The influence of particle size on the spread distance and angle of friction of granular materials

To produce a hazard map and thus provide mitigation measures against natural catastrophic events such as landslides, avalanches, and debris flows, it is necessary to determine the spread distance of such flows. There is a well-documented relationship between the angle of friction and the debris flow volume, allowing one to determine the possible distance a debris flow can travel. However, the effect of mean particle size (d₅₀) and the sorting coefficient (S_c) on the final spread distance (D_f) has received mere attention. In this study, a mini conical-shaped mould was used to measure the final spread distance (D_f) and angle of friction (α_f) for various dry granular material samples both in air and in water. Experimental results indicated that a granular material sample with a smaller d₅₀ travel a longer distance compared to a sample with a higher d₅₀. However, a sample with a smaller d₅₀ results in a smaller angle of friction than the one with a higher d₅₀. Conversely, a well-sorted sample has a smaller spread distance and larger angle of friction. Results also indicated that the spread distance in air is slightly larger and the angle of friction is slightly smaller than in water.