dmrb volume 10 section 1 part 2 - ha 56/92 - new roads

DESIGN MANUAL FOR ROADS AND BRIDGES
VOLUME 10 ENVIRONMENTAL
DESIGN AND
MANAGEMENT
SECTION 1 NEW ROADS
PART 2
HA 56/92
NEW ROADS
PLANTING, VEGETATION AND SOILS
SUMMARY
This Advice Note gives guidance on the environmental
design of planting, vegetation and soils for new roads.
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February 2001
DESIGN MANUAL FOR ROADS AND BRIDGES
HA 56/92
THE HIGHWAYS AGENCY
THE SCOTTISH EXECUTIVE DEVELOPMENT
DEPARTMENT
THE NATIONAL ASSEMBLY FOR WALES
CYNULLIAD CENEDLAETHOL CYMRU
THE DEPARTMENT FOR REGIONAL DEVELOPMENT*
New Roads
Planting, Vegetation and Soils
* A Government Department in Northern Ireland
Summary:
This Advice Note gives guidance on the environmental design of planting,
vegetation and soils treatment for new roads.
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Part 2 HA 56/92
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Part 2 HA 56/92
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December 1992
DESIGN MANUAL FOR ROADS AND BRIDGES
VOLUME 10 ENVIRONMENTAL
DESIGN
SECTION 1 THE GOOD ROADS
GUIDE - NEW ROADS
PART 2
HA 56/92
THE GOOD ROADS GUIDE
NEW ROADS
PLANTING, VEGETATION AND SOILS
Contents
General Preface to the Good Roads Guide series of
Advice Notes
Chapter
December 1992
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
General Preface
The Good Roads Guide
Volume 10 Section 1
Part 2 HA 56/92
GENERAL PREFACE TO THE GOOD ROADS GUIDE
SERIES OF ADVICE NOTES
Structure of the Guide
0.1 The Good Roads Guide is the name given to the
series of documents contained in Sections 1, 2 and 3 of
Volume 10 of the Design Manual for Roads and
Bridges. The Guide is written in nine Parts each of
which is published as an Advice Note. The Guide is
written to be read as a whole. The Parts of the Good
Roads Guide are as follows:Section 1
NEW ROADS
Part 1
Part 2
Part 3
HA 55/92
HA 56/92
HA 57/92
Part
Part
Part
Part
4
5
6
7
HA 58/92
HA 59/92
HA 60/92
HA 61/92
Landform and Alignment
Planting, Vegetation and Soils
Integration with Rural
Landscapes
The Road Corridor
Nature Conservation
Heritage
Contract and Maintenance
Implementation
Section 2
MOTORWAY WIDENING
Part 1
HA 62/92
Section 3
IMPROVING EXISTING ROADS
Part 1
HA 63/92
Environmental Design
Widening Options and
Techniques
Environmental Design
Improvement 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 and
cross references have been provided.
0.3 The first Chapter of each Part of the Guide
reviews the issues and topics covered. The subsequent
chapters deal with a particular topic. Within each
chapter, the key issues are first listed and then discussed
with illustrations drawn from roads throughout the UK.
0.4 The Good Roads Guide is not a step-by-step
guide on how to build a road or a substitute for
professional advice. It is intended to be used by the
designer to help in the identification of areas and issues
where careful consideration of environmental factors is
required. The division of the Guide into Parts and the
Parts into topics has been done to aid this process.
0.5 Environmental design of roads is a matter of
respecting the special character of each individual
location. The illustrations included show solutions
devised to meet the requirements of specific sites. The
use of standard solutions, irrespective of the location, is
not appropriate.
Implementation
0.6 The principles set out in this Advice Note should
be taken into account in the preparation of all schemes
for the construction and improvement of trunk roads,
including motorways.
0.7 Where conflicts exist between environmental
design, costs, engineering feasibility and safety
requirements, and competing options are available, the
Design Organisation will need to advise the Overseeing
Department accordingly.
Application in Wales
0.8 Requirements in Wales are primarily covered by
the publications "Roads in Upland Areas: Design Guide"
(published by the Welsh Office 1990) and "Roads in
Lowland Areas: Design Guide" and "Rock Profiling and
Vegetation Re-establishment" (both due for publication
by the Welsh Office in 1993). This Advice Note
supplements these Design Guides.
Application in Scotland
0.9 The Scottish Office Roads Directorate endorses
the practice given in the Good Roads Guide. More
specific guidance is provided by the Roads Directorate's
Landscape Officer.
0.10 The Scottish Office discussion document
published in February 1992 "Roads, Bridges and Traffic
in the Countryside" addresses related issues.
