Cohesive Soils

For cohesive soil deposits, such as clays and silts which are modestly characterised by the undrained shear strength parameter cu, the evaluation of the shaft friction and the tip resistance is explained in the following sections.

Shaft Friction in Cohesive Material

The friction acting along the pile shaft in a cohesive layer can simply be estimated by:

$q_s = \alpha \cdot c_{u \text{, avg}} \label{eq:unit-shaft-resistance-cohesive}$

Factor of adhesion is equal to:

$\alpha = \begin{cases} 0.55 & \text{for } \frac{c_u}{p_a} \leq 1.5 \\ 0.55 + 0.1 \left( \frac{c_u}{p_a} - 1.5 \right) & \text{for } 1.5 < \frac{c_u}{p_a} \leq 2.5 \end{cases} \label{eq:factor-of-adhesion}$

General notes for shaft friction in cohesive soil layers

Reference for equations $(\ref{eq:unit-shaft-resistance-cohesive})$ and $(\ref{eq:factor-of-adhesion})$: O’Neill and Reese (1999), pp.B-27-28.
Ultimate unit skin friction $q_s$ has a limiting value of 380 kPa or 55 psi in cohesive soils.

Tip Resistance in Cohesive Material

The tip resistance of a pile resting in a cohesive layer is generally taken as:

$q_b = N_c \cdot c_u \label{eq:unit-tip-resistance-cohesive}$

Bearing capacity factor $N_c$ is equal to:

$N_c = \begin{cases} 9.0 & \text{for } h_\text{penet.} \geq 3B \\ 6 + \frac{h_\text{penet.}}{B} & \text{otherwise} \end{cases} \label{eq:bearing-capacity-factor-nc}$

General notes for tip resistance in cohesive soil layers

Reference for equations $(\ref{eq:unit-tip-resistance-cohesive})$ and $(\ref{eq:bearing-capacity-factor-nc})$: Fleming et al. (2009), p.108.
Ultimate unit tip resistance $q_b$ has a limiting value of 4000 kPa or 580 psi in cohesive soils.