New thoughts for impact force estimation on flexible barriers

Flexible barriers have received increasing attention in debris flow control because they are more economical and easier to install when compared with rigid barriers. However, in the design of a flexible barrier, the debris impact force is difficult to estimate, even if sophisticated numerical analysis is employed. In this paper, suggestions for simplified impact force estimations are given. At first, the existing approaches to estimate the impact force for impermeable rigid barriers are modified to cater for the case of a flexible barrier. We consider that there are two key characteristics of flexible barriers when compared with rigid barriers: Flexibility and permeability. Flexibility exemplifies itself in a longer duration of impact. A simple spring-mass system is used to represent the interaction of the debris flow and barrier and observed impact times are considered. It is deduced that the impact force on a flexible barrier should be less than half of that for a rigid barrier, both being impacted by the same/similar debris flow. Furthermore, for a ring net which is impacted by a debris flow of substantial mass and velocity, it is considered that the impact load is proportional to the elastic deformation of the flexible barrier in the direction of flow. Impact force calculated using the preceding assertion has been compared with the impact force in published results, and a satisfactory comparison is found. Large-scale experiments are proposed so that the validity of the above assertions can be ascertained. Permeability, the other key characteristic of a flexible barrier, can also influence the impact force as less force will be imposed on the barrier if less debris mass is retained by the barrier. Large-scale experiments are also proposed to investigate the relationship between the barrier net opening size and the debris impact force. Besides, existing approaches for estimating debris flow loading on impermeable rigid barriers are reviewed and improved by introducing a drag force which can impede the impact force. Then the largest force combination during the impact process cannot be simply determined as the largest dynamic loading or the largest earth pressure loading, and it can only be decided by calculating the largest force of all three stages.