Load-carrying capacity of piles

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

2) 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.

3) In-situ penetration test:

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.

4) Pile load tests:

It is the most reliable and efficient method for estimation of pile capacity. Here the test pile is embedded and then loaded to failure. The pile capacity is related to the ultimate load or the load at which the settlements do not exceed the permissible limits.

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