A drainage field (also known as a soakaway system) is an essential part of a septic tank or sewage treatment plant, allowing wastewater to be safely and effectively absorbed into the ground. Over time, drainage fields can become clogged, saturated, or damaged, leading to system failure and potential environmental hazards. If your drainage field is no longer functioning properly, it may need to be replaced to comply with UK regulations (BS 6297:2007 & Environment Agency General Binding Rules).
A drainage field consists of a network of perforated pipes laid in gravel-filled trenches, allowing treated wastewater from a septic tank or sewage treatment plant to disperse into the surrounding soil. The natural filtration process removes contaminants before the water re-enters the environment.
Over time, drainage fields can become clogged with sludge, compacted, or waterlogged, leading to system failure. Common signs that your drainage field needs replacement include:
There are a host of reasons you might need to replace your drainage field, more common examples are:
Drainage field installations requite a lot of thought and planning. the process is very involved and incudes communication with the environment agency, local planning and thorough checks to ensure the location is not in a restricted area. the below is a simplified guide to what you should expect in a basic drainage field installation.
Before installing a drainage field, a percolation test is carried out to check how well the soil absorbs water. UK regulations prohibit drainage fields in clay-heavy or waterlogged areas, as poor drainage can lead to system failure. If the site is unsuitable, alternative drainage solutions may be required.
Based on the percolation test results, the drainage field is designed to the correct size, layout, and depth. Factors such as property size, daily wastewater volume, and soil conditions are taken into account to ensure long-term performance. A poorly designed system can cause blockages, odours, and groundwater contamination.
Trenches are dug at least 200mm below ground level, ensuring proper wastewater dispersion.
Perforated pipes are laid within a washed gravel bed (20mm–50mm stone) to improve filtration.
A geotextile membrane is placed over the gravel to prevent soil from clogging the system.
Trenches are backfilled with soil to restore the landscape and ensure stability.
The drainage field is connected to the outlet pipe of a septic tank or sewage treatment plant. UK regulations require that only treated effluent is discharged into the drainage field to avoid groundwater pollution.
Once installed, the system is tested to confirm proper water flow and absorption. It must meet BS 6297:2007 and Environment Agency General Binding Rules to ensure environmental safety.
We have dedicated pages explaning all the important documents where you will find all the information you need here but you can also take a look at the following links from gov.uk and british water guidelines:
Alternatively give us a call and we will be happy to go though any deails or parts to ensure you completely understand the requirements and best options for you and your property.
A percolation test (or perc test) is essential for evaluating whether your soil is suitable for a septic system soakaway or drainage field. This test measures how quickly water drains through the soil—a key factor in wastewater treatment system design.
Dig at least two test holes, each 300 mm x 300 mm wide and at least 300 mm below the proposed invert level (the bottom of the infiltration pipe). Space the holes along the planned layout of your subsurface irrigation system. While digging, record any changes in soil structure and note the depth of the water table, if present.
To simulate long-term soakaway conditions, fill each hole with water to a minimum depth of 300 mm. Allow the water to completely drain. This pre-saturates the soil for accurate testing.
If water drains away in under 10 minutes, refill the hole up to 10 times. If rapid drainage continues, the soil is too permeable and may pose a risk of groundwater contamination—making it unsuitable for a drainage field. If water has not drained after 6 hours, the soil is likely too dense or impermeable, also rendering it unsuitable for soakaway systems.
Refill each test hole with water to a depth of at least 300 mm. Measure the time (in seconds) it takes for the water to fall from 75% to 25% full (a 150 mm drop). Then, divide this time by 150 to calculate the percolation value (Vp)—the average number of seconds it takes water to drop 1 mm.
Perform the measurement at least three times per hole. Average the results to calculate a reliable Vp figure.
Document all data, including hole depths, soil characteristics, times recorded, and final percolation values. These results may be needed for planning permission or review by environmental authorities.
If your Vp readings vary significantly (more than 50% above or below the average), conduct additional tests—at least three more—at different points within the proposed drainage field.
A Vp between 15 and 100 seconds/mm indicates that the site is suitable for a drainage field. A Vp below 15 means the soil drains too quickly, risking insufficient treatment and pollution. A Vp above 100 suggests poor soakage, likely leading to surface ponding and system failure.
If the Vp is less than 15 or more than 100, contact your local environmental authority. They can advise on alternative wastewater disposal options, such as treatment units, mounded systems, or constructed wetlands.
Accurate percolation testing is critical for designing a safe and compliant septic drainage system. It helps prevent system failure, protects the environment, and ensures your site meets planning regulations.
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Whether you're planning a new septic system, replacing a failed drainage field, or simply ensuring your current setup stays compliant, Clearmasters is here to help. Our team specialises in accurate percolation testing, efficient drainage field installations, and reliable sewage treatment plant maintenance.
We operate a modern fleet of vehicles equipped with the latest technology, allowing us to deliver rapid, clean, and precise service on every job—whether it’s a routine maintenance visit or an urgent system issue.
Our experienced office staff are always available to offer expert advice, answer your questions, or help schedule your site assessment, percolation test, or drainage field servicing at a time that suits you.
On site, our fully trained and friendly engineers will carry out all work to the highest industry standards, keeping disruption to a minimum. We’re proud to support our customers with a professional, personal service that keeps your wastewater system safe, legal, and long-lasting.
A percolation test measures how quickly water drains through soil. It's essential before installing or replacing a drainage field, ensuring your septic system meets UK regulations and treats wastewater effectively.
You’ll need a percolation test before installing a new drainage field, replacing a failed one, or making significant changes to an existing septic system. It helps determine whether the soil is suitable for safe wastewater disposal.
Vp indicates how fast water drains. Values between 15–100 seconds/mm are suitable. Below 15 is too fast, risking pollution. Above 100 means poor soakage and likely surface flooding—both require alternative solutions.
Drainage field pipes are typically laid 600–700 mm below ground level. They must remain above the water table and in oxygen-rich soil to allow proper treatment and dispersion of effluent.
Possibly, but only if the soil conditions have improved or weren't the cause of failure. A new percolation test must be carried out to determine if the same area is still suitable for use.
A percolation test usually takes at least two days. It includes soil saturation, multiple measurements, and observing water infiltration to calculate accurate and compliant Vp values.
