Soakaway Design According to BRE Digest 365

A soakaway is a simple yet effective drainage solution designed to manage surface water by allowing it to seep into the ground naturally. Typically constructed as a pit or a chamber filled with porous materials such as gravel or rubble, a soakaway enables rainwater and runoff to disperse slowly into the surrounding soil, preventing flooding and waterlogging.

Soakaways are commonly used in residential, commercial, and agricultural settings as part of sustainable drainage systems (SuDS). They are especially useful for areas with limited access to main drainage systems, helping reduce the pressure on urban infrastructure while promoting groundwater recharge. In addition to their functional benefits, soakaways play a key role in mitigating environmental issues, such as reducing water pollution by filtering out debris and contaminants before water reenters the soil.

Proper soakaway design and maintenance are essential to function effectively. Factors like soil permeability, rainfall intensity, and the volume of water to be managed must be carefully considered during installation. With their cost-effectiveness and eco-friendly advantages, soakaways have become a popular choice for modern drainage challenges.

Soakaway Design According to BRE Digest 365

1.1 Usage of Soakaway

Soakaways are used to store the immediate surface water run-off from hard surfaced areas, such as roofs or car parks, and allow for efficient infiltration into the adjacent soil. They discharge their stored water sufficiently quickly to provide the necessary capacity to receive run-off from a subsequent storm.

The time taken for discharge depends upon the soakaway shape and size, and the surrounding soil’s infiltration characteristics.

1.2 Shape of Soakaway

A soakaway is typically a structure designed to manage surface water by allowing it to percolate back into the ground, rather than flowing into stormwater systems. The shape of a soakaway is often determined by site-specific conditions, soil characteristics, and water management requirements. Common shapes include:

Rectangular or Square Soakaway

Cylindrical Soakaway

• Borehole-style soakaway: Drilled vertically into the ground, especially in areas with permeable strata such as chalk or sand below.
• Advantages: Ideal for deep infiltration in areas with high groundwater flow capacity.
• Usage: Suitable for sites with limited horizontal space or dense urban areas.

Trench Soakaway

Long, narrow excavations filled with gravel or perforated modular crates.

• Advantages: Covers larger areas and spreads the water discharge over a wide zone.
• Usage: Common in commercial and agricultural installations.

Modular Soakaway (Crate Systems)

A modern design comprising lightweight, modular plastic crates assembled into any shape, though typically rectangular.

• Advantages: Lightweight, customizable, and high void ratio (usually 95%).
• Usage: Popular in new builds and sustainable urban drainage systems (SuDS).

Factors Affecting Shape Selection:

• Soil Permeability: Highly permeable soils allow flexibility in shape, while less permeable soils may need deep or elongated structures.
• Available Space: Compact plots may require vertical or modular solutions.
• Volume of Water: Larger catchment areas necessitate bigger soakaways.
• Regulatory Requirements: Some areas mandate specific dimensions and designs for soakaways.

1.3 Media of Soakaway

They can be filled with rubble, Granular (gravel or brickwork), plastic cells, perforated pre-cast concrete ring units or any similar structure that collects rainwater and run-off. The structures are built to allow rainwater to infiltrate directly into the ground. Soakaways can also be deep bored.

Perforated Pre-cast Concrete Ring Units	Plastic Cells	Gravel or Brickwork

1.4 Soakaway may not be an appropriate solution:

There are times when a soakaway may not be an appropriate solution,

• In areas of ground that have low permeability,
• Where surface water could be contaminated.
• The maximum seasonal water table should be above the base of the soakaway; contaminants in the ground could be mobilised,
• In areas of instability.

1.5 Cost and performance

The size and complexity of the soakaway largely dictates the costs involved.

• Overall the cost of a soakaway can be described as low/medium compared with conventional drainage.
• Larger soakaways cost more to construct due to:
  • Higher labour costs and the use of more construction materials;
  • Disposal of excavated soil may also be an issue.

Running and maintenance costs are low, with routinely undertaken tasks as follows:

• Removal of sediments and debris from pre-treatment devices (leaf screens, sedimentation chambers, filter strips and swales).
• Cleaning of gutters or filters on downpipes.
• Removal of roots causing blockages.
• Monitoring performance.

