The Mam Tor landslide, North Derbyshire

Abstract

The ancient but still active landslide at Mam Tor, in Namurian mudstones, is a massive example of a slump-earthflow. It has a length of 1000 m, a maximum width of 450 m, and an average slope of 12 degrees from the toe to the foot of the back scarp. The upper, slump sector has moved about 160 m on a curved slip surface within a shear zone of brecciated clay; the shear zone, 2 m thick and in places at a depth of 30 m, lies above hard mudstone of the Edale Shales; weathered mudstone and any superficial deposits, and several metres of unweathered mudstone, having been removed by shearing and incorporated in the slide debris. By contrast the lower, earthflow sector, which is up to 18 m in thickness, has spread downslope by at least 400 m with little disturbance of the original ground. The landslide probably started about 3600 years ago as a sudden large slip in the steep hillslope. Degradation and softening of the slip mass would rapidly lead to secondary slips and initiation of the earthflow. Subsequent movements can be attributed to a succession of slips, becoming smaller with time, in response to winter rainstorms. Radiocarbon dating of an Alder root in fossil soil beneath the earthflow demonstrates that the cumulative effect of such slips has resulted in the landslide toe advancing 320 m during the past 3200 ($\pm $200) calendar years. A static analysis of stability shows the slide to be in a state of limiting equilibrium under normal winter groundwater levels, with a residual strength on the slip surface represented by an angle of shearing resistance $\phi _{\text{r}}^{\prime}$ = 14 degrees; a value in good agreement with tests on clays of similar composition. However, records of movements in the present century indicate that slips leading to displacements typically of about 0.3 m are still taking place, an average at four-year intervals, in winter months with more than 200 mm rainfall. An analysis of the mechanics of these `storm-response' movements is given in terms of the transient rise in water level and the reaction of residual shear resistance to increasing rates of displacement. Some other large Pennine landslides in Namurian mudstones are known to be considerably older than the Mam Tor slide, and on examination they are found to be permanently stable. By analogy with Mam Tor, this condition has been attained after a long sequence of secondary slips leading progressively to more stable configurations in which reactivation occurs only in heavier (and less frequent) rainfall, until finally the slide mass remains stable even in the most severe rainstorms. The timescale for this process to be completed appears to be of the order of 8000 years.