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 Performance of granular columns in dispersive soils(Thomas Telford Services Ltd ttjournals@ice.org.uk, 2014) Nayak, S.; Dheerendra Babu, M.R.; Shivashankar, R.; James, N.The soils found abundantly along the Konkan belt in peninsular India are lateritic soils and lithomargic clays. The locally available lithomargic clayey soils are problematic in the sense that their strength reduces drastically under saturation conditions, which is typical behaviour of the dispersive type of soil. Most foundations are placed on this soil layer. This paper presents results from a series of laboratory plate load tests carried out in unit cell tanks to investigate the behavior of granular columns in these weak (lithomargic clay) grounds. Tests are carried out with two types of loading: with the entire area in the unit cell tank loaded, to estimate the stiffness of the improved ground; and with only the granular column area loaded, to estimate its limiting axial capacity. Investigations were carried out by varying the area ratio (or spacing), diameter of granular columns, end condition and column configuration. The load-settlement behaviour, stiffness and bulging behaviour of granular columns are analyzed. It is found that the ground treated with granular columns exhibits a high load-carrying capacity and stiffness, and a significant reduction in settlement, compared with the untreated ground. © Ice publishing: All rights reserved.Item Shear Response of Pervious Concrete Column Improved Ground(Springer, 2021) Rashma, R.S.V.; Shivashankar, R.; Jayalekshmi, B.R.This study deals with numerical analysis of the shearing resistances of pervious concrete column improved ground vis-à-vis ordinary stone column improved ground. Analysis is done by numerically simulating a large shear test model, representing pervious concrete column improved ground. The parameters varied in this study are the 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. The shear response of improved ground is quantified by the applied strain controlled vertical load to the entire width of large shear test model that induces shear movements within the tank model. It is observed that the pervious concrete column improved ground shows better shear performance than ordinary stone column improved ground. It is also 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. © 2020, Indian Geotechnical Society.Item Liquefaction Mitigation Potential of Improved Ground Using Pervious Concrete Columns(Springer, 2022) Rashma, R.S.V.; Jayalekshmi, B.R.; Shivashankar, R.In this study, liquefaction mitigation potential of improved ground using pervious concrete column is being investigated. 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 and excess pore water pressure generation of pervious concrete column improved ground on a mildly sloping soil strata of infinite extent under seismic loading. The soil strata considered is fully saturated sand with an inclination of 4°. The parameters influencing seismic performance of improved ground like area ratio, founding depth of columns, diameter of columns and hydraulic conductivity of columns are considered. 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. 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. © 2021, Indian Geotechnical Society.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 Three dimensional analyses of geocell reinforced encased stone column supported embankments on lithomargic clay(Taylor and Francis Ltd., 2023) Vibhoosha, M.P.; Bhasi, A.; Nayak, S.Geocells are a superior form of reinforcement due to their cost-effectiveness and three-dimensional confining properties. However, numerical modeling of geocell is always challenging due to its three-dimensional honeycomb structure. The limitations of the equivalent composite approach (ECA) led to the recent development of full 3D numerical models, which consider geocell-infill material interaction. This paper discusses the time-dependent performance of geocell-reinforced encased stone column-supported embankment considering the actual 3D nature of geocells using the finite element program ABAQUS. Parametric studies were carried out to study the stress transfer mechanism, vertical deformation of the foundation soil, and stress-strain variation inside the geocell pockets. It is found from the analyses that with the provision of a geocell layer on top of Geosynthetic Encased Stone Columns (GESC), the stress concentration ratio improved by 47% at the end of consolidation compared to GESC alone. Also, an 80% reduction in foundation surface settlement is observed with geocell-sand mattresses. The geocell-sand mattress decreased the bulging of the stone columns, and almost 80% of the stone column bulging occurred by the end of the embankment construction. The proposed model’s numerical results show that the equivalent composite approach overestimated the stress concentration ratio and bearing capacity. The tensile stresses are non-uniformly distributed in the geocell pockets, and the maximum tensile force was mobilised at the geocell mid-height. Among the various geocell infill materials analysed, the aggregates were best suited considering the stress concentration ratio and vertical settlement. The numerical results supported the idea that encased stone columns with geocells at the embankment base can perform similarly to a geosynthetic reinforced piled embankment system, which is costlier but very efficient. When the modular ratio is more than 40, geocell-reinforced encased stone column-supported embankment is similar to GRPES. © 2022 Informa UK Limited, trading as Taylor & Francis Group.