HS2 | BTS
concurrently with the tunnel construction by using a moveable bridge under which a section of invert (including the carrier drain and catch-pits) was cast. This methodology has significant programme benefits,
allowing TBM operations to carry on unhindered as the invert pours were cast below the bridge. Align elected to use a slip-forming machine to
cast the walkways, obtaining excellent productivity, whereas for logistical reasons SCS are using a moveable formwork bridge.
SCHEME DEVELOPMENT The increase in quantum of tunnelling as the scheme developed was described. In 2012, before the Hybrid Bill was submitted, the amount of cut-and-cover and bored tunnels was 40% less than the current scheme. Tunnels that have subsequently been added include: 1. The entire Northolt Tunnels East – the original alignment was to be at surface level between OOC station and Greenpark Way shaft in Greenford, west London;
2. Bromford Tunnel – as the original alignment was to thread its way through to Curzon Street at grade, negotiating several network rail lines and the M6;
3. The Chiltern Tunnels have been extended by 2.7km; and,
4. New cut-and-cover tunnels at Greatworth and Copthorne.
AERODYNAMICS A short description was given on some of the aerodynamic issues that affect tunnel design for a
high-speed railways, including: 1. Control of aural comfort of passengers – this is the sensation you feel in your ears as a train enters a tunnel;
2. Pressure loading on the walls of the tunnel structures and equipment from the passage of trains; and,
3. Micro-pressure waves. These are sonic booms that, unless mitigated, can be heard at the exit portal as a train enters the tunnel (possibly many km away) unless mitigated. Mitigation of these is achieved with a porous portal at the tunnel entrances.
GEOLOGICAL SETTING The geology of HS2’s Phase 1 alignment between Birmingham and London was described, showing it spanning between older Triassic Marl and Sandstone beds to younger Tertiary sediments - resulting in quite varied geological conditions for the three areas of HS2’s bored tunnels. Ground Investigation (GI) to examine this geology included 6085 boreholes, drilled to a total depth of 160km.
South The NTW tunnels are bored through the London Clay Formation, Lambeth Group and Chalk Group, whilst NTE and Euston Tunnels are bored in London Clay. In the varied NTW geology, the Lambeth Group
consists of only Reading Formation – the Upper and Lower Mottled Beds, which contain sand channels. These sand channels are common in the mottled clays, having been deposited as sands, silts and clays
Chiltern Tunnel Porous Portal
Opposite top: Long
Itchington Wood Tunnel cutterhead being transported through the village of Ufton to start second drive
Opposite bottom: Invert Bridge viewed from above
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