Bulk Friction Angles in Dry, Drained, and Saturated Gravel Beds

We examined the effect of capillary action and lubrication of grains on bulk friction angles through tilting chute experiments. In each experiment, we screed a bed of 5mm gravels in 65cm long x 18cm wide tilting chute with fixed roughness and slowly tilted the chute until a granular avalanche occurred. We performed these experiments under three conditions: with dry grains, with a bed that had been submerged and subsequently drained such that no water occupied the pore space, and with the entire apparatus submerged under water such that the bed is saturated. In addition, for each of these cases, we performed experiments with 5, 10, and 15cm bed thicknesses. In the dry case, the bed failed at ~ 41º, and bed thickness did not have a significant effect on failure angle. In the drained case, friction angles increased from 46.5º to 50.9º with increasing bed thickness. In the submerged case, the bed failed at angles not significantly different than those from the dry case, and they did not vary with bed thickness. The increase in friction angles between the dry and drained cases suggests that addition of the water induces a cohesive effect on the grains. Because the pore pressure from the saturated bed removes capillary effects but retains lubrication effects, the submerged case data suggest that capillary action is primarily responsible for the observed increases in friction angle and effects from grain lubrication are negligible. Further study is ongoing to fully understand the effect of capillary action on bulk friction angles in unsaturated gravel and why it appears to increase with bed thickness.