Experiments on the impact process of dry and saturated mixtures against obstacles of various shape

Debris flows exert enormous impact forces on obstacles such as bridge piers, defensive walls and buildings. An estimation of these forces is necessary for designing reliable defence structures and debris flow resistant constructions. As field monitoring and measurements in mountain areas are generally difficult and expensive, reduced scale experiments are needed to improve the knowledge of debris flow impact forces. This work describes laboratory experiments aimed to analyse impact processes of saturated granular flows dams. Small-scale tests were carried out in a chute at the University of Pavia, and a fixed volume of saturated etherogeneous mixtures was released and eventually impacted on PVC dams with different cross sections. A deposition plane 0.60 m wide, 1.20 m long, was placed at the chute outlet with a steepness of 8.5°. Two mixtures grading curves were used with the aim to represent two typical field conditions. One of the mixtures was characterized by a widely ranging grain size similar to the one usually employed in the preparation of concrete; the other mixture was characterized by a greater porosity and by a prevailing coarser fraction. A load cell, fixed to the chute with an oscillating lever system, measured the average impact force on the dams, while high resolution pressure gages recorded the over-shoots at the impact. Video-recordings the chute side allowed to explore wave run-up on the dam and to obtain front speed. Flow depth was monitored by a laser level transducer, and video recordings from above the deposition plane were used to get the shape of the deposition area.