Design Manual for Roads and Bridge

May 2006

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 4 GEOTECHNICS ANDDRAINAGE

SECTION 2 DRAINAGE

PART 8

HA 118/06

DESIGN OF SOAKAWAYS

SUMMARY

This advice note gives design guidance on howsoakaways may be incorporated into systems used totreat and store road runoff prior to discharging toground. It describes the steps needed to protectreceiving groundwater and the constraints these mayplace on soakaway design and construction. Thisdocument provides general design advice, howeverthere are a number of design procedures describedherein which must be adopted as mandatory practice,for which reference is made to HD 33 (DMRB 4.2) asappropriate.

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HA 118/06

Design of Soakaways

Summary: This advice note gives design guidance on how soakaways may beincorporated into systems used to treat and store road runoff prior todischarging to ground. It describes the steps needed to protect receivinggroundwater and the constraints these may place on soakaway design andconstruction. This document provides general design advice, however there area number of design procedures described herein which must be adopted asmandatory practice, for which reference is made to HD 33 (DMRB 4.2) asappropriate.


THE HIGHWAYS AGENCY

TRANSPORT SCOTLAND

WELSH ASSEMBLY GOVERNMENTLLYWODRAETH CYNULLIAD CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELAND

Volume 4 Section 2Part 8 HA 118/06

May 2006

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date ofNo incorporation of No incorporation of

amendments amendments

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VOLUME 4 GEOTECHNICS ANDDRAINAGE

SECTION 2 DRAINAGE

PART 8

HA 118/06

DESIGN OF SOAKAWAYS

Contents

Chapter

1. Introduction

2. Description of Soakaway Systems

3. Design and Construction of Soakaway Systems

4. Construction Practices

5. Maintenance and Management of SoakawaySystems

6. References

7. Glossary

8. Enquiries


May 2006


Chapter 1

ne may cause current guidance to be superseded.

2.4 Design of other drainage structures such as linearains is discussed in HD 33 (DMRB 4.2). The use of1. INTRODUCTION

1.1 General

1.1.1 This Advice Note gives guidance on howsoakaways may be incorporated into systems used totreat and store road runoff prior to discharging toground. This Advice Note has been compiled tosummarise and review the current available designguidance in use within the UK on the selection andconstruction of soakaways. The design guidanceprovided in this document focuses on point dischargesto the ground from single soakaways (or combinationsof these). These are defined as drainage structures (pits,chambers and trenches) that allow infiltration to theground through their base and sides and that incorporatebelow ground storage.

1.1.2 The Advice Note originates from a review ofavailable information on the fate and transport of roadcontaminants and the design of existing road soakawaysystems. The review focussed primarily on anunderstanding of the processes operating in theunsaturated zone (i.e. above the water table) and howthese may influence soakaway design and preventgroundwater pollution.

1.1.3 In most of the UK the term highways isequivalent to Scottish roads. In this document, as withthe guidance provided in HA 216: Road Drainage andthe Water Environment (DMRB 11.3.10), the termroads will be used as standard terminology.

1.1.4 This document provides general design advice,however there are a number of design proceduresdescribed herein which must be adopted as mandatorypractice, for which reference is made to HD 33: Surfaceand Sub-Surface Drainage Systems for Highways(DMRB 4.2) as appropriate.

1.2 Background

1.2.1 There is growing awareness by theEnvironmental Protection Agencies (EPAs) that roadrunoff may, under certain circumstances, have anadverse effect on receiving waters, includinggroundwater. This has arisen both because of improvingknowledge of the polluting content of road runoff andbecause of improved treatment of other sources ofpollution. Contact details for the various EPAs can be

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found on their respective web-sites:Introduction

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2.2 Overseeing Organisations have a duty underllution protection legislation to ensure that roadnoff does not pollute receiving waters. This can ariseth from the effects of routine runoff and fromillages on the road. For discharges to ground, Theroundwater Regulations 1998 (SI 1998 No 2746) inngland, Scotland and Wales or The Groundwateregulations (Northern Ireland) 1998 (Statutory Rule NI98 No 401) include lists of substances whose entry tooundwater must be prevented or controlled. Thesebstances are commonly referred to as List I and List substances. Details of these substances are set out inA 216 (DMRB 11.3.10). Because consents are notquired for the discharge of road runoff, thesponsibility is with the Overseeing Organisations toscharge their duty not to pollute receiving waters bysuring that systems for the treatment of road runoffe installed when necessary. EPAs should, however, bensulted about proposals for discharging road runoff, they have the power to serve prohibition notices inspect of discharges that are in breach of pollutiongislation. Implementation into UK law of the Waterramework Directive (2000/60/EC) (e.g. The Waternvironment (Water Framework Directive) (Englandd Wales) Regulations 2003 (SI 2003 No 3245);imilar regulations pertain to Scotland and Northerneland) has given added weight to the need to protectisting water bodies.

2.3 The disposal of road runoff through soakawaystems is reasonably well established within the UK.owever, the attenuation mechanisms operating on roadnoff to reduce pollutant loadings at the dischargeint below the soakaway, and within the unsaturatedne above the water table, are currently poorlyderstood. Changes in design practice, as well as

gislation controlling the discharge of pollutants tontrolled waters and the results of future research intoe fate of pollutants in soakaways and the unsaturated1/1

vegetated systems to treat road runoff is discussed inHA 103: Vegetated Drainage Systems for Highway


Chapter 1IntroductionRunoff (DMRB 4.2). HA 216 (DMRB 11.3.10)provides guidance on assessing the risks to groundwaterfrom road runoff.

1.3 Design Concepts

1.3.1 There are three major objectives in the drainageof roads:

the speedy removal of surface water to providesafety and minimum nuisance for the road user;

provision of effective sub-surface drainage tomaximise longevity of the pavement and itsassociated earthworks;

minimisation of the impact of road runoff on thereceiving environment.

