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A Finite Element Study of Beam on Reinforced Granular Beds with Sand Drains

Sarvesh Chandra
Professor, Dept. of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur, India
C. S. Upadhyay
Associate Professor, Dept. of Aerospace Engineering, Indian Institute of Technology Kanpur, Kanpur, India
Imran Ahmad
Former Post Graduate Student, Dept. of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur, India
Arindam Dey
Research Scholar, Dept. of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur, India

Source IITB.AC.IN

Keywords: Geosynthetic reinforced granular fill-soft soil system, Finite element method, Sand drains

ABSTRACT: This paper presents the settlement analysis of a beam resting on geosynthetic reinforced granular fill-soft soil system. Each subsystem of the reinforced fill-soil system is idealized by elastic membrane, Pasternak shear layer, Winkler springs and dashpots, as applicable. The suggested model incorporates various aspects of the behavior of the geosynthetic-reinforced granular fill-soft soil system such as horizontal stress induced in the granular fill, the compressibility of the granular fill, and the time-dependent behavior of the subgrade. The differential equations governing the settlement response of the beam resting on two layered reinforced foundation soil has been formulated by incorporating deformation compatibility conditions. The numerical solutions are obtained using Finite Element Method and results are presented in non-dimensional form. The parametric studies are carried out to enumerate the effects of parameters on the settlement response of the system. Results indicate that over a large number of various parameters under large deformation the proposed model evaluate the settlement of the system and horizontal displacement of membrane with reasonable accuracy. It is observed that compressibility, shear modulus and thickness of granular fill, pre-stressing and tension modulus of reinforcement have appreciable influence on the settlement of the system and horizontal displacement of membrane. It is observed that the horizontal displacement of the geosynthetic membrane is negligible as compared to that of the vertical settlement. The model is also analyzed for the case of a sand drain in the soft soil which indicated that the elapsed time and the radius of the sand drain significantly affects the settlement response of the system.

A foundation constructed on soft soils may experience excessive settlement and possible bearing capacity failure under a surcharge load. One technique that is mostly used nowadays to improve the strength of soft foundation soils is the placement of engineered granular fills containing geosynthetic reinforcement (e.g. geogrid, geotextiles) on the soft soil. Model tests carried out by various researchers (Fragaszy and Lawton, 1984; Love et al. 1987; Mandal and Sah, 1993; Adams and Collin, 1997) exhibited improved behavior of the soft soil system with respect to their load settlement response whenever a geosynthetic, with or without pretension, is provided along with the granular fill. The calculation of settlement and ultimate bearing capacity of the geosynthetic reinforced granular fill over soft soil, along with the requirement of stability in terms of settlement, is an important issue for the design engineers. Such issues can be, and are commonly taken care of by using the approach of analyzing the beams on elastic foundations and obtaining the flexural response of the system under different loading conditions, and by using different solution techniques.

  • Introduction
Several concepts have been developed to explain the reinforcing mechanism of the reinforcement used in the soil. One of the approaches to study the interaction problem is by idealizing the behavior of the soil. The linear
elastic idealization of the supporting soil medium is usually represented by one-parameter or two-parameter mechanical or mathematical model, such as Winkler model, Filolenko – Borodich model, Pasternak model, Kerr model (Kerr, 1964) etc. Over times, various combinations of the above simplified models with varying degree of complexity has come into existence in order to study the influence of the material properties of the soil such as shape, size, configuration, stress history, soil moisture, and permeability on the behavior of foundation systems. Several analytical and numerical works have been carried out on this aspect by various researchers (Biot, 1937; Gazis, 1958; Vesic, 1961; Cheung and Nag, 1968; Rao et al., 1971; Sharma and Dasgupta, 1975; Giroud and
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