Solifluction shears at Carsington, Derbyshire

Abstract

A clayey solifluction deposit of Devensian age on the lower slopes of a broad valley in Namurian mudstone is about 1.2 m thick and near its base are shear surfaces, gently undulating and occupying on average 40% of the length (56 m) examined in trial pits. Within a shear zone up to 2 mm in thickness the clay mineral aggregates are aligned sub-parallel to the shear surface; otherwise the aggregates are in random orientation. Tests show that strength on the shear surface falls to the residual value (c′ = 0, φr′ = 12°) after small displacements. The resultant peak strength parameters of the deposit, allowing for the undulating and discontinuous nature of the shears, are c′ = 6 kPa and φ′ = 18°; as compared to c′ = 10 kPa and φ′ = 20° for the intact clay. Average index properties are w = 40, LL = 75, PL = 32 and clay fraction = 62%.

Engineering geology of Quaternary soils: I. Processes and properties

“…Skempton et al (1991) have carried out an extensive study of the Carsington (Derbyshire) dam failure and movement along a shear zone has also been recognized by Spink (1991) but at a greater depth.…”

“…Some of these features have been attributed to the shearing of the at University of Iowa on May 24, 2015 http://egsp.lyellcollection.org/ Downloaded from Both Skempton et al (1991) and Spink (1991), rely upon the development of residual shear strength as opposed to peak strength as the controlling factor in the sliding failure mechanism of slopes. This is a realistic view as many investigations have shown.…”

Macro‐ and Micromorphologic Interpretation of Relict Periglacial Slope Deposits from the Holy Cross Mountains, Poland

“…The individual slip planes/shear zones associated with the detachment at the base of the diamicton are defined by the range of microstructures observed, including striated b‐fabric with a parallel arrangement of grains, granostriated b‐fabric and ‘turbate’ structures. Previous studies show that such planes are characterised by striated b‐fabric and exhibit a linear or curvilinear arrangement (Skempton et al ., ; Harris and Lewkowicz, ). Features caused by rotation, such as ‘turbates’ and granostriated b‐fabric, mark the initial stages of deformation (e.g.…”

“…Harris and Ellis, 1980;Harris, 1998;Van Vliet Lanoë, 1976, 1998Bertran, 1993;Elliott, 1996;Bertran and Texier, 1999). The micromorphology of deposits formed as a result of active-layer detachment, however, has only been described by Harris and Lewkowicz (1993a) in the Canadian Arctic and by Skempton et al (1991) in Derbyshire, England. Additionally, the deposits formed by a shallow landslide in the French Alps were described by Bertran and Texier (1999).…”

7. Exploration

“…In addition, mass movements under periglacial conditions resulted in the clays being permanently reduced in mass strength. The particular features that may be observed, and the associated weathering effects are summarized in Spink & Norbury (1993); an example of the result of not assessing such features occurred at Carsington Dam on residual Carboniferous Mudstones (Skempton et al 1991).…”