Experimental and Numerical Study of Rock breakage by Pulsed Water Jets

Rock breakage is, arguably, the most important step in the mining process. Poor blasting practice produces large boulders that are difficult and expensive to handle. The presence of oversized boulders in draw-points can stop production in underground caving mines. Blasting and heavy impact hammers are the main techniques used in mines to fracture oversized rocks, but both impose undesirable extra costs as a secondary rock-breakage operation. Blasting is time-consuming and can damage the infrastructure, whilst impact hammers are cumbersome and cannot be used where boulders are not readily accessible. It is important, therefore, to develop more effective secondary rock-breakage techniques. Preliminary research demonstrated that pulsed water jets are routinely able to break large (approximately 1m3) boulders of competent hard rocks in less than a minute. Therefore, they can potentially be a more effective method for secondary rock-breakage. They can deliver high power intensity with a low reaction force meaning that a lightweight, flexible apparatus can be employed. The breakage mechanism of rocks under the impact of water pulses is, however, not yet clear. A pulsed water jet consists of a series of discrete, large water drops travelling at high velocity. These apply cyclical impact forces of short duration onto the target material. The water impacts generate stress waves inside the rock samples and these waves can, potentially, influence the rock- breakage process. The contribution of the stagnation water pressure on the target after impact may also play a crucial role in fracture initiation and propagation in the rock. The geometrical properties of the jet are controlled and influenced by the method that is used to generate the water pulses. The original contribution of this research is the determination of the processes of water pulse formation and the discovery of key factors that influence rock fracture initiation and propagation. A straightforward method was employed to generate a single water pulse using a hammer that stroke a piston, rested on top of a water-filled chamber. This impacting action pressurised the water, causing it to be expelled at high velocity through a nozzle that was mounted on the chamber axis opposite the piston. A theoretical investigation was undertaken to improve the understanding of this system for generating water pulses. A computational model, on the basis of continuity and momentum equations for a compressible viscous flow, was developed to simulate the pressure dynamics in the chamber. This model was used to optimise the relative sizes of the hammer, the piston, and the height of the water column in order to produce the largest and the highest-velocity water pulses. The model was validated experimentally using a purpose-built apparatus. Research was conducted to measure the impact loading of the water pulses on a target material. The main challenge was to measure this load for an event that lasted for less than one microsecond. This obviously required an extremely fast-response sensing system to capture the induced stress wave from the water hammer pressure before it reduced to the stagnation pressure. Customised sensing and data acquisition equipment was developed for these measurements. PVDF (Polyvinylidene fluoride) shock gauges were selected as the most appropriate sensors because of their unique rapid response, large stress range, and large signal to noise ratio. PVDF polymer films are piezoelectric material and generate electrical charge in response to applied stress. A current-mode measurement method was chosen because of the high-speed nature of the phenomenon of interest. The derivative of the stress was measured and the signal was then integrated numerically. This measurement, in conjunction with the high-speed photography of the water pulses, was applied to a study of the coherence of the generated water pulses. In the final stage of the research, experiments were conducted to examine the damage caused to confined rock specimens by sequences of water pulses of various pulse lengths and pulsation frequencies. These experiments were undertaken for different rock types. The observed rock damage was then used to construct an explanatory model of the mechanisms of rock fracture. The breakage mechanism was found to be controlled by the number of water pulse impacts and by the duration of stagnation water-pressure on the target. The successive high-energy impacts of the water pulses were found to create localised fracture zones in the vicinity of the impact surface. These impacts also initiated fatigue in the target, introducing micro-structural damages. The initial impacts of water pulses created a damaged area at the point of impact. The stagnation pressure from the water flow supplied the crack opening pressure, which controlled the crack opening process and thus affected the development of the failure zone. However, crack growth could be interrupted by energy dissipation and by toughening mechanisms. It was found that effective rock-breakage depended upon the physical properties of the target rock, in particular the brittleness, and that, ideally, the length and frequency of the water pulses should be tuned to accommodate these rock properties to optimise the rock-fracture process

