An experimental investigation on mass failures occurring in a riverbank composed of sandy gravel

This paper reports the results and interpretation of laboratory experiments carried out on a model of a relatively coarse (sandy gravel) riverbank, with the aim of investigating the basic processes and possible factors of instability. After a series of initial tests, three main experiments were carried out in a glass walled tank, where a bank model was built with bank angles varying from 75° to 90°, bank height of 70 cm, and same sediment mixture (60% gravel, 40% sand), but with the addition of 1% of cement in the third experiment only. During the experiments, the bank was subject to a given hydrograph associated with a static oscillation of the water level and corresponding variations in pore water pressures were measured. Results show the occurrence of a large variety of processes (including erosion and failures because of loss of matric suction, cantilever, slab and slide failures, and granular flows) during the ascending phase of the hydrograph. Bank instability was related to a superimposition of initial (geometric) factors, and progressive reduction of apparent cohesion during the rising water stage. Apparent cohesion was sufficient to maintain a stable bank in loose material, but only for low bank height and/or slopes, whereas a limited percentage of cement is able to explain a markedly different response in terms of stability and mechanisms of failure.