Drainage below ground

5.3

Also see:

BRE Digest 365

5.3.11Surface water soakaways

Soakaway drainage shall be sited and constructed to provide adequate short term storage for surface water and adequate percolation into the surrounding ground. Issues to be taken into account include:

soakaway location
soakaway design.

Soakaway location

Soakaways should be:

built on land lower than, or sloping away from, buildings

sited at least 5m from the foundations of a building

sited to take account of topography, ensuring that water is drained away from the building

in soil of low permeability, only be provided where no alternative system is available.

Soakaway design

NHBC may require a percolation test for a soakaway, especially where there is:

doubt about the ground,

a large quantity of run-off into the soakaway which may swamp the ground.

Where the ground is free draining and granular, a test may not be necessary.

In soil, chalk and fill material subject to modification or instability, the advice of a specialist geotechnologist should be sought regarding the siting and suitability of soakaways.

Small soakaways

Small soakaways are holes filled with granular material, e.g. broken brick, crushed rock or gravel, with particle size 10mm to 150mm.

PVC sheet or concrete blinding should be laid over the fill to prevent topsoil being washed down into the soakaway.

Large soakaways

Large soakaways consist of a pit lined with dry jointed or honeycomb brickwork.

Alternatively, precast perforated concrete rings or segments may be laid dry and surrounded with granular material.

The volume of large soakaways should be calculated to ensure suitable capacity.

Percolation test procedure for surface water soakaway

The rate at which water will disperse into the ground depends on the permeability of the ground, which varies with soil type. The percolation test provides an assessment of how the ground drains.

As the test hole can be used as part of a soakaway, it should be:

dug in a place that could be used as a soakaway

at least 5m from the foundations of a building

to the same depth as the proposed drain.

Percolation test procedure for surface water soakaways

Step 1	Bore a hole 150mm in diameter with an auger, to a depth of one metre.
Step 2	Fill with water to depth of 300mm. As an aid, mark a stick 300mm from one end, place in the hole and fill up to the mark. It takes approximately 5.5 litres to fill a volume of this size.
Step 3	Observe the time taken in minutes for the water to soak away.
Step 4	Where possible, the test should be repeated and the average time used.
Step 5	A second group of tests are carried out after the hole has been bored out to a depth of two metres, still using a 300mm depth of water.
Step 6	Where the soil appears to become more permeable with depth, it may be useful to deepen and retest the bore in one-metre stages.

Design of soakaway

The relationship between the diameter or effective depth required for a soakaway, to suit a given collection area, e.g. roof or paved surface, and the average time (T) resulting from the test is shown in the graph below.

The diameter and effective depth below invert level are assumed to be the same dimension (D).

Example

Test time (T) = 900 minutes

Plan area to drain = 150m2

From the graph below, the diameter and effective depth of the soakaway (D) are both 2.8m.

Where the ground is of low permeability; dig separate soakaways to drain smaller but distinct parts, for example:

one side of a roof to one soakaway

the driveway or yard to a third soakaway.

the other side to a second soakaway

Where the permeability of the ground increases with depth; tests in the deepened trial holes will give shorter percolation times. It may be more cost effective to build a smaller soakaway at a greater depth below the surface.