NUMERIČKA STUDIJA MEHANIKE LOMA OKO MINSKIH BUŠOTINA TE ISTRAŽIVANJE POJAVE DISKONTINUITETA NA UZORKU TAKVIH LOMOVA

The mechanism of rock fragmentation around blastholes is of prior importance in the evaluation of drilling and blasting performance in open pit and underground mines. This paper aims to numerically investigate the crack propagation mechanism around a single blasthole using the distinct element method (DEM). In this study, two specimens with different borehole diameters were considered and the effects of the stress waves on their cracking mechanism were simulated. To validate the numerical model, the length of the radial cracks around each blasthole was measured and compared against an analytical fracture mechanics model. The fractured zones around the blasthole were also compared against previous experimental tests and good agreement was observed. The effect of a single discontinuity on the crack propagation mechanism was also studied and it was found that the discontinuity normal stiffness plays a significant role in the fractured zones around the blasthole. For low values of normal stiffness, the discontinuity surface acted as a free surface, and the shock wave was significantly reflected, while at high values of normal stiffness, cracks propagate across the discontinuity surface.Mehanika loma stijena oko minskih bušotina važan je čimbenik u ocjeni varijabli bušenja i miniranja, bilo na površini ili pod zemljom. U radu je numerički ispitana mehanika širenja lomova u blizini jedne takve bušotine uporabom metode diskretnih elemenata. Simulirana su dva uzorka različitih promjera bušotina te način širenja tlačnih valova i nastanak pukotina. Numerički model provjeren je mjerenjem dužine radijalnih pukotina oko bušotine te usporedbom dobivenih vrijednosti s analitičkim modelom mehanike loma. Zone pucanja oko bušotine uspoređene su s prethodnim eksperimentalnim testovima te je opaženo dobro podudaranje. Također je istražen i utjecaj pojedinačnoga diskontinuiteta na širenje pukotina te je utvrđeno kako čvrstoća diskontinuiteta ima veliku ulogu u opisanim zonama lomova oko bušotina. Kod nižih vrijednosti čvrstoće zone diskontinuiteta djeluju kao slobodne zone gibanja s kojih se tlačni valovi uglavnom reflektiraju. Zone veće čvrstoće ne sprječavaju pukotine da prolaze kroz njih

Distinct element modelling of the mechanical behaviour of intact rocks using voronoi tessellation model

This paper aims to study the mechanical behaviour and failure mechanism of intact rocks under different loading conditions using the grain based model implemented in the universal distinct element code (UDEC). The grain based numerical model is a powerful tool to investigate complicated micro-structural mechanical behaviour of rocks. In the UDEC grain based model, the intact material is simulated as assemblies of a number of polygonal blocks bonded together at their contact areas. To investigate the ability of such a numerical framework, uniaxial and triaxial compression tests as well as direct tensile test were simulated in UDEC and then the results were compared with the laboratory experiments undertaken on Hawkesbury sandstone. There was a good agreement between the experimental and numerical under different loading conditions. In order to investigate the effect of micro-properties of the grain based model, blocks and contacts, on the laboratory scale intact rocks, a set of parametric study was undertaken. The results from this analysis confirmed that the block size is an intrinsic characteristic of a model which has significant effects on the mechanical behaviour of the numerical models. Also, it was concluded that the cohesion and friction angle of contact surfaces control both uniaxial and triaxial compressive strengths. Finally, it was found that in the triaxial compression test, as the applied confining pressure increases, the effect of contact cohesion on the strength decreases while the effect of friction angle increases

Geological discontinuity persistence: Implications and quantification

Persistence of geological discontinuities is of great importance for many rock-related applications in earth sciences, both in terms of mechanical and hydraulic properties of individual discontinuities and fractured rock masses. Although the importance of persistence has been identified by academics and practitioners over the past decades, quantification of areal persistence remains extremely difficult; in practice, trace length from finite outcrop is still often used as an approximation for persistence. This paper reviews the mechanical behaviour of individual discontinuities that are not fully persistent, and the implications of persistence on the strength and stability of rock masses. Current techniques to quantify discontinuity persistence are then examined. This review will facilitate application of the most applicable methods to measure or predict persistence in rock engineering projects, and recommended approaches for the quantification of discontinuity persistence. Furthermore, it demonstrates that further research should focus on the development of persistence quantification standards to promote our understanding of rock mass behaviours including strength, stability and permeability

Numerical study of the mechanical behaviour of rock joints and non-persistent jointed rock masses

