Analytical Study on the Influence of Rigidity of Foundation and Modulus of Subgrade Reaction on Behaviour of Raft Foundation

Abstract

Shallow foundations are commonly used for supporting structures unless soil, loading and serviceability requirements necessitate deep foundations. Shallow foundations may be designed as rigid or flexible foundations depending on loading conditions as well as relative stiffness of foundation and supporting soil subgrade. For design of flexible foundations, soil is modelled as per Winkler approach or as a continuum. For analysis of most foundations, Winkler approach is found acceptable, wherein modulus of subgrade reaction is evaluated and applied as soil springs. The different factors which affect design of foundation include superstructure geometry and stiffness; type, magnitude and location of loading; rigidity of foundation and modulus of subgrade reaction. The current study attempts to understand the influence of rigidity of foundation and modulus of subgrade reaction on behaviour of raft foundation. A multistoreyed structure is analysed in Staad Pro V8i software for different combinations of foundation rigidity and modulus of subgrade reaction; and parameters such as base pressure, settlement, shear stress and bending moment have been compared. It is concluded from the study that modulus of subgrade reaction has higher influence on variation in foundation base pressure as compared to rigidity of foundation. It is also noted that impact of variation in modulus of subgrade reaction on structural design of foundation is negligible. However, the rigidity of foundation influences the shear stress and bending moment in foundation. It is opined that such studies would help in developing decision matrix to account for various parameters in optimization of foundation design.