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DESIGN MANUAL FOR ROADS AND BRIDGES
VOLUME 10 ENVIRONMENTALDESIGN ANDMANAGEMENT
SECTION 1 NEW ROADS
PART 2
HA 56/92
NEW ROADSPLANTING, VEGETATION AND SOILS
SUMMARY
This Advice Note gives guidance on the environmentaldesign of planting, vegetation and soils for new roads.
INSTRUCTIONS FOR USE
1. Remove existing title page, content page andGeneral Preface page on the Goods Roads Guideseries of Advice Notes.
2. Insert new title page.
3. Archive this sheet as appropriate.
Note: New contents pages for Volume 10 containingreference to this document are available withHA 55/92.
HA 56/92
New RoadsPlanting, Vegetation and Soils
DESIGN MANUAL FOR ROADS AND BRIDGES
THE HIGHWAYS AGENCY
THE SCOTTISH EXECUTIVE DEVELOPMENTDEPARTMENT
THE NATIONAL ASSEMBLY FOR WALESCYNULLIAD CENEDLAETHOL CYMRU
THE DEPARTMENT FOR REGIONAL DEVELOPMENT*
Summary: This Advice Note gives guidance on the environmental design of planting,vegetation and soils treatment for new roads.
* A Government Department in Northern Ireland
Volume 10 Section 1Part 2 HA 56/92
December 1992
REGISTRATION OF AMENDMENTS
Amend Page No Signature & Date of Amend Page No Signature & Date ofNo incorporation of No incorporation of
amendments amendments
Registration of Amendment
Volume 10 Section 1Part 2 HA 56/92
December 1992
REGISTRATION OF AMENDMENTS
Amend Page No Signature & Date of Amend Page No Signature & Date ofNo incorporation of No incorporation of
amendments amendments
Registration of Amendment
VOLUME 10 ENVIRONMENTALDESIGN
SECTION 1 THE GOOD ROADSGUIDE - NEW ROADS
PART 2
HA 56/92
THE GOOD ROADS GUIDENEW ROADSPLANTING, VEGETATION AND SOILS
Contents
General Preface to the Good Roads Guide series ofAdvice Notes
Chapter
1. Planting, Vegetation and Soils: Introduction
2. Screening with Vegetation
3. Off-site Planning
4. The Right Vegetation for the Countryside
5. Formal Planting and the Urban Fringe
6. Retaining Existing Vegetation
7. Establishing Woodland
8. Scrub and Tree Groups
9. Hedges
10. Grass and Heathland
11. Steep Slopes
12. Effective Tree and Shrub Establishment
13. Soils
14. Enquiries
DESIGN MANUAL FOR ROADS AND BRIDGES
December 1992
Volume 10 Section 1Part 2 HA 56/92
December 1992
GENERAL PREFACE TO THE GOOD ROADS GUIDESERIES OF ADVICE NOTES
Structure of the Guide
0.1 The Good Roads Guide is the name given to theseries of documents contained in Sections 1, 2 and 3 ofVolume 10 of the Design Manual for Roads andBridges. The Guide is written in nine Parts each ofwhich is published as an Advice Note. The Guide iswritten to be read as a whole. The Parts of the GoodRoads Guide are as follows:-
Section 1 NEW ROADS
Part 1 HA 55/92 Landform and AlignmentPart 2 HA 56/92 Planting, Vegetation and SoilsPart 3 HA 57/92 Integration with Rural
LandscapesPart 4 HA 58/92 The Road CorridorPart 5 HA 59/92 Nature ConservationPart 6 HA 60/92 HeritagePart 7 HA 61/92 Contract and Maintenance
Implementation
Section 2 MOTORWAY WIDENING
Part 1 HA 62/92 Environmental DesignWidening Options andTechniques
Section 3 IMPROVING EXISTING ROADS
Part 1 HA 63/92 Environmental DesignImprovement Techniques
How to use the Good Roads Guide
0.2 Many of the design ideas put forward in Section 1- New Roads are also relevant to the other Sections andcross references have been provided.
0.3 The first Chapter of each Part of the Guidereviews the issues and topics covered. The subsequentchapters deal with a particular topic. Within eachchapter, the key issues are first listed and then discussedwith illustrations drawn from roads throughout the UK.
0.4 The Good Roads Guide is not a step-by-stepguide on how to build a road or a substitute forprofessional advice. It is intended to be used by thedesigner to help in the identification of areas and issueswhere careful consideration of environmental factors isrequired. The division of the Guide into Parts and theParts into topics has been done to aid this process.
0.5 Environmental design of roads is a matter ofrespecting the special character of each individuallocation. The illustrations included show solutionsdevised to meet the requirements of specific sites. Theuse of standard solutions, irrespective of the location, isnot appropriate.
Implementation
0.6 The principles set out in this Advice Note shouldbe taken into account in the preparation of all schemesfor the construction and improvement of trunk roads,including motorways.
0.7 Where conflicts exist between environmentaldesign, costs, engineering feasibility and safetyrequirements, and competing options are available, theDesign Organisation will need to advise the OverseeingDepartment accordingly.
Application in Wales
0.8 Requirements in Wales are primarily covered bythe publications "Roads in Upland Areas: Design Guide"(published by the Welsh Office 1990) and "Roads inLowland Areas: Design Guide" and "Rock Profiling andVegetation Re-establishment" (both due for publicationby the Welsh Office in 1993). This Advice Notesupplements these Design Guides.
Application in Scotland
0.9 The Scottish Office Roads Directorate endorsesthe practice given in the Good Roads Guide. Morespecific guidance is provided by the Roads Directorate'sLandscape Officer.
0.10 The Scottish Office discussion documentpublished in February 1992 "Roads, Bridges and Trafficin the Countryside" addresses related issues.
