These are the traditional soakaways, typically between 1.50m and 2.50m depth and can take relatively large volumes of water from roofs and paved areas etc. The length and depth of the trench is varied to suite the ground conditions and the volume of water to be accommodated.

A trench soakaway is usually a gravel filled excavation with perforated pipe, but modern applications also include plastic crates which have a much higher storage capacity. The trench is usually lined with a geotextile fabric to prevent mixing with the surrounding ground. Older style soakaways may simply comprise a rubble filled hole in the ground but these are somewhat old fashioned.

Suitable testing types: BRE365, FHST (for preliminary design), VP (for small soakaways)

Ring soakaways are perforated precast concrete rings set into a square excavation which is backfilled around with granular material and a concrete “biscuit” over the top. The benefit is the increase in storage volume compared to a fully gravel-filled pit, but there may be other pro’s and con’s to consider. The typical depth is 1.50-2.50m. Water discharges horizontally through the perforations and vertically through the open base. Water discharges horizontally through the perforations and vertically through the open base. Usually permitted up to within 5m of buildings and 2m from pavements.

Suitable testing types: BRE365, FHST (for preliminary design), VP (for small soakaways)

Borehole soakaways are like a water well, just used in reverse! More accurately these are a borehole drilled in 200-300mm diameter installed with a liner and usually with a concrete ring catch pit at ground level. The filter is usually 100-200mm PVC pipe, slotted with a filter sock and pea shingle surround in the lower parts, and the upper part is plain unslotted pipe with a bentonite cement grout. At the top of the liner is usually a siphon head and a debris filter.

These are normally used where there is a capping of impermeable ground overlying more permeable strata at depth (for instance, Clay-With-Flints overlying Chalk). The deeper strata must be dry. Given the relatively limited surface area, these can either end up quite deep and/or requiring multiple boreholes at sufficient spacing with individual catch pits.

Suitable testing types: BHST (for more standard sized soakaways), FHST (for shallow wells only)

Note: the test borehole can be installed on completion of the satisfactory test.

These are usually employed for septic tank installations, but can also be used as part of a SuDS scheme if space is available. These comprise a network of shallow gravel filled trenches with perforated pipes.

Sustainable Drainage System (SuDS) 

In natural environments, rain falls on permeable surfaces and soaks into the ground; a process called through infiltration. 

In urban areas where many surfaces are sealed by buildings and paving, natural infiltration is limited. Instead, drainage networks consisting of pipes and culverts, divert surface water to local watercourses. In some cases, this has resulted in downstream flooding and deterioration in river water quality caused when foul sewers are overwhelmed by surface water leading to a release of dirty water into rivers. 

Sustainable drainage systems aim to alleviate these problems by storing or re-using surface water at source, by decreasing flow rates to watercourses and by improving water quality. 

There is no single type of SuDS and are likely to comprise a series of steps such as collecting water for irrigation, green roofs, vegetated swales (ditches), filter trenches, retention ponds and wetlands, as well as soakaways and permeable paving. 

Typically these are shallow systems and will require a holistic approach. 

Suitable testing types: BRE365, FHST (for preliminary design), VP (for small soakaways and percolation trenches)  

The problem in collecting rainwater is then having to put it somewhere. Permeable paving avoids this, by allowing water to drain through directly into the ground. Surfacing may be gravel or grass or permeable blockwork, and sometimes an orthogonal geogrid is used to keep the surface bound and protected from damage. In favourable conditions, the pavement can be used as the soakaway for other areas such as roofs.

Suitable testing types: VP (most appropriate), BRE365 (suitable but overkill), FHST (for preliminary design)

French Drains are sometimes considered along with soakaways, but typically these are gravel filled trenches laid with a fall in order to convey water away from an area and often discharge out over ground level or to a ditch, etc.

Micro-drainage is increasingly discussed and these are essential shallow small soakaways built into built environment.