Investigation of rock fragmentation during rockfalls and rock avalanches via 3‐D discrete element analyses

Abstract

This paper investigates the characteristics of dynamic rock fragmentation and its influence on the postfailure fragment trajectory. A series of numerical simulations by discrete element method (DEM) were performed for a simple rock block and slope geometry, where a particle agglomerate of prismatic shape is released along a sliding plane and subsequently collides onto a flat horizontal plane at a sharp kink point. The rock block is modeled as an assembly of bonded spherical particles with fragmentation arising from bond breakages. Bond strength and stiffness were calibrated against available experimental data. We analyzed how dynamic fragmentation occurs at impact, together with the generated fragment size distributions and consequently their runout for different slope topographies. It emerges that after impact, the vertical momentum of the granular system decreases sharply to nil, while the horizontal momentum increases suddenly and then decreases. The sudden boost of horizontal momentum can effectively facilitate the transport of fragments along the bottom floor. The rock fragmentation intensity is associated with the input energy and increases quickly with the slope angle. Gentle slopes normally lead to long spreading distance and large fragments, while steep slopes lead to high momentum boosts and impact forces, with efficient rock fragmentation and fine deposits. The fragment size decreases, while the fracture stress and fragment number both increase with the impact loading strain rate, supporting the experimental observations. The fragment size distributions can be well fitted by the Weibull's distribution function.

Dynamic Fragmentation of Jointed Rock Blocks During Rockslide‐Avalanches: Insights From Discrete Element Analyses

“…The kinetic energy of each subdomain of the rock block is calculated as the summation of the translational and rotational kinetic energy of all particles in the subdomain, , with being the moment of inertia and v j and ω j being the translational and angular velocities of particle j . In addition, the elastic strain energy stored in each particle bond and contact are denoted as E b j and E c j , respectively, as in Zhao et al (). At any time, the cumulative energy dissipated in each subdomain is calculated as where Nb i and Nc i are the number of bonds and contacts in subdomain i , respectively.…”

“…In addition, under the current model configuration, the presence of lateral confinement can constrain the lateral motion of fragments, so that large fragments are mainly deposited near the slope toe region. However, in unconfined conditions fragments can spread laterally in a subcircular fan‐like pattern, with shorter runout distances (Zhao et al, ).…”

“…The current study attempted a detailed analyses of rock fragmentation using the so‐called rock damage ratio ( D , see the definition in section 3.2), which is also associated with the final runout distance ( L C and L ). This parameter is useful for evaluating the potential destructive power of a rock avalanche event (Zhao et al, ). For rock blocks with preexisting joints (e.g., the regular and inclined joints), the initial debonding of particles within the joint gaps can be considered as the initial rock damage ( D ini , calculated as the ratio of the number of debonded to the initial number of bonds of an intact rock block).…”

Numerical Investigation of Caved Rock Mass Friction and Fragmentation Change Influence on Gravity Flow Formation in Sublevel Caving

“…In general, DEM assumes the creation of the models with different element shapes that are subjected to Newton's law of motion and with appropriate constitutive models at their contacts. This method is widely used for the modeling of problems related to granular materials [28][29][30], coal mining [31] and different rock and soil mechanics problems [32][33][34][35][36]. DEM modeling of sublevel caving was demonstrated by Hustrulid, Sellden, DeGagne and Bobadilla [5,[37][38][39][40].…”

Extremely Energetic Rockfalls

“…Since experimental observations demonstrate that the surface energies depend on many factors such as the rate of deformation (Grady & Kipp, ; Zhao et al, ), the stress distribution in the original body after the impact, and the fragment size, then the definition of an expression capable of representing the evolution of the fragmentation specific energy with the diameter could provide a more general framework. Following the observation of a scale dependency of strength on block size (Bieniawski, ; McDowell & Bolton, ; Weiss et al, ), we introduce a power law function characterized by a parameter α influencing the rate of fragmentation specific energy decay, and a representative diameter D 0 where the surface density of fragmentation energy is known ( γ 0 ).…”