Pile cap

This article is about structural engineering. For a headgear, see field cap.

A pile cap is a thick concrete mat that rests on concrete or timber piles that have been driven into soft or unstable ground to provide a suitable stable foundation. It usually forms part of the foundation of a building, typically a multi-storey building, structure or support base for heavy equipment. The cast concrete pile cap distributes the load of the building into the piles. A similar structure to a pile cap is a "raft", which is a concrete foundation floor resting directly onto soft soil which may be liable to subsidence.

Design

A geological survey must be carried out first to establish the stability of the proposed site for the support cap. The cap thickness wil be determined by the load that it has to support and the number of piles used to distribute the load into the underlying soil. Other considerations, such as any localised loading that any part of the mat must support are taken into account. Some soil is so fluid in nature (such as clay and sand), that screw shaped piles are used, these resist the tendency for the pile to sink under the added weight of the cap and the load placed upon it. Standard engineering practice is followed with regard to the square area of the cap, thickness, and its design loading. From a set of appropriate calculations the sizes will be determined and the quantity of concrete required calculated.

Construction

The mat is made of concrete which is an aggregate of small rocks and cement. This mixture has to be supported by a framework to avoid sagging and fracture whilst setting. This process is known as shuttering and reinforcing. The materials used are long twisted steel bars between the piles held in shape by thinner tie wires. Once this steel mat is laid, timber is attached around the perimeter to contain the wet concrete mixture. Once poured, (usually as a series of small loads), the concrete is stirred to remove any air pockets that might weaken the structure when set. The concrete undergoes a chemical change as it hardens and this produces a lot of heat. Sometimes, if the mass of concrete is very large, pipes carrying refrigerant coolant are used in the mass to assist the setting process to prevent the concrete from cracking.