It is known that the modulus of subgrade reaction ks is not a soil constant but is a function of the contact pressure and settlement. It depends on foundation loads, foundation size and stratification of the subsoil. The main modulus of subgrade reaction ksm for a rectangular foundation on layered subsoil can be obtained from dividing the average contact pressure qo over the settlement so under the characteristic point on the foundation, which had been defined by Graßhoff (1955). Clearly, this procedure is valid only for rectangular foundations on a layered subsoil model. Determining the main modulus of subgrade reaction ksm for irregular foundation on an irregular subsoil model using another analysis is also possible by ELPLA.

In this example, settlement calculations at the characteristic point on the raft, using Steinbrener's formula (1934) for determining the settlement under the corner of a rectangular loaded area with the principle of superposition, are used to verify ELPLA analysis for determining the main modulus of subgrade reaction ksm. Consider the square raft in Figure 17, which has area of Af = 8 *12 [m ] and thickness of d = 0.6 [m].