# 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:

Coefficient of lateral earth pressure $$K$$ is defined as:

### 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:

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

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:

### 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.