Madhira R. Madhav

Deformation analysis of soft ground reinforced by columnar inclusions

Abstract A simple theoretical approach to predict the deformation behaviour of soft ground reinforced by columnar inclusions such as stone columns./granular piles, sand compaction piles, lime or cement columns, etc., is presented in this paper. The analysis is performed based on the deformation properties of the column material and the surrounding soil. The interaction shear stresses between the column and the surrounding soil are considered to account for the stress transfer between the column and the soil. The solution is obtained by imposing compatibility between the displacements of the column and the soil for each element of the column-soil system. Numerical evaluations are made for a range of parameters to illustrate the influence of various parameters on the predictions. The proposed method is verified with finite element analysis and a reasonable agreement is obtained between the predictions.

A new model for geosynthetic reinforced soil

Abstract It is common to use geosynthetics to reinforce soft-soils or peat with a view to improve their load — settlement response. A new foundation model element — the rough membrane, is proposed to represent the response of the geofabric. Combining this element with Winkler springs and Pasternak shear layers to model respectively the soft soil and the granular fills, a new foundation model is presented for the geosynthetic — granular fill — soft soil system. Analysis of results at small displacement indicates the effect of granular fill to be more and significant than that of the membrane thus confirming large scale model test results of Jarrett and results based on F.E.M. (Boutrup and Holtz). The effect of the membrane increases with the load or decreasing soil stiffness.

Analysis of soft ground-granular pile-granular mat system

Abstract A simple theoretical approach to analyze soft ground reinforced by granular piles with granular mat on top is presented. The interaction shear stresses between the granular pile and the surrounding soil are modified due to overburden pressure by the granular mat, which in turn affects the stress transfer between the granular pile and the soil. The granular mat is assumed as a rigid, smooth layer. Numerical evaluations are made to evaluate the effect of the presence of the granular mat on treated ground responses. The proposed method is validated by comparisons with other numerical models, finite element analysis and field test results.

Settlement response of a reinforced shallow earth bed

Abstract Presently, reinforcement in the form of extensible (e.g. geosynthetics) or apparently inextensible (e.g. metallic strips) inclusions are applied widely for strengthening shallow foundation beds. The effective use of reinforcement requires the understanding of its performance in combination with the strength characteristics of the soil to be reinforced. This paper is concerned with the development of a simple mathematical model to account for the membrane effect of a reinforcement layer on the load-settlement response of a reinforced granular fill-soft soil foundation system. The nonlinear loading pressure-settlement response of the granular fill and soft soil, respectively, for plane strain loading condition are incorporated into the formulation. Parametric studies for a uniformly loaded strip footing show that membrane action of reinforcement causes reduction in the settlement below the footing which is over and above the effect of granular fill.

Analysis of inextensible sheet reinforcement subject to transverse displacement/force: linear subgrade response

Abstract Reinforced earth fills, ground and slopes counteract the destabilizing forces by mobilising tensile forces in the reinforcement. In most studies, the pull-out resistance due to only axial pull is considered in the analysis for stability. However, the kinematics of failure clearly establishes the reinforcement being displaced obliquely. In this paper, a new approach is presented for the analysis of sheet reinforcement subjected to transverse force. Assuming a simple Winkler type response for the ground and the reinforcement to be inextensible, the resistance to transverse force is estimated. The response to the applied force is shown to depend not only on the interface shear characteristics of the reinforcement but also on the deformational response of the ground. A relation is established between pull-out resistance and transverse free end displacement. A parametric study quantifies the contributions of depth of embedment, length and interface characteristics of the reinforcement, stiffness of the ground, etc. on the over all response. It is established that reinforcement in dense granular fills subjected to a transverse pull generates pull-out resistances larger than purely axial pull-out capacity in reinforced earth construction.

Modified pasternak model for reinforced soil

The paper studies the influence of introducing a geosynthetic to into a geotechnical structure composed of a granular fill overlying a soft soil strata. It is shown that the geosynthetic component effectively reduces the magnitude of the settlement due to loads carried by the system through increasing the lateral confining pressure in the granular fill. The study is an extension of a previous study by the same authors, which presents a general model of the layered system and which reduces to Winkler, Filenko-Borodich and Pasternak models for particular cases.

Reinforced granular fill-soft soil system: confinement effect

Abstract A new mathematical model has been developed for the analysis of a reinforced foundation bed by incorporating the confinement effect of a single layer of reinforcement. It is quantified in terms of the average increase in confining pressure due to the reinforcement from which modified shear stiffnesses of the granular soil surrounding the reinforcement are obtained. Parametric studies for plane strain conditions show that the confinement effect of reinforcement leads to an improved mechanical response over the unreinforced granular fill-soft soil foundation system.

Reinforced granular fill-soft soil system: Membrane effect

Abstract A new model for a reinforced shallow foundation bed is proposed by incorporating the ‘rough membrane’ element for a single layer reinforcement. Mechanics of the rough membrane element with the assumption of horizontal shear stress transfer at the soil/reinforcement interfaces are explained and formulated. The model is generalized by incorporating nonlinear responses of the soft soil and of the granular fill under plane strain loading conditions. The membrane effect is quantified in terms of the improvement in the load-settlement responses of the composite foundation system over the unreinforced one. Parametric results indicate that reinforcement while in tension spreads the load over a larger area, leading to a reduction in the settlement beneath the footing.

Generalized stability analysis of reinforced embankments on soft clay

By modifying Janbus generalized procedure of slices (GPS), the stability of a reinforced embankment constructed on a non-homogeneous clay deposit of finite depth is analysed to compute the factor of safety. The non-circular critical surface corresponding to the minimum factor of safety is obtained by a sequential unconstrained minimization technique in conjunction with a conjugate direction method for a multidimensional search and a quadratic interpolation technique for a unidimensional search. The effect of the thickness of the desiccated zone and the variation of undrained shear strength of clay with depth on the factor of safety is considered in the analysis. The influence of the tensile reinforcement force on the location of the critical surface is presented. The effects of the tensile reinforcement force and its orientation as well as the number of reinforcing layers on the stability of embankment have also been studied. The results obtained from the present analysis are compared with solutions reported in the literature.

Ground subsidence due to nonlinear flow through deformable porous media

An analytical solution is presented for the problem of time rate of ground subsidence and piezometric pressure distribution caused by a sudden increase in the effective stress due to an instantaneous lowering of the groundwater table in an aquifer-aquitard-aquiclude system of finite thickness, obeying a nonlinear flow law of the type v = Min. Unlike for Darcian flow the degree of subsidence and the nondimensional pore pressure distribution are found to be dependent on both the amount of groundwater fall and the thickness of the deforming medium.