Geotopics 101 – What is soakaway testing?

'What is a soakaway?'

Before we get into soakaway testing, we first need to understand what exactly a soakaway is…

Essentially a soakaway is a system that stores excess surface water runoff and allows it to infiltrate into the nearby soils. These pits are dug out and filled with a void forming material, usually either lined with brickwork or loose rubble, or as has become more common, plastic geo-cellular units which have a higher void ratio.

These soakaway drainage systems are a more sustainable alternative to traditional piped drainage that connects to sewer systems, more closely mimicking natural drainage by allowing water to soak into permeable ground. They enable excessive rainwater to be dealt with at the source instead of being delivered into, and potentially overwhelming, sewer systems.

The design of a soakaway depends on several factors including ground permeability, groundwater levels, type of ground, contamination, space, nearby foundations, and the potential risk of ground instability.

Site investigation and testing and testing must be conducted prior to design and construction. Desk studies and ground investigations are used to assess the water table depth, potential contamination risks and the suitability of the strata for discharging the soakaway, including its permeability.

There are two main types of soakaways: traditional and borehole.

A traditional soakaway collects surface water in a designated area and drains it into the surrounding soil. This type is usually used in areas where surface water cannot be discharged into existing sewers or watercourses.

A borehole soakaway transfers water into the ground from a drilled hole. This type is used when the top layers of soil are impermeable, and the soakaway needs to reach a deeper layer of permeable material.

To determine the best type and size of soakaway for a site, a soakaway test will be needed.

Soakaway Tests

Field investigations are required to confirm infiltration rates. To do this, it is recommended that you excavate a soakage trial pit to the same depth as anticipated in the full soakaway. Sometimes, if necessary for stability, granular material can be used to fill the trial pit. In that case you would use a vertical observation tube embedded in the granular material so that the water level can be monitored with dip tape.

You then fill the soakage trial pit and allow it to drain three times until its almost empty, recording the water level and time from filling at clearly defined intervals. You then calculate the soil infiltration rate from the time taken for the water to fall from 75% to 25% full.

Get in touch