Mikael Rinne

Multi-Region Boundary Element Analysis for Coupled Thermal-Fracturing Processes in Geomaterials

This paper describes a boundary element code development on coupled thermal–mechanical processes of rock fracture propagation. The code development was based on the fracture mechanics code FRACOD that has previously been developed by Shen and Stephansson (Int J Eng Fracture Mech 47:177–189, 1993) and FRACOM (A fracture propagation code—FRACOD, User’s manual. FRACOM Ltd. 2002) and simulates complex fracture propagation in rocks governed by both tensile and shear mechanisms. For the coupled thermal-fracturing analysis, an indirect boundary element method, namely the fictitious heat source method, was implemented in FRACOD to simulate the temperature change and thermal stresses in rocks. This indirect method is particularly suitable for the thermal-fracturing coupling in FRACOD where the displacement discontinuity method is used for mechanical simulation. The coupled code was also extended to simulate multiple region problems in which rock mass, concrete linings and insulation layers with different thermal and mechanical properties were present. Both verification and application cases were presented where a point heat source in a 2D infinite medium and a pilot LNG underground cavern were solved and studied using the coupled code. Good agreement was observed between the simulation results, analytical solutions and in situ measurements which validates an applicability of the developed coupled code.

Modelling Fracture Propagation in Anisotropic Rock Mass

Anisotropic rock mass is often encountered in rock engineering, and cannot be simplified as an isotropic problem in numerical models. A good understanding of rock fracturing processes and the ability to predict fracture initiation and propagation in anisotropic rock masses are required for many rock engineering problems. This paper describes the development of the anisotropic function in FRACOD—a specialized fracture propagation modelling software—and its recent applications to rock engineering issues. Rock anisotropy includes strength anisotropy and modulus anisotropy. The level of complexity in developing the anisotropic function for strength anisotropy and modulus anisotropy in FRACOD is significantly different. The strength anisotropy function alone does not require any alteration in the way that FRACOD calculates rock stress and displacement, and therefore is relatively straightforward. The modulus anisotropy function, on the other hand, requires modification of the fundamental equations of stress and displacement in FRACOD, a boundary element code, and hence is more complex and difficult. In actual rock engineering, the strength anisotropy is often considered to be more pronounced and important than the modulus anisotropy, and dominates the stability and failure pattern of the rock mass. The modulus anisotropy will not be considered in this study. This paper discusses work related to the development of the strength anisotropy in FRACOD. The anisotropy function has been tested using numerical examples. The predicted failure surfaces are mostly along the weakest planes. Predictive modelling of the Posiva’s Olkiluoto Spalling Experiment was made. The model suggests that spalling is very sensitive to the direction of anisotropy. Recent observations from the in situ experiment showed that shear fractures rather than tensile fractures occur in the holes. According to the simulation, the maximum tensile stress is well below the tensile strength, but the maximum shear stress is probably enough to displace mica contact.

RESEARCH ON MODELLING OF SPATIAL DYNAMIC STRUCTURAL MECHANICS AND SPATIO-TEMPORAL EVOLUTION OF COAL MINE STOPES

Original scientific paper Major hazardous accidents in coal mines are basically attributable to a lack of understanding of or the failure to establish a complete spatial structural mechanical model of the stope during the course of exploitation, as well as of the spatial movement over time caused by the mining itself, mining at the wrong time, as well as improper roadway maintenance and advance of the work face. To efficiently study and analyze the mechanism behind major stope disasters, a method based on monitoring the stress and displacement of stopes was adopted to deduce the process whereby the overlying strata fracture, and a method of ascertaining stope stability based on qualitative identification of the dynamics was further proposed. The study found that the stability of stopes during the process of mining the face can be divided into two stages: (1) Unstable stage: i.e. the distance the work face has advanced Lx width of work face L0. These research results provide a basis for reasonable determination of a space-time relationship for the mining process.

Coupling Rock-Fracture Propagation with Thermal Stress and Fluid Flow

AbstractThis paper describes the theory behind a recent extension of a two-dimensional (2D) boundary-element code, FRACOD, to enable simulations of either coupled fracture (F)-hydraulic (H) processes or coupled F-thermal stress (T) in rocks. This extension is the next step in the ongoing development of a three-dimensional (3D) fracture mechanics code that couples F-H-T processes and predicts fracture initiation and propagation under thermal and hydraulic loadings. The original FRACOD simulated both mode I (tensile) and mode II (shear) fracture propagation that only involved mechanical processes in rock masses. In this study, the F-T coupling in FRACOD was developed using an indirect boundary-element method based on fictitious heat sources. The F-H coupling in FRACOD focused on fluid flow in explicit rock fractures using a cubic law. An explicit iteration method is used to simulate the fluid flow process in fractures and its interaction with mechanical deformation. Several verification and application case...

