Geotechnical Engineering

Soakaway design and drainage

Soakaways are used to collect surface water runoff prior to being discharged into the surrounding soil. Soakaways are required in areas where the surface water run-off cannot be disposed into existing sewers or watercourses.

A conventional shallow soakaway comprises a partially perforated cylindrical chamber. The chamber will need to be large enough to store the run-off before the water is able to be drained into the surrounding soil. In areas where impermeable soils are underlain by a permeable strata, soakage may be achieved using a borehole soakaway. A deep bored perforated plastic or steel pipe will need to be installed in order to discharge the water from a sealed storage chamber.

Soakage or permeability tests are required in order to determine the soil infiltration rate and the drainage capacity of the soils or the rate of permeability. We can use a number of test methods to determine whether the soil conditions are suitable for soakaway drainage. The testing is normally carried out in accordance with either BRE Digest 365 ‘Soakaway design’ or BS 5930 ‘Code of practice for site investigation’.

The BRE Digest 365 soakage test is normally carried out in trial pits typically filled with gravel to ensure the sides of the trial pit remain stable during charging and draining. The soil infiltration rate is determined by filling the pit with clean water from a bowser or tanker and determining the length of time for a proportion of the volume of water to drain away.

BS 5930 field permeability or percolation tests are normally carried out in boreholes using either the falling-head or constant-head test method. Falling-head permeability tests determine the permeability or percolation rate of an area by filling a borehole with water and recording the rate at which it drains away. This test method is most suitable for use in soils with medium to low permeability.

Constant-head permeability tests determine the permeability or percolation rate of an area by maintaining a constant head of water in a test pit or borehole. The water that drains out of the test hole is replenished at the same rate from a water source. The stable flow rate of water entering the hole is measured over time to determine the permeability of the soil. This test method is most suitable for use in rock areas or soils with high permeability.

Conventional trial pit soakage tests are relatively expensive and cause some disruption to the site. The borehole tests in accordance with BRE Digest 365 are generally less accurate than trial pit tests; however for most soil types they still give reasonable estimates of infiltration rates.