Sustrans Design Manual Chapter 8 - Bridges and other structures

1September 2014

Sustrans Design Manual Chapter 8

Bridges and other structures (draft)February 2015

Sustrans Design Manual • Chapter 8: Bridges and other structures (2014, draft)

2 February 2015

About SustransSustrans makes smarter travel choices possible, desirable and inevitable. We’re a leading UK charity enabling people to travel by foot, bike or public transport for more of the journeys we make every day. We work with families, communities, policy-makers and partner organisations so that people are able to choose healthier, cleaner and cheaper journeys, with better places and spaces to move through and live in.

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ContentsThis chapter of the Sustrans Design Manual should be read in conjunction with Chapter 1 “Principles and processes for cycle friendly design.” That chapter includes key guidance on core design principles, whether to integrate with or segregate from motor traffic, the space required by cyclists and other road users as well as geometrical considerations. Readers are also directed towards the “Handbook for cycle-friendly design” which contains a concise illustrated compendium of the technical guidance contained in the Design Manual. This chapter has initially been issued as a draft and it is intended that it be reviewed during 2015; feedback on the content is invited and should be made by 31 May 2015 to [email protected]

1. Key principles

2. Introduction

3. Underbridges and overbridges

4. Headroom, parapets and gradients

5. Bridges: design

6. Bridges: utilising existing opportunities

7. Bridges: construction materials

8. Retaining structures

9. Boardwalks

10. Subways / underpasses

11. Wheeling ramps

12. Tunnels

13. Key references

3February 2015


1. Key principles• structures should enable creation of a continuous network

• derestricted roads carrying in excess of 10,000 vehicles per day are difficult barriers to negotiate and it is often more practical to look at using structures to resolve crossing issues

• modern structures should be as lightweight as possible and feel spacious to the user even at busy times

• desire lines can be difficult to reflect, but any deviation from a direct route should be limited

• minimise the effect of any approach gradients – 1 in 20 is preferred, and appropriate for all user groups, however where necessary steeper gradients are acceptable where achieving 1 in 20 would be disproportionally expensive

• design widths should acknowledge suppressed demand and allow for growth in user numbers

• deck width should allow for effect of parapets with a minimum width of 3.5m

• avoid right angled turns on approach paths, which are difficult to negotiate

• where desirable heights for parapets cannot be achieved on existing structures this should not necessarily preclude their use as crossings for cyclists

• consider maintenance and how it will be looked after

• when there is a requirement for a structure to be lit, lighting columns should not restrict the usable width of the path. Lighting sources can be imaginative and be an integral part of the structure, rather than an “add on”

• not all usable structures are new. Farm accommodation crossings may need to be adapted to create approach gradients that suit walking and cycling, but these structures offer plenty of opportunity, especially in rural areas

• road bridges can often be adapted to meet the requirements of a traffic free route, particularly where carriageway space can be reallocated


4 February 2015

2. Introduction2.1A traffic free path is likely to encounter any number of physical barriers along its route. Railways, rivers, canals, small watercourses and roads all create breaks in connectivity.

2.2Dealing with the natural and artificial topography alongside routes requires thought, as bridges and retaining structures, no matter how small, are essential to solving some of the fundamental connectivity issues that restrict non-motorised movement.

2.3Structures on traffic free routes need to provide high quality continuous routes that are fit for purpose. Particular attention needs to be paid to their alignment and design, as well as construction details. Installing structures can be complicated. Even on quieter urban roads a closure is often necessary.

2.4There may be particular constraints when re-using existing structures including the presence of legally protected wildlife, such as bats and birds.

3. Underbridges and overbridges3.1How a structure is defined depends upon how a traffic free route passes across a barrier. The terms ‘underbridge’ and ‘overbridge’ date back to how railway lines crossed obstacles, for which the structure is described from the perspective of the train driver. This chapter describes the structure from the perspective of the path user. Table 3.1 identifies particular issues that may need to be considered in each case.

Table 3.1: Over and underbridges: items to consider

Overbridge (over line) Underbridge (under line)

Traffic free routes pass under a road or railway line

Traffic free routes pass over a road, railway, river or canal

Likely to generate issues of

• headroom

• personal security

• lighting

• drainage

• path width (if on canals / riverside paths)

• visibility

• vandalism / broken glass

Likely to generate issues of

• headroom (road/canal users’ requirements)

• parapet heights

• visual impact

• approach gradients

• path alignment and available land

5February 2015


4. Headroom, parapets and gradients4.1Headroom and parapet requirements will vary depending upon the type of barrier encountered. Re-using old structures, especially railway and canal infrastructure or ancient road bridges, can be an effective way of creating continuity for a traffic free route. These do not necessarily comply with the guidance around headroom or parapets for new structures.

4.2Similarly, whilst advice on preferred gradients should normally be followed, there are other factors to consider in making a decision, such as additional distance for users, aesthetics and the need for rest areas. There may be particular constraints when re-using existing structures.

Headroom: underbridges4.3The amount of height gain required to achieve a minimum headroom clearance will have a significant effect upon the length of any approach ramps, and therefore upon land required, aesthetics and planning requirements.

4.4Clearances are measured from road, rail or water level, and these may bear little relationship to the surrounding ground levels. It is therefore important to establish the exact difference along the approaches to ensure that any lengths of ramp are designed with the end user in mind.

4.5Table 4.1 below sets out various scenarios, and the minimum clearances normally required to the underside of any new structure.

