1. DETERMINATION OF THE LOAD CARRYING CAPACITY OF THE PILE
FOUNDATIONS
The load carrying capacity of piles comprises of two parts: skin friction and
end bearing. A wide range of method of analyses for determining skin friction
and end bearing is currently available for cast in-situ piles. Each of these
methods provides different outcome load capacities for driven piles and situ
piles. However, following equations may be used for all practical purposes:
Ultimate Skin Friction
A. Cohesive soil:
sus AcQ ××=α
where,
Qs = ultimate skin friction of pile (kN)
α = empirical adhesion factor [0.45 for cast in-situ piles and 0.50 to 1.0
for driven piles]
Cu = undrained cohesion (kN/m2
)
As = circumferential area of pile (m2
)
B. Cohesionless soil:
scorrs ANAQ ××= ; [A = 1.0 for situ pile and 2.0 for driven pile]
where,
Qs = ultimate skin friction of pile (kN)
Ncorr = corrected SPT for overburden pressure
As = circumferential area of pile (m2
)
Ultimate End Bearing Capacity
A. Cohesive soil:
pup AcQ ××=9
where,
Qp = ultimate end bearing capacity of pile (kN)
cu = undrained cohesion (kN/m2
)
Ap = tip area of pile (m2
)
B. Cohesionless soil:
pcorrp ANBQ ××= ; [B =120 for situ piles and 400 for driven piles]
where,
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2. Qp = ultimate end bearing capacity of pile (kN)
Ncorr = corrected SPT for overburden pressure
Ap = tip area of pile (m2
)
Design Load Capacity
Total ultimate load capacity of pile:
Qu = Qs + Qp
Design load capacity,
FS
Q
Q u
d =
where, FS = Factor of Safety (=4.0)
Courtesy: Professor Abdul J Khan, BUET
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