December 1992
0/1
General Preface
The Good Roads Guide
Volume 10 Section 1
Part 2 HA 56/92
Application in Northern Ireland
0.11 The principles set out in this Advice Note are
endorsed as good practice by the Department of the
Environment (NI). The guidance will be taken into
account in preparing schemes for the construction or
improvement of all roads in Northern Ireland.
Acknowledgements
0.12 The following photographs have been reproduced
with permission:Chapter 11 Page 1
Timber retaining wall with concrete piers, planted with
willow previously published in Use of Vegetation in
Civil Engineering published by CIRIA/Butterworths.
Chapter 11 Page 2
Two photographs supplied by MMG Erosion Systems:
Soil restored by honeycomb geotextile before
hydroseeding and after.
Photograph showing the use of fibre mat by permission
of Richards, Moorehead and Laing Ltd, previously
published in Landscape Design.
Two photographs supplied by Comtec (UK) Ltd
showing hydroseeding before and after.
0/2
December 1992
CHAPTER 1 PLANTING, VEGETATION AND SOILS: INTRODUCTION
1.1 SCOPE
l This Part gives guidance on the environmental design of planting, and vegetation, and soil treatment for
new roads.
1.2 MAIN ISSUES
l 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 in
planting aims to integrate the road with adjacent landscape and not create a separate roadside
landscape.
l Planting and seeding need clear objectives. Design of woodland planting, for instance, should take into
account amenity, wildlife or screening. Different types of grassland are needed for specific functions
such as nature conservation interest or return to agriculture.
l Soil conditions are the most important elements in establishing vegetation. If soil structure is damaged or
if 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 the
case on road schemes. By such practices as the use of high shrub densities and low tree densities at
planting, 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 to
establish and are often unsuccessful.
l Planting outside the highway boundary may be necessary to ensure integration with the surrounding
landscape 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 SOILS
VOLUME 10 SECTION 1
PART 2 HA 56/92
1.4 DESIGN OBJECTIVES
l 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, as
possible. Retaining land adjacent to the highway should always be considered, in order to provide
integration 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 current
best practice.
l To ensure soil restoration using matching soil types wherever possible.
l To mitigate secondary impacts on retained vegetation.
1.5 MITIGATION
l Design for effective long-term maintenance.
l Provide adequate conditions for plant growth through preparation of uncompacted soils to appropriate
depths. 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 1:2
embankments.
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 least
10 m deep. An eye-level shrub screen can be achieved with a minimum width of 5 m.
l Direct loss of vegetation
l Respect the existing pattern of vegetation and use mainly native species in rural areas.
l Creation of unsuitable soil conditions and adverse gradients that are too steep for new planting.
l Design earthworks with the type and extent of planting in mind: the shallower the gradient the easier it is
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 mature
woodland 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.
to establish vegetation.
1.6 STATUTORY BODIES
l 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 adviser
for Wales, Scotland and Northern Ireland.
l Opportunities to reintroduce predominantly native planting in areas such as intensive arable land at the
urban fringe.
DECEMBER 1992
1/1
VOLUME 10 SECTION 1
PART 2 HA 56/92
CHAPTER 2 SCREENING WITH VEGETATION
2.1 PRINCIPLE
l 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 a
screen which is not itself an intrusion into the landscape.
2.2 KEY ISSUES
l 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 shrub
screen 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 differences
in 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.
Good practice: A52, Nottingham This dense, well-maintained hawthorn screens the
settlement very well from this busy road
2.4 SPECIES FOR SCREENING
For 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
2.3 LOCATION AND DENSITIES OF SCREENS
A2, Kent Typical screening problems where the road runs close to property
A12, Essex Housing, road and railway are close together in a narrow corridor with little
screening or planting structure
Good practice Typical arrangement of a well-considered screen
Improvement by screen planting
DECEMBER 1992
2/1
VOLUME 10 SECTION 1
PART 2 HA 56/92
CHAPTER 2 SCREENING WITH VEGETATION
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 dominant
element, will be lost. It might have been better either to have accepted the view or to have broken it up with
intermittent planting
2.7 SCREENING FOR FLAT LANDSCAPES
Screen 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 design
planting around existing features.
A17, Norfolk In such an open landscape screening needs to be based around existing tree groups and settlement
Improvement
DECEMBER 1992
2/2
CHAPTER 3 OFF-SITE PLANTING
VOLUME 10 SECTION 1
PART 2 HA 56/92
3.1 PRINCIPLE
l 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 benefit
from this planting.
3.2 KEY ISSUES
l 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’s
expense. After a minimum three-year maintenance period, the landowner is obliged to maintain the
planting 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 road
with the landscape, as discussed in Pt 3.
