Pile cap
Encyclopedia
A pile cap is a thick concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 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
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...

 pile cap distributes the load of the building into the piles
Deep foundation
A deep foundation is a type of foundation distinguished from shallow foundations by the depth they are embedded into the ground. There are many reasons a geotechnical engineer would recommend a deep foundation over a shallow foundation, but some of the common reasons are very large design loads, a...

. 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
Subsidence
Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...

.

Design

A geological survey
Geological survey
The term geological survey can be used to describe both the conduct of a survey for geological purposes and an institution holding geological information....

 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
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...

. 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 under goes 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
Refrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...

 coolant
Coolant
A coolant is a fluid which flows through a device to prevent its overheating, transferring the heat produced by the device to other devices that use or dissipate it. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, and chemically inert, neither causing nor...

are used in the mass to assist the setting process to prevent the concrete from cracking.
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