This study provides some elements for understanding the behavior of water–debris mixtures containing clay, silt, sand, and boulders at high solid concentrations. Accurate, simple shear rheometrical results for various clay–water mixtures and fine debris flow fractions with different added sand concentrations, in the shear rate range from 10−2 to 10−2 s−1 are presented. In this shear rate range, the behavior of these fluids is similar to the behavior of the initial fluid (without sand), i.e., it may be well represented by a Herschel–Bulkley model (with a power parameter close to 1/3). With the initial fluids (yield stress from 20 to 200 Pa) the suspension yield stress increases exponentially with the increase in sand (diameter between 100 and 200 μm) concentration, as long as the latter does not exceed 30%. However the rate of increase is less than the corresponding rate for the initial fluid and is correspondingly smaller as the grain size distribution is less well sorted. Diagrams showing the increase of yield stress with solid concentration may help to estimate the yield stress of coarser suspensions as long as the solid fraction is not too close to the maximum packing fraction. Key words : clay–water suspensions, water–debris mixtures, rheology, yield stress, sand addition, rheometry.
