I. Onederra

Modelling the size of the crushed zone around a blasthole

A new model to predict the extent of crushing around a blasthole is presented. The model is based on the back-analysis of a comprehensive experimental program that included the direct measurement of the zone of crushing from 92 blasting tests on concrete blocks using two commercial explosives. The concrete blocks varied from low, medium to high strength and measured 1.5 m in length, 1.0 m in width and 1.1 m in height. A dimensionless parameter called the crushing zone index (CZI) is introduced. This index measures the crushing potential of a charged blasthole and is a function of the borehole pressure, the unconfined compressive strength of the rock material, dynamic Youngs modulus and Poissons ratio. It is shown that the radius of crushing is a function of the CZI and the blasthole radius. A good correlation between the new model and measured results was obtained. A number of previously proposed models could not approximate the conditions measured in the experimental work and there are noted discrepancies between the different approaches reviewed, particularly for smaller diameter holes and low strength rock conditions. The new model has been verified with full scale tests reported in the literature. Results from this validation and model evaluations show its applicability to production blasting.

Measuring blast fragmentation at Esperanza mine using high-resolution 3D laser scanning

Abstract Image analysis as a technique for fragmentation measurement of rock piles has been the subject of research since the 1980s, and to date, run of mine (ROM) fragmentation optimisation studies have primarily relied on particle size measurement using photographic-based 2D imaging systems. Disadvantages of 2D imaging systems include particle delineation errors because of variable lighting and material colour and texture variation; no direct measure of scale and perspective distortion; and inability to distinguish overlapped particles, non-overlapped particles, and areas of fines. With the development of 3D imaging technologies, there is an opportunity to develop techniques that could improve data collection and overcome the limitations of existing 2D image-based systems. This paper describes the first attempt to use 3D high-resolution laser scanning techniques to quantify ‘whole of muckpile’ fragmentation from full-scale production blasting. During two monitoring campaigns in 2013, high-resolution laser scanning data were collected from production blasts at Esperanza mine (Antofagasta Minerals Group). Fully automated analysis of the 3D data was possible in all cases where the data were of sufficiently high resolution. Manual pre-processing was required when the data were of low resolution to specify the region of fines. Overall results indicated that ROM fragmentation requirements were meeting specified targets, despite the marked differences in powder factors. This was particularly the case for those blasts conducted in similar geological domains. This work has demonstrated that high-resolution laser scanning can be used as an alternative technique to measure ‘whole of muckpile’ fragmentation in production blasting.

Blast Induced Damage and Dynamic Behaviour of Hangingwalls in Bench Stoping

This paper presents the results of a comprehensive monitoring program designed to investigate the extent of blast induced damage experienced by rock masses extracted by bench stoping methods. An array of triaxial geophones and extensometers were used to monitor blast vibration attenuation and measure hangingwall deformations during stope extraction. In addition, pre and post surveys of the hangingwall rock mass were conducted using a TV borehole camera and cavity survey instrumentation. These surveys were later used to calibrate damage profiles into the stope hangingwalls. Peak particle velocity, hangingwall deformation measurements and stope surveys were used to develop a site specific damage model that allowed engineers to asses drilling and blasting configurations to minimise the extent of pre-conditioning and damage. In addition the study included the analysis of the frequency response, displacements and accelerations experienced by the excavation as extraction and mine filling progressed. This work aimed at improving our understanding of the influence of blasting on the dynamic behaviour of stope hangingwalls. The study demonstrated that estimates of the maximum extent of rock mass pre-conditioning and/or damage made through the application of the Holmberg-Persson approach compared well with measured results. In addition, the study found that dynamic loading imparted on an exposed hangingwall from subsequent stope blasting was also expected to contribute to rock mass weakening and that mine filling was crucial to arrest further deterioration. Hangingwall accelerations were used to demonstrate that larger openings may be more susceptible to dynamic loading.

