Impact of Coefficient of Subgrade Reaction on Raft Foundation Design

Winkler method is widely used in computer analysis of raft foundations
due to its simplicity. In this method, soil is represented as medium
consisting of infinite number of elastic, identical, but independent
springs. The stiffness of the springs is the coefficient of subgrade
reaction, ks. The inadequacy of this modeling to account for the elastic
continuity of soil is its serious limitation. The computation of ks is very
difficult as it depends on several factors like stiffness of raft and
superstructure, soil properties and depth of embedment of foundation.
Hence the accuracy of analysis of raft foundation depends on the
estimate of representative ks considering the above factors. The present
study was carried out to investigate the impact of changing coefficient
of subgrade reaction values in the design of raft foundation. General
trends were established for the variation of bending moment, shear and
settlement of raft with change in values of ks. The Winkler method is
then compared with continuum modeling of soil. Finally two rafts were
designed varying ks values and its impact on structural design and cost
of execution is presented.