Seepage

Drainage

Buried Flexible Pipeline: Part 1: Structural Design

$$w_{g}=\kappa\gamma H$$

where

$$\kappa = \frac {1-e^{-2\frac{H}{B^{\prime}}K_{0}\tan \delta}} {2\frac{H}{B^{\prime}}K_{0}\tan\delta}$$

B' = width of slip zone at top of pipe $\kappa$ = silo reduction factor for trench fill load effect e = base of natural logarithms $K_{0}$ = ratio of lateral to vertical soil pressure (has value between the active and passives oil resistance ratios) $\delta$ = friction angle on the slip plane, 0 $\le$ $\delta$ $\le$ $\phi$ in degrees $\phi$ in soil friction angle for fill material, in degrees

According to AS 3725 and AS 4060 (Refs. 1 and 2), the product of $K_{0}$ and tan$\delta$ varies between 0.11 for soft clay and 0.16 for crushed aggregate

Where a superimposed distributed dead load is applied to a pipe in a trench, the silo reduction factor, which enables trench wall friction to be taken into account, can be calculated from the following equation:

$$k_{s}=e^{-2 \frac{H}{B^{\prime}} K_{0} \tan \delta}$$

where $k_{s}$ is silo reduction factor for area load effect

In practice, the use of the silo reduction factors for buried flexible pipe calculations is usually limited to deep pipe trenches where H/B $\le$ 10.