1.3.2 These design objectives require that to operatesuccessfully, soakaways should be sited in porous andpermeable ground of sufficient depth and lateral extentto be able to accommodate potential maximumdischarges under storm conditions. Furthermore, whatis good for the physical disposal of road drainage (i.e.rapid dispersal of flow) is the exact opposite of what isrequired with respect to the in-ground attenuation ofpollutants. Where slow infiltration rates are utilised tooptimise these in-ground processes, the use ofbalancing ponds or other flow attenuation mechanismsmay be required to accept the quantity of runoffgenerated under peak flow conditions by the roaddrainage.

1.3.3 This design guidance allows the designer toconsider those processes that may occur in the passageof road runoff through the soakaway system andthrough the unsaturated zone, prior to its ultimatedischarge into groundwater. However, application of therisk assessment methodology described in HA 216(DMRB11.3.10) may demonstrate the requirement forpollution prevention measures to be incorporated in thedesign of the drainage system. These should beincorporated upstream of the soakaway discharge. Suchmeasures could include structures for the control andcontainment of accidental spillages, or introducemethods for the treatment of pollution derived fromroutine road runoff. Guidance for these measures maybe found in HD 33 (DMRB 4.2) and HA 103 (DMRB4.2).

1.3.4 Recently much attention has been paid to thedesign of drainage systems with more emphasis oncontrolled transmittal of runoff to discharge points1/2

(notably to surface waters). Such systems place agreater reliance on retaining water in storage ponds andallowing slow recharge to groundwater and have inparticular been applied to the reduction of the flashynature of runoff from urban drainage. Guidance on theuse of these systems for roads is provided in HA 103and there is also published guidance available from theEnvironment Agency (A Guidance Manual forConstructed Wetlands), and additional information ontheir website.

1.3.5 The design of soakaways, of any description,should also take into account constraints arising fromthe possible impact on landscape and ecology.Discharges to groundwater may potentially affectbiodiversity (e.g. through impacts on wetland habitats)and any such effects should be considered both in thelocation and the function of the soakaways.

1.3.6 In spite of the promotion and introduction ofmeasures that allow for the slow infiltration of roaddrainage into the ground, there is little data availablethat provides a sufficient basis to be able to quantifythese effects, either in the short or longer term. Asdiscussed above, further research, particularly on thefate of contaminants on the unsaturated zone, is neededto determine what these effects may be.

1.4 Purpose

1.4.1 The purpose of this note is to provide advice onthe design and construction of soakaways comprisingan excavated pit or trench rather than systems such asattenuation ponds for which there is design adviceavailable elsewhere (HA 103 (DMRB 4.2)). ThisAdvice Note also discusses the measures that can bemade to complement the pollutant attenuation capacityof the soakaway system, including taking into accountthe underlying unsaturated zone, in order to prevent therisk of groundwater pollution. The note shouldtherefore be read in conjunction with HA 216 (DMRB11.3.10) which provides an overview of the currentknowledge of road runoff in the UK, the groundconditions that are believed to affect its nature and aguide to assessing the risk of pollution arising fromroutine road runoff and accidental spillages.

1.4.2 The design of soakaways cannot be prescribed.Each situation should be considered individually asthere are many factors that should be taken intoconsideration, for example, depth of unsaturated zone,geology, traffic flows.May 2006


1.5 Scope

1.5.1 The principles outlined in this Advice Note applyto all schemes of Overseeing Organisations for TrunkRoads including Motorways. They may also be appliedgenerally to other new highway schemes and by otherhighway authorities for use during the preparation,design and construction of their own comparableschemes. Soakaways may be installed during majormaintenance works or as a retro-fit. Designers andSpecifiers of DBFO schemes should note that it is notconsidered currently feasible to specify performancestandards for these systems.

1.6 Implementation

1.6.1 The Advice Note should be used forthwith for allschemes currently being prepared provided that, in theopinion of the Overseeing Organisation, this would notresult in significant additional expense or delayprogress. In making such an assessment, it should beborne in mind that the Overseeing Organisations have aduty not to pollute groundwater. Design Organisationsshould confirm its application to particular schemes

Chapter 1Introductionwith the Overseeing Organisation.May 2006 1/3

Volume 4 Section 2 Chapter 2Description of Soakaway Systems

ncrete footing formed and then segments lowered one top of the other until the hole is filled. The areatween the outside of the rings and the excavation canPart 8 HA 118/06

2. DESCRIPTION OF SOAKA

2.1 General

2.1.1 There are a range of drainage systems which mayact as soakaways, where the primary discharge from thesystem is to groundwater. Fundamentally, these aim toencourage efficient hydraulic contact between thedrainage system and the underlying ground in order toprovide effective drainage. Such options include forexample:

combined surface and groundwater filter drains;

fin drains;

filter drains;

informal drains or over the edge drainage.

In addition there may be systems wherein discharge togroundwater occurs but is incidental to the function ofthe drainage (e.g. unlined ditches).

2.1.2 Further details of both these types of system areprovided in HD 33 (DMRB 4.2).

2.1.3 Additionally a number of surface water drainagefeatures also incorporate discharges to the unsaturatedzone for example:

retention ponds;

sedimentation ponds;

infiltration basins;

wetlands;

swales/grassed channels.

2.1.4 Further details of these are provided in HA 103(DMRB 4.2).

2.1.5 These options all provide varying degrees ofprotection to groundwater resources, primarily throughmaximising the depth of unsaturated zone beneath thedrain. Guidance on the design of these types of drainageis provided in HD 33 (DMRB 4.2) and HA 103 (DMRB4.2). In order to produce a suitable design and tominimise the impact of road drainage on groundwaterquality, whilst accounting for the attenuation properties

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of the unsaturated zone, soakaway drainageWAY SYSTEMS

corporating any of these options should also embracee design principles described in Chapter 3.

1.6 As described in Section 1.1.1, this guidancecuses on soakaway structures that essentially providescharge to the ground at a single point, either throughambers or trenches or into open pit type structures.

1.7 There are a wide range of these soakaway typesat have been put into use including, for example:

segmental concrete chambers within excavatedpits (with the chamber used as storage);

pre-cast concrete perforated ring units;

brickwork within previously created excavations;

trenches;

open pits;

rubble/aggregate filled pits;

pits/trenches with proprietary geo-cellular units.

