Abstract

The spectacular Mam Tor landslip, near Castleton, Derbyshire, formed over 3000 years ago on an oversteepened slope left after the last ice age. A section of the Namurian Edale mudstones and overlying Mam Tor sandstones has collapsed, leaving an 80 m high scar on the eastern side of Mam Tor. The slip is about 750 m long and nearly 300 m wide and has the old main road from Manchester to Sheffield built across it. The slipped mass is in a state of year-on-year creep motion, which over the 190 years since the construction of the highway has led to extensive damage to the road, culminating in its closure in 1979. Since 1996 we have carried out annual monitoring by electronic distance measurement of the movement of a network of some 30 stations on the slipped mass. The average movement rate of the whole mass is about 10 cm a−1, with the central region moving significantly faster, at almost 50 cm a−1. Thus substantial readjustments of mass are taking place within the landslip. Local vertical displacements are systematically related to horizontal offsets, and the ratio of the two allows inference of the local attitude of the basal slip surface. The development of surface morphological features (pressure ridges and irregular topography, shear offsets on the highway, bulging of the road surface) reflects the lateral variations in displacement rate. Comparison of our survey of the present-day road position with that recorded in the topographical survey of 1880 shows a total 40 m downhill displacement of the highway over 122 years. This is consistent with extrapolation of our measured displacement rates back to 1880. Displacement rates within this period are, however, clearly higher than during the past 3000 years. There is a fairly clear correlation between vertical and horizontal displacement rates and annual variations in rainfall, with accelerated displacements following winter rainfall above a critical threshold level. Using survey points to define nodal points of a network of triangles, we have analysed the distribution of strain within the slipped mass. This revealed a pattern of continuous strain variations comparable with that found in flowing glaciers. In the lower part of the slip, horizontal strains are contractional and triangle areas are decreasing, causing some uplift of the ground relative to the general downhill flow on the basal slip surface. In the uphill part, strains are extensional and triangle areas are increasing. The new data allow an estimate of the time-dependent rheological behaviour of the sheared mudstone in the basal shear zone. The shear stress vs. shear strain rate relationship is very non-linear, with significant shear strain rates occurring only at shear stress levels within 10% of that required for rapid (catastrophic) shearing.

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