A Study of the Behavior of Stone Column in Local Soft and Loose Layered Soil Pradip Das Post Graduate Student Civil Engineering Department, National Institute of Technology, Agartala, India e-mail: [email protected]

Dr. Sujit Kumar Pal Associate Professor Civil Engineering Department, National Institute of Technology, Agartala, India e-mail: [email protected]

ABSTRACT In the modern civilized world ground improvement technique is a great concern to the geotechnical engineers. Stone columns are found effective, feasible and economical to improving the soft and loose layered soil. Stone columns increase the unit weight and the bearing capacity of soil. It can densify the surrounding soil during construction. The improvement of a soft soil by stone columns is due to different sizes of aggregate (size between 2 to 10 mm) in the soft soil. This paper presents the utilization of stone column to improve the load capacity of sandy silt soil with clay in naturally consolidated state. Load tests through the compression testing machine are performed on single un-encased stone column in sandy silt soil with clay (i.e., sand = 37.29%, silt = 33.00% and clay = 29.71%). Un-encased and encased (with geotextile) stone column behavior on layered soil also discussed in this investigation. In case of un-encased stone column load carrying capacity increases with the increasing diameter of the stone column but in un-encased and encased layered soil load carrying capacity decreases with the increasing diameter of the stone column. The load bearing capacity of stone column in un-encased and encased layered soil decreases with the increasing diameter of stone column.

KEYWORDS:

Stone column, soil, Un-encased and encased layered soil, Bearing

capacity.

INTRODUCTION Stone columns have been increasingly used for improvement of soft soils to increase the load bearing capacity and to reduce the settlements. Improvement of soft clay deposits by the installation of stone columns is one of the most popular techniques followed worldwide. The stone columns not only act as reinforcing material increasing the overall strength and stiffness of the compressible soft soil, but also they promote consolidation through effective drainage. - 1777 -

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Potential applications include stabilizing foundation soils to support embankments and approach fills, supporting retaining structures (including reinforced earth), bridge bent and abutment structures on slightly marginal soft to stiff clays and loose silty sands, landslide stabilization and reducing liquefaction potential of clean sands. Also, stone columns under proper conditions can greatly decrease the time required for primary consolidation. This ground improvement technique has been successfully applied for the foundations of structures like oil storage tanks, earthen embankments, raft foundations etc where large settlement is possible. Ambily and Gandhi (2007) presented experimental and finite-element analyses carried out to study the effect of shear strength of soil, angle of internal friction of stones, and spacing between the stone columns on the behavior of stone columns. Tan and Tjahyono (2008) proposed two simplified conversion methods to obtain the equivalent plane-strain model of the unit cell, and investigated their applicability to multicolumn reinforced ground. The acceleration of consolidation rate by stone columns mostly analyzed within the framework of a basic unit cell. Hussein et al. (2009) presented the laboratory measurements of the properties of such clays and their settlements at different applied stresses. Wang (2009) presented an analytical solution for the consolidation of soft soil foundations reinforced by stone columns under time-dependent loadings. The differential equations of the foundations reinforced by stone columns are obtained including smear and well resistance under arbitrary applied loadings. Murugesan and Rajagopal (2010) investigated qualitative and quantitative improvement of individual load capacity of stone column encasement through laboratory model tests conducted on stone columns installed in clay bed prepared in controlled condition in a large scale testing tank. Shivashankar et al. (2011) presented a experimental results from series of laboratory plate load tests carried out in unit cell tanks to investigate the behaviour of stone columns in layered soils, consisting of weak soft clay overlying a relatively stronger silty soil, for various thicknesses of the top layer. Indraratna et al.(2013) presented a novel numerical model finite-difference method to analyze the response of stone column reinforced soft soil under embankment loading, adopting the free strain approach and considering both arching and clogging effects. Marto et al (2013) introduced the assumptions, procedures and results of the numerical analysis for simulating the behaviour of un-encased versus geogrid encased stone column in soft clay. Bhattacharyya and Pal (2012) investigated the improvement of silty clay soil by installation of un-encased and encased stone column in different diameter.

