What is a Borehole Soakaway

A borehole soakaway is a deep vertical infiltration structure designed to disperse surface water, treated effluent or clean drainage into permeable geological layers located beneath the ground surface. Unlike shallow soakaways that rely on near surface soils, borehole soakaways penetrate through impermeable strata to reach deeper layers of sand, gravel, chalk or fractured rock where infiltration is far more effective. This approach is particularly valuable in areas with limited space, poor soil permeability or high surface water loadings that cannot be managed using conventional drainage methods.

Borehole soakaways are widely used in sustainable drainage, commercial and industrial developments, stormwater attenuation systems and rural drainage solutions. Their ability to handle significant volumes of water within a compact footprint makes them an essential engineering tool in challenging ground conditions.

This article explores the purpose, design and construction of borehole soakaways, explains their hydraulic behaviour, outlines regulatory considerations, discusses advantages and limitations, and examines their role in modern drainage engineering.

The purpose of a borehole soakaway

The primary function of a borehole soakaway is to remove excess water from the surface or drainage networks by directing it deep into the ground. Many soils, particularly clay based or compacted soils, have poor infiltration rates at shallow depths. Water tends to accumulate at the surface, increasing the risk of flooding, erosion and structural damage.

By drilling vertically to reach permeable strata, engineers create a pathway through which water can drain efficiently. The deeper layers usually possess larger pore spaces and greater hydraulic conductivity, allowing water to disperse more rapidly and consistently than in surface soils. This method ensures reliable drainage performance even during prolonged rainfall events or peak flow periods.

In addition to stormwater management, borehole soakaways may also be used for treated wastewater disposal where environmental conditions and regulations permit.

Geological and hydrogeological principles

Successful borehole soakaway design depends on understanding the geological and hydrogeological conditions at the site. The ability of the system to absorb water is directly linked to the permeability, depth and thickness of the target strata. Sands, gravels, chalk and fractured limestone typically offer the best infiltration characteristics due to their interconnected pore structures.

During the design phase, site investigations are conducted to establish the depth of impermeable layers, identify suitable infiltration zones and determine seasonal groundwater levels. Engineers must ensure that the borehole does not intersect groundwater in a manner that could lead to contamination. The base of the soakaway should remain above the maximum groundwater table to maintain unsaturated flow conditions, enabling effective infiltration and natural filtration of water as it passes through the soil.

Hydrogeological modelling may be used to predict long term performance and ensure that water discharged to the ground will not exceed the natural recharge capacity of the aquifer.

Design and construction of borehole soakaways

Borehole soakaways vary in depth and diameter depending on the volume of water to be managed and the site specific geological conditions. Depths commonly range from 5 to 50 metres, with diameters from 150 mm to over 600 mm. The borehole is drilled using cable percussion or rotary methods depending on ground hardness.

Once drilled, the borehole is cased through unstable or impermeable layers to prevent collapse and ensure water flows only into the target permeable zone. The lower section is left open, screened or filled with graded gravel to promote uniform infiltration. A headworks chamber or inspection chamber is installed at the surface to receive water from drainage systems and distribute it evenly into the borehole.

Flow control features may include silt traps, filters or vortex separators to prevent fine particles entering the borehole and reducing infiltration capacity. Maintenance access is essential to ensure long term performance.

Hydraulic behaviour and performance

The performance of a borehole soakaway is governed by Darcy’s Law and unsaturated flow dynamics. Water entering the soakaway moves outward into the soil at a rate determined by hydraulic conductivity, head pressure and the geometry of the borehole. The vertical nature of the system creates a large wetted surface area relative to its footprint, allowing significant volumes of water to be absorbed into the surrounding strata.

During high flow conditions, the borehole temporarily stores water while infiltration continues. In well designed systems, water levels within the chamber subside rapidly once inflow stops. Poor performance may occur if the receiving strata become saturated, if silt accumulates, or if groundwater levels rise unexpectedly.

