Lu Zheng

Joints are one of the most important factors that influence the fragmentation mode of rocks during impact. Using physical experiments and an OpenMP-based parallel discontinuous deformation analysis (DDA) incorporating virtual joints, this study investigates crack propagation mechanisms and fragmentation modes in rocks with various joint distribution patterns during rockfall impacts. The results indicate that (1) the degree of rock fragmentation can be quantitatively described by the surface area ratio of the equivalent volume sphere ( η ). η increases with increasing joint connectivity rate ( k ), decreases with increasing joint-slope angle ( β ), and tends to increase with increasing joint spacing ( s ). But when k is low, η decreases once s increases to a critical threshold. (2) Rockfall fragmentation modes can divided into three categories according to the destructive morphologies of rocks with different joint distributions. (3) The degree of particle fragmentation within the joint set ( ρ ) quantitatively characterizes the rockfall fragmentation mode. The inherent factor that determines the rockfall fragmentation mode is the length-to-width ratio, b , of the blocks between joints. (4) The three failure modes are internal fragmentation within joint sets ( b < 0.25 and ρ > 0.2), expansion along outlines of joint sets (0.25 ≤ b ≤ 1 and 0.01 ≤ ρ ≤ 0.2), and expansion along specific joints ( b > 1 and ρ < 0.01). (5) Different joint geometric parameters significantly affect fragment energy and velocities after impact. This study can serve as a reference for particle fragmentation mechanisms in rocks with inherent joints.

Due to the nonlinear nature of contacts, the real contact information is indeterminate and there is no analytical solution for discontinuous computation. In the context of two-dimensional discontinuous deformation analysis (2D DDA) for arbitrary polygon block systems, the contact indeterminacy arises from at least two aspects, namely kinematics and geometry. This study provides a specific numerical solution for 2D DDA to address the indeterminacy problem of contacts between arbitrary polygon blocks. First, local contact boundary is established by defining the solid angle of 2D blocks and the entrance solid angle between two 2D solid angles of two blocks. Second, the linearized first entrance formulas are derived to estimate the first entrance time and position. Third, an enhanced shortest exit strategy based on entrance angles is introduced to provide a unified solution of the contact points and direction of closed contacts. Some extreme and challenging examples, including oblique collision contacts, undue penetration contacts, genuine vertex–vertex contacts (including absolutely symmetric contacts), extremely abnormal block contacts (i.e., contacts between extremely short edges and/or sharp angles), multi-connected block contacts and numerous singular contacts, demonstrate the accuracy, robustness and application prospects of the proposed method in addressing difficult contact problems.

Rock avalanches pose significant threats to infrastructure due to their high kinetic energy. Baffle structure serves as an effective engineering solution for mitigating rock avalanche hazards. In this study, a new mangrove guard baffle (MGB) was proposed by combining traditional baffle (TB) designs with the prop root structure of mangroves. Using EDEM software, simulations were conducted to analyze the impact of baffle type and particle size on the movement velocity, deposition, impact force, and kinetic energy of rock avalanches. The results indicated that the process of rock avalanche impact on a baffle could be divided into four stages: initiation, acceleration, impact, and static deposition. The maximum average front velocity obtained by the rock avalanche in the acceleration phase was about 4.5 m/s, and the maximum average tail velocity was about 3 m/s. In addition, the MGB created a larger dead zone and reduced the deposition area. Compared to the STB, under the SMGB, the deposition areas of M1–M4 decreased by 17.9%, 12.7%, 17.1%, and 16%, respectively. MGB demonstrated superior impact force dissipation. When the number of baffle rows was three, the impact force dissipation rate of SMGB on M4 reached 99%. Compared to TBs, the momentum of the rock avalanche was reduced by 1%–3%. Considering the economic benefits and protection performance, the arrangement of two rows of baffles could be the best solution. These findings provided insights into the design of baffle structures against rock avalanches.