1. Preliminary study

A Preliminary investigation of the stability problems of natural slopes in Southern England was carried out in 1968/69 using air photographs in conjunction with existing literature and a limited field study. General photoreconnaissance included the study of slopes consisting of the Inferior Oolite overlying the Lias south of Leicester, but with special attention to the scarp face of the Cotswold Hills between Broadway and Bath, the Lower Greensand overlying the Weald clay on the escarpment south and west of Sevenoaks in Kent and the Upper Greensand overlying the Gault on the Isle of Wight and in Dorset.

This study indicated that instability on slopes consisting of a clay overlain by a bed of significantly greater permeability was widespread enough to merit special attention. The following hypothesis was suggested to define the scope of further investigation.

2. Hypothesis and explanation

The hypothesis states:

wherever precipitation is sufficiently high, slopes consisting of a clay overlain by a bed of significantly greater competency and/or mass permeability, are likely to be actively degraded by mass movement (e.g. PL la-lf).

(In this context the following should be noted:

1. Competent and incompetent are used as relative terms. Competent materials are considered to be strong, to have a low deformability and to have brittle characteristics of failure. Incompetent materials are considered to be relatively weaker than competent materials, to have lower shear strengths, a higher deformability and to fail in a plastic way.

2. Mass permeability is taken to include both permeability and

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