An investigation into the one-dimensional compression properties of the reconstituted and intact blue clay of the Maltese islands

Abstract

Stress-strain relationships allow analysis of various materials to predict their behaviour when stress is imposed. In the case of soil, such constitutive relationships are complex as there are many characteristics which affect mechanical behaviour. A one-dimensional tension state does not exist for soils, but the equivalent state in compression is of utmost validity and importance. One must also understand and appreciate that structural materials can be engineered for the given project. That is, for the required design, the material is engineered to balance safety, costs and complexity of construction to achieve an efficient and feasible design. The materials making up the Maltese Islands have been studied extensively from a geological perspective, resulting in a qualitative description of the various strata. However, there is yet much to be discovered from a geotechnical point of view. This type of analysis allows for a quantitative understanding of the mechanical characteristics of the stratigraphy of the Maltese Islands. The following work investigates the Blue Clay formation of the Maltese Stratigraphic sequence from a geotechnical point of view. This material is peculiar by way of its relatively soft nature and also by its impermeable nature, which implies that, deformation under applied stress can be substantial and that it will occur over extended time periods as pore pressures are slowly dissipated. Time is therefore of utmost importance in considering the behaviour of this natural material. The influence that structure has on the strength of the clay is investigated in this dissertation since the influence it is known to affect the compression and shear characteristics of a soil. This is done by comparing clay samples to a base material – the reconstituted equivalent. Two different samples were obtained from different depths in order to represent the weathered and unweathered versions of the Blue Clay. Even though they belong to the same stratigraphic layer, the materials are expected to behave differently due to variances within the structure emanating from stress history subjection and the exposure to climactic conditions. Clay samples collected were subjected to several tests in order to determine index and compression properties of the specimens. They are also analysed briefly from a microstructural perspective in an attempt to understand how fabric and bonding both effect the mechanical behaviour of clay materials. Results from the various tests and investigations are compared to theoretical expectations, and other materials investigated by various authors. Conclusions relate to the composition of the Blue Clay (such as the degree of activity, and the relationship between carbonate content and plasticity index), and the compression characteristics (such as behaviour beyond yield, and the effect of structure in samples).