3.3 LOCATION OF OFF-SITE PLANTING
Points of good practice showing off-site planting used in a variety of ways to conserve or
enhance landscape character
Typical off-site planting benefits and constraints
Good practice: M40, Banbury Bypass shows off-site planting used to screen both property
and public open space
DECEMBER 1992
Good practice: M40, Warwickshire Off-site planting on the Burton Dassett Hills has
conserved the character of a country park and provided screening for local settlement
3/1
VOLUME 10 SECTION 1
PART 2 HA 56/92
CHAPTER 4 THE RIGHT VEGETATION FOR THE COUNTRYSIDE
4.1 PRINCIPLE
l 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 setting
of the road. Integration with the landscape (see Ch 3), provision of nature conservation benefits and
driver interest are essential considerations.
4.2 KEY ISSUES
l 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 be
better suited to their surroundings.
l Native species should generally be used in rural areas. In addition, they must be native to the region and
occur naturally on the soil type adjacent to the road. For example, beech is not native north of the
midlands and should not be planted in woodlands beyond this region, but it is widely used in designed
landscapes throughout the country. Similarly, shrubs of limestone such as the wayfaring tree should only
be used on these soils.
Good practice: M6, Cumbria Apt use of low-maintenance grassland on the right soil
conditions has ensured a fit to the landscape, as well as providing nature conservation
interest
4.4 HEATHLAND
l The choice of seed for wildflower mixes presents a very complex problem, and advice should be sought
from the Wild Flower Handbook published by the Department of Transport as well as from English
Nature.
l Although many semi-natural plant communities contain a lot of species, others such as beechwood or
heathland, have only a few. Good design must work with, not against, the composition of natural
vegetation types and consider the use of natural groupings, not arbitrary mixtures, grids or matrices, in
preparing planting layouts.
4.3 GRASSLAND
Grassland is often the right vegetation type to fit the existing landscape and must be
established as a site-specific type.
landscape
Heathland is a rapidly-windling resource,
easier to establish than many other
vegetation types. The Department of
Transport can therefore make a positive
contribution to nature conservation and
character in the right circumstances.
4.5 SCRUB
Scrub communities can be just as varied as woodland, but they can also be large, uniform
areas of common species like hawthorn and blackthorn. Careful appraisal of local conditions
is needed and arbitrary introduction of species is not good practice.
4.6 WOODLAND
Most woodlands are products of either a long period of management of self-regenerating
trees 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 a
special 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 practice
dictates that the right range of local species is established for a project by consultation with English
Nature and other bodies;
l natural woodland structure is a mosaic of groups of the same species responding to local changes in
soil, topography and drainage. Beech woodland, for instance, is almost entirely dominated by one
species, while ash with field maple woodland is more varied.
Rank, coarse grassland is typical of much highway land and stands out in marked contrast
to the surrounding agricultural land, rather than blending with it
DECEMBER 1992
4/1
VOLUME 10 SECTION 1
PART 2 HA 56/92
CHAPTER 5 FORMAL PLANTING AND THE URBAN FRINGE
5.1 PRINCIPLE
Establishing avenues in rural areas
l 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 roadcorridor landscape, or the use of non-native species, is appropriate because it provides a sense of place
and a distinctive style.
5.2 KEY ISSUES
l 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 urban
backlands.
l Well-sited planting can highlight landmarks, signal the approach to settlement and focus the driver’s
attention.
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 the
roadside landscape
DECEMBER 1992
Good practice: A40, Buckinghamshire Near the historic landscape of West Wycombe Park
this well-maintained lime avenue is entirely appropriate to its setting
5/1
VOLUME 10 SECTION 1
PART 2 HA 56/92
CHAPTER 5 FORMAL PLANTING AND THE URBAN FRINGE
5.4 APPROACH TO SETTLEMENT
Structure planting can buffer the road from development, improve the road corridor and
structure views from the road: see Pt 4, Ch 12
Warrington New Town An adequate width of planting between the industrial units and the
road provides a pleasant environment
DECEMBER 1992
5.5 THE NEED FOR PLANTING
M25, Waltham Abbey section Structure planting would have made all the difference to this
road corridor
Western Avenue, A40 Planting on the roadside edge here would break up the scale of the
built environment and focus views on the buildings of interest
5/2
VOLUME 10 SECTION 1
PART 2 HA 56/92
CHAPTER 5 FORMAL PLANTING AND THE URBAN FRINGE
5.6 CHANGING TO AN URBAN ENVIRONMENT
5.7 BACKLANDS
New roads can cut through the backlands of urban and suburban areas where structure
planting may be necessary to improve the environment for both drivers and residents.
