Rheological investigations of debris flow materials

A 2.0 m diameter viscometer/rheometer was designed, constructed, and used to test the behaviour of coarse grained debris flow materials. A 1/5 scale model was also constructed and used to test fluid-only and grain-fluid mixtures. The 30° inverted cone-and-plate viscometric system produced results comparable to those obtained from more standard viscometric systems. Internal flow behaviour was shown to be similar to standard cone-and-plate geometries and values of internal shear rate were close to those predicted from theory. The behaviour of highly concentrated fine suspensions (clay slurries) confirmed the findings of previous workers that these materials have a plastic or Bingham plastic rheology. Clay slurries containing coarse grains, and artificial grain-fluid mixtures, display many features of granulo-viscous behaviour such as stick-slip phenomena, structural changes, and fluctuations in torque values. Flow curves displaying dilatant behaviour occurred almost universally in coarse grain-fluid mixtures at shear rates greater than about 5 s⁻¹. Flow curves for fresh concrete were similar to those identified by others and might well fit the Bingham plastic model. Flow curves for debris flow materials were extremely sensitive to water content. Debris flow materials that have a bimodal grain size distribution are likely to have a dilatant plastic rheology while those which have a low content of coarse material and a unimodal grain size distribution are likely to have a plastic or viscoplastic rheology