LAST UPDATED: 9th January 2023 by The Editorial Team
Summary: Step by step guide and video showing how to build a septic soak away, calculate the size and construct a within regulations and complete a percolation test.
How to construct a septic tank soakaway
A sewage effluent soak away is not the same as a soak away used for surface water dispersal. The only function of a surface water soak away is to disperse the water into the ground, but a sewage effluent soak away has two functions.
The first is to soak the septic tank liquid away. The second function is to further treat the effluent to remove suspended solids, bacteria and viruses, nitrogen and phosphorous using natural aerobic bacteria which grow within the soak away. Only then can the waste be soaked away into the ground. This is to avoid groundwater contamination, as much of our drinking water is sourced from it.
Sewage effluent soak aways must conform with Section H of the Building Regulations – Sewage and Drainage issues and BS 6297 : 2007
Sizing of a soakaway
The size of the soak away, (i.e. the area in square metres), is determined by the number of bedrooms in the house and the porosity of the soil. Currently, The Environment Agency requires this to be one person per bedroom plus 0.5 persons per house. The porosity of the soil can only be determined by a percolation test.
Minimum soakaway site distances
- 10m (34ft) from a watercourse or ditch.
- 50m (164ft) from water abstraction points.
- 15m (50ft) from any building, and sufficiently distant from any other soak away, including roof water soak aways.
- 2m (7ft) from a boundary.
- The soak away or drain field area should be downslope of any groundwater source.
- No water supply pipes or underground services should be within the soak away area.
- No access roads, driveways or paved areas should be within the soak away area. This includes fields with tractor and agricultural traffic.
- The water table or bedrock must not, at any time, be within 1.2m (4ft) of the bottom of the soak away trench.
Construction of soakaways and drain fields
Sewage effluent soak aways are carefully designed constructions consisting of trenches containing pipes laid on gravel or stone beds, covered with more stone and geotextile membrane and backfilled with topsoil.
- Use solid perforated foul drainage pipe, with the slots/holes at the bottom, lay in trenches of a uniform gradient not steeper than 1:200. Perforated ‘Flexicoil’ type pipe is not allowed.
- Pipes should be laid at a minimum depth of 200mm (8in) and a maximum depth of 700mm (28in) to enable aerobic contact between the effluent and the soil particles.
- The length of a single trench should not exceed 30m (98ft).
- Pipes should be laid on 300mm (12in) of clean shingle, gravel or broken grade 1 stone (20mm– 50mm).
- Soak away trenches should be filled to a level of 50mm (2in) above the pipe and covered with a layer of geotextile membrane to prevent silt entry.
- The remainder of the soak away trench can be filled with topsoil.
- Drainage trenches should be between 300mm (12in) and 900mm (3ft) wide with areas of undisturbed ground of minimum 1m wide between parallel trenches.
- An inspection/distribution chamber should be installed between the septic tank and the drain field.
- The soak away layout should be set out as a continuous loop or grid fed from the inspection chamber, not a straight pipe run with an ‘end’.
- The soak away pipes should be fed by a distribution chamber.
- The area of the soak away is calculated after carrying out a minimum of three percolation tests in the proposed drain field area, over three consecutive days and applying a formula as detailed on the percolation test information below.
Percolation tests for a soakaway design
A soil percolation test is required to be carried out on the land where the septic tank or treatment plant soak away is intended to be built. It is a test that determines the rate in which soil absorbs a known volume of water. This is to ensure that the land (soil) is suitable for a soak away, and to ensure that the soak away is designed properly.
A percolation test is always required for septic tanks.
How to carry out a percolation test
A percolation test should not be carried out during abnormal weather conditions and if done in summer, then the times should be increased by 30% to 50%, depending on the recent weather.
The percolation test method
- Excavate a hole 300mm (12in) square and 300mm (12in) below the proposed outlet from the septic tank or sewage treatment plant.
- Dig the test hole vertically to the appropriate depth. Remove all loose debris.
- Mark the hole 75mm (3in) from the bottom and 75mm (3in) from the top by pushing 6 inch nails into the sides of the of the percolation test hole.
- Fill the test hole with water to a depth of at least 300mm (12in). Allow to seep away overnight.
- Next day, refill the test section with water to a depth at least 300mm (12in). Observe the time, in seconds, for the water to seep away from 75% full to 25% full.
- Divide this time by 150mm (6in), (i.e. 50% of the 300mm (12in) depth). The answer gives the average time in seconds (V) required for the water to drop 1mm.
- Carry out the test at least 3 times, with at least 2 trial holes. The average figure from the tests should be taken.
Drainage field soak away disposal should only be used when percolation tests indicate average values of V of between 15 and 100 and the preliminary site assessment report and trial hole tests have been favourable. A percolation test is deemed to have failed if the results are less than 12 or more than 100.
This minimum value ensures that untreated effluent cannot percolate too rapidly into ground water. Where V is outside these limits effective disposal is unlikely to take place in a soak away drainage field.
* Some Councils insist that percolation tests are carried out by ‘qualified persons’ and do not accept your own test results. You may be required to contact a contractor to carry out the percolation test. Please check with your Local Authority.
Calculating the size of the soakaway
The calculation gives the area (A), in square metres, required for the soak away trench.
V = The time in seconds for the water in the test hole to drop by 1mm.
p = The maximum number of persons that the unit is designed to serve.
For sewage treatment units
Area (A) = Vp X 0.20
For septic tanks
Area (A) = Vp X 0.25
Further calculations are required to give the length of pipes required depending on the width of the trench, e.g. for a 600mm (2ft) wide trench, the area would be divided by 0.6.
Never breathe septic tank gases as they can overcome you very quickly.