Question

The design of shallow foundation is governed by the bearing capacity of soil. The bearing capacity of soil is the maximum average contact pressure between the foundation and the soil which should not produce shear failure in the soil. However, some cases at site, the suitable bearing capacity only can be found at 3 m depth from the surface or more. Based on your understanding, discuss this situation and how to resolve the issue in perspective as a consultant engineer

Answer 1

The issue depends on type of construction work which the site is going have in the future.Based on the type of building the foundation depth is usually decided. Let us say based on geotechnical results it has been found out that good strata is availabe 3m below the ground level and above that say black cotton soil is present.

Case 1: Building to be constructed is not a high rise building :- If this is the case then loads on the foundation will be compartively on lower side. If areal extent of the side is on the higher side then excavation of entire soil for a depth of greater than 3 m is not feasible. Then in that case Raft foundation(floating foundation) can be be designed. As the name says the foundation is floating, hence it is not usuallly affected by underlaying soil conidtions. Raft founadtion being a complete slab of concrete all the criteria for single shear, double shear and punching shear failure are satisfied in general.

Case 2: Building to be constructed is a high rise building :- If sufficient strength is not availabe even at 3m, then with increase in depth excavation becomes more and more costly. In this case deep foudations such as end bearing piles, friction piles or combined can be constructed to bypass all the loose soil and tansfering the load to high strength rock at the bottom.

Based on feasibility studies and load calculations the depth and type of foundation can be decided by the consulting engineer.