HS2 has completed the first tunnel cross passages. Credit: High Speed Two Ltd.

UK-based High Speed 2 (HS2) has concluded work on the first of 38 underground connections between the northbound and southbound tunnels.

The milestone was attained after tunnelling machines Florence and Cecilia under the Chilterns neared the four-mile mark.

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Work on HS2’s longest tunnels extending ten miles under the Chilterns entered a new phase with the commencement of work on the 38 passages.

The Chiltern tunnels will enable operations of high-speed trains between London and the North with maximum speeds of 200mph (320km/h).

The trains will run through two parallel tunnels connected by short passages for use in emergencies.

A remotely controlled excavator was used by the miners’ expert team to break out of and excavate from one running tunnel to the adjacent tunnel.

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The ground is supported using a sprayed concrete lining (SCL) as the excavation is advanced.

HS2’s main works contractor Align is responsible for works on each passage, which is between 15 and 20 metres in length.

After the SCL-lined tunnel was finished, a water-proof membrane has been deployed followed by a secondary concrete lining built by putting concrete behind formwork installed in the cross passage.

HS2 tunnel engineering head Martyn Noak said: “HS2 is making huge progress, with 25,000 jobs supported by the project, construction in full swing between London and Birmingham and now the start of this new phase of tunnelling work under the Chilterns.”

Last month, HS2 completed a one-mile dig under Long Itchington Wood in Warwickshire utilising a 2,000t custom-made tunnel boring machine known as Dorothy.