General PrefaceThe Good Roads Guide
0/1
Volume 10 Section 1Part 2 HA 56/92
December 1992
Application in Northern Ireland
0.11 The principles set out in this Advice Note areendorsed as good practice by the Department of theEnvironment (NI). The guidance will be taken intoaccount in preparing schemes for the construction orimprovement of all roads in Northern Ireland.
Acknowledgements
0.12 The following photographs have been reproducedwith permission:-
Chapter 11 Page 1Timber retaining wall with concrete piers, planted withwillow previously published in Use of Vegetation inCivil Engineering published by CIRIA/Butterworths.
Chapter 11 Page 2Two photographs supplied by MMG Erosion Systems:Soil restored by honeycomb geotextile beforehydroseeding and after.
Photograph showing the use of fibre mat by permissionof Richards, Moorehead and Laing Ltd, previouslypublished in Landscape Design.
Two photographs supplied by Comtec (UK) Ltdshowing hydroseeding before and after.
General PrefaceThe Good Roads Guide
0/2
DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 1 PLANTING, VEGETATION AND SOILS: INTRODUCTION
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1.1 SCOPEl This Part gives guidance on the environmental design of planting, and vegetation, and soil treatment for
new roads.
1.2 MAIN ISSUESl The pattern and species composition of vegetation in the English landscape often changes dramatically
over short distances. This pattern and species grouping needs to be reflected in design. Good practice inplanting aims to integrate the road with adjacent landscape and not create a separate roadsidelandscape.
l Planting and seeding need clear objectives. Design of woodland planting, for instance, should take intoaccount amenity, wildlife or screening. Different types of grassland are needed for specific functionssuch as nature conservation interest or return to agriculture.
l Soil conditions are the most important elements in establishing vegetation. If soil structure is damaged orif the wrong soil type is reinstated, it may be impossible to establish suitable vegetation.
l Long-term management must always be considered in planting design. This has not always been thecase on road schemes. By such practices as the use of high shrub densities and low tree densities atplanting, maintenance can be concentrated in the most effective areas.
l There is often public pressure for instant landscapes using large trees. Such landscapes are difficult toestablish and are often unsuccessful.
l Planting outside the highway boundary may be necessary to ensure integration with the surroundinglandscape and this can be achieved by using the Department of Transport’s off-site planting powers.
1.3 EFFECTS OF ROAD DEVELOPMENT ON VEGETATION AND SOILSl Direct loss of vegetation
l Creation of unsuitable soil conditions and adverse gradients that are too steep for new planting.
l Disruption of the existing pattern of vegetation in the landscape.
l Indirect effects on retained vegetation through changes in drainage pattern and exposure of maturewoodland to windblow.
l Exposure of the road to views, such as those from properties, which require screening.
l Opportunities for restructuring the vegetation of degraded landscapes.
l Opportunities to reintroduce predominantly native planting in areas such as intensive arable land at theurban fringe.
1.4 DESIGN OBJECTIVESl To define areas needed for effective mitigation by planting.
l To restore as much of the pre-existing pattern of field boundaries, woodland, heathland and moorland, aspossible. Retaining land adjacent to the highway should always be considered, in order to provideintegration with the landscape.
l To establish a clear design objective and maintenance regime for each area of vegetation established.
l To reinstate soil to the highest possible standard by stripping, storing and reinstating it in line with currentbest practice.
l To ensure soil restoration using matching soil types wherever possible.
l To mitigate secondary impacts on retained vegetation.
1.5 MITIGATIONl Design for effective long-term maintenance.
l Provide adequate conditions for plant growth through preparation of uncompacted soils to appropriatedepths. Where these cannot be provided, an alternative solution should be sought.
l Recognise the limitations of prevailing conditions: eg large trees cannot be established on compacted 1:2embankments.
l Use a high standard of maintenance for rapid establishment.
l Design for easy maintenance.
l Allow adequate areas for planting for specific purposes. For example, a tall screen needs to be at least10 m deep. An eye-level shrub screen can be achieved with a minimum width of 5 m.
l Respect the existing pattern of vegetation and use mainly native species in rural areas.
l Design earthworks with the type and extent of planting in mind: the shallower the gradient the easier it isto establish vegetation.
1.6 STATUTORY BODIESl Within this Part, reference to the Department of Transport, English Nature, English Heritage and the
National Rivers Authority should also be read as referring to the appropriate statutory authority or adviserfor Wales, Scotland and Northern Ireland.
DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 2 SCREENING WITH VEGETATION
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2.1 PRINCIPLEl Planted screens can be effective in restricting both views of the road and views from the road. Good
design will achieve both of these, while reflecting local character, maintaining good views and creating ascreen which is not itself an intrusion into the landscape.
2.2 KEY ISSUESl The most effective screen is one close to the viewer rather than to the object viewed. A screen at the
bottom of the garden is more effective than one at the roadside.
l A minimum thickness of 10 m is required for a tree screen which can be adequately maintained. A shrubscreen needs to be at least 5 m wide.
l Vegetation needs to be at least 4.5 m tall to screen heavy goods vehicles.
l Plants vary greatly in their foliage density and branching habit. Hawthorn, for example, makes a good,dense screen; ash does not.
l Variations in width and height are essential when fitting a screen into an established vegetation pattern.
l When used in combination with walls or with closed-boarded fences, planting should reflect differencesin the landscape character on the two sides of the screen.
l Vegetation does not reduce noise significantly, but it may reduce people’s perception of noise.
l Best design practice provides screening while maintaining attractive, long-distance views.
