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(UK) LIMITED

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EX2 5AZ

United Kingdom

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By email

David Fish

Town Hall,

Walliscote Grove Road,

Weston-super-Mare,

BS23 1UJ

Our ref: UA008579-01

Date: 03 March 2016

Dear David

Subject: Tutshill Sluice Cycleway Feasibility Assessment

North Somerset Council is investigating options to construct a cycle way along an

existing embankment at Yeo Bank Farm, Kingston Seymour.

This letter report contains a high-level appraisal of the feasibility and relative costs of

three options to construct a new cycle way. The report includes a high level appraisal

with input from the following disciplines- structural, geotechnical, flood risk and ecology.

Yours sincerely

Aimee Hart

Flood Risk Consultant

Email: [email protected]

Direct line: 01392 374627

Enc. 0001-UA008579-01

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Letter_03.03.16.docx 2

Tutshill Sluice Cycleway Feasibility Assessment

Introduction The purpose of this appraisal is to inform a proposal to construct a cycleway and

bridleway along an existing embankment between the Tutshill Sluice and Wick Lane

access points, at Yeo Bank Farm, Kingston Seymour. This will form a section of the

strategic Weston-super-Mare to Clevedon cycle route, which shall be free of main roads

and remove the need for cyclists to negotiate the busy A370 and B3133 roads (which

form part of the current signed cycle route).

The proposed cycleway measures approximately 1km in length. The central national

grid reference is ST 37950 65750. The following sections contains information to form a

feasibility study for the three options to construct a cycleway and bridleway listed below

(Figure 1):

1. Option 1- Widening the embankment on the landward (eastern) side by

importing additional fill.

2. Option 2- Widening the embankment on the landward (eastern) side by using

plastic ‘log piling’, BaFix or gabions.

3. Option 3- Reinstating a cycle track bridge spanning the Congresbury Yeo along

the line of the original railway bridge, using durable lightweight materials.

Additional to Options 1 and 2 are sub-options to construct new cycle track bridges

around the two existing sluice structures on the landward side. The channel of the

Tutshill sluice is approximately 10m wide and the channel of the Sampson Sluice is 15m

wide.

Figure 1- Site Location

Aerial photograph of the site © Google (2016); Image © Infoterra Ltd and Bluesky



The site is relatively low-lying and flat, and is cut by a number of creeks and drainage

ditches. The surrounding land is currently protected from flooding at times of extreme

tidal events by the Congresbury Yeo tidal embankments. The top level of the

embankment is approx. 8.45m AOD and the toe level approx. 4-5m AOD. The levels of

the existing embankment were recently raised by the Environment Agency to provide a

1 in 75 year (1.33% AEP) standard of protection. The level of the Tutshill Sluice is

8.54mAOD and the level of the Sampson Sluice is 7.55mAOD.

It is likely that the footprint of the new cycleway would largely be confined to the existing

embankment. Option 1 would have a larger footprint than Option 2, as the embankment

requires a 1 in 4 slope to tie the widened section into the surrounding land.

The following sections include a high level appraisal, with assessment of the three

options from a structural, geotechnical, flood risk and ecological perspectives.



Options Option 1 - Embankment Widening / Slope Re-Grading The embankment could be widened by extending the footprint or re-grading the existing

slope. The slope angle / grade of the widened earthwork would depend on the material

imported to the site and aesthetics.

It is assumed that the current earthwork bund is sufficiently impermeable to act as a

flood prevention structure thus the material imported to site to undertake the widening

could be either cohesive or granular in nature. The widening will likely have to be the

same as the current slope angle, 1(V) to 4(H) so that the overall stability is not impaired

although with the use of suitable granular fill, it may be possible to steepen the slope

angle to minimise the volume of fill that will need to be imported. If this is possible this

would limit the additional land-take required. It has been estimated that approximately

5,000m3 of material is required to carry out widening of embankment

A detailed assessment should be undertaken on the stability of the earthwork to

optimise the slope angle and import requirements. It should be note that the widening

option imposes additional load to the ground causing settlement of the existing bund.

This will be a medium term effect occurring probably over a few years and maybe

irregular leaving an uneven final level that will be subject to ponding following heavy

rainfall. It should also be noted that the material imported to undertake the widening

would have to be suitably tested to confirm its properties, and consequent compaction

requirement.

Option 2a – Widening the embankment crest in the same footprint using Log Piles

The use of plastic sheet piles has been widely adopted by associations such as the

National Trust and the Environment Agency as flood prevention methods. They also

being adopted by organising such as Highway England and used for small retaining

walls. There are a number of different types of plastic sheet piles which offer varying

structural integrity but generally they suffer from poor bending ability and thus can only

retain relatively shallow fills (<500mm). The “log pile” (see Figure 2) offers the most in

terms of structural functionality as they are hollow and steel tubes can be inserted

through the centre to enhance their bending ability.

Figure 2: Installed Plastic Log Piles

http://www.liniar.co.uk/plastic-piling/log-pile/



The “log pile” would generally be used for retained heights no greater than 1.5m but this

would allow the footprint of the earthworks to be significantly smaller than for the

embankment widening / re-grading option. Furthermore the backfill to the plastic piles

will need to be granular material.

A “log pile” wall would be constructed at the crest of the slope thereby limiting the

backfill material required on site (see Figure 3). However further slope stability analysis

would be required to confirm this would not make the slope unstable in the long term (as

fill is being added to the crest of the slope).

Alternatively if the analysis yielded unsatisfactory results it could be positioned at the toe

of the slope which while reducing the land take when compared with a widening / re-

grade option alone, does require greater volumes of fill materials that an option at the

crest of the slope. If widening of the crest is adopted consideration would need to be

given for a handrail.

Figure 3: The use of log piles to widen the crest of the embankment

Standard driving techniques (as developed for steel sheet piles) include impact,

vibratory or hydraulic drivers. It has been known that damage to the heads of the piles

can occur while driving and there is also an issue with driving the piles off-line and

losing verticality. Often specialist plant is used to minimise the stresses on the plastic

piles.

There is some debate in the industry with regards to the design life of plastic sheet piles

most probably due to their limited time in the market place. There is particular concern

as to how they suffer creep movement as they degrade over time. Sources referred to

within the TRL report (Carder et al, 2002) have claimed design life ranging from 35

years to 100 years, however this depends on the loading they are subject to and their

serviceability requirements.

Option 2b – Widening the embankment crest in the same footprint using BaFix

BaFix is a solution offered from Asset International Structured Solutions, and is

generally adopted in rail environments to form walkways next to the railway line. This is

structural system using steel facing grids similar to gabions (Figure 4). This solution

would slightly increase the load at the crest of the slope thus reducing the factor of

safety and as such further slope stability assessment would be required to confirm its

suitability.



Figure 4: BaFix System

www.assetint.co.uk/

The suppliers of BaFix suggest that a 6m long element without further preparation could

be installed in approximately 15 minutes, and is therefore considered reasonably quick

to install. The design life of this structural elements can be guaranteed for 120 years.

Figure 5: The use of BaFix to widen the crest of the embankment

Other solutions such as crib waling or gabion baskets would also be suitable at the crest

of the slope and would typically be very similar to the BaFix solution described above

and have not therefore been described in detail further. All these options would place

additional loading on the existing slope.



Option 3 – Reinstating a foot / cycle track bridge avoiding the sluices

It is proposed that the footbridge follows the former railway bridge alignment over the

Congresbury Yeo tidal estuary (Figure 6).

Figure 6 - Embankment and Proposed Crossing Locations

Aerial photograph of the site © Google (2016); Image © Infoterra Ltd and Bluesky

Starting from Mud Lane north of the Tutshill Sluice the footpath follows the old railway

alignment up to the estuary’s north embankment and then goes east to cross over the

Tutshill Sluice. The path then turns west along the left bank of the Congresbuury Yeo

and re-joins the old railway alignment after crossing the Sampson Sluice. The proposed

footbridge follows the old railway path when crossing the estuary, and as such reduces

the length of the detour on the path by 70%, from around 400m to 125m.