The mechanical behaviour of a rock mass is significantly controlled by the presence of discontinuities, and understanding the effect of discontinuities is essential in the economical and reliable design of rock structures. This research investigated the effect of roughness and persistency of rock joints on the mechanical behaviour of rock structures using the Particle Flow Code (PFC). In PFC, the intact material is represented by an assembly of particles bonded together. Traditionally, joints have been modelled in PFC using either the bond removal method or the smooth joint model. Analyses undertaken in this research showed that these approaches are not able to reproduce the shear behaviour of rock joints. To overcome this problem, a new shear box genesis approach was proposed. The ability of the proposed method in reproducing the shear behaviour and asperity degradation of rock joints was investigated by undertaking a comparative study against analytical and empirical models, as well as experimental direct shear tests on synthetic joints under different normal stresses whereby good agreement was found.To overcome existing problems in the determination of the shear behaviour of rock joints, a combination of photogrammetry and numerical modelling was proposed. Results indicated this approach has the ability to reproduce the shear behaviour of rock joints. The proposed method was employed to investigate the effect of scale on the shear behaviour of rock joints. It was found that the mobilisation of different order asperities for different joint lengths resulted in the negative scale effect on the peak shear strength.The effect of joint geometrical parameters of non-persistent joints on the mechanical behaviour of jointed rock masses was numerically investigated. A validation study was undertaken by uniaxial and biaxial compression tests, and the numerical analyses were found to correlate well with physical experimentation at low confining pressures. This was followed by a sensitivity study on the effect of joint configuration parameters on the failure mode, unconfined compressive strength, and deformation modulus.The insights presented in this study have improved understanding of the effect of joint roughness on the shear behaviour, and the process of damage evolution of rock joints as well as the effect of joint geometrical parameters on the mechanical behaviour and failure mechanism of non-persistent joints

Numerical assessment of rupture mechanisms in Brazilian test of brittle materials

Due to the difficulties associated with performing a direct uniaxial tensile test, the tensile strength of brittle materials is commonly estimated indirectly. The Brazilian test is probably the most widely accepted indirect technique among others. Since the development of the Brazilian test, the test has received considerable attention due to its ease of sample preparation and interpretation. However, the accuracy of this method has also been criticized for a long time in the literature. This paper aims to numerically study some of these criticisms by mainly focusing on failure mechanisms, the effect of contact loading conditions and stress distributions induced in the Brazilian test. Brazilian discs having contact angles ranging from 1 to 30° (i.e. concentrated to distributed loading scenarios) were simulated in the Fast Lagrangian Analysis of Continua (FLAC) based on the finite difference model. Results of this study show that at low contact loading angles, zones of high compressive stresses are generated at the vicinity of loading rims which is followed by an initiation and propagation of the cracks from these high-stressed zones. With an increase in the contact loading angle, the magnitude of compressive stresses at the vicinity of loading rims decreases leading to the disc's tensile rupture initiation at its centre. Also, it was found that the transition in the observed failure mode from cracking under the loading rims to central tensile cracking is material dependent in a Brazilian test. Finally, the ratio of uniaxial compressive strength to tensile strength was suggested as a controlling parameter for the evaluation of failure transition in a Brazilian test

Evaluation of air blast parameters in block cave mining using particle flow code

Air blast, a sudden mass movement of air, can occur in underground mining system where caving develops an extensive mass of unsupported rock spanning a large void. Air blast can result in injury to mine personnel, damage to equipment or disrupts mine operation. Evaluation of air blast parameters is, therefore, an essential part to develop strategies to mitigate the hazard. The properties of a muckpile or a caved zone are significant factors affecting the magnitude of air blast in particular on the undercut and extraction levels. This research investigates the effect of muckpile properties on air flow using the numerical code, PFC2D. The critical parameters such as thickness, block size and porosity (swell factor) of the muckpile have been studied to quantify how much they could change the magnitude of air pressures and velocities while the air flows through the muckpile. It was found that the porosity of the muckpile is the most effective parameter on the magnitude of air blast and by designing a thick layer of blasted rock with low porosity in the caved zone, the intensity of the air blast can be significantly reduced. The findings of this study can be used to design air blast plugs or bulkheads in order to isolate any potential air blast from the active workings, or to quantify the minimum thickness of the muckpile above extraction levels to manage air blast hazards

DEM Analysis of Backfilled Walls Subjected to Active Translation Mode

In this paper, the problem of a retaining wall under active translation mode is investigated numerically. To this end, a series of numerical models is conducted using the discrete element code, PFC2D. The backfill soil is simulated by an assembly of separate cohesionless circular particles. Backfill soil was prepared by pouring soil particles from a specific height under gravity force and giving them enough time for appropriate settlement. Different heights of retaining walls are simulated and the lateral earth pressure on the wall is observed under both at-rest and active conditions. Numerical results compared with predictions from some analytical methods and measurements from physical models. The active state of earth pressure is defined as the earth pressure distribution corresponding to the values of wall displacement where the failure zone in the backfill is fully developed. The numerical results showed that the fully active state of earth pressure occurred at a wall displacement corresponding to the strains required for reaching the critical state in biaxial compressive tests