlement response, bearing capacity and bulging of the stone column. The optimum thickness of the reinforced and unreinforced sand bed has also been determined.
191
300 250 200 150 100 50 0 10
15
20
25
30
35
Water content (%)
2. Experimental investigation
Fig. 1. Variation of unconfined compressive strength of clay with water content.
2.1. Material used Clay, sand, stone and geogrid were used for the experimental investigations. The properties of clay have been presented in Table 1. Unconfined compressive strength (UCS) tests were carried out on clay samples at different water content and the variation of UCS of the clay with water content has been presented in Fig. 1. Water content of the clay was maintained at 30% throughout the series of tests and the corresponding UCS value of the clay (19 kPa) has been determined from Fig. 1. The bulk unit weight of the clay at 30% water content was determined to maintain identical unit weight in all the tests. Sand particles passing through 4.75 mm sieve were used to prepare the sand bed placed over the stone columnimproved soft clay. Crushed stone materials of size 2 mme6 mm were chosen to prepare the stone column. The properties of sand and crushed stone materials have also been presented in Table 2. From the particle size distribution curves of sand and stone column materials (as shown in Fig. 2), the uniformity coefficient, Cu and the coefficient of curvature, Cc values have been determined and presented in Table 2. Biaxial geogrid, made of high-density polyethylene, was used as a reinforcement layer. The properties of geogrid reinforcement have been presented in Table 3. 2.2. Experimental setup To prepare the soft soil bed, a square tank of 525 mm 525 mm size and 400 mm high was used in all the tests. A 50 mm diameter auger was used to dig the circular hole for preparing the stone column. Steel pipe of diameter 50 mm was used to finish the internal surface of the hole made by auger before filling it with stones. The stone column was installed up to the end of clay bed. Compactors with different sizes and weights were used to compact the clay, stones and sand to achieve the required density of the Table 1 Properties of clay.
materials. Drilling guide was used to support the auger and place it vertical during drilling of hole in clay bed. Steel circular plate of diameter 100 mm and thickness 12.5 mm was used as footing to apply the load. Three arms were attached with the footing to fix up the dial gauges for measuring the settlement of footing during the application of load. Mechanical jack-frame arrangement was used to apply load on the soil stratum through the footing plate (as shown in Fig. 3). The load was applied through plunger and proving ring of 7.5 kN capacity. Three dial gauges were fixed at 120 angles to each other. The diameter of stone column was chosen to be 50 mm in all the tests and the depth of clay bed was maintained at 300 mm. The first test was carried out on clay bed without any improvement techniques and the load-settlement behaviour was investigated. Thereafter, other tests were carried out on soft soil improved by stone column alone and on soft soil improved by stone column along with unreinforced and geogrid-reinforced sand bed. Summary of the tests conducted has been presented in Table 4. Fig. 3 shows the schematic diagram of the experimental setup. 2.3. Preparation of clay bed In all the tests, identical technique was adopted to prepare the clay bed. To maintain similar properties throughout the tests, clay bed was prepared at 30% water content in all the cases. The bulk unit weight at 30% water content was found as 19.8 kN/m3. Before filling the tank with clay, polythene sheet was laid on internal walls of the tank to avoid any friction between clay and walls of tank and to prevent loss of water. To maintain same unit weight of clay in each test, the tank was filled in six equal layers of 50 mm thickness and the required weight of clay in each layer was calculated based on bulk unit weight of 19.8 kN/m3. Each layer was compacted with steel rammers of diameter 45 mm, 70 mm, and square hammer of 150 mm 150 mm to achieve the required thickness. Smaller Table 2 Properties of sand and stone.
Parameters
Value
Specific gravity Liquid limit (%) Plastic limit (%) Plasticity index Optimum moisture content (%) Maximum dry unit weight Bulk unit weight at 30% water content Undrained cohesion Compression Index Classification based on plasticity characteristics (USCS)
2.57 43.3 19.5 23.8 18.3 16.7 kN/m3 19.8 kN/m3 9.5 kPa 0.244 CL
Parameters
Specific gravity Maximum dry unit weight Minimum dry unit weight Internal friction angle (f) at 70% relative density Bulk unit weight at 70% relative density Uniformity coefficient (Cu) Coefficient of curvature (Cc)
Values Sand
Stone
2.75 19.8 kN/m3 16.17 kN/m3 42
2.70 17.2 kN/m3 15.1 kN/m3 45
18.55 kN/m3 3.6 0.63
16.5 kN/m3 2.1 0.96
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2.6. Test procedure
100 90
diameter hammer was used to remove the air bubbles induced at the time of filling of tank.