To assess suitability, percolation tests and pumping tests are often undertaken to measure infiltration rates. These tests help determine required borehole diameter, depth and the number of soakaways required for a given development.

Environmental and regulatory considerations

In the United Kingdom, borehole soakaways must comply with both drainage and environmental regulations. The Environment Agency governs discharge of water into the ground, ensuring protection of groundwater resources. Any system designed for infiltration must avoid direct discharge into aquifers used for drinking water abstraction unless specific permissions are obtained.

Planning authorities may also require evidence that borehole soakaways will not increase flood risk or cause instability in nearby structures. Building regulations specify requirements for drainage layout, separation distances from foundations and surface water management.

Where treated effluent is involved, additional controls apply. The system must demonstrate adequate purification capacity through soil filtration and attenuation before water reaches groundwater. Protective buffer zones may be required around watercourses and protected habitats.

Applications in modern drainage engineering

Borehole soakaways are commonly used in urban environments where land availability is limited. Roof drainage, car parks, industrial yards and commercial buildings often rely on deep soakaways when surface infiltration is not feasible. In rural settings, they help manage runoff from farm buildings, tracks and agricultural hardstanding.

They are also incorporated into sustainable drainage systems to reduce surface water discharge into sewers, supporting compliance with policies aimed at mitigating flooding and improving environmental resilience.

Industrial facilities may use borehole soakaways to manage uncontaminated process water or cooling water, subject to regulatory approval. In such cases, strict monitoring and quality control are required.

Advantages of borehole soakaways

Borehole soakaways offer several important advantages over conventional shallow systems. Their ability to reach deep permeable layers allows them to function reliably even where surface soils have poor infiltration capacity.

Key benefits include:

• High infiltration capacity within a small surface area, ideal for constrained sites

• Effective management of large stormwater volumes with minimal land take

• Reduced discharge to surface water sewers, supporting sustainable drainage goals

• Long operational lifespan when properly designed and maintained

These characteristics make borehole soakaways attractive for large developments and locations where alternative drainage solutions would be impractical or cost prohibitive.

Limitations and potential challenges

Despite their effectiveness, borehole soakaways also present challenges. Their performance is heavily dependent on underlying geology and may be unsuitable in areas with high groundwater tables, clay dominated geology or sensitive aquifers. Drilling costs can be significant, especially where deep permeable layers are required.

Silting is a common issue. Without effective pre treatment, fine particles can accumulate in the borehole and reduce infiltration rates. Maintenance access must therefore be incorporated. In some cases, periodic jetting or redevelopment of the borehole may be required.

Another limitation is the potential for groundwater contamination if the system is poorly designed or used to dispose of inappropriate effluent. Strict adherence to environmental regulations is essential to avoid long term environmental harm.

Maintenance and long term operation

Regular inspection and maintenance are necessary to ensure ongoing performance. Typical activities include clearing silt traps, inspecting headworks chambers, checking access covers, removing accumulated debris and monitoring water levels during rainfall events. Periodic hydraulic testing may also be performed to assess changes in infiltration capacity.

If performance declines significantly, remediation may involve airlifting, backwashing, jetting or replacing gravel packs. In some cases, redeveloping the borehole using specialist techniques can restore its capacity.

The role of borehole soakaways in future drainage systems

As climate change increases rainfall intensity and urban development continues to place pressure on drainage infrastructure, borehole soakaways will play an increasingly important role. Their ability to manage high volumes of water underground supports resilience, reduces surface flooding and complements sustainable drainage strategies aimed at restoring natural infiltration.

Innovations in drilling technology, flow control, monitoring and geotechnical assessment are improving reliability and reducing installation costs. Advanced modelling tools allow engineers to predict long term behaviour with greater accuracy, ensuring sustainable performance over the lifespan of the system.

Borehole soakaways represent a sophisticated yet practical solution for modern water management. When designed and operated correctly, they provide a powerful means of returning water to the environment safely and sustainably while supporting infrastructure resilience in both urban and rural settings.