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.
Safe load capacity:
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
q_{a}=α.Č.A_{s}
Where, Č = cohesion in kN/m^{2}
α = reduction factor
A_{s} = surface area of pile stem in m^{2}.
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
q_{f}=ΣK x PD_{i} x tanδ x A_{si}
_{ D}
Where, K is the coefficient of earth pressure
PD_{i} is the effective overburden pressure in kN/m^{2} for i^{th} layer
δ is the angle of wall friction between pile and soil in degree
A_{si} is the surface area of pile stem in m^{2} in the i^{th }layer
End bearing component of ultimate load carrying capacity is given by the formula:
q_{b}=A_{b}(0.5D x γ x N_{γ}x PD x N_{q})
Where, D is the pile stem diameter in metre
γ is the effective unit weight of soil at tip level in Mg/m^{3}
PD is the effective overburden pressure at pile tip level in kN/m^{2}
N_{γ} is bearing capacity factor depending upon the angle of internal friction, , at pile tip level
A_{b} is the area of the pile tip in m^{2}.
Design of Pile Foundations:
P_{u}=0.4f_{ck}A_{c}+0.6f_{sy}A_{sc}
Where P_{u }= axial load on the member,
A_{sc} = area of longitudinal reinforcement for column,
A_{c} = area of concrete,
f_{ck} = characteristic compression strength of the concrete, and
f_{sy} = characteristic strength of the helical reinforcement, but not exceeding 415 N/mm^{2}.