Pile foundations consist of piles that are dug into the soil till a layer of stable soil is reached. Pile foundations transfer building load to the bearing ground with greater bearing capacity. Pile foundations are useful in regions with unstable upper soil that may erode, or for large buildings.
Pile foundations have been used for many years, for carrying and transferring of loads to soil considered to be weak in structure due to the soil conditions. In the early stages of development, villages and towns were located in the close vicinity of lakes and rivers due to the availability of water, and, also, to ensure proper protection of the area. Therefore, the weak bearing ground was reinforced by the use of timber piles that were either manually forced into the ground, or fixed in holes that were filled with stones and stand. The primitive methods of pile installation were modified after the industrial revolution, and techniques of installation by steam or diesel driven machines were introduced. With the advancement in the technologies of soil mechanics and other related disciplines, superior piles and pile installation systems have been developed.
Piles are generally used when the bearing capacity of the soil is considered to be inadequate for the structural load of heavy construction. The piles transfer the load to the solid ground located at a depth. If the shallow soil is not stable, or the settlement estimated is not tolerable, then the use of piles may be the only practicable solution. Furthermore, if the conditions of soil necessitate extensive ground development that is expensive, the use of piles may be more economical. Use of piles is not only beneficial in unstable shallow soil, but also helpful in normal ground conditions to withstand vertical and horizontal loads, or foundations over water like jetties.
The pile foundations should be carefully designed in accordance with the soil and load conditions, and the cost. To ensure the reliability of the piles foundation that should perform as a unit, the pile caps should be joined with beams or a reinforced concrete slab that could perform in tension and compression. The piles should be designed to carry axial, shear, and bending stresses that may develop by the relative horizontal movement of piles between the layers in the soil. Piles can be made from various materials, like steel, timber, and concrete, each possessing different characteristics that should be considered.
The pile installation technique is an important feature in the design of pile foundations that should be selected carefully by taking into consideration the resistance to achieve the desired penetration, pile characteristics, space available at the site, and the disturbance due to noise. The generally used method for the installation of piles is a pile driver. These use the dropping weight technique in which an appropriate weight is hoisted at a suitable height in a sleeve, is released to hit the pile head, and this then drives the pile in the ground. The technique can be single-acting dropping weight or double-acting, driven by steam, compressed air, or hydraulically. In the double acting technique, compressed air is also used during the descending stroke of the weight.