Direct determination of rheological characteristics of debris flow

In order to predict debris flow characteristics it is necessary to determine the rheological properties of these mixtures. It is here suggested that the mixture composing a debris flow (which often contains boulders surprisingly large) can be obtained by adding to clear water successively coarser particles derived from a sample of the debris flow material. At each addition different suspensions are obtained that can be considered as progressively coarser interstitial fluids of the complete mixture. These suspensions are easier to be tested and their rheological properties can thus be determined. The behavior of the complete material can then be inferred from these results. This procedure has been adopted for a debris flow that occurred in Italy in 1995; several suspensions have been generated using a sample of the debris flow and a wide range of rheometrical techniques have been used to determine their rheological characteristics: laboratory rheometer, inclined plane test, large-scale rheometer, field tests. Some experimental difficulties have appeared at each step, thus only approximate results could be obtained with this method. The rheological parameters and behavior type of the complete material have been inferred from the yield stress-solid fraction curve and from the flow curve evolution. The behavior has been found to be mainly viscoplastic and representable by a Herschel-Bulkley model.