The long-term stress-strain behaviour of chalk

Chalk is a weakly bonded calcareous rock primarily composed of skeletons of microscopic planktonic organisms. Its sedimentary highly anisotropic nature is reflected in its significant range of porosity, bonding strength and textural variation. These intrinsic properties trigger a complex short-term engineering response that has been the subject of numerous studies, whilst its long-term response has attracted lesser attention. A long-term laboratory test programme of conventional compression and extension tests was carried out (duration: 8 to 22-months) in a purpose built environmentally controlled facility, with specially designed loading frames and modified triaxial cells. Furthermore, index as well as triaxial and unconfined tests were also conducted in order to establish the short-term mechanical response. The results were compared and complimented with those of similar studies. In addition, Scanning Electron Microscope (SEM) techniques were employed in an effort to investigate the micro-mechanical response. This led to the formulation of a conceptual micromechanical model associated with the short and long-term deformation of chalk. It was concluded that each variety of chalk has a differing short-term failure envelope controlled by factors such as grain bonding, mineralogy and fabric texture. It was also demonstrated that chalk is not an ideal elastic material and its pseudo-elastic response was associated with the onset of creep. Creep strains appeared to trigger an ageing process that produces elevated post-creep strength and stiffness irrespective of the applied stress path. This was associated to an increase in the frictional interlock of the material. The SEM study revealed a pervasive fabric evolution throughout the post creep/aged samples. This suggested a more uniform stress distribution within the longterm tested samples, than that occurring in the short-term tested samples. The only .discernable effect that could be attributed to the applied stress paths during creep testing was related to the magnitude of the creep rates.EThOS - Electronic Theses Online ServiceGBUnited Kingdo