In all the cases, the prepared test bed in tank along with stone column and overlying sand bed was placed under a loading frame. Loading was applied through a footing resting on the prepared soil bed and resistance offered by test bed with or without stone column was measured with the help of proving ring. Short-term loading test was conducted in all the cases. Load was applied in equal increments and each increment of the load was maintained until negligible change in the settlement was observed. The settlement due to increment of each equal interval of loading step was observed through three mechanical dial gauges having least count of 0.01 mm fixed on the footing at 120 angle to each other. Loading was applied until the total settlement of the footing attained was at least 20% of footing diameter. To observe the bulging of the stone column after testing, thin cement slurry was added in the stone column without disturbing the column and allowed to get sufficient strength so that even after removal of clay surrounding the stone column, the shape of the tested stone column remained unaffected. After removal of the stone columns from the clay, the bulge depth and bulge diameter of the columns were measured. Fig. 4 shows the photograph of a stone column after testing.
2.4. Preparation of stone column
3. Results and discussion
After preparing the clay bed of 300 mm, a cylindrical hole of diameter 50 mm was made at the centre of the clay bed by an auger of 50 mm diameter. The hole inside the clay bed was smoothly finished from inside with the help of 50 mm diameter steel pipe. The relative density of stones was maintained as 70%. The unit weight of stones was determined at 70% relative density and using the known volume of the hole, the total weight of stone required to fill up the hole was determined. Total weight of stone material was divided into six equal batches to fill up the hole. Each batch of stone was poured and compacted with steel-tamping bars of diameter 10 mm and 30 mm in such a manner that the finished height of each layer of stone column was 50 mm.
3.1. Load-settlement characteristics
Sand Percentage finer
80 70 60
Stone column material
50 40 30 20 10 0 0.01
0.1
1
10
Particle size (mm) Fig. 2. Particle size distribution curves for sand and stone column material.
2.5. Preparation of sand bed The weight of sand required to form a certain thickness of the bed was determined with the known unit weight of sand (at 70% relative density). For different thicknesses of sand, the required weight of sand was calculated and preparation of bed was carried out in layers. Each layer was compacted with a wooden square hammer with equal efforts of compaction to achieve the required relative density of sand bed. In case of geogrid-reinforced sand bed, initially a sand layer of 5 mm thickness was placed in between geogrid layer and clay bed as it has been reported in literature (Han and Gabr, 2002) that in field, a thin granular layer is generally placed over the soft soil before placing the geosynthetic reinforcement layer. Thereafter, the circular geogrid layer was placed in such a way that the centre of geogrid coincides with the centre of stone column. Same procedure was followed to prepare the sand bed over the geogrid layer upto required thickness.
Fig. 5 shows the load-settlement characteristics of the unimproved clay bed, clay bed improved by stone column alone and clay bed improved by stone column along with 30 mm thick unreinforced and geogrid-reinforced sand bed. The diameter of the geogrid layer has been taken as 500 mm. The improvement in load-carrying capacities under different conditions has been computed at 20 mm settlement. From Fig. 5, it has been observed that the placement of sand bed over stone column-improved soft clay increases the loadcarrying capacity of the improved soil and the use of geogrid layer within the sand bed is effective in further increment of the same. As compared to unimproved clay bed, an improvement of 69% in loadcarrying capacity has been observed when the clay bed is improved with stone column only. As compared to unimproved clay bed, 141% improvement in load-carrying capacity has been observed when unreinforced sand bed is placed over stone column-improved soft clay and for reinforced sand bed the improvement is 233%. For a loading intensity of 0.5 kN, as compared to unimproved soil, the settlement has been reduced by 67%, 91% and 94% when the soil is improved by only stone column, by stone column along with unreinforced and geogrid-reinforced sand bed, respectively. For a loading intensity of 1.0 kN, as compared to unreinforced sand bed, 44% reduction in settlement has been observed when geogrid-reinforced sand bed is used; whereas for a loading intensity of 1.3 kN, the reduction in settlement is 55%. Thus, it can be said that the geogrid reinforcement is more effective for higher loading intensity than for lower loading intensity. Similar behaviour has been observed by Deb et al. (2007) in the developed model for geosynthetic-reinforced granular fill-soft soil system with stone column. 3.2. Optimal thickness of the unreinforced and geogrid-reinforced sand bed
Table 3 Properties of geogrid. Parameters
Value
Mesh aperture size Thickness Weight Ultimate tensile strength Strain at maximum load
1 mm 1 mm (square) 1.0 mm 261 gm/m2 1.84 kN/m 15%
To determine the optimum thickness of unreinforced sand bed, the thickness of sand bed was varied from 0.2 to 0.8 times the diameter of the footing (as shown in Table 4). Fig. 6 shows the loadsettlement characteristics of the unreinforced sand bed of different thicknesses placed over stone column-improved clay. The loadcarrying capacity at 20 mm settlement has been calculated and
K. Deb et al. / Geotextiles and Geomembranes 29 (2011) 190e196
193
Fig. 3. Schematic diagram of the test setup.
