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.
Table 1: Bearing capacity factor vs. angle of internal friction for granular soils, adapted from NAVFAC (1986), p.7.2-194.
| \(\phi\) | 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 |
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.
- Ultimate unit tip resistance \(q_b\) has a limiting value of 4312.5 kPa or 625 psi as advised by Mullins (2006), p.307.