Generally it is highly recommended that an infiltration test is undertaken before detailed design of a soakaway is completed. However it is sometimes useful to calculate a design infiltration rate from geotechnical principles. This may be useful at preliminary or feasibility stage, or if onsite testing is limited or impossible. In some countries such as the US, calculations such as those shown in this section are routinely used in place of onsite testing.

The infiltration rate of water through saturated soils is a common problem and can be solved directly through Darcy’s Law. Flow from a soakaway pit however should be through unsaturated layers as the base of the soakaway should always be above the groundwater level. The equations governing unsaturated flow are more complex non-linear differential equations which are impractical to solve directly in this case.

There is however an empirical solution which has been proposed to turn theoretically correct values for saturated flow into unsaturated flow useful for soakaway design. The proposed design procedure as used in some of the states of the US is detailed below;

• First the saturated hydraulic conductivity of the underlying soils must be ascertained using one of the below methods;
  • This can be done by taking samples and measuring the saturated hydraulic conductivity in the laboratory using either the constant head or falling head permeability test.
  • Alternatively there are a number of field tests which can estimate the saturated hydraulic conductivity value.
  • Where existing site investigation data must be used the saturated hydraulic conductivity can be determined from soil grading information using the below empirical relationship;