Summary

Field and laboratory investigations of the Walton's Wood landslide led, for the first time, to the determination of the residual strength of a clay by (1) tests on natural slip surfaces, (2) reversal shear tests on previously unsheared specimens, and (3) stability calculations. The shear strengths determined by all three methods were practically identical, and could be represented by the parameters cr′ = 0, r′ = 14°. The strength of the clay in its ‘fully-softened’ or ‘critical’ state was approximately c′ = 0, ′' = 20°. The much lower residual strength is due to the development of a continuous slip surface at large displacements and re-orientation of clay particles along the slip surface.

The landslide had probably been initiated by the erosion of an ice-marginal drainage channel during a retreat stage of the Last (Weichselian) Glaciation. The consequent oversteepening of the hillside, in mudstones of the Upper Coal Measures, led to landslips and rapid weathering, so that eventually the slope became blanketed by a thick layer of colluvium inclined at an angle of about 11° to the horizontal. Final equilibrium had not yet been attained when, in 1961, construction of a motorway embankment began on the old landslide. Failure of the partially built embankment occurred. A description is given of the remedial measures which enabled the motorway to be completed at this locality in November 1963. Typical index properties of the clay in the landslide mass are: w = 29, LL = 57, PL = 27, clay fraction = 70 per cent. The predominant clay mineral is kaolinite.

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