1.8 Sections 2.2 and 2.3 provide typical designnsiderations for pre-cast and trench type soakaways,though the designer should consider those designsost suitable to ground conditions, space, dischargequirements and other site specific criteria.

1.9 Soakaways for draining roads will commonlyrvice relatively large areas requiring significantorage volumes. Such soakaways may not, inemselves provide such storage capacity, althoughere are examples where large storage volumesmprise part of the soakaway.

2 Pre-cast Perforated Concrete Ring Typeoakaway

2.1 This system (see Figure 1) is commonly used fors simplicity of construction and ease with whichmpatible components can be supplied from variousurces. An excavation to the required depth is made, a2/1

be backfilled with aggregate.


Chapter 2Description of Soakaway Systems

2.5 Physical Attenuation Processes2.3 Trench Type Soakaway

2.3.1 Trench type soakaways should be constructedwith inspection tubes at regular intervals. Theseinspection tubes (observation wells) should beconnected by a horizontal perforated or porousdistributor pipe laid in the top of the granular fill alongthe trench as shown on Figure 2 (BRE Digest 365). Theextreme ends of the trenches should be identified byinspection tube covers or other access covers.

2.3.2 Trenches are generally constructed with ahorizontal base (CIRIA Report 156) and the volumebetween the structure and the excavation backfilledwith granular material. Typically a minimum width of300mm will be considered (CIRIA, Report 156).

2.3.3 Trenches tend to require a lower volume ofexcavation and granular fill material for a givendischarge capacity than a soakaway with a squareprofile. The narrower and longer the trench, the moreefficient it is in terms of outflow performance andconstruction cost compared to wider and shortertrenches (CIRIA C522). A narrower trench, of forexample, 0.3m width, requires a reduced storagevolume relative to wider trenches, of 0.6m or 1.0m,because it has an enhanced outflow performance.

2.4 Contaminant Attenuation Processes

2.4.1 Once they enter the unsaturated zone around asoakaway, pollutants within road runoff are dependantupon ground conditions, subject to transport throughvarious pathways prior to entering groundwater in thesaturated zone. Such pathways are illustrated in Figure3. Within these pathways, and with the unsaturated zoneacting as a substrate, a number of contaminantattenuation processes occur which may act to reduce theconcentration of pollutants arriving at the saturatedzone.

2.4.2 Natural attenuation results from the combinedimpacts of physical processes (e.g. filtration), chemicalreactions (e.g. oxidation of sulphides) and biochemicaltransformations (e.g. the degradation of compoundsunder aerobic or anaerobic conditions). The action ofthese various attenuation mechanisms may influencethe design of a soakaway system for instance bymaximising the contact with strata that may provideenhanced attenuation potential or preventing directdischarge into a strata that does not offer any potential.2/22.5.1 The physical attenuation of contaminants may beby processes associated with adsorption onto the solidmatrix through which flow takes place, or by filtration.

2.5.2 Adsorption retains contaminants within thematrix of the unsaturated zone. In general terms,materials of mixed mineralogy, especially thosecontaining a proportion of clay minerals, providegreater opportunity for retention or retardation ofparticles by adsorption than formations composedprincipally of silica (e.g. clean sands and sandstones) orcalcium carbonate (clean limestones, particularly theUpper Chalk).

2.5.3 Filtration removes suspended particles, trappingthem within the pore spaces of the drainage medium. Ingeneral, shallow groundwater, protected by a thinunsaturated zone and composed generally of coarsegrained or heavily fissured materials is likely to bemore vulnerable to contamination by particulates (dueto lesser potential filtration) than is groundwater infiner grained, more massive materials and with asignificant depth of unsaturated zone.

2.5.4 Volatilisation will allow partial attenuation ofvolatile organic compounds through venting toatmosphere from a porous matrix. The effectiveness ofthis process depends on the relative temperature of theground with respect to ambient air and the depth ofunsaturated zone.

2.6 Chemical and Biochemical AttenuationProcesses

2.6.1 Chemical and biochemical processes ofattenuation act mainly on organic contaminants such ashydrocarbons, although metals may be transformedthrough processes such as oxidation.

2.6.2 Biodegradation is typically the most importantprocess acting to reduce contaminant mass. The processacts to reduce contamination levels by oxidation-reduction reactions and is dependent on groundwatergeochemistry, microbial population, and contaminantproperties. Biodegradation can occur under aerobic andor anaerobic conditions and may ultimately result incomplete degradation of many organic contaminants.May 2006


2.6.3 Abiotic degradation can result in partial orcomplete degradation of contaminants through chemicaltransformations. These reactions are dependent on thecontaminant properties and groundwater geochemistry.Rates of degradation are typically much slower thanthose associated with biodegradation.

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Volume 4 Section 2 Chapter 3

ould encourage the development of new habitats. Themplications of any such developments should bevaluated both with respect to the potential forroviding new habitats for protected species (such asater Voles and Great Crested Newts) and with respectPart 8 HA 118/06

3. DESIGN AND CONSTRUCSYSTEMS

3.1 General

3.1.1 In the past soakaways have been designedprimarily on hydraulic grounds; i.e. simply to transmitrunoff efficiently and facilitate drainage into theunderlying unsaturated zone using 1 in 10 year returnperiods for volume requirements. Previous designs ofsoakaway chambers have even incorporated boreholesto by-pass the unsaturated zone once the storagecapacity of the chambers has been reached, ensuringthat the roadway remains clear of water but potentiallyallowing contaminants to enter the saturated zone withlittle impedance. Under the current legislative regimeand with the introduction of tighter controls within theWater Framework Directive, allowing direct dischargeto groundwater through structures such as boreholes isconsidered inappropriate and should not be used withina soakaway system design. HD 33 (DMRB 4.2)provides mandatory requirements with respect to suchdirect discharges.

3.1.2 Traditionally little regard has been made tothe effects the soakaway may have in particularhydrogeological situations, although more recentlydesigns have been influenced by the need to meetrequirements set by the regulators.

