CD 530 Design of soakaways

This document is published by National Highways.

This document supersedes HA 118/06 which is withdrawn.

This document forms part of the works specification. It does not purport to include all the necessary provisions of a contract. Users are responsible for applying all appropriate documents applicable to their contract.

Road runoff can, under certain circumstances, have an adverse effect on receiving waters, including groundwater. This can arise both from the effects of routine runoff and from spillages on the road. Overseeing Organisations have a duty under pollution protection legislation to ensure that road runoff does not pollute receiving waters. Further requirements and advice regarding legislation governing discharges to the ground are provided in LA 113 [Ref 10.N].

The disposal of road runoff through soakaway systems is reasonably well established within the UK. However, pollution attenuation mechanisms affecting road runoff at the discharge point below the soakaway and within the unsaturated zone above the water table have, in the past, been less well understood.

This document originates from a review of available information on the fate and transport of road contaminants and the design of existing road soakaway systems. The review focused primarily on an understanding of the processes operating in the unsaturated zone (i.e. above the water table) and how these may influence soakaway designs and prevent groundwater pollution. This has been backed up by recent research Byrns & Brewin 2010 [Ref 6.I] that identified significant pollution attenuation mechanisms operating within soakaway drainage systems which should be taken into account during soakaway design.

Requirements and advice for the design of other drainage structures such as linear drains are provided in CG 501 [Ref 1.N]. Requirements and advice for the design of infiltration basins and other vegetated systems to treat road runoff are provided in CD 532 [Ref 12.I]. Requirements and advice for the assessment of risks to groundwater from road runoff are provided in LA 113 [Ref 10.N].

This document provides requirements for and advice on the design of soakaways that may be incorporated into systems used to treat and store road runoff prior to discharging to ground. This document is based upon and refers directly to current available design guidance.

The assumptions made in GG 101 [Ref 6.N] apply to this document.

1.1 This document shall be used for the design of soakaway systems that comprise drainage structures (pits, chambers and trenches) that allow infiltration of road runoff to the ground through their base and sides and that incorporate below-ground storage.

1.2 Soakaway systems shall only be used where space or other site specific constraints prevent the use of a broad and shallow infiltration system.

NOTE 1 Broad and shallow infiltration systems, such as infiltration ponds, offer increased potential for pollution attenuation (see Appendix A) and are the preferred option where the road drainage is to be discharged to ground.

NOTE 2 Other documents provide requirements and advice on the design of alternative infiltration systems that utilise infiltration, such as infiltration ponds in CD 532 [Ref 12.I], grassed channels in CD 521 [Ref 4.N] and reservoir pavements in CD 531 [Ref 9.I].

1.3 Before undertaking the drainage design, an assessment of the flood and pollution risk from highway runoff shall be undertaken in accordance with LA 113 [Ref 10.N].

NOTE 1 The requirements and advice document LA 113 [Ref 10.N] provides an overview of the current knowledge of road runoff in the UK, the ground conditions that are believed to affect its nature, and provides requirements and advice for the assessment of the pollution risk arising from routine road runoff and accidental spillages.

NOTE 2 Methods for assessment of pollution risk and impacts from both routine runoff (chronic impact) and from spillage (acute impact) are provided in LA 113 [Ref 10.N].

1.4 Design proposals for discharging road runoff to the ground, including the selection of the soakaway site, shall be the subject of consultation with the relevant environmental protection agency (EPA).

1.5 The design shall incorporate measures to address constraints identified by the EPA.

NOTE Although under UK legislation consents are not required for the discharge of road runoff, the EPA has the power to serve prohibition notices in respect of discharges that are in breach of pollution legislation.

1.6 The design of the soakaway system shall be based upon site-specific conditions.

NOTE The design of soakaways cannot be prescribed. Each site-specific situation is evaluated individually as there are many factors that will influence soakaway design. These include, for examples: available space; depth of unsaturated zone; geology; discharge rate and volume; and traffic flows.

1.7 This document shall be implemented forthwith on all schemes involving the design of soakaways on the Overseeing Organisations’ motorway and all-purpose trunk roads according to the implementation requirements of  GG 101 [Ref 6.N].