Signalling change Planting at roundabouts (see also Pt 1, Ch 13) can signal change from
rural to urban areas and provide landmarks. Simple ground cover, shrub and specimen tree
planting is inexpensive to maintain
Chesterfield Bypass, Derbyshire
Urban planting: Milton Keynes More formal planting, using robust ornamental species
tolerant of roadside conditions is appropriate where roads pass through urban areas. See
also Chs 1 and 12
DECEMBER 1992
Improvement Planting of highway land and adjacent derelict areas could greatly improve the
character of the road corridor
5/3
VOLUME 10 SECTION 1
PART 2 HA 56/92
CHAPTER 6 RETAINING EXISTING VEGETATION
6.1 PRINCIPLE
l 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’s
alignment is being selected and in order to safeguard individual trees near the carriageway.
6.4 EXCAVATION AND TREES
Most tree roots lie in the top 600 mm of soil and 90 per cent are in the top metre. Even very
shallow disturbance can cause irreparable damage.
6.2 KEY ISSUES
l 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 of
the road.
l Young trees and scrub require as much attention as mature trees. They may not be much to look at but
they 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 for
development given in BS 5837:1991.
l Retaining an existing tree near the road can be an expensive and time-consuming process. It must be
storm damage on the remaining trees.
6.3 ALIGNMENT NEAR TREES AND WOODLAND
Careful alignment allows maximum use to be made of existing woodland, trees and hedges
to improve the landscape setting of a road.
➤
tackled in the right way.
l Where the edge has been taken off a wood, special attention must be paid to the effects of windblow and
Poor practice: A22, Sussex
The root system has been severed by
the road construction, damaged further and
eventually suffocated by the construction of
a bund
➤
Improvement
It should have been
accepted that it was not feasible to retain
the tree and a new landscape scheme
devised
The principle The road has been aligned to fit within a landscape of mature parkland trees
DECEMBER 1992
Good practice: A16, Boston, Lincolnshire A fine specimen tree was retained by good
practice. It was necessary to construct a scaffolding cage to prevent damage during
construction
6/1
VOLUME 10 SECTION 1
PART 2 HA 56/92
CHAPTER 6 RETAINING EXISTING VEGETATION
6.5 INSTANT MATURITY
6.6 EXPOSING A NEW WOODLAND EDGE
Where a road has to pass through a woodland the trees at the newly exposed centre, which
are 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 plantation
has given a very unattractive result
➤
Good practice: A69, Brampton Bypass, Cumbria Mature trees have been retained within
the working area of viaduct construction by co-operation between designers and contractors
Good practice Careful thinning in
advance of road construction has
produced an attractive and more stable
edge
➤
Points of good practice are
llustrated here. Scarification can play an
important part in preparing ground
intended to develop as woodland edge
A23, West Sussex Mature trees have been retained in a wide central reserve, giving instant
maturity to landscape: see also Pt 1, Ch 12
DECEMBER 1992
6/2
VOLUME 10 SECTION 1
PART 2 HA 56/92
CHAPTER 7 ESTABLISHING WOODLAND
7.1 PRINCIPLE
7.4 WOODLAND TYPE
l 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 ISSUES
l 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, realistic
layouts 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 clear
design 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 adjacent
woodland and is appropriate for the maintenance regime proposed.
l Correct planting distances are essential for effective maintenance and good establishment. If trees are
allowed 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 be
suppressed and the wrong balance of surviving trees will result.
l Successful establishment is achieved by the rigorous application of basic horticultural principles. See
Pt 7, Ch 3.
7.3 VARIETY AND GROUPING
High forest has trees providing
a closed canopy with little or no
shrub layer. Regular plantations
such as those of poplar, or
species with dense shade like
beech look like this and should
be imitated if they occur nearby
Coppice with standards This is
the traditional method of English
woodland management with
about 40 trees/ha and shrubs
cut (coppiced) on a 7- to 25-year
cycle. It is appropriate in many
lowland situations because it will
marry in with existing woodland.
The standard trees have single
stems, and the coppice is multistemmed.
Scrub woodland: scrub with
emergent trees. The scrub is
unmanaged and gradually
dies out as trees grow up.
This can be a very effective
means of woodland
establishment, creating
significant wildlife interest
without major maintenance
implications. It is the method
generally recommended for
new woodland planting, with
a 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.
DECEMBER 1992
7/1
VOLUME 10 SECTION 1
PART 2 HA 56/92
CHAPTER 8 SCRUB AND TREE GROUPS
8.1 PRINCIPLE
8.4 TREE GROUPS
l 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 an
essential element for integrating the road with parkland.
