Faculty Publications
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Item Stone Columns with Vertical Circumferential Nails: Laboratory Model Study(2010) Shivashankar, R.; Dheerendra Babu, M.R.D.; Nayak, S.; Manjunath, R.This paper presents results from a series of laboratory plate load tests carried out in unit cell tanks to investigate the improvement in stiffness, load carrying capacity and resistance to bulging of stone columns installed in soft soils. A new method of reinforcing the stone columns with vertical nails installed along the circumference of the stone column is suggested for improving the performance of these columns. Tests were carried out with two types of loading (1) the entire area in the unit cell tank loaded, to estimate the stiffness of improved ground and (2) only the stone column loaded, to estimate the limiting axial capacity. It is found that stone columns reinforced with vertical nails along the circumference have much higher load carrying capacity and undergo lesser compression and lesser lateral bulging as compared to conventional stone columns. The benefit of vertical circumferential nails increases with increase in the diameter, number and depth of embedment of the nails. The improvement in the performance of stone column was found to be more significant, even with lower area ratio. It is found that reinforcing stone column with vertical circumferential nails at the top portion to a depth equal to three times the diameter of stone columns, will be adequate to prevent the column from excessive bulging and to improve its load carrying capacity substantially. © 2010 Springer Science+Business Media B.V.Item Performance of stone columns with circumferential nails(2011) Nayak, S.; Shivashankar, R.; Dheerendra Babu, M.R.D.Stone columns are often used as an effective technique for improving the performance of soft ground. Stone columns derive their load-carrying capacity due to lateral confinement from the surrounding soil. Very soft soils offer very low lateral confinement, leading to large settlements and low load-carrying capacities. In this paper, an alternative method of enhancing the performance of stone columns in soft soils by reinforcing the stone columns with circumferential nails driven vertically is suggested. The method was developed in laboratory-scale model tests and a series of plate load tests were performed in unit cell tanks to investigate the performance of stone columns reinforced with circumferential nails. The investigation was carried out by varying the depth of nails below ground level, the number of nails and the diameter of nails with different diameter stone columns and area ratios (orspacing). It was found that the circumferentially reinforced stone columns have much higher load-carrying capacity with a significant reduction in settlement and less lateral bulging in comparison with plain stone columns.Item Experimental Studies on Behaviour of Stone Columns in Layered Soils(2011) Shivashankar, R.; Dheerendra Babu, M.R.D.; Nayak, S.; Rajathkumar, V.Stone columns are found to be effective and economical ground improvement technique in soft grounds. Understanding its behaviour when they are installed in stratified soils, in particular when the upper layer consists of weak soil, will be of great practical significance. This paper presents results from a 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. Tests were carried out with two types of loading (1) the entire area in the unit cell tank loaded, to estimate the stiffness of improved ground and (2) only the stone column loaded, to estimate the limiting axial capacity. Laboratory tests were carried out on a column of 90 mm diameter surrounded by layered soil, for an area ratio of 15%. It is found that the depth of top weak layer thickness has a significant influence on the stiffness, load bearing capacity and bulging behavior of stone columns. © 2011 Springer Science+Business Media B.V.Item Some studies on engineering properties, problems, stabilization and ground improvement of lithomargic clays(Southeast Asian Geotechnical Society seags@ait.ac.th, 2015) Shivashankar, R.; Ravi Shankar, A.U.; Jayamohan, J.The study area for this paper is coastal Karnataka in India. The area has laterites and lateritic soils, and also a large number of sporadic lateritic hillocks. The soil stratification mainly consists of lithomargic clay sandwiched between the weathered laterite at top and the hard granitic gneiss underneath. Quite often the top laterites are removed in this area for use as bricks for construction purposes, thus exposing the underlying lithomargic clay. This coastal area receives copious amount of rainfall and a lot of developmental activities are taking place. These lithomargic clays, locally called as 'shedi soils' are also used as fill material in low lying areas, very often adjacent to water bodies. These soils behave as dispersive soils and are also highly erosive. A lot of engineering problems - such as foundation problems, subgrade problems, erosion and slope stability problems are being faced due to the presence of these shedi soils. Some laboratory studies on the engineering and strength