1.6 Soakaway Design

1.6.1 Principles

The design of a soakaway depends on a number of factors including the following:

• Permeability of the ground.
• Groundwater level (preferably undertaken when water levels will be highest, during winter to spring)
• Type of ground
• Contamination
• Space restrictions
• Building foundations (It is not desirable to locate a soakaway close to a building. A minimum distance of 5 m is most often quoted)
• Risk of ground instability and other hazards.

Perforated pre-cast concrete ring unit soakaways should be installed within a square pit, with sides about twice the selected ring unit diameter.

• Granular fill can be separated from the surrounding soil by a suitable geotextile to prevent migration of fine particles into the soakaway.
• The top surface of the granular fill should also be covered with geotextile to prevent the ingress of fill material during and after surface reinstatement.
• Geotextile should not be wrapped around the outside of the ring units as it cannot be cleaned satisfactorily or removed when it has become blocked.

In order to limit any possible alteration to the quality of groundwater, attention should be paid to the source of the run-off water that is to be collected.

• If it is from a paved surface where there is a risk of oil or fuel spillage, a light liquid separator should be provided.
• Domestic drives and paths should not need a light liquid separator, but municipal roads and car parks will require them.

For drained areas less than 100 m², soakaways can be:

• square or circular pits, either filled with rubble or lined with dry-jointed brickwork, or perforated pre-cast concrete ring units surrounded by suitable granular fill.

For drained areas above 100 m², soakaways can be:

• perforated pre-cast ring units or trench type and not substantially deeper than soakaways that serve small areas:
  • 3 to 4 m is adequate if ground conditions allow.

There is an increasing number of soakaways that are being built using plastic cells.

• These are typically lightweight modular water storage cells with a high void ratio.
• Plastic cells can be used for either attenuation or infiltration of surface water in residential, commercial, industrial and retail applications.
• Proprietary components such as silt traps, flow control units and adaptors will normally be used as part of these systems.

1.6.2 Site Investigation and Testing

Site investigation and testing should be carried out prior to design or construction work taking place; this is part of the design process.

The site investigation will be required to assess the following:

• Water table depth and perched water table presence/depth (based on the worst annual case, ie during April or May). Groundwater levels should be investigated to ensure that the base of the proposed infiltration component is at least 1 m above the maximum anticipated groundwater level.
• Chemical contamination risks
• Suitability of strata for soakaway discharges, including permeability.

In the site investigation, boreholes and/or soakage trial pits can be used to determine the ground condition, soil material and presence and depth of groundwater.

1.6.3 Risk assessment

The information gathered during the site investigation should be used to prepare a risk assessment, which may be required by planning and building development authorities. Issues of interest are chemical contamination, ground failure features and the effects of adjacent development.

• The soakaway should not connect a contamination source to a groundwater target, ie creating a source-pathway-receptor linkage.
• Ground and slope instability, flooding and wash out-induced settlements should be considered.

1.6.4 Soil infiltration

Site testing for soil infiltration rates should give representative results for the proposed site of the soakaway. This is achieved by the following:

• Excavating a soakage trial pit of sufficient size to represent a section of the soakaway.
• Filling the soakage trial pit several times in quick succession while monitoring the rate of seepage.
• Examining site data to ensure that the area surrounding the soakaway has been assessed for variations in soil conditions, areas of filled land, preferential underground seepage routes, variations in the level of groundwater, and any geotechnical and geological factors likely to affect the long-term percolation and stability.

1.6.5 Infiltration Rate test

Field investigations are required to confirm infiltration rates. The procedure recommended in this Digest is to excavate a soakage trial pit to the same depth as anticipated in the full-size soakaway.

• For run-off from 100 m² this will be 1 m to 1.5 m below the invert level of the drain discharging to the soakaway.
• Overall depths of excavation will be typically 1.5 m to 2.5 m for permeable areas >100 m² draining to the soakaway.
• The soakage trial pit should be 1 m to 3 m long and 0.3 m to 1 m wide. It should have vertical sides trimmed square and, if necessary for stability, should be filled with granular material.

Fill the soakage trial pit and allow it to drain three times to near empty. Each time record the water level and time from filling, at intervals sufficiently close to clearly define water level versus time. The filling of the soakage trial pit should be on the same or consecutive days.

1.6.6 Sizing

The design method for sizing a soakaway is based upon the equation of volumes:

I – O = S

where:

• I = the inflow from the impermeable area drained to the soakaway
• O = the outflow infiltrating into the soil during rainfall
• S = the required storage in the soakaway to balance temporarily inflow and outflow.