2.3 LOCATION AND DENSITIES OF SCREENS
A2, Kent Typical screening problems where the road runs close to property
Good practice Typical arrangement of a well-considered screen
Good practice: A52, Nottingham This dense, well-maintained hawthorn screens thesettlement very well from this busy road
2.4 SPECIES FOR SCREENINGFor screening, useful trees are beech, holly, whitebeam, hornbeam and field maple.Hawthorn, blackthorn and dogwood are useful shrubs. Choice of species does, however,depend on the character of the surrounding vegetation: see for example, Ch 8.
2.5 SCREENING A ROAD CORRIDOR THROUGH DEVELOPMENT
A12, Essex Housing, road and railway are close together in a narrow corridor with littlescreening or planting structure
Improvement by screen planting
CHAPTER 2 SCREENING WITH VEGETATION
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2.6 SCREENING WHILE MAINTAINING VIEWS
A typical integrated design that maintains views The problem: M40, Oxfordshire When this planting matures an attractive view, in which the road is not a dominantelement, will be lost. It might have been better either to have accepted the view or to have broken it up withintermittent planting
2.7 SCREENING FOR FLAT LANDSCAPESScreen on flat landscapes can be difficult. They often have very little vegetation cover, so that planting near the road merely draws attention to the problem. It is essential, therefore, to designplanting around existing features.
A17, Norfolk In such an open landscape screening needs to be based around existing tree groups and settlement
Improvement
VOLUME 10 SECTION 1PART 2 HA 56/92
DECEMBER 1992
DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 3 OFF-SITE PLANTING
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3.1 PRINCIPLEl Off-site planting is used to screen property and public places at some distance from the road. Designers
need to be fully aware of their off-site planting powers and the maintenance required to get full benefitfrom this planting.
3.2 KEY ISSUESl Under Section 253 of the Highways Act 1980 the Department of Transport can enter into agreement with
a landowner affected by a new trunk road for planting on his/her land at the Department of Transport’sexpense. After a minimum three-year maintenance period, the landowner is obliged to maintain theplanting for 25 years.
l Off-site planting is most often achieved on the land of people who wish to be screened from the road.Planting on third-party land where the owner receives no benefit from it can be difficult to arrange.
l Off-site planting is an opportunity not only to provide mitigation by screening but also to integrate the roadwith the landscape, as discussed in Pt 3.
3.3 LOCATION OF OFF-SITE PLANTING
Typical off-site planting benefits and constraints
Good practice: M40, Banbury Bypass shows off-site planting used to screen both propertyand public open space
Points of good practice showing off-site planting used in a variety of ways to conserve orenhance landscape character
Good practice: M40, Warwickshire Off-site planting on the Burton Dassett Hills hasconserved the character of a country park and provided screening for local settlement
DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 4 THE RIGHT VEGETATION FOR THE COUNTRYSIDE
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4.1 PRINCIPLEl Before preparing detailed planting and seeding strategies (these are dealt with in Chs 7-13) it is essential
to gain a clear understanding of the type of vegetation and species’ composition appropriate to the settingof the road. Integration with the landscape (see Ch 3), provision of nature conservation benefits anddriver interest are essential considerations.
4.2 KEY ISSUESl Large-scale tree and shrub planting may not be the best landscape strategy for a new road. Where
screening is not an issue, it may be appropriate to create grassland, heathland or scrub, which may bebetter suited to their surroundings.
l Native species should generally be used in rural areas. In addition, they must be native to the region andoccur naturally on the soil type adjacent to the road. For example, beech is not native north of themidlands and should not be planted in woodlands beyond this region, but it is widely used in designedlandscapes throughout the country. Similarly, shrubs of limestone such as the wayfaring tree should onlybe used on these soils.
l The choice of seed for wildflower mixes presents a very complex problem, and advice should be soughtfrom the Wild Flower Handbook published by the Department of Transport as well as from EnglishNature.
l Although many semi-natural plant communities contain a lot of species, others such as beechwood orheathland, have only a few. Good design must work with, not against, the composition of naturalvegetation types and consider the use of natural groupings, not arbitrary mixtures, grids or matrices, inpreparing planting layouts.
4.3 GRASSLANDGrassland is often the right vegetation type to fit the existing landscape and must beestablished as a site-specific type.
Rank, coarse grassland is typical of much highway land and stands out in marked contrastto the surrounding agricultural land, rather than blending with it
Good practice: M6, Cumbria Apt use of low-maintenance grassland on the right soilconditions has ensured a fit to the landscape, as well as providing nature conservationinterest
4.4 HEATHLAND
Heathland is a rapidly-windling resource,easier to establish than many othervegetation types. The Department ofTransport can therefore make a positivecontribution to nature conservation and
landscape character in the right circumstances.
4.5 SCRUBScrub communities can be just as varied as woodland, but they can also be large, uniformareas of common species like hawthorn and blackthorn. Careful appraisal of local conditionsis needed and arbitrary introduction of species is not good practice.
4.6 WOODLANDMost woodlands are products of either a long period of management of self-regeneratingtrees and shrubs or of deliberate planting. They do not have a natural distribution of species.New planting is an opportunity to create a more natural woodland type which will give aspecial character to the area and be of high wildlife interest.
The important points to remember are:l oak is a common woodland tree but is not the dominant species it is often believed to be. Small-leaved
lime, ash, field maple, and other species were originally equally common in some areas. Good practicedictates that the right range of local species is established for a project by consultation with EnglishNature and other bodies;
l natural woodland structure is a mosaic of groups of the same species responding to local changes insoil, topography and drainage. Beech woodland, for instance, is almost entirely dominated by onespecies, while ash with field maple woodland is more varied.
DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 5 FORMAL PLANTING AND THE URBAN FRINGE
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5.1 PRINCIPLEl Most inter-urban roads run through the countryside where they need to blend with their surroundings in
the ways illustrated in Chs 2, 3 and 4. However, three are some situations where a distinctive road-corridor landscape, or the use of non-native species, is appropriate because it provides a sense of placeand a distinctive style.
