1. OVERVIEW OF FOUNDATIONS
The word foundation is derived from a latin word fondare meaning to set or ground on something solid. A foundation is that part of a structure which transmits the weight of the structure to the ground in a manner that the soil below does not fail in shear and the settlement is within the safe limits. Foundations are broadly classified into two categories: • Shallow Foundation
• Deep Foundation
A shallow foundation, according to Terzaghi is one whose width is greater than its depth ie. D/ B < or = 1. The main types of shallow foundations are footings are footings and raft or mat foundation. Footings can be further subdivided as shown below: • Strip Footing
• Spread or Isolated footing
• Combined Footing
• Strap or Cantilever Footing.
The loads must be "spread" to the soil in a manner such that its limiting strength is not exceeded and resulting deformations are tolerable. Shallow foundations accomplish this by spreading the loads laterally, hence the term spread footing. Whereas spread footing (or simply footing) supports a single column, a mat is a special footing used to support several randomly spaced columns or to support several rows of parallel columns and may underlie a portion of or the entire building. The mat may also be supported, in turn, by piles or drilled piers. Foundations supporting machinery and such are sometimes termed bases. Machinery and the like can produce substantial load intensity over a small area, so the base is used as a load-spreading device similar to the footing. Deep foundations are analogous to spread footings but distribute the load vertically rather than horizontally. The terms drilled pier and drilled caisson are for the pile type member that is constructed by drilling a 0.76m diameter hole in the soil, adding reinforcing as necessary, and backfilling. On the other hand, a deep foundation is that which transmits the load at considerable depth below the ground surface. The main distinction between a deep foundation and a shallow foundation is generally made according to Terzaghi’s criterion which as discussed earlier termed shallow foundation as that which its depth equals or is less than its width. A very prominent example of deep foundation that will be discussed extensively in this work is the pile foundation.
WHAT IS A PILE?
A pile is a slender structural member made of steel, concrete or wood which transfer the load to a deeper soil or rock of larger bearing capacity. Piles are generally driven, drilled or jacked into the ground. Depending on the type of soil, pile material, load transmitting characteristics etc., piles are classified accordingly as will be discussed later. Alongside piles, pile cap is a vital component of a pile foundation. Pile foundations have been used as load carrying and load transferring systems for many years. In the early days of civilization, from the communication, defense or strategic point of view villages and towns were situated near to rivers and lakes. It was therefore important to strengthen the bearing ground with some form of piling. Timber piles were driven in to the ground by hand or holes were dug and filled with sand and stones. In 1740 Christoffoer Polhem invented pile driving equipment which resembled to days pile driving mechanism. Steel 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 exploit lands with poor soil characteristics. This has led to the development and improved piles and pile driving systems. Today there are many advanced techniques of pile installation. 1.3
FUNCTION OF PILES
Just like other types...
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