Abstract:
High Speed 2 (HS2), Britain’ s new high-speed railway, is the biggest current transport infrastructure project in the country. It will connect London and Birmingham and put passengers first with more reliable, faster and frequent services. HS2 trains will run on zero-carbon electricity helping to decarbonise the rail network. HS2 involves building major structures including 32 miles of tunnels, embankments, cuttings, bridges and viaducts.
The C1 contract crossing the Chilterns includes a requirement for 16 km of tunnel requiring 4 x 18.2m internal diameter vent shafts constructed using Diaphragm Walls up to 78.5m deep and a Diaphragm Wall headwall at the North Portal including Low Headroom works and anti ovalisation barrettes, required due to very soft ground. Cement bentonite break in / break out (BIBO) and TBM parking plugs were also required.
This paper will discuss the logistical, geological and practical constraints that were identified during the project preparation and overcome during the execution of the specialist geotechnical works for the 4 shafts and portal.
Pre-treatment grouting was required at all sites to mitigate the risk of major support fluid losses. Post treatment grouting at BIBO blocks were also required. Geology comprised superficial deposits over structureless chalk including dissolution features; historical erosional features later backfilled with sands / gravels and alluvial deposits.
Shaft design included the requirement for soft eyes to allow the Tunnel Boring Machine’ s TBM’ s to shave the side of the shaft. Challenging verticality tolerances of 1 in 300 to 1 in 400 were required. Each shaft required 2 no single bite primaries, 6 no 3 bite primaries and 8 No single bite closing panels. The rational that determined this panel arrangement will be discussed.
Delivery of the works had to accommodate challenging logistics on very small sites in environmentally sensitive areas. A hydraulic grab was used for the first 6 m of excavation and for stop end removal. Thereafter excavation used a Trench cutter with milled joints and polystyrene void formers.
The project encountered reinforcement cage supply issues due to industry capacity as it was the 4th of 5 major HS2 diaphragm wall contracts to be let. The reinforcement design developed during delivery of the project from 3 individual cages per large primary to 2 no cranked cages, simplifying installation. Cages required significant coupler density for internal structures; this required cautious bentonite management, concrete placement and shop drawing detail.
Spoil and slurry treatment and disposal were critical, bentonite plant pipework and cable layouts were pre planned to maximise efficiency and accommodate a desander, desilter and centrifuge.
Management of bentonite density influenced the rate of excavation. Milled joints resulted in increased bentonite contamination form Chalk fines and concrete fines in closing panels. A bespoke recycler utilising gravity between scalper and desander was developed to prevent particle cutting by pumps and reducing plant footprint. Addition of polymer to bentonite mud facilitated 2 micron / µ m cut point treatment (30 m3/h). Management of stored volumes when the cutter was not in operation were critical necessitating a trade-off between disposal costs and rental of centrifuge equipment and consumables. Bentonite mud was transferred between shaft locations for reuse to minimise final disposal.
Concrete was supplied from the Southern portal batch plant, leading to travel time to more northern shafts being problematic. The site team had to work to ensure that this did not reduce the concrete quality in situ.
This paper will detail the considerations assessed and implemented to overcome the key factors mentioned above.