Introduction to Rock Fracture Mechanics

This chapter provides the basic theories and principles behind rock fracture mechanics. It starts with introducing the Griffith flaws and energy balance theory, which is the foundation of the modern fracture mechanics. Then the concept of stress intensity factor for linear elastic fracture mechanics is introduced, followed by a description of the criteria for fracture propagation. Also described in this chapter is the subcritical crack growth which dominates the time-dependent long term stability of a fractured medium.

Guidelines to design the scope of a geotechnical risk assessment for underground mines

Risk assessment has been seen as an important tool in reducing accidents. Use of risk assessment tools also has its significant presence in the underground mining industry in preventing work related hazards. Geotechnical uncertainty however is among the leading cause behind major accidents such as roof collapse, airblast etc., which leads to multiple fatalities and financial loss. Geotechnical risk assessment done at stages as early as mine design can help justify a different mine design aspect such as support methods for a risky area different from the rest of the mine. The aim of this paper is to organize the geotechnical risk assessment process to suit the underground mining needs. A numerical ranking system has been developed to plan the risk assessment process and choose amongst the risk assessment tools. The risk assessment process has been redefined into four sections namely — hazard identification tool, risk assessment approaches, risk assessment parameters and risk representation tool. Risk identification tools have been shortlisted to suit underground mining needs through literature review. Risk assessment approaches have been defined into deterministic, probabilistic and possibilistic approaches with relevance to geotechnical assessment. Elements to be considered in an underground mine for a geotechnical risk assessment have been structured into classes. Establishment of scope of a geotechnical risk assessment based on these classes has been explained. Selection guideline for the appropriate risk identification tool has been defined based on the scope of risk assessment. The significance of the risk assessment approach has been explained and a numerical ranking system has been formulated which can be used by an underground mine to choose among the probabilistic, possibilistic and deterministic approaches.

Intelligent Scheduling for Underground Mobile Mining Equipment

Many studies have been carried out and many commercial software applications have been developed to improve the performances of surface mining operations, especially for the loader-trucks cycle of surface mining. However, there have been quite few studies aiming to improve the mining process of underground mines. In underground mines, mobile mining equipment is mostly scheduled instinctively, without theoretical support for these decisions. Furthermore, in case of unexpected events, it is hard for miners to rapidly find solutions to reschedule and to adapt the changes. This investigation first introduces the motivation, the technical background, and then the objective of the study. A decision support instrument (i.e. schedule optimizer for mobile mining equipment) is proposed and described to address this issue. The method and related algorithms which are used in this instrument are presented and discussed. The proposed method was tested by using a real case of Kittilä mine located in Finland. The result suggests that the proposed method can considerably improve the working efficiency and reduce the working time of the underground mine.

Thermo-Mechanical Simulations of Pillar Spalling in SKB APSE Test by FRACOD

Abstract This paper summarises a study on pillar spalling using fracture propagation code FRACOD. The rock mass response in a heated pillar between two large boreholes in the planned Aspo Pillar Stability Experiment by SKB is modelled. To model all the planned loading phases, the code has been improved in many ways. Today it predicts the explicit fracturing process including fracture initiation. Barton-Bandis model has been applied to estimate the fracture properties for newly initiated fractures. It also simulates the development of Acoustic Emission (AE) events. A stress reconstruction technique has been developed to transfer excavation and heat induced stresses from other models into FRACOD The modelling results suggest that the excavation induced stresses will cause slight fracturing in the pillar walls. Fracture propagation driven by thermal loading may lead to minor spalling

An Approach to Realizing Process Control for Underground Mining Operations of Mobile Machines

The excavation and production in underground mines are complicated processes which consist of many different operations. The process of underground mining is considerably constrained by the geometry and geology of the mine. The various mining operations are normally performed in series at each working face. The delay of a single operation will lead to a domino effect, thus delay the starting time for the next process and the completion time of the entire process. This paper presents a new approach to the process control for underground mining operations, e.g. drilling, bolting, mucking. This approach can estimate the working time and its probability for each operation more efficiently and objectively by improving the existing PERT (Program Evaluation and Review Technique) and CPM (Critical Path Method). If the delay of the critical operation (which is on a critical path) inevitably affects the productivity of mined ore, the approach can rapidly assign mucking machines new jobs to increase this amount at a maximum level by using a new mucking algorithm under external constraints.

A method to downscale joint surface roughness and to create replica series using 3D printed molds

In order to determine the in-situ shear strength of rock joints, large scale testing is required. However, this is both expensive and difficult to execute. One possible method to overcome this may ...