Table 4.1: Clearance / headroom requirements

Bridge Over Min Headroom (to soffit of walking / cycling bridge)

Watercourse / stream Bridge soffit - 1 in 100 year flood + 600mm freeboard

Non navigable river Bridge soffit - 1 in 100 year flood + 600mm freeboard

Navigable / Tidal river Depends on location

Canal 2.7m from towpath preferred

Road 5.7m from top of kerb

Road - bus route 5.7m from top of kerb

Trunk Road 5.7m from top of kerb

Non electrified railway 4.78m from rail level

Electrified Railway 4.78m from rail level

Subway 2.4m (cycles)


6 February 2015

Headroom: overbridges4.6The recommended headroom for subways is 2.3m for pedestrians and 2.4m for cyclists, increasing to 2.6m and 2.7m for lengths in excess of 23m. A headroom of 3.7m is required for mounted equestrians. However, there are many examples of structures on public roads and on traffic free routes with headroom well below 2.4m, with appropriate warning signs, which operate without incident for cyclists, so low headroom bridges should not prevent a route from being developed or from being included for development with future funding streams. The overall design of subways is covered in Section 10 below.

4.7LTN 2/08 Section 10.10.2 takes a flexible approach to headroom around subways and overbridges stating that “the headroom around existing pedestrian subways is typically 2.3m, and routes under canal bridges often have less clearance. The restricted height should not lead to automatic rejection of a proposed permit to cycle. It may represent the best available option if potential risk to users can be managed.”

4.8On arched structures that have reduced headroom good through visibility to the path beyond can ensure that users can use the section of path with the greatest height, although this is not always possible.

NorthamptonCentral Glasgow

Newton Abbot Peak Forest Canal, Hyde, Manchester

7February 2015


Case Study: Reach Lode, CambridgeshireIn Cambridgeshire, the local fenland environment has dropped following centuries of peat removal, drainage and agricultural use. The Lodes are a network of ancient waterways linking the fens that now sit several metres above the surrounding landscape. Several are navigable, including Reach and Burwell Lodes. Designing structures to cross these needs to factor in the clearance required for navigation, but in such a way that they still have a positive aesthetic impact upon a very flat landscape.

Case Study: adapting an existing structure, RugbyAdapting existing structures can also lead to designs for improved access requirements being constrained by the surrounding topography. In these images of Rugby, the main bridge structure needed to clear overhead power lines for the West Coast main line. The ground levels beyond the Network Rail boundary were considerably lower at this point, extending the length of access ramp required.

A height gain of approximately 9m was necessary. A single ramp at 1 in 20 would be 180m, impractical in most locations. To create a solution that worked the ramp gradient was steeper than recommended, up to 1 in 15, and a “zig zag” utilised to save space.

Land ownership constraints were a significant factor in the design process, with a developer unwilling to co-operate beyond what was absolutely necessary under their planning remit – sometimes third party involvement is all about the commercial impact. It also explains the erection of the palisade fencing, albeit until the adjacent land is turned into housing.


8 February 2015

Parapet Height4.9Parapet height for new bridges is normally 1.15m for pedestrians, 1.4m for cyclists, or 1.8m for equestrians. On existing structures being converted to cycle use this parapet height cannot always be achieved, but it should not necessarily preclude their use as crossings for cyclists; advice is given in Sustrans Technical Information Note 30 Parapet Heights on Cycle Routes.

4.10Re-used old structures, especially railway infrastructure or ancient road bridges, often do not comply with this guidance. There are many examples of historic bridges on public roads with parapets below 1.4m and no footway, which operate without incident for cyclists.

4.11Where the 1.4m parapet height cannot be achieved cost effectively, a risk assessment should be undertaken and there may be ways to mitigate the main risks, for example:

• old railway structures can be adapted to improve existing parapet heights. Where original ballast remains in situ, this is often at a greater depth than is necessary to support walking and cycling loadings

• reducing the depth of the ballast layer can have a significant impact upon the overall height of the parapet (Figure 4.1)

• take account of the thickness of the parapet walling; a thick wall increases the effective height of the parapet

• where width permits, locate the paths in the middle of a structure and discourage users from straying near the edges

Lambley Viaduct, Northumberland

Bristol (Bedminster)

Topsham

Overall parapet height using old railway track bed level is below 1.40m

Dished channel formed in surfacing to remove surface water away from parapets. Use existing drainge system to carry water away from structure if in suitable condition

Reduce existing railway formation to create extra height to parapet sides

Check original waterproofing and add minimal depth of construction to surface. Surface between parapet

walls to increase longevity of structure

Alternatively raise the existing parapet height by adding new railings

Original track bed level

Fig 4.1 Indicative sketch for works to the Nidd Viaduct, Harrogate

Bridges for cyclists should ideally have a parapet height of 1.4m (1.8m if also providing for equestrian use). On existing structures this cannot always be achieved, but it should not necessarily preclude their use as crossings for cyclists. LTN 2/08 Section 10.8.2,

9February 2015


Hockley Viaduct, Winchester

On traffic free routes that accommodate horses, but have structures that are not designed to permit mounted horse use, provision of timber or stone mounting blocks each side of a structure can aid mounting, Kenilworth

Simple adaptation of existing structures, and the parapets, can result in a visually appealing solution. Using parapets and bridge decks as “art projects” could open up other funding streams that would not necessarily be considered for route development. Clydach, South Wales

Sub-standard parapets on cycle route, Bristol

4.12Adapting parapets is possible, but costs time and money to resolve, although damaged stone parapets can be replaced with modern installations that allow users better views of the local environment. Where it is deemed appropriate to raise the parapets, the following points should be considered:

• different parapet styles can be used within the same structure depending upon the state of the original parapet walls

• lead-in rails on the approach to a structure will give a better aesthetic when they are designed to the same style as the main structure

• timber post and rail fences are effective, but lack aesthetic quality and can become a maintenance liability

• post and wire fencing has less visual impact and is a low cost solution, but can also be a maintenance liability

• retain open views wherever possible, but restrict access to steep sides, deep/fast flowing water or major roads

• does the structure contain legally protected wildlife, such as bats and birds?

