A case history is presented illustrating the role of engineering geology in the construction N of Cardiff of some 6.6 km of motorway cuttings in glacial moraine and bedrock of Lower Old Red Sandstone age.

The proposed cut at Wenallt, located at the foot of Wenallt Reservoir, had featured prominently in the Public Inquiry. Subsequently this proved to be the only location at which a large failure occurred. Differences between the assumed and actual geological conditions are described. Due to the geological structure and the low residual strengths of c′ = 5 kN/m2 and ϕ′ = 10.5° in the Devonian marl, the only practical solution involved major earthworks to remove the slipped material. The excavated mass was partially replaced with a toe berm which incorporated a variety of drainage measures.

Experience from this contract has substantiated the need for detailed geotechnical monitoring during construction of major cuts formed in folded and faulted soft rocks. This enables checks to be made on design assumptions, permits revised assessments of stability and facilitates redesign if required.

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