Similar to shallow foundation, the pile foundation has to be safe against shear
failure and also the settlement needs to lie within the allowable limits. The
methods to determine the load-carrying capacity of pile foundation is
categorized into four groups;1) Static
method:

The static method provides the ultimate capacity of an individual pile and
depends on the characteristics of the soil. The ultimate load capacity is given
by,

Qu = Qp + Qs

Where, Qu = ultimate failure load

QP = point/base/tip resistance of the pile

Qs = shaft resistance induced by friction/adhesion between the soil and pile
shaft

Qp = qp.Ap

Qs = fs.As

Qp = ultimate bearing capacity of soil at pile tip

Ap = Area of pile tip

Fs = average unit skin friction between sand and pile surface

As = effective surface area of the pile in soil contact

1. Dynamic formulas:

The ultimate capacity of piles driven in certain types of soils is related to
the resistance against penetration developed during driving operation. The
ultimate load capacity formulas are based on the principle that the resistance
of a pile to further penetration by driving depends upon the energy imparted to
the pile by hammer. Assumption is made that the load-carrying capacity of the
pile is equal to the dynamic resistance during driving. However, the dynamic
formulas are not efficiently reliable.

1. In-situ penetration test:
[http://3.bp.blogspot.com/-O2PC77CW2Qs/VRQ8Hz2klXI/AAAAAAAABYw/_fOKqRvKkV4/s1600/images-11.jpg]

The pile capacity may be calculated from the results of in-situ standard
penetration test. Here the empirical formulas are applied for determining the
point resistance whereas the shaft resistance is found from the standard
penetration number (N). Alternative method can be by using the static formulas
after determining the N-value, since this value is related to the angle of
shearing resistance (). The cone penetration tests may be used to calculate the
pile capacity.