To mitigate the flood risk, it is proposed that the bridge’s superstructure soffit to be

designed above the current top level of the tidal embankments. In order for the

superstructure to extend from top of tidal embankment to top of tidal embankment, the

bridge would have initially required to be approx. 90m in length. Upon review, an

embankment bund could be constructed at the north section with embedded drainage

pipes. This reduces the bridge’s length by 40% resulting in a 55m long structure. See

Figure 7 below for elevation view of the potential solution.

Figure 7- Proposed Vertical Arrangement



Structural

A structural assessment has been undertaken to assess the footbridge solutions.

The design brief is for a footbridge to be used for cyclists and equestrians. The following

design requirements have been taken into consideration:

• 3.50m wide deck (clearance between parapets’ faces), this is a code requirement

specified in BD 29/04.

• 1.80m handrail height for the parapet (to accommodate the equestrian traffic)

• Anti-skid surface for the deck (suitable for equestrians).

• Live Loads: UDL=5kN/m2 (for pedestrians) and PL=8.12kN (for equestrians).

• Minimum headroom of 2.7m with dismount provision or 3.7m mounted (to suit the

equestrians).

The assessment considers the most suitable option is a three span arrangement and

the use of Fibre Reinforced Polymer (FRP) materials for the whole superstructure

reduces the self-weight, which typically represents 60% of its overall load using

conventional material, on the foundations in comparison with single span and the use of

steel for the main structural elements.

Incorporating the decking within the “I” beam height in a U-frame configuration, reduces

the overall depth of superstructure. Furthermore, smaller span lengths results in smaller

“I” beam section sizes which improves aesthetics aspect of the bridge. Steel may be the

most suitable material when taking into account the conventional materials but the

substantial benefit with FRP materials is beginning to be recognised by the industry,

especially in footbridge design. These materials can match and even exceed steel’s

versatility for structural adequacy, pre-fabrication and on site constructability. Given the

possible access constraints, remoteness of site location, and potential environmental

impact the FRP option is the most favourable (Figure 8).

Figure 8 - Proposed “I” FRP girders option

While a relatively new material, a number of footbridges have been constructed using

FRP materials, replacing conventional material such as steel and concrete. FRP is a

very versatile material which can potentially open up options to clients where aesthetics

are an important part of design.

For this reason, the FRP option is recommended for concept design stage.

In comparison with traditional bridge superstructure materials, FRP is seen as

innovative option due to its lightweight nature whilst still achieving the same level of

performance as conventional materials. The initial building cost might be higher for this

type of material but the whole life cost is considered to be substantially less. The

materials do not corrode (particularly important in a saline environment where steel will

have a limited life) or require regular maintenance, such as painting, and inspection

costs are drastically reduced when compared with the conventional options. The



properties of FRP materials will increase the sustainability, safety and cost benefits of

the structure. The capital costs for the FRP option have been estimated to be £636,525

and with maintenance costs over a 120 year period the total whole life costs are

£786,525.

For further details regarding options for the bridge design and the drawing specifications

refer to the technical report enclosed (4000-UA008579-UU41-01).

Additional to Options 1 and 2 is a sub-option to construct a new cycle track and

equestrian bridges around the sluice structures on the landward side. To bypass the

Tutshill sluice the bridge would need to span approximately 10m and to bypass the

Sampson Sluice the bridge would need to span 15m. Assuming the deck of the both

cycle track bridges are 3.5m wide (code requirement specified in BD 29/04) an

indicative cost for the Tutshill bridge is £60k and the indicative cost for the Sampson

bridge is £90k.



Geotechnical

The ground conditions have been assessed from the Congresbury Yeo Tidal Banks –

Ground Investigation Report – (Royal Haskoning DHV, March 2015) and from the

Ground Investigation No. AC0711 Factual Report – Congresbury Yeo (CJ Associates,

December 2014).

Records from British Geological Survey (BGS) confirm the site is predominantly Tidal

Flat Deposits comprising SILT and CLAY. The ground conditions comprise up to 4m of

firm cohesive embankment material, overlying circa. 20m of soft to firm silty CLAY with

bands of PEAT, overlying stiff gravelly CLAY. Gravel is of MUDSTONE. The ground

condition of the nearby Tutshill Sluice footbridge are as follow:

Table 1- Ground Conditions

The following geotechnical testing has been undertaken during the previous

investigation comprising;

• Moisture Content

• Liquid Limit, Plastic Limit, Plasticity Index

• Particle Size Distribution

• Sedimentation by Hydrometer

• BRE SD1

• Total Organic Content

• Dry Density

• One Dimensional Consolidation Properties

• Triaxial tests ‘Quick Undrained’.

• In addition Geo-Environmental Testing was also undertaken.

The Geotechnical testing showed that in areas the Tidal Flat Deposits are particularly

soft in nature (<10kPa). The existing embankment prior to the heightening was

assessed as having an undrained strength of 40kPa and the new material used in the

heightening works an undrained strength of 60kPa. The coefficient of volume

compressibility (mv) for the underlying material ranged from 0.41 to 0.55 m2/MN

indicating a significantly compressive material.

The existing earthwork has already been heightened to comply with the new EA flood

levels. The widening has been undertaken using cohesive material and it is assumed to

have been compacted to the earthwork specification issued (HE Earthwork

Specification). The earthwork is underlain by soft CLAY and SILT with undrained

strengths less than 10kPa. The historic boreholes also show over 2m thickness of peat,

however this was at depth and is assessed as unlikely to have a significant effect on

these fairly shallow works. The coefficient of volume compressibility values, for the

shallow Tidal Flat deposits suggest that settlement will occur across the earthwork even

with small additional loads. This would have to be carefully assessed at the detailed

design stage and the risk managed through construction. It is understood that

settlement plates have been installed to monitor the performance of the earthwork

(installed at 200m centres).



As part of this high level assessment of options it is assumed that the flood bund is fit for

purpose (i.e. is impermeable) and that the slopes are currently stable (i.e. are not

suffering slope stability problems). The solutions available include re-grading or the use

of shallow retaining structures such as plastic piles or earth retaining structure such as

BaFix, crib walling or gabion baskets.



Flood Risk

Based on a desk based assessment a summary of the flooding issues at the proposed

site are discussed in Table 2.

Table 2- Flood Risk

Flooding issues

Source of flooding

Flood risk Comments

Further Assessment

Required Low Medium High

Rivers �

Small parcels of the site are located in high risk Flood Zone 3 of the Congresbury Yeo. The site could be at risk of flooding in an extreme flood. Options involve construction on the banks and in the channel of the Congresbury Yeo and Oldbridge River.

Yes- Initial Assessment below.

Sea �

The site is located in Flood Zone 3 and is at high risk of flooding from the tidal Severn Estuary.

Yes- Initial Assessment below.

Surface water �

Environment Agency mapping indicates the site is at Very Low risk of flooding from surface water. The site is drained by network of land drains (rhynes). It is considered that the principal risk is from tidal and fluvial sources.

No

Groundwater �

The site is underlain by a Secondary B Aquifer, predominantly lower permeability layers which may store and yield limited amounts of groundwater (Mercia Mudstone). Borehole testing indicated groundwater is stuck at approximately 9m below ground level.

No

Artificial sources �

The site is surrounded by a complex pumped drainage system (rhynes). However, it is considered the principal risk is from tidal and fluvial sources.

No

The high level assessment undertaken indicates that the principal flood risk at the site is

considered to be from tidal and fluvial sources and this is discussed further in the

sections below.

The existing embankment is located in high risk Flood Zone 3 and the site is at risk of

tidal flooding from the Severn Estuary. Mapping undertaken for the North Somerset

Strategic Flood Risk Assessment (SFRA) and the Environment Agency’s (EA) online



flood mapping indicates the embankment is within tidal Flood Zone 3a (at risk during the

1 in 200 year (0.5% AEP) return period flood event) and the primary risk to the site is

tidal flooding from the Severn Estuary (Figure 9).

Figure 9- Flood Mapping

Source- North Somerset SFRA (Royal Haskoning, 2008)

The existing Congresbury Yeo tidal embankment is an EA flood defence asset. It forms

part of a wider defence system which protects the low lying Somerset levels from

frequent tidal flooding. The EA completed works to the Congresbury Yeo tidal banks in

2015 to raise the height of the bank to approx. 8.45m AOD (1 in 75 year (1.33% AEP)

standard of protection).

