The mechanical behaviour and critical state of glacial sediments

This research contributes to the debate on glacier dynamics, which have a significant effect on global climate change and sea-level changes. Glacier advances and retreats have great effects which can be viewed not only from their influence upon human and habitats within their immediate locality but their much more pervasive influence on all global habitats due to the effect of modern ice masses on global climate and sea-level. One of the most fundamental characteristics of glaciers and ice sheets is their ability to move. In the early models adopted to simulate glacier flow, glaciers were thought to rest on clean bedrock. However, borehole studies in different glacial areas revealed the presence of a bed of deformable sediment underneath some glaciers. Although the old common model assumed that glacier movement is generated by sliding processes, recent research proved that a high percentage of glacier movement is attributed to the shearing process of the saturated bed sediment. In the glacial environment, sediment production, deformation and deposition cannot be separated conceptually. Sediments produced by the direct erosion of lithified material by glacier ice typically contain particles spanning a large size distribution. Frequent interaction, between particles and between particles and the rigid bed, results in their substantial modification during transport. Inter-particle stresses are often high enough to cause fracture and abrasion of particles. The grain size distribution of sediments is a fundamental control on sediment deformation properties. The evolution of particle size distribution of glacial sediment, due to glacier movement, would result in an increase in the fine modes of their particles with the increase of transport distance, but there appears to be a lower size limit beyond which no further particle crushing occurs, regardless of transport distance. This complies with those findings which studied breakage potentials of some granular soils. Tests carried out on samples of a glacial sediment which were collected from Langjokull-Iceland, indicated that this sediment had been subjected to pervasive deformation underneath glaciers, which involved sediment particle breakage, resulting in this sediment reaching a terminal grading state such that no further significant particle breakage could be achieved under any further pressure or strains. The sediment behaviour accordingly was completely different to that expected for granular soils which follow a Critical State (CS) framework. The sediment exhibited a stiff behaviour represented by the lack of a clear yielding point in its compression curves, and no unique Normal Compression Line was observed, indicating that a transitional behaviour can be obtained from this sediment which has reached its terminal grading. On the other hand, a differently graded sample from the same sediment exhibited a behaviour similar to granular soils which deform following a CS framework, in which a clear yielding point, which is associated with particle breakage, and a tendency to form a unique Normal Compression Line is observed, an implication that the grading of the sediment is a key factor in defining the mechanical behaviour of the sediment. The research also addresses the rheology of glacial sediment deformation by investigating the influence of strain rate on its mechanical behaviour, and comparing the results obtained from Langjokull sediment with existing data of tests which were carried out on another glacial sediment which was collected from Ice Stream B-West Antarctica.