How to carry out a soil infiltration rate test to assess the site feasibility for Soakaway

This page gives general guidance on how to conduct soakaway tests and detail that should be included in the test report. This information is intended for developers, agents and contractors to ensure proper design and installation of soakaways for surface water drainage.

The following infiltration test recommendation is based on guidance contained within BRE3651 soakaway test guidelines.

The soakaway test report submitted to support a planning application must include the following minimum information.  

The Soakaway test should be done in a way that gives a representative result for the proposed site soil infiltration capacity.  

A Soakaway Test should be carried out in accordance with the following points: 

• The infiltration test must be undertaken/ overseen by either a suitably qualified engineer/ consultant or a company with relevant knowledge and experience.
• Excavating a trial pit using the recommended dimensions by BRE365 Guidelines. See for more information in the figure below. 

Running the test

Fill the pit with water three times in quick succession to represent and simulate soil moisture typical of the wet soil condition of the site when the soakaway becomes operative. 

A large volume of water will be needed to measure the soil infiltration rate, which may need the use of a water tanker. The water should be added quickly to ensure the pit is filled completely in a short amount of time. It's important to take precautions during the filling process to prevent the walls of the pit from collapsing. 

Keep in mind that the soil infiltration rate can be influenced by the modifications made to the landscape of the site. This includes factors such as landfills, existing underground seepage pathways (like caves), fluctuations in groundwater levels, and various geotechnical and geological elements that may affect long-term percolation. Therefore, it is crucial to examine the site's soil characteristics and geological conditions thoroughly. 

Evaluate the impact of proposed soakaway in terms of short- and long-term land stability of the site and surrounding area as the soakaway should not compromise land stability. 

The soakaway must be at least 5m away from any building. 

The water table should be at least 1m below the base of the proposed soakaway. 

Soakaway test reports must include: 

• trial pit log records,  
• water level measurement records,  
• a graph of water level versus time,
• a picture of the test pit with a timestamp. 

The report must clearly show the site infiltration rate estimation calculations and half empty time of the soakaway. 

The dimensions of the test pit must be the same for all trial test pits. It is crucial to ensure that the estimated infiltration rate remains consistent so that comparisons can be made between the tests. This will help to identify the minimum infiltration rate among the three trials. 

Recommended Soakaway trial pit size for proposed development 

The soakaway trial pit size will be subject to the proposed type of development. 

The trial pit should be excavated to the same depth of the anticipated full-size soakaway below the invert of the drain. 

The trial pit should be between 0.3m to 1m wide and 1m to 3m long. Guidance regarding the exact dimension can be found in BRE365. 

The walls of trial pit should have vertical sides at right angles to the base. If necessary for stability, the trial pit should be filled with granular material. 

If granular material such as gravel is not needed, it is recommended to mark the 75% and 25% effective depth levels in the trial pit. This helps maintain the accuracy of water level readings. 

The trial pit should be filled, and allowed to drain three times to near empty, with readings taken as follows: 

• The time it takes the water level to lower to 75% of effective depth.
• The time it takes the water level took to lower to 25% of effective depth. 

When granular material is used, a full height, perforated, vertical observation tube should be installed in the pit so that water levels can be monitored with a dip tape. 

Important considerations

Long-term maintenance and inspection must be considered during the design, construction and operation of the soakaway. 

The potential pollution impact of surface-run off from paved surfaces into the soakaway should be considered so that the quality of groundwater is not harmfully affected. 

Maintenance of silt traps, gully pots and interceptors will improve the long-term performance of the soakaway. The use of wet well chambers within the soakaway system can further assist in pollutant trapping and extended operating life. 

Care must be taken so that the introduction of large volumes of surface run-off into the soil does not disrupt the existing sub-surface drainage patterns; it may be advantageous to use extended trench soakaway systems.  

The effect of sloping ground must be considered to ensure that flood risk of downhill areas is avoided and does not give rise to land instability. 

In soil types such as chalk, Wealden Clay, or similar, and where land consists of fill materials that may be unstable, the advice of a specialist Geotechnical Engineer should be sought, during the design stage, regarding the suitability and placement of a soakaway. 

Site investigations must be undertaken thoroughly and competently so that all aspects of soil properties, geotechnology and hydrogeology are adequately reviewed alongside the hydraulic designs of soakaways.