Building foundations are supported on different types of soils encountered under construction site. We are always suspicious about loose sands, clay deposits, collapse potential soils etc. and always look for firm ground surface to avoid deep foundation. Even when there have no alternative to avoid deep foundation, we will try to support tip of such foundation over a firm deposit. Such firm deposit is rock; now should we fill safe when building is founded on rock?
Rock deposit is not always safe, rock may be weathered out, may be disturbed by construction, some may be of expansive nature and various causes should be investigated before placing foundation. But extent of investigation should be based on type of structure;here we will differentiate structure by loading i.e. for lightly loaded and heavily loaded building.
Lightly loaded foundation:
In case of light buildings, foundation performance on sound and hard rock is very good. The total settlement and maximum value of differential settlement should be essentially zero. Sometimes, rocks supporting concrete foundation have more strength than foundation materials (i.e. concrete). When such light buildings are supported on sound and hard rock, comprehensive investigation is not considered important in usual condition and the reference is used from building code; the allowable bearing capacity recommended by international building code (TABLE 1806.2 in IBC) can be taken as design reference.