Estimation of fines generated by blasting-applications for the mining and quarrying industries

Abstract This paper introduces an engineering approach to estimate the proportion of fines generated during the blasting process. The proposed framework is based on the combination of two Rosin-Rammler based distribution functions to model the full range of fragments expected to be produced during this process. This particular approach, which has been successfully applied for a number of years by the Julius Kruttschnitt Mineral Research Centre (JKMRC), has been improved with the introduction of a new model to predict the potential volume of crushed material resulting from the crushing and shearing stages of blasting. Other sources of fines including liberation of infilling from discontinuities, particle collisions and post-blast processes have been excluded to simplify the modelling process. Validation analysis of the proposed framework has shown that there is good agreement between model predictions and the measured distribution of fines. In three distinct cases, results verified the hypothesis that a single index of uniformity can be used to describe the distribution of fragments in the range of 1 mm through to the expected post-blast mean fragment size (x50). Although some limitations have been noted, the approach appears to provide useful approximations for continuous improvement analysis and applications. The practical application of the proposed modelling framework is demonstrated with an engineering study aimed at assessing the impact of blast fragmentation on the overall production of fines in a hard rock quarry. Results from simulations showed that less crushing requirements due to an overall increase in fragmentation contribute to a decrease in the specific crushing energy and hence a reduction in power consumption requirements. This analysis helped demonstrate the importance of addressing the impact of blast fragmentation distribution on overall quarry productivity requirements; and highlights the importance of adopting a holistic approach when addressing the blast optimisation problem.

Understanding main causes of nitrogen oxide fumes in surface blasting

Abstract Post-blast nitrogen oxide fumes (NOx) from surface blasting activities have become an important operational issue in Australian Coal mines. Post-blast fumes are a direct product of the detonation process which can be easily identified as the resultant yellow to orange post-blast clouds. There is general agreement that the conditions leading to fumes are associated with fuel deficiencies or incomplete detonation of the explosive product. From a practical perspective this can be due to one or a combination of factors such as explosive product characteristics, confinement effects, ground conditions, inappropriate blast design parameters, explosive product selection, on-bench practices and potential contamination of explosive product in the blasthole. This paper presents the preliminary results of a project funded by the Australian Coal Association Research Program (ACARP). The main objective of this project is to gain a better understanding of the principal causes of post-blast fumes at the operational level so that incidences can be minimised. Results from a comprehensive literature review, an industry survey and analysis of newly established blast registers have indicated that an appropriate matching of product to ground is essential to minimise fume incidences. For this to occur there must be a clear understanding of the potential impact of the characteristics of key components in any given product formulation and ANFO/Emulsion blends. Analysis has confirmed the impact of confinement and ground conditions as well as the potential contamination of product with fine drill cuttings near the stemming region. Trends associated with increases in the likelihood of fume incidences from long sleep times were identified but could not be entirely confirmed with the available data. The analysis on overburden blasts showed that it is fair to assume that the likelihood of high level fume incidences may increase when a product is slept for >10 days and that decisions to have product sleeping for longer periods should be supported by a good understanding of the product characteristics and ground conditions (geology and hydrogeology). Further work is required to build capabilities to verify on-bench product specifications and characteristics including the ability to measure in hole density changes, product moisture, verify AN/Emulsion blending ratios and undertake a more through characterisation of ground conditions. It is also important to extend the range of measurement systems to evaluate the performance of the detonation process in situ. To that end the authors have embarked in the development of instrumentation to better understand the impact of changing characteristics of explosive products in situ; a brief description and preliminary results from this research effort are also discussed in this paper.

Modelling fracturing, disturbed and interaction zones around fully confined detonating charges

Abstract In order to gain better insights into the complex mechanisms at play under fully confined blasting conditions in mining applications, several models were constructed and analysed using the hybrid stress blasting model (HSBM). A disturbed zone or microdamage criterion was proposed and used in the modelling analysis. It combined a lattice bond contact tensile failure criteria and a simple peak particle velocity based approach. Estimates were made of the extent of fracturing, disturbed and interaction zones from fully confined blastholes spaced at distances of up to 18 m, simultaneously initiated with primers positioned every 8 m. Continuous interaction was evident along the explosive column at spacings of <15 m. When blastholes were spaced at 18 m, the attenuation of the stress wave showed only partial or limited interaction along the column. With regards to the influence of in situ stress magnitudes in the range of 500–1500 m, modelling results appeared to capture the impact of stress intensity on the final extension of the macrofracturing zone. At depths of 1200 m and above, the degree of continuous interaction was diminished at the level of the initiating primers. For the simulated geotechnical conditions, the orientation of radial fractures with respect to the principal stress direction is more evident at anisotropy ratios greater than two. However, the extent and shape of the disturbed zone does not appear to be influenced by anisotropy, which showed a deficiency in the implementation of in situ stresses in the current modelling framework and further work is being conducted to address this limitation.

