The Bromford tunnel is a high-speed railway tunnel under construction in North Warwickshire and Birmingham, England, and will serve to bring the High Speed 2 rail line into Birmingham upon completion.
The contract to build the tunnel, as part of the wider N1 and N2 lots on the HS2 programme, was awarded to the Balfour BeattyVinci JV on 1 April 2020, valued at c.£5 billion.[1] This followed them being named by HS2 as part of the intention to award in July 2017.[2]
The route was initially envisioned in the High Speed Rail (London - West Midlands) Act 2017 to be a tunnel, 2.86 km (1.78 mi) long (in Work No. 3/203) and partly on viaduct, less than 30 m (98 ft), in length (in Work No. 3/157).[3][4]
On 20 January 2022, HS2 Ltd made a Transport and Works Act Order application to remove the words "partly on viaduct" in the description of the works, in a bid to extend the tunnel by around 3 km (1.9 mi) instead.[4] This decision was claimed to reduce land take, minimise impacts on the Park Hall Nature Reserve, avoid the need to redirect the River Tame, and reduce construction traffic.[5]
The two Herrenknecht tunnel boring machines (TBMs) used to construct the tunnel were named Mary Ann, after Mary Ann Evans (with the pen name, George Eliot) and Elizabeth, after Elizabeth Cadbury.[7][8] Most of the second TBM to be delivered, Elizabeth, was repurposed from TBM Dorothy used to construct the Long Itchington Wood tunnel, with a new outer cutter-head and shield ordered.[9][10]
Design
The tunnel passes underneath the Park Hall Nature Reserve, River Tame, and M6 motorway, and runs up to 40 m (131 ft) underground.[11]
The tunnel will have 13 cross-passages, spaced every 500 m (1,640 ft), and 5.5 m (18.0 ft) wide.[12]
The eastern portal at Water Orton will have a porous portal, likely in the form of a perforated concrete structure, to mitigate the sound and shockwave produced by the piston effect of moving trains, travelling 230 km/h (145 mph).[5][11]
The decision the extend the tunnel to over 3 km resulted in the need for an intermediate shaft to allow for adequate airflow.[13] The tunnel will have a 47 m (154 ft) deep, 18.6 m (61 ft) diameter ventilation shaft at Castle Vale, around the halfway point of the tunnel. The headhouse, located in the Castle Bromwich Business Park, will have pre-cast concrete walls with an overlaid "flexi-brick" lattice facade made of ceramic tiles and a 'green roof'.[14][15]
Construction
Tunnelling and excavation is primarily through Mercia Mudstone, at mostly BGS grade II weathering. The groundwater level is, at most, 30 metres (98 ft) above the tunnels' roof.[12]
The variable pressure TBMs used to construct the tunnel have a cutter-head diameter of 8.62 metres (28.3 ft), and weight approximately 1,600tonnes each. They are both expected to take 16months to complete the tunnel, employing around 450people.[10]
The TBM Mary Ann started boring in August 2023 from an underground box structure, measuring 160 m × 30 m × 15 m (525 ft × 98 ft × 49 ft), from the eastern portal at Water Orton towards the 22 m (72 ft) deep portal at Washwood Heath. This was followed by TBM Elizabeth in March 2024.[5][16][17]
A total of 41,594 concrete segments, produced by the joint venture's pre-cast concrete factory in Avonmouth, Bristol, will be used to support the tunnels. These will form 5,942 rings, weighing 49tonnes each. Forty percent of the cement used was replaced with ground granulated blast-furnace slag (GGBS), claimed to reduce the overall carbon footprint involved. The TBMs are expected to excavate 1.87milliontonnes of material, which will be treated by the on-site slurry treatment plant for use on HS2's delta junction, south of Water Orton.[10][18]
Mary Ann is expected to break through by the end of 2024, while Elizabeth is expected to finish the drive by autumn 2025.[5]
