Experimental Study on the Influence of Clay Minerals on the Yield Stress of Debris Flows

It is common knowledge that a relationship exists between the volumetric sediment concentration, the percentage of clay mineral content, and the activity of clay minerals (which is related to the clay type) and the yield stress of a debris flow mass. In this paper, the effect of four types of common clay minerals (montmorillonite, illite, chlorite, and kaolin) on the yield stress of mixtures were investigated in a quantitative way. Laboratory experiments were carried out with single and mixed clay mineral types, various volumetric concentrations, and percentages of clay mineral content. For the same clay content and same volumetric concentration, a decreasing effect was found, respectively, of montmorillonite>illite>kaolin on the yield stress of the flow mixture, whereas the effect of chlorite was equal to that of kaolin. A concept of equivalent clay mineral percentage (P0) is introduced in this paper, in which the value of 1 is taken as the characteristic coefficient of kaolin in the experiments. Compared to kaolin, the coefficients of montmorillonite, illite, and chlorite appeared to be 1.7, 1.3, and 1, respectively. An equation is proposed to calculate the yield stress of debris flows as a function of the volumetric concentration and equivalent clay mineral percentage. The formula is applicable for the calculation of yield stress of single clay and mixed clay type minerals in debris flows. The formula proved to be accurate after validation with data from other studies.