presented in Fig. 8. It has been observed that as the ratio of thickness of sand bed to the diameter of the footing (Hs/D ratio) increases from 0.2 to 0.5, the load-carrying capacity increased by 54%, whereas only additional 1.3% increment in load-carrying capacity has been observed when Hs/D ratio increased from 0.5 to 0.6. Thus, the optimum thickness of the unreinforced sand bed is 0.5 times the diameter of the footing. Fig. 7 shows the load-settlement characteristics of the geogridreinforced sand bed of different thicknesses placed over stone column-improved clay. The load-carrying capacity at 20 mm settlement has been calculated and presented in Fig. 8. To determine the optimum thickness of the geogrid-reinforced sand bed, the diameter of geogrid reinforcement was chosen as 5 times the diameter of the footing. It has been observed that as the Hs/D ratio increases the load-carrying capacity also increases up to a value of 0.3, whereas beyond this value as the thickness of the sand bed increases the load-carrying capacity decreases. Thus, the optimum thickness of geogrid-reinforced sand bed is 0.3 times the diameter
of the footing. At low sand bed thickness, large defection has occurred in the geogrid reinforcement directly underneath the footing. The large deflection of the geogrid reinforcement would mobilize the membrane action and induce more mobilized tension in the geogrid layer. The vertical component of the tensile force
Table 4 Summary of experimental programme. Sl. No
Thickness of clay bed (mm)
Diameter of stone column (mm)
Thickness of sand bed (mm)
Diameter of geogrid (mm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
300 300 300 300 300 300 300 300 300 300 300 300 300 300
e 50 50 50 50 50 50 50 50 50 50 50 50 50
e e 20 30 40 60 80 20 30 40 30 30 30 30
e e e e e e e 500 500 500 200 250 300 400 Fig. 4. Stone column after testing.
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Load (kN)
Load (kN) 0
0.5
1
1.5
2
0
2.5
1
1.5
2
0
0 5
5
Settlement (mm)
Settlement (mm)
0.5
10 15 20
15
20mm thick sand bed + 500 mm diameter geogrid
20
Clay bed only Clay bed + stone column Clay bed + stone column + 30 mm thick sand bed Clay bed + stone column + 30 mm thick reinforced sand bed
25
10
30mm thick sand bed + 500mm diameter geogrid 40mm thick sand bed + 500mm diameter geogrid 25
30
Fig. 7. Load-settlement characteristics of reinforced sand bed of different thicknesses.
Fig. 5. Load-settlement characteristics of various reinforced conditions.
