Aerial view of tunnel site

Excavation

Tunnel section

• Clients
  • Union Railways
• Main contractors
  • Skanska UK
• Consultants
  • Rail Link Engineering
  • (Arup Bechtel Halcrow Systra)
• WJGL roles
  • Dewatering subcontractor
• Project reference
  • J545

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The main contract involved constructing 1.3 km of cut and cover railway tunnel and approach retained cuts. Side support was by contiguous piles and excavation depth was 10–18 m. Ground conditions comprised Hythe Beds sand overlying Atherfield and Weald clays with pore pressures 2–3 m below ground level. Control of pore pressures in the Weald clay allowed significant savings by reducing the extent of temporary propping required during construction. WJ installed an array of 200 ejectors to 30 m depth on both sides of the tunnel. Discharge flows were only 2–5 l/min/well but this was sufficient to achieve the target requirement of 10–15 m reduction in pore pressures. Wellpoint dewatering was used to control groundwater levels in the Hythe beds.

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