PILE FOUNDATIONS

Introduction:

Piles are used to carry vertical loads through weak soil to dense strata having high bearing capacity. In normal ground conditions, they can resist large uplift and horizontal loads, hence, can be used as foundations of multistoried buildings, transmission line towers, retaining walls, bridge abutments.

Piles will derive their load carrying capacities partly through adhesion and partly through friction between pile shaft and the surrounding soil and also through end bearing between pile tips and the bottom soil. In cohesive deposits, ultimate load through adhesion is given by the formula as per

B-2 of BIS: 2911 (Part-I/Sec 2) – 1979

qa=α.Č.As

Where,  Č = cohesion in kN/m2

α = reduction factor

As = surface area of pile stem in m2.

In granular deposits, ultimate load through friction is given by the formula as per B.1.1 of BIS: 2911 (Part-I/Sec 2) – 1979:

i=1

qfK x PDi x tanδ x Asi

D

Where, K is the coefficient of earth pressure

PDi is the effective overburden pressure in kN/m2 for ith layer

δ is the angle of wall friction between pile and soil in degree

Asi is the surface area of pile stem in m2 in the ith layer

End bearing component of ultimate load carrying capacity is given by the formula:

qb=Ab(0.5D x γ x Nγx PD x Nq)

Where, D is the pile stem diameter in metre

γ  is the effective unit weight of soil at tip level in Mg/m3

PD is the effective overburden pressure at pile tip level in kN/m2

Nγ is bearing capacity factor depending upon the angle of internal friction,  , at pile tip level

Ab is the area of the pile tip in m2.

Design of Pile Foundations:

1. First obtain the load taken by each pile
2. The reinforcement in pile is obtained by the following equation:

Pu=0.4fckAc+0.6fsyAsc

Where Pu = axial load on the member,

Asc = area of longitudinal reinforcement for column,

Ac = area of concrete,

fck = characteristic compression strength of the concrete, and

fsy = characteristic strength of the helical reinforcement, but not exceeding 415 N/mm2.

1. Pile caps are to be designed based on the principles involved in the design of footings and rafts. Its depth is based on shear and development length for the column bars.
2. The beam interconnecting all the pile caps of a system of piles is referred to as a grade beam. If grade beams are supported during construction, max. B.M. is taken as wl2/50. If grade beams are not supported during construction, max. B.M. is taken as wl2/30.