Trench soakaways are very common and can be used for commercial developments, linear features such as roads and sometimes for larger domestic applications, where a garden or driveway soakaway would be too small.

Trench Soakaway Details

A trench soakaway consists of a shallow narrow trench. Usually this is filled with granular material and includes a layer of geotextile. Inspection chambers should be included at least at the two ends of the trench so that the infiltration performance and condition of the unit can be monitored.

 A perforated distributor pipe is often included running the length of the infiltration trench. This quickly distributes the runoff to the whole length of the trench. If no distributor pipe is included in storm conditions the runoff may not be able infiltrate through the gravel backfill quickly enough to prevent the inflow drain from becoming surcharged. This also increases the effective infiltration rate as the whole of the trench length is being used at all times.

Where a distributor pipe is used, it is recommended that a chamber including a T-piece inlet is used to minimise the amount of sediments and floating debris entering the pipe. This will prolong the life of the pipe by preventing it becoming blocked with debris.

Trench Soakaway Applications

A trench soakaway may be preferred to a conventional soakaway pit in the following conditions;

• Where there are restrictions on the depth of the soakaway, for example where there are layers of contaminated soils or impermeable strata. In these conditions a long shallow trench soakaway will achieve greater infiltration rates than a single shallow soakaway pit.
• Where the development layout favours a long shallow trench rather than a large deep single soakaway pit. This may be the case for linear features such as roads or railways, or around a large site periphery such as around a car park.