Influence of the mineralogy of fines on sediment slurrying and slurry behaviors

Fine debris is an important component of natural debris flows. Previous studies focused primarily on the clay minerals found in the fines, and non-clay minerals were often neglected. The effects of mineralogy of fines on debris-mass slurrying and flow behaviors of the resultant slurries are examined herein. The fines (≤0.04 mm) in the < 5 mm fraction of the Dongyuege Creek debris-flow deposit is replaced with five other mineral powders with the same maximum particle size. Four types of separate and sequential experiments related to debris slurrying and slurry behaviors are carried out with the prepared clastic materials. The obtained slurrying index ranging from 0.08 to 0.18 shows that non-clay minerals also can function as the fine fractions of debris-flow materials, so long as the requirement of grain size distribution is met. Equidimensional, non-clay minerals making up fines of debris flows can increase the upper solid concentration limits of slurrying (with a maximum of 0.692) and peak values of relative excess water pressure (measured maximum mean peak value is 0.99), leading to higher momentum and higher competence, and thereby more destructive catastrophe. The sediments with platy non-clay mineral-dominated fines have potential for mobilizing into small- to medium-size debris flows with a relatively small competence. Clay minerals in the fines may indeed enhance the liquefaction potential of debris masses by expanding the difference between upper and lower solid concentration limits of slurrying (0.413 and 0.238, respectively, for pure kaolinite), but they significantly suppress the momentum, competence, and destructive power of potential debris flows by lowering upper solid concentration limit of slurrying of debris masses. Alpine catchments rich in non-clay minerals, notably those releasing dolomite into loose sediments, may be more prone to threatening and destructive debris flows. The basin producing clay minerals should be more susceptible to low-magnitude/high-frequency debris flows with less devastating consequences.