Pile Load Capacity Calculation – Single Pile and Group Pile

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The Pile load capacity calculation will determine the ultimate load the pile foundation can take under service load condition. This capacity is also called as bearing capacity of piles. The piles installed can be a single pile or group piles. Hence the load calculation for a single pile and group pile will be different. This is done for given load condition or for the size of foundation. Here the load carrying capacity calculation for both single piles and group piles.

Single Pile Load Capacity Calculation

Here the vertical load and the horizontal load acting on the pile has to be determined.

Calculation of Vertical Load

The allowable resistance to compression  R_ac of a single pile is provided by the end bearing, F_eb and the skin friction for each strata, F_sf. Thus,

Rac = Feb + Total (Fsf )        Eq.1

Therefore, the maximum compressive service load that can be supported by a single pile is its total resistance R_{ac ,} less the self-weight of the pile, W. Thus

Nser  < Rac -W = Rnet   Eq.2

A pile can also resist a tensile load. The maximum tensile service load, that can be resisted by pile is

Rat = Total (Fsf )+ W    Eq.3

The soil exploration details will provide the details of the end bearing and the skin friction value. These values are obtained by test loadings and pile driving energy procedures. These ultimate values are divided by a partial factor of safety of between 2 and 3 to give the allowable values  F_eb and F_sf.

 Calculation of Horizontal Load

The two main factors that limit the horizontal capacity of the pile are:

1. Maximum deflection on the structure
2. Structural Capacity of the pile

The maximum horizontal capacity for a given deflection is determined from the modulus of subgrade reaction (kN/m3). There are several methods to determine the modulus of subgrade reaction.

Group Pile Load Capacity Calculation

In order to support heavy loads, the piles are arranged in groups. The piles are arranged in groups so that the size and the cost of construction of pile cap can be reduced. Undisturbed Bearing capacity and required driving conditions are obtained by providing minimum Clear Distance between the piles. This distance will be equal to the twice the pile diameter. 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         Eq.4

Where k1 and k2 are soil coefficients. Individual pile loads within the group are limited to the single pile capacity.