3.2 Overall Requirements

3.2.1 There are three fundamental principles thatshould be applied to soakaway discharge system design,which are to:

ensure that the hydraulic performance of thesoakaway outfall allows sufficient storageand infiltration capacity such that the systemwill have the capacity to drain the designstorm (and hence prevent flooding of thecarriageway);

ensure protection of receiving groundwater;and

provide measures to prevent the possibility ofan accidental spillage passing through thedischarge system. The level of controlprovided should be commensurate with therisk identified at a soakaway. Guidance on the

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risk of pollution from accidental spillages isprovided in HA 216 (DMRB 11.3.10).Design and Construction of Soakaway Systems

TION OF SOAKAWAY

.2.2 The most fundamental hydraulic designrinciple for soakaway discharge systems for roadspplications is to provide sufficient storage capacity tollow the removal of storm runoff from the carriageway,uickly and effectively. The principal hydraulic designriterion is therefore to provide sufficient capacityithin the system to cover peak runoff. This isenerally achieved by constructing large detentiononds, to temporarily store the water discharging fromhe road, upstream of the soakaways or by constructingnderground chambers with porous sides or bases thatlso have sufficient internal capacity to store the runoff.he size and number of retention ponds or chambershould be determined to provide the required capacityf the drainage system within the design constraints ofhe location.

.2.3 The requirement to provide effective drainageannot, however, override the need to protectroundwater, which is an explicit legal requirement.his applies to both pollutants carried by routinerainage (the focus of this Advice Note) and pollutantshat may enter the system through accidental spillage.

.2.4 As described in Section 1.3.3, the riskssessment methodology in HA 216 (DMRB 11.3.10)rovides a means to determine the level of risk toroundwater from chronic pollution derived fromoutine road runoff. This should be carried out by apecialist and the potential risks discussed with the EPArior to selection of the soakaway site. The assessmentay demonstrate the requirement for pollution

revention measures to be incorporated in the design ofhe drainage system. These should be incorporatedpstream of the soakaway discharge.

.2.5 The soakaway design should take intoonsideration potential impacts on ecology, habitats andiodiversity. These could arise from, for example,otential effects of soakaway drainage on the quality ofeceiving groundwaters which subsequently emerge athe surface (e.g. providing flow to wetlands).

.2.6 Certain soakaway designs (e.g. open pits)3/1

to the potential for encouraging invasive species.

Volume 4 Section 2Chapter 3Design and Construction of Soakaway Systems3.2.7 Whilst the soakaway itself may have littlemanifestation at the surface, fencing, maintenance,access routes and signage may all have landscapeimpacts. Sympathetic design should be adopted,particularly in sensitive locations, with advice soughtfrom the Overseeing Organisation as necessary.

3.3 Site Specific Factors

3.3.1 There are a number of potential limitingfactors in the design of a soakaway in order to minimisethe pollution potential and also to maximise the scopefor attenuation within the unsaturated zone, theseinclude:

The hydrogeological potential of the site. Isthe substrate an aquifer?

The location of the site. Is the site locatedwithin the bounds of a source protection zone(SPZ)? The discharge of List II (see 1.2.2)substances to groundwater is permitted withadequate risk assessment, and the presence ofan SPZ will affect the level of permissiblerisk.

The depth to groundwater at the site(thickness of the unsaturated zone).

The soil or rock type and thicknessencountered at the site.

The microclimate of the region. Climaticfactors including precipitation and surfaceevaporation rates are important considerationswhen assessing the amount of road runoff,particularly peak storm flows.

Would a soakaway lead to mobilisation ofpollutants in existing contaminated land?

3.3.2 Note the Water Framework Directive is suchthat it essentially treats all groundwater with equalweight i.e. a certain minimum level of protection isrequired. Where source protection zones may beimpacted, additional protection may be warranted withrespect to the protection of human health. Theseconcepts are explained in more depth in DMRB11.3.10.

3.4 Site-Specific Design

3.4.1 Once the general area where a soakaway is to3/2

be situated is identified, the detailed specific design Part 8 HA 118/06

is

required. In the past designs have tended to be generalfor a whole scheme with no variations to take intoaccount differences in ground conditions along theroute. As a road generally traverses a wide range ofground conditions the design of the discharge systemshould also change to reflect this. In low lying areasnear rivers there may be little unsaturated zoneavailable and a soakaway should perhaps be broad andflat in configuration whereas on high ground, the depthof unsaturated zone is large enough for smaller deeperstructures, reducing the amount of land take.

3.4.2 The pre-treatment of drainage water shouldalso be considered as a means of reducing or removingsome potential contaminants, as well as possiblyproviding some short-term storage capacity within thedrainage system. Treatment systems may requireperiodic maintenance to remove accumulated pollutantsbefore the capacity is exceeded. As an example, the useof vegetative treatment systems in road drainage isdiscussed in HA 103.

3.4.3 There are a number of factors to beconsidered when selecting a soakaway for a specificlocation. The key factors are the topography and theshape of the area available adjacent to the road. Thedesign should ensure that the:

soakaway selected suits the site dimensions;

soakaway is not within 3-6m of a building, tomeet practices nationally (see note below);

road sub-base remains unsaturated when thesoakaway is at its maximum design capacity;

vertical distance between the soakaway andthe ground water is maximised;

soakaway does not lead to (harmful)groundwater emergence downgradient;

soakaway does not surcharge groundwaterleading to (harmful) waterlogging orexacerbate groundwater flooding;

soakaway does not lead to the washing out offines (causing instability) or lead to (harmful)dissolution of the subsurface.

Some of these aspects of soakaway design are critical tominimise impacts on nearby structures and the waterenvironment. HD 33 (DMRB 4.2) provides mandatoryguidance with respect to a number of these designelements.May 2006


Chapter 3Note:

According to BS EN 752-4:1998: If drainage is to be asoakaway, the subsoil and the general level of thegroundwater should be investigated. It is not desirableto locate a soakaway cloaser than 3m to 6m from abuildings foundations, nor in any other position suchthat the ground below foundations is likely to beadversely affected.

BRE Digest 365 recommends that: Soakaways shouldnot normally be constructed closer than 5m to buildingfoundations.

