Septic Tank Soakaway Problems: Causes, Signs & How to Fix Them

What Is a Septic Tank Soakaway?

A soakaway — also called a drainage field or leach field — is the area of ground where treated liquid from your septic tank is discharged and naturally filtered through the soil. It's a critical part of your septic system, and when it fails, the consequences can be serious and expensive.

A functioning drainage field consists of a network of perforated pipes buried in trenches filled with clean gravel, typically at a depth of 300–600mm. The treated effluent from your septic tank trickles through these pipes, percolates through the gravel, and is further cleaned by soil bacteria before reaching the water table.

Common Causes of Drainage Field Failure

1. Biological Clogging (Biomat Formation)

Over time, a biological layer called a 'biomat' naturally forms around the gravel and pipes. This is normal in small amounts, but if your septic tank isn't emptied regularly, partially treated sewage with high solid content reaches the drainage field and causes excessive biomat growth. This clogs the soil pores and prevents absorption.

2. Compaction

Driving vehicles, parking, or placing heavy structures over the drainage field compresses the soil, destroying the air spaces that are essential for the biological treatment process. Even regular foot traffic on a wet drainage field can cause compaction over time.

3. High Water Table

If the water table rises to the level of the drainage pipes — which can happen seasonally in many UK locations — the effluent has nowhere to go. The ground becomes saturated and the drainage field effectively stops working during wet periods.

4. Unsuitable Soil

Heavy clay soils absorb water very slowly. If the original percolation test was inadequate or conditions have changed, the soil simply cannot handle the volume of effluent being discharged. Sandy soils, conversely, can drain too quickly without adequate treatment.

5. Tree Root Intrusion

Tree and shrub roots are naturally attracted to the nutrient-rich moisture in a drainage field. Roots can block and break perforated pipes, disrupting the even distribution of effluent across the field.

6. Age

Even well-maintained drainage fields have a finite lifespan. Most last 15 to 25 years before the soil becomes too clogged with accumulated biomat to function effectively.

Signs Your Drainage Field Is Failing

• Soggy, waterlogged ground over the drainage field, even in dry weather
• Sewage surfacing — liquid or solid waste visible on the ground
• Foul smells in the garden, particularly near the drainage field area
• Unusually lush, green grass over the drainage field compared to the rest of the garden
• Slow drains in the house — if the drainage field can't accept effluent, it backs up through the system
• Standing water in inspection chambers between the tank and drainage field

How to Fix Drainage Field Problems

Short-Term Fixes

• Empty the septic tank — this reduces the load on the drainage field and may temporarily resolve symptoms
• Reduce water usage — shorter showers, fewer laundry loads, fix dripping taps
• Remove trees and shrubs growing near or over the drainage field
• Rest the drainage field — if you have a secondary area, alternating between fields allows recovery

Long-Term Solutions

Preventing Drainage Field Problems

• Empty your septic tank regularly — this is the single most important thing you can do. A well-maintained tank sends cleaner effluent to the drainage field
• Never drive or park over the drainage field
• Keep trees at least 3 metres away from drainage pipes
• Don't pave or build over the drainage field — it needs oxygen to function
• Avoid excessive water use — spread laundry loads across the week
• Use septic-safe cleaning products — chemicals that kill tank bacteria result in poorer effluent quality

Do I Need Planning Permission to Replace a Drainage Field?

In most cases, replacing a like-for-like drainage field does not require planning permission. However, you will need:

• Building Regulations approval — the new field must meet BS 6297 and Building Regulations Part H
• A percolation test — to confirm the soil at the new location is suitable
• Environmental considerations — the field must be a minimum distance from watercourses, boundaries, and water supplies

If your drainage field is failing, don't ignore it. Failed drainage fields can cause pollution — a criminal offence under the General Binding Rules. Contact a local operator for an assessment.