Studies on the Behaviour of Pervious Concrete Column Improved Ground Subjected to Static Shear and Seismic Load

Abstract

Granular piles or stone columns are extensively used across the globe for improving soft soils, especially for supporting embankments on soft grounds, because of its ease of construction and inherent advantages. Various modified stone columns and their behaviour under vertical loading are extensively reported in literature. However, the behaviour of improved ground under lateral loading conditions are limited. Additionally, among all the reported externally/internally improved stone columns, very few modified stone columns like encased stone columns, deep cement columns and rigid columns are generally used in practice. Therefore, present study considers modified stone column as pervious concrete column which is reported as an alternative to conventional stone column owing to its comparable permeability characteristics with stone column in addition to its higher vertical load carrying capacity. The behaviour of pervious concrete column improved ground under static shear and seismic loading conditions are carried out and compared with the performance of conventional stone column improved ground. In the first part of the study, the behaviour of stone column and pervious concrete column under static shear loading conditions are investigated. The shearing resistances of pervious concrete column improved ground vis-à-vis ordinary stone column improved ground under static shear loading conditions are assessed. Numerical analyses were carried out by simulating direct shear test model and large shear test model, representing pervious concrete column improved ground using ABAQUS software. The single column modelled in the study represents the column placed beneath the toe of the embankment, where shear loading is predominant. Inclined direct shear tests are also analyzed by varying the slope (+/-) of potential failure surface with horizontal to represent the actual practical conditions. A total of 378 direct shear test models are analyzed to study the effect of normal pressure, effect of diameter, effect of reinforcement and effect of shear surface inclinations. The ultimate shear strength of pervious concrete column improved ground is found to be higher than ordinary stone column improved ground. It is found that the pervious concrete column improved ground under zero normal pressure has significant shear resistance than ordinary stone column improved ground and could be provided beneath the toe of the embankment for better shear performance. In order to study the performance of improved ground with floating and end-bearing pervious concrete columns, large shear test tank model with increased depth is analyzed. The shear response of improved ground is quantified, and the parameters considered are depth of pervious concrete column/pile, floating and end bearing piles, diameter, single pile and two pile group and distance from the edge of loading area in the model. It is observed that the pervious concrete column improved ground exhibits better shear performance than ordinary stone column improved ground. It is found that the pervious concrete column undergoes very small lateral deflections. It is also observed that more number of pervious concrete columns, and closer they are to the loaded area, better is the shear performance. The end bearing pervious concrete column improved ground is found to have significantly higher shear resistance than floating pervious concrete column improved ground. Therefore, it is suggested to provide full depth of pervious concrete column up to the bearing strata for achieving better shear performance. Pervious concrete columns show significantly lesser lateral displacements compared to ordinary stone columns. Peak lateral displacements in case of pervious concrete column are at the surface and the deflected profile of the column is very much like that of a rigid pile with a free or unrestrained head condition. Stone columns are highly recommended for mitigating liquefaction and the feasibility of pervious concrete column in preventing liquefaction is addressed in the second part of this study. Liquefaction induced lateral spreading causes catastrophic damages during and after earthquakes. Therefore, the effectiveness of pervious concrete column remediation in soil strata for mitigating liquefaction-induced lateral spreading is emphasized. The seismic performance of pervious concrete column improved ground is compared with conventional stone column improved ground. Three-dimensional finite element analysis using OpenSeesPL software is conducted to study the ground lateral deformation, excess pore water pressure generation and shear-strain behaviour of pervious concrete column improved ground on a mildly sloping soil strata of infinite extent under seismic loading. The parameters influencing the seismic performance of improved ground like area ratio, founding depth of columns, diameter of columns and hydraulic conductivity of columns are considered. The efficacy of pervious concrete column on three types of soil strata in mitigating liquefaction along with parameters influencing ground lateral deformation such as thickness of sandwiched liquefiable soil layer, permeability of surrounding soil, ground surface inclination, peak ground acceleration and surcharge load are reported. The influence of earthquake characteristics such as frequency content, significant duration, time of peak ground acceleration and arias intensity on lateral displacement, excess pore pressure dissipation and shear stress-strain behaviour of modelled ground are also studied. Total stress analysis is also conducted and compared with effective stress analysis on maximum response profile along the depth of improved ground with column inclusions when subjected to earthquake loading conditions. The stone column gets distorted during seismic loading due to shearing and causes dilation. The distorted gravel structure of stone column increases the length of the drainage path and retards the dissipation of excess pore water generated due to shaking. Whereas the pervious concrete column structure is not distorted due to seismic shaking and the pervious concrete column inclusion reduces drainage path for excess pore water to dissipate quickly. Therefore, the seismic shear strains developed in the surrounding soil is drastically reduced. The limited excess pore pressure generation and relatively higher effective confinement reduces the lateral displacement of pervious concrete column improved ground significantly. It is found from various response parameters that the pervious concrete column improved ground has better seismic performance than conventional stone column improved ground. The lateral deformation profile of pervious concrete column is found to be similar to that of concrete pile, allowing excess pore water pressure to dissipate through the pores of pervious concrete column. Liquefaction-induced lateral deformation is found to be lesser in pervious concrete column improved ground in comparison with stone column improved ground. The lateral deformation of pervious concrete column remediated ground is found to be independent of surrounding soil permeability. The pervious concrete column inclusion is found to be a better alternative to stone column in mitigating liquefaction in susceptible soils like loose sand, medium-dense sand, sandwiched sand deposits and silt strata. It is also found that the pervious concrete column remediation is a better alternative than stone column in seismically active regions even with peak ground acceleration of 0.6g. It is found that the generation of excess pore pressure reaches near zero values when the permeability of pervious concrete column is greater than 0.3 m/s irrespective of the characteristics of the earthquake events. From total stress analysis and effective stress analysis, it is observed that for column improved ground, in addition to pore pressure build-up, the maximum response profile is highly influenced by significant duration and frequency of seismic excitation. It is also concluded that pervious concrete columns could be used as an alternative to conventional stone columns to mitigate liquefaction to a larger extent.