Carters Bridge was built in 1868 as a two span overbridge comprising steel beams supporting a timber deck. Constructed for the opening of the Lancashire Union Line, it is the only means of access for a farmer between two sections of his land.
By the 1950s, the mass brick abutments showed significant signs of deterioration and were propped by timbers. The distress is likely to have resulted from ground movements caused by historic mining. The superstructure was replaced in the 1960s by a pair of steel beams supporting timber decking. The brick abutments were partially broken down and replaced by bank seats at the top of the railway cutting.
The structure spans the Wigan to Huyton lines, which were electrified in 2012 in Phase 2 of the North West Electrification Programme. However, the structure was found to be understrength and the parapet was inadequate over the electrified lines.
So, in 2015, Network Rail commissioned Murphy and Tony Gee and Partners to design and construct a replacement bridge. The GRIP 2 document (feasibility) stated that the bridge would be replaced by a two-span steel composite structure supported in the existing abutments. This solution required extensive temporary works and would cause considerable disruption to the farmer’s business as it would have required a 10-week closure of the bridge.
Instead, the team developed a 39-metre single-span solution, to be constructed parallel to the existing bridge. This minimised the disruption to the farmer as the new bridge could be built while maintaining access using the old bridge. However, the design team was constrained in two ways – only two 29-hour closures of the railway would be permitted for the scheme, and the new structure would have to be installed over the electrified railway.
The bridge is situated on the Pennine Lower Coal Measures Formation (PLCM) of the Langsettian age of the South Lancashire Coalfield, composed of sandstone, siltstone, mudstone and coal beds. Covering this solid geology there is a thin veneer, less than a few meters thick, of superficial glacial deposits (diamicton).
Due to historic mining activity in the area and the predicted presence of coal seams beneath the structure, a comprehensive site investigation was undertaken. The ground investigation comprised eight boreholes to depths of between 31 and 40 metres. A combination of rotary coring and open hole drilling was employed, with the objective of providing geotechnical information for foundation design and checking for the presence of worked or unworked coal seams beneath the structure.
The materials encountered in the boreholes generally corroborated the anticipated superficial and solid geology from the desk study, which had been developed based upon the examination of geological maps and borehole records in the public domain.
The proposed north abutment founding level was confirmed as being situated, in its entirety, in an eight-metre-thick layer of ‘extremely weak’ to ‘weak’ mudstone. The intact coal seams and various lithologies encountered in the four boreholes located on the northern side of the cutting could be readily correlated.
However, correlation of the materials encountered in the boreholes situated on the southern side of the cutting was extremely difficult, with boreholes only 10 metres apart showing significant differences in stratigraphy. These differences were explained at design stage by the presence of two mapped geological faults in close proximity to the location of the south abutment, one was noted to be ‘uncertain’.
Irrespective of the aforementioned, the two boreholes closest to the position of the south abutment both gave the founding level stratum to be a ‘medium-strong’ sandstone of variable thickness.
Having assessed the results of the geological survey, both abutments were designed with shallow foundations, the north abutment on mudstone with an allowable bearing pressure of 320kPa and the south abutment on a sandstone founding stratum with an allowable bearing pressure of 1,000kPa.
Due to the variability detected in the boreholes, a risk was identified that the founding stratum could vary from that anticipated in design. Consequently, once the excavations were nearing founding level, they were inspected and mapped to confirm they complied with the minimum acceptable geotechnical conditions in terms of rock lithology, strength and degree of fracturing identified on the drawing.
Preliminary calculations were also undertaken for the south abutment to confirm that, in the event that the actual founding stratum was a mudstone, a workable shallow foundation solution could be found without the necessity to revert to piles.
Bridge deck solution
Tony Gee developed a concept where the main weathering steel beams would be lifted into place with the stringcourse and parapet in place. This removed the requirement for temporary works over the railway during construction and allowed the bridge to be installed in a single 29-hour closure of the railway.
A 3D BIM model was used throughout the design process to facilitate buildability discussions within the delivery team. This concept was developed by Tony Gee working in close collaboration with Murphy International (steelwork fabricator) and Shay Murtagh (pre-cast concrete manufacturer).
The two pairs of beams were fabricated with the stringcourse in two sections in the precast yard to allow delivery. These were spliced together on site and a small in-situ stitch section was cast in the stringcourse at ground level. The aluminium parapets were installed prior to lifting.
The entire substructure was designed to be precast as modular elements to limit lift weights.
To enable the pair of girders, stringcourse and parapet to be lifted as a single unit, the stringcourse was designed as a continuous upstand. This provided lateral stability to the main beam during installation. It required innovative collaboration between the fabricator and designer, ensuring the units could be fabricated in two pieces and then transported to site in Wigan.
To communicate the new bridge construction process effectively, a QR code was added to information posters. This allowed site team members and site visitors to scan the QR code which redirected them to a website which had an interactive 3D installation model for the bridge. This displayed a user-friendly, step-by-step installation procedure enabling people to fully understand each stage in the construction process.
Environmental management and sustainability
The environmental management plan (EMP) was developed at the earliest opportunity, ensuring that requirements of key stakeholders, including the operators of a buried fuel line which crossed the site were identified and addressed.
All legal requirements were met prior to the start of construction. This included obtaining a hedgerow removal notice from St Helens council to allow for the widening of the site access for the transportation of the crane to site. In addition, a T7 exemption was obtained which would allow for the crushing and screening of bricks from the demolished abutments on site. This diverted 500 tonnes of waste from landfill as the material was reused in the reinstatement of the bridge approach slopes and for use as hardstanding via U1 exemption.
The main beams and entire new substructures were installed in a single 29-hour possession using a 1,000 tonne crane. On completion, the stone for the crane berthing area was reused for the approach roads.
The scheme was successfully completed in May 2016 through close collaboration between the designer and contractor and by an innovative challenge to the GRIP 2 proposal.
The scheme was subsequently awarded Medium Project of the Year at the ICE North Awards 2017. Darrell Matthews, North West regional director of the Institution of Civil Engineers, said: “The new Carter’s Bridge is a great example of how civil engineers solve problems. In this case, an old bridge had to be replaced while causing the least possible disruption to the people who needed access to it and the railway that passed under it.
“The engineers achieved this by ingenuity in design and construction, including the prefabrication of bridge components off-site so they could be installed using a large capacity crane during a single 29-hour closure of the railway line.
“Meanwhile, the old bridge was left in place while the new one was constructed, after which the old bridge and the remains of an even older structure were demolished. The whole thing was delivered on time, on budget, with an excellent safety record and minimal impact on the environment.”
This article was written by Lee Barraclough, associate director at Tony Gee and Partners. Additional material kindly supplied by Delyth Bowen of J Murphy & Sons.
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