BEHAVIOUR OF FOUNDATIONS ON IMPROVED GROUND UNDER STATIC AND MOVING LOADS

Abstract

The soft soils are often reinforced either only with stone columns or stone columns with geosynthetic. The geosynthetic-reinforced granular fill are often placed on soft soils / soft soils improved by stone columns for the construction of shallow footings, embankments, unpaved roads, railroads, oil drilling platforms, heavy industrial equipments, parking lots etc. in many parts of the world. In this thesis an attempt has been made to study the response of a beam resting on these reinforced earth beds. A modeling approach has been adopted and each sub-system of the reinforced soft soil system is idealized by the mechanical foundation model elements, such as soft soil by Kelvin-Voigt body, stone columns by Winkler springs, granular fill by Pasternak shear layer and the geosynthetic layer by rough elastic membrane, which are commonly adopted for solving many soil-foundation-structure interaction problems in geotechnical engineering. Various aspects of case the behavior of the granular fill-stone column improved soft soil system and the geosynthetic-reinforced granular fill-stone columnreinforced soft soil system are incorporated, which may be necessary in many field situations. These include the flexural rigidity of foundation, ultimate resistance of soft soil, stone columns and the granular fill layer, nonlinearity in the behavior of soft soil, stone columns and granular fill, interfacial friction coefficient, diameter and spacing to diameter ratio of stone columns, relative stiffness of stone columns with respect to surrounding soft soil, average degree of consolidation, magnitude and velocity of the applied load, damping etc. Three models have been developed and presented in this study. In the first model, the foundation, idealized as a beam of finite length has been analyzed which is resting on a granular fill layer on top of stone column treated soft soil and subjected to static concentrated loads. Nonlinear behavior of soft soil, stone columns and the granular fill layer has been presented by hyperbolic constitutive relations. The foundation has been subjected to two concentrated loads on its edges (one bay footing) in one and to five concentrated loads (four bay footing) in another case of analysis. Considering the equilibrium of different elements under the applied load, the equations governing the response function of the model are derived and solved with the help of appropriate boundarv conditions. Numerical solution of these equations has been obtained employing finite difference method. The results have been presented in non-dimensional form and various parametric studies have been conducted to observe the influence of magnitude of applied loads, flexural rigidity of foundation beam, diameter and spacing to diameter ratio for stone columns, ultimate resistance of soft soil, stone columns and the granular fill, relative stiffness of stone columns with respect to surrounding soil and the average degree of consolidation on the response of the model. In the second model, combined footing, idealized as a beam of finite length has been analyzed. The beam is resting on geosynthetic-reinforced granular fill-stone column improved soft soil system and subjected to five concentrated loads (four bay footing). Nonlinear behavior of soft soil, granular fill and the stone columns has been given due consideration in the analysis. The geosynthetic has been assumed as a rough elastic membrane and inextensible in nature. The governing differential equations have been derived considering the equilibrium of each sub system of the model and have been solved applying appropriate boundary and continuity conditions. The numerical solutions have been obtained by using iterative finite difference scheme and all the results have been presented in nondimensional form. Various parametric studies have been carried out to observe the effect of magnitude of applied loads, flexural rigidity of foundation beam, diameter and spacing to diameter ratio for stone columns, ultimate resistance of soft soil, stone columns and the granular fill, relative stiffness of stone columns with respect to surrounding soil and the average degree of consolidation on the response of proposed model. The influence of inclusion of geosynthetic layer in the granular fill has also been studied. The third model deals with an infinitely long beam resting on granular fill-stone column improved soft soil system / geosynthetic reinforced granular fill-stone column improved soft soil system subjected to a concentrated load moving with constant velocity. In this model the geosynthetic layer has been assumed as an inextensible membrane. The governing differential equations have been derived considering the equilibrium of each sub system of the model and have been solved applying appropriate boundary conditions. The numerical solutions have been obtained by using iterative finite difference scheme and all the results have been presented in non-dimensional form. The model has been first validated with the available studies. The influence of employing the stone columns in the soft soil has been studied along with the influence of inclusion of geosynthetic layer in the granular fill placed on top of soft soil system. The effect of magnitude and velocity of applied load, viscous damping, diameter and spacing to diameter ratio of stone columns, ultimate resistance of soft soil, granular fill and the stone columns, interfacial friction coefficient, lateral earth pressure coefficient, average degree of consolidation and the relative stiffness of stone columns with respect to surrounding soil has been studied on the response of the proposed model. II! Wherever possible the flexural responses of the beams have been compared with the studies of similar nature, suggested in past for all the three models. The parametric studies have been carried out for each model to bring out clearly the influence of the various parameters. From the present study, it has been found that the proposed foundation models, for the beams of finite as well as infinite lengths resting on granular fill-stone column treated ground / geosynthetic-reinforced granular fill-stone column improved soft soil system subjected to concentrated loads; stationary and moving, are well suited to evaluate the responses over a large range of various parameters. Comparisons of the settlement predictions by the proposed foundation models with those obtained using the existing models showed similar trend of results for common model parameters. The magnitude of applied load and the finite flexural rigidity of the foundation, in the first model, have been found to affect the response of the proposed model significantly. Diameter and spacing to diameter ratio of stone columns have influenced the response quite significantly. Optimum value of spacing to diameter ratio for stone columns has been suggested based on detailed parametric study. Ultimate resistance of soft soil and stone columns, relative stiffness of stone columns and the average degree of consolidation affected the response of the model significantly. However, the ultimate resistance of granular fill in shear and its shear modulus has not found to influence the response of soil-foundation system. Similar observations with respect to above mentioned parameters have been made with reference to second model in which a geosynthetic layer has been sandwiched in the granular fill layer. Apart from this detailed parametric study, the influence of inclusion of geosynthetic layer has also been studied. It was observed that with the inclusion of geosynthetic layer in the soil-foundation system, the deflection further reduces to large extent. Inclusion of geosynthetic layer has been found to be more effective at larger loads and for smaller values of flexural rigidity of foundation. Relative stiffness of stone columns with surrounding soil and the ultimate shear resistance of granular fill have not been found to influence the response of soil-foundation system. In the third model an infinitely long beam resting on granular fill-stone column treated ground ' geosynthetic-reinforced granular fill-stone column improved soft soil system subjected to a concentrated load moving with constant velocity has been analyzed. The response has been found to be greatly affected by magnitude of applied load intensity, higher values oi' velocity of the applied load, viscous damping, diameter and spacing of stone IV columns, ultimate resistance of soft soil and the stone columns, relative stiffness of stone columns and the average degree of consolidation. Shear modulus of the granular fill and its ultimate resistance in shear, interfacial friction coefficients have not been found to influence the response of the model significantly. The geosynthetic has been found to act more as a separator and drainage measure rather than as the reinforcement.