Geotechnical and
Environmental Services

Specialists in the investigation, assessment and reclamation
of development land.


Soakage or percolation tests for design of soakaways

With the modern emphasis on Sustainable Urban Drainage schemes (SUDs), there has been an increase in the use of soakaways and permeable pavements for the disposal of surface water.  In order to design such systems, it is necessary to determine the permeability of the soil i.e the rate water will pass through the ground.

If you are planning to use a soakaway, then contact BRD for intial advice as to the likelihood of their successful use can often be determined by interpretation of the geology alone.  If the ground is likely to be suitable, then BRD can undertake soakage tests either as a standalone project, such as supporting information to a flood risk assessment, or integrated into a more general ground investigation.

For soakaway design, the most common test is the BRE Digest 365 ‘Soakaway design’ test that essentially involves excavating a trial pit of known dimensions, filling it with water and recording how quickly the water seeps into the ground.  The data collected is analysed to calculate a soil infiltration rate employed in the design methodology.  This test has some problems with its practical application including the need for a large volume of water, ground conditions rarely allowing the repeat tests required to be completed within one day, safety of tests lasting more than one day and collapse of the pit sides during the test.

Other types of test are available to determine permeability including falling head tests in boreholes, shallow depth NHBC percolation test, correlations from laboratory particle size determinations and rising head tests in boreholes.  Again BRD experience can guide Clients on the best approach for a particular situation.

In some areas of the country, the near surface soils are effectively impermeable (i.e negligible permeability) such as clays, but at depth there may be permeable soils which could accept surface water.  A common example is the presence of superficial deposits of Clay-with-flints over Chalk rock.  In such circumstances a deep borehole soakaway design may be appropriate.  BRD have extensive experience of undertaking suitable tests within boreholes to confirm such a design is viable.

The Environment Agency do not permit the use of soakaways in contaminated ground, where direct discharge to groundwater is possible and in certain sensitive locations.  BRD can advise on these issues and help negotiate an appropriate SUDs scheme.