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Cohesive Soils

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

Undrained Cohesion Profile

The undrained cohesion can optionally increase linearly with depth within a layer:

\[ c_u(z) = c_u + c_{u\text{, inc}} \cdot z \label{eq:undrained-cohesion-profile} \]

where \(z\) is the depth below the top of the layer and \(c_{u\text{, inc}}\) is the rate of increase with depth. When \(c_{u\text{, inc}} = 0\), the undrained cohesion is constant throughout the layer.

Shaft Friction in Cohesive Material

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

\[ q_s = \alpha \cdot c_u \label{eq:unit-shaft-resistance-cohesive} \]

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 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 580 psi in cohesive soils.