Tree clumps are necessary in the reinstatement or construction of parkland landscapes or in
providing a broken edge to a woodland to fit in with local character, as shown below:
Keeping local character: for this tree groups
are best established by the traditional
method ofplanting in a matrix of scrub which
is eventually shaded out or removed. This
reduces maintenance requirements and
allows better establishment, thus:
8.2 KEY ISSUES
l 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 effective
maintenance.
l Intermittent planting is particular important for landscapes like downland and wetland, where large-scale
planting is usually inappropriate.
8.3 SCRUB AND WOODLAND EDGES
Planting with scrub
Scrub can be a significant element in softening new woodland when planted at the same
time, or in softening newly-exposed edges. It can be maintained easily by rotational coppicing
as set out in Pt 7, Ch 5. An example is shown below.
Principles put into practice
Maturing clump
8.5 INTERMITTENT PLANTING
Intermittent planing is appropriate where a strong vegetation pattern would over-emphasise
the road.
Typical intermittent planting on an
open landscape with sparse vegetation
➤
Good practice: Using scrub to soften a woodland edge
➤
➤
DECEMBER 1992
Successful intermittent planting in the
open landscape of the Pevensey levels
A27, Brighton Bypass Here
hawthorn scrub is a longestablished vegetation type.
New scrub is the most
appropriate planting to absorb
the road into the landscape
8/1
VOLUME 10 SECTION 1
PART 2 HA 56/92
CHAPTER 9 HEDGES
9.1 PRINCIPLE
l 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 ISSUES
9.5 OVERGROWN HEDGES
Hedges 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 and
maintaining them can be a good way of integrating the road with the landscape and of
screening property.
l 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 the
farm.
l Hedgerow trees are being lost gradually in many parts of the country. Roadside hedges offer an
opportunity to provide replacements and enhance landscape character as a result.
l Species composition should reflect that of neighbouring hedges. Although overgrown hedges of mixed
species 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 farmed
landscape, where these are predominant features, should be on integration with existing landscape
pattern.
Integrated planting
9.3 ESTABLISHMENT
Cultivate a strip 600 mm wide and 300 mm deep at a distance of 300 mm from the fence line
and incorporate a 75 mm layer of bulky organic matter. Plant in two lines 300 mm apart to
form a staggered, double row. The transplants in each line should be 450 mm apart, giving a
total of five plants per running metre.
9.4 FENCING
9.6 HEDGEROW TREES
Hedgerow trees can be planted with the staggered double row of transplants when a hedge is
planted. They must be indicated clearly by marker stakes so that they are not cut accidentally
during hedge trimming. However, their retention relies on the goodwill of the farmer and the
attention of his hedging contractor, and it may be advisable to offset the trees from the
hedges so that they lie just within the highway land.
A standard post-and-rail fence is unnecessary and inappropriate where a hedge is to form a
boundary. Light metal posts and stockproof wire are the best solution and allow the hedge to
grow through the fencing on the highway side.
On the farmer’s side it will usually be necessary to put up a temporary stockproof fencing to
allow the hedge to become established.
M26, Kent Light metal posts and stockproof wire provide a more-than-adequate fence which
does not impede hedge growth
Well-formed hedgerow trees can help integrate the road with the surrounding landscape. It
is worth solving the problems of their establishment and maintenance
DECEMBER 1992
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CHAPTER 10
VOLUME 10 SECTION 1
PART 2 HA 56/92
GRASSLAND AND HEATHLAND
10.1 PRINCIPLE
l 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 take
advantage of current practice in amenity and nature conservation grassland establishment and
management.
10.4 MANAGEMENT IMPLICATIONS
10.2 KEY ISSUES FOR GRASSLAND
l 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 management
requirements. A realistic objective for roadside grassland is not to attempt to recreate these grasslands
but 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 are
naturally-fertile soils it is realistic to aim for grassland of moderate species diversity, made up of plants
with wide tolerances.
l Unmanaged grassland eventually turns to scrub. However low its fertility, a management programme is
required.
l On steep slopes, rapid establishment may be necessary to avoid surface erosion. In general, the use of
short-term nurse crops like Italian rye-grass is not recommended and a site-specific solution, such as the
use 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-dominated
A45 The steep-sided bund in the central reservation is difficult to get to and cannot be cut
with conventional mowing equipment. A dense hedge would have been better, or the use of a
low-fertility, low-maintenance design.
Any grassland needs cutting to maintain it. The frequency and height of cutting depend on
the objective, the site’s fertility and flowering period of the plants present. These must be
taken into account in specifications and are discussed in Pt 7, Ch 4
10.5 PUBLIC ACCEPTABILITY
mixtures and can look untidy in autumn and winter. Education of the public is therefore a significant
requirement.
