What is the Relationship between Soakaways & Percolation Testing?

A percolation test is required to determine if a new or replacement soakaway can be installed.

What is a Percolation Test?

A Percolation Test is required on land where a septic tank or sewage treatment plant is proposed to be built. Permeability Testing or Percolation Testing is required to allow design of soakaway drainage systems for both clean and grey water.

The test determines the rates within which the soil absorbs a known amount of water. This is required to ensure that the land intended for the soakaway is suitable for this use.  This is important because if the ground is too wet or too impermeable the wastewater will not be able to pass into it and will return to the septic tank.

A Percolation Test is always required prior to the installation of septic tanks.

Percolation Tests are essential for ensuring that the correct drainage systems are installed at a property.

How Is A Percolation Test Undertaken?

A Percolation Test consists of digging two or more holes in the soil of the proposed leach field to a specified depth.

Percolation testing is a requirement when a septic tank draining system is to be installed.  The testing involves of the excavation of a 300mm square pit and filling with clean water.  The water level change is recorded over time until the pit has drained.  The testing is usually undertaken three times.  On completion the pit is backfilled with arisings.

The weather on a day of testing can be very influential. Therefore, tests should not be carried out in periods of heavy rainfall as it can affect the results by making the ground more saturated than usual. It can also result in false results by filling up the test pits.

The British Standard Code of Practice for the Design & Installation of Drainage Fields for Use in Wastewater Treatment sets out in detail how a percolation test should be carried out. This can be found here

Variable Head Tests

Variable Head Tests are split into rising and falling Head Tests. A Rising Head Test is undertaken in a borehole that has a standing groundwater level. The water in the borehole is bailed or pumped out and the rise of the water is recorded at over time until it has reached the original level.

A falling head test is typically undertaken on a dry borehole, or one with a low water level.  Clean water is poured into the hole and the water level is recorded over time, up until it has reached the original level.

What Happens If A Percolation Test Fails?

If a Percolation Test fails, this means that the drainage area or field is not appropriate for the property and therefore a septic tank would not be suitable for the property.

What is a Soakaway?

There are two different types of Soakaway; foul water soakaways (grey water) and surface water soakaways.

Foul water soakaways deal with a small volume of water over a long period, whereas rainwater soakaways may be required to deal with a large amount of water in a very short space of time.

How Is A Soakaway Test Carried Out?

A soakaway test is normally carried out by mechanically excavating a test pit to a predetermined depth and filled with clean water from an IBC or bowser.  The change in water level is recorded at specific intervals until the pit is empty, or if the drainage is very slow, until the test pit has lost 75% of the volume of water.  The test is typically carried out three times on each test pit. Once the testing is complete, any remaining water is pumped out and the test pit is backfilled.

Sustainable Drainage Design (SUDS)

In considering planning permission, local authorities will often require the developer to demonstrate that all sustainable urban drainage systems (SUDS) have been considered before granting permission. This can readily be achieved by undertaking soakaway infiltration tests to BRE 365 in shallow trial pits. The trial pits can often serve a dual purpose to provide foundation design information for low rise housing developments.

How EMS Geotech Can Help

EMS offer a range of tests including:

Full scale soakaway / NHBC percolation testing in accordance with BRE365:2015 and BS6297:2007 + A1:2008; and

Falling or rising head permeability tests in boreholes or borehole standpipes

EMS will be happy to recommend the most suitable test for your site, based on the ground conditions, the type of soakaway drainage anticipated and the available access.

Many Councils & Local Authorities insist that Percolation Tests are carried out by ‘qualified’ individuals.  EMS’s qualified consultants are able to satisfy this.