Flow characteristics of 2-phase granular mass flows from model flume tests

Abstract Changes in basal fluid pressure and normal flow depth in 2-phase granular mass flows, the equivalent coefficient of friction and the equivalent coefficient of friction between gravity centres before and after failure were investigated for pumiceous gravel, and a mixture of pumiceous gravel and volcanic ash by means of large-scale model flume tests. The proportion of finer soil particles, the inclination of slope segments, and the width of horizontal segments were selected as the controlled conditions. No excess of basal fluid pressure was observed in the pumiceous gravel. On the other hand, an excess of pressure head was generated in the mixture of pumiceous gravel and volcanic ash. This was due to the suspension of the fine particles in the pore fluid increasing its effective density and the quasi-undrained shear deformation due to the low permeability of the mixture in the mass of the granular flow during its motion. In tests with a slope of 30°, both the pumiceous gravel and the mixture of pumiceous gravel and volcanic ash travelled less distance in the narrow flume than in the wide one. This was likely because, in the wide flume, no clogging occurred where the width of the flume changed, and less frictional collisions against side-walls and resulting grains' contact took place. In tests with a narrow flume, the pumiceous gravel clogged and most of material was deposited around the constriction. However, the mixture passed through the constriction smoothly. This could be because less frictional resistance was generated in the mixture due to the lowering of effective stress between soil particles associated with the excess of pore-fluid pressure. Although the test on a slope of 35°had the largest potential energy before the release of the materials and the test on a slope of 25°had the least, the test with the 35-degree slope produced the largest equivalent coefficient of friction and the smallest value was obtained from the test with a 25-degree slope. This finding suggests that the dissipation of kinetic energy, especially in the vertical direction, due to the collisions and the following abrupt deformation around the connection of the flume between the slope and horizontal segments may play an important role in the travel distance