10.3 CLEAR OBJECTIVES
Wildflower grassland does not establish as easily or as rapidly as a lawn. Informing the
public can help make such temporary features more acceptable
Poor practice This grassland, dominated by false oat grass, has very little wildlife interest or
amenity value. It has developed from rye grass sown on topsoil, a technique more appropriate
for heavily used public open space
DECEMBER 1992
Tidiness or naturalness? French motorways have a neat and tidy appearance. However,
the close mowing is inimical to wildlife interest
10/1
CHAPTER 10
VOLUME 10 SECTION 1
PART 2 HA 56/92
GRASSLAND AND HEATHLAND
10.6 ESTABLISHMENT AND FERTILITY
l 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 and
cultivated to a depth of 100 mm. On nutrient-poor subsoils, such as chalk, a layer of topsoil, no deeper
than 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 required
for 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 have
more 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 flora
can be developed using the right preparation, seeding and management
10.7 NATURALLY-FERTILE SOILS
On naturally-fertile soils, such as most clays, a mixture of robust and colourful species can
be used to give a pleasing effect. Many of these species form stable and long-lasting
herbaceous vegetation which requires limited maintenance. A selection of species which have
been used with some success is given in the table opposite, together with some of their
requirements. Taller species must be set back from the verge and kept out of sight lines.
DECEMBER 1992
Key
Persistence: *-****
Showiness: *-****
Geographical distribution: W-widespread; S-below 55º
Moisture demand: *-***
10/2
CHAPTER 10
VOLUME 10 SECTION 1
PART 2 HA 56/92
GRASSLAND AND HEATHLAND
10.8 NATURALLY INFERTILE SOILS
10.10 KEY ISSUES FOR HEATHLAND
Good practice Re-use of poor
moorland soil seeded with
appropriate grass species has
allowed the re-establishment of
heathland vegetation including
heather, tormentil and lady’s mantle
l 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 new
heathland habitats.
l The best examples of heathland are the results of complex management regimes. These cannot be
created 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 by
using 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.
Chalk grassland: A27 Chalk
grassland is disappearing rapidly as a
result of farming improvements and
the abandoning of grazing. Chalk
grassland can become established
on suitably treated highway land
10.9 MAKING THE MOST OF OPPORTUNITIES
Road development can create conditions for species-rich grassland, a rapidly disappearing
habitat.
A21, Kent Heathland has established on the poor, acid soils of the Tunbridge Wells Sand
Wetland: M6 Low ground at centres of roundabouts can be much moister than the
surrounding land. Here, wet grassland may be created. A similar habitat can be established
around balancing ponds
DECEMBER 1992
If bladed material is available this should be spread in a prepared seed bed of subsoil as
described for grassland, if seed alone is to be used, it may be best to use a mulch and
emulsion to stabilise the light sandy soils while vegetation established. More information on
heathland establishment is contained in Pt 5, Ch 2.
10/3
CHAPTER 11
VOLUME 10 SECTION 1
PART 2 HA 56/92
STEEP SLOPES
11.1 PRINCIPLE
l 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 the
road to the landscape and in stabilising soil.
On some continental and North American roads, willow, which will grow vigorously from stem
cuttings (setts) has been used to stabilise clay banks. Brushwood can also be used with
gabions or a timber slope grating.
11.2 KEY ISSUES
l 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 arrive
on 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, in
which case the use of geotextiles and other techniques can create the right microsites for natural
regeneration.
l Some of the most satisfactory results can be obtained by the use of vegetation together with modular
retaining systems like gabions.
l Hydroseeding is a widely-used method of establishing grass on steep slopes. The principle of
establishing grassland for nature conservation and amenity, set out in Ch 10, should be applied to the
issues raised by vegetating steep slopes.
11.3 STABILISING WITH VEGETATION
Vegetation can help surface stability where it can penetrate to the bedrock.
Impenetrable bedrock Vegetation does not
help to stabilise this
DECEMBER 1992
Penetrable bedrock eg clay. Roots can
act as toe piles and can be a major
element in the slope’s stability
Brushwood combined with timber slope grating and
concrete piers can make a visually attractive roadside
edge
Simple methods of establishing
shrub cover with brushwood
11/1
CHAPTER 11
VOLUME 10 SECTION 1
PART 2 HA 56/92
STEEP SLOPES
11.4 GEOTEXTILES
11.5 HYDROSEEDING
Honeycomb geotextiles or matting is appropriate where a rapidly-established, dense grass
cover is required.
Hydroseeding is widely used to establish good grass cover. The choice of emulsion, organic
matter and seed used as well as their proportions in the mix are, however, crucial for good
results. A precise specification chosen for the site and a properly-qualified, specialist subcontractor are essential.
