HS2: Chiltern Tunnel digging reaches Chalfont St Peter
The tunnels are the longest on the high speed rail route
Excavation work on HS2's Chiltern Tunnels has completed its first stage, reaching a ventilation shaft at Chalfont St Peter.
Two 170-metre Tunnel Boring Machines, named Florence and Cecilia, have travelled 3.6 miles under the Chilterns after being launched at the Southern End of the tunnels last Summer.
The 78m deep shaft at Chalfont St Peter is the first of five that will provide ventilation and emergency access to the ten-mile-long twin tunnels – which are the longest on the project. Once complete, the shaft will be covered by a headhouse designed to resemble local farm buildings.
Designed specifically for the geology of the Chilterns, each TBM is a 170m long self-contained underground factory, digging the tunnel, lining it with concrete wall segments and grouting them into place as it moves forward.
The TBMs named ‘Florence’ and ‘Cecilia’ by local school children launched in May and June last year and are expected to break out at the north portal in around two and a half years.
Welcoming the progress, HS2 Ltd Project Client David Emms said: “The Chiltern tunnel will take HS2 underneath the hills and safeguard the woodlands and wildlife habits above ground as well as significantly reducing disruption to communities during construction and operation of the new railway.
“It’s great to see how much progress has been made by Florence and Cecilia – and the teams excavating the five ventilation shafts – and I’d like to thank everyone involved in getting us this far.”
Each of the separate northbound and southbound tunnels will require 56,000 precision engineered, fibre-reinforced concrete wall segments – which are all being made in purpose build factories on site at the south portal, located just inside the M25.
During their first 3.6 miles, Florence and Cecilia have combined installed more than 20,000 separate segments, each weighing around 8.5 tonnes.
Approximately 2.7 million cubic metres of material will be excavated during the construction of the tunnels and used for landscaping on the south portal site. Once construction is complete, this will help create around 90 hectares of wildlife-rich chalk grassland habitats.
Chalk grassland used to be widespread across the hills of south east England and are considered habitat of international conservation significance with just 700ha left across the Chilterns.