Klaus Thoeni

Rockfall Hazard Analysis From Discrete Fracture Network Modelling with Finite Persistence Discontinuities

Developing an accurate representation of the rock mass fabric is a key element in rock fall hazard analysis. The orientation, persistence and density of fractures control the volume and shape of unstable blocks or compartments. In this study, the discrete fracture modelling technique and digital photogrammetry were used to accurately depict the fabric. A volume distribution of unstable blocks was derived combining polyhedral modelling and kinematic analyses. For each block size, probabilities of failure and probabilities of propagation were calculated. A complete energy distribution was obtained by considering, for each block size, its occurrence in the rock mass, its probability of falling, its probability to reach a given location, and the resulting distribution of energies at each location. This distribution was then used with an energy–frequency diagram to assess the hazard.

Network Design and Quality Checks in Automatic Orientation of Close-Range Photogrammetric Blocks

Due to the recent improvements of automatic measurement procedures in photogrammetry, multi-view 3D reconstruction technologies are becoming a favourite survey tool. Rapidly widening structure-from-motion (SfM) software packages offer significantly easier image processing workflows than traditional photogrammetry packages. However, while most orientation and surface reconstruction strategies will almost always succeed in any given task, estimating the quality of the result is, to some extent, still an open issue. An assessment of the precision and reliability of block orientation is necessary and should be included in every processing pipeline. Such a need was clearly felt from the results of close-range photogrammetric surveys of in situ full-scale and laboratory-scale experiments. In order to study the impact of the block control and the camera network design on the block orientation accuracy, a series of Monte Carlo simulations was performed. Two image block configurations were investigated: a single pseudo-normal strip and a circular highly-convergent block. The influence of surveying and data processing choices, such as the number and accuracy of the ground control points, autofocus and camera calibration was investigated. The research highlights the most significant aspects and processes to be taken into account for adequate in situ and laboratory surveys, when modern SfM software packages are used, and evaluates their effect on the quality of the results of the surface reconstruction.

New Developments of the Boundary Element Method for Underground Constructions

AbstractIn this paper, an innovative modeling approach is presented for the simulation of underground construction with the emphasis on tunneling. The boundary element method (BEM) is used, and the theoretical background is discussed first. This is followed by test examples, where the results are compared with results from available software, showing the efficiency and accuracy of the new method. Finally, a practical example considering nonlinear material, sequential excavation, and installation of ground support is presented.

Qualitative Rockfall Hazard Assessment: A Comprehensive Review of Current Practices

Rockfall phenomena represent a major hazard in mountainous areas because they can cause severe damage to infrastructure and buildings as well as serious injuries and fatalities. Rockfalls do not pose the same level of economic risk as large-scale landslides, yet they are responsible for a similar number of accidents and fatalities. Therefore, appropriate land-use planning is necessary to protect people, buildings and facilities from rockfall hazards. Over the last two decades, several methodologies have been proposed to assess rockfall hazards, identify potentially dangerous areas (i.e., rock cliffs with failure-prone blocks) and provide guidelines for choosing and installing the most appropriate mitigation measures. This paper provides a comprehensive review of the existing rockfall hazard assessment methodologies. In particular, the review focuses on qualitative methods that allow a rapid evaluation of a rockfall hazard without costly and time-consuming numerical simulations. The most commonly adopted methodologies in Europe and North America are described and critically analyzed to highlight their differences and similarities and to identify their primary advantages, limitations and fields of application.

A rapid approach to estimate the rockfall energies and distances at the base of rock cliffs

ABSTRACT This paper presents a new methodology for estimating the expected energies and first impact distances at the base of a rock cliff, subject to the geometry and properties of the cliff and the representative block being known. The method is based on a sensitivity analysis, conducted by means of kinematic simulations and carried out for a large range of input parameters and their combinations, taking into account the uncertainty associated with their estimate. The proposed approach is validated by comparing predictions to experimental data and shows great potential for a quick qualitative hazard assessment.