Mapping contained within the SFRA indicates there is a small parcel of land around the

Tutshill sluice structure within fluvial flood Zone 3b (functional floodplain) of the

Congresbury Yeo. This zone comprises land where water has to flow or be stored in

times of flood. It is defined as land which would flood with an annual probability of 1 in

20 (5%) or greater in any year (Figure 10).



Figure 10- Mapped Fluvial Flood Zone

Source- North Somerset SFRA (Royal Haskoning, 2008)

The table below summarises the flood risk appraisal for the various options to construct

a cycle way along an existing embankment between the Tutshill Sluice and Wick Lane

access points.

Table 3- Flood Risk Appraisal

Option Flood Risk Mitigation and Opportunities 1. Widening the embankment on the landward side

• Works required to the flood defence asset (embankment).

• All construction works located in the tidal floodplain. Largest footprint in the floodplain of the options considered.

• The embankment is located in fluvial Flood Zone 3b (functional floodplain) in the vicinity of the Tutshill Sluice.

• Floodplain compensation likely be required for works in fluvial Flood Zone 3b.

• Ecological improvements could be incorporated into compensation works.

• Flood defence consent required.

• Flood Risk Assessment required.

2. Widening the embankment on the landward side by using plastic ‘log piling’, BaFix or gabions

• Works required to the flood defence asset (embankment).

• All construction works located in the tidal floodplain. Use of ‘log piling’ or BaFix results in the works having less footprint in the floodplain compared to Option 1.

• Floodplain compensation likely to be required for works in fluvial Flood Zone 3b.





Option Flood Risk Mitigation and Opportunities

• The embankment is located in fluvial Flood Zone 3b (functional floodplain) in the vicinity of the Tutshill sluice.


3. Reinstating a foot / cycle track bridge

• Works within the channel and banks of the Congresbury Yeo.

• Principal flood risk is from tidal flooding.

• Location of the bridge minimises work to the embankment in the fluvial floodplain of the Congresbury Yeo.

• Hydraulic modelling may be required to determine impact of the structure on the tidal channel.

• The proposed embankment required robust drainage measures and the impact on the drainage system will require assessment.

• Assessment of tidal flood levels required to set the level of the bridge structure.

• MMO license required for work below tidal MHW.



Sub-option to construct a new cycle track bridges around both sluice structures on the landward side.

• Works on the banks of the Congresbury Yeo at Tutshill Sluice and Oldbridge River at the Sampson Sluice.

• Works located in fluvial Flood Zone 3b (functional floodplain) in the vicinity of the Tutshill sluice.

• Floodplain compensation likely to be required for works in fluvial Flood Zone 3b.


• Assessment of fluvial flood levels required to set the level of the bridge structure.



To summarise all options are located within the tidal floodplain of the Severn Estuary

(Flood Zone 3). Options 1 and 2 could involve work to the existing embankment on the

landward side of the Tutshill Sluice, which is located within the fluvial functional

floodplain of the Congresbury Yeo. Works located within the functional floodplain, if

permitted by the EA, would require floodplain compensation to compensate for water

displaced by widening and earthworks within the floodplain.

A Flood Risk Assessment (FRA) will be required to assess all options under

consideration. An indicative cost for a Level 1 Flood Risk Assessment (not including

hydraulic modelling) is £2,500. Hydraulic modelling may be required to assess the

impact of the bridge options on the banks and channel of the Congresbury Yeo and

Oldbridge River. It is recommended that the need for hydraulic modelling is established

with the Environment Agency to define the scope of the FRA prior to any assessment

work being undertaken.



Ecology

The tidal channel of the Congresbury Yeo (downstream of the sluice structure) is within

the Severn Estuary Special Area of Conservation (SAC) and Site of Special Scientific

Interest (SSSI). Due to the sensitivity of the environment surrounding the sluice and

embankment the following section recommends the tasks required to allow a full

ecological assessment of the options.

To assess the impacts of the options it is recommend that a Preliminary Ecological

Appraisal (PEA) is undertaken at each option location, which will involve a desk study

and a walkover field survey of the proposed construction footprint. The walkover field

survey would include the following:

• Phase 1 Habitat survey of the option to identify habitats present. Option 3 lies within

the Severn Estuary Special Area of Conservation (SAC) – the walkover would

additionally assess if the habitats have affinities to those for which the SAC is

designated;

• Identification and mapping of any invasive species of plants, such as Japanese

Knotweed, where present;

• Assessment/identification of the possible presence of protected species or species

of conservation concern, such as great crested newts, reptiles, birds, dormice and

bats or habitats suitable for such species.

A PEA report would be prepared following the site visit identifying any constraints,

mitigation requirements and enhancement opportunities to the proposed cycleway. In

order to fully assess the impact of cycleway proposals on protected species, further

surveys may be necessary dependent on the option chosen and the outcome of the

desk study exercise. For example, if data indicates that populations of

waterfowl/waders use habitats in proximity to the options, then further targeted bird

surveys may be required and/or a Habitat Regulation Assessment (HRA) may be

required on account of potential impacts to Special Protection Area (SPA) birds/SAC

habitats and species. Similarly if data indicates that great crested newts are present in

the local area (which sounds possible based on what the farmer suggested with regards

to newt fencing for works associated with the EA flood alleviation bund), further surveys

for great crested newts might be necessary. We would be able to advise on the need

for such surveys once we have received the data from Somerset Environmental

Records Centre (SERC), Wetland Bird Survey (WEBS), British Trust for Ornithology

(BTO) and any recommendations for further surveys and impact assessments would be

included within the report.

As part of the desk study we would contact SERC for information relating to protected

species and non-statutory designated sites within 1km. In addition and based on the

proximity of the site to the Severn Estuary SPA, we would request data from

WEBS/BTO on waterfowl and waders, the populations of which are an internationally

designated feature of the SPA. A web-based search to obtain supplementary

information regarding statutory designated sites of nature conservation importance

would also be undertaken.

The Wick St Lawrence Cycle Route - Phase 1 Habitat Survey and Protected Species

Ecological Appraisal Report dated April 2010 identified the potential for disturbance to

various protected species (otter, badger) and natural habitats and ecosystems in

breeding seasons during both the construction and operational phases of the cycleway.

An accurate planning of the construction phase will help to minimise the disturbance.

Fencing and pollution control methods should be in place prior to the commencement of



works. The estimated costs associated with the tasks described above are contained

within the table below.

Table 4- Ecology Task Estimates

Task Fees (excl.

VAT)

Desk study covering all Options (including direct SERC and WEBS costs*) £1,180

Field survey (per Option) £375

PEA report (per Option) £1,500

TOTAL per Option £3,055

*We estimate that direct costs for the provision of SERC and WEBS/BTO data would be £880

(excl. VAT)



Conclusions and Recommendations

The costs provided are for indicative purposes only and detailed costing is required

before the costs can be confirmed for the options proposed. The findings of the initial

appraisal are concluded in Table 5 below. Please note the embankment costings have

been produced using the Environment Agency Unit Cost Database, which gives a range

of between £39/m3 and £122/m3 with a mean of £94/ m3 for embankment fill. Therefore

there is considerable uncertainty to cost at this stage. Bridge and plastic piling costs

have been estimated with the assistance of suppliers.

Table 5- Appraisal Summary Table

Option Indicative

Capital Cost

Comments

1. Widening the

embankment on the

landward side by

importing additional fill.

£730k (£2,920 per

metre of widening – including

subsidiary bridges but

not including land costs)

The embankment slope will likely have to be the same as the current slope angle, 1(V) to 4(H) so that the overall stability is not impaired. The footprint widening option imposes additional load to the ground likely to cause settlement of the existing bund. This will be a medium term effect occurring probably over a few years and maybe irregular leaving an uneven final level that will be subject to ponding following heavy rainfall. Good drainage and firm foundations will be required to ensure the path remains passable in all weather conditions, although use of more granular material for the widened section should help this. Option located in high risk tidal Flood Zone 3. Largest footprint in the floodplain for the options considered. Works to the embankment in the vicinity of the Tutshill Sluice are at high risk of fluvial flooding from the Congresbury Yeo. Flood Risk Assessment is required to assess the impact of the option on flood risk to the works and to third parties. Works to the landward side of the bund are not within the SSSI or SAC. However, a PEA will be required to assess the impacts at the location of the option.

