Multi-scale laboratory characterisation of soft calcareous mudstone

Abstract

The construction industry of Abu Dhabi is thriving and its coastline has some of the most ambitious structures in the world. Whilst the sub-surface evaporitic and calcareous soft rocks of this region are of great geological interest, they are relatively poorly understood from a geotechnical engineering perspective, forcing foundation designs to be overly conservative. Understanding a lithology’s’ stiffness at small strains is of great importance for the accurate estimation of ground movements around excavations and foundations, and yet post-SI laboratory testing programmes in this part of the world tend to centre around basic rock mechanics practices such as UCS tests. These procedures are generally unsuitable due to the friable and moisture sensitive nature of the native lithologies, and rarely obtain parameters representative of in-situ behaviour. Measuring the development of local strains on samples is particularly problematic due to these physical attributes. The calcareous mudstones of this thesis have mechanical and structural characteristics falling between that of soil and rock and as such require a geotechnical testing approach that combines methods from both soil and rock mechanics disciplines. A custom framework has been developed that acts as a methodology for the laboratory testing of a soft calcareous mudstone. The mineralogical, micro-structural and mechanical characteristics of this lithology are examined via a suite of testing techniques, including XRD, SEM, advanced triaxial, DIC and Bender Elements along with industry standard procedures. Shearing, tensile and consolidation behaviours are explored. Examination of the micro-macro scale features of this material show it to be highly structured, with strength and stiffness being controlled by inter-granular bonding of Dolomite grains, as well as by mean effective stress state and rate of strain. The presence of fibrous Palygorskite acts to reduce the degree of bonding, causing specimens rich in this clay mineral to behave more mechanically ductile.