This work forms part of the research programme into the properties of colliery discards undertaken in Durham since the Aberfan disaster of 1966. The discussion relating to residual strength in the paper 'Colliery Spoil Tips - after Aberfan' was summed up by the authors, McKechnie Thomson and Rodin, with the words "The information currently available on residual strength poses a number of questions and further studies are needed to clarify such factors as the effect of normal pressure and magnitude ofstrain". The current work attempts to clarify some of these problems. Large strain (1.5 to 2.5 metres total displacement) 12-inch shear-box tests have been undertaken on selected samples from different National Coal Board coalfield areas. Supplementary programmes of tests in a 60mm shear-box were used to confirm the results of the large-scale tests. Similarly, additional tests have been conducted at normal stresses which are generally higher than those customarily adopted for normal soils testing. The results show that at low normal stresses (80 kN/m (^2), shear strength reduction in most discards is limited (15 - 25 per cent) for a displacement of one metre. The exception is extreme low rank material from the West Midlands coalfield, for which a strength reduction of over 40 per cent was obtained. Tests revealed a marked increase in the rate of shear strength reduction for most discards at a normal stress value between 200 - 300 kN/m (^2). Above 500 kN/m (^2) increase in the rate of breakdown is limited. Correlation of shear strength characteristics with the rank code number of the associated coal show generally negative results, the exception being extreme 'low rank' discard from collieries in the West Midlands. Statistical treatment of the results has revealed significant correlations between the shear strength properties and the grading parameters. In practical terms, vehicular activity is unlikely to cause shear planes to develop in the majority of new or existing tips during emplacement or regarding operations