MATERIALS AND EXPERIMENTAL PROGRAME Materials Soil: Two types of soil have been used, namely, sandy silt with clay soil (i.e., sand = 37.29%, silt = 33.00 % and clay = 29.71%) and silty clay soil (i.e., sand = 17.21%, silt = 50. 25% and clay = 32.54%). The sandy silt with clay soil has collected from NIT Agartala campus, Tripura, India and Silty clay has collected from river bank near Agartala city, Tripura, India. Physical properties like specific gravity, liquid limit and plastic limit, and engineering properties like maximum dry density, optimum moisture content and shear strength characteristics (i.e., cohesion and angle of internal friction) are found out and shown in Table 1. Stone aggregates: Crushed stone aggregates of sizes between 2 mm and 10 mm have been collected from market near Agartala, Tripura, India to form stone column. The finer fraction passing through 2 mm was removed by wet sieving and used after drying. Properties of the aggregate for the stone column are given in Table 2. The stone aggregates were compacted to a density of 15.01kN/m3 to form stone columns.

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Experimental Programme The load-deformation behavior of the column treated soil has been studied by applying vertical load with the help of a loading frame. A cubic tank (height 45 cm, 32 cm × 32 cm inner dimensions and 5cm thick) made of perspex sheet has been used for the experiment (Figure 1). For making a stone column a soil bed was prepared for a depth of 5 cm then galvanized iron made tube placed over the soil bed. Soil was filled surrounding the small tube. Care has been taken that no soil enter into the small tube and soil have filled up to the required height in the cubic tank. Water was sprinkled over the filled soil to make the soil get compacted by natural consolidation. Smaller tube filled with aggregate which are at centre of the cubic tank. Aggregates tamped with tamping rod of 60 cm length and 1.6 cm diameter falling though a height 2.5 cm for 25 blows after each pouring of equal volume aggregates (for maintaining a particular density of 15.01 kN/m3). Level of aggregate in the smaller tube kept same with the level of the surrounded soil and while pouring the aggregates in the smaller tube simultaneously tube has been taken out. The load was applied through a proving ring at a constant displacement rate. Settlements were monitored for equal intervals of loads up to the failure. Load was applied over the entire area, with 30 mm thick sand layer placed over the entire surface. A steel plate of 5 mm thickness was placed over the sand blanket. The loading was applied in a similar way until the settlement reaches at 25 mm under the compression testing machine. Load is measured by the proving ring and displacement measured by the dial gauge. The test is repeated for different diameter of stone column.

RESULTS AND DISCUSSIONS Table 1: Physical and engineering properties of soils Soil properties Physical properties: Sand (%) Silt content (%) Clay content (%) Specific gravity Liquid limit (%) Plastic limit (%) Plasticity index (%) Engineering properties: Optimum moisture content (%) Maximum dry density (kN/m3) Angle of internal friction, ϕ (in degree) Cohesion, c (kN/m2)

Sandy silt with clay

Silty clay soil

37.29 33.00 29.71 2.60 30.10 16.46 13.64

17.21 50.25 32.54 2.56 39.50 20.57 18.93

17.10 17.26

18.50 16.77

24.00

19.37

13.79

15.04

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Figure 1: Load test on single stone column in test tank

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Table 2: Properties of aggregate Properties of Aggregate Specific gravity 2.613 Water absorption 2.147 Angularity number 4.23 c = 9.86 kN/m2 Shear strength ϕ = 46.94°

Behavior of un-encased stone column in sandy silt with clay soil Test was carried out on end bearing stone columns of length 300 mm and with diameters of 50, 60, 70, 80, 90 and 100 mm. The load has been applied mechanically in compression testing machine, and the resulting load-settlement curves were compared in Figure 2. The load carrying capacity for 25 mm settlement of stone column in un-encased stone column increases with the increasing diameter of stone column. Influence of diameter of the on un-encased stone column is discussed herein.

