Investigation of flowslides from the failure of mine tailings' dams.

Progress to further understand the complex nature of the flowslides has, in the past, been restricted by a shortage of good quality experimental and field data. This thesis describes a series of small scale laboratory flowslide experiments, using a realistic test material of fine sand and water, which was performed to elucidate aspects of this phenomenon. Following initiation of the flowslide, liquefaction of the static material occurred and the flowslide accelerated under the influence of gravitational driving forces without any segregation of the components of the two-phase material. It was observed that movement of the flowing material was not arrested 'en masse', but unsteady deposition of sand and pore fluid at the lower flow boundary occurred, being dependent on deceleration of the flowslide. This resulted in thinning of the flowing layer. The rate of deposition also increased with the solid volume concentration of the flowing material. The final stages of flowslide motion were influenced by the presence of surface water derived from consolidated material. It was established that excess pore pressures were present during flowslide motion. Within the limitations of experimental measurement, the magnitude of these excess pore water pressures at the lower flow boundary was such that the intergranular friction was sufficiently reduced to allow motion. This reduction of intergranular friction explained the fluid-like motion observed at bed slopes lower than the equilibrium slope of the granular fluid. Some shear resistance mechanism other than Coulomb friction must therefore play an important role in bringing the flowslide to a halt.