2. Widening the

embankment on the

landward side by using

plastic ‘log piling’,

BaFix or gabions.

£515k (£2,060 per

metre of widening– including

subsidiary bridges but

not including land costs)

A “log pile” wall or BaFix system could be constructed at the crest of the slope thereby limiting the backfill material required on site. Alternatively if the slope stability analysis yielded unsatisfactory results the piling or other retaining structure could be positioned at the toe of the slope. This would reduce landtake when compared with a footprint widening option alone, but would cost more as it would require more material to be imported. This would place the cost at approximately £650k. Good drainage and firm foundations will be required to ensure the path remains passable in all weather conditions. Option located in high risk tidal Flood Zone 3. Works to the embankment in the vicinity of the Tutshill Sluice are at high risk of fluvial flooding from the



Option Indicative

Capital Cost

Comments

Congresbury Yeo. A Flood Risk Assessment is required to assess the impact of the option on flood risk to the works and to third parties. Works to the landward side of the bund are not within the SSSI or SAC. However, a PEA will be required to assess the impacts at the location of the option. Environment Agency approval would be required to embed piles or gabions within their flood defence embankment, as there is a risk this could impair the integrity of the embankment.

3. Reinstating a foot /

cycle track bridge

spanning the river

along the line of the

original railway bridge,

using durable

lightweight materials.

£735k (including

40m of new embankment, not including land costs)

The proposed footbridge reduces the length of the detour on the path by 70%, from around 400m to 125m. It also eliminates two ‘pinch points’ at the sluice crossings where conflict between cycle path users and farm animals could occur. The bridge option is £220k more expensive when compared to the embankment widening using piling (including the cost sub option for two bridges around the existing sluice structures). It is recommended that an embankment bund is constructed at the north end with embedded drainage pipes. This reduces the bridge’s length by 40% resulting in a 55m long structure. The assessment considers the most suitable option is a three span arrangement and the use of FRP materials for the whole superstructure. Option located in high risk tidal Flood Zone 3. Works located within the channel and banks of the Congresbury Yeo and the principal flood risk is from tidal flooding. The location of the bridge minimises work to the embankment in the fluvial floodplain of the Congresbury Yeo. A Flood Risk Assessment is required to assess the impact of the option on flood risk to the works and to third parties. Works are located within the SAC and SSSI and there is the potential for high impact on the wildlife population. Impacts to the designated sites will require detailed assessment, appropriate mitigation and input from key stakeholders. A PEA will be required to assess the impacts at the location of the option. Although the construction cost is higher than the embankment crest widening option there is less potential for long-term impact on the flood defence embankment as the bridge will reduce usage of the flood embankment and avoid additional loading affecting the embankment through settlement. Using FRP gives a long, durable life and avoids corrosion.



Option Indicative

Capital Cost

Comments

Sub-option to

construct new cycle

track bridges around

both sluice structures

on the landward side.

£150k

(Estimated cost for the Tutshill Sluice bridge is £60k and Sampson Sluice bridge is £90k)

Option located in high risk tidal Flood Zone 3. Works on the banks of the Congresbury Yeo at Tutshill Sluice and Oldbridge River at the Sampson Sluice. Works located in fluvial Flood Zone 3b (functional floodplain) in the vicinity of the Tutshill sluice. Works to the landward side of the bund are not within the SSSI or SAC. However, a PEA will be required to assess the impacts at the location of the option. This cost is included within the Option 1 and 2 totals above.

Following the initial assessment it is considered the main site constraints are:

• Soft ground conditions – The nearest borehole identified approximately 20m of

soft ground consisting of soft to firm silty sandy gravelly clay and very loose silty

sand interbedded with soft sandy clayey silt.

• High risk of flooding – The site is located in tidal Flood Zone 3. According to the

Environment Agency’s Flood Map from Planning the area is assessed as having a

1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year.

An area around the Tutshill Sluice is located in the fluvial functional floodplain of the

Congresbury Yeo (1 in 20 (5%) or greater in any year).

• Environmental impact – The tidal channel of the Congresbury Yeo (downstream of

the sluice structure) is within the Severn Estuary Special Area of Conservation

(SAC) and Site of Special Scientific Interest (SSSI). There is the potential for

disturbance of natural habitats and ecosystems in breeding seasons.

Findings

• The lowest cost option is to widen the embankment using plastic piling or a similar

method to retain the wider crest section within the existing embankment footprint.

However, there is considerable uncertainty in the costs of such a construction and

this option carries the highest risk of affecting integrity of the existing flood defence.

• The option to reinstate a bridge on the alignment of the original railway bridge has

the highest capital cost but this is marginal compared with the embankment

widening without piling (or similar), and is expected to have a lower maintenance

cost. It would also produce the least impact on the existing flood defence and it

would enable cycle path users to have a separate, dedicated route.

The recommendations arising from the initial assessment are listed below:

• It is recommended that further works are undertaken to determine the ground

conditions at the substructure location of the proposed bridges (river bed for the

piers and estuary’s embankments for the abutments) and the suitability of the new

embankment that significantly reduces the length of the Option 3 bridge. A site

investigation survey needs to be carried out to determine location of the old railway

bridge’s foundations to mitigate risk of clashes between the new and old

substructures.

• A topographic survey of the river bed and embankments is required to determine the

superstructure’s soffit level and the height of the substructures. The survey is also

required to determine exactly how the proposed new embankment will impact the



mean high water level and the overall flood related risks. Since the proposed

embankment will include robust drainage measures, increasing risk of flooding is

unlikely, but needs to be confirmed by specialist study. Alternative options such as

an additional span or a series of arches can be explored at the Concept Design

stage.

• Whilst the relative impacts of floodplain loss in the context of the wider environment

are likely to be very small, a FRA is recommended to determine the current and

anticipated flood levels at the location of all options and the impact of the widened

embankment bund on the wider floodplain. Further discussions with the

Environment Agency should be undertaken to determine the mitigation requirements

for all options.

• An in depth analysis will be required to confirm the suitability, the final structural

configuration and elements’ dimensions of all three options listed and the sub

options. This analysis should be carried out in conjunction with the FRA.

• Further ground investigation is required as follows: CPTs every 40m along the route

to a depth of 10m (approximately 25 CPTs). These can then be correlated against

the existing CPTs and the geotechnical laboratory testing already undertaken at the

site.

• A new topographic survey capturing the work undertaken in heightening of the

earthwork will be required to determine the footprint for widening and the slope

angles / grades of the widened earthwork, alternatively the retained heights of the

wall if chosen as the preferred option.

• Information from the monitoring undertaken by Team Van Oord would be useful in

assessing the likely effect of widening the earthwork.

• It is recommended that a PEA report is prepared to identify any ecological

constraints, mitigation requirements and enhancement opportunities to the proposed

cycleway.

References

• Carder, DR, Darley, P,Barker, KJ, 2002. Structural Use of Plastic Sheet Piling in

Highway Applications (TRL 533).

• CJ Associates, December 2014. Ground Investigation No. AC0711 Factual Report –

Congresbury Yeo.

• Royal Haskoning DHV, 2008. North Somerset Strategic Flood Risk Assessment

(SFRA).

• Royal Haskoning DHV, March 2015. Congresbury Yeo Tidal Banks – Ground

Investigation Report.

