NEED & FUNCTIONS OF PILE FOUNDATION
When it becomes impossible to provide the suitable surface foundation for a structure; the use of pilefoundations becomes necessary, this situation arose from either the soil condition or the order of bottom layers, the nature of the loads transferred to the soil or the nature of the site and operational conditions
Many factors prevent the selection of surface foundation as a suitable foundation such as the nature of soil and intensity of loads, we use the piles when the soil have low bearing capacity or in building in water like bridges and dams
The main components of the foundation are the pile cap and the piles. Piles are long and slender members which transfer the load to deeper soil or rock of high bearing capacity avoiding shallow soilof low bearing capacity. The main types of materials used for piles are Wood, steel and concrete. Piles made from these materials are driven, drilled or jacked into the ground and connected to pile caps. Depending upon type of soil, pile material and load transmitting characteristic piles are classified accordingly.
Pile foundations have been used as load carrying and load transferring systems for many years. !n the early days Timber piles were driven in to the ground by hand or holes were dug and filled with sand and stones. “teel piles have been used since 1800 and concrete piles since about 1900.
The industrial revolution brought about important changes to pile driving system through the invention of steam and diesel driven machines. More recently, the growing need for housing and construction has forced authorities and development agencies to e’ploit lands with poor soil characteristics. This has led to the development and improved piles and pile driving systems. Today there are many advanced techni(ues of pile installation.
Function of Pile Foundation:
As other types of foundations, the purpose of pile foundations is:
– To transmit the buildings loads to the foundations and the ground soil layers whether these loads vertical or inclined
– To install loose cohesion less soil through displacement and vibration.
– To control the settlements; which can be accompanied by surface foundations.
– To increase the factor of safety for heavy loads buildings
The selection of type of pile foundation is based on site investigation report. Site investigation report suggests the need of pile foundation, type of pile foundation to be used, depth of pile foundation to be provided. The cost analysis of various options for use of pile foundation should be carried out before selection of pile foundation types.
Unless the ground condition is rocks, for heavy construction and multi-storied buildings, the bearing capacity of soil at shallow depth may not be satisfactory for the loads on the foundation. In such cases, pile foundation has to be provided. The number of piles in a pile groups required is calculate from the pile capacity of single pile and the loads on the foundation. Piles are a convenient method of foundation for works over water, such as jetties or bridge piers.
Important Factors To Consider During Pile Design
A lightly loaded column may, in some instances, require only a single pile. However, since under field conditions the actual position of a pile may be as much as several inches from its planned location, an eccentric loading can hardly be avoided. Consequently, the heads of single piles are usually braced in two directions by grade beams. If only two piles are needed, their heads may be connected by a concrete cap braced by grade means in only one direction, perpendicular to the line joining the two piles Fig.1(b). Clusters containing three or more piles are provided with reinforced concrete caps, as shown in Fig.1(c), and are considered stable without support by grade beams.
Vertical piles may also be used to resist lateral loads as for example, beneath a tall chimney subject to wind. Compared to their axial capacity, the lateral capacity is usually small. Where large lateral loads are to be used (Fig.1d.) Batters of 4 horizontal : 12 vertical represent about the greatest inclination that can be achieved with ordinary driving equipment. Economy usually favors smaller inclinations even if more piles have to be battered.