Marine geotechnical investigations on the Mersey

RSK’s geoengineering specialists Structural Soils helped to design and construct the Mersey Gateway bridge by providing marine geotechnical investigations. Right from the outset, the team knew the complex site, including its environment and isolated position, would provide plenty of technical and logistic challenges, so it engaged the wider RSK group of companies for its extensive skillset.

Difficult terrain; specialist tools

The dynamic site is a 1-km-wide tidal estuary with an irregularly shaped large sandbank in the centre. Deeper water surrounds the sandbank, which is submerged by high spring tides. The riverside on both sides is salt flats, and the nearest harbour is more than 20 km downriver.

Considering the timeframe and the site location, RSK applied a multidisciplinary approach. Deploying specialist equipment, including a jack-up platform and a spud-legged barge, made accessing the site and drilling easier. Using the jack-up platform, engineers drilled 28 boreholes in the deep-water channels and moved the platform according to the tidal cycle. Directly from the sandbank’s surface, engineers drilled boreholes through the sandbank using equipment positioned on the spud-legged barge. In many boreholes, in-situ, high-pressure rock dilatometer testing was undertaken incrementally as the boreholes progressed. Once completed, specialists began double packer permeability testing and geophysical logging, including optical and acoustic logging.

Transporting the drilling equipment to the site involved a 20-km journey upriver from the nearest dock. RSK appointed local contractors with good knowledge of the river and access to low draught vessels. Boats with outboard engines could not be used owing to a build up of sediment deposits on the riverbed, so a hovercraft and helicopter, operated by RSK Orbital, transported engineers from shore to the sandbank and the drilling platform. To ensure the highest level of environmental safeguarding, RSK implemented a wide range of control measures and contingency plans for potential pollution incidents affecting the estuary and undertook noise monitoring and abatement measures to ensure that the ground investigations did not set a poor precedent of noise impacts on nearby residents and businesses.

“I’d like to thank every single person who got us to this point, on what has been a long and sometimes challenging journey,” offered a delighted local official.