UK: Construction of HS2 Ltd’s Green Park Way ventilation shaft at Greenford in west London has entered a crucial phase, as preparations ramp up ahead of the arrival and subsequent recovery of four tunnel boring machines at the site.
The TBMs are excavating the 13·5 km long twin-bore Northolt Tunnel which will eventually take HS2 trains from West Ruislip to the Victoria Road crossover box, situated to the west of Old Oak Common station.
The work is being carried out by the Skanska Costain Strabag joint venture, which is responsible for constructing more than 20 km of twin-bore HS2 tunnels in London.
The Green Park Way ventilation shaft is one of four shafts along the Northolt Tunnel route, but it is from there that the four TBMs will be extracted. The ventilation shaft consists of two access routes: a main shaft, located above the Up running line towards Old Oak, and a satellite access passage reached from the Down bore.
The process of extracting the TBMs will be somewhat unusual and is thought to be a UK first. This is due to the high water table, which in places exerts as much as 3 bar of pressure. Due to the volume of groundwater, the TBMs will not be able to break through the sides of the ventilation shafts as they normally would. Instead, steel ‘cans’ are being constructed within the shafts to receive the TBMs; these will stop the shafts from flooding with water.
The cans are being built in sections on the surface, and the SCS JV is now starting to lower them into place and complete final assembly. Once complete, the cans will be filled with either foamed concrete or stabilised sand, which will provide resistance as the TBMs break through the sides of the shafts.
To extract the cutter head and tunnelling shield, the top part of the can will then be removed. However, this can only take place once a watertight seal has been formed between the tunnel and shaft.
It is anticipated that Sushila will be the first TBM to arrive, with the breakthrough expected to occur later this year; the second will be TBM Caroline, expected early 2025. Both of the TBMs were launched from West Ruislip in 2022.