5.2 KEY ISSUESl An avenue can give a distinctive character to a road corridor. Large trees should be placed at least 7.5 m
away from the edge of the carriageway.
l Bold planting can give structure to the urban fringe and bring character and a sense of place to urbanbacklands.
l Well-sited planting can highlight landmarks, signal the approach to settlement and focus the driver’sattention.
l Good design can signal the change from rural to urban environments.
5.3 AVENUES
Oxford ring road For roads through cities, avenues give a distinctive character to theroadside landscape
Establishing avenues in rural areas
Good practice: A40, Buckinghamshire Near the historic landscape of West Wycombe Parkthis well-maintained lime avenue is entirely appropriate to its setting
DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 5 FORMAL PLANTING AND THE URBAN FRINGE
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5.4 APPROACH TO SETTLEMENT
Structure planting can buffer the road from development, improve the road corridor andstructure views from the road: see Pt 4, Ch 12
Warrington New Town An adequate width of planting between the industrial units and theroad provides a pleasant environment
5.5 THE NEED FOR PLANTING
M25, Waltham Abbey section Structure planting would have made all the difference to thisroad corridor
Western Avenue, A40 Planting on the roadside edge here would break up the scale of thebuilt environment and focus views on the buildings of interest
DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 5 FORMAL PLANTING AND THE URBAN FRINGE
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5.6 CHANGING TO AN URBAN ENVIRONMENT
Signalling change Planting at roundabouts (see also Pt 1, Ch 13) can signal change fromrural to urban areas and provide landmarks. Simple ground cover, shrub and specimen treeplanting is inexpensive to maintain
Urban planting: Milton Keynes More formal planting, using robust ornamental speciestolerant of roadside conditions is appropriate where roads pass through urban areas. Seealso Chs 1 and 12
5.7 BACKLANDSNew roads can cut through the backlands of urban and suburban areas where structureplanting may be necessary to improve the environment for both drivers and residents.
Chesterfield Bypass, Derbyshire
Improvement Planting of highway land and adjacent derelict areas could greatly improve thecharacter of the road corridor
DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 6 RETAINING EXISTING VEGETATION
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6.1 PRINCIPLEl Retaining existing vegetation can play an essential part in maintaining landscape character and local
landmarks. Making the best use of established vegetation needs to be considered both when a route’salignment is being selected and in order to safeguard individual trees near the carriageway.
6.2 KEY ISSUESl Where trees are to be retained as landscape features, inspection by a qualified arboriculturalist is needed
to ensure that the effort is worthwhile. In general, a tree should last at least 15 years after completion ofthe road.
l Young trees and scrub require as much attention as mature trees. They may not be much to look at butthey will develop to maturity well in advance of new planting.
l Alignment near groups of trees and woodland edges must conform with the safe distances fordevelopment given in BS 5837:1991.
l Retaining an existing tree near the road can be an expensive and time-consuming process. It must betackled in the right way.
l Where the edge has been taken off a wood, special attention must be paid to the effects of windblow andstorm damage on the remaining trees.
6.3 ALIGNMENT NEAR TREES AND WOODLANDCareful alignment allows maximum use to be made of existing woodland, trees and hedgesto improve the landscape setting of a road.
The principle The road has been aligned to fit within a landscape of mature parkland trees
6.4 EXCAVATION AND TREESMost tree roots lie in the top 600 mm of soil and 90 per cent are in the top metre. Even veryshallow disturbance can cause irreparable damage.
Poor practice: A22, SussexThe root system has been severed bythe road construction, damaged further andeventually suffocated by the construction ofa bund
Improvement It should have beenaccepted that it was not feasible to retainthe tree and a new landscape schemedevised
Good practice: A16, Boston, Lincolnshire A fine specimen tree was retained by goodpractice. It was necessary to construct a scaffolding cage to prevent damage duringconstruction
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DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 6 RETAINING EXISTING VEGETATION
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6.5 INSTANT MATURITY
Good practice: A69, Brampton Bypass, Cumbria Mature trees have been retained withinthe working area of viaduct construction by co-operation between designers and contractors
A23, West Sussex Mature trees have been retained in a wide central reserve, giving instantmaturity to landscape: see also Pt 1, Ch 12
6.6 EXPOSING A NEW WOODLAND EDGEWhere a road has to pass through a woodland the trees at the newly exposed centre, whichare usually tall and slender with narrow crowns, can be blown over in strong winds.
Poor practice: new residential road, Hampshire Exposure of the dense, leggy plantationhas given a very unattractive result
Good practice Careful thinning inadvance of road construction hasproduced an attractive and more stableedge
Points of good practice arellustrated here. Scarification can play animportant part in preparing groundintended to develop as woodland edge
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CHAPTER 7 ESTABLISHING WOODLAND
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7.1 PRINCIPLEl The objectives of woodland planting for road development are to integrate the road with the landscape, to
provide visual interest and to provide wildlife benefits. Commercial forestry objectives are not appropriate.Native trees and shrubs should generally be used.
7.2 KEY ISSUESl Trees and shrubs grow best in small blocks of the same species (this is the way they are found in
nature), and not in apparently random mixtures. Planting plans must, therefore, create simple, realisticlayouts that can be achieved and supervised by planting on a grid.
l The structure and species composition intended for the mature woodland must be established as cleardesign objectives using species native to the locality.
l There are several types of woodland structure. It is important to choose the one that fits in with adjacentwoodland and is appropriate for the maintenance regime proposed.
l Correct planting distances are essential for effective maintenance and good establishment. If trees areallowed to grow too close together they become very tall and slender-stemmed, with very narrow crowns.
l Different species grow at different rates. If poorly-designed mixtures are used, some species will besuppressed and the wrong balance of surviving trees will result.
l Successful establishment is achieved by the rigorous application of basic horticultural principles. SeePt 7, Ch 3.
