Yorkshire Geotechnical Group – A63 Castle Street Improvement

Synopsis

A 400m long underpass is currently being constructed on the A63 in the centre of Hull. Problematic ground involving very soft clay, organics, glacial clays, running sand and a confined chalk aquifer with 5m tidal range introduce significant geotechnical risks to the scheme. The underpass is formed from diaphragm walls with a RC base slab, tied down with tension piles. A jet grout layer forms a sub-surface prop to limit deflection, improve base stability and limit groundwater ingress. Deep dry mixing was adopted to maintain stability and limit ground movement during diaphragm wall construction.

The project’s ground risks were significant, from a cost, programme and reputation point of view. So rather than go into the construction with so much uncertainty, the trial was planned to happen while the design was being finalised, in advance of the main works

With Designer, Main Contractor, Specialist Contractor, Client and Industry experts working together as one team, all of the geotechnical elements were trialled. Some significant issues were encountered. Excellence in teamwork, foresight and thoroughness allowed solutions to be developed and assumptions validated during the trial. The knowledge gained from the trial has directly informed the design and execution of the mainline works, which will enable the delivery of a challenging project with much reduced risk and greater cost and programme certainty.

Richard will describe the scheme, it’s risks and challenges, the trial and how the outcomes were fed into the main construction. Providing perspectives from all sides of the project team.