How to Calculate Pile Capacity in Granular Soils? Explained in 1-minute

Piles are a form of deep foundations used for supporting buildings when there are heavy loads and weak soil strata. Sometimes, granular soils like sand or silt may be too weak, or column loads too high, in which case a structural designer may opt for piles. In this article, we describe a method to calculate pile capacities based on the SPT-N values for granular soils.

Ultimate Pile Capacity

A simple and relatively conservative method of estimating the ultimate capacity of piles is as follows:

                (kN)
                    (kN)
 

Where Ab is the base area of the pile (), and As is the pile shaft area (), both in units of m2. D is the pile diameter in meters and L is the pile length.

Allowable Pile Capacity

The allowable pile bearing capacity can be simply calculated as:

The safety factor, S.F., may be taken as 2.50

Example

A pile cap with two piles supports a single column, with a working load (1.0*dead + 1.0*live) of 4,500 kN. Each pile is 750 mm in diameter and 20 m long, bearing fully in dry medium dense sand with an N value of 30. Calculate the capacity of a single pile.