Scottish Building Standards for domestic dwellingsrefer to both the above standards but also recommendthat, for single dwellings: the finished soakawaysshould be a minimum of 5m from the dwelling and theboundary. However, this dimension may br reducedslightly on small sites where ground conidtions allow,such as very well draining soil.

Given the potentially high discharge volumes and ratesthat may be generated by road run-off, designers mustconsider the proximity of buildings on a site specificbasis, particularly with respect to ground conditions.

3.4.4 CIRIA Report 156 includes a number offlowcharts to aid the design process, including theselection of a suitable system.

3.5 Soakaway Design

3.5.1 The risk assessment process described inHA 216 (DMRB 11.3.10) includes evaluation of thegeological setting of the site, which should be afundamental consideration in the development of thedesign.

3.5.2 Based on the criteria detailed in the followingsections, and subject to the risk assessment, the keyelements in the design and construction of an effectivesoakaway are:

where identified as necessary, the introductionof containment and control measures forpotential pollution from accidental spillage;

where identified as necessary, pre-treatment toremove non-soluble and particulatecontaminants;

sufficient capacity to accommodate thequantity of design runoff;May 2006 sufficient drainage paths/ports to allow waterto infiltrate into surrounding ground;

filter or settlement mechanisms to prevent theblockage of drains or siltation of the drainagepaths plus the surrounding ground;

maximising depth of unsaturated zone;

allowance for the controlled overflow ofextreme storm events;

the provision of observation wells/pipes(inspection tubes/chambers) to allowinspection and maintenance.

3.6 Infiltration Capacity

3.6.1 The performance of a soakaway system willdepend to a large extent on the ability of water toinfiltrate through the unsaturated zone, which is in turndependant on the physical properties of the ground andthe surface area in contact with the soakaway. Theability of a soakaway to transmit water will beinfluenced by a number of factors, such as the numberand size of drainage ports, the amount of sedimentallowed to settle and remain in the chambers and thedegree of choking that occurs immediately outside thechamber in the surrounding ground. For example,special soakaway manhole rings are available from pre-cast concrete suppliers. These have sufficient outlets toallow the water to infiltrate into the ground. If non-standard pre-cast concrete units are used, or a sitespecific design for the soakaway chamber isundertaken, then the capacity to discharge into theground should be considered.

3.7 Vertical and Horizontal Drainage

3.7.1 The permeability of a rock formation mayvary between the horizontal and vertical, dependantupon the precise lithology and structure. In sedimentaryformations consisting of interbedded layers, thehorizontal (along bedding) component may besignificantly higher than the vertical. The effectivenessof soakaways in layered systems will be heavilyinfluenced by the degree of interconnection betweenlayers of high transmissivity, through fractures andfissures.

3.7.2 In areas of significant fracturing, for examplein some sandstones or granite, in an otherwisehomogenous lithology, the soakaway performance willbe determined largely through interception of one or

Design and Construction of Soakaway Systemsmore fracture systems.

3/3

Volume 4 Section 2Chapter 33.7.3 Natural geological systems may be complex,with a variety of flow components contributing todrainage capacity around the soakaway. Figure 4 showssome possible permutations in drainage characteristicsaround a soakaway. Site specific information will berequired to optimise soakaway design to local flowconditions.

3.8 Aspect Ratio

3.8.1 It is considered good practice to maximise thedepth of unsaturated zone below a soakaway device toallow the maximum attenuation of pollutants to occur.This may mean that the depth and size of chambersneeds to be varied so that in areas with less unsaturateddepth, the soakaway system comprises a number ofshallow interconnected chambers to provide sufficientshort-term storage, whilst maximising the depth ofunsaturated zone. In areas with a deeper unsaturatedzone the soakaway may comprise fewer deeperchambers, so requiring less land, whilst stillmaintaining sufficient attenuation capacity. This isillustrated schematically in Figure 5. It should be noted,however, that further land may be necessary for accessand maintenance.

3.8.2 Surface infiltration systems (e.g. lagoons,infiltration ponds etc) also depend on surface area toprovide sufficient drainage capacity, so that a largeshallow infiltration pond will allow rapid dispersal ofwater through the semi-permeable base whereas a deepnarrow pond will retain water for much longer.

3.9 Storage

3.9.1 The drainage system must provide a balancebetween sufficient infiltration rate and storage capacityto allow the fast and efficient removal of water from thesurface of the road. The storage capacity must bedesigned to cope with peak runoff from the maximumdesign storm events defined, for no flooding of the roadsurface without the drainage network backing up.Storage is thus essential where the discharge rate fromthe road exceeds the infiltration capacity of thesoakaway. The rate of runoff from the road surfaceshould be calculated using an appropriate designmethodology/drainage modelling programme using theactual design rainfall values. The calculated outflowsfrom the drainage system should be utilised in thedesign of the soakaway instead of following themethods given in the published design guidance (BRE365 and CIRIA Report 156). This will ensure that thesoakaways are designed to suit the actual road or

Design and Construction of Soakaway Systemssection thereof under consideration.

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3.10 Spillages

3.10.1 Measures to control and contain spillageshould be installed where the combination of theprobability of the occurrence of a major spillage andrisk to the receiving waters is sufficient to justify them.Such measures will enable polluting material to beintercepted before it reaches the soakaway orinfiltration zone, providing sufficient time foremergency spill responses to be implemented. Guidanceon the necessity for the provision of spillage controland containment is provided in HA 216 (DMRB11.3.10).

3.11 Design Procedures

3.11.1 Soakaways store storm water runoff andprovide for its infiltration into the surrounding soil. Theinfiltration must occur sufficiently quickly to providethe necessary capacity within the drainage system tocope with the expected runoff, based on expectedrainfall intensity and frequency or the outflowcalculations from the use of modelling as described in3.9.1. Providing adequate storage volume andsubsequent discharge are the two design parameters thatgovern the calculations for soakaway design. Design ofsoakaways and infiltration trenches should beundertaken in accordance with:

BRE Digest 365 Soakaway Design;

CIRIA, Report 156 Infiltration Drainage Manual of Good Practice.

