WJ provided geotechnical and groundwater control for the Conwy Crossing, the UK’s first major immersed tube tunnel, carrying the A55 across the River Conwy estuary. Opened in 1991, the project required excavation, groundwater control, and innovative dewatering techniques to manage complex geological conditions.
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The project involved three large excavations:
Dewatering and pore pressure control were critical to ensuring excavation stability, preventing groundwater ingress, and limiting tidal seepage from the estuary.
A combination of groundwater control techniques was employed:
A trial of 15 ejectors in the northeast corner of the casting basin confirmed effective dewatering in glacial lake deposits. It also highlighted that the underlying rockhead functioned as a recharge boundary rather than an impermeable layer. The system successfully maintained the required drawdown levels and excavation stability.
The project was the first ever large scale application of ejector dewatering in the UK and the project demonstrated the effectiveness of the system for large excavations in challenging ground conditions. The innovative approach allowed for successful tunnel construction with minimal seepage or instability, ensuring the long-term durability of the structure.
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