Soakaway and Percolation test
Soakaways have long been the traditional way to dispose of storm water from buildings and paved areas remote from a public sewer or watercourse. In recent years, soakaways have been used within urban, fully-sewered areas to limit the impact on discharge of new upstream building works and to avoid costs of sewer upgrading outside a development. Soakaways are seen increasingly as a more widely applicable option alongside other means of storm water control and disposal.
Soakaways must store the immediate storm water run-off and allow for its efficient infiltration into the adjacent soil. They must discharge their stored water sufficiently quickly to provide the necessary capacity to receive run-off from a subsequent storm. The time taken for discharge depends upon the soakaway shape and size, and the surrounding soil’s infiltration characteristics.
BRE Digest 365 Soakaway and Percolation tests are a part of a site investigation and should be carried out prior to a build commencing.
The different types of Soakaway/Percolation testing available are as follows:
Falling/rising head permeability test
A falling head test is conducted in drilled borehole. These are generally required when the permeable strata is too deep to perform in a hand dug or machine dug pit or trench. For this test the borehole or standpipe is filled with water and the time taken for it to soak away is measured and used to calculate the coefficient of permeability.
A rising head test is used in areas with a high-water table. To complete this test water is pumped from a borehole or standpipe. The time it takes for the water to rise is then recorded and used to calculate the coefficient or permeability.
A percolation test is performed in a hand excavated trial pit. These are used for small scale developments or septic tank installations. The trial pit is excavated to 300mm square. Water level is monitored from 75% to 25% level. The time taken for the water to soak away between these levels is used to calculate the percolation test rate.
BRE 365 Soakage test
The BRE 365 test is the most common performed on sites. It requires use of an excavator to dig a test hole to the predesigned installation depth for the soakaway. The method of determination must give representative results for the proposed site of the soakaway.
This is achieved by:
1. Excavating a trial pit of sufficient size to represent a section of the design soakaway.
2. Filling the pit several times in quick succession whilst monitoring the rate of seepage, to represent soil moisture conditions typical of the site when the soakaway becomes operative.
3. Examining site data to ensure that variations in soil conditions, areas of filled land, preferential underground seepage routes, variations in the level of groundwater, and any geotechnical and geological factors likely to affect the long-term percolation and stability of the area surrounding the soakaway have been assessed.
Local building control and/or planning authorities should advise where fluctuations in groundwater level may cause a problem in the long-term for any proposed depth of excavation.
All the above are always performed using potable water .