The Stability of Natural Slopes

Abstract

URING recent years research into the stability of foundations and earthf works has been carried out at the Building Research Station of the Department of Scientific and Industrial Research. This work has covered a wide field in civil engineering construction and has also been applied to problems of a larger geographical-geological scale, such as landslips, coast erosion, and river bank erosion. The object of this paper is to present, in the light of this recent research, a general picture of the factors which control the movements of natural slopes, with particular reference to those examples which have been studied by the Building Research Station. In terms of geological time it may be said that no slope is perfectly stable. The problem is largely a question of degree of stability; slow movements may continue almost unnoticed in a lifetime, but over a long period they accumulate and mould the topography; more sudden movements, recurring at inter? vals, may be catastrophic and seriously disturb man and his works. It is largely these latter movements that the civil engineer has studied. The subject appears to have developed along two paths, one followed by the geologist and geographer, and the other exploited by the engineer; both have strayed a little and the ground between has never been completely cleared. The former has traversed most of the earth and described the scenery and site conditions, but has recorded few detailed measurements. The latter has cut his own path, disturbed nature, and kept slopes.to a maximum to save his labours, but deep in his cuttings has tended to forget the scenery and site conditions. Frequently he has had to return to deal with landslips, and maintenance has been costly, but through these experiences he has accumulated detailed measurements of his failures and has recently evolved a rational method of design. The geologist and geographer would benefit from a knowledge of the engineering methods and could assist by taking measurements on a large scale. They would then begin to understand more clearly some of the most important processes in the history of the earth's crust. The engineer could equally profit from a fuller appreciation of the longterm view of the geologist and consider the site conditions and changing climate in relation to his works. Whereas the geologist and geographer accept landslips without any sense of responsibility as a natural process, the engineer is aware that he may have created instability and because of this responsibility he has been obliged to develop methods of estimating slope stability. With the introduction of the railways, and their restricted gradients, the engineer started to make widespread large cuttings and embankments, and the history of railway construc? tion abounds with records of slope failures. The earlier canals with their numerous locks and contour planning had generally avoided deep cuttings or embankments. All this construction produced very little advance towards a rational method of estimating the stability of slopes. It was not until the early