TUTSHILL FOOTBRIDGE OPTIONS

Feasibility Study Report DOC. REF: 4000-UA008579-UU41-01– Feasibility Study Report

FEBRUARY 2016

CONTACTS

JON ROYDS Associate Technical Director

dd +44 (0)1483 803 121 df +44 (0)1483 803 000 m +44 (0)789 425 1475 e [email protected]

Arcadis. The Surrey Research Park 10 Medawar Road Guildford GU2 7AR United Kingdom

Arcadis Consulting (UK) Limited is a private limited company registered in England & Wales (registered number 02212959). Registered Office at Manning House, 22 Carlisle Place, London, SW1P 1JA, UK. Part of the Arcadis Group of Companies along with other entities in the UK. Copyright © 2015 Arcadis. All rights reserved. arcadis.com

TUTSHILL FOOTBRIDGE OPTIONS Feasibility Study Report

Author Oliver Wells

Checker Razvan Capra

Approver Jon Royds

Report No 4000-UA008579-UU41-01

Date FEBRUARY 2016

VERSION CONTROL Version Date Author Changes

01 09/02/2016 Oliver Wells First issue

02 18/02/20016 Razvan Capra EA flood risk works information amended

This report dated 05 February 2016 has been prepared for North Somerset Council (the “Client”) in accordance with the terms and conditions of appointment dated 05 January 2016 (the “Appointment”) between the Client and (“Arcadis”) for the purposes specified in the Appointment. For avoidance of doubt, no other person(s) may use or rely upon this report or its contents, and Arcadis accepts no responsibility for any such use or reliance thereon by any other third party.

CONTENTS

1 EXECUTIVE SUMMARY ........................................................................................... 1

2 BACKGROUND INFORMATION .............................................................................. 4

Site location ...................................................................................................................................................... 4

Ground conditions ............................................................................................................................................ 4

Environmental conditions................................................................................................................................ 5

3 OPTION EVALUATION ............................................................................................. 6

3.1 Span arrangement ................................................................................................................................ 7

3.2 Materials ................................................................................................................................................ 7

3.3 Footbridge options ............................................................................................................................... 8

3.3.1 Option 1 – Truss ................................................................................................................................. 8

3.3.2 Option 2 – Arch Bridge ....................................................................................................................... 8

3.3.3 Option 3 - Cable Stayed Bridge .......................................................................................................... 9

3.3.4 Option 4 – Steel “I” girders .................................................................................................................. 9

3.3.5 Option 5 – FRP Bridge ........................................................................................................................ 9

4 COST ESTIMATES ................................................................................................. 12

5 CONCLUSIONS ...................................................................................................... 14

5.1 Option 1, 2 and 3 ................................................................................................................................. 14

5.2 Option 4 ............................................................................................................................................... 14

5.3 Option 5 ............................................................................................................................................... 14

6 RECOMMENDATIONS............................................................................................ 16

7 REFERENCES ........................................................................................................ 17

APPENDICES

General Arrangement

Options Matrix

1 EXECUTIVE SUMMARY Arcadis has been commissioned by North Somerset Council to undertake a feasibility study for a footbridge as part of the Tutshill Crossing Scheme.

This reports identifies the options for a shared cycle/ equestrian footbridge crossing the Congresbury Yeo tidal estuary on a flood plain west of the Tutshill Sluice, North Somerset.

The bridge forms part of the Weston to Clevedon Cycle Route – Tutshill Crossing Scheme. It is proposed that the footbridge follows the former railway bridge alignment over the Congresbury Yeo tidal estuary. The proposed arrangement reduces the total length of the footpath that crosses the estuary by 70%, from 400m to 125m.

The distance between the tidal embankments opening is approximately 90m in length. Upon review, an embankment bund is recommended at the north section, with embedded drainage pipes, to reduce the total length of the bridge to 55m.

In relation to the localised land use, access and ground conditions, the most suitable proposal is a 3 span FRP (Fibre Reinforced Polymer) bridge with an overall length of 55m, adopting spread foundations for the abutments and piers. When compared with conventional building materials (timber, steel, concrete), the use of FRP materials for the whole superstructure significantly reduces the loading on the substructure mitigating the risks associated with the site’s soft ground conditions.

The current code requires a 3.50m clear width between parapets to accommodate the pedestrians, cyclists and equestrian traffic. The height for the parapet shall be 1.8m and an anti-skid surface is adopted to suit equestrian use. The provision of a 600mm high solid infill panel, measured from deck level, as advised by the British Horse Society is considered appropriate.

The most important site constraints are:

• Soft ground conditions – nearest borehole identified approximately 20m of soft ground consisting of soft to firm silty sandy gravelly clay and very loose silty sand interbedded with soft sandy clayey silt.

• High risk of flooding – the site is located in a Flood Zone 3 area. According to the Environment Agency’s Flood Map from Planning the area is assessed as having a 1 in 100 or greater annual probability of river flooding (>1%), or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year.

• Environmental impact – potential for disturbance of natural habitats and ecosystems in breeding seasons.

The high risk of flooding can be mitigated by detailing the bridge soffit above the top of tidal embankments. Adopting the Fibre Reinforced Polymer (FRP) materials for the superstructure, reduces the loadings on the substructure, mitigating the soft ground condition issue. The high strength-to-weight ratio, high corrosion resistance and the benefits of lightweight FRP materials over conventional ones reduce the environmental impact during construction as it requires lighter plant for transporting and erection. It also reduces maintenance cost over the life of the structure. The structure can be prefabricated and delivered on site for assemble therefore reducing the construction time and cost. Detailed planning of the construction phase would ensure minimum disturbance of natural habitats and ecosystems. A railway bridge was constructed in the 1880’s which passed over the River Yeo. The steel lattice bridge had 7 spans and was 240ft (73m) long, supported on cast iron piers. It should be noted that soon after completion of construction, two of the piers sank and the bridge needed repairs. It retained a sagging appearance and the bridge was removed in 1943.

1

Figure 1 – The now demolished River Yeo Railway Bridge (River Yeo Bridge)

The bridge’s metal supporting columns still exist but are on private land. Consideration of removing these unsightly columns can be, subjected to public consultation, factored in to the project.

Figure 2 – Cast Iron Piles left in ground from the demolished River Yeo Railway Bridge (River Yeo Bridge)

2

Figure 3 – General view of proposed FRP bridge

The use of FRP materials can potentially replicate the shape of the previous bridge superstructure. The former span arrangement can also be maintained. Replicating the aesthetic of the original bridge structure could restore some cultural heritage within the area for locals.

Further works It is advised that further works are undertaken to determine the ground conditions at the substructure location of the future bridge (river bed for the piers and estuary’s embankments for the abutments) and the suitability of the new embankment that significantly reduces the bridge’s length. A site investigation survey needs to be carried out to determine the old bridge’s foundations locations to mitigate risk of clashes between the new and old substructures. A flood study is recommended to determine the current and anticipated 1:100 flood level and the impact of embankment bund. A topographic survey of the river bed and embankments needs to be undertaken to determine the superstructure’s soffit level and the height of the substructures. The survey is also required to determine exactly how the proposed new embankment will impact the mean high water level and the overall flood related risks. Since the proposed embankment will include robust drainage measures, increase risk of flooding is unlikely, but needs to be confirmed by specialist study and further discussions with the Environment Agency should be undertaken. Alternative options such as an additional span or a series of arches can be explored at the Concept Design stage. An in depth analysis will be required to confirm the suitability of Option 3, the final structural configuration and elements’ dimensions. This analysis must be carried out in conjunction with the flood safety improvement study that is currently being developed by a third party.

3

2 BACKGROUND INFORMATION Site location The site is located north of Wick St Lawrence, North Somerset, approximately 1mile from the coast. The surrounding area predominantly comprises marsh land, largely below the level of mean high water springs. The land is protected from flooding by tidal earth embankments. The bridge is proposed to span across the Congresbury Yeo tidal estuary west of the Tutshill Sluice, spanning between the embankments.#

Figure 4 – Location map

The top level of the embankment is approx. 8-8.5m AOD and the toe level approx. 4-5m AOD.

Ground conditions Records from British Geological Survey (BGS) confirm the site is predominantly Tidal Flat Deposits comprising SILT and CLAY. However these boreholes are some way from the line of the bridge foundations. The ground conditions comprise up to 4m of firm cohesive embankment material, overlying circa. 20m of soft to firm silty CLAY with bands of PEAT, overlying stiff gravelly CLAY. Gravel is of MUDSTONE.

