The Northern Hub programme and electrification scheme will provide an upgraded rail network in the north of England, particularly in the Manchester area, to provide better connectivity, faster journey times and improved capacity. The new track layout, which will allow up to 700 more trains to run each day through central Manchester, will primarily be on masonry viaducts and the new Ordsall Chord railway elevated viaduct, which links together Manchester’s Piccadilly, Oxford Road and Manchester Victoria stations.
Lundy Projects was sub-contracted by Northern Hub Alliance member Skanska/BAM to fabricate and install signal gantry structures, OLE (overhead line equipment) structures and associated foundations. Lundy had also contributed to the design of these structures in conjunction with architectural designer BDP and project lead designer AECOM Mott MacDonald JV.
Having installed its first structure for the project (signal gantry MC466) during the 2014 Christmas blockade, Lundy has commissioned 14 further signal gantries since then.
Gantry mounting
The structures are predominantly sited on Victorian elevated masonry viaducts, which has inherent design and construction constraints. To achieve the end user requirements and to successfully install the structures within a restrictive environment, a variety of substructure and superstructure solutions were required.
Where gauging prohibited the gantry legs to be positioned within the parapet walls of the viaduct, the structures were designed so that the legs would be outboard of the arch piers, mounted on fully welded steel bracket assemblies which were mechanically fixed to the masonry viaduct piers with using a drilled Cintec anchor system and cementitious grout.
The piers were core drilled with an oversized hole and an anchor tendon, formed of a stainless steel threaded bar and an encapsulating fabric sock, which was then inserted into the cored void. Cementitious grout was mixed and injected, under low pressure, through veins to the base of the sock, so filling the void. Once cured, the anchor was then pull-tested to ensure the design strength had been reached.
The stainless-steel bars projected from the wall face, so that the steel mounting bracket could be attached. As the brackets were fixed above quoin level, this design omitted street level foundations, securing the railway from trespass and resultant vandalism.
Project manager Tony Boyle stated: “The works required the fabrication and installation of OLE portals, TTCs (two-track cantilevers) and piles. With only a short lead-in time of two weeks, and installation to be carried out over five 30-hour blockades, both the fabrication and installation teams, including our client Skanska/BAM, had to work in complete unison to deliver the project.”
The steel CHS (circular hollow section) vibro-driven piles were fabricated in Lundy’s robotic facility, which allowed a quick turnaround and mobilisation of site crews within two weeks.
As the REFOS (Rail Edge to Face of Structure) dimensions at some locations resulted in the reach being too great to install the piles from a rail machine, so an off-track piling solution was needed. Skanska/BAM constructed a new haul road and piling mat to allow Lundy to bring in a 35-tonne tracked machine to install the piling and mast in one possession.
The on-track piling was then undertaken with Lundy’s own T10000 Colmars, each fitted with a Movax and BSP hammer attachment to allow the CHS steel piles to be vibro-driven to design total depth. A long-reach Colmar was then used to install the masts.
Once the as-built dimensions had been taken from the piling, this information was immediately relayed back to the fabrication works, allowing the booms to be finalised, galvanised and delivered within a week. Tony Boyle commented: “This close working collaborative relationship forged between Lundy, Skanska/BAM and the client effectively reduced the number of possessions and programme duration from what would otherwise normally be required.”
Architectural harmonisation
Due to the architectural importance of Manchester’s viaducts as they run through the city centre, positioning a visually exposed signal gantry in the heart of this infrastructure required an ‘out of the box’ approach. Signal gantry MC665 is sited on the newly installed Trinity Way steel bridge, part of the new Ordsall Chord track and elevated viaduct, so, although the design had to meet all signal sighting, OLE electrical clearances and gauging requirements, aesthetics was still a major concern.
Signal gantry MC665 is a two-track portal, with three signal dropper cages serving the Up/Down Ordsall Chords in both directions. The route is AC electrified, so the signal dropper cages had to be coordinated around pantograph envelopes while the entire structure needed OLE mesh and solid screens for protection. Fortunately, being sited on a newly designed steel bridge, gauging was not a concern, which allowed the unconventional structure design to work.
MC665 structure was to be harmonised with its surroundings. Working to the architect’s vision, Skanska/BAM and Lundy Projects supplied a bespoke solution.
Unfortunately, from the architect’s perspective, the colour scheme had to remain Network Rail standard black for the boom and signal cages and grey for the legs. However, providing that the structure’s shape and signal position conformed to the standard gauging and sighting requirements, the project was free to change the actual shape. This is where the trapezoidal prism gantry was born!
The gantry’s primary steelwork was fabricated entirely from steel plate, welded together to form the boom cord sections. The legs were again formed from steel plate, shaped as an inclined A-frame, but, significantly, they were designed to be streamlined, without any of the diagonal bracing which is normally used to provide rigidity.
The column shafts were inclined in both cross track and along track directions, forming a trapezoidal prism gantry. The column shafts were also cranked at the base – as they were unseen from the street below, they returned to vertical, which allowed a perpendicular baseplate connection to the bridge deck.
The fabrication was undertaken in one of Lundy’s two fabrication facilities, using the latest software technology and machines to produce the quality needed on such an aesthetically demanding structure. Special jigs and templates were designed to accommodate the different axial directions of the individual plates forming the column and boom section profiles, ensuring they were welded within the tight fabrication tolerances.
To maintain the sight lines of the legs being continued to the boom floor level, each column was spliced below the boom level, with an end-plated connection. This resulted in the ‘join’ being hidden, maintaining the continuity of the leg profile. The leg sections above the splice continued to the top of the boom level and the boom shafts were fully welded between the inside faces of the columns.
The boom sections, which were fabricated from steel plate, were designed with a flange on one edge. The cable trays were fixed to the lip of the flanges which allowed them to be fully accessible but still be hidden from view from below.
To follow the lines of the leg members and the trapezoid shape, the walkway containment handrailing and OLE mesh panels were also set to the same incline as the legs, resulting in a truly bespoke structure that fits in with the design of the new viaduct and also with its historical location in Manchester city centre.
This article was written by Jon Andrews.