acting in the geogrid reinforcement partially counterbalances the superimposed load exerted by the overlying soil. As a result, the vertical stress is reduced in the zone below the reinforcement due to combined action of mobilized tension in the reinforcement and membrane action in its curvature (Burd, 1995; Lee et al., 1999; Basudhar et al., 2008). However, when the sand bed thickness increases such that Hs/D > 0.3, a major portion of the shear failure zone of the soil is developed above the reinforcement layer and the deflection of the reinforcement also decreases. This led to reduction in the utilization of membrane action and less mobilized tension in the geogrid has been induced (Lee et al., 1999). This phenomenon reduces the effectiveness of the geogrid layer causing reduction in bearing capacity. Thus, the stone column under geogrid-reinforced sand bed having Hs/D ¼ 0.4 produces less bearing capacity than that under geogrid-reinforced sand bed having Hs/D ¼ 0.3. Studies show that as the thickness of the reinforced sand bed is equal to or greater than the optimum thickness of the unreinforced sand bed, the bearing capacity of unreinforced and reinforced sand bed is almost same (Lee et al., 1999). This is due to the fact that as the thickness of the reinforced sand bed increases, the deflection of the reinforcement decreases and the effectiveness of the reinforcement also decreases. When the thickness of the reinforced sand bed is equal to or greater than the optimum thickness of the unreinforced sand bed the effectiveness of the reinforcement is almost insignificant. Thus, the geogrid-reinforced sand bed with Hs/D 0.5 will
give almost same bearing capacity as compared to that under an unreinforced sand bed with Hs/D 0.5. The improvement in load-carrying capacity, as compared to unimproved soft clay, at 20 mm settlement is 196% and 233% when unreinforced and geogrid-reinforced sand beds with optimum thickness have been placed over stone column-improved soft clay, respectively. Lee et al. (1999) reported similar observation based on numerical and model studies of strip footing resting on reinforcedgranular fill-soft soil system without stone column inclusions. Due to presence of stiffer stone column in the soft clay, lower optimum thickness of the sand bed has been required as compared to the optimum thickness under without stone column condition to get the maximum improvement in load-carrying capacity of improved ground. However, from the present study and the results reported by Lee et al. (1999), it has been observed that the ratio of optimum thickness of the unreinforced to geogrid-reinforced sand bed is almost similar for both the cases under with and without stone columns. 3.3. Optimal extent of geogrid Fig. 9 shows the load-settlement characteristics of sand bed reinforced by geogrid reinforcement of various diameters. While determining the optimal extent of the geogrid reinforcement, the optimum thickness of the sand bed was considered. From the loadsettlement characteristics, it has been observed that for a particular
2
Load (kN) 0
0.3
0.6
0.9
1.2
1.5
1.75
1.8
0
1.5 5
Load (kN)
Settlement (mm)
1.25 10 15
1 0.75
20
0.5
25
0.25
30
20mm thick sand bed 40mm thick sand bed 80mm thick sand bed
30mm thick sand bed 60mm thick sand bed
35 Fig. 6. Load-settlement characteristics of unreinforced sand bed of different thicknesses.
Unreinforced sand bed Geogrid-reinforced sand bed
0 0
0.2
0.4
0.6
0.8
1
Thickness of sand bed / Footing diameter Fig. 8. Variation of load-carrying capacity with thickness of sand bed/footing diameter (at 20 mm settlement).
K. Deb et al. / Geotextiles and Geomembranes 29 (2011) 190e196
Load (kN) 0
0.3
0.6
0.9
1.2
1.5
1.8
2.1
0
Settlement (mm)
5
10
15
500mm diameter geogrid 400mm diameter geogrid 300mm diameter geogrid
20
250mm diameter geogrid 200mm diameter geogrid
25 Fig. 9. Load-settlement characteristics of reinforced sand bed with different diameter of geogrid.
settlement, the load-carrying capacity increases as the diameter of the geogrid increases upto 3 times the diameter of the footing, whereas beyond this value the load-carrying capacity does not change. Thus, the optimal extent of the reinforcement is 3 times the diameter of the footing; and, beyond this diameter any additional reinforcement is ineffective. 3.4. Bulging of stone column It has been observed that, due to application of axial load, a bulge is produced at the top of a single stone column up to a depth of 2 to 3 times the diameter beneath the surface (Barksdale and Bachus, 1983). In the present experiment, the effect of unreinforced and geogrid-reinforced sand bed placed over stone column on bulging behaviour of the column has been studied. Thus, the length-to-diameter ratio was taken greater that 3. In the field, most constructed stone columns have length-to-diameter ratios equal to
a
Bulge diameter (mm) 0
50
b
or greater than 4 to 6 and a bulging failure usually develops depending on whether the tip of the column is floating in soft soil or resting on a firm bearing layer (Barksdale and Bachus, 1983). Thus, the chosen length-to-diameter ratio (equal to 6) of the stone column in the present experimental study (where the tip of the column is resting on firm layer) is relevant to field applications. The bulge of stone column after each test was measured to study the effects of sand bed and reinforcement on bulging of the column. Fig. 10 shows the bulging patterns of the stone column under various test conditions. The vertical axis represents the length of the stone column and horizontal axis represents the bulge diameter of the stone column. In case of only stone column-improved soft clay, a maximum bulge of 12 mm has been observed at a depth of 60 mm from top of