HS1 Ashford Tunnels

The HS1 railway between London St Pancras and The Channel Tunnel serves both national and international high-speed routes.

The railway connects London, Paris and Brussels as well as providing much improved domestic commuter services between London and Kent. This huge infrastructure project, one of the most complex in modern engineering times, demanded only the very best solutions to often challenging problems. WJ’s innovative wellpoints and ejector system proposals for the Ashford Tunnel elements gave Union Railways the confidence to commission us.

Our primary focus was on the design and construction of a 1.3km ‘cut & cover’ railway tunnel, which also included approach retained cuts. With this solution drawn up by our strategic planning team, we got to work on the side support of the tunnel with continuous piles and an excavation depth of between 10 to 18 metres. A key challenge in ensuring the success of our approach was how to manage the prevailing ground conditions. Comprising Hythe Beds sand overlying Atherfield and Weald clays, WJ’s accurate measurements identified pore pressures 2 – 3 m below ground level. By reducing the extent of temporary propping during the construction phase, we were able to effectively control the pore pressures in the Weald clay. This, in turn, meant significant savings for the client.

The next phase was the installation of an array of 200 ejectors to depths of over 30 meters on both sides of the tunnel. Although we had accurately calculated that the discharge flows would only be between 2-5ltr per minute per well, we knew that this would be enough to achieve the target requirement of 10-15m reductions in the pore pressures. To compliment this, we used wellpoint drilling to directly control the groundwater levels in the Hythe beds.

WJ are proud to have been a part of delivering the UK’s first high speed rail system. From design to implementation, our solutions guaranteed the best possible environment for the construction work to continue.

• LOCATION: Ashford Kent
• CLIENT: Union Railways
• MAIN CONTRACTOR: Skanska Construction