The ground condition of the nearby Tutshill Sluice footbridge are as follow:

Depth Ground Conditions SPT N Cu Phi mv 0 – 1.8m Made Ground: soft to firm silty sandy gravelly CLAY 12 40kPa - -

1.8 – 13.2m Soft to firm silty sandy CLAY with bands of Peat - 10kPa - 0.41–0.88

13.2 – 21.2m Very loose silty SAND interbedded with soft sandy clayey SILT 2, 2, 4, 7 - 30◦ -

21.2 – 22.4m Firm reddish brown sandy slightly gravelly CLAY. Gravel is of MUDSTONE. >50 150kPa - -

The above ground conditions have been assessed from the Congresbury Yeo Tidal Banks – Ground Investigation Report – Royal Haskoning DHV (March 2015) and from the Ground Investigation No. AC0711 Factual Report – Congresbury Yeo (CJ Associates) dated December 2014.

Tutshill Sluice

Sampson Sluice

Proposed Footbridge location

4

Environmental conditions The bridge is to be located in an area with a high risk of flooding, primarily from the sea. The risk associated with the flooding can be mitigated by designing the bridge’s superstructure above the current top level of the tidal embankments and considering the necessity for bridge piers.

The Wick St Lawrence Cycle Route - Phase 1 Habitat Survey and Protected Species Ecological Appraisal Report dated April 2010 identified the potential for disturbance to various protected species (otter, badger) and natural habitats and ecosystems in breeding seasons during both the construction and operational phases of the cycleway. An accurate planning of the construction phase will help to minimise the disturbance. Fencing and pollution control methods should be in place prior to the commencement of works. This report concluded that the structure will have a low overall environmental impact.

5

3 OPTION EVALUATION The design brief is for a footbridge to be used for cyclists and equestrians. The following design requirements are taken into consideration for all the options:

• 3.50m wide deck (clearance between parapets’ faces), this is a code requirement specified in BD 29/04. • 1.80m handrail height for the parapet (to accommodate the equestrian traffic) • Anti-skid surface for the deck (suitable for equestrians) • Live Loads: UDL=5kN/m2 (for pedestrians) and PL=8.12kN (for equestrians) • Minimum headroom of 2.7m with dismount provision or 3.7m mounted (to suit the equestrians). Starting from Mud Lane the footpath follows the old railway alignment up to the estuary’s north embankment and then goes west to cross over the Tutshill Sluice. The path re-joins the old railway alignment after crossing the Sampson Sluice. The proposed footbridge follows the old railway path when crossing the estuary, as such reduces the length of the detour by 70%, from around 400m to 125m.

Figure 5 – Proposed alignment

It has been assumed that the tidal estuary is not navigable. The overhead vertical clearance does not accommodate the passing of vessels. The superstructure’s soffit level is determined by the mean high water level and allowing for debris carried by flood waters.

To mitigate the flood risk, it is proposed that the bridge’s superstructure soffit to be designed above the current top level of the tidal embankments. In order for the superstructure to extend from top of tidal embankment to top of tidal embankment, the bridge would have initially required to be approx. 90m in length. Upon review, an embankment bund could be constructed at the north section with embedded drainage pipes. This reduces the bridge’s length by 40% resulting in a 55m long structure. See Figure 6 below for elevation view of potential solution.

Figure 6 – Proposed vertical arrangement

Proposed crossing, L=125m

Existing crossing, L=400m

6

The following site constraints have been identified:

• Soft ground to a depth of 20m – 2m below ground level in BH4, closest to bridge location (according to BH log sheet, Ground Investigation No. AC0711 Factual Report – Congresbury Yeo (CJ Associates) dated December 2014).

• High risk of flooding. • Environmental issues during construction – potential for disturbance of natural habitats and ecosystems,

particularly breeding season. • Buildability issues – access to and from rural site. Construction methodology need to be considered in the

planning and design stages. • Importing of fill for section of embankment – no suitable site won material likely to be available.

3.1 Span arrangement Various span arrangements can be adopted for the 55m long structure. When considering the site’s constrains, suitable span arrangements are analysed as follows:

• Single span – this type of arrangement requires a greater self-weight of the superstructure. The loads from the superstructure are concentrated and transmitted to only two supports. Because of the soft ground conditions this option will require substantial piled foundations at the two abutments. To reduce the loads on the substructure, a truss or a bowstring arch can be adopted for the superstructure. The single span arrangement does provide a clear cross section for unhindered flow of the river.

• Two spans – this arrangement reduces the total self-weight of the superstructure and redistributes the loads onto two abutments and one pier. The two spans can be simply or continuously supported. Multiple structure types can be designed to accommodate this option. A more appealing architectural design can be achieved by considering a single pier cable stayed bridge.

• Three spans – the most efficient use of material to reduce the self-weight of the structure can be achieved by adopting the continuously supported 3 spans arrangement. The main risk associated with this option is that the structure is sensitive to different settlements of the supports. The risk can be mitigated by using piled foundations.

• Multiple spans – increasing the number of spans reduces the superstructure’s self-weight, the loads on each substructure, and further mitigates the risks associated with the soft ground conditions. By adopting a simple supported span arrangement, the effects of differential settlement are eliminated. The increased number of substructures will impact the construction costs. The works required to construct the all the substructures will have a greater environmental impact and restrictions to water flow.

3.2 Materials Together with the span arrangement, the chosen structural material has a great influence on the overall sustainability of the design. Both conventional and new materials have been analysed as follows: • Concrete – A concrete superstructure will have a negative impact on the soft ground conditions risks due

to its inherently greater self-weight. The use of concrete should be limited to the substructure. The piers and abutments are likely to be formed of reinforced concrete founded on top of reinforced concrete piles and pile caps or pad foundations. Reinforced concrete is inherently robust and durable.

• Steel – this represents one of the most sustainable options for the superstructure’s main structural elements when referring to the site’s constrains. Steel provides a lighter superstructure when compared with concrete and all the analysed span arrangements can be adopted. Steel requires corrosion protection in the form of a paint system, requiring maintenance after about 20years. To further increase the structure’s sustainability, the use of weathering steel should be considered, to remove the need for painting and to reduce maintenance. The use of steel should be limited to the main structural elements and lighter materials should be adopted for the decking and parapets.

7

• Timber – this material is suitable for use as the main structural elements of the superstructure only when choosing a multiple span arrangement (5 spans or more). Timber can also be used for the decking or parapet elements to further reduce the overall weight of the structure. An option is to use steel for the superstructure’s main structural elements together with a timber deck and parapet.

• Fibre reinforced polymer – this material is light weight and has increased corrosion resistance properties. This option reduces the self-weight of the superstructure to a minimum and reduces the maintenance costs as well. FRP material has been used in the decks and superstructure members of bridges since the mid1970s. It is envisaged that FRP would form the entire bridge superstructure including deck and parapets. However due to the variety of structural properties available from FRP, testing should be recommended as part of the design stage to ensure a robust and durable form of construction is achieved.

3.3 Footbridge options Footbridge options have been devised from a suitability matrix considering; structural arrangement, material, and form of construction. A summary of the conclusions of the five possible options are indicated below:

Option No. of Spans Type Girder Decking Parapet Foundation

1 1 Truss Steel FRP FRP Concrete, piled

2 1 Arch Steel FRP FRP Concrete, piled

3 2 Cable stayed Steel FRP FRP Concrete, piled

4 3 “I” girders Steel FRP FRP Concrete, piled

5 3 Through U-Frame / “I” girders FRP FRP FRP Concrete, spread / piled

3.3.1 Option 1 – Truss This option consists of a single span of 55m length. It is proposed that the main structural elements are of steel construction with the decking and parapet made out of lightweight materials such as aluminium or FRP.

For this configuration the girders are constructed from circular hollow steel sections. A suitable decking surface is applied to minimise echoing when equestrians cross the bridge. The single span arrangement lends itself to the use of piles, offering better design efficiency. A possible configuration is indicated in Figure 7 below.

Figure 7 – Proposed truss configuration

3.3.2 Option 2 – Arch Bridge The superstructure consists of a bowstring arch bridge with steel box girders for the main structural elements and a lightweight decking solution. Piles is recommended to accommodate the vertical loads of the superstructure. A possible configuration is indicated in Figure 8.