3.11.2 It is also possible to determine informationrelating to the capacity for a given lithology to transmitwater through a falling or rising head test using agroundwater monitoring standpipe or borehole. Thistest is generally carried out by surcharging a piezometerwith water and monitoring the fall in head as thecolumn of water equilibrates with the watertable overtime. A procedure for undertaking this test is providedin BS 5930: Code of Practice for Site Investigation.This test may potentially be useful at initial designstages as many road schemes will have boreholecoverage along the length of the proposed route andcalculations of the aquifer permeability can be made ateach location.

3.11.3 BRE Digest 365 provides advice on the designof soakaways in urban environments, which, whilst notstrictly applicable to roads, does provide methods fordetermining the size of the soakaway required to dealwith anticipated levels of runoff. The methodology usesMay 2006


Chapter 3Design and Construction of Soakaway Systems

3.11.7 The design guidelines in BRE 365 and CIRIAReport 156 may not always give similar results for aspecific circumstance because the two methods treat thefactor of safety in different ways. CIRIA C521 forScotland and Northern Ireland and C522 for Wales andEngland, design manuals for sustainable drainagereference both methods as applicable without favouringeither one.a 10 year return period for a 15 minute duration stormto determine the required storm flow capacity. This maynot be appropriate for roads design.

3.11.4 The BREs methodology does not allow forattenuation of flow within the drainage system itself.Road drainage systems will very often comprise longdrain runs which run to a low point in the road to wherethe soakaway drainage is provided. A significantamount of water will be attenuated as it flows throughthe drainage run, effectively reducing the peak flow atthe soakaway. The BREs methodology, therefore, overdesigns the required soakaway capacity based on inflowvolume from the road and in order to accuratelycalculate the required soakaway size the inflowcalculation used in the guidance should be replacedwith the design flows calculated for the road based onguidance in HD 33 (DMRB 4.2).

3.11.5 CIRIA Report 156 provides brief guidance ondesigns of drainage systems to remove pollutants, basedon physical and biological systems, such as sedimenttraps, interceptors, soakaways and vegetative treatmentsystems for a range of drainage scenarios, includingroads. This report provides methods for determining therequired size for a drainage system based on the amountof rainfall and infiltration characteristics of the ground.In terms of groundwater protection this report reliesupon the Environment Agencys Policy and Practice forthe Protection of Groundwater, which provides basicguidance on the suitability of discharges to soakaway interms of providing protection to water abstractionsources and groundwater resources in general. CIRIAReport 156 concentrates on source protection and doesnot provide detailed advice on treatment systems.Implementation of the Water Framework Directiverequires a different approach whereby all groundwateris protected regardless of use and the Agency will beproducing new guidance on groundwater in light of this.

3.11.6 The methodology within CIRIA Report 156uses the lower portion of the chamber walls plus thebase of the soakaway to calculate the infiltration rateinto the ground and hence the required storage capacity.It should be noted that over time the base of a soakawaycan become silted, unless adequate maintenance orupstream protective measures are implemented.Incorporating the base into the contact area can thuslead to overestimating the rate of infiltration andunderestimating the volume of storage. This method cantherefore under estimate the size of soakaway requiredfor a given inflow runoff rate.May 2006 3/5


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Chapter 3Design and Construction of Soakaway SystemsMay 2006

Figure 4: Variation in Drainage Characteristics in Multi-Layered and Fracture Lithologies

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Figure 5: Maxim

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Chapter 4Construction Practices

Health and Safety procedure in the design, constructionand maintenance of soakaway systems is described inHD 33 (DMRB 4.2).4. CONSTRUCTION PRACT

4.1 General Construction Guidelines

4.1.1 Soakaways should be constructed sufficiently faraway from buildings or other structures such as bridgesin order to prevent the risk of undermining foundations.Building practices recommend that a minimumdistance of 3m-6m between a subsurface drainagesystem and a building should be adopted, depending onground conditions (see Section 3.4.3).

4.1.2 Soakaways should not normally be deeper than 3to 4m in order to maximise the length of the flowpath tothe watertable through the unsaturated zone. Thegreater the thickness of unsaturated zone available, thegreater the potential for pollutant attenuation. The issueof fate and transport of pollutants from road drainage isdiscussed fully in HA 216 (DMRB 11.3.10).

4.1.3 The long term performance of the soakawaydepends on maintaining the initial storage volume bykeeping the pores clear within the granular fill. Anymaterial that is likely to clog the pores of the drainagematerial or seal the interface between the storage andthe adjacent soil should be intercepted before dischargeto the soakaway (CIRIA, C522) in order to maximisethe effective life of the soakaway between cleaning. Anassessment of the risk of pollution and need for pretreatment is discussed in HA 216 (DMRB 11.3.10).Consideration of the need for sediment traps and, whereappropriate, oil interceptors to treat the surface waterprior to discharge to soakaways may be required(CIRIA, Report 156) based on an assessment of therisks. Vegetative systems are discussed in HA 103(DMRB 4.2) and treatment efficiencies for variousdrainage systems are provided in HD 33 (DMRB 4.2)for a range of common pollutants.

4.1.4 Whilst the interception of sediments prior toentry into the soakaway is an essential pre-requisite togood design, some soakaway designs may incorporatethe use of geotextiles to prevent the migration of finematerials. Geotextiles may be used to:

separate granular backfill materials from groundmaterial in the walls of excavated pits;

prevent fines within the soakaway frommigrating outward into granular surroundmaterials hence reducing clogging of thoseMay 2006

materials;ICES

lay over the top surface of a granular fill toprevent downward ingress of backfill materialduring and after surface reinstatement.

The requirements for the use of geotextiles will bespecific to the type of soakaway design adopted.

4.2 Health and Safety Considerations

4.2.1 As required by The Construction (Design andManagement) Regulations 1994 (SI 1994 No 3140),The Construction (Design and Management)Regulations (Northern Ireland) 1995 (SI 1995 No. 209),Designers and Specifiers should be aware of the risks tothe public, road maintainers and those implementing thedesign. A risk assessment should be undertaken at thetime the systems are designed or specified.

