Pile can not used singularly like as column because it is extremely difficult to drive the pile absolutely vertical and to place the foundation exactly at the centre. If there is eccentricity occurs between the load and pile, it may fails the structure. So to minimise this risk, piles are installed in groups with a min number of three. The load is transferred to the pile group through a RCC slab. This slab is known as a pile cap.
When the piles are spaced a sufficient distance, the group capacity may approach to the sum of individual capacities. On the other hand, if the piles are closely spaced, then the stress transmitted by the piles to the soil may overlap and it may reduce the load carrying capacity of the pile.