1.8 The requirements contained in GG 101 [Ref 6.N] shall be followed in respect of activities covered by this document.

2.1 The design of the soakaway shall promote efficient hydraulic contact between the drainage system and the underlying ground in order to provide effective drainage.

2.2 The primary discharge from the soakaway shall be via infiltration to the sub-surface.

NOTE 1 There are a range of other drainage systems where the primary discharge is to the sub-surface, these include: combined surface and groundwater filter drains; fin drains; filter drains; informal drains or ‘over the edge drainage’. There can be other drainage systems from which discharge to ground occurs but is incidental to the overall function of the drainage (e.g. unlined ditches). Requirements and advice for all these drainage systems is provided in CG 501 [Ref 1.N].

NOTE 2 Surface water drainage features can also incorporate discharges to the sub-surface for example: retention ponds; sedimentation ponds; infiltration basins and wetlands. Requirements and advice for these systems is provided in CD 532 [Ref 12.I].

NOTE 3 Similarly, swales and grassed channels can incorporate discharge to the ground. Requirements and advice for these systems is provided in CD 532 [Ref 12.I].

2.3 Soakaways shall be sited in porous and permeable ground of sufficient depth and lateral extent to be able to accommodate potential maximum discharges under design storm conditions.

2.3.1 Where slow infiltration rates occur, the use of balancing ponds or other flow attenuation mechanisms may be required to accept the quantity of runoff generated under peak flow conditions by the road drainage.

NOTE What is good for the physical disposal of road drainage (that is, rapid dispersal of flow) is the opposite of what is required with respect to the in-ground attenuation of pollutants. Slow infiltration rates can be utilised to optimise these in-ground attenuation processes.

2.4 Soakaway design shall allow for pollution attenuation processes active within the unsaturated zone.

NOTE 1 Pollution attenuation processes can be enhanced by maximising contact with strata with the greatest attenuation potential or preventing direct discharge into strata that do not offer any potential. Further advice is available in LA 113 [Ref 10.N] and CIRIA C753 [Ref 13.N].

NOTE 2 Appendix A provides information on the pollution attenuation processes active in the unsaturated zone which can be taken into account in soakaway design.

2.5 The depth of the unsaturated zone beneath the soakaway shall be maximised.

NOTE The effectiveness of natural attenuation processes can be enhanced through increased depths of unsaturated zone beneath the soakaway.

2.6 The depth from the base of the soakaway to the maximum likely groundwater level shall be a minimum of 1.2 m.

NOTE 1 In accordance with CIRIA C753 [Ref 13.N], adopting a minimum (1 m) depth to groundwater minimises the risk of loss of storage in the soakaway from rising groundwater, reduces the potential for direct discharge into groundwater and reduces the risk of groundwater contamination.

NOTE 2 The Environment Agency sets out a series of position statements with respect to the protection of groundwater: LIT 7660 2018 [Ref 11.I]. Within this document, Section G refers to discharges of sewage effluent into the ground, which in turn refers to BS 6297 2007 [Ref 4.I] that sets a minimum depth of 1.2 m between the base of an infiltration trench and the seasonally highest groundwater level. This minimum depth to groundwater is adopted within this requirement.

2.7 Site specific information shall be gathered to determine the depth to groundwater beneath the base of the soakaway.

2.7.1 To determine the maximum level of groundwater, information may be gathered by adopting a lines of evidence approach.

NOTE 1 The lines of evidence approach can include, for example, site-specific ground investigations and desk studies undertaken by a groundwater specialist.

NOTE 2 Depth to groundwater is variable and seasonal. Short term monitoring alone can be insufficient to determine a representative maximum groundwater level. All available evidence is used to determine this maximum level.

2.8 The design of soakaways, their location and functionality shall incorporate measures to minimise or mitigate potential impacts on landscape and ecology.

NOTE Discharges to groundwater can potentially affect biodiversity (such as down gradient groundwater-dependent terrestrial ecosystems (GWDTE)) through impacts arising from changes in groundwater flow and/or groundwater quality). Impacts on GWDTE can in turn have impacts on landscape. These impacts can be identified through the risk assessment process carried out under LA 113 [Ref 10.N] and mitigation measures incorporated into the design. Open, rubble-filled pits could have direct impacts on biodiversity and landscape.

