Engineering properties and slope stability of lower coal measures rocks from monk wood cutting,Unstone-Dronfield by-pass,Derbyshire

This 30 m deep cutting has been the subject of a detailed geotechnical analysis, in view of mass-sliding wedge failures which had occurred in cuttings in similar geological situations in other parts of Britain.The sequence of rocks includes two coal seams, the upper being thin (0.3 m) and the lower thick (2 m), the latter with a recognisable seatearth (underclay). There is a thick sandstone at the top of the succession, with siltstone, mudstone and black mudstone also present.Both field and laboratory testing was carried out. The most probable failure mechanism within the more competent sandstone sequence to the south of the site would probably have been in the form of wedge failures on pre-existing discontinuities within the rock mass. It was necessary to evaluate the shear strength which could be mobilised along these planes and a field rock shear box was used. Drained-soil shear tests were also made on potential failure-surface materials, namely seatearth and clay shear zones. Weathered mudstone samples were tested to provide design parameters for the analysis of rotational slips. Additional laboratory testing carried out comprised bulk density, classification tests, slake durability and point-load indices. X-ray diffraction analyses were made on a range of materials such as seatearth, mudstone, shear zone and sandstone bedding-plane shear surface. This extensive test programme enabled parameters to be accurately determined for use in stability analyses. The latter show the critical effects of water conditions on the factor of safety.The performance of the cut has been monitored since early 1974 and no recordable movement has been noted.