If the loads transmitted by the columns in a structure are so heavy or the allowable soil pressure so small that individual footings would cover more than about one-half of the area, it may be better to provide a continuous footing under all columns and walls. Such a footing is called a raft or mat foundation. Raft foundations are also used to reduce the settlement of structures located above highly compressible deposits. Since rafts are usually at some depth in the ground, a large volume of excavation may be required. If weight of the excavated soil is equal to the weight of the structure and that of the raft, and the centres of gravity of excavation and structure coincide, settlement would be negligible. Where complete compensation is not feasible, a shallower raft may be accepted if the net increase in loads in small enough to lead to tolerable settlement.