The northern section of the Bedford Western Bypass was officially completed and opened by the Mayor of Bedford Borough on 25 April 2016. The bypass has featured in local and national plans for over 30 years, with the first southern section linking the A421 to the former A428 completed in 2009, the second northern phase then linking the A4280 at Biddenham to the A6.
The new route permits traffic travelling from the north or south to travel round the perimeter of Bedford, alleviating congestion in the busy town centre. It also opens up areas for 1,200 new homes, affordable dwellings and an employment park for 650 new jobs.
A significant obstacle on the northern section was the crossing of the Midland main line, which serves as the only primary route between London St Pancras station, the East Midlands and parts of South Yorkshire. It was therefore vital that the proposed bridge had minimal impact on the daily operation of the railway line.
The design would have to be both future-proof and low-maintenance, and provide access by road, cycle and utilities to the adjacent development land. At this location, the bypass would be a single 9.3 metre wide carriageway and 3.5 metre cycleway crossing the four railway lines (Up/ Down Fast and Slow).
The main contractor for this £18.6 million scheme was Breheny Civil Engineering, along with subcontractor MPB, for Bedford Borough Council. Tony Gee and Partners was responsible for civil and structural designs for various structures on the scheme, working with lead designers Waterman Infrastructure and Environment.
Tony Gee had been involved with the project since its inception back in 2006, developing conceptual designs for the railway crossing at an early stage, with close liaison with the council, design team and the relevant authorities, through to detailed design and eventual construction in 2014. Various construction forms were considered during the initial design stage with regards to the buildability and impact on the railway. A 48 metre long single-span composite weathering steel bridge without splices, supported on two in-situ concrete abutments, was considered to be the best solution.
The construction depth of the deck and carriageway alignment was optimised to minimise the extent of fill required to form the abutments, which were to be built up from grade. 80 per cent (90,000 cubic metres) of the fill was site-won material, excavated from three new drainage attenuation ponds forming a new country wildlife site. The design also catered for future overhead electrification of the railway line north of Bedford to Sheffield in respect of the minimum headroom clearances required.
The stratigraphy at this location consists of large areas of limestone and mudstone at relatively shallow depths. This sound formation permitted the abutments to be constructed on spread footings distributing the loads rather than using piles. The length of span and geotechnical conditions precluded the use of an integral bridge abutment; however the longer span configuration permitted all works associated with the abutments (some 1,200 cubic metres of concrete) to be constructed outside of the operational railway land.
The deck was supported on 1.85 metre deep, 50 metre long weathering steel girders weighing 45 tonnes each with plate thicknesses up to 60mm. The detailing considered potential moisture traps, and run-off plates were provided to prevent moisture migrating back to the bearings to avoid rust staining and increased corrosion.
Weathering steel was selected to maximise the long- term durability of the structure. The naturally forming rust patina provides 120-year design life and mitigates the potential costs associated with the maintenance of protective coatings over the busy railway.
The girders were designed and detailed to be erected in k-braced pairs weighing over 100 tonnes. The combined weight and design of the girders was limited by the maximum reach of a 1,000 tonne mobile crane, with 300 tonne superlift ballast, situated to the side of the railway so that they could be quickly installed during limited railway possession hours.
Between 160 to 180mm of pre-camber was applied to each girder at fabrication to mitigate the large self-weight deflections, and careful consideration was required on installation to ensure the tapered bearing plates on the soffit landed in the correct positions.
The 250mm thick 300 cubic metre in-situ reinforced concrete deck was formed using permanent GRP formwork to provide an instant working area. With the approval of Network Rail, this permitted deck construction to progress during continued operation of the railway.
The edge girders were also pre- installed with cantilever working platforms, providing soffit formwork for the copings and handrails to provide instant access. The platforms reduced the requirement for further lifts and work at height.
The deck also contains multiple service ducts for future utility connections to the adjacent development land. In accordance with the DMRB (Design Manual for Roads and Bridges) durability requirements, the structure also contains stainless steel reinforcement in exposed areas, such as the parapet copings and the bearing plinths, to minimise the extent of future maintenance and to provide an asset that will last for the required design life.
The deck copings support H4a very-high containment parapets on either side, capable of withstanding impact from HGVs, with anti-climb plates to the rear and top hat copers. The parapets tie in with transitions at each end to the adjacent vehicle restraint system to prevent errant vehicles leaving the carriageway and finding their way onto the track.
Whilst the bridge is, perhaps, not remarkable for its geometry and form, the project has been able to progress without problems due to the careful planning of the construction method and design detailing, minimising its impact on the existing infrastructure and public.
The team at Tony Gee and Partners was very pleased to have been involved in the scheme and to have the satisfaction of seeing it built -10 years after its original conception.
Written by Tim Burgess, principal engineer at Tony Gee and Partners