PROBAWLISTtGANALY5S OF STRIP FOOTING ON LAYERED SOIL MEDIA USING MONTE CARLO SIMULATION

Abstract

Strip footing is a shallow footing subjected to uniformly distributed load. Usually these are analyzed under plain strain condition. Conventional methods of analysis and design of foundations employ the values of soil parameters as average values emanating from multiple tests conducted for this purpose. However, due to inherent variability in the soil strata, the soil properties cannot be considered as deterministic quantities. Further, consideration of mean value of any soil property leads to the use of quite high values of the factor of safety in the analysis and design. In view of these, it is more appropriate to consider the soil parameters as random variables. In the present work, the analysis-df a strip footing resting on a three layered soil media has been carried out considering the soil parameters as a random variables using Monte Carlo simulation. The deterministic model used for this purpose employs modulus of elasticity as the soil parameter, which has been considered as random variable having lognormal distribution. Influence of various parameters like relative thickness of soil layers and relative elastic modulii has been studied by means of a detailed parametric study. Factors of safety with respect to settlement of the footing and the interfacial stresses have been determined and these were related to the associated risk factor and coefficient of variation of the random variable. A detailed parametric study revealed that for a given risk level, the factors of safety can be expressed in terms of the coefficient of variation of elastic modulus and only mildly upon other geometric parameters of the soil — foundation system. This facilitated the development of closed form equations for the upper bounds on factors of safety only in terms of allowable risk of failure and the coefficient of variation of elastic modulus.