Glen Ogle


10 February 2015

Gradients and approach paths4.13Approach paths designed to a maximum gradient of 1 in 20 will be negotiable by all user groups, but depending upon the extent of height gain required, this could create lengthy ramps. Recommended gradients are included in Table 4.2.

4.14Whilst advice on preferred gradients should normally be followed, there are other factors to consider in making a decision, such as additional distance for users, aesthetics, planning requirements and the need for rest areas. There may be particular constraints when re-using existing structures including the presence of legally protected wildlife, such as bats and birds.

4.15However, providing gradients that exceed the maximum recommended should be the last resort, after exploring all other solutions, including additional land purchase.

4.16Approaches that are too steep or straight can generate a public perception about speeding cyclists. Too flat and they will require additional land, or become meandering zig zags, often caged in by parapet sides resulting in expensive, aesthetically poor solutions.

4.17In construction, ramped accesses into a new building or property are required to provide level landing areas where they are at a gradient of 1 in 20 or greater. On a structure this is not necessarily appropriate and a ramp that has several landing areas over its length can be uncomfortable for cyclists. Each landing area adds extra length, so that it is very easy for a 1 in 15 ramp to be similar in length to one of 1 in 20, with no landing areas.

5. Bridges: design5.1Bridges can provide very useful connections along footpaths or cycle tracks away from the road, avoiding conflicts at major roads and taking routes across other barriers such as railways and waterways. Where the topography is favourable the need for approach ramps can be minimised and good natural surveillance improves personal security. New bridges can be designed as features along a route and may become attractors in their own right. New bridges are generally considerably cheaper than new subways.

5.2Particular benefits of bridges include:

• provides a conflict free crossing of a major barrier• a new bridge may provide an opportunity for a landmark feature• a bridge will often be cheaper than a subway• good personal security

Table 4.2: Recommended gradients

3% Preferred maximum

5% Normal maximum - up to 100m

7% Limited gradient - up to 30m

>7% For short lengths

A maximum gradient of 3% is recommended, but this can rise to 5% for up to a maximum of 100m. Where steeper slopes are unavoidable then the limiting gradient is 7% over a distance of up to 30m. Steeper gradients are not recommended, except over short distances. LTN 2/08 Section 8.7.2

The preferred maximum gradient for off-carriageway routes is 3%, with an acceptable maximum of 5%. Where new routes are constructed alongside the existing carriageway, the gradient will need to reflect the conditions of the road. As such where it is not practicable to provide gradients not steeper than 5%, steeper gradients may be considered over shorter distances. DMRB TA90/05 Section 5.4

11February 2015


5.2Key design features:

• bridges require considerable investment and should normally cater for both pedestrians and cyclists

• where a bridge will attract high numbers of pedestrians and cyclists the aim should be to provide effective segregation between them so that each group can travel at their preferred speed

• where usage will be lower, an unsegregated bridge offers opportunities for users to stop on either side to take in the view, and requires less width than segregation

• bridge approaches and decks should be straight or nearly straight. Right angled turns are difficult for cyclists to negotiate

• gradients should be in accord with the maximum values given in Table 4.2, depending on slope length. Steeper gradients than 7% are not recommended, except over very short distances

• where the topography is favourable the need for approach ramps can be minimised

5.3The key dimensions for bridge widths are summarised in Figures 5.1 and 5.2 and noted below:

• pedestrian only: minimum width of 2m, with additional width for busy routes

• unsegregated pedestrian/cycle bridge: the width should reflect the level and type of use forecast with a minimum of 4m width on main cycle routes, or 3.5m on less busy secondary routes. On particularly heavily trafficked routes it should be increased to 5m or more

• segregated pedestrian/cycle bridge:

• footway width should reflect the level and type of use forecast with a minimum of 2m width, increasing to 3.5m width where there is frequent use by groups

• cycle track width should be sufficient to accommodate the forecast level of use with a minimum of 3m, preferably 4m or more

• a bridge parapet has little impact on pedestrians’ width but reduces the usable width for cyclists by 500mm on each side

5.4Parapet height for new bridges is normally 1.15m for pedestrians, 1.4m for cyclists, or 1.8m for equestrians. On existing structures being converted to cycle use this parapet height cannot always be achieved, but it should not necessarily preclude their use as crossings for cyclists; advice is given in Sustrans Technical Information Note 30 Parapet Heights on Cycle Routes. Parapet heights of existing and new structures are discussed more fully in Section 4.

