Strain Distribution in the Mam Tor Landslip, Derbyshire
E.H. Rutter, C. Arkwright and R.F. Holloway
Rock Deformation Lab., Dept. of Earth Sciences,
University of Manchester M13 9PL
e.rutter@man.ac.uk
The spectacular Mam Tor landslip, near Castleton, Derbyshire, formed about 3000 years ago on an oversteepened slope left after the last ice age. A section of the Namurian Edale shales and overlying Mam Tor sandstones has collapsed, leaving and 80 m high scar on the eastern side of Mam Tor. The slip is about 500 m long and 250 m wide and has the old main road from Manchester to Sheffield (unwisely) built across it. The slipped mass is in a state of continuous creep motion, which over the 180 years since the construction of the highway has led to extensive damage to the road, culminating in its closure in 1977. The landslip, with its spectacular road damage caused by movement on localized arcuate slips within the main mass, is heavily frequented by school and university field parties.
Since 1996 we have carried out an annual monitoring by electronic distance measurement of the movement of a network of some 30 stations on the slipped mass, in order to obtain a clearer picture of the creep motions within it. A previous study between 1991 and 1998, by Nottingham Trent University only used stations on the road, but we have been able to tie our survey into theirs, giving a complete record of displacements on the common stations from 1991 to 2002.
The average movement rate of the whole mass is about 20 cm per year, with the central region moving significantly faster, at almost 1 m per year. 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 is approximately equal to the local attitude of the basal slip surface. The development of surface morphological features (shear offsets on the highway, bulging of the road surface) reflect 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. Present day displacement rates are, however, clearly higher than during historic times. 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 used the methods of geodetic and glacier strain analysis to analyse the distribution of strain within the slipped mass. This revealed a pattern of continuous strain variations comparable to 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. This strain partitioning may explain the recent acceleration of movements in the central part of the slip, which may have been exacerbated by the local reforming of the topography associated with the original construction of the highway.