3.3 The hydraulic performance of the soakaway system design shall provide sufficient storage and infiltration capacity to allow drainage of the design storm from the carriageway.

3.4 The soakaway drainage system shall be designed to manage surface water runoff from the 1:10 year storm return period, or greater.

3.5 The soakaway system design shall allow for the effects of climate change on peak rainfall intensity and volume over the lifetime of the development.

NOTE Advice on climate change allowance is provided in CG 501 [Ref 1.N] and CIRIA C737 [Ref 12.N].

3.6 The soakaway design shall incorporate measures necessary to provide spillage and pollution control to protect receiving groundwater.

NOTE The level of pollution prevention and control provided is to be commensurate with the risk to groundwater at each site. Guidance on the risk of pollution from accidental spillages is provided in LA 113 [Ref 10.N].

3.7 The soakaway and its associated elements shall adopt a design sympathetic to the local landscape.

NOTE 1 Landscaping requirements and advice can be found in LA 107 [Ref 7.N] and LD 117 [Ref 7.I].

NOTE 2 Whilst the soakaway itself can have little manifestation at the surface the associated elements such as fencing, maintenance facilities, access routes and signage can all have landscape impacts.

3.8 General site selection shall include assessment of the following factors:

NOTE The 2000/60/EC [Ref 3.N] is such that it essentially treats all groundwater with equal weight – i.e. a certain ‘minimum level’ of protection is required. The discharge of non-hazardous substances to groundwater is permitted with adequate risk assessment. Where source protection zones could be impacted, this will affect the level of permissible risk and additional protection can be warranted with respect to the protection of human health. Further advice can be found in LA 113 [Ref 10.N].

3.9 Evaluation of the geological setting of the site and the groundwater risk assessment process described in LA 113 [Ref 10.N] shall be fundamental to the development of the design.

NOTE Further Information on variations in ground conditions and their influence on design is provided in Appendix B.

3.10 Following general site selection, the site-specific design process shall be followed.

3.11 Soakaway design along the selected route shall allow for variations in ground conditions and the range of factors described in CG 501 [Ref 1.N].

NOTE 1 In the past, designs have tended to be generic for a whole scheme with no variations to take into account differences in ground conditions along the route. As a road generally traverses a wide range of ground conditions, the design needs of the discharge system will change to reflect this.

NOTE 2 Changes along the route could be such that in low lying areas near rivers there could be little unsaturated zone available and a soakaway broad and flat in configuration can be used (or a soakaway not used at all); on high ground, the depth of unsaturated zone can be large enough for smaller deeper structures, reducing the amount of land take.

3.12 The design of the soakaway shall be selected such that:

3.13 The proximity of buildings to the soakaway site shall be evaluated on a site-specific basis, particularly with respect to ground conditions.

NOTE High discharge volumes and rates can be generated by road run-off. Prescribed limits with respect to the proximity of buildings are not always appropriate to a specific site.

3.14 A minimum of 5 m shall be allowed between the soakaway and any buildings or similar structures.

NOTE 1 BS EN 752 [Ref 5.I] no longer provides for a minimum displacement between a building and a soakaway, but identifies specific risks associated with below-ground infiltration systems that concentrate significant volumes of water in a small area (such as soakaways) and recommends obtaining specialist advice.

NOTE 2 BRE DG 365 [Ref 11.N] recommends that soakaways are constructed at least 5 m from building foundations.

NOTE 3 Both Building Regs Doc. H [Ref 1.I] and the Technical Handbook of SBS TH2017 [Ref 10.I] provide a recommendation that soakaways are a minimum of 5 m from buildings.

3.15 The soakaway design shall include the following elements:

3.16 The design shall include allowance (outlets and/or overflows) for the controlled overflow of exceedance events.

3.17 Any discharge from overflows for exceedance events shall be routed safely to avoid flooding the road and minimise impact upon adjacent land.

3.18 The drainage system as a whole shall be assessed for the consequences of exceedance for storm return periods in excess of 1 in 100 years in accordance with CG 501 [Ref 1.N] and LA 113 [Ref 10.N].