Gateshead Millennium Bridge

Diglis Bridge, Worcester

Glasgow city centre

Killamarsh, Sheffield


12 February 2015

5.5Other design considerations that are important for bridges include:

• design widths should take account of suppressed demand and allow for growth in user numbers including potential new developments along the route

• exposure of users to the weather should be considered - covered bridges will be beneficial

• any new bridge over a road should also provide a good quality links to that road

• designs can be as simple or as complex as the budget allows, but aesthetics can be crucial. A bridge over a key road corridor can make a statement; a simple design might be more appropriate in areas over looked by housing

• clear signing for existing walkers and cyclists will give them key destinations along the road and the traffic free route

• when designing a structure always ensure that the client, the designer and the contractor know where utilities are located

• access onto structures should generally be barrier free

• where structures lift or rotate gated control is necessary. Locating these controls requires careful thought so that the ability for all users to access the bridge easily is not impaired

Parapet height (h)

• 1.4m preferred for cyclists, but many existing bridges operate well with lower heights

• 1.8m for equestrian use (mounted)

• effective width of bridge reduced by 500mm at each parapet

• for advice on substandard parapet heights, refer to Sustrans Technical Information Note 30

h

4m or more preferred 3.5m min

Figure 5.1 Width requirements for bridges: unsegregated

3m min

Figure 5.2 Width requirements for bridges: segregated

2m min

Appropriate lead-in barriers to the bridge parapet should be considered, particularly if the approach is on an incline

Gradient 5% or less(preferred gradient 3%)

Steps

Figure 5.3 Design of ramps

Guard rail may be appropriate

Pont y Weirin, Cardiff

13February 2015


Millennium Bridge, Newcastle/Gateshead

New River, Cheshunt

Bradford

• the location of the bridge should take into account legally protected wildlife either within existing embankments and structures or using the feature being crossed, such as otters along a river

5.6Similar criteria apply to the conversion of footways over road bridges to shared use, and these are considered further in Section 6.

Peace Bridge, Derry/Londonderry

Hob Moat, York


14 February 2015

6. Bridges: utilising existing opportunitiesIntroduction6.1When a new cycle bridge is required it may be possible to make use of existing infrastructure to reduce the outlay. In some cases this may be at a location where the existing embankments can be used when replacing a demolished railway bridge, or where an existing busy road bridge can be adapted to carry cyclists safely, or where use may be made of a farm accommodation bridge. This section discusses issues that may need to be considered when taking advantage of these opportunities.

Avebury to Chippenham

Weymouth

Kenilworth

Cable Stay design, Scunthorpe

Ecological considerations in existing structures6.2Where an existing structure is to be used, in particular structures made from brick or stone such as old railway bridges or abutments, these should be surveyed for wildlife at the earliest possible stage. The presence of bats in a structure can significantly delay a construction project and will lead to an increase in the complexity and cost of proposals. It is unlikely that the presence of a protected species will prevent construction but the sooner an issue is identified the easier it is to deal with.

6.3Similarly where existing embankments are to be used a survey should be conducted for wildlife within the embankment or using the feature that is being crossed (in the case of rivers or green space). In many instances moving a development a few metres can substantially reduce ecological impacts, allowing construction to take place whilst protecting local wildlife.

Road bridges that retain the path level6.4In some locations, notably where a traffic free route uses an old railway formation on which a bridge has been removed to allow a road improvement, it may be feasible to construct a new bridge that needs little or no change to the path level. In such situations the traffic free route benefits from not having to drop down to the road to cross and climb up again, with a better environment for walking and cycling. Traffic on the road benefits from not being interrupted by pedestrians and cyclists, particularly where a signalled crossing would be required. Such a bridge will require a greater initial outlay than a controlled crossing, but will need less regular maintenance. Convenient connections to the road should be retained from both sides of the bridge.

15February 2015


Adapting an existing road bridge6.5There are many locations across the UK that utilise existing structures for walking and cycling, but the quality of provision is poor. Often cited arguments are that shifting kerb lines and changing the dynamics of a structure is impractical, that it is costly or that there are utilities affected. However, where there is a desire to achieve a continuous and high quality route many of these apparent obstacles can be overcome.

6.6The scale of intervention may range from merely moving the kerb line further into the carriageway to removal of a full lane of traffic in order to provide a high quality route for cyclists that is segregated from both traffic and pedestrians.

6.7Shifting a kerb line to create better space requires consideration of the following factors:

• the benefits that cohesive routes built to a high standard can give

• the amount of space that can be taken from the carriageway, bearing in mind the level and type of traffic using the road

• political will to take space from the motorist for pedestrian and cycle use

• minimising the costs associated with shifting or protecting existing utilities

• structural assessments of both bridge and parapets

• timescales, times of day when works are restricted

Parapet height (h)

• 1.4m preferred for cyclists, but many existing bridges operate well with lower heights

• 1.8m for equestrian use (mounted)• effective width of bridge reduced by 500mm at each

parapet• for advice on substandard parapet heights, refer to

Sustrans Technical Information Note 30

2.0m min one way

3.0m min two way

Unsegregated cycle track/footway

Margin 0.5m where practical (widen into carriageway if needed)

0.5m

Not to scale

hRedcliffe Bridge, Bristol (before)

A127 Hall Lane, Upminster (before)

A127 Hall Lane, Upminster (after)

Redcliffe Bridge, Bristol (after)

Figure 6:1 Footway on existing bridge: improving for shared use


16 February 2015

Adapting a farm accommodation bridge6.8Re-using farm accommodation crossings can be key to retaining route continuity in rural areas. Existing approach gradients are often steep and will require extensive works to create new embankments with better gradients, and land requirements will always need to be considered with any purchase costs factored in.