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Figure 2: Load settlement response of untreated and stone column treated sandy silt with clay soil Influence of the diameter of the un-encased stone column The results were obtained by applying load with a loading plate to the stone column. Tests have been conducted so as to determine the improved bearing capacity of the sandy silt with clay soil due to inclusion of stone column. The loading on the stone column shows a clear failure indicating ultimate load and shows the significant raise in the load carried by the stone column up to 25mm settlement. Figure 2 shows load-settlement behavior of untreated and treated with stone column for different diameters of stone column. The load carrying capacity for 25 mm settlement of treated soil increased with the increase in diameter of stone column. The increment of ultimate load carrying capacity between 50 mm to 60 mm diameter of stone column is 14.29%, increment of ultimate load between 60 mm to 70 mm diameter of stone column is 17.08%, increment of ultimate load between 70 mm to 80 mm diameter of stone column is 10.32%, increment of ultimate load between 80 mm to 90 mm diameter of stone column is 3.23% and increment of ultimate load between 90 mm to 100 mm diameter of stone column is 4.76%. It seen that the responses of stone column with increasing diameter shows a higher load carrying capacity for 25mm settlement. Similar trend have also been observed by Murugeson and Rajagopal (2010), Bhattacharyya and Pal (2012), and Marto et al (2013) in their findings of model tests on stone column of different diameters.

Behavior of un-encased stone column in layered soil made of silty clay soil at top and sandy silt with clay soil at bottom In case of layered soil stone column diameters are same as used in previous experiment. The load carrying capacity for 25 mm settlement of column in layered soil decreases with the increase in diameter of stone column. But here load carrying capacity of column is lesser as compared to

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the previous experiment i.e. without layered soil. Influence of diameter of the stone Column on layered soil is discussed in this section.

Influence of diameter of the stone column on layered soil The load carrying capacity of different diameter of layered stone column decreases with the increasing diameter of stone column. The decrease in load capacity of different diameter of stone column in layered soil between 50mm and 60mm, 60mm and 70mm,70 and 80mm,80and 90 are 32.10%, 10.42%, 13.08% and 23.53% respectively. This is due to poor lateral confinement offered by the weak silty clay soil because of bulging occurs in the top soil. In case of stone column in layered soil improvement of soil condition largely depends upon the smaller diameter of stone column. As the diameter increases improvement with the stone column decreases. Similar observation noticed by Shivashankar et al. (2011) in their experimental studies on behaviour of stone columns in layered soils. load in KN

Settlement in MM

Layered stone column 50 mm dia sc 60 mm dia  sc 70 mm dia  sc 80 mm dia  sc 90 mm dia  sc

Figure 3: load settlement response of stone column treated layered soil made of silty clay soil at top and sandy silt with clay soil at bottom

Table 3: Bearing capacity of stone column with different diameters in layered soil Diameter of stone column (mm)

Bearing capacity (kN/m2) for 25mm settlement

50

1782.53

60

937.25

70

623.63

80

427.72

90

267.22

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Effect of geo-textile in un-encased and encased layered soil The encasement effect on load carrying capacity of the stone column in different diameter up to 25 mm settlement is shown in Figure 3. The Figure shows that 60mm diameter of stone column in encased with geo-textile increases load carrying capacity to 61.06%, for 70mm diameter it is 33.75% and for 80mm diameter it is 28.77%. In case of encased stone column the load carrying capacity decreases with the increasing diameter of stone column in layered soil. This is due to poor lateral confinement offered by the soft clay soil. Smaller diameter of stone column carrying higher load than higher diameter of stone column. Murugeson and Rajagopal (2010) reported similar type of results in their studies on the behavior of single and group of geosynthetic encased stone columns. Similar trends have also been observed by Bhattacharyya and Pal (2012) in their studies of a single stone column.

Layered stone column

load in KN

Settlement in MM

60 mm dia unencased  sc 70 mm dia unencased  sc 80 mm dia unencased  sc 60 mm dia encased sc 70 mm dia encased sc 80 mm dia encased sc

Figure 4: load settlement response of un-encased and encased stone column treated layered soil m of silty clay soil at top and sandy silt with clay soil at bottom Table 4: Bearing capacity of un-encased and encased (with geotextile sheet) stone column with different diameters in layered soil Stone column (un-encased)

Diameter (mm) D=60

Diameter (mm) D=70

Diameter (mm) D=80

926.64

623.63

435.69

D=60

D=70

D=80

1485.2

836.70

561.02

Bearing capacity in 25mm settlement (kN/m2) Stone column (encased) Bearing capacity in 25mm settlement (kN/m2)

Bulging Behavior of Stone Column It is observed that in case of un-encased stone column, bulging occurs at a different depth of the stone column. In case of stone columns in layered soils, at different diameter the entire bulging was noticed in the top weak layer zone only. It can be seen that, in case of homogeneous

