What is the Difference between Soakaway & Percolation Testing?

A soakaway test is required to determine if a new or replacement soakaway can be installed for disposing of surface run-off water. A Percolation Test is required where a septic tank or sewage treatment plant is proposed for treating foul water.

What is a Percolation Test?

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 septic tank and drainage field 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?

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 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 may not be appropriate for the property and therefore a septic tank might 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.

Special Measures in the River Lugg Catchment

Herefordshire County Council, in collaboration with Natural England and the Environment Agency and others, have developed a Nutrient Management Plan to tackle the ongoing phosphate pollution problem blighting the Rivers Lugg and Wye (you can read more on this topic in our article here).

As of April 2021, any proposed developments for household foul water discharge to drainage field in the River Lugg Catchment Area must meet the seven criteria set by Herefordshire Council:

  1. The drainage field is more than 50m from the designated site boundary (or sensitive interest feature) and;
  2. The drainage field is more than 40m from any surface water feature e.g. ditch, drain, watercourse, and;
  3. The drainage field is in an area with a slope no greater than 15%, and;
  4. The drainage field is in an area where the high water table groundwater depth is at least 2m below the surface at all times and;
  5. The drainage field will not be subject to significant flooding, e.g. it is not in flood zone 2 or 3 and;
  6. There are no other known factors which would expedite the transport of phosphorus for example fissured geology, insufficient soil below the drainage pipes, known sewer flooding, conditions in the soil/geology that would cause remobilisation phosphorus, presence of mineshafts, etc and;
  7. To ensure that there is no significant in combination effect, the discharge to ground should be at least 200m from any other discharge to ground.

 

River Lugg Phosphates

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.