Development of the Streethay Cutting design: from retained to open cut

Streethay Cutting is a 2.5km long cutting that lies approximately 1km to the east of Lichfield, Staffordshire. It is at the northern part of HS2 Phase One, beyond which the route continues north to Curborough where the route splits: the mainline interfaces with Phase Two; and a spur connects to the West Coast Mainline at Handsacre. Streethay Cutting was originally a retained cut at hybrid Bill. There were significant sections of diaphragm and secant piled walls up to 10m deep due to the quantity and control of groundwater, all of which would add significant capital cost and impact the programme. However, new GI data showed a greater proportion of mudstone versus sandstone across the length of the cutting, and additional groundwater monitoring data indicated more favourable groundwater conditions that are not hydrostatic from ground level. The updated ground model enabled the integrated project team (IPT) to have improved understanding of the groundwater infiltration rates, and to better understand some risks and uncertainties to be managed through detailed design.

This key information and further analysis have shown that a significant proportion of the cutting can now be changed from a retained cut to an open cut, removing the need for 1.2km of retaining wall. By reverting to the open cut, the design team has been able to realise significant cost and programme saving for HS2. This paper will cover the geotechnical constraints and challenges, as well as the wider issues that have been solved to change the design concept. The summary will include the environmental, sustainability and carbon savings, and some wider impacts made as a result of moving to an open cutting.