4.2.2 Ease of access for maintenance is important, notonly to encourage regular maintenance, but also for thesafety of the maintenance operatives. It will also aidemergency personnel in carrying out any measures tomitigate the effects of a spillage. Appropriate access tosystems remote from the main carriageway is essentialin all locations, and in many locations a gated accesscould be provided. Designers should ensure that there isadequate, safe access for both workers and plant andthat provision is made for all maintenance operations tobe carried out without disruption to the safety and freeflow of traffic on the adjacent carriageway.

4.2.3 Not withstanding the above, any work carried outaround soakaway systems, whether during constructionor as part of maintenance procedures should be carriedout with due consideration of health and safety. TheManagement of Health and Safety at Work Regulations1999 (SI 1999 No 3242), The Management of Healthand Safety at Work Regulations (NI 2000), TheConfined Spaces Regulations 1997 (SI 1997 No 1713)and The Confined Spaces Regulations (NorthernIreland) 1999 (SR 1999 No 13) are particularlypertinent. Advice should also be sought from theOverseeing Organisations.

4.2.4 The mandatory implementation of appropriate4/1


ecologists and, as necessary landscape specialists.Further advice on these aspects of maintenance isprovided in HA 103 (DMRB 4.2).

5.1.8 It is essential to ensure that sediments cleaned5 MAINTENANCE AND MASOAKAWAY SYSTEMS

5.1 Routine Maintenance

5.1.1 Road runoff contains a significant amount ofparticulate material, in the form of road stone and tyrefragments, mud and dust. Without periodic removal ofthis, the discharge system will eventually becomeblocked. In designing a discharge system, an effectivemeans of trapping sediment should be provided, in aneasily accessible area for periodic emptying andmaintenance.

5.1.2 A Management Plan should be developed forthe entire drainage system for each road (refer alsoHA 103 (DMRB 4.2)). This Management Plan shouldset out a systems objectives, formulate an annualprogramme of maintenance and provide opportunitiesto review behaviour of the drainage system. This shouldinclude for advice to be adopted for the managementand maintenance of soakaways, any pre-dischargetreatment and any other elements associated with thesystem. The Management Plan should prescribe thevarious maintenance operations which may be required.Specific maintenance requirements are suggestedbelow, however these should be adapted as necessary tosite and system specific requirements.

5.1.3 The frequency of inspection of soakawaysshould be determined during the immediate postconstruction period. It is suggested (CIRIA C609) that,for the first six months, monthly inspection is carriedout to determine the rate of accumulation of sediment inboth soakaway and pre-discharge treatment devices(particularly silt traps). This procedure will then allowthe long-term frequency of inspection/cleaning to bedetermined. This frequency may be subsequentlyreduced if conditions allow. Cleaning requirements andfrequency will depend upon the size and type of areadrained. A suggested maintenance schedule is set outbelow, although these requirements should not beinterpreted rigidly, and a proactive approach based onsite specific requirements, is to be encouraged:

removal of debris from the floor of chamberand sediment traps. (minimum annualfrequency);

check observation wells/inspection tubes forMay 2006

clogging and to ensure soakaway is emptying(annual);NAGEMENT OF

inspect area around soakaway for groundsettlement or sediment loss (annual);

removal and washing of exposed stones on thetrench surface (annual);

trimming any roots that may be causingblockages (annual).

If annual inspections show significant performancedeterioration, granular materials surrounding thesoakaway may have become clogged these may needreplacement and or overhaul. The frequency for thesemore intrusive actions can only be assessed on a sitespecific basis and will be very variable, perhaps in therange between 10-30 years.

5.1.4 Maintenance will usually be carried out byhand, although a suction tanker can be used for debrisremoval. If maintenance is not undertaken for very longperiods, deposits might become hard-packed andrequire considerable effort to remove.

5.1.5 The area draining to the infiltration deviceshould be regularly swept to prevent silt being washedoff the surface.

5.1.6 A problem frequently encountered with drainsand sewers is the ingress of tree roots through poorjoints or cracks in the network. This occurs becauseroots are drawn to the presence of water and hence is acommon problem with soakaways where a permeablestructure is a design feature and roots can grow throughthe soakaway walls, reducing the passage of water.

5.1.7 In the maintenance and cleaning of open pittype soakaways, advice should be sought from anecologist, or other appropriately qualifiedenvironmental specialist, to ensure that the operationmay be carried out with safeguards in place to protectprotected species or breeding birds that may havecolonised the pit. Similarly plant removal or trimmingshould be undertaken following advice of both an

Chapter 5Maintenance and Management of Soakaway Systemsout from one part of the system are not allowed tomigrate further downstream in the drainage system or

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from moving down the drainage system. Any suchpollution control devices incorporated within a drainagesystem will require signing to allow quick identificationand location by emergency services so that controlmeasures can be safely and effectively deployed as soonas possible. Requirements for signage of these systemsis provided in HD 33 (DMRB 4.2).

Chapter 5Maintenance and Management of Soakaway Systemsallowed to enter the water environment. Considerationmust be given to the disposal of sediment and plantwaste as these will retain contaminants from the roadrunoff. Assuming it is planned to send the removedmaterial to landfill for disposal, this waste will besubject to The Landfill Regulations (England andWales) 2002 (SI 2002 No 1559) (as amended). TheLandfill Regulations (Scotland) 2003 (SSI 2003No.235) (as amended) and The Landfill Regulations(Northern Ireland) 2003 (Statutory Rule 2003 No 496)(as amended) apply in these respective countries.

5.1.9 According to the level of contaminationwithin the waste, it may be classified as inert, non-hazardous or hazardous, in accordance with wasteacceptance criteria set out in amendments to TheLandfill Regulations. Advice of the EnvironmentalProtection Agency and the Overseeing Organisationshould be sought with respect to classification of anywaste generated during maintenance and cleaningoperations.

