Some landslides move imperceptibly downslope, whereas others accelerate catastrophically. Experimental landslides triggered by rising pore water pressure moved at sharply contrasting rates due to small differences in initial porosity. Wet sandy soil with porosity of about 0.5 contracted during slope failure, partially liquefied, and accelerated within 1 second to speeds over 1 meter per second. The same soil with porosity of about 0.4 dilated during failure and slipped episodically at rates averaging 0.002 meter per second. Repeated slip episodes were induced by gradually rising pore water pressure and were arrested by pore dilation and attendant pore pressure decline.

ABSTRACT. A ring-shea r device was used to study the factors that co ntrol the ultimate (steady) streng th of till at high shea r strains. Tests a t a steady strain ra te and at different stresses norm al to th e shearing direction yielded ultim ate friction angles of26.3° and 18.6° for tills conta ining 4% and 30% clay-sized particles, respectivcly. Other tests at steady normal stresses a nd va ri abl e shear-strain rates indi cated a tendency for both tills lO weaken slightly w ith increasing stra in rate. This weakening may be due to sma ll increases in till porosity.Th ese res ults provide no evidence o f v iscous behavior and suggest that a Coulombplas ti c idealization is reasonable [or till deformation. Howeve r, vi scous behavior has often been sugges ted on th e basis of di stributed shear stra in observed in subglaci a l till. We hyp ot hes ize that deform ation may becom e distributed in till th at is deformed cyc licall y in response to flu ctu ations in basal wa ter pressure. During a deform ation event, transient dil ation of discre te shea r zones should cause a reduction in intern al pore-water pressu re that should streng then th ese zo nes rela tive to the surro unding till, a process call ed dilarant harde ning. Consequent changes in hea r-zo ne positi on, when integrated over time, may yield the obse rved di tributed stra in.

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Quarrying rate from adhesive wear theory: Equation 2 of the articleErosion of solid surfaces in sliding contact is referred to as "wear" in materials science. Owing to asperities (bumps) on solid surfaces, the real area of contact along the sliding interface is less than the total area. The theory of adhesive wear uses the frequency of asperity-junction formation during slip and posits a probability k of asperity breakage at a junction to determine the volume of wear fragments Q v :where x is the slip displacement, N is the number of asperity junctions formed per unit distance of slip, and V e is the volume of material eroded if an asperity breaks, which depends on the load normal to the slip surface, the hardness of the material, and the asperity geometry (Rabinowicz, 1965). In materials science applications involving microscopic asperities, k is a wear constant. If a specific asperity geometry and spacing are assumed, the value of k can be determined through experiments in which solids are slid past each other and surface erosion is measured (for example, k for metals is typically 0.5-160 ×10 -3 ) (Rabinowicz, 1965).For the two-dimensional stepped glacier bed of the model (Fig. 1), with steps of tread length L, the total number of steps is n s = L T /L, where L T is the total length of the bed. For each unit distance of displacement, ice will traverse a step 1/L times, so that N = n s /L, or

ABSTRACT. The field observations of G.S. Boulton stimulated widespread interest in deformable beds. Shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure. During unsteady deformation, pseudo-viscous shear resistance can be caused by dilation of consolidated tills and resultant pore-pressure decline. This effect is probably uncommon, however, because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective-pressure transients. Stick-slip motion at Whillans Ice Stream, Antarctica, indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip. Recurrence intervals for rapid-slip episodes there (6-18 hours) indicate that till-strength variations, if driven by changes in pore pressure either related or unrelated to basal freezing, are focused in the uppermost several centimeters of the bed. Ploughing of grains at the bed surface and associated excess pore pressures in adjacent till can account for rate-weakening during rapid slip, with pore-pressure decay causing strengthening between slip episodes. By promoting shallow, sluggish subglacial water flow and low effective pressure, soft beds may help sustain themselves by slowing their own transport. Soft-bed shear resistance, kinematics and continuity are problems rooted in subglacial hydrology.

Subhourly measurements of bed deformation, bed shear strength, subglacial water pressure, and surface speed at Storglaciären, a glacier in northern Sweden, showed that the shear-strain rates of the bed decrease during periods of high water pressure and surface speed. High water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing. The instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation, as has been commonly thought.

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Water-pressure fluctuations beneath glaciers may accelerate rock fracture by redistributing stresses on subglacial bedrock and changing the pressure of water in bedrock cracks. To study the potential influence of water-pressure fluctuations on the fracture of subglacial bedrock, ice flow over a small b edrock step with a water-filled cavity in its lee is numerically modeled, and stresses on the bedrock surface are calculated as a function of transient water pressures in the cavity. Stresses on the bed are then used to calculate principal stress differences within the step. Rapid reductions in cavity water pressure increase principal s tress differences in the bed, increasing the likelihood of crack growth in the step and the formation of predominantly vertical fractures. Relatively impermeable bedrock may be most susceptible to fracturing during water-pressure reductions b ecause high water pressure in cracks within the rock can be maintained, as water pressure decreases in cavities. These results, when considered in conjunction with the strong likelihood that increases in water pressure accelerate the removal of rock fragments loosened from the bed, suggest that in zones of ice-bed separation where waterpressure fluctuations typically are large, rates of quarrying may be higher than along other parts of glacier beds.