7.3 VARIETY AND GROUPING
7.4 WOODLAND TYPE
High forest has trees providing Coppice with standards This is Scrub woodland: scrub witha closed canopy with little or no the traditional method of English emergent trees. The scrub isshrub layer. Regular plantations woodland management with unmanaged and graduallysuch as those of poplar, or about 40 trees/ha and shrubs dies out as trees grow up.species with dense shade like cut (coppiced) on a 7- to 25-year This can be a very effectivebeech look like this and should cycle. It is appropriate in many means of woodlandbe imitated if they occur nearby lowland situations because it will establishment, creating
marry in with existing woodland. significant wildlife interestThe standard trees have single without major maintenancestems, and the coppice is multi- implications. It is the methodstemmed. generally recommended for
new woodland planting, witha ratio of 8 shrubs to 1 tree:see Pt 7, Ch 5
Good practice: M27, Hampshire A variety of species has been used. The numbers of individuals in each species group and the size of the clumps, are in keeping with the scale of the road.
VOLUME 10 SECTION 1PART 2 HA 56/92
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DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 8 SCRUB AND TREE GROUPS
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8.1 PRINCIPLEl The use of scrub and small groups of trees is often essential in softening abrupt edges of woodland
planting and integrating the road with pre-existing landscape character. Clumps of trees can be anessential element for integrating the road with parkland.
8.2 KEY ISSUESl Scrub is an effective way of giving a more natural edge to woodland and increasing its wildlife interest. It
is also the present vegetation character of a number of places which have thin, poor soils.
l Tree clumps and single trees need particular attention for successful establishment and effectivemaintenance.
l Intermittent planting is particular important for landscapes like downland and wetland, where large-scaleplanting is usually inappropriate.
8.3 SCRUB AND WOODLAND EDGESScrub can be a significant element in softening new woodland when planted at the sametime, or in softening newly-exposed edges. It can be maintained easily by rotational coppicingas set out in Pt 7, Ch 5. An example is shown below.
Good practice: Using scrub to soften a woodland edge
A27, Brighton Bypass Herehawthorn scrub is a long-established vegetation type.New scrub is the mostappropriate planting to absorbthe road into the landscape
8.4 TREE GROUPSTree clumps are necessary in the reinstatement or construction of parkland landscapes or inproviding a broken edge to a woodland to fit in with local character, as shown below:
Keeping local character: for this tree groupsare best established by the traditionalmethod ofplanting in a matrix of scrub whichis eventually shaded out or removed. Thisreduces maintenance requirements andallows better establishment, thus:
Principles put into practice
8.5 INTERMITTENT PLANTINGIntermittent planing is appropriate where a strong vegetation pattern would over-emphasisethe road.
Typical intermittent planting on anopen landscape with sparse vegetation
Successful intermittent planting in theopen landscape of the Pevensey levels
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Planting with scrub
Maturing clump
DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 9 HEDGES
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9.1 PRINCIPLEl Hedges can be essential for integrating the road with agricultural landscapes. They need to be planted
and managed in a way that reflects local character.
9.2 KEY ISSUESl Hedges are normally provided as part of accommodation works by negotiation with the farmer. Their
layout should ensure that he/she can maintain them easily employing methods used elsewhere on thefarm.
l Hedgerow trees are being lost gradually in many parts of the country. Roadside hedges offer anopportunity to provide replacements and enhance landscape character as a result.
l Species composition should reflect that of neighbouring hedges. Although overgrown hedges of mixedspecies may be appropriate in intensively farmed areas, hedges in many areas are hawthorn-dominated.
l Low machine-cut hedges have very little wildlife interest and the emphasis in intensively farmedlandscape, where these are predominant features, should be on integration with existing landscapepattern.
9.3 ESTABLISHMENTCultivate a strip 600 mm wide and 300 mm deep at a distance of 300 mm from the fence lineand incorporate a 75 mm layer of bulky organic matter. Plant in two lines 300 mm apart toform a staggered, double row. The transplants in each line should be 450 mm apart, giving atotal of five plants per running metre.
9.4 FENCINGA standard post-and-rail fence is unnecessary and inappropriate where a hedge is to form aboundary. Light metal posts and stockproof wire are the best solution and allow the hedge togrow through the fencing on the highway side.On the farmer’s side it will usually be necessary to put up a temporary stockproof fencing toallow the hedge to become established.
M26, Kent Light metal posts and stockproof wire provide a more-than-adequate fence whichdoes not impede hedge growth
9.5 OVERGROWN HEDGESHedges are normally best planted outside highway land as accommodation works (see Ch 3).However, overgrown hedges with mature trees are frequently found in the countryside andmaintaining them can be a good way of integrating the road with the landscape and ofscreening property.
Integrated planting
9.6 HEDGEROW TREESHedgerow trees can be planted with the staggered double row of transplants when a hedge isplanted. They must be indicated clearly by marker stakes so that they are not cut accidentallyduring hedge trimming. However, their retention relies on the goodwill of the farmer and theattention of his hedging contractor, and it may be advisable to offset the trees from thehedges so that they lie just within the highway land.
Well-formed hedgerow trees can help integrate the road with the surrounding landscape. Itis worth solving the problems of their establishment and maintenance
DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 10 GRASSLAND AND HEATHLAND
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10.1 PRINCIPLEl Grassland and heathland of nature conservation and amenity interest can be developed in many road
landscapes. Clear design and management objectives are necessary for each area. These should takeadvantage of current practice in amenity and nature conservation grassland establishment andmanagement.