3.19 The depth and size of soakaways shall be adjusted to allow for variations in depth to groundwater.

NOTE 1 It is good practice to maximise the depth of unsaturated zone below a soakaway device to allow the maximum attenuation of pollutants to occur. On this basis, design depths of no more than 3 to 4 m provide the optimum opportunity for these attenuation processes to occur.

NOTE 2 In areas with less unsaturated depth, the soakaway system can comprise a number of shallow interconnected chambers to provide sufficient short-term storage, whilst maximising the depth of unsaturated zone beneath the soakaway. In areas with a deeper unsaturated zone, the soakaway can comprise fewer deeper chambers, so requiring less land, whilst still maintaining sufficient attenuation capacity. This is illustrated schematically in Figure 3.19N2. Further land can still be needed for access and maintenance.

NOTE 3 Surface infiltration systems (as examples lagoons, and infiltration ponds) also depend on surface area to provide sufficient drainage capacity, so that a large shallow infiltration pond will allow rapid dispersal of water through the semi-permeable base whereas a deep narrow pond will retain water for much longer. Further details on the design of these shallow systems is provided in CD 532 [Ref 12.I].

3.20 The design of the soakaway drainage system shall provide a balance between sufficient infiltration rate and storage capacity to allow the fast and efficient removal of water from the surface of the road.

3.21 The cumulative storage capacity of the soakaway drainage system shall include storage within the soakaway itself and any additional upstream storage.

NOTE Storage is essential where the discharge rate from the road exceeds the infiltration capacity of the soakaway and provides temporary storage until the generated volume of stormwater is dissipated. Upstream storage can include, for example, detention ponds, oversized pipes, underground chambers or other means.

3.22 The storage capacity within the soakaway system shall be designed to manage peak runoff and rainfall volume from the defined design storm events.

NOTE Guidance in CIRIA C753 [Ref 13.N] suggests that the infiltration component discharges from full to half full within 24 hours so that subsequent rainfall events can be managed. This capacity can be built into system storage upstream of the soakaway (such as in ponds or oversized pipes) and can be included in design calculations.

3.23 The rate of runoff from the road surface shall be calculated with an appropriate design methodology / drainage modelling programme using the actual design rainfall values.

3.24 The modelled outflows from the drainage system shall be utilised in the design of the soakaway system.

NOTE To ensure that a soakaway is designed to suit the actual road or section under consideration, the modelled / calculated result can be used instead of following the methods given in the published design guidance BRE DG 365 [Ref 11.N] and CIRIA R156 [Ref 5.N]).

3.25 Containment and control measures for potential pollution from accidental spillage shall be provided upstream of the soakaway where the risk assessment carried out in accordance with LA 113 [Ref 10.N] identifies these measures as necessary.

NOTE Containment and control measures will enable spillages to be intercepted before reaching the soakaway or infiltration zone, providing sufficient time for emergency spill responses to be implemented. Guidance on the necessity for the provision of spillage control and containment is provided in LA 113 [Ref 10.N].

3.26 Pre-treatment to remove non-soluble and particulate contaminants (chronic pollutants) shall be provided where the risk assessment carried out in accordance with LA 113 [Ref 10.N] identifies these measures as necessary.

NOTE The risk to receiving groundwater from chronic pollutants can also take into account the potential pollution attenuation capacity of the unsaturated zone.

3.27 The design of soakaways and infiltration trenches shall be undertaken in accordance with either BRE DG 365 [Ref 11.N] or CIRIA R156 [Ref 5.N].

3.27.1 The design procedures in either BRE DG 365 [Ref 11.N] or CIRIA R156 [Ref 5.N] may be selected based on the proposed soakaway design.

NOTE 1 The methodology in BRE DG 365 [Ref 11.N] and CIRIA R156 [Ref 5.N] are similar, but there are important differences between the methods. These are further described in Appendix C.

NOTE 2 The infiltration capacity of a given lithology can be determined by applying either a falling or rising head test via a groundwater monitoring standpipe or borehole. A procedure for undertaking falling and rising head tests is provided in BS 5930 [Ref 3.I]. These tests can potentially be useful at initial design stages as many road schemes will have borehole coverage along the length of the proposed route and initial calculations of the aquifer permeability can be made at each location.