Good practice A steep slope has been successfully grassed over by hydroseeding
Soil restored by honeycomb geotextiles
before hydroseeding
Good practice The geotextile has
established a stable surface, permitting
natural regeneration by ash
Biodegradable geotextiles can be used for surface stability while seeding becomes
established, as shown in the photograph below:
11.6 ROCK OUTCROPS
Rock outcrops can vegetate naturally. This is usually the most effective method, and certainly
gives the best visual effect.
A470, Glamorgan Natural
➤colonisation usually provides the
best visual effects
➤
Where weathering is likely to cause
many fragments to fall down the
surface, vegetation can be established
by creating microsites to be colonised
naturally
Temporary surface stability on a site with self-evident risk of erosion
DECEMBER 1992
11/2
CHAPTER 12
EFFECTIVE TREE AND SHRUB ESTABLISHMENT
This section is a brief aide memoire for details of current planting and establishment.
12.1 DISTANCE FROM THE CARRIAGEWAY
Shrubs must be not planted within 3 m of the carriageway and trees not within 5 m of it.
12.2 PLANT SIZES
Bare-root transplants, 450-900 mm high, for shrubs and trees are the recommended size and
specification, 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 early
impact. Trees taller than this will provide short-term benefit only. The special case of the use of larger
stock in relation to motorway widening is dealt with in Section 3: Pt 1 and avenues are discussed in Ch 5.
12.3 STOCK HANDLING
Bare-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 PREPARATION
The growing medium needs to be cultivated to a depth of 450 mm to allow adequate tree and shrub
growth. Where this is not possible, there is no point in creating planting pits extending into compacted
layers.
VOLUME 10 SECTION 1
PART 2 HA 56/92
12.10 WEED CONTROL
The importance of a high standard of weed control cannot be over-emphasised. A 750 mm diameter area
around 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 or
similar material may be required. In rural areas chemical weed control is recommended, in preference to
mulch mats or other physical methods.
12.11 PEST CONTROL
It is necessary to protect woody transplants on sites where rabbits, hares, voles or deer are present. The
type of protection to be used (eg individual guards or fencing-off planting blocks) will depend on the size
of area and abundance of the pests. Guidance can be found in the relevant Forestry Commission
publications.
12.12 STAKING
Staking is unnecessary for the sizes recommended and prevents proper establishment of smaller trees
and shrubs. It should only be used for larger sizes. Stakes should be no taller than one quarter the height
of the tree.
12.13 PLANTING AND THINNING GRIDS
12.5 PRE-PLANTING WEED CONTROL
Before planting into a dense grass sward or weed-infested area, a herbicide spray must be used to give a
weed-free planting area of at least 750 mm diameter for each plant.
12.6 TIME OF PLANTING
Planting is best carried out in November and December. Planting from January to March can be much
less satisfactory and it should not take place after the end of March.
12.7 FERTILISERS
On most disturbed soils, an overall base application of a balanced granular fertiliser should be given at
the manufacturer’s recommended rate, either incorporated into the surface, or as a top dressing.
12.8 PLANTING METHODS
Pit planting is generally recommended. Pits should be 300 x 300 x 300 mm with the sides and bottom
broken up. The back-fill material should be 75 percent topsoil, or approved subsoil (see Ch 13) and 25
percent bulky organic matter. The latter will vary from one part of the country to another because of local
availability.
12.9 TREE SHELTERS
These should generally be used only on exposed sites or where rapid establishment is imperative.
DECEMBER 1992
12/1
CHAPTER 13
13.1 PRINCIPLE
l 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 a
construction material, so detailed information about the mechanical properties of the available soil is
obtained for constructing a road. However, as soil is also a complex biological system supporting plant
growth, an equal understanding of its biological system supporting plant growth, an equal understanding
of its biological properties is needed for successful vegetation establishment.
13.2 KEY ISSUES
l For each soil type to be stripped and respread, the topsoil and subsoil layers need to be identified and
l
l
l
l
VOLUME 10 SECTION 1
PART 2 HA 56/92
SOILS
clearly defined. A strategy for their use and storage must be developed before the contractor moves onto
a site.
Topsoil is needed to grow trees and shrubs, but usually in only very limited quantities for grassland
establishment.
Soils for reinstatement should be those derived from the site and no soil should be moved from the site
until it is clear that all the resoiling requirements can be met from this source. If soil has to be imported it
must be the right one for the vegetation being planted or sown.
Clear guidance on soil-handling constraints, particularly under wet conditions, must be available to the
contractor and resident engineer.
The risk of surface soil slippage can be reduced by producing a very rough finish to the formation to allow
the surface soil to key into it, and by ensuring that the soil is sufficiently uncompacted to allow roots to
penetrate to the full depth of the soil profile and have maximum lateral spread within it (see Steep Slopes,
Ch 11).
