HS2’s first giant TBM – which launched almost 3 years ago – broke through on 27 February at the end of her 16 km (10-mile) journey under the Chilterns, excavating the longest tunnel on the ambitious new rail project.
The 2,000 t TBM, named Florence, is one of 10 machines excavating the 51,5 km (32 miles) of tunnel on the new railway linking London Euston with Birmingham and was the first to launch, in May 2021.
The HS2 project will almost halve journey times between Britain’s two largest cities, while freeing up space for more local services on the most crowded part of the existing West Coast Main Line.
Two identical TBMs were used to create the twin-bore Chiltern tunnel, which stretches from the South Portal near the M25 to South Heath in Buckinghamshire. Parallel tunnels will carry north and south bound trains with a second TBM, named ‘Cecilia’, due to breakthrough in the coming weeks.
Designed specifically for the geology under the Chilterns, each TBM is an underground factory – excavating the tunnel, lining it with 56,000 pre-cast concrete segments and grouting them into place as it moves forward at an average speed of 16m per day.
Four similar TBMs are being used for the London approach tunnels, while another two will work on Birmingham’s Bromford tunnel. Preparations are also underway for the launch of two more machines to excavate the Euston tunnels.
The breakthrough comes a week after HS2 published new research which shows how the impending arrival of high speed rail will drive a EUR11.68 billion (GBP10 billion) economic boost for the West Midlands during the next 10 years, with a huge increase in new development around the two station sites.
The TBMs Florence and Cecilia, launched from the South Portal - are operated by HS2’s main works contractor, Align – a joint venture formed of three international infrastructure companies: Bouygues Travaux Publics, Sir Robert McAlpine, and VolkerFitzpatrick.
They two machines - built by Herrenknecht - incorporate a number of innovations and technologies that have been introduced on TBMs in the UK for the first time, to enhance performance and safety. This includes ‘semi-continuous boring’, allowing our TBMs to build the rings that line the tunnels without pausing.
Each TBM is operated by a crew of around 17 people, working in shifts to keep the machines running 24/7. They are supported by over 100 people on the surface, managing the logistics and maintaining the smooth progress of the tunnelling operation.
The 3 million cubic metres of chalk and other material removed during the tunnelling is being used to create an ambitious grassland restoration project at the south portal, which will include 127 hectares of new landscaping, wildlife habitat and biodiverse chalk grassland.
The chalk, which is pumped back through the tunnel in a slurry mixed with water, is processed at a slurry treatment plant at the south portal site where the flint is removed and water extracted before it can be reused in landscaping.
Chalk grassland – a type of calcareous grassland - is habitat of international conservation importance mainly found on limestone and chalk valleys of south-east England and the Isle of Wight.
Lime-rich, but low in nutrients, the thin soil holds little water and heats up quickly. These conditions encourage a huge variety of smaller herbs and wildflowers and over 40 species can be found in one square metre of grassland, including some of the UK’s rarest orchids and invertebrates. Only 700 hectares of chalk grassland exist across the whole of the Chilterns AONB.
At its deepest point, the tunnel is 80m beneath the Chilterns and passes under the M25, local railway lines and twice under the River Misbourne. Extensive water quality, groundwater level and surface water flow monitoring was put in place prior to the start of construction, and there has been no significant change to water quality during the tunnelling work.
For further information please click here for a video, here and uk/65 for the tunnelbuilder archive and visit https://www.hs2.org.uk/. 09/24.
