Soakaways do not work below the water table. Most current guidance suggests that the base of a soakaway must be above the seasonal high-water table. Previously there used to be a minimum unsaturated (dry) zone but this has been relaxed. The water table does vary seasonally; in some areas this may be less than a metre, but in many areas the fluctuations can be much more considerable.

Coarse granular materials such as flood plane gravels will tend to have a moderate to high permeability, meaning they are excellent for soakaways. These materials can be capped with clay (head deposits) and there can be a shallow water table being in a flat and low-lying area.

Finer granular materials are generally associated with sandstone bedrock. These are widely present in many areas of the UK. The permeability of these materials can vary, as any silt and fine sand can easily block up the pore spaces which are needed for a material to be permeable.

Clays and silts are collectively described as cohesive soils and these materials tend to have a poor soakage potential. Unfortunately, these materials are also widely distributed throughout the UK. Tests carried out in these materials tend either to be abandoned after 1 day (because the water level doesn’t move) or have to extend for the full 4 days in order to get any meaningful data.

Chalk can vary in soakage potential, depending on the prevalence of fissures and fractures. Some of the fastest soakage tests can be in hard blocky rubbly chalk rock. However, a weathered low-density chalk putty can behave like a cohesive soil. Soils towards the centre of values are likely to be more weathered. Also, there can be a variable zone of frost weathering resulting a cap of low permeable clays that can be over 3m deep in places. This capping is so extensive in places it is mapped as “Clay-with-Flints” on geological maps.

Like chalk, bedrock can vary with soakage potential – however is rarely impermeable. This variation is due to the presence and the abundance of fractures and joints throughout the rock. Excavating a pit to depth can be a challenge depending on the strength of the rock so if these conditions are anticipated we need to know.

Soakage tests cannot be carried out in contaminated soils or on contaminated land. As they involve the excavation of a pit which is then filled with water, they provide an easy route for the spreading of contamination if conducted here.

Soakage tests cannot be carried out within 5m of a building or a road and must be located at least 2.5m from a boundary. Also, they must not be located near (no exact figures given) any drainage fields, drainage mounds, or any other soakaways – this is to prevent exceeding the soakage capacity of the ground and reducing the effectiveness of those features.

Due to stability concerns, they cannot be located near basements (again, no exact figures given). Soakaways should also not be located on the active side of a retaining wall near to the wall itself in order to avoid instability, this is the same principle as the basement concerns.

Soakaways must not be located in an area of unstable land or where the groundwater table reaches the bottom of the excavated pit.