Development of pressure and temperature gauges to monitor in situ performance of commercial explosives

Abstract A better understanding of the detonation performance of an explosive charge can be gained by directly measuring pressure, temperature and velocity of detonation (VOD). This is particularly important with explosives used in the mining industry because their performance is directly influenced by the degree of confinement given by the borehole diameter and the surrounding rock mass. A project funded by the Australian Coal Association Research Program (ACARP) was initiated in early 2009 with the view to design and build cost effective prototype instrumentation to measure the relative differences in detonation pressures and temperatures of commercial mining explosives. The project’s primary focus was on low density explosives. Low density (also referred to as low shock) explosives have been available in various forms for nearly 20 years. There have been significant advances in the availability, reliability and flexibility of these explosives which now offer a range of densities and degrees of water resistance. Their detonation performance is not well understood and cannot be accurately predicted with current ideal and non-ideal detonation codes. It was therefore viewed as important to be able to directly measure pressure and temperature during the detonation process of these complex mixtures in production blastholes. Identifying the pressure release patterns of both conventional and low density explosives under different geotechnical conditions should provide the necessary information to both validate detonation codes and better define input parameters for breakage and fragmentation models. This paper gives a general description of the prototype instrumentation developed and reports on the results obtained to date in both laboratory and full scale conditions.

An Alternative Approach to Determine the Holmberg-Persson Constants for Modelling Near Field Peak Particle Velocity Attenuation

A new approach to determine the Holmberg and Persson site specific constants (K and α) for modelling near field peak particle velocity (PPV) attenuation due to blasting is presented. Currently the Holmberg-Persson approach requires the measurement of peak particle velocity at several locations resulting from a known explosive source. The objective is the determination of the attenuation characteristics of the rock mass in a specified direction and for a given explosive type. The new alternative approach significantly reduces instrumentation and monitoring requirements. It uses an analytical method to predict the peak particle velocity at the point at which rock crushing ceases and combines this with a known value of PPV measured at a pre-defined location. Four case studies which include both model scale and full scale blasting conditions are used to demonstrate and validate this alternative approach.

Design methodology for underground ring blasting

Abstract This paper discusses a systematic approach to underground ring design as well as a methodology for the continuous improvement of designs as conditions change. The methodology is applicable to designs for prefeasibility and feasibility stages as well as designs for currently producing mines. The proposed method still recognises the role of experiential guidelines but provides additional and novel empirical techniques to improve the first pass approximations such that they better suit the prevailing geotechnical conditions. The strength of this method is that the designer is able to assess the impact of the design in terms of the expected fragmentation and potential damage to the surrounding rock mass.

Development of a novel mining explosive formulation to eliminate nitrogen oxide fumes

Abstract Post-blast nitrogen oxide fumes (NOx) from coal overburden blasting may occur in a variety of geological conditions with the use of bulk ammonium nitrate (AN) based explosive products. In Australia, government directives to stop blasting activities because of NOx fume incidences have led to costly delays in production, which has directly impacted on the ability of operations to meet production targets. Nitrogen oxide and nitrogen dioxide can cause serious health risks to persons exposed, with excessive levels of NO2 also affecting the viability of flora and root systems. A number of research projects in Australia have focussed on minimising the risk of NOx fumes by better understanding the behaviour of current explosive products. The main outcomes from these projects have been the development and implementation of guidelines or administrative controls to minimise the NOx fume risk and reduce the potential exposure to the hazard. This paper describes preliminary work to provide a step-change solution that has the potential to completely eliminate the NOx hazard. A novel formulation that substitutes the use of AN with oxygenated water (OW) as the main oxidising agent has been developed and recently patented as part of a PhD program at The University of Queensland. The detonation properties of mixtures made with OW and fuel were studied. Unconfined velocities of detonation (VODs) tests of OW sensitised mixtures were conducted. It was found that for reliable detonation to occur, a minimum level of sensitisation must be accomplished. Adequately sensitised mixtures, with a water content of 47% by weight, were able to detonate at velocities in the range of 2600–5000 m s−1, with a critical diameter of the order of 23 mm. The recorded detonation velocities were clearly dependent on the mixture density and charge diameter, similar to the non-ideal behaviour of AN-based commercial