stone column. However, in case of stone column along with 30 mm unreinforced and geogrid-reinforced sand bed, the maximum bulge is 10.5 mm at 110 mm depth and 8 mm at 145 mm depth, respectively. Thus, in case of only stone columnimproved clay maximum bulging occurs at a depth of 1.2 times the diameter of column, whereas for unreinforced and geogrid-reinforced sand bed case the depth is 2.2 and 2.9 times the diameter of column, respectively. As compared to only stone column-improved soil, 12.5% reduction in maximum bulge diameter of the stone column has been observed when sand bed is placed over the stone column-improved soft clay. Additional 21% reduction in maximum bulge diameter has been observed when geogrid reinforcement is placed within the sand bed. It can be concluded that the maximum bulge diameter of stone column reduces and the depth of bulge increases with the application of sand bed. Inclusion of geogrid reinforcement further reduces the bulge diameter and increases the bulge depth. Shahu et al. (2000) shows that adequate thickness of granular mat reduces the load carried by granular pile both at the top and bottom and helps to reduce the failure of the granular pile due to bulging of the pile. Very high stress concentration has been observed near the top of the columns (Shahu et al., 2000), which causes high bulging. However, when sand bed is placed over the stone column-improved soft clay significant reduction in stress
Bulge diameter (mm) 0
100
50
c
Bulge diameter (mm) 0
100
50
100
0
0
0
195
1.2 D
2.2 D 100
Depth (mm)
Depth (mm)
100
150
200
250
250
2.9 D
1.21 D
150
200
100 1.16 D 150
200
250 D = 50 mm
D = 50 mm
300
50
50
Depth (mm)
50
1.24 D
300
D = 50 mm
300
Fig. 10. Bulging patterns of the stone column when soft clay has been improved with (a) stone column alone (b) stone column with 30 mm unreinforced sand bed (c) stone column with 30 mm geogrid-reinforced sand bed.
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concentration on top of the column has been observed and the variation of stress concentration with depth is more uniform (Shahu et al., 2000). Thus, placement of sand bed on top of the stone column-improved soft clay reduces the bulge diameter and increases the bulge depth of the stone column. The findings reported in this paper are based on small-scale model tests and are subjected to limitations of scale and boundary effects. Thus, the findings cannot be directly applied for field cases. The present study can be extended for prototype-scale model footing resting on unreinforced or geogrid-reinforced sand bed-soft soil system with stone column in single or group to avoid the scaling and boundary effects. However, in the present study, the model tank has been taken as sufficiently large to reduce the boundary effects. To reduce the scaling effects, the dimensions of the various components have been chosen proportionally with the prototype dimensions. In the present experimental study, small aperture size and thin model geogrid with relatively low stiffness has been used to avoid the size effect in the model experimental results. However, in case of field application comparatively large aperture size and thicker geogrids with higher stiffness are usually used. Thus, the chosen model geogrid properties used in the present experiments are suitable to achieve the same performance results as compared to full-scale geogrid. The results of the present laboratory tests show similar trend with the results reported from numerical and analytical studies. Thus, the results of the present laboratory model study are useful to investigate the behaviour of the unreinforced and geogrid-reinforced sand bed resting over stone column-improved soft clay. 4. Conclusions Based on the experimental results the following conclusions can be drawn: 1. The presence of stone column in soft clay improves the loadcarrying capacity and decreases the settlement of the soft soil. The placement of sand bed further increases the load-carrying capacity and decreases the settlement of the stone columnimproved soil. The inclusion of geogrid as reinforcing element in the sand bed significantly improves the load-carrying capacity and reduces the settlement of the soil. As compared to unimproved soft clay, 69%, 141% and 233% improvement in loadcarrying capacity have been observed (at settlement equal to 20% of the footing diameter) when soft clay is improved by stone column alone, by placing of unreinforced and geogrid-reinforced sand bed of optimum thickness over stone column, respectively. 2. The optimum thickness of unreinforced sand bed placed over the stone column-improved soft clay is 1.7 times the optimum thickness of the geogrid-reinforced sand bed. The optimum thickness of unreinforced and geogrid-reinforced sand bed is 0.5 and 0.3 times the diameter of the footing, respectively. Under optimum thickness of geogrid-reinforced sand bed, the optimum diameter of the reinforcement is 3 times the diameter of the footing. 3. Decrease in bulge diameter and increase in depth of bulge have been observed due to placement of sand bed over stone column-improved soft clay. Further decrease in maximum bulge diameter and increase in depth of bulge have been observed due to application of geogrid. The maximum bulge has been observed at a depth of 1.2, 2.2 and 2.9 times the diameter of stone column in case of soil improved by stone column alone and by placing of unreinforced and geogridreinforced sand bed over stone column, respectively. However, the findings of the present experimental study are affected by the various factors, such as, diameter of the stone columns, footing diameter to stone column diameter ratio, length-to-diameter
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