Granular flowsdeveloped during phenomena such as a landslide or rock avalanche can apply tremendous impact forces on an obstacle in their flow path. Previous researches already disclosed that the force exerted on a retaining wall can consist of a drag force and an active or passive earth force, and a gravity- and friction-induced force. However, up-to-date, only the force normal to the retaining wall surface has been primarily studied, while the influence of the force tangential to the retaining wall has hardly been investigated. Therefore, a specific study was carried out by Dr. Jiang Yuanjun and Prof. Zhao Yu from Institute of Mountain Hazards and Environment, Chinese Academy of Sciences (IMHE, CAS) to see the effect of tangential force. 

Dr. Jiang and Prof. Zhao have investigated granular flow impact against a retaining wall through the measurement of both normal and tangential sub-forces. It is found that the tangential sub-forces change from positive to negative with respect to the wall in the impact process, which can be classified into two impact states according to the development of the stagnant zone. In the process, the interface friction between the granular material and the wall is calculated according to normal and tangential forces and defined as equivalent interface friction angle, which is observed to vary, and it is smaller than the value measured by interface friction test. The absolute value of the equivalent interface friction angle decreases with the slope angle. Jiang and Zhao also observe that reduction in the interface friction angle of the wall has negligible influence on the impact force calculation, while reduction in the interface friction angle of the flume base leads to a significant overestimation of the force. It is believed that their findings can significantly aid the study of granular flow and its applications. 

The output of this research was published in “Géotechnique Letters”, and awarded as the best paper of quarter one in 2015. This research is mainly supported by the West Light Foundation of Chinese Academy of Sciences and the Science Foundation of the Institute of Mountain Hazards and Environment for Young Scholars.