6.9In developing proposals to use an accommodation bridge the following points should be considered:

• parapet heights may need to be raised, or existing systems replaced

• it is worth stripping the existing surfacing back to bridge deck and waterproofing the whole structure

• kerblines are not always necessary, but will prevent cyclists from getting too close to the parapets, and will allow space for pedestrians to stand if they want to stop. Always retain a clear route with a minimum usable width of 2.5m

• it may not always be possible to achieve a 1 in 20 gradient, but this should not necessarily deter use of the structure

• if more land is required talk to adjacent land owners, but always go prepared with drawings showing what is actually required – and have them in a format that farmers and land owners understand

Sleaford, Lincolnshire (before)

Sleaford, Lincolnshire (after)

17February 2015


7. Bridges: construction materials7.1Bridge design should be delivered to BS5400, the code of practice for steel, concrete and composite bridges; in part some of this is superseded by Euro codes. Designers should be encouraged to start with this, but then allow a flexible approach to provide for pedestrian and cycle movements.

7.2Larger structures will give more scope for something creative, especially if the client wants an iconic design. Smaller structures, perhaps up to 20m in length, can be “bought” from supplier catalogues, at a unit cost. Some of the UK’s most iconic structures provide striking solutions, space for pedestrians and cycles to mix freely and have been the catalyst to the expansion of a wider walkway and cycle network.

7.3Often bridge designers will choose materials that they know will best do the functional job required, but that shouldn’t necessarily preclude other options from being considered. A bridge built from concrete and steel will be significantly heavier than one built entirely from steel, or timber, with obvious implications for delivering to site, or the size of crane required to lift it into place.

7.4The simplest structures are often the easiest to transport from workshop to site location, and will require very little effort to install. Considering that most walking and cycling barriers are minor watercourses, a simple lightweight timber, fibre reinforced plastic or steel structure will often suffice. Materials and products should be sympathetic to the location.

1400

1928

410

Rails: 45x120 redwood, fixed to posts using coach bolts

Posts: 70x120 redwood fixed to rails using coach bolts

Bearers: 50x150 hardwood fixed @ 600 centres

Post packers (hardwood)

Main beams

Deck planks 45x145 redwood Hi-Grip Excel

Fig 7.1: Typical construction detail, lightweight timber structure

410

410

All dimensions in mm


18 February 2015

Reinforced plastic7.5Several small structures around Watermead Park in Leicester use a design consisting of green oak timber handrails and a glass reinforced plastic decking material more commonly found in marinas. Using a simple steel lattice frame as a basis these materials produce an aesthetic yet extremely lightweight structure, ideal for the type of loadings associated with walking and cycling.

7.6Structures designed at 2.5m wide may suit some locations where usage is likely to be very low, but designers should always seek to use the maximum width available. Increasing to 3.5m for this style would require longer transverse beams.

Fig 7.2: Cross section through one of the Watermead Park lightweight bridges 14

002500

Mesh to BS 7818

“Marina Deck”

decking

Transverse beam

Welded on bolt heads to recieve timber parapets

600 x 300 fabricated box stringer

75 x 38mm timber rail



19February 2015


Steel7.7Pure steel structures can be aesthetically pleasing, or simply functional as the images below show. A simple warren truss design may lack visual beauty but fabrication costs are considerably less. Iconic structures will have a visual impact, but these come with a hefty price tag.

Bath - Two Tunnels

Northampton - River Nene & Grand Union Canal Newton Abbot - River Teign

Northwich - River Dane

Workington - River DerwentOmagh - River Strule


20 February 2015

Concrete7.8Concrete, or a composite of concrete and steel, produces heavy structures often partially, or wholly, built in situ, creating huge construction sites, but the structures that they produce are often iconic and blend into the landscape or urban environment with relative ease. Such structures are often part of a much wider series of enhancements.

Parapets7.9The choice of parapet style and deck surface treatment is important. A sympathetic approach to both can leave a lasting impression of a structure that has a good aesthetic quality.

7.10Long structures over rivers and tidal areas that have used glass or open sides, will allow pedestrians and cyclists to stop and enjoy the environment through which they are passing. Even in urban environments, structures over major roads need to be designed to allow the user to feel that they remain connected to the outside world.

Glasgow

Shoreham

21February 2015


8. Retaining Structures8.1Retaining structures are often needed to support a new path or the adjacent embankment, and the different options available produce differing aesthetics and have a wide range of costs. As Table 8.1 illustrates, there are a variety of options available and each type can work in a variety of situations. Some are easier to construct (gabions), and others easier to maintain (brickwork). Brickwork can attract graffiti and become unsightly, but the ability to plant “crib walling” can soften the aesthetics of the structure.

Table 8.1 Wall construction types and approximate costs

Type of retaining structure Approximate cost per linear metre

Gabion boxes (1m x 1m x 1m + class 6G stone fill) £150

Steel I beam & timbers (175mm x 175mm I / 2.4m timber) £120

Brick retaining wall £300-400

Criblock wall £300-400

8.2Banks up to 1 in 2 do not generally require retaining structures, providing that they are planted; the root systems will bind together the sub soils creating a natural stability.

Gabion box8.3The most common solution is a wire mesh and stone filled gabion box. These are relatively cheap and easy to install, and have been successfully used in a wide variety of situations. Aesthetically they are harsh, perhaps more suited to an urban environment, but functional.

Timber post and rail fence

1100

Layer 4 1x1x1

Layer 3 1.5 x1x1

Layer 2x1x1

Layer 1.5x1x1 &1x1x1

Figure 8.1 Typical gabion box layering

Cycle path

Original ground profile

Temporary works batter 1

2.5

Possible continuous line of fence sheeting

Hastings

Exmouth

Killamarsh, Sheffield

Powderham, Devon

6°


22 February 2015

Timber wall8.4The simplest solution may still be a series of timber sleepers or steel I beams set vertically, approximately 1700mm apart with horizontal timbers, roughly 150 x 200 in size. The adjacent bank can be re-graded so that it suits multiples of the timber size.

