What is a Percolation Test

A percolation test is a field-based method used to evaluate the rate at which water filters through soil. It is a key diagnostic procedure in drainage and wastewater planning, particularly for determining whether a site is suitable for infiltration systems such as soakaways or drainage fields. The test measures how quickly water is absorbed by the ground and helps guide the design of systems intended to disperse surface water or treated effluent into the soil.

This test is a regulatory requirement in many countries, including the UK, for the installation of septic tanks, package treatment plants and certain Sustainable Drainage Systems (SuDS).

Purpose of a percolation test

The main purpose of a percolation test is to establish whether the soil at a specific location can safely and effectively absorb water. It determines:

  • The infiltration rate of the soil

  • Whether the soil is suitable for a soakaway or drainage field

  • The size and design requirements for the proposed drainage system

  • The risk of waterlogging, ponding or pollution from poorly draining ground

If a site fails the percolation test, infiltration systems are typically ruled out, and alternative drainage solutions must be considered.

When a percolation test is needed

A percolation test is typically required when:

  • Installing a new septic tank or wastewater treatment plant

  • Planning a replacement for a failed soakaway

  • Designing surface water soakaways or infiltration-based SuDS

  • Applying for planning permission in rural or unsewered locations

  • Complying with Building Regulations and Environment Agency guidelines

In most cases, the test must be conducted before construction begins and form part of the drainage design submitted for approval.

How a percolation test is performed

A percolation test involves excavating one or more trial holes in the proposed drainage area and measuring the rate at which water drains from them. The standard procedure, particularly for drainage fields under BS 6297, includes the following steps:

  1. Digging test holes
    A number of test pits are dug to a depth of the proposed soakaway or drainage field trenches, usually 300 mm wide and 250 mm deep below the bottom of the proposed pipe level. The sides must be vertical and the base flat.

  2. Saturating the soil
    The holes are filled with water and allowed to drain away to ensure the soil is saturated. This process may be repeated over several hours or days depending on ground conditions.

  3. Measuring the soak time
    The holes are then refilled with water to a specified depth (usually 75% of the depth) and the time taken for the water level to fall by a set amount (e.g. 75 mm) is recorded. This process is repeated at least three times for each hole to obtain an average value.

  4. Calculating the percolation value
    The result, expressed as Vp (minutes per mm), is used to determine whether the soil is within the acceptable range for a soakaway or drainage field. In the UK, Vp should be between 15 and 100 for a standard drainage field.

This hands-on test provides real-world data that reflects site-specific soil and groundwater conditions.

Tools and equipment

A percolation test does not require complex machinery but does involve the following:

  • Shovels or a mini-excavator to dig test holes

  • Measuring tape or dipstick

  • Stopwatch or timer

  • Buckets or hose to add water

  • Level and straightedge for accurate hole depth

  • Record sheets for data logging

In some cases, especially on large developments or sites with complex geology, percolation testing may be accompanied by additional soil investigations or infiltration testing as per BRE Digest 365.

Understanding the results

The result of a percolation test determines:

  • Whether a drainage field is permitted

  • The required length and layout of the drainage field trenches

  • The risk of effluent reaching the surface or contaminating nearby watercourses

  • The long-term sustainability of the proposed drainage system

If the soil drains too slowly (high Vp), effluent may back up or pond at the surface. If it drains too quickly (low Vp), there may be insufficient treatment time before water reaches groundwater. Both outcomes can lead to environmental or regulatory issues.

Regulatory context

In the UK, percolation testing is mandated under:

  • Building Regulations Approved Document H
    Requires proof of infiltration performance before approval of non-mains drainage.

  • BS 6297:2007+A1:2008
    Sets out the method for testing and designing drainage fields for small wastewater treatment systems.

  • Environment Agency General Binding Rules
    Define conditions for effluent discharge and prohibit it where percolation is inadequate.

Planning authorities and environmental regulators may require percolation test reports as part of the approval process for new builds, especially in areas without access to mains sewerage.

Limitations and common issues

Although the test is simple in principle, several factors can affect accuracy:

  • Soil moisture must be consistent with typical seasonal conditions

  • Disturbed or compacted ground may give misleading results

  • Weather extremes can skew measurements

  • Only testing in one location may not reflect the variability across the site

  • Failing to saturate the soil properly before timed tests can invalidate results

For large or sensitive developments, professional soil assessments are often recommended in addition to percolation testing.

Alternatives when percolation is unsuitable

If percolation results fall outside the acceptable range, alternatives include:

  • Installing a raised drainage field with imported granular material

  • Using a packaged treatment plant with surface discharge (subject to permits)

  • Connecting to a mains sewer if available

  • Constructing a sealed cesspool or holding tank (last resort due to maintenance burden)

The choice depends on site constraints, regulations and long-term maintenance considerations.

Conclusion

A percolation test is a practical and essential method for assessing the suitability of soil to support infiltration-based drainage solutions such as soakaways and drainage fields. By measuring how quickly water drains through the soil, it ensures that systems are designed to operate effectively, protect public health and avoid environmental harm.

Whether you are planning a small rural home or a large development, a properly conducted percolation test provides the foundation for safe, compliant and sustainable drainage design.