Granular Soils

For granular soil deposits, such as sands and gravels, the estimation of the shaft friction and the tip resistance are presented in the following sections.

Shaft Friction in Granular Material

The friction acting along the pile shaft in a granular layer can be derived from:

$$q_s = K \cdot \sigma_{v \text{, avg}}' \cdot tan{\phi'} \label{eq:unit-shaft-resistance-granular}$$

Coefficient of lateral earth pressure \(K\) is defined as:

$$K = 0.85 \left( 1 - sin{\phi'} \right) \label{eq:coefficient-of-lateral-earth-pressure}$$

General notes for shaft friction in granular soil layers

• Reference for equations \((\ref{eq:unit-shaft-resistance-granular})\) and \((\ref{eq:coefficient-of-lateral-earth-pressure})\): Tomlinson and Woodward (2008), pp.165-171.
• Ultimate unit skin friction \(q_s\) has a limiting value of 200 kPa or 29 psi in granular soils.

Tip Resistance in Granular Material

The tip resistance of a pile resting on a granular layer is estimated by:

$$q_b = N_q \cdot \sigma_{v}' \label{eq:unit-tip-resistance-granular}$$

Bearing capacity factor \(N_q\) depends on the angle of internal friction. This relationship is presented in Table 1.

\(\phi\)	26	28	30	31	32	33	34	35	36	37	38	39	40
\(N_q\)	5	8	10	12	14	17	21	25	30	38	43	60	72

Ultimate unit tip resistance \(q_b\) can be also calculated using the standard penetration test (SPT) value of the bearing stratum at the pile toe. This value should be an average representing a thickness of 3 times the pile diameter \(( 3B )\) beneath the pile toe. The SPT correlations are defined as:

$$q_b = \begin{cases} 57.5 \cdot N_\text{SPT} & \left(q_b \text{ in kPa} \right) \\ 8.33 \cdot N_\text{SPT} & \left(q_b \text{ in psi} \right) \end{cases} \label{eq:spt-correlations}$$

General notes for tip resistance in granular soil layers

• Reference for equation \((\ref{eq:unit-tip-resistance-granular})\): Prakash and Sharma (1990), pp.221-222.
• Reference for equations \((\ref{eq:spt-correlations})\): O’Neill and Reese (1999), p.B-22.
• Ultimate unit tip resistance \(q_b\) has a limiting value of 4312.5 kPa or 625 psi as advised by Mullins (2006), p.307.