Vertical settlements at various depths above the centre-line of a shallow caving longwall face, were measured by levelling and by using an anchor extensometer system installed in two underground boreholes drilled from abandoned pillar workings 75m above the face. Differential settlement between the upper and lower levels was found to be greater than expected and this is attributed partly to the presence of strong, good quality sandstone strata in the Coal Measures cyclothem. Considerable damage and collapse of workings at the upper level occurred 30-4Om after passage of the face at the lower level and coincided with the predicted zone of peak tensile strain.
Design of support systems for faces and access roadways for longwall coal mine workings requires a detailed understanding of the mechanisms of ground deformation and yield around and particularly above collapsing longwall excavations. Whilst various analytical (1,2) , statistical (3) and empirical (4) methods have been proposed to describe the phenomena associated with this deformation, very little experimental evidence has been obtained in sufficient detail to allow an accurate description of the mechanism of deformation. In particular the effect of the geology and geotechnics of the overlying strata has in many cases been ignored. At Lynemouth Colliery (5) in Northumberlandt he first face of a new undersea development in the Brass Thill seam 75m beneath existing pillar workings in the combined High Main and Main seam, presented an ideal opportunity for installation of instrumentation (6) to observe strata deformation above a 18Om long retreating longwall face. The depth of the workings was approximately 200m below mean sea level, and 160m below sea bed, at a distance of approximately 5km due east from the shoreline. The extraction height was 1.6m and the roof was caved behind standard Gullick- Dobson 180 ton 5-leg chock supports.