5.1.10 It is likely that sediment and plant waste willrequire pre-treatment prior to disposal at a landfill site.This can take place either as the material is extracted orat the landfill site itself. The disposal of non-hazardousor hazardous waste is expensive and disposal facilitiesare limited. The benefits of testing, screening,separation and mechanical de-watering of the sedimentsusing mobile plant, should be considered. This not onlyfacilitates the separation of the materials into high andlow contamination levels (thereby minimizing disposalcosts) but also reduces the volume and weight of anymaterial that has to be landfilled by removing excesswater. The sand and pressed cake so produced is in aform that can be accepted by landfill sites under theterms of the Landfill Regulations, which ban highmoisture content wastes

5.1.11 This maintenance guidance is primarily aimedat chamber and trench type soakaways. Procedures willneed to be adapted to the particular soakaway designused. Guidance on maintaining vegetative systems forhighways is provided in HA 103 (DMRB 4.2) withsuggested frequencies for inspections of differentcomponents of these drainage systems.

5.2 Spillage Control

It is important to prevent gross pollution, such as mayoccur following a major road accident, from entering asoakaway. HD 33 (DMRB 4.2) discusses the provisionof control systems, such as notched weirs or penstocks,to prevent drainage water that is grossly contaminated5/2 May 2006


Chapter 6References

he Landfill Regulations (England and Wales) 2002SI 2002 No 1559) (as amended).

he Landfill Regulations (Scotland) 2003 (SSI 2003 No35) (as amended).

he Landfill Regulations (Northern Ireland) (Statutoryule NI 2003 No 496) (as amended).

he Management of Health and Safety at Workegulations 1999 (SI 1999 No 3242).

he Water Environment (Water Framework Directive)England and Wales) Regulations 2003 (SI 2003 No245).

he Water Framework Directive (2000/60/EC).

ritish Standard BS EN 752-4:1998: Drain and Sewerystems Outside Buildings Part 4: Hydraulic Design6. REFERENCES

Building Research Establishment (BRE) Digest 365Soakaway Design.

British Standard 5930 Code of Practice for SiteInvestigation.

Construction Industry Research and InformationAssociation (CIRIA): Report 156 Infiltration drainage manual of good practice.

CIRIA C521 Sustainable urban drainage systems design manual for Scotland and Northern Ireland.

CIRIA C522 Sustainable urban drainage systems design manual for England and Wales.

CIRIA C523 Sustainable urban drainage systems bestpractice manual.

CIRIA C609 Sustainable drainage systems Hydraulic,structural and water quality advice.

Design Manual for Roads and Bridges (DMRB)

HD 33 Surface and Sub-surface DrainageSystems for Highways (DMRB 4.2)

HA 103 Vegetative Drainage Systems forHighway Runoff (DMRB 4.2)

HA 216 Environmental Assessment Techniques:Road Drainage and the WaterEnvironment (DMRB 11.3.10).

Policy and Practice for the Protection of Groundwater,Environment Agency, 1998.

Guidance Manual for Constructed Wetlands. R&DTechnical Report P2-159/TR2 Environment Agency.2003.

The Construction (Design and Management)Regulations 1994 (SI 1994 No 3140).

The Confined Spaces Regulations 1997 (SI 1997 No1713).

The Groundwater Regulations 1998 (SI 1998 No 2746).

The Groundwater Regulations (Northern Ireland) 1988

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(Statutory Rule NI 1998 No 401).nd Environmental Considerations.

he Scottish Building Standards: Technical Handbook:omestic.6/1


ltration Herein used as synonymous withpercolation the generallydownward flow of water throughthe unsaturated zone to the watertable.

meability A measure of an aquiferscapacity to transmit groundwaterthrough a unit metre of itssaturated thickness (units in m/day).

aturated zone The zone between the top of anaquifer (limited above by theground surface) and the watertable (i.e. the top of the saturatedzone).

tilisation Changing of liquid to gaseousphase.

Chapter 7Glossary7. GLOSSARY

Aerobic In the presence of oxygen (air).

Advection The process by which solvents(e.g. dissolved pollutants) aretransported by the bulkmovement of flowinggroundwater.

Adsorption The uptake and retention of onesubstance onto the surface of aanother.

Anaerobic In the absence of oxygen (air).

Capillary fringe The zone at the interface betweenthe saturated and unsaturatedzones where water is drawnupward by capillary force.

Controlled waters In England, Wales and Scotlandsurface waters and groundwaterare collectively known asControlled Waters (this term isnot used in Northern Ireland, butin this document these representgroundwater and surface water).

Diffusion (In groundwater) the dispersionof a solvent caused by the kineticactivity of the ionic or molecularconstituents of the solvent andgroundwater. (In vapour) mixing caused by the kineticactivity of ionic or molecularconstituents.

Dispersion The spreading and mixing ofchemical constituents ingroundwater caused bydifferential velocities within andbetween the pore spaces in anaquifer.

Geo-cellular unit Proprietary units, usuallymodular and of plasticconstruction, which may be usedas infill to excavated soakawaysof trenches. These provide highvoid space and surfaceperforations to allow both high

Infi

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water storage and rapid dispersal.7/1


May 2006 8/1

8. ENQUIRIES

All technical enquiries or comments on this Advice Note should be sent in writing as appropriate to:

Divisional Director(Safety & Information)Highways AgencyRoom 4B, Federated HouseLondon RoadDorking A J PICKETTSurrey RH4 1SZ Divisional Director

Chief Road EngineerTransport ScotlandVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer

Chief Highway EngineerTransport WalesWelsh Assembly GovernmentCathays Parks M J A PARKERCardiff Chief Highway EngineerCF10 3NQ Transport Wales

Director of EngineeringThe Department for Regional DevelopmentRoads Service HeadquartersClarence Court10-18 Adelaide Street G W ALLISTERBelfast BT2 8GB Director of Engineering

Chapter 8Enquiries

CONTENTSINTRODUCTIONDESCRIPTION OF SOAKAWAY SYSTEMSDESIGN AND CONSTRUCTION OF SOAKAWAY SYSTEMSCONSTRUCTION PRACTICESMAINTENANCE AND MANAGEMENT OF SOAKAWAY SYSTEMSREFERENCESGLOSSARYENQUIRIES

Design Manual for Roads and Bridge

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