An overview on advances in computational fracture mechanics of rock

Due to its complexities, rock fracturing process still poses many pressing challenges despite intense research efforts. With the rapid development of computational mechanics, numerical techniques have gradually become robust tools for the investigation of rock fracture. Nevertheless, not all of the devised methods are capable of adequately modelling the rock fracture process. For an accurate simulation of the process, a numerical method needs to be capable of modelling crack initiation, propagation, bifurcation, coalescence and separation. This paper provided a review of recent advances in computational analysis of the rock fracture process, which is built upon a number of literature on numerical modelling of mechanics of failure in rock and other brittle materials. After briefly discussing the fundamentals of rock fracture mechanisms, the basic structure of the existing and recently developed numerical techniques such as finite element method, boundary element method, distinct element method, combined methods and multi-scale coupled method are illustrated. Finally, the strengths and weaknesses of these numerical techniques are discussed and the most promising methods are highlighted

Development of a 3D Hybrid Finite-Discrete Element Simulator Based on GPGPU-Parallelized Computation for Modelling Rock Fracturing Under Quasi-Static and Dynamic Loading Conditions

As a state-of-the-art computational method for simulating rock fracturing and fragmentation, the combined finite-discrete element method (FDEM) has become widely accepted since Munjiza (2004) published his comprehensive book of FDEM. This study developed a general-purpose graphic-processing-unit (GPGPU)-parallelized FDEM using the compute unified device architecture C/C ++ based on the authors' former sequential two-dimensional (2D) and three-dimensional (3D) Y-HFDEM IDE (integrated development environment) code. The theory and algorithm of the GPGPU-parallelized 3D Y-HFDEM IDE code are first introduced by focusing on the implementation of the contact detection algorithm, which is different from that in the sequential code, contact damping and contact friction. 3D modelling of the failure process of limestone under quasi-static loading conditions in uniaxial compressive strength (UCS) tests and Brazilian tensile strength (BTS) tests are then conducted using the GPGPU-parallelized 3D Y-HFDEM IDE code. The 3D FDEM modelling results show that mixed-mode I-II failures are the dominant failure mechanisms along the shear and splitting failure planes in the UCS and BTS models, respectively, with unstructured meshes. Pure mode I splitting failure planes and pure mode II shear failure planes are only possible in the UCS and BTS models, respectively, with structured meshes. Subsequently, 3D modelling of the dynamic fracturing of marble in dynamic Brazilian tests with a split Hopkinson pressure bar (SHPB) apparatus is conducted using the GPGPU-parallelized 3D HFDEM IDE code considering the entire SHPB testing system. The modelled failure process, final fracture pattern and time histories of the dynamic compressive wave, reflective tensile wave and transmitted compressive wave are compared quantitatively and qualitatively with those from experiments, and good agreements are achieved between them. The computing performance analysis shows the GPGPU-parallelized 3D HFDEM IDE code is 284 times faster than its sequential version and can achieve the computational complexity of O(N). The results demonstrate that the GPGPU-parallelized 3D Y-HFDEM IDE code is a valuable and powerful numerical tool for investigating rock fracturing under quasi-static and dynamic loading conditions in rock engineering applications although very fine elements with maximum element size no bigger than the length of the fracture process zone must be used in the area where fracturing process is modelled

Analytical and experimental study of pressure dynamics in a pulsed water jet device

Pulsed high-velocity water jets are of interest for breaking rocks and other materials. This paper describes a straightforward way of generating single water pulse with a hammer impacting a piston that rests on top of a chamber filled with water. This impacting action pressurises the water, expelling it at high velocity through a nozzle. A theoretical investigation is outlined aimed at gaining a better understanding of this system for generating water pulses. A computational model is developed to simulate the pressure dynamics in the chamber based on continuity and momentum equations for a compressible viscous flow. This model is used to optimise the relative sizes of the hammer and piston as well as the height of the water column to produce the highest velocity water pulse. The model was validated by building an experimental apparatus. In these experiments maximum pressures of about 200 MPa were measured inside the chamber over a time period of about 560 μs. This produced a water pulse with maximum velocity of 600 m/s. Experiments were conducted with nozzle diameters between about 1 mm and 4 mm to study the effect of discharge volume on the pressure history. The results illustrate that although the peak attainable pressure decreases with an increase in nozzle diameter, the duration of the elevated pressure remains similar for all nozzles