properties of these lithomargic clays and stabilized soils, Ground Improvement on shedi grounds are made and reported.Item Influence of Earthquake Characteristics on Pervious Concrete Column Improved Ground(Springer Science and Business Media Deutschland GmbH, 2022) Rashma, R.S.V.; Jayalekshmi, B.R.; Shivashankar, R.In this paper, the influence of earthquake characteristics on the seismic performance of ground improved with pervious concrete columns in place of conventional stone columns is presented. Two scaled earthquake ground motions with different seismic characteristics are applied to the finite element models of ground with and without column inclusions. Total stress analysis is also conducted and compared with effective stress analysis on maximum response profile along the depth of column improved ground. The study is further extended to sandwiched liquefiable soil deposits of varying thickness. It is noted that the average lateral displacement reduction of the pervious concrete column improved ground is 90% when compared to unimproved sand strata when subjected to two different earthquake excitations. 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. The pervious concrete column performed better in homogeneous sand deposit as well as sandwiched liquefiable soil of varying thickness when subjected to different seismic excitations with different characteristics. © 2022, The Author(s), under exclusive licence to Springer Nature Switzerland AG.Item A Numerical Study on the Shear Strength of Pervious Concrete Column in Weak Ground(Southeast Asian Geotechnical Society, 2022) Rashma, R.S.V.; Jayalekshmi, B.R.; Shivashankar, R.In this study, the response of pervious concrete column-treated ground under shear loading is examined by employing a series of numerical analyses. The shear behaviour of pervious concrete column-treated ground is compared with stone column-treated ground and weak ground. Two types of analyses were carried out to assess shear strength of the composite ground. Conventional direct shear test model and large shear test models were evaluated using ABAQUS software. The pervious concrete column-treated ground is observed to have greater shear strength than the mere stone column-treated ground. The lateral deflection pattern of the pervious concrete column is also noticed to be very much lesser than conventional stone columns under static shear loading. The overall shear performance of the pervious concrete column-treated ground is found to be improved than the typical stone column-treated ground. © 2022, Southeast Asian Geotechnical Society. All rights reserved.Item Liquefaction-Induced Lateral Spreading Mitigation Using Pervious Concrete Column Inclusion in Sloping Strata(Taylor and Francis Ltd., 2023) Rashma, R.S.V.; Jayalekshmi, B.R.; Shivashankar, R.The effectiveness of pervious concrete column remediation in homogeneous and sandwiched soil strata for mitigating liquefaction-induced lateral spreading is being investigated in this study. The seismic performance of pervious concrete column improved ground is compared with stone column improved ground. The efficacy of pervious concrete column on three types of soil strata in mitigating liquefaction along with the 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. Three-dimensional nonlinear finite element software OpenSeesPL is used to analyze remediated ground with stone column and pervious concrete column inclusions. 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, silt strata and sandwiched liquefiable soil deposits. © 2022 Taylor & Francis Group, LLC.Item Limit equilibrium slope stability analysis of column-supported embankment on weak ground(Emerald Publishing, 2024) Rashma, R.; Jayalekshmi, B.R.; Shivashankar, R.Purpose: The study aims to analyse the stability of embankments over the improved ground with stone column (SC) and pervious concrete column (PCC) inclusions using limit equilibrium method. The short-term stability of PCC-supported embankment system is rarely addressed. Therefore, the factor of safety (FOS) of column-supported embankment system is calculated using individual column and equivalent area models. Design/methodology/approach: The stability analysis of column-supported embankment system is conducted using PLAXIS LE 2D. The various geometrical and shear strength parameters influencing the FOS of these embankment systems such as diameter of columns, spacing between columns, embankment height, friction angle of column material, undrained cohesion of weak ground and cohesion of PCC are considered. Findings: The critical failure envelope of PCC-supported embankment system is observed to be of toe failure, whereas the failure envelope of stone column-supported embankment system is generally of deep-seated nature. Originality/value: It is found that for PCC embankment system, FOS and failure envelope are not influenced by the geometrical/shear strength parameters other than height of embankment. However, for stone column-supported embankment system, FOS and failure envelope are dependent on all the shear strength and geometrical parameters considered in this study. © 2023, Emerald Publishing Limited.