DEEP FOUNDATIONS: Pile Foundation.

Pile foundation is that type of deep foundation in which the loads are taken to a low level by means of vertical members which may be of timber, concrete or steel. Pile foundation may be adopted (i) instead of a raft foundation where no firm bearing strata exists at any reasonable depth and the loading is uneven, (ii) when a firm bearing strata does exist but at a depth such as to make strip or spread footing uneconomical, and (iii) when pumping of sub-soil water would be too costly or timbering to excavations too difficult to permit the construction of normal foundations.

Piles used for building foundation may be of four types:

(i) End bearing pile [Fig. 2.8 (a)]
(ii) Friction pile [Fig. 2.8 (b)]
(iii) Combined end bearing and friction pile [Fig. 2.8 (c)] and
(iv) Compaction piles [Fig. 2.8 (d)]

End bearing piles [Fig. 2.8 (a)] are used to transfer load through water or soft soil to a suitable bearing stratum. Such piles are used to carry heavy loads safely to hard strata. Multi-storeyed buildings are invanably founded on end bearing piles, so that the settlements are minimised.

FIG.  2.8  PILE FOUNDATIONS.

Friction piles [Fig. 2.8 (b)] are used to transfer loads to a depth of a friction-load-carrying material by means of skin friction along the length of the pile. Such piles are generally used in granular soil where the depth of hard stratum is very great.

Fig. 2.8 (c) shows a pile which transfers the super-imposed load both through side friction as well as end bearing. Such piles are more common, specially when the end bearing piles pass through granular soíls.

Compaction piles [(Fig. 2.8 (d)] are used to compact loose granular soils, thus increasing their bearing capacity. The compaction piles themselves do not carry a load. Hence they may be of weaker material (such as timber, bamboo sticks etc.)-sometimes of sand only. The pile tube, driven to compact the soil, is gradually taken out and sand is filled in its place thus forming a ‘sand pile’.