The influence of diagenetic and mineralogical factors on the breakdown and geotechnical properties of mudrocks

Abstract

AbstractMudrocks comprise fine-grained, sediments, in which the modal grain size is <0.063 mm and clay minerals are often major constituents. In geotechnics the term defines a generic group of argillaceous lithologies ranging from stiff clay-soils to strong, partly metamorphosed rocks. In the UK outcrops occur extensively and they also lie concealed beneath later deposits. In engineering applications mudrocks can present challenging forms of behaviour, including rapid deterioration and structural breakdown during sampling and preparation for tests. Attempts to measure their geotechnical properties are often frustrated and misleading results appear in the literature. A review of the engineering properties of UK mudrocks and studies involving mudrocks of varying induration demonstrates the importance of composition and genesis in controlling physical behaviour and provides a framework for understanding variation in mudrocks. Various index tests have been appraised in terms of their value for predicting mudrock durability. It is newly proposed that style of breakdown provides guidance for predicting the mechanical properties and that simple index tests can supplant relatively expensive and time-consuming undisturbed sampling and testing, even where this is possible. This approach has potential for use in the prediction of fracture formation owing to changes in stress conditions and pore water pressure.