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soil beds, bulging of stone column was predominant in weaker soil, due to lesser lateral confinement offered by that soil. On the other hand, the encased stone columns have undergone much lesser lateral expansion near the ground surface. It is observed that stone column encased with geo-textile sheet of different diameter the lateral bulging occur at deeper depths as compared to the un-encased stone column. It has been happened for the reason that the applied surface load is transmitted deeper into the column due to encasement effects. As the stiffness of the encasement increases, the lateral stresses transmitted to the surrounding soil are found to decrease. This occurrence makes the load capacity of encased columns lesser dependent on the strength of the surrounding soil as compared to un-encased stone columns. Similar trend observed by Murugeson and Rajagopal (2010) in their finding of model tests on stone column of different diameter. Bhattacharyya and Pal (2012) also noticed similar observations in their studies of a single stone column for un-encased and encased stone column. Marto et al (2013) in their studies of performance analysis of reinforced stone columns using finite element method also reported the same.

Figure 5(a): Bulging behavior of unencased stone column in sandy silt with clay soil

Figure 5(b): Bulging behavior of un-encased stone column treated layered soil made of silty clay soil at top and sandy silt with clay soil at bottom

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Figure 5(c): Bulging behavior of encased stone column treated layered soil made of silty clay soil at top and sandy silt with clay soil at bottom

CONCLUSIONS The experimental analyses have been carried out to study the behavior of stone column on the different diameter of stone columns for ground improvement and stabilization of soil. Based on the above results and discussions, the following conclusions may be made: 1. The stone column treated soil can carry more load than untreated soil. 2. The load carrying capacity of treated soil increase with the increase in diameter of stone column. 3. When column area is loaded, failure of bulging occurs within the entire column area. 4. The load settlement behavior of entire loaded area is almost linear. 5. The load carrying capacity of treated layered soil decreases with the increasing of diameter of stone column. 6. The encased stone column in layered soil also decreases with the increasing diameter of stone column.

REFERENCES 1. Ambily, P. and Gandhi, S. R (2007) “Behavior of stone columns based on experimental and fem analysis”, Journal of Geotechnical and Geoenvironmental Engineering, ASCE 133(4):405– 415.

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2. Black, J. A.; Sivakumar, V.; Madhav, M. R. and Hamill, G. A. (2007) “Reinforced stone column in weak deposit: laboratory modal study”, Journal of Geotechnical and Geoenvironmental Engineering, ASCE Vol. 133, (9). 3. Tan, S. A.; Tajahyono, S. and K. K. (2008) “Simplified plane-strain modeling of stone column reinforced ground”, Journal of Geotechnical and Geoenvironmental Engineering, ASCE Vol. 134, (2):185–194, 1154–1161. 4. Murugesan, S. and Rajagopal, K. (2010) Studies on the behavior of single and group of geosynthetic encased stone columns”, Journal of Geotechnical And Geo Environmental Engineering, ASCE , Vol.136(1):129-139. 5. Wang, G. (2009) “Consolidation of soft clay foundation reinforced by stone columns under time-dependent loading”, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol.135, (12):1922–1931. 6. Shivashankar, R.; Dheerendra Babu M. R.; Nayak, S. and Rajathkumar, V. (2011) “Experimental studies on behaviour of stone columnsin layered soils”, Geotechnical Geological Engineering, 29:749-757. 7. Hussein, H. K.; Mohammad, M. M. and Raida, G. R. (2009) “Soft clay soil improvement using stone column sand dynamic compaction techniques”, Engineering and technical Journal,Vol.27,(14). E 8. Deb, K.; Basudhar P. K. and Chandra, S. (2007) “Generalized Model for GeosyntheticReinforced Granular Fill-Soft Soil with Stone Columns”, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 7, (4): 266–276 9. Indraratna, B.; Basack, S. and Rujikiatkamjorn, C. (2013) “Numerical solution of stone column–improved soft soil considering arching, clogging, and smear effects”, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 139, (3): 377–394 10. Marto, A.; Moradi, R.; Helmi, F.; Latifi, N. and Oghabi, M. (2013) “Performance Analysis of Reinforced Stone Columns Using Finite Element Method”, Electronic Journal of Geotechnical Engineering, Vol. 18. 11. Bhattacharyya, A.; and Pal, S.K. (2012) “A study of single stone column”, Indian Geotechnical Conference held at IIT, Delhi, India. Vol.1, 612-615.

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