8

Figure 8 – Proposed arch configuration

3.3.3 Option 3 - Cable Stayed Bridge The substructure consists of an “A” shape pier and two abutments to support both ends of the superstructure. The distribution of spans needs to be reviewed to ensure best design efficiency against site constraints. This arrangement will help reduce the dead load of the whole structure. The decking elements are proposed to be constructed from FRP or aluminium. The main pier is a piled foundation whilst the end abutments are spread footings. A possible configuration is indicated in Figure 9 below.

Figure 9 – Proposed cable stayed configuration

3.3.4 Option 4 – Steel “I” girders A 3 span arrangement has been proposed in order to spread the loading across more foundations. The superstructure’s main girders are designed as steel “I” girders, spanning continuously over intermediate supports. This results in a more efficient use of the material that reduces the self-weight of the structure. For the decking, the same lightweight materials are proposed. The abutments can be spread footings and piles for intermediate piers. A possible configuration is indicated in Figure 10 below.

Figure 10 – Proposed “I” steel girders option

3.3.5 Option 5 – FRP Bridge For this option, the conventional materials used for the main structural elements are replaced by FRP materials. This option is an extension of Option 4 where FRP is used for the main girders instead of steel. This reduces the self-weight of the main structural elements by 75%. As the spans are simply supported,

9

differential settlements of the foundation locations can be tolerated. Spread footings can be used for all substructures therefore simplifying design and reducing the cost and time of construction. A possible configuration is indicated in Figure 11 below.

Figure 11 – Proposed “I” FRP girders option

While a relatively new material, a number of footbridges have been constructed using FRP materials, replacing conventional material such as steel and concrete. FRP is a very versatile material which can potentially open up options to client where aesthetic is an important part of design.

A footbridge was recently unveiled at Dawlish station on the Great Western railway constructed entirely of FRP. The bridge resembles the old steel structure that needed to be replaced in a manner sympathetic to the original. The 17.5m span bridge is lightweight and resistant to the salt water environment.

Figure 12: Dawlish station new FRP footbridge (rail.co.uk)

Another modern design example is the GFRP Lleida Pedestrian Bridge in Lleida, Spain. The structure is a tied-arch 38 m long and 6.2 m rise. The bridge crosses the Madrid-Barcelona high-speed rail link and it was fabricated in three months and erected by crane in just three hours.

Figure 13: Lleida Pedestrian Bridge in Lleida, Spain (en.wikipedia.org)

10

In 2014, a composite pedestrian bridge was opened in Nijmegen, the Netherlands. It connects Nijmegen to Ooypoort, a nature reserve located on the banks of the Waal River. The bridge’s structure consists purely of glass fibre-reinforced polyester and the bridge has a single span of 56m. The bridge is designed in such a way that it can be partly submerged in case of high water, without any damage to the structure.

Figure 14: Ooypoort composite pedestrian bridge crossing the Waal River in Nijmegen, the Netherlands (en.wikipedia.org)

11

4 COST ESTIMATES The estimates are given in Table 1 below.

Table 1 – Options’ cost estimates

Table 1 gives an indicative price for the analysed design options suggested in this report. This is an approximation of the construction and maintenance costs, and will need to be considered further should the bridge enter a detailed design phase. Actual values could deviate by up to +/- 40%

Item Item Detail

Item Price (£)

Opt

ion

1 Tr

uss

Opt

ion

2 A

rch

Opt

ion

3 C

able

st

ayed

Opt

ion

4 St

eel I

gi

rder

s

Opt

ion

5 FR

P

1. General Contractor Prelims £5000 p/w for 10 weeks 50000 50000 50000 50000 30000

Consultant Fees Design Fees, Supervision, Project Management 20000 20000 30000 15000 40000

2. Superstructure Truss Steel circular sections 110000 - - - -

Arch Box girders, transverse I girders - 120000 - - -

Cable stayed Main steel girders, cable system - - 140000 - -

Steel I girders I steel girders, transverse I girders - - - 90000 -

FRP girders Main I girders, transverse stiffeners - - - - 290000

Decking 41000 41000 41000 41000 41000 Parapet 66000 66000 66000 66000 66000 3. Substructure Spread Foundation Concrete - - - 13750 11000 Piled Foundations Concrete 71500 71500 77000 55000 27500 Elevations Concrete 27500 27500 49500 27500 11000 4. Embankment Construction Import fill, construct embankment

Import and compact fill for embankment bund 25000 25000 25000 25000 25000

Install drainage Install drainage 12000 12000 12000 12000 12000

Sub-Total 423000 433000 493250 395250 553500 Contingency, inflation and risk (15%) 63450 64950 73987.5 59287.5 83025

TOTAL CONSTRUCTION COSTS 486,450 497,950 567,238 454,538 636,525 5. Maintenance Inspections assumed 120 years 48000 51600 60000 48000 30000 Maintenance and repairs assumed 120 years 456000 504000 504000 456000 120000

TOTAL MAINTENANCE COSTS 504000 555600 564000 504000 150000

TOTAL COSTS 990,450 1,053,550 1,131,238 958,538 786,525

12

The price does not include: detailed design of the bridge, design meetings, walkovers, and surveys. A whole life cost exercise for the maintenance activities has not been undertaken.

Due to the assumed construction and maintenance prices used to develop the cost estimates it is advised that a better reference system should be used when comparing the Total Costs of the options. In Table 2 the total costs value is compared with the one for Option 5 (reference value).

Table 2 – Cost estimate ratios

Table 2 shows Option 4 to be the solution to take forward if considering the construction costs only. When taking into account the overall costs associated with each option, the low maintenance works required for Option 5 make this option the most suitable one. The use of FRP materials has a great influence on reducing the substructure’s costs and represents the best solution to mitigate the risks associated with the site’s ground conditions. The impact of the high superstructure’s construction costs for Option 5 is reduced when considering the inspection and maintenance costs for the whole life of the structure. The FRP superstructure will require minimum maintenance.

The risks that were previous identified have an important impact on the final prices’ values that were used to determine the costs for each option. We would recommend that more accurate cost estimates are undertaken based upon the further design development of the preferred options to be taken forward.

Cost estimates ratios

Item Price (£)

Opt

ion

1 Tr

uss

Opt

ion

2 A

rch

Opt

ion

3 C

able

st

ayed

Opt

ion

4 St

eel I

gi

rder

s

Opt

ion

5 FR

P

Construction costs 0.76 0.78 0.89 0.71 1.00 Maintenance costs 3.36 3.70 3.76 3.36 1.00

TOTAL COSTS 1.26 1.34 1.44 1.22 1.00

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5 CONCLUSIONS 5.1 Option 1, 2 and 3 These options propose to cross the tidal estuary using a single 55m span for the truss and arch configurations and a two span arrangement for the cable stayed one. The main advantage of these configurations is that the river is not constricted by piers and a more appealing architectural design can be achieved.

The site’s ground conditions do not favour this span arrangements, mainly due to the high dead weight that need to be accommodated by the substructures. A longer span requires the use of more construction material therefore increases the self-weight. The weight from the superstructure is equally shared on two supports for the truss and arch configurations, resulting in higher design loads on the abutment compared to the other options. As for the cable stayed configuration, the pier’s foundation attracts a greater proportion of the load resulting in bigger foundation for the central pier.

The proposed configurations can be achieved most suitably using piled foundations for the substructure. An option study for ground improvement could be investigated. The use of piles or ground improvement works increases the construction costs. When compared with the other options, the materials and construction costs required for the main structural elements are likely to be significantly higher.

Another constraint that has a great impact for this option is the limited site access for the heavy plant and components to form the final structure. The use of heavy cranes with suitable reach is required and the ground condition might not be suitable, particularly the soft terrain between the two embankments.

The inspection and maintenance costs are high when taking into account the structure’s whole life cycle. It is recommended that this option is discounted due to the high construction and maintenance costs and due to the site constrains that will have increased construction risk.

5.2 Option 4 A 3 span option reduces the self-weight applied onto the foundations compared with the single span options. The spans take the form of 16m for the end-spans and 23m for the mid-span to make a better use of the materials by balancing the loads on the main girders. The continuous beam configuration is sensitive to the differential settlement of the supports. This can be mitigated by adopting piles for the piers and spread footings for the abutments (lower reactions’ values at the ends of the bridge). The superstructure can bear directly on two piles that are extended to act as piers – mimicking the supports of the original rail bridge.

