DHARM
PILE FOUNDATIONS 657
both theoretically and by load tests (Bjerrum, 1957). Both theory and experience demonstrate
that buckling rarely occurs because of the effective lateral support of the soil; it may occur only
in extremely slender piles in very soft clays or in piles that extend through open air or water.
Therefore, the ordinary pile in sand or clay may be designed as though it were a short column.
The pile transfers the load into the soil in two ways. Firstly, through the tip-in compres-
sion, termed ‘end-bearing’ or ‘point-bearing’; and, secondly, by shear along the surface, termed
‘skin friction’. If the strata through which the pile is driven are weak, the tip resting on a hard
stratum transfers most part of the load by end-bearing; the pile is then said to be an end-
bearing pile. Piles in homogeneous soils transfer the greater part of their load by skin friction,
and are then called friction piles; however, nearly all piles develop both end-bearing and skin
friction.
The following is the classification of the methods of determining pile capacity:
(i) Static analysis (ii) Dynamic analysis
(iii) Load tests on pile (iv) Penetration tests
The first two are theoretical approaches and the last two are field or practical approaches.
16.5.1Static Analysis
The ultimate bearing load of a pile is considered to be the sum of the end-bearing resistance
and the resistance due to skin friction:
Qup= Qeb + Qsf ...(Eq. 16.1)
where Qup = ultimate bearing load of the pile,
Qeb = end-bearing resistance of the pile, and
Qsf = skin-friction resistance of the pile.
However, at low values of load Qeb will be zero, and the whole load will be carried by
skin friction of soil around the pile. Qeb and Qsf may be analysed separately; both are based
upon the state of stress around the pile and on the shear patterns that develop at failure.
Meyerhof (1959) and Vesic (1967) proposed certain failure surfaces for deep foundations.
According to Vesic, only punching shear failure occurs in deep foundations irrespective of the
density index of the soil, so long as the depth to width ratio is greater than 4 (This is invariably
so for pile foundations).
Qeb = qb. Ab ...(Eq. 16.2)
Qsf = fs As ...(Eq. 16.3)
Here, qb = bearing capacity in point-bearing for the pile,
fs = unit skin friction for the pile-soil system,
Ab = bearing area of the base of the pile, and
As = surface area of the pile in contact with the soil.
The general form of the equation for qb presented by various investigators is:
qb = cNc +
1
2
γ bNγ + q. Nq ...(Eq. 16.4)
which is the same form as the bearing capacity of shallow foundations.