Designing a Soakaway

Soakaways are water reservoirs designed to store waste water for discharge into the soil over a prolonged period. They ideally require an incoming pipe and an overflow pipe, and a means of preventing soil entering it whilst allowing water to percolate out slowly into surrounding permeable ground strata. The incoming pipe might also have an oil trap, grit trap or filter appropriate to the end use.

Soakaways are best designed as long narrow trenches constructed along the contour (see plan view).

The soakaway should be filled with non degradable non compressible materials - usually single sized drainage stone or hardcore but substitutes such as whole waste glass bottles or slate waste or shale can be suitable. Give consideration to the need for eventual refurbishment due to a build up of solids in the voids.

The soakaway should be lined (as shown in the diagram) with a suitable geotextile such as Terram 500S or equivalent, to protect the fill materials from the ingress of soil from around the trench and from the backfill material above.

Trench cross section dimensions

Geotextiles come in a standard width of 4.5m (4500mm) wide. To line the cross section of your soakaway with a single width of 4.5m-width geotextile, therefore you will want to ensure that the top plus the bottom plus the two sides plus an overlap of at least 0.6mm (600mm):

(2×w) + (2×d) + 600mm overlap = 4500mm

Example: you know that you want the width of the soakaway w = 600mm

d = [4500 - (2×w) - 600]/2 = [4500 - (2×600) - 600]/2 = 1350mm

Closing the ends

Allow the length of the trench (l) plus (2×d) to allow for end closure. Fold up the ends like a Christmas parcel.

Length of geotextile (L):

L = l + (2×d)

Larger cross sections

For large cross sections the geometry must be calculated individually.

Where the geotextile must be overlapped, allow a minimum of 600mm per overlap for good measure.

Example: lining a trench cross section of 1800mm by 900mm:

Use a full width of geotextile for the bottom section

4500mm = 450 + 450 + 900 + 900 + 1800

Cap it with a half width of geotextile:

2250mm = 225 + 225 + 1800 Overlap Check! 450 + 225 = 675mm > 600mm