Wednesday, October 24, 2012

DEEP FOUNDATIONS: Pier Foundation (drilled caisson foundation).

A Pier foundation consisis of a cylindrical column of large diameter to support and transfer large super-imposed loads to the firm strata below. The difference betwcen pile foundaiion and pier foundation lies in the method of construction. Though pile foundations transfer the load through friction and/or bearing, pier foundations transfer the load only through bearing. Generally, pier foundation is shallower in depth than the pile foundation. Pier foundation is preferred in a location where the top strata consists of decomposed rock overlying a strata of sound rock. In such a condition, it becomes difficult to drive the bearing piles through decomposed rock. In the case of stiff clays, which offer large resistance to the driving of a bearing pile, pier foundation can be conveniently constructed.

Pier foundations may be of the following types:

(i) Masonry or concrete pier
(ii) Drilled caissons.

These are shown in Fig. 2.9.

When a good bearing stratum exists upto 5 m below ground level, brick, masonry or concrete foundation piers in excavated pits may be used [Fig. 2.9 (a)]. The size and spacing of the piers depends upon the depth of hard bed, nature of overlying soil and super-imposed loads.

The terms drilled caissons, foundation pier or sub-pier are interchangeably used by engineers to denote a cylindrical foundation.

A drilled caisson is largly a compressed member subjected to an axial load at the top and reaction at the bottom. Drilled caissons are generally drilled with the mechanical means. Drilled caissons may be of three types : (i) concrete caisson with enlarged bottom [Fig. 2.9 (b)], (ii) caisson of steel pipe with concrete filled in the pipe [Fíg. 2.9 (c)]  and (iii) caisson with concrete and steel core in steel pipe [Fig. 2.9 (d)].


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