Crib walling8.5For structures greater than 2.0m in height, or where there is a need to provide something more aesthetically pleasing, using a system called “criblock walling” may be an option. This is a wall set roughly at 15 degrees from the vertical and manufactured by several UK companies. It is generally a timber solution, but the system can also be concrete. Planting softens the aesthetics, creating a green wall over time.

8.6Crib walling gives a flexible construction that can withstand some degree of settlement and movement without detriment to the stability of the wall. It is best applied in areas where dry subsoils exist.

Conwy

Exmouth

Padiham, New criblock walling ramp supports the main path and enables access ramp construction

23February 2015


Wildlife enhancements8.9Retaining structures and enhancements offer opportunities for creating habitats for wildlife along a route. The use of subsoil to create low nutrient conditions on an embankment will encourage wild flowers and butterflies. Gabion boxes and criblock walling create small holes and crevices that can be used by small mammals, reptiles and amphibians to hibernate. Retaining structures can also be used as the basis for a ‘green wall’ where vegetation is encouraged through the introduction of soil, retained by a barrier, to a vertical surface.

Paving slabs8.7Low earth embankments may be effectively retained by use of paving slabs set no steeper than 7 degrees from the vertical.

Reinforced earth8.8Reinforced earth, where embankments are constructed in layers, and the ground strengthened by installing primary and secondary layers of a geogrid is another approach that has been proven to be successful and will allow slopes of up to 35°. It can be extremely useful when wanting to re-work existing railway embankments, especially those that were cut back following removal of an original structure many years ago.

Reinforced earth approach to reconstructing original embankment, Bath

Green wall, NCN 71 Whitehaven to Rowrah in Cumbria

Paving slabs retaining a low embankment, Bedford


24 February 2015

9. Boardwalks9.1Boardwalks and similar elevated structures are often viable solutions in, or through, areas of ecological and environmental importance, or within floodplains. Hardwood timbers and recycled plastics can work equally well.

9.2Boardwalks often require parapets. Where the risk of injury from falling is minimal parapets are not necessary, however there is a risk that users will step off a path if it is busy.

9.3The minimum width for a boardwalk is normally 3.5m, but greater width may be needed if it is expected to be busy. Many locations for boardwalks are in coastal environments which can be popular during summer months.

9.4Boardwalks are not cheap to install, and a path that meanders through sensitive areas may still be more practical.

9.5Verges left between new boardwalks and existing structures or fences require maintenance.

9.6Boardwalks may be built at ground level where walking, cycling and mobility access would otherwise have been difficult.

Exe Estuary Trail Southampton

Burton Point, North WalesBetween Newport and Caerleon, with River Usk in flood

25February 2015


10. Subways / underpasses10.1Subways/underpasses can provide very useful connections for footpaths or cycle tracks away from the road, avoiding conflicts at major roads and taking routes across other barriers such as railways. Where the topography is favourable the need for approach ramps can be minimised and good natural surveillance is essential for personal security. Often this option will involve the conversion of an existing pedestrian subway or an underpass provided for private access.

10.2Poorly designed subways in particular can be intimidating places; those with tight blind corners have a higher perceived safety concern than those constructed higher and wider than the minimum guidance suggests.

10.3Particular benefits of subways include:

• provides a conflict free crossing of a major barrier

• avoids exposure to the weather

• the longitudinal profile of an underpass (down then up) is more comfortable for cyclists than bridges with approach ramps

New at-grade crossings provided with existing subway retained, Croydon


26 February 2015


• underpasses require considerable investment and should normally cater for both pedestrians and cyclists

• underpasses can attract high numbers of pedestrians and cyclists and the aim should be to provide effective segregation between them so that each group can travel at their preferred speed

• underpass approaches and the crossings themselves should be straight or nearly straight. Right angled turns are difficult for cyclists to negotiate

• gradients should be in accord with the maximum values given in Table 4.2, depending on slope length. Steeper gradients than 7% are not recommended, except over very short distances

• where the topography is favourable the need for approach ramps can be minimised

10.5The key minimum dimensions for new subways are summarised in Figures 10.1 and 10.2, and noted below:

• subways for pedestrians only require headroom of at least 2.3m (2.6m for lengths over 23m) and a width of 3.0m (2.3m for light use)

• subways for use by cyclists require headroom of 2.4m (2.7m for lengths over 23m) and width of at least 4.0m (3m for light use) if unsegregated

• segregated: the width for pedestrians should be at least 2m, the cycle track 2.5m and the margin strip 0.5m

• headroom for cyclists and pedestrians as above

• a headroom of 3.7m is required for mounted equestrians

4.0m (3.0m with light usage)

2.4m (2.7m)

Figure 10.2 Minimum requirements for new shared use subways: unsegregated

Note: dimensions in brackets apply to subway lengths>23m

0.5m margin

2.5m cycle track

2.0m footpath

2.4m (2.7m) 2.3m (2.6m)

Figure 10.1 Minimum requirements for new shared use subways: segregated

Note: dimensions in brackets apply to subway lengths>23m

Bridge with sub-standard headroom on cycle route, Nottingham

Artwork in subway, Southampton

27February 2015


Changing the environment to permit cycling can be valuable, with clear demarcation that cycling is permissible

Dover before

Dover after

New subway under the main railway line, Royston

10.6The headroom in existing pedestrian subways is typically 2.3m; the restricted height or width available should not lead to automatic rejection of a proposal to permit cycling. There are many examples of structures on public roads and on traffic free routes with headroom well below 2.4m, which operate without incident for cyclists. Any restricted headroom should be clearly signed. The Cyclists Dismount sign should not be used.