1.6.6.1 Inflow to the Soakaway

Q=CIA/360

where:

• Q = maximum rate of runoff (m³/s)
• C = runoff coefficient
• I = rainfall intensity with a duration equal to the time of concentration (mm/hr)
• A = drainage area (ha)

Type of Drainage Area	Runoff Coefficient
Business
Downtown areas	0.70 – 0.95
Neighbourhood areas	0.50 – 0.70
Residential
Single-family areas	0.30 – 0.50
Multi-units, detached	0.40 – 0.60
Multi-units, attached	0.60 – 0.75
Suburban	0.25 – 0.40
Apartment dwelling areas	0.50 – 0.70
Industrial
Light areas	0.50 – 0.80
Heavy areas	0.60 – 0.90
Parks, cemeteries	0.10 – 0.25
Playgrounds	0.20 – 0.40
Railroad yards	0.20 – 0.40
Railroad yards	0.20 – 0.40
Unimproved areas	0.10 – 0.30
Lawns
Sandy soil, flat, 2%	0.05 – 0.10
Sandy soil, average, 2-7%	0.10 – 0.15
Sandy soil, steep, 7%	0.15 – 0.20
Heavy soil, flat, 2%	0.13 – 0.17
Heavy soil, average, 2-7%	0.18 – 0.22
Heavy soil, steep, 7%	0.25 – 0.35
Streets
Asphaltic	0.70 – 0.95
Concrete	0.80 – 0.95
Brick	0.70 – 0.85
Drives and walks	0.75 – 0.85
Roofs	0.75 – 0.95

1.6.6.2 Outflow from the Soakaway

O = a_s50 × f × D

where:

• a_s50 = the internal surface area of the soakaway to 50% effective storage depth: this excludes the base area which is assumed to clog with fine particles and become ineffective in the long term
• f     = the soil infiltration rate determined in a soakage trial pit at the site of the soakaway
• D   = the storm duration.

1.6.6.3 Required storage volume in the soakaway, S

Storage must be equal to, or greater than, inflow minus outflow, defined above in sections 3.6.1 and 3.6.2, and is the required effective volume available between the base of the soakaway and the invert of the drain discharging to the soakaway.

There are four steps in the design procedure:

1. Carry out a site investigation to determine the soil infiltration rate;
2. Decide on a construction type (eg filled pit in square, circular or trench form, or concrete ring units with granular surround);
3. Calculate required storage volume, S, from inflow minus outflow for a range of durations of 10-year design storms to determine the maximum storage predicted for the type of soakaway (or as per the design manual);
4. Review the design to ensure its overall suitability considering space requirements, site layout and time for emptying.

O = W * L * d * (V%)

Where:

• W    = Width of the soakaway;
• L     = Length of the soakaway;
• D    = Depth of the soakaway (Soakaway height); (Trench soakaway depths should generally be between 1 and 2 m. (According to SuDS)
• V% = Void ratio of fill material (voids volume/total volume) According to SuDS

             Manual, the void ratio of the different types of media as the following table:

Material	Porosity, v
Geocellular systems	0.90 – 0.95
Uniform gravel	0.30 – 0.40
graded sand or gravel	0.20 – 0.30

For Example: A perforated concrete ring soakaway may be installed in a square or rectangular plan excavation and the gap between the rings and the soil filled with clean stone. Under these circumstances an effective porosity, v’, applies.

where

• r’   = radius of the ring sections (m)
• W = width of the excavation (m)
• L   = length of the excavation (m)

1.6.6.4 Time of Emptying of Soakaway

The infiltration component should discharge from full to half􀀐full within a reasonable time so that the risk of it not being able to manage a subsequent rainfall event is minimized. It is usual to specify that half emptying occurs within 24 hours (or as per the design manual).

Check on time of emptying half storage volume, t_s50:

As Per Dubai Municipality, The following criteria shall be considered for sizing of Modern Plastic Soakaways:

Parameter	Design Criteria
Design with overflow arrangement ARI	10-year
Design without overflow arrangement ARI	25-year
Emptying time at low level	5 days
Emptying time at high level	2-3 weeks

It shall be noted that the emptying time for the higher level is approximate. Actual emptying time may vary depending on the infiltration rate or outlet sizing based on the lower level emptying time.

References

1. BRE (1991) Soakaway design, BRE Digest 365
2. CIRIA SuDS Manual 2015