Preparation for the arrival of HS2’s first tunnelling machines has passed a milestone with the completion of the 17m high headwall and ground reinforcement at what will become the south portal of the 10-mile long Chiltern tunnel.
The site, just inside the M25 in North-West London has been excavated down so that the tunnel boring machine can slope downwards as it starts digging under the M25 and then on towards Birmingham.
In order to reinforce the ground behind the headwall, more than 636 ‘soil nails’ – some up to 20m long – were driven into the wall and connected to the concrete lining. This reinforcement is required to hold the ground and the surface of the wall in place as the TBMs begin to break through.
The two tunnel boring machines are due to arrive on site later this year and will launch early next year.
The ‘twin-bore’ Chiltern tunnels will be the longest and deepest tunnel bores on the route, with separate northbound and southbound tunnels and five ventilation shafts. Tunnelling is expected to take around 3 years to complete.
Once work is complete, the whole construction site will be landscaped with material excavated from the tunnels and trees planted in order to blend it in with the surrounding countryside. The huge tunnel facing wall will vanish underground and likely never be seen again.