10.7Other design considerations that are important for subways include:

• lighting should be vandal proof

• no corners/recesses

• exit must be visible on entering the subway

• where segregation is required a shallow, or 45º kerb face, is normally sufficient. A full height, or 90º, kerb presents a barrier to cycles and a trip hazard to pedestrians

• generous headroom and width will be highly beneficial in terms of subjective safety, natural surveillance and personal security

• a greater width or walls diverging towards the top increases natural light

• light wells are desirable to maximise natural illumination

10.8Where an existing subway is being adapted for use by cyclists consideration needs to be given to wildlife that might use the feature. In particular where a subway is not currently well lit it may act as a roost site or commuting route for local wildlife that would be disturbed or prevented from using the feature should it become well lit throughout the night.

10.9Where it is proposed to permit cyclists to use a pedestrian-only subway this takes time and is likely to generate strong opinions on both sides. Any such change needs to fully assess the suitability of alternative options for providing connectivity for cyclists, to demonstrate that the value of permitting cycling outweighs the disadvantages.

10.10Maintenance can be a big concern; graffiti covered walls, broken glass, broken lights, blind corners and litter give out a very different message to the public when compared to something that has good quality lighting, both naturally and artificially, has artwork rather than graffiti, is accessible by street sweepers, and retains a straight through route.


28 February 2015

11. Wheeling ramps 11.1Where cycle routes are introduced onto routes originally designed for pedestrian use only, such as canal towpaths or railway footbridges, flights of steps are sometimes unavoidable, at least in the short term. To assist cyclists, wheeling ramps should be added to one or both sides of the flights using steel sections or by forming them in concrete.

11.2This though is by no means the best solution; it still forces cyclists to dismount and many are poorly located, making them difficult to use. Steps are still a barrier to wheelchair users and non-standard cycles, and difficult to negotiate for parents with small children and buggies.

11.3A design for a retrofit steel ramp is included as Figure 11.1, with a new build concrete ramp in Figure 11.2.


• locating the wheeling ramp close to the wall minimises the trip hazard for pedestrians

• the distance between the ramp and the wall should be enough to ensure that the pedals and handlebars do not clash with the wall or handrail while the bike is being held reasonably vertically

• the wheeling channel needs to extend beyond the top and bottom steps to provide a smooth transition

• steel sections should have a nonslip surface so that the tyres grip the ramp on descent

• in most cases the ramp is fitted to one side, usually on the right for people climbing, but on well used routes a ramp on each side should be considered

HandrailSee detail for top and bottom ends

100 x 50 steel channel bolted to existing steps

200

Section A A

steps

200

100

100 x 50 steel channel bolted to existing steps

Section AA

100mm flat end for fixing to the ground

Channel end rounded off

100 x 50 steel channel fixed to existing steps

Elevation

Bottom end detail

Top end detail

Channel end flattened off

100mm flat end for fixing to the ground

Fig 11.1 Steel wheeling ramp, retrofit

29February 2015


11.5Other considerations that are important when considering wheeling ramps include:

• wheeling ramps should not obstruct convenient access to the handrail nor be located in the centre of the steps where they might form a trip hazard

• where a ramp is constructed in metal, a continuous piece is preferred

• joints in the metal can damage tyres if they are left un-maintained, and all fixings should be counter sunk and left flush, or ideally fixed from below

• in some instances timber and stone surfaces blend better with the original construction

• the considerable effort required from cyclists, especially with luggage

• they are of no benefit to many non-standard cycles such as tricycles, cargo bikes and cycles with trailers

• signing can be a great benefit, especially if route users can be advised in advance and given alternative routes as an option

• ramps need regular checking; loose fixings, collections of broken glass, and damaged sections can all be problematic for cyclists

Handrail

Steps

50mm dia semi-circular channel formed in concrete

200

Section AA

200 100

50

Section A - A

Edge of steps

50mm dia semi-circular channel formed in concrete

Elevation

Fig 11.2 Concrete wheeling ramp, new build

Cast concrete strip sited away from the handrail, Hamilton

Aesthetically pleasing facility is set far enough away from the wall to enable a cycle to be moved upright, Nottingham

Ramp highlighted with colour, Ladygrove


30 February 2015

12. Tunnels12.1In a similar way that re-opening old railway viaducts and bridges provides a connection for walking and cycling routes above ground, railway tunnels provide a similar, subterranean advantage. Where the bridge or viaduct allows a path to continue across a valley without significant level changes, the tunnel can provide the connection beneath hillsides.

A cage protects path users from falling rocks, Peak District

Fencing prevents access onto the tunnel entrance, Bath

Anti-vandal lighting, Dartford

Concreted refuges, Bath

31February 2015


12.2Victorian railway engineers have already undertaken the hardest part, and in many cases there is no technical reason why today’s engineers should shy away from re-opening structures that are structurally sound enough for walking and cycling infrastructure.

12.3Understanding the challenges faced in developing a potential idea into a coherent and spectacular route is only a small part of the process, and the levels of risk that often get associated with such a project can be widely off the mark. In order for a project to be successful the delivery team will need to identify the key risks from a technical (engineering & construction), ecological (bats) and end user perspective.

Electric box in secure cage, Bath

Bat box, Bath

Fencing to prevent boulders from falling onto path, Peak District

Notice for users, Bath


32 February 2015

Understanding the tunnel

• previous inspections/examinations – have they highlighted any areas of concern?

