Whist this is not a structural calculation it is one that is sometimes asked for by local authorities when a Building Regulation application is submitted for either a new build or extension. It may be requested on either residential or commercial projects. The calculation is the same for either, its normally just the size of the soakaway which is different.

The calculation is based on a field porosity test which enables an assessment ot the permeability of the ground to be made. This then allows the soakaway to be designed.

The field test is not difficult to do, simply requiring the excavation of a hole which is filled with water and then the time taken for the water level to drop is measured at regular intervals.

It should be remembered that some types of soils (clays) are impermeable and tests in these types of soil are likely to be a waste of time. Sands and gravels on the other hand are very permeable and these types of soil are ideal for the use of soakaways.

We need the following information to design a soakaway:

field permeability test results (full details of what is required will be provided)

Design information:

All design work is carried out in accordance with BRE Digest 365