What are the reasons of attribution of raft foundations by rigid method?

The overdesign of raft foundations by rigid method can be attributed to three major reasons:

a. There is uncertainty in conventional analysis; even more costly sophisticated analyses have their short comings,

b. The extra cost of overdesign of the raft foundation will generally be small relative to the total project cost, and

c. Additional safety factor is provided for the additional cost.

The raft can also be designed as a plate or beams on elastic foundation using either finite difference or finite element method. However these methods require the use of a computer.