Pile load capacity calculation is done to find the ultimate load the pile foundation can support when loaded. It is also known as the bearing capacity of piles. Pile load capacity calculation is done for single pile or a group of piles based on the requirement of number of piles for the given load or size of foundation. We will discuss here the load carrying capacity of both single pile and group of piles.
SINGLE PILE CAPACITY CALCULATION:
The allowable resistance to compression Rac of a single pile is provided by the end bearing, Feb and the skin friction for each strata,Fsf. Thus,
Therefore, the maximum compressive service load that can be supported by a single pile is its total resistance less the self weight of the pile, W. Thus
A pile can also resist a tensile load. The maximum tensile service load, that can be resisted by pile is
Values of the end bearing and skin friction resistance are obtained from soil exploration, pile driving energy and test loadings. These ultimate values are divided by a partial factor of safety of between 2 and 3 to give the allowable values Feb and Fsf.
The horizontal capacity of a single pile is limited by maximum deflection that the structure can accommodate and also by the structural capacity of the pile.
There are several methods of determining the modulus of subgrade reaction (kN/m3) from which maximum horizontal capacity for a given deflection can be determined.
GROUP PILE CAPACITY CALCULATION:
Piles are generally arranged in groups to support heavy vertical loads. To minimize the size, and therefore the cost of the pile cap, the piles are arranged as close together as possible.
To provide undisturbed bearing capacity and suitable driving conditions, the recommended minimum clear distance between the piles is twice the diameter of the pile.
The total vertical service load on a group of piles should not exceed the group capacity, which is given by:
Group capacity= group friction capacity + group end bearing capacity
= 2D(L+K)k1 + BLk2
Where k1 and k2 are soil coefficients. Individual pile loads within the group are limited to the single pile capacity.