HS2 tunnels will pass below an existing main line railway intersection bridge near Euston, known as Bridge 7, constructed in the early 20th century. An assessment of tunnelling induced ground movement predicted potential adverse effects on the fragile historic structure. The challenge for the bridge mitigation team was to develop a methodology for logistics, design, and construction of the mitigations, including confirming existing conditions during a six-week blockade below the bridge but with no interruption to the main line above. The challenge was increased by the low headroom, buried obstructions, and severely limited access and working space.
Analysis showed the masonry bridge abutments and wing walls would have low resilience to predicted ground movements. After considering several options, a mitigation was developed using prestressed ground anchors to provide a permanent restoring force to the existing retaining walls. The sequence of pre-coring, anchor installation, proof testing and stressing was closely co-ordinated with the site team. The anchors will be continuously monitored during HS2 tunnelling construction, using an observational approach, agreed with Network Rail, to avoid overstressing the historic masonry structures.
The bridge mitigation works included the creation of gaps at the back of the main girders at both abutments to let the bridge deck slide over the bearing plates, and some localised steelwork strengthening to enable the bridge to resist differential settlements. Finally, jacking systems were inserted beneath the six westernmost columns to mitigate against unforeseen ground movements.
Careful selection of methods helped to manage risks during site works and severe programme risks associated with overrun of the blockade. Work was successfully completed within the six-week blockade.
Introduction
This paper is presented as part of the works to deliver the Main Works Civils Contract (MWCC) for the southern section of High Speed Two (HS2) Phase One which includes the Northolt Tunnels and the Euston Tunnel and Approaches, being delivered by the SCS Integrated Project Team.
HS2 Contract S1 (Euston Tunnels and Approaches) involves the construction of tunnels to connect the HS2 running lines into the new low-level terminus in Euston, London. As part of this contract, three new SCL tunnels (the “crossover tunnels”) are to be constructed directly beneath the existing Network Rail main lines just north of Euston station (Figure 1).Zoom Figure 1
The crossover tunnels will run in close proximity to an existing Network Rail intersection bridge (ELR LEC1 0m 1147y) “Bridge 7”, which enables “Line X” to cross beneath four of the main lines leading to Euston’s platforms. The bridge is within the zone of influence of all three HS2 tunnels as shown in Figure 2.Zoom Figure 2
Bridge 7 was constructed between 1902 and 1904 as part of the extensive programme of works carried out by the London & North Western Railway to widen and improve the approach to Euston Station. It is a highly skewed, two-span riveted steel structure (Figure 3). The deck comprises two main longitudinal external girders and four pairs of longitudinal internal trough girders, one pair supporting each main line track A, B, C and D. The rails are supported on ballast and concrete, or timber sleepers. Lines A to D and X are all AC overhead electrified; lines C and D are also 3rd rail DC electrified.Zoom Figure 3
Zoom Figure 4
The main girders span continuously between abutments and are bolted to a central pier. The abutments are solid masonry gravity retaining walls with concrete footings and stone capping to support the girder bearings. The central steel trestle of braced vertical columns is supported on bedstones and a masonry foundation with a concrete footing (Figure 4). The abutment and pier footings are propped at foundation level by concrete stretcher beams (Figure 5a).Zoom Figure 5
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