What is a Soakaway

A soakaway is a type of drainage solution designed to manage surface water by allowing it to percolate naturally into the surrounding ground. Typically constructed as a pit or trench filled with rubble, gravel or modular crate systems, a soakaway collects rainwater runoff from roofs, driveways, patios or other impermeable surfaces and facilitates its gradual dispersal into the soil. It plays a vital role in sustainable drainage strategies by reducing pressure on piped drainage systems and helping to prevent localised flooding.

Soakaways are commonly used in both domestic and commercial settings, especially in areas with limited access to public stormwater sewers. They are relatively simple in concept but must be properly designed and maintained to be effective over the long term.

How a Soakaway Works

The principle of a soakaway is based on infiltration. When surface water reaches the soakaway, it enters a void-filled structure below ground that temporarily holds the water. The stored water then seeps into the surrounding soil through natural drainage, replenishing the groundwater and avoiding runoff into surface water bodies.

The key elements of a typical soakaway include:

  • Inlet pipe or gully: Channels water from roof gutters, drains or surface collection points into the soakaway

  • Void space: Created using coarse aggregates or plastic crates to temporarily store water

  • Geotextile membrane: Often used to prevent fine soil from entering the void and causing blockage

  • Surrounding soil: Acts as the final filter and allows for infiltration

The speed and volume of infiltration depend on the soil type, rainfall patterns, and the soakaway’s design capacity. In permeable soils such as sand or loam, soakaways function effectively. In clay soils or areas with high water tables, performance may be poor or require enhanced design features.

Types of Soakaways

Soakaways come in several variations depending on the available space, expected water load and ground conditions.

1. Traditional Rubble-Filled Pit

This is the oldest and simplest form. A hole is dug and filled with coarse stone or rubble to create voids for water storage. It is inexpensive but may offer limited storage capacity and can clog over time without proper filtering.

2. Modular Soakaway Crates

Modern systems use engineered plastic crates or cells that interlock to form a stable structure with high void ratios (typically around 95 percent). These are wrapped in permeable geotextile and buried below ground. They are lightweight, easy to install and provide high capacity in compact areas.

3. Soakaway Trenches

Used for linear infiltration, these are long, narrow trenches filled with gravel or modular crates. Ideal for receiving runoff from long surfaces such as roads or car parks.

4. Permeable Paving with Integrated Soakaway

Some systems integrate soakaways beneath permeable surfaces, such as block paving or gravel stabilisation grids, allowing water to percolate vertically into the ground.

Each type has its own advantages and is selected based on site-specific requirements and constraints.

Where Soakaways Are Used

Soakaways are typically installed where there is a need to manage rainwater sustainably and where discharge to a sewer or watercourse is not feasible or desirable.

Common applications include:

  • Residential housing developments

  • Rural properties without access to surface water sewers

  • Industrial estates and commercial buildings

  • Roads and highways drainage

  • Sports fields and green infrastructure

  • Car parks and hard landscaping projects

They are particularly important in developments governed by planning rules that require on-site surface water management or compliance with Sustainable Drainage Systems (SuDS) standards.

Design Considerations

To be effective, a soakaway must be carefully designed with consideration of several key factors:

  • Soil infiltration rate: Tested on site, often using a percolation or infiltration test

  • Roof or surface area drained: Determines the volume of runoff to be managed

  • Depth to groundwater or bedrock: Soakaways must not interfere with the water table or structural foundations

  • Soil type: Sandy or loamy soils drain well, while clay soils hinder infiltration

  • Size and configuration: Must be adequate to store water from peak storm events

  • Distance from buildings and boundaries: Typically at least 5 metres from foundations to avoid undermining

British Standards such as BS EN 752 and CIRIA guidance documents provide methodologies for soakaway design. For example, storage volume is typically calculated to handle a 1-in-10 or 1-in-30 year rainfall event depending on local policy.

Advantages of Soakaways

Soakaways offer numerous benefits from both environmental and infrastructure perspectives.

  • Reduces surface water runoff: Limits the burden on public sewers and helps prevent localised flooding

  • Improves water quality: Filters pollutants naturally as water passes through the soil

  • Promotes groundwater recharge: Supports aquifers and helps maintain base flows in rivers

  • Cost-effective: In many cases, cheaper than constructing new drainage connections

  • Supports SuDS strategies: Encouraged in planning policies as part of green infrastructure

These advantages make soakaways a favoured solution in sustainable urban development and water-sensitive landscape design.

Limitations and Challenges

Despite their many benefits, soakaways are not suitable for all locations. Limitations include:

  • Soil permeability: In clay soils or compacted ground, infiltration is too slow or negligible

  • High groundwater levels: Can prevent effective drainage and pose a pollution risk

  • Space requirements: Large soakaways require adequate land away from buildings

  • Blockage risk: Without filtration, fine particles can clog the system over time

  • Maintenance needs: Overlooked soakaways may become ineffective due to silt or root ingress

In such cases, alternative SuDS features such as swales, rain gardens or detention basins may be more appropriate.

Maintenance and Inspection

Although soakaways are passive systems with no moving parts, they require occasional inspection and maintenance to ensure ongoing performance.

Recommended practices include:

  • Cleaning gutters and gullies regularly: Prevents debris from entering the system

  • Checking inlets for blockages or siltation: Especially after heavy storms

  • Inspecting geotextile membranes (where accessible): Ensures no root or soil ingress

  • Monitoring performance: Noticing signs such as surface ponding or damp areas near the soakaway may indicate failure

In modern installations, access points or inspection chambers are often provided for easier monitoring.

Regulatory and Planning Context

In the UK, the use of soakaways is supported by planning policy and environmental regulations aimed at managing flood risk and water quality.

  • Building Regulations (Part H): Require that surface water is managed on-site where possible, with soakaways as a preferred method

  • Sustainable Drainage Systems (SuDS) guidance: Promotes infiltration-based systems for new developments

  • Environment Agency guidance: Sets restrictions on soakaway use in sensitive areas, such as Source Protection Zones for drinking water

  • General Binding Rules: Apply to soakaways used for discharging treated effluent from septic tanks, with conditions around location and percolation performance

Local authorities may require percolation tests, design calculations and drainage plans as part of the planning approval process.

Environmental Considerations

Soakaways contribute to more natural hydrological cycles in urban areas by:

  • Reducing flow volumes and velocities that can cause stream erosion

  • Limiting pollution from first flush surface runoff

  • Allowing natural breakdown of organic pollutants in soil

  • Supporting vegetation growth and soil moisture balance

However, they must be correctly sited and constructed to avoid unintended consequences such as groundwater contamination or structural damage to nearby buildings.

Soakaways and SuDS

Soakaways are a core component of Sustainable Drainage Systems (SuDS), which aim to mimic natural water movement and improve resilience to climate change impacts.

In a SuDS context, soakaways are often used in combination with:

  • Permeable paving

  • Rainwater harvesting

  • Green roofs

  • Filter drains and trenches

This multi-layered approach allows for greater flexibility in managing rainwater and tailoring systems to individual site constraints.

Conclusion

A soakaway is a practical and environmentally sound method of managing surface water drainage by allowing it to infiltrate naturally into the ground. Whether constructed using traditional rubble or modern modular crates, a well-designed soakaway can reduce flood risk, protect watercourses and support sustainable development goals.

To ensure long-term functionality, soakaways must be correctly sized, installed in suitable soil conditions and maintained periodically. As climate variability increases and urbanisation continues, soakaways will remain an important part of holistic water management strategies across the UK.