Client: Anglian Water Services Limited
Framework: Long-term Framework Agreement with AWS (25+ years)
This project involved a 4.5 km flood relief tunnel along Clacton seafront. The scheme presented significant geotechnical challenges related to the stability of the London Clay cliffs and the presence of water-bearing granular deposits along the proposed tunnel alignment.
Challenges and Site Conditions:
The site presented significant challenges, with steep coastal slopes prone to landslips and variable clay strata with perched groundwater complicating stability. These conditions heightened community concerns over potential tunnel-induced instability.
Our Approach
- Completed 150 cable percussive boreholes (10–20m deep), many equipped with piezometers.
- Identified a subsurface water-bearing channel deposit and modelled its influence on tunnel stability.
- Installed extensive instrumentation (140 inclinometers, 70 pneumatic piezometers, 450 crack gauges) to monitor ground movement and pore pressure.
- Performed detailed settlement and slope stability analyses to confirm the safety of tunnelling beneath the cliff toe.
- Worked closely with Anglian Water and local stakeholders to communicate findings and address public concerns.
Outcome
The investigation and monitoring programme confirmed that the proposed tunnel would not impact cliff stability. This assurance enabled the project to proceed safely, mitigating risk and maintaining public confidence. The scheme was delivered on time and to budget, contributing to Anglian Water’s wider flood resilience objectives.