HOW DOES A SEPTIC TANK SOAKAWAY ACTUALLY WORK?
What happens inside a septic tank?
A septic tank is a primary settlement chamber designed to separate solids from liquid wastewater. Inside the tank:- Heavy solids settle and form sludge
- Oils and fats rise and form scum
- Partially clarified liquid (effluent) remains in the centre
What is a soakaway or drainage field?
A soakaway — often referred to as a drainage field — is an underground dispersal system that allows treated effluent to infiltrate the soil gradually. It typically consists of:- Perforated distribution pipes
- Gravel-filled trenches
- Geotextile membrane
- Surrounding permeable soil
How does the soil treat wastewater?
The soil acts as a natural biofilter. As effluent moves through unsaturated soil:- Pathogens are reduced
- Organic matter continues to break down
- Nutrients are filtered
- Harmful bacteria decline through biological activity
Why does soil type matter?
Soakaway performance depends on permeability. Free-draining soils such as sandy loam allow steady infiltration and oxygen exchange. Clay soils drain poorly. If a soakaway is installed in unsuitable ground:- Effluent cannot disperse properly
- Trenches become saturated
- Surface pooling may occur
- Drainage backs up
What is percolation testing?
Percolation testing measures how quickly water drains through soil. A test hole is excavated and filled with water. The drop in water level over time determines whether the ground is suitable and how large the drainage field must be. Without correct percolation testing, systems may be undersized or installed in unsuitable locations, increasing risk of early failure.How long should a soakaway last?
A correctly designed drainage field can last 20 to 30 years. However, lifespan may be reduced by:- Increased occupancy beyond design capacity
- Poor septic tank maintenance
- Excess solids entering the field
- Root intrusion
- Surface water ingress
- Natural silt accumulation
What are the signs of soakaway failure?
Common warning signs include:- Slow-draining sinks and toilets
- Gurgling sounds in pipework
- Persistent odours
- Waterlogged ground
- Unusually lush grass over the drainage area
- Surface discharge of wastewater
How is a soakaway assessed during a survey?
During a specialist homebuyer septic tank survey, assessment includes:- Visual inspection of the drainage area
- Evaluation of discharge arrangements
- Internal drain performance checks
- Identification of soil saturation indicators
- Review of system age and layout
- Compliance review
Final thoughts
A septic tank without a functioning soakaway is only half a drainage system. If you are buying a property with off-mains drainage, understanding how the soakaway works — and whether it is performing correctly — is essential. Boxall Ward provides specialist homebuyer septic tank surveys across Sussex, Kent and the South East, offering clear professional assessment before you commit to purchase.Frequently asked questions
- How far should a soakaway be from a house?
- Drainage fields are typically positioned several metres away from buildings to prevent structural impact and allow safe infiltration, depending on site layout and regulations.
- Can you drive or park over a soakaway?
- No. Vehicle weight can compact the soil, damage pipes, and reduce infiltration performance.
- Does heavy rain affect soakaway performance?
- Yes. Prolonged rainfall can saturate soil, reducing infiltration capacity and temporarily slowing drainage.
- Can trees damage a soakaway?
- Yes. Tree roots can enter pipes or compact soil, leading to blockages and reduced performance.
