SOIL MECHANICS IN RELATION TO GEOLOGY

Abstract

Summary: Soil mechanics consists of the study of the physical and mechanical properties of uncemented deposits such as clays, silts and sands, and the application of these studies to the solution of civil engineering problems. The paper commences with a brief outline of the methods of measuring shear strength, and consolidation: the two most important mechanical properties of soils. Detailed consideration is then given to five problems concerning clays and silts where soil mechanics is able to contribute toward a more complete understanding of certain geological phenomena. Firstly the application of the Atterberg Limits and particle size analysis to the quantitative description of cohesive soils is discussed. A new term, the “activity” of clays is introduced, based on the above-mentioned tests, and it is shown that activity is related to the mineralogy and the geological history of clays. Secondly, a recent investigation of landslips in the boulder clay of west Durham is described. Two basic types of movement were observed, referred to as deep rotational slip and surface slips. Their mechanism is considered, and their geometrical proportions are compared with, and shown to be similar to, those of other land-slips in widely differing clays. The sequence of valley formation by landslipping is also considered. Thirdly, quantitative data on the consolidation (or compaction) of clays and silts are presented, which supplement those given by the author in a paper to the Geological Society in 1944. Next the questions of thixotropy and sensitivity of clays are discussed in the light of recent laboratory and field work, and it is shown that the extra-sensitive clays of marine origin (found in Canada and Scandinavia) exhibit only slight thixotropy; their unusual properties being chiefly the result of leaching out of soluble salts, subsequent to deposition. Finally the development of fissured structure in clays is briefly considered.