The Influence of Shape on the Inherent Rolling Potential of Loose Rocks

The likelihood that rolling of a rock will be initiated and/or sustained on a slope depends on many factors related to the characteristics of the block and the slope. However, all other things being equal, some solid shapes have a greater potential to roll on a slope than others. This paper describes the results of a systematic laboratory study to determine how shape affects the ease with which rolling of ball-like blocks can be initiated, and its likelihood of being sustained. A simple scheme is presented to group basic shapes with similar rolling tendencies. Through systematic tests with polyhedral blocks on a frictional ramp with a range of inclinations, different basic ball-shaped forms are compared in terms of the ease with which rolling can be initiated, and the likelihood that it will be sustained. The results show that “ball” shapes (with principal dimensions of roughly similar size) are more prone to rolling but that even between shapes within this group, such as cubes and octahedra, the tendency to roll is strongly influenced by other factors including the number of faces. The importance of the starting position on the initiation of rolling is also demonstrated and quantified.

Grain Learning: Bayesian Calibration of DEM Models and Validation Against Elastic Wave Propagation

The estimation of micromechanical parameters of discrete element method (DEM) models is a nonlinear history-dependent inverse problem. In order to reproduce the experimental measurements with high accuracy, this work aims to develop a machine learning-based calibration toolbox named “Grain learning”, which can extract grains from X-ray computed tomography (CT) images and perform Bayesian parameter estimation for DEM models of dry granular materials.

A Simplified Scheme for Piezoelectric Anisotropic Analysis in Human Vertebrae Using Integral Methods

This paper outlines a computational model for the analysis of the piezoelectric behaviour of the vertebral body remodelling process. Particular attention is paid to the algorithms for the simulation of the stress energy density for each point of the geometry and the distribution of the density in the bone. In addition, the model takes into account the piezoelectric effect and the anisotropy (transversal isotropy) of the bone. A model for internal anisotropic piezoelectric bone remodelling of a human vertebra is discussed in detail. The model consists of the implementation of an algorithm which includes the elastic and electric variables in a single equation using boundary element method. The presented results show a good agreement with biological data and the model does not include any electric additional charge.

Numerical Investigation of Rockfall Impacts on Muckpiles for Underground Portals

Small-scale waste rock piles or muckpiles are commonly used as energy absorption barriers in various surface mining applications. This paper numerically investigates the impact behaviour of blocks on muckpiles used as cushion layer on top of underground portal entries. A three-dimensional discrete element model is implemented into the open-source framework YADE and validated using full-scale experimental data. The model allows estimating the energy absorption capacity of the muckpile and the impact forces acting on the portal structure. It also provides valuable information on the rebound characteristics which are useful for the definition of the potential safety areas in the vicinity of an underground entry. In order to show its capabilities, the model is applied to a large number of cases representing potential design conditions. The influence of block mass, impact velocity and absorbing cushion thickness on the forces at the base of the muckpile and the rebound trajectories after impact are investigated.

Discrete Modelling of Soil-Inclusion Problems

A generalised approach for the modelling of arbitrary shaped deformable structures in the framework of the discrete element method is presented. Minkowski sums of polytopes and spheres are used to describe the geometry of rounded cylinders and particle facets. In the current formulation, these new elements can be deformable. Their deformation is defined by the set of positions and orientations of their nodes. The elements can be connected to form arbitrary structures, such as grids and membranes. The constitutive behaviour of such connections is defined via an elastic perfectly plastic beam model. Contacts between other not connected structures or particles are detected based on three simple primitives: spheres, cylinders and thick rounded facets. The introduction of a virtual sphere at the contact point not only allows for straightforward contact handling but as well for the use of standard contact models based on sphere–sphere interactions. Hence, there is no need for developing new contact models. The approach is implemented into the open-source framework YADE. The capability of the newly developed approach for the modelling of soil–inclusion problems is presented.