13.6 SOIL STRIPPING AND STORAGE
The subsoil and topsoil should be stripped and stored separately under favourable weather conditions so
that a proper soil profile can be re-established. Storage-mound depth is related to the method used to
make it. Loose tipping is the ideal way to make a storage mound, when it may be up to 4 m high at the
centre of the tipped heaps. Otherwise, mounds should not exceed 2 m in height The structure of clay
soil, in particular, is irreversibly damaged by stripping when it is too wet. The contract should specify that
soils with a clay fraction of more than 25 per cent should not be handled when their moisture content
exceeds a specified level.
Once completed, soil mounds must not be trafficked by any machinery and should be kept free of
injurious weeds by the application of herbicide if necessary. If they are to be left for longer than nine
months they should be sown with annual rye-grass or another appropriate species.
13.7 SOIL SPREADING
The formation on which soil is spread must have the surface roughened to 150 mm deep, to allow it to
key in properly. The same constraints apply to spreading soil as apply to stripping it. Appropriate
machinery with the lightest possible footprint must be used. Compaction must be avoided.
13.8 ALLEVIATION OF COMPACTION
Sites that have become compacted, such as contractors’ compounds, require breaking up before tree
planting can take place. They should be ripped with tines not further apart than 750 mm to be depth of
450 mm in two passes at right angles. If this is not possible, shallower ripping should be carried out and
consideration given to shrub or grass establishment only.
13.3 UNDERSTANDING TOPSOIL AND SUBSOIL
Soils vary greatly in their characteristics, depths and vertical structures. A soil under mature woodland
or 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 when
mixed with ameliorants, C may be suitable. The materials from some B and C horizons are suitable for
establishing 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 with
the advice of a soil scientist and horticulturalist. Definition of soil should not be based on arbitrary depths.
Excavation of pits into compacted surfaces is not recommended, since plants roots will only grow within
the area of the excavated pit. This is likely to become waterlogged and the roots will die.
13.4 USING THE RIGHT TOPSOIL
Topsoil is generally not required for grass establishment (see Ch 10). However, where a dense sward
that will require regular mowing is necessary, topsoil depth should not exceed 100 mm. Deeper soil
leads to excess growth and hence the need for increased maintenance.
Soils can vary considerably along the line of a route. It is essential to keep these different soils separate
and restore them to the right parent area. This will ensure that the roadside vegetation marries with its
surroundings.
13.5 AMELIORATING THE SOIL
When a subsoil has insufficient structure to allow plants to establish (eg a heavy clay), incorporation of
other material to ameliorate it can be much more effective than the expensive process of importing new
topsoil. 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 the
subsoil’s structure.
DECEMBER 1992
13.9 SOIL DEPTHS
The standard depth for soil for tree and shrub establishment is 300 mm over the 150 mm roughened
formation surface. This will not allow very large trees to develop in the way that they would on natural
soils and this must be taken into account in design and management. In special circumstances, such as
the rapid establishment of screening on level sites a greater depth may be possible. This is described in
Section 3: Pt 1.
13.10 RESTORATION TO AGRICULTURE
Restoration to productive agriculture is an essential part of good practice. Land is acquired under license
with the compulsory purchase order and offered back to the farmer on completion. The restoration
needs to be to a high standard not only to ensure it fits with the surrounding landscape, but also to
ensure that the landowner will take it back. A widely accepted code of practice has been developed by
the minerals extraction industry.
13/1
Chapter 14
Enquiries
Volume 10 Section 1
Part 2 HA 56/92
14. ENQUIRIES
All technical enquiries or comments on this Advice Note should be sent in writing as appropriate to:-
Head of Highways Policy and
Environment Division
The Department of Transport
2 Marsham Street
London SW1P 3EB
J ROBINS
Head of Highways Policy
and Environment Division
The Deputy Chief Engineer
The Roads Directorate
Scottish Office Industry Department
New St Andrew’s House
Edinburgh EH1 3TG
J INNES
Deputy Chief Engineer
Head of Roads Engineering (Construction) Division
Welsh Office
Y Swyddfa Gymreig
Government Buildings
Ty Glas Road
Llanishen
Cardiff CF4 5PL
B H HAWKER
Head of Roads Engineering
(Construction) Division
Superintending Engineer Works
Department of the Environment for
Northern Ireland
Commonwealth House
Castle Street
Belfast BT1 1GU
D O’HAGAN
Superintending Engineer Works
Orders for further copies should be addressed to:
DOE/DOT Publications Sales Unit
Government Buildings
Block 3, Spur 2
Lime Grove
Eastcote HA4 8SE
December 1992
Telephone No: 081 429 5170
14/1