10.2 KEY ISSUES FOR GRASSLANDl The lack of clear objectives in the establishment and management of grassland in the past has led to the
domination of extensive areas by vigorous, coarse species.
l Semi-natural, species-rich grassland have developed over a long time and with complex managementrequirements. A realistic objective for roadside grassland is not to attempt to recreate these grasslandsbut to develop extensive areas of moderate species richness and diversity.
l Control of soil fertility is the key to establishing a diverse grassland with low maintenance requirements.Conventional topsoil is required only in some circumstances, and then sparingly. Where there arenaturally-fertile soils it is realistic to aim for grassland of moderate species diversity, made up of plantswith wide tolerances.
l Unmanaged grassland eventually turns to scrub. However low its fertility, a management programme isrequired.
l On steep slopes, rapid establishment may be necessary to avoid surface erosion. In general, the use ofshort-term nurse crops like Italian rye-grass is not recommended and a site-specific solution, such as theuse of emulsions to hold the surface while cover establishes, should be sought.
l The types of grassland described in this section are slower to establish than conventional rye-dominatedmixtures and can look untidy in autumn and winter. Education of the public is therefore a significantrequirement.
10.3 CLEAR OBJECTIVES
Poor practice This grassland, dominated by false oat grass, has very little wildlife interest oramenity value. It has developed from rye grass sown on topsoil, a technique more appropriatefor heavily used public open space
10.4 MANAGEMENT IMPLICATIONS
A45 The steep-sided bund in the central reservation is difficult to get to and cannot be cutwith conventional mowing equipment. A dense hedge would have been better, or the use of alow-fertility, low-maintenance design.
Any grassland needs cutting to maintain it. The frequency and height of cutting depend onthe objective, the site’s fertility and flowering period of the plants present. These must betaken into account in specifications and are discussed in Pt 7, Ch 4
10.5 PUBLIC ACCEPTABILITY
Wildflower grassland does not establish as easily or as rapidly as a lawn. Informing thepublic can help make such temporary features more acceptable
Tidiness or naturalness? French motorways have a neat and tidy appearance. However,the close mowing is inimical to wildlife interest
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10.6 ESTABLISHMENT AND FERTILITYl Advice on establishment is to be found in Pt 7, Ch 4.
l Subsoil (see Ch 13) must be separated from topsoil, spread after the completion of the earthworks andcultivated to a depth of 100 mm. On nutrient-poor subsoils, such as chalk, a layer of topsoil, no deeperthan 50 mm, should be spread as a seed bed.
l Cultivation of the surface 50 mm to provide a seed bed is required, but a fine tilth such as that requiredfor the seeding of lawns, is not needed. No fertiliser should be used.
l While many grasses will germinate in spring or autumn over a long period, many broadleaf plants havemore exacting requirements, so it is recommended that seeding takes place in August/early September:whenever practical.
l A range of standard seed mixes is available. However a site-specific mix, prepared with specialist advice,is usually the best option.
Good practice: A22, Willingdon Roundabout On low-fertility chalk soils an attractive floracan be developed using the right preparation, seeding and management
10.7 NATURALLY-FERTILE SOILSOn naturally-fertile soils, such as most clays, a mixture of robust and colourful species canbe used to give a pleasing effect. Many of these species form stable and long-lastingherbaceous vegetation which requires limited maintenance. A selection of species which havebeen used with some success is given in the table opposite, together with some of theirrequirements. Taller species must be set back from the verge and kept out of sight lines.
KeyPersistence: * -**** Geographical distribution: W-widespread; S-below 55ºShowiness: * -**** Moisture demand: * -***
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10.8 NATURALLY INFERTILE SOILS
Good practice Re-use of poormoorland soil seeded withappropriate grass species hasallowed the re-establishment ofheathland vegetation includingheather, tormentil and lady’s mantle
Chalk grassland: A27 Chalkgrassland is disappearing rapidly as aresult of farming improvements andthe abandoning of grazing. Chalkgrassland can become establishedon suitably treated highway land
10.9 MAKING THE MOST OF OPPORTUNITIESRoad development can create conditions for species-rich grassland, a rapidly disappearinghabitat.
Wetland: M6 Low ground at centres of roundabouts can be much moister than thesurrounding land. Here, wet grassland may be created. A similar habitat can be establishedaround balancing ponds
10.10 KEY ISSUES FOR HEATHLANDl Heathland is a declining habitat nationally and most significant areas are now under statutory protection.
Where new roads are put through former heathland their construction is an opportunity to develop newheathland habitats.
l The best examples of heathland are the results of complex management regimes. These cannot becreated on roadside landscapes, but there are opportunities for developing simpler systems.
l There is often a shortage of suitable seed sources for heathland and best effects are usually achieved byusing chopped-up fragments of heathland vegetation and surface soil (blading) from site clearance.
l Management must always be borne in mind, since heathland on well-drained sites (ie most highway land)is particularly vulnerable to invasion by birch and pine.
l Widening into existing heathland is discussed in Section 2: Pt 1, Ch 9.
A21, Kent Heathland has established on the poor, acid soils of the Tunbridge Wells Sand
If bladed material is available this should be spread in a prepared seed bed of subsoil asdescribed for grassland, if seed alone is to be used, it may be best to use a mulch andemulsion to stabilise the light sandy soils while vegetation established. More information onheathland establishment is contained in Pt 5, Ch 2.
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VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 11 STEEP SLOPES
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11.1 PRINCIPLEl Steep slopes are sometimes necessary in road development, although they should be avoided wherever
possible. Establishing vegetation on them can be a difficult process, but it is often essential in fitting theroad to the landscape and in stabilising soil.
