A percolation test (also called a perc test) measures the rate at which water drains through the soil at a specific depth and location. While the term is sometimes used interchangeably with infiltration testing, percolation tests are most commonly associated with the design of drainage fields (also called soakaway fields or infiltration fields) for foul water treatment — specifically for septic tank and sewage treatment plant effluent disposal.

The percolation test procedure (as specified in BS 6297 and Building Regulations Approved Document H) involves:

Test hole excavation: A hole is dug to the proposed drainage field invert depth (typically 250-750mm below the proposed distribution pipe level)
Pre-soaking: The hole is filled with water and allowed to drain completely, then refilled. This saturates the surrounding soil and provides a consistent starting condition
Timed drainage: The hole is filled to a set level and the time taken for the water level to drop a specified distance (typically 25mm) is recorded
Vp calculation: The average time (in seconds) for the water level to fall 1mm is calculated. This value (Vp) determines the required area of the drainage field

Percolation test results must fall within an acceptable range for a drainage field to be viable:

Vp below 15: Soil drains too quickly — effluent will not receive adequate treatment
Vp between 15 and 100: Suitable for drainage field design
Vp above 100: Soil is too impermeable — drainage field not viable

For surface water infiltration design, the BRE365 test is the standard method rather than the percolation test. However, both tests assess the same fundamental property — the ability of the ground to accept and transmit water — and are often carried out during the same site visit.

Aegaea carries out percolation testing alongside BRE365 soakaway testing to provide a complete picture of ground permeability for both surface water and foul drainage design.

Percolation Test