A study of residual shear strength of Namurian shale in respect of slopes in north Derbyshire
This study deals with the classification and stability of number of landslips in North Derbyshire. The engineering properties of materials controlling stability are investigated with particular reference to residual shear strength. The topography of the study area comprises a series of valleys eroded into Carboniferous (Namurian) sandstones and shales. Landslippage occurs on valley sides where thick units of sandstone overlie comparatively weaker shale. Geomorphological studies using aerial photographs, topographic maps and field observations have served as primary data for establishing the style and geometry of movement together with the present day stability condition. The landslip at Mam Tor has been given special attention in view of its present instability. Two boreholes were drilled in which piezometers were installed and movements have been monitored using EDM equipment. The stability of each landslip as analysed iii terms of a factor of safety against movement for both first time, and subsequent movements, using the limit equilibrium method. An extensive investigation of the engineering properties of the mudrocks involved in the landslides has been carried out. These studies include determination of remoulded residual shear strength, grading, composition, and index properties of samples from boreholes, the quarry at Hope Valley Cement Works and surface outcrops. Both the ring shear and shear box apparatuses were employed and besides test method and sample pretreatment, other causes of variation of residual shear strength have been investigated. These include normal effective stress value, loading sequence, rate of displacement as well as sample composition, grading and classification. It is concluded that ring shear tests give lower residual shear strength values than the shear box method. Also shale disaggregated by tumbling in water produces a lower residual shear strength values than crushed material. It would appear that in conventional tests on indurated shale a high proportion of clay aggregations remain intact even when sheared. The study indicates that the mobilized shear strength in the field is little higher than the laboratory ring shear one for thoroughly disaggregated shale. However, it suggested that standard preparation methods and shear box tests yield erroneously; high residual shear strength for the indurated shale studied.