11.2 KEY ISSUESl Woody vegetation can help stabilise material with shallow-seated instability (less than 2 m), especially on
clay soils, but it has no effect on deep-seated instability.
l The use of geotextiles plays an integral part in vegetating steep slopes. New materials continually arriveon the market and the solutions given here are only indicative.
l Part 1, Ch 18 deals with rock exposures. Sometimes it is appropriate to establish vegetation on them, inwhich case the use of geotextiles and other techniques can create the right microsites for naturalregeneration.
l Some of the most satisfactory results can be obtained by the use of vegetation together with modularretaining systems like gabions.
l Hydroseeding is a widely-used method of establishing grass on steep slopes. The principle ofestablishing grassland for nature conservation and amenity, set out in Ch 10, should be applied to theissues raised by vegetating steep slopes.
11.3 STABILISING WITH VEGETATIONVegetation can help surface stability where it can penetrate to the bedrock.
On some continental and North American roads, willow, which will grow vigorously from stemcuttings (setts) has been used to stabilise clay banks. Brushwood can also be used withgabions or a timber slope grating.
Penetrable bedrock eg clay. Roots canact as toe piles and can be a majorelement in the slope’s stability
Impenetrable bedrock Vegetation does not help to stabilise this
Simple methods of establishingshrub cover with brushwood
Brushwood combined with timber slope grating andconcrete piers can make a visually attractive roadsideedge
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11.4 GEOTEXTILESHoneycomb geotextiles or matting is appropriate where a rapidly-established, dense grasscover is required.
Soil restored by honeycomb geotextiles Good practice The geotextile hasbefore hydroseeding established a stable surface, permitting
natural regeneration by ash
Biodegradable geotextiles can be used for surface stability while seeding becomesestablished, as shown in the photograph below:
Temporary surface stability on a site with self-evident risk of erosion
11.5 HYDROSEEDINGHydroseeding is widely used to establish good grass cover. The choice of emulsion, organicmatter and seed used as well as their proportions in the mix are, however, crucial for goodresults. A precise specification chosen for the site and a properly-qualified, specialist sub-contractor are essential.
Good practice A steep slope has been successfully grassed over by hydroseeding
11.6 ROCK OUTCROPSRock outcrops can vegetate naturally. This is usually the most effective method, and certainlygives the best visual effect.
A470, Glamorgan Naturalcolonisation usually provides the
best visual effects
Where weathering is likely to causemany fragments to fall down the
surface, vegetation can be establishedby creating microsites to be colonisednaturally
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DECEMBER 1992
VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 12 EFFECTIVE TREE AND SHRUB ESTABLISHMENT
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This section is a brief aide memoire for details of current planting and establishment.
12.1 DISTANCE FROM THE CARRIAGEWAYShrubs must be not planted within 3 m of the carriageway and trees not within 5 m of it.
12.2 PLANT SIZESBare-root transplants, 450-900 mm high, for shrubs and trees are the recommended size andspecification, except for evergreens which should be container-grown and can be used at smaller sizes.Plants taller than 900 mm are not recommended except where light, feathered trees are needed for earlyimpact. Trees taller than this will provide short-term benefit only. The special case of the use of largerstock in relation to motorway widening is dealt with in Section 3: Pt 1 and avenues are discussed in Ch 5.
12.3 STOCK HANDLINGBare-root plants must be protected from drying out right up to the time that they are put into the ground.Forestry bags are recommended.
12.4 PREPARATIONThe growing medium needs to be cultivated to a depth of 450 mm to allow adequate tree and shrubgrowth. Where this is not possible, there is no point in creating planting pits extending into compactedlayers.
12.5 PRE-PLANTING WEED CONTROLBefore planting into a dense grass sward or weed-infested area, a herbicide spray must be used to give aweed-free planting area of at least 750 mm diameter for each plant.
12.6 TIME OF PLANTINGPlanting is best carried out in November and December. Planting from January to March can be muchless satisfactory and it should not take place after the end of March.
12.7 FERTILISERSOn most disturbed soils, an overall base application of a balanced granular fertiliser should be given atthe manufacturer’s recommended rate, either incorporated into the surface, or as a top dressing.
12.8 PLANTING METHODSPit planting is generally recommended. Pits should be 300 x 300 x 300 mm with the sides and bottombroken up. The back-fill material should be 75 percent topsoil, or approved subsoil (see Ch 13) and 25percent bulky organic matter. The latter will vary from one part of the country to another because of localavailability.
12.9 TREE SHELTERSThese should generally be used only on exposed sites or where rapid establishment is imperative.
12.10 WEED CONTROLThe importance of a high standard of weed control cannot be over-emphasised. A 750 mm diameter areaaround each plant should be weed free at the start of the growing season and be maintained that way.Where ornamental planting is appropriate, such as at roundabouts, a blanket mulch of course bark orsimilar material may be required. In rural areas chemical weed control is recommended, in preference tomulch mats or other physical methods.
12.11 PEST CONTROLIt is necessary to protect woody transplants on sites where rabbits, hares, voles or deer are present. Thetype of protection to be used (eg individual guards or fencing-off planting blocks) will depend on the sizeof area and abundance of the pests. Guidance can be found in the relevant Forestry Commissionpublications.
12.12 STAKINGStaking is unnecessary for the sizes recommended and prevents proper establishment of smaller treesand shrubs. It should only be used for larger sizes. Stakes should be no taller than one quarter the heightof the tree.
12.13 PLANTING AND THINNING GRIDS
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VOLUME 10 SECTION 1PART 2 HA 56/92CHAPTER 13 SOILS
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13.1 PRINCIPLEl The handling, storage and preparation of soil is the most important factor in establishing vegetation, since
mistakes can rarely be put right. Engineers are interested in the physical properties of soil as aconstruction material, so detailed information about the mechanical properties of the available soil isobtained for constructing a road. However, as soil is also a complex biological system supporting plantgrowth, an equal understanding of its biological system supporting plant growth, an equal understandingof its biological properties is needed for successful vegetation establishment.
