1 Bearing capacity of soft clays improved by stone columns: a parametric analysis Suresh Prasad Singh, Indraneel Sengupta, & Mrinal Bhaumik Department of Civil Engineering, National Institute of Technology, Rourkela, Odisha, India, ABSTRACT A detailed numerical analysis has been performed on a unit cell stone column by varying parameters such as shear strength of clay, angle of internal friction of the column material, slenderness ratio (L/d) of the column, area replacement ratio, and the modular ratio, Poisson's ratio of clay and column material. A parametric study is conducted for floating stone column by using the finite element package PLAXIS. A drained analysis was performed using Mohr-Coulomb criterion for both the materials. Validation was done by taking experimental data for a single unit cell stone column. From the analysis, it is observed that the bearing stress increases with increase in the aspect ratio (L/d) up to a particular range beyond which there is negligible change in the bearing capacity. Bulging is the main cause of failure of a single stone column when loaded alone. An increase in the area ratio caused an increase in the bearing value of the stone column reinforced soil due to higher relative stiffness. The most influential parameters for the design of stone column reinforced soil are the angle of internal friction of the column material and area replacement ratio whereas the cohesion of surrounding material, slenderness ratio, and Poisson's ratio contribute to a lesser magnitude. 1 INTRODUCTION Soft clay deposits are mainly found in the marine environment and in India, they are found mainly along the Eastern and Western coastal regions. Soft clay possesses very low bearing capacity, undergoes large settlement, is highly compressible and has very low shear strength. Stone columns are inserted to improve the bearing capacity of the soft clays and to reduce the settlement. They act as a stiffer medium and also accelerate the rate of consolidation, thereby, increasing the shear strength of the surrounding clay. Various researchers have studied the behaviour of stone columns. Hughes and Withers (1974), Balaam et al. (1978), Ambily and Gandhi (2007) have conducted model tests on stone columns to understand its behaviour based on the unit cell concept. Priebe et al. (1995) performed numerical analysis based on the unit cell concept to estimate the settlement considering an infinite number of stone columns whereas Hanna et al. (2013) conducted numerical study to identify the mode of failure of a single column and a group of stone columns based on the unit cell concept. Hanna et al. (2014) also developed an analytical model to predict the bearing capacity of reinforced soft soil with stone column based on the limit equilibrium method. The present paper discusses the improvement of soft clay with the introduction of stone columns and the most influential parameters governing the bearing capacity of stone columns. A detailed numerical analysis has been carried out on a single unit cell stone column under a circular rigid raft considering the triangular pattern of stone columns. A parametric study is conducted by using the finite element package PLAXIS. The parameters varied are the shear strength of clay, the angle of internal friction of the column material, slenderness ratio (L/d) of the column, area replacement ratio, and the modular ratio, Poisson's ratio of clay and column material. The diameter of the column was taken as 1m and the length varied as 2m, 4m, 5m, 10m, 15m and 20m. The spacing is varied as 1.74m, 2.13m, 3m and 4.26m to have area replacement ratios of 30%, 20%, 10% and 5% respectively, whereas the undrained shear strength was varied as 10kPa, 20kPa, 30kPa and 40kPa. Friction angle of the column was varied as 40 o , 45 o and 50 o . Failure pattern of a single column when loaded alone is also assessed and reported in this paper. 2 VALIDATION For checking the accuracy of the finite element analysis, validation was done on a unit cell model taken from published work done by Ambily and Gandhi (2007). The Figure 1 shows the model for the unit cell stone column and Figure 2 shows the comparison between bearing stress versus settlement curve for S/d=2 and Cu =30kPa. The properties of the material used for the validation is shown in Table 1 given below.
Bearing capacity of soft clays improved by stone columns: a parametric analysis
Jun 28, 2023
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