What is raft foundation and what are its advantages??Are this type foundations costly??

Question

What is raft foundation and what are its advantages??Are this type foundations costly??

Answer 1

A raft foundation is essential a mat foundation.  The purpose of a mat foundation is when you a building built on a site with low soil bearing conditions or when a foundation.  A raft foundation is use to distribute the buidling pressure over a large area so the soil can bear the stress.  Think of it this way.  If you had a building that has a site soil capacity of two thousand pounds per square foot and the building weight is 8 thousand pounds.  What foundation would put less stress on the soil?  a 500 square foot foundation or a 1000 square foot foundation.  A 500 square foot will give you a stress of 16 pounds per square foot while the 1000 will give you 8 pounds per square foot.  Less stress is imposed on the building with the large foundation.  Also, the raft foundation sometimes have haunches  that go below the slab portion to resist punching shear from column or large concentration loads or wide beams along edges and through the intermediate in the slab to resist bending forces due to expansive soils (clay soils) and bending forces imposed the structure above.