Nonlinear Analysis and Behaviour of Reinforced soil Structures

Abstract

Soil is a complicated material that behaves non-linearly and often shows anisotropic and time dependent behaviour when subjected to stresses. It exhibits non-linear behaviour well below failure condition with stress dependent stiffness. To describe behavior of a material suitably, it is necessary to establish constitutive models (constitutive relations or equations) representing mathematical descriptions of their behavior under external load. Reinforced soil is a composite material formed by the association of frictional soil and tension resistant elements in various forms such as sheets, strips, nets or mats of metal, synthetic fabrics, arranged in the soil mass to reduce or suppress the tensile strain which might develop under gravity and boundary forces. The past few decades have shown tremendous improvements in the reinforced soil systems from using stiff to the more flexible reinforcing elements and geosynthetic reinforcements. In most of the investigations, the effect of reinforcement on the behaviour of soil mass was studied by pull out tests, direct shear tests, equivalent homogeneous method. Existing methods have been found to over predict the stress in the reinforcement, because of which, designs based on these methods are very uneconomical. In this research, an effort has been made to improve our understanding of the internal stress-strain distribution in reinforced soil structures by carrying out linear and nonlinear analyses. Reinforced soil systems like Retaining wall, MSE Embankment, Reinforced soil foundation have been studied by developing programs in Fortran 77. The programs developed are RWPT-LIN, RWSW-LIN and RWSW-INT to study the unreinforced and reinforced soil retaining wall. Programs MSE-PRO and RSF-PRO have been developed to conduct studies on MSE wall and reinforced soil foundation. Linear and Nonlinear SSI of unreinforced soil have been carried out by developing programs SSI-LIN and SSI-NLIN. Linear and Nonlinear SSI of reinforced soil has been carried out by developing programs RSSI-LIN and RSSI-NLIN. The nonlinear analysis is being carried out using DruckerPrager constitutive relation. Goodman’s interface element has been used in the linear analysis of Retaining wall. The results obtained from the developed programs are validated either with experimental studies or by comparing with previously published work. Pilot studies have been carried out on unreinforced soil retaining wall and reinforced soil retaining wall subjected to point loads and self-weight separately. Studies have also been carried out on a mechanically stabilized MSE wall (AIT wall available in literature) to validate the developed program for linear and nonlinear analysis respectively. For the reinforced soil foundation, the bearing capacity ratio, effect of top layer spacing on bearing capacity, variation of modulus of subgrade reaction of reinforced soil under square and circular footings resting on a reinforced granular bed overlying weak soil have been studied. Parametric studies have been carried out to study the effects of type, number and length of reinforcement layers on foundation soil both numerically and experimentally. The developed programs are working well for the retaining wall, the MSE wall, the foundation soil and the soil structure interaction for unreinforced and reinforced cases. It has also been validated with the experimental studies on reinforced foundation soil. All the studies carried out have proved that the results of nonlinear analysis are closer to experimental and field studies. Interface element is found to enhance the performance of the reinforced soil and increases the compressive stresses. Soil structure Interaction (SSI) refers to the effect of soil and the foundation on its structure. It is the difference between the actual response of the structure and the response of the structure when fixed base is considered. In this connection, Soil structure interaction analysis for fixed base structure, unreinforced and reinforced soil foundation have been carried out both linearly and nonlinearly by adopting the macro element concept. Encouraging results have been obtained for the chosen structure and foundation soil. Fixed base structure shows very less moments, shear and axial forces as it is verystiff. The structure resting on unreinforced soil shows the maximum displacements, moments, shear and axial forces. The structure resting on reinforced soil shows a reduction in displacements, moments, shear and axial forces which may promote more economical design of structures.