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Sunderland safety factor

CORUS RAIL Consultancy (CRC) has carried out the ground investigation and design for a rock slope excavation on part of the Tyne & Wear Metro extension to Sunderland.

Sited close to a multi-storey car park, the excavation and steepening of a 50m long rock face on the Sunderland Direct extension from Sunderland to South Hylton was one of the main challenges on the 5.1km long project.

CRC produced a preliminary design for an anchor reinforced cutting and detailed the excavation stages while the site investigation was carried out. It showed that that the number and size of the anchors could be reduced.

The investigation, comprising two sub-horizontal and three vertical boreholes, revealed that ground conditions consisted of various grades of weathered dolomitic limestone, which gave difficulties with core recovery.

An optical televiewer was used down the boreholes to obtain discontinuity and dip information of fractures and slip planes within the limestone. This showed that only potential plane and small wedge failure geometries would pose a risk to the stability of the slope.

Designs were based upon increasing the factor of safety of the rock face against plane failure to an acceptable level and minimising any potential movement along rock discontinuities by installing anchors.

The staged excavation used two rows of anchors with a rock fall netting face and a catch fence at the toe of the excavation. Shotcreting was restricted to localised failures to minimise the environmental impact of the cutting.

The two-year, £98M design and build public-private partnership project to extend the Tyne and Wear Metro to Sunderland is funded by Railtrack, Government, Nexus (the Tyne & Wear Passenger Transport Executive) and a grant from the European Regional Development Fund.

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