The engineering geology of the lower greensand of south-east England : with particular reference to the microfabric, geotechnical index properties and shear strength characteristics

Abstract

This is an Engineering Geological study of the arenaceous members of the Lower Greensand (Lower Cretaceous) of South-East England. The investigations carried out were of the microfabric, geotechnical index properties and shear strength characteristics of the Folkestone Beds, Hythe Beds and Sandgate Beds. These tend to fall in the interesting transitional area between sands and sandstones. The Folkestone Beds are quartz rich, medium to fine-medium sands that contain iron-oxide and kaolinite cement. They have been subject to mechanical compaction which has led to grain re-arrangement, increases in the number of straight contacts and porosity reduction. Further reductions in porosity and increases of straight contacts were due to chemical compaction by pressure solution. Decreases in Tangential Index (the number of tangential contacts amongst all grain to grain contacts) result from increases in compaction. The interlocked nature of many of the sands results in a significant cohesion while some cohesion is also provided by the cementation. Large gains in cohesive intercept values, obtained by direct shear testing, occur as the Tangential Index reduces below 34%. By contrast, increases in cohesion with cement contacts is of a lesser order. In the sands that contain significant proportions of cement, compaction processes have been inhibited. This has led to increases in the Tangential Index and decreases in cohesion due to interlocking. Two of the sands contain little intergranular cement, are highly interlocked and classed as `locked' sands. The majority of the Folkestone Beds are classed as slightly cemented sands. The Hythe Beds are fine grained, glauconitic sandstones which contain quartz cement in the form of syntaxial overgrowths. They have been subject to both mechanical and chemical compaction processes resulting in increasing numbers of straight and concavo-convex contacts and decreases in porosity. Chemical compaction leading to pressure solution is the probable source of the quartz cements. The Hythe Beds with quartz cement have the highest uniaxial compressive strengths and increases in quartz cement content correspond to increasing strength values. The Sandgate Beds are glauconitic and calcareous cemented sands and sandstones. They contain little quartz cement but significant amounts of calcitic cement. The cement content in these sediments is associated with higher values of Tangential Index and is presumed to be due to the early cementation inhibiting the development of an interlocked fabric. The higher Tangential Index, despite the calcite content, results in lower uniaxial compressive strengths demonstrating the importance of interlocking to the development of strength.

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