A Laboratory Study of Till Deformation

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Deformation of water-saturated sediment (till) beneath glaciers may, in effect, lubricate glacier beds. Such deformation has been measured beneath a number of modern glaciers and may also have contributed to the fast flow of Pleistocene ice masses with a possibly profound effect on regional climate. Support is requested to study with a rotary (ring-shear) device that shears a water-saturated till sample. Because the device can deform a large sample to infinitely high strains, it can be used to test a much broader range of hypotheses than traditional soil testing devices. For example, it has been used successfully to study stress concentrations in shearing sediment and the consequent evolution of till grain size. Special features of the device allow continuous observation of the distribution of shear strain, measurement of local stresses normal to the direction of shear, and isolation of wall effects. In experiments aimed at better understanding till rheology, both clay-rich and clay-poor tills will be sheared with the primary goal of determining the relation between deformation rate and the steady (residual) till strength. In some experiments the effective normal stress will be varied to investigate the role of transient pore-pressure gradients on the apparent till rheology. In a second set of experiments, the evolution of contacts, the preservation of structures (e.g. sublithified sand clasts), and the development of fabric will be studied to test field criteria for the identification of highly strained Pleistocene tills. In addition, grain communication will continue to be studied with the additional important goal of quantifying the progressive degradation of selected diatom species as a function of shear strain.