• have remedial works been carried out as a result?

• who owns it? - private landowners still have a responsibility to undertake regular inspections of any tunnels that they own, and act accordingly when remedial works are necessary

• what does the tunnel go through? • bored through rock • brick lined structure

• single or double bore (i.e. did it have one or two railway lines through it?)

• does it have ventilation shafts or side adits?

• historical records

• local interest groups

• local libraries

• old land plans

• national archives

• the presence of statutory designations such as Sites of Special Scientific Interest (SSSIs)

• are there legally protected species, especially bats? are there any historic records of bats in the local area?

Understanding the tunnel entrance and the immediate approaches

• are wingwalls (side walls at the tunnel entrance) free from vegetation?

• do wingwalls have visible signs of cracking, which may indicate settlement or movement? Are tunnel portals free of vegetation or will remedial works be required to clear away self-seeded vegetation?

• is the ground above the tunnel entrance suitably stable or will remedial works be necessary to remove any loose material, including rocks / boulders?

• are approaches free draining or is there any sign of standing water?

• has the tunnel been blocked off with access gates, partially or wholly filled in?

• what materials were used to fill in a tunnel entrance – inert or contaminated? (Mining areas may have utilised colliery waste leading to methane or coal gas build ups)

• noxious gases from rotting vegetation/timbers?

• asbestos based materials?

• if a tunnel has been filled in around the entrance is this because of structural issues with the tunnel mouth (this may not be noted in recent reports)?

Technical Risks: Checklist

33February 2015


Inside the tunnel

• is the tunnel floor dry? if not, identify where water has entered from and assess implications if left alone.

• does the tunnel have a drainage system - can it be traced, repaired and re-used?

• refuges (niches in the brickwork that allowed railwaymen to step away from trains) can be places for people to hide - can they be blocked up or made inaccessible?

• are there ventilation shafts? if so do they have timber / metal sheets attached that would have deflected incoming rainwater away from the shaft - are they securely fixed or can they be removed?

• does the air flow through the tunnel?

• are any previous repairs obvious?

• wedged brickwork • colliery arches • timber beams • RSJs • concrete beams

• is it obvious what defects exist? • spalled brickwork • loose masonry • bulging brickwork • failure of rock or brick lining

• is the tunnel used by bats to roost, breed, swarm and/or hibernate at different times of year?

Construction

• identify essential works to make a tunnel safe for opening (List A)

• identify works that are required, but not essential (List B)

• use organisations that understand what it takes to re-open a tunnel. Local Authorities’ approach to undertaking such projects is based on using in-house term contractors and engineering consultancies and can be very risk averse. Sustrans’ approach uses ex-railway engineers, who understand railway structures

• understand the difference between essential and non-essential works – it can greatly reduce the amount of work that you think you need to do, with an obvious impact on costs

• understand who the client is (it may or may not be the tunnel owner)

• ensure that all known information, including historical records, is given to the designer and contractor

• ensure that the design team understand the hazards including access, operational plant, materials

• where there is spalling to brickwork or the mortar face is damaged/ missing, it may not be necessary to repair or replace every little defect

• if a tunnel is wet when first opened good ventilation during construction can help to dry it out

• ensure that everyone working in a tunnel understands that they are working in a confined space environment

• use natural falls where possible to ensure that paths within tunnels remain dry

• railway tunnels are likely to be lined with soot; a light clean up to 3m above path level is all that is required, it is not necessary to clean every last piece of brickwork

• can the former trackbed material be re-used as a path sub base?

• where bats are present can mitigation be put in place to avoid harming them and do we need to apply for a licence from Natural England?


34 February 2015

Maintenance

• regular inspections are important, with annual routine inspections and a 6 yearly principal inspection

• general visual inspections should be carried out to assess the following:

• path surface

• drainage concerns (especially after heavy rainfall, or in winter for snow/ice)

• lighting - faults should be reported immediately

• vegetation - remove any that appears around wingwalls, abutments and tunnel portals

• signing

• items identified as non-essential should be programmed when determined appropriate

• emergency access paths are kept free and usable at all times, ensure that any locked gates have keys!

• engage emergency services/power suppliers/local authorities to ensure that everyone knows what to do if an emergency arises

Public perception and concerns

• how are you going to communicate to the public the length of the tunnel and time it takes to walk/cycle through it?

• how will the public communicate an emergency?

• how will anti-social behaviour be managed (piped classical music played on a continuous loop has been used to deter loitering, for example)?

• is personal security an issue?

• is the tunnel lit? all day or timed on/off?

• is there CCTV provision?

• are there contact numbers clearly visible – landlines where mobile signals are poor?

• how is a route signed within the local area? good signing will encourage more use, so that a route becomes self-policing

35February 2015


13. Key ReferencesTechnical Information Note 29: Lighting of Cycle Paths, Sustrans 2012

Technical Information Note 30: Parapet Heights on Cycle Routes, Sustrans 2012

Bats in Bridges, Bat Conservation Trust (undated)

Bats and Lighting in the UK, Bat Conservation Trust, 2009

Forestry Bridges, Forestry Commission

Path Bridges, Paths for All, 2006

Subways for Pedestrians and Cyclists Layout and Dimensions, TD36/93, Highways Agency 1993

Design Criteria for Footbridges, BD 29/04, Highways Agency 2004

Inclusive Mobility, DfT, 2002

Documents

Sustrans Design Manual Chapter 8 - Bridges and other structures