Evaluation of the viscosity of tropical soils for debris flow analysis: a new approach

To analyse the debris flow type of landslides it is necessary to know the viscosity of the fast-moving soil mass. Such rheological parameters are important in the evaluation of characteristics of propagation of the movement downhill. This paper presents a proposal for the assessment of soil viscosity based on a modified set-up of the well-known concrete slump test. Through the definition of shearing rates in the slump tests, soil viscosity was determined on a plate–plate rheometer. Four natural soils from Rio de Janeiro, Brazil, with differing grain size distribution and plasticity, were used in the investigation. The results obtained are compared with experimental data from the literature, and show that care must be taken in defining viscosities based only on data related to grain size. Correlations between soil moisture and shearing rates, defined in the modified slump tests, and viscosity, defined in the rheometer tests, were obtained. As a result, a fairly good relationship between shearing rate and viscosity was established. Comparing the obtained data with numerical results available in the literature it is concluded that the developed experimental methodology may provide a simple way to evaluate the viscosity of soils for debris flow analysis.