Experimental studies of the influence of entrained sediment on ice friction

The importance of entrained sediment in controlling the resistance to glacier sliding has been highlighted recently by the results of a novel set of field studies [e.g. Iverson et al., Science, 2003; Cohen et al., J. Geophys. Res., 2005]. Here, we report on a series of simple table-top experiments in which we measured the effective dynamic friction coefficient of laboratory-grown ice with entrained particles of sieved lithic sand pulled across a smooth glass substrate. We measured the effective sliding resistance of ~ 107 mm diameter cylindrical specimens at fixed normal load as a function of particle concentration and grain size. To minimize stick-slip behavior, sliding experiments were conducted at relatively rapid velocities of up to 0.72 m/s, and low normal stresses of order 300 Pa. Experiments using fine sand (0.25-0.42 mm diameter) with particle concentrations ranging between 1 and 10 percent by weight showed no significant variation in effective friction coefficient: the ratio of frictional resistance to normal load. Experiments conducted at constant particle concentration exhibited an increase in the effective friction coefficient with grain size; at 10 weight percent sediment loading the frictional force divided by the normal force for very fine sand (0.125-0.25 mm dia.), fine sand, medium sand (0.42-1.0 mm dia.), and coarse sand (1.0-1.4 mm dia.) were 0.40± 0.08, 0.49± 0.08, 0.58± 0.09, and 0.64± 0.14 respectively (± 1σ). These preliminary results confirm that even relatively small quantities of sediment significantly alter the effective frictional resistance. However, under laboratory conditions we find that the effect saturates at sediment loads lower than 1 percent by weight. Efforts are ongoing to constrain the effect of basal melt rate, which was held approximately constant in the foregoing experiments at 0.064 cm/minute. We discuss the implications of these results to our understanding of the mechanisms controlling the frictional behavior of dirty ice and models of glacier sliding.