Understanding Percolation Tests to BRE 365 and Their Importance in Drainage

What is a Percolation Test?

A percolation test, often referred to as a "perc test," is a method used to assess the rate at which water drains through soil. This test is crucial for determining the suitability of the ground for various drainage solutions, particularly soakaways. The procedure involves digging a test pit, filling it with water, and measuring the time it takes for the water to percolate through the soil. The results provide valuable data on the soil's infiltration rate, which is essential for designing effective drainage systems.

BRE 365: The Standard for Soakaway Design

BRE Digest 365 is the most widely recognised guideline for designing soakaways in the UK. It provides detailed instructions on how to conduct percolation tests and use the results to design soakaways that can effectively manage surface water runoff. According to BRE 365, the test involves excavating a trial hole to the anticipated depth of the soakaway, filling it with water, and recording the time taken for the water level to drop from 75% to 25% full.

Why Are Percolation Tests Needed?

Percolation tests are essential for several reasons:

1. Soil Suitability: They determine whether the soil can adequately absorb and drain water. This is crucial for preventing waterlogging and ensuring the effectiveness of drainage systems.

2. Environmental Protection: Proper drainage helps prevent groundwater contamination by ensuring that wastewater is adequately filtered through the soil before reaching the water table.

3. Regulatory Compliance: Many local authorities require percolation tests to be conducted before approving the installation of drainage systems, ensuring that they meet environmental and safety standards.

Why Do Local Authorities Insist on Percolation Tests?

Local authorities insist on percolation tests for several important reasons:

1. Public Health and Safety: Ensuring that drainage systems are effective helps prevent waterlogging and the associated health risks, such as the breeding of mosquitoes and other pests

2. Environmental Protection: Properly designed drainage systems prevent the contamination of groundwater and surface water, protecting local ecosystems and water supplies

3. Infrastructure Integrity: Effective drainage systems help maintain the integrity of public infrastructure, such as roads and buildings, by preventing water damage and erosion

4. Regulatory Compliance: Local authorities have a duty to ensure that new developments comply with national and local regulations regarding drainage and water management

The Role of Percolation Tests in the Hierarchy of Drainage

The hierarchy of drainage is a framework used to prioritise drainage solutions based on their sustainability and effectiveness. The National Planning Practice Guidance outlines this hierarchy, which aims to manage surface water runoff in the most sustainable way possible. The hierarchy is as follows:

1. Infiltration into the Ground: The most sustainable option, where water is allowed to percolate into the ground through soakaways or other infiltration systems. Percolation tests are critical here to ensure the ground can handle the water load.

2. Discharge to a Surface Water Body: If infiltration is not feasible, the next best option is to discharge water into a nearby watercourse.

3. Discharge to a Surface Water Sewer or Drainage System: When the first two options are impractical, water can be directed to a surface water sewer or drainage system.

4. Discharge to a Combined Sewer: The least preferred option, used only when no other methods are viable.

By conducting percolation tests, engineers and planners can determine the most appropriate drainage solution for a given site, ensuring compliance with the hierarchy and promoting sustainable water management practices.

Conclusion

Percolation tests to BRE 365 are a fundamental part of designing effective and sustainable drainage systems. They provide essential data on soil infiltration rates, helping to ensure that drainage solutions are both effective and environmentally friendly. 

By fitting into the hierarchy of drainage, these tests play a crucial role in managing surface water runoff and protecting our natural water resources.