Pile Capacity in Deep Foundations

Piles are long, slender structural members used to transfer loads from superstructures down through weak, compressible soil layers to deeper, stiffer, and less compressible soil or rock.

1. Components of Pile Capacity

The ultimate load-carrying capacity of a single pile ($Q_u$) is derived from two distinct sources of resistance provided by the surrounding soil:

1. Point Bearing Capacity ($Q_p$): Resistance provided by the soil just below the pile tip.
2. Skin Friction Capacity ($Q_s$): Resistance generated by the friction and adhesion between the pile shaft surface and the surrounding soil layers.

The total ultimate capacity is the sum of these two components:

$Q_u = Q_p + Q_s$

2. Point Bearing Capacity ($Q_p$)

The point bearing capacity is similar to the bearing capacity of a deep, small-diameter footing. It is calculated as:

$Q_p = A_p q_p$

Where:

• $A_p$ is the cross-sectional area of the pile tip.
• $q_p$ is the unit point resistance of the soil.

For deep foundations, $q_p$ is heavily dependent on the effective overburden pressure and the soil's internal friction angle for sands, or the undrained shear strength ($c_u$) for clays.

3. Skin Friction Capacity ($Q_s$)

The skin friction is the summation of the frictional resistance over the entire embedded length of the pile.

$Q_s = \sum p \cdot \Delta L \cdot f$

Where:

• $p$ is the perimeter of the pile section.
• $\Delta L$ is the incremental length of the pile in a given soil layer.
• $f$ is the unit skin friction resistance in that layer.

Methods for Determining Friction ($f$):

• Alpha Method (Clays): Uses an empirical adhesion factor ($\alpha$) multiplied by the undrained shear strength ($c_u$) to find $f = \alpha c_u$.
• Beta Method (Sands/Clays): Relates friction to the effective overburden stress using an empirical coefficient ($\beta$).

4. Factor of Safety

Because soil mechanics involves significant uncertainties, a high Factor of Safety (FS) is typically applied to determine the allowable design load ($Q_{all}$):

$Q_{all} = \frac{Q_u}{FS}$

Commonly, an FS between 2.5 and 4.0 is used, depending on the reliability of the soil data and whether pile load tests have been performed.