Although the general perception is that steel “I” girders do not have the same architectural value as an arch bridge or a cable stayed one, the structure blends in to the surroundings and is not visually intrusive to the area’s landscape. The required depth of the main girders is relatively small and the possibility of installing the parapet directly on girder’s top flange keeps the overall depth of the superstructure low. The architectural aspect is highly improved making the bridge more visually pleasing.

The site access requirements do not pose as greater constraint as compared with Options 1 to 3. By adopting slimmer and lighter structural elements, the plant and equipment required for construction can all be reduced. The current access is likely to be suitable for a haul route.

The whole life cost is considered to be less than Options 1 to 3 by having to inspect and maintain a simpler and smaller structure (smaller elements, smaller area to inspect or paint). It is recommended that this option is to be developed further in the concept design stage.

5.3 Option 5 This option derives from Option 4 but uses FRP instead of steel. It further reduces the superstructure self-weight by 75% compared to other options. This weight reduction makes a three equally long, simply supported spans arrangement most suitable. The simply support structure is more robust to differential settlement between supports. This configuration allows the use of spread footings for both abutments and all intermediate supports.

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The use of FRP for bridge design is becoming increasingly popular due to the many advantageous properties it exhibits. As previously indicated, successful example such as the Dawlish Footbridge in UK and Lleida Pedestrian Bridge in Spain, have been constructed across the world as a result. The process from concept design to construction will have to be a joint effort between Client, bridge designers, FRP materials specialists and manufactures. As a requirement, the structure, or parts of the structure (main structural elements) will have to be tested to ensure the design is in accordance with the bridge’s real behaviour. This will increase the construction costs.

In comparison with traditional bridge superstructure materials, FRP is seen as innovative option due to its lightweight nature whilst still achieving the same level of performance as conventional materials. The initial building cost might be higher for this type of material but the whole life cost is considered to be substantially less. The materials do not corrode or require regular maintenance, such as painting, and inspection costs are drastically reduced when compared with the conventional options. The properties of FRP materials will increase the sustainability, safety and cost benefits of the structure.

It is recommended that this option is to be taken forward in the concept design stage.

15

6 RECOMMENDATIONS Following the analysis of the suitability matrix of the options, a risk/impact analysis has been carried out to determine the most sustainable, cost efficient & safe option. The results are presented in Appendix B.

Option 5 has been selected as the most suitable for a number of reasons. A three span arrangement and the use of FRP materials for the whole superstructure reduces the self-weight, which typically represents 60% of its overall load using conventional construction material, on the foundations in comparison with single span (Option 1 and 2) and the use of steel for the main structural elements (Option 4).

Incorporating the decking within the “I” beam height in a U-frame configuration, reduces the overall depth of superstructure. Furthermore, smaller span lengths results in smaller “I” beam section sizes which improves aesthetics aspect of the bridge. Steel may be the most suitable material when taking into account the conventional materials but the substantial benefit with FRP materials is beginning to be recognised by the industry, especially in footbridge design. These materials can match and even exceed steel’s versatility for structural adequacy, pre-fabrication and on site constructability. Given the possible access restraints, remoteness of site location, and potential environmental impact Option 5 is the most favourable.

For this reason, Option 5 is recommended for concept design stage.

Option 4 also presents a suitable solution, when considering a more conventional approach. This option implies a shorter pre-construction stage and reduces the construction costs compared to Option 5. The sustainability of Option 4 is further increased by adopting the weathering steel for the superstructure’s main structural elements and by using FRP materials for the decking and the parapet.

It is recommended to consider Option 4 for further design stages should a more conventional solution is to be adopted.

Figure 14 – General view

Figure 15 – View of the deck

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7 REFERENCES rail.co.uk. (n.d.). Retrieved from UK’s First Grade II Listed Plastic Station Footbridge :

http://www.rail.co.uk/rail-news/2013/new-plastic-station-footbridge/ River Yeo Bridge. (n.d.). Retrieved from Weston Clevedon & Portishead Railway:

http://www.wcpr.org.uk/Yeo%20bridge.html en.wikipedia.org (n.d.) Retrieved from GFRP Lleida Pedestrian Bridge:

https://en.wikipedia.org/wiki/GFRP_Lleida_Pedestrian_Bridge

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General Arrangement

Options Matrix and Risk/Impact analysis

Tutshill Footbridge Feasibility Matrix

Suitability 0-1 = unsuitable

2-3 = could work

4-5 = suitable

Topic Description / Issues

Superstructure

Comments

Tim

ber

Stee

l

Con

cret

e

FRP

Ground Conditions

Embankment Material

(Up to 4m thick) Firm slightly sandy CLAY with occasional cobble sized pockets of stiff CLAY. Cu = 60kPa.

3 3 1 5

Soft ground until approx. depth of 21m. Superstructure needs to be lightweight, to reduce loading / settlement carried to foundations.

Cutting Material (Superficial Deposits)

(Approx. 20m thick) Soft to firm silty sandy CLAY with bands of PEAT. Cu = 10 - 40kPa.

Cutting Material (Bedrock)

MUDSTONE or stiff gravelly CLAY of Mudstone. N > 50

Span Options

Single Span Eliminate need for intermediate foundations 0 3 1 3 Not possible for timber unless offset cable

stay used

2 spans Reduces loads on bridge / foundations 1 3 2 4

3 spans (18.3m) Reduces loads on bridge / foundations 2 4 3 5


Superstructure

Comments

Tim

ber

Stee

l

Con

cret

e

FRP

5-6 spans (9-11m)

Reduces loads on bridge / foundations further 5 5 4 5 Concrete least suitable due to option for

lighter materials

Superstructure Loading

Pedestrian / Equestrian UDL = 5kN/m2, Point Load = 8.12kN 5 5 5 5 Suitable superstructures can be

designed

Foundations

Spread Foundations

Soft clay - settlement likely, make lightweight 3 3 1 5

If foundations supports in embankment layer, reasonable strength of soil achievable (Cu up to 60kPa) Piled

Foundations

Soft clay exhibited to 20m + depth. For heavyweight structures, piles into mudstone layer

5 5 5 5

TOTAL 24 31 22 37

Tutshill Footbridge Risk/Impact Matrix


Option

Comments

Trus

s

Arc

h

Cab

le

stay

ed

Stee

l I

gird

ers

FRP

Ground Conditions

Differential settlement

Soft ground conditions are likely to produce unequal settlements for the foundations

1 1 2 3 1 The simple supported span arrangement is the most suitable when considering this risk

Plant and equipment bearing

Soft ground conditions might not be able to accommodate large plant and equipment

4 4 4 2 1 Reduced structural element weights will require lighter plant and equipment

Substructure

Loading from the superstructure

Large substructures due to concentrated loads on fewer elements

4 4 4 2 1 FRP will result in the most light structure

Requirement of piles Increases the costs, important lengths 5 5 5 5 3 Only Option 3 might not require piles for

the footings

Substructure self-weight Increases the load on the footings 4 4 5 3 2

The loads from the superstructure and the structural configuration have an important impact on the substructures dimensions

Risk/Impact 4-5 = high

3-2 = medium

1 = low


Option

Comments

Trus

s

Arc

h

Cab

le

stay

ed

Stee

l I

gird

ers

FRP

Construction and maintenance costs

Design costs A more complex structure will have greater design costs 3 4 4 3 5 FRP materials require specialist

consultants

Testing and checking the design

The newly developed materials require more investigations

1 2 2 1 4

FRP materials require more testing than other conventional materials. The manufacturer will be involved in the design process

Construction costs

The complexity of the design affects these costs by means of plant, access, transportation, man-hours

3 4 5 3 3

Although FRP is more expensive than other materials, the costs related with the work (transport, erecting) on site are lower

Access to area 4 4 4 3 2 This will be overcome in any situation, however FRP might be the most viable due to the option for transporting sections Access onto flood plain 4 4 4 3 2

Maintenance and inspection costs

A more complex structure implies greater inspection and maintenance costs.

3 4 5 3 1 FRP materials are the most cost efficient when regarding the maintenance costs

TOTAL 36 40 44 31 25

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T: +44 (0)1483 803 000

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