13.2 KEY ISSUESl For each soil type to be stripped and respread, the topsoil and subsoil layers need to be identified and
clearly defined. A strategy for their use and storage must be developed before the contractor moves ontoa site.
l Topsoil is needed to grow trees and shrubs, but usually in only very limited quantities for grasslandestablishment.
l Soils for reinstatement should be those derived from the site and no soil should be moved from the siteuntil it is clear that all the resoiling requirements can be met from this source. If soil has to be imported itmust be the right one for the vegetation being planted or sown.
l Clear guidance on soil-handling constraints, particularly under wet conditions, must be available to thecontractor and resident engineer.
l The risk of surface soil slippage can be reduced by producing a very rough finish to the formation to allowthe surface soil to key into it, and by ensuring that the soil is sufficiently uncompacted to allow roots topenetrate to the full depth of the soil profile and have maximum lateral spread within it (see Steep Slopes,Ch 11).
13.3 UNDERSTANDING TOPSOIL AND SUBSOILSoils vary greatly in their characteristics, depths and vertical structures. A soil under mature woodlandor permanent grassland consists of layers, referred to as its A, B and C horizons, over bedrock.Horizons A and B are suitable for tree and shrub growth and in some circumstances, especially whenmixed with ameliorants, C may be suitable. The materials from some B and C horizons are suitable forestablishing species-rich grassland (or heathland on sandy soils).However, soils vary greatly from this typical pattern and each site must be investigated.It is essential that the characteristics of all the soils along the line of the route are taken into account withthe advice of a soil scientist and horticulturalist. Definition of soil should not be based on arbitrary depths.
13.4 USING THE RIGHT TOPSOILSoils can vary considerably along the line of a route. It is essential to keep these different soils separateand restore them to the right parent area. This will ensure that the roadside vegetation marries with itssurroundings.
13.5 AMELIORATING THE SOILWhen a subsoil has insufficient structure to allow plants to establish (eg a heavy clay), incorporation ofother material to ameliorate it can be much more effective than the expensive process of importing newtopsoil. Suitable ameliorants can, under appropriate circumstances, range from bulky organic matter,such as approved sewage sludge, to inert materials such as pulverised fuel ash, which open out thesubsoil’s structure.
13.6 SOIL STRIPPING AND STORAGEThe subsoil and topsoil should be stripped and stored separately under favourable weather conditions sothat a proper soil profile can be re-established. Storage-mound depth is related to the method used tomake it. Loose tipping is the ideal way to make a storage mound, when it may be up to 4 m high at thecentre of the tipped heaps. Otherwise, mounds should not exceed 2 m in height The structure of claysoil, in particular, is irreversibly damaged by stripping when it is too wet. The contract should specify thatsoils with a clay fraction of more than 25 per cent should not be handled when their moisture contentexceeds a specified level.
Once completed, soil mounds must not be trafficked by any machinery and should be kept free ofinjurious weeds by the application of herbicide if necessary. If they are to be left for longer than ninemonths they should be sown with annual rye-grass or another appropriate species.
13.7 SOIL SPREADINGThe formation on which soil is spread must have the surface roughened to 150 mm deep, to allow it tokey in properly. The same constraints apply to spreading soil as apply to stripping it. Appropriatemachinery with the lightest possible footprint must be used. Compaction must be avoided.
13.8 ALLEVIATION OF COMPACTIONSites that have become compacted, such as contractors’ compounds, require breaking up before treeplanting can take place. They should be ripped with tines not further apart than 750 mm to be depth of450 mm in two passes at right angles. If this is not possible, shallower ripping should be carried out andconsideration given to shrub or grass establishment only.
Excavation of pits into compacted surfaces is not recommended, since plants roots will only grow withinthe area of the excavated pit. This is likely to become waterlogged and the roots will die.
13.9 SOIL DEPTHSThe standard depth for soil for tree and shrub establishment is 300 mm over the 150 mm roughenedformation surface. This will not allow very large trees to develop in the way that they would on naturalsoils and this must be taken into account in design and management. In special circumstances, such asthe rapid establishment of screening on level sites a greater depth may be possible. This is described inSection 3: Pt 1.
Topsoil is generally not required for grass establishment (see Ch 10). However, where a dense swardthat will require regular mowing is necessary, topsoil depth should not exceed 100 mm. Deeper soilleads to excess growth and hence the need for increased maintenance.
13.10 RESTORATION TO AGRICULTURERestoration to productive agriculture is an essential part of good practice. Land is acquired under licensewith the compulsory purchase order and offered back to the farmer on completion. The restorationneeds to be to a high standard not only to ensure it fits with the surrounding landscape, but also toensure that the landowner will take it back. A widely accepted code of practice has been developed bythe minerals extraction industry.
Volume 10 Section 1Part 2 HA 56/92
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14.ENQUIRIES
All technical enquiries or comments on this Advice Note should be sent in writing as appropriate to:-
Head of Highways Policy andEnvironment Division
The Department of Transport J ROBINS2 Marsham Street Head of Highways PolicyLondon SW1P 3EB and Environment Division
The Deputy Chief EngineerThe Roads DirectorateScottish Office Industry DepartmentNew St Andrew’s House J INNESEdinburgh EH1 3TG Deputy Chief Engineer
Head of Roads Engineering (Construction) DivisionWelsh OfficeY Swyddfa GymreigGovernment BuildingsTy Glas Road B H HAWKERLlanishen Head of Roads EngineeringCardiff CF4 5PL (Construction) Division
Superintending Engineer WorksDepartment of the Environment for
Northern IrelandCommonwealth HouseCastle Street D O’HAGANBelfast BT1 1GU Superintending Engineer Works
Orders for further copies should be addressed to:
DOE/DOT Publications Sales UnitGovernment BuildingsBlock 3, Spur 2Lime GroveEastcote HA4 8SE Telephone No: 081 429 5170
Chapter 14Enquiries
