Reasons for providing Raft foundations

 
1.   Low Soil bearing capacity -----------i) Good soil but γ_d is low
                                                             ii) High groundwater table/ seepage pressure
                                                            iii) Filled up soil / organic soil like peat, muck  

2.  Very heavy loads such that overlapping of footing area

3.  Sum of area of individual shallow footings is more than 50 % of land area ( Thumb rule)

4.  Presence of loose compressible pockets. If these pockets are within range of pressure bulb of individual shallow footing, it can create a large settlement. But if we provide raft footing, this effect is minimized due to large pressure bulb area

5. To resist large hydrostatic uplift pressure

6.  Some structures by nature require raft foundations. Ex. Elevated water tank, chimney, silos etc.

7.   When structure and equipment to be supported are very sensitive to differential settlement