A number of methods have been proposed based primarily on two approaches of a simplified and truly elastic foundation, i) simplified elastic foundation ii) truly elastic foundation.

The modulus of subgrade reaction (Ks) is applicable to the case of load through a plate of size 30 × 30 cm or beam 30 cm wide on soil area.

The soil in a simplified elastic foundation is assumed to be continuous elastic medium obeying Hooke’s law.

According to IS: 2950-1965 code of practice, the maximum settlement should not exceed 40 mm in the foundation on clayey soils and 25 mm in foundations on clayey soils.

The value of Ks can be found corresponding to a different size and shape for both cohesive and cohesionless soils.

Conventional and flexible methods are to be used in case the foundation is comparatively flexible and the loads tend to concentrate over small areas.

The effect of shape utilizes the principal of superposition.

The behavior of a raft foundation being complicated, a number of simplifying assumption has to be made in design. There are two approaches for design-conventional method and the elastic method or the soil line method.

The conventional method is based on the following two basic assumptions:

The actual reinforcement provided should be twice of that worked out theoretically in the case of design by conventional method.