4.1 Soakaway design shall incorporate measures to intercept sediments before they discharge into the soakaway.

4.1.1 Sediment traps, vortex separators or similar sediment treatment devices may be placed upstream of the soakaway.

4.1.2 Vegetative systems and ponds may be used to intercept sediments upstream of the soakaway.

NOTE 1 The long-term performance of a soakaway depends on maintaining the initial storage volume by keeping the pores clear within the granular fill and by removing any material that is likely to clog the pores of the drainage material or seal the interface between the storage and the adjacent soil.

NOTE 2 Advice on treatment methods is provided in CG 501 [Ref 1.N] and specific advice on vortex separators is given in CD 528 [Ref 13.I]. Advice on vegetative treatment systems is given in CD 532 [Ref 12.I].

NOTE 3 Assessment and advice on the need for treatment of pollutants and spillage is provided in LA 113 [Ref 10.N] and advice on sediment transport and deposition is given in CD 523 [Ref 2.N].

4.2 When geotextiles are used, their incorporation shall be specific to the adopted soakaway design.

4.2.1 Geotextiles may be used to:

NOTE Whilst the interception of sediments prior to entry into the soakaway is an essential pre-requisite to good design, some soakaway designs can incorporate the use of geotextiles to prevent the migration of fine materials within the soakaway.

4.3 The design of the soakaway and its immediate infrastructure and surroundings shall provide safe access for both workers and plant.

4.4 The design of the soakaway and its immediate surroundings shall allow access for emergency personnel and equipment to be able to mitigate the effects of a spillage.

4.5 Provision shall be made for all maintenance operations to be carried out without disruption to the safety and free flow of traffic on the adjacent carriageway.

NOTE Ease of access for maintenance is important, not only to encourage regular maintenance, but also for the safety of the maintenance operatives.

4.6 The soakaway design shall facilitate safe access for inspection and maintenance, and include access for the management and removal of sediment.

4.7 During design, an initial management plan shall be developed for the entire drainage system that:

Once they enter the unsaturated zone beneath and around a soakaway, pollutants within road runoff are subject to transport through various pathways, prior to entering groundwater in the saturated zone. These pathways are dependent upon ground conditions and are illustrated generically in Figure A1. Within these pathways, and with the unsaturated zone acting as a substrate, a number of contaminant attenuation processes occur which may act to reduce the concentration of pollutants arriving at the saturated zone.

Natural attenuation results from the combined impacts of physical processes (e.g. filtration), chemical reactions (such as oxidation of sulphides) and biochemical transformations (such as the degradation of compounds under aerobic or anaerobic conditions).

The performance of a soakaway system will depend to a large extent on the ability of water to infiltrate through the unsaturated zone, which is in turn dependent on the physical properties of the ground and the surface area in contact with the soakaway. The ability of a soakaway to transmit water will be influenced by a number of factors, such as the number and size of drainage ports, the amount of sediment allowed to settle and remain in the chambers, and the degree of choking that occurs immediately outside the chamber in the surrounding ground. For example, special soakaway manhole rings are available from pre-cast concrete suppliers. These have sufficient outlets to allow the water to infiltrate into the ground. If non-standard pre-cast concrete units are used, or a site-specific design for the soakaway chamber is undertaken, then the capacity to discharge into the ground should be considered.

The permeability of a rock formation may vary between the horizontal and vertical, dependent upon the precise lithology and structure. In sedimentary formations consisting of interbedded layers, the horizontal (along bedding) component may be significantly higher than the vertical. The effectiveness of soakaways in layered systems will be heavily influenced by the degree of interconnection between layers of high transmissivity, through fractures and fissures.

In areas of significant fracturing, for example in some sandstones or granite, in an otherwise homogenous lithology, the soakaway performance will be determined largely through interception of one or more fracture systems.

Natural geological systems may be complex, with a variety of flow components contributing to drainage capacity around the soakaway. Figure B.1 shows some possible permutations in drainage characteristics around a soakaway. Site specific information will be required to optimise soakaway design to local flow conditions.