The Folkestone Bed sands: microfabric and strength

Abstract The Folkestone Beds formation is a marine shallow-water deposit of Cretaceous age. It consists mostly of poorly lithified sands which classify onto the sand/sandstone borderline, having properties neither akin to the classical concept of an engineering soil nor being strong enough to be labelled a rock . Intact samples were obtained by block sampling at 17 locations. Studies of the microfabric were made from thin sections prepared after epoxy resin impregnation. Although predominantly quartzose, the samples have a very wide range of grain sizes, size distributions, porosity and grain contact relations. The random inter-relationships amongst the microfabric parameters suggest that shelf sands, such as this formation, do not fit the patterns reported for deeply buried, and hence more diagenetically altered, sandstones. The majority are weakly cemented but samples from two locations possess negligible binding cement and are classed as locked sands . Measurements of the peak shear strength were carried out on air-dry samples of the intact sand using a 60 mm square, direct shear box. The peak shear strengths were strongly influenced by the intensity of interlocking: this factor being more significant than either porosity or total cement content. The samples possess a tensile strength, uniaxial compressive strengths up to 725 kPa and a cohesion intercept at zero normal stress in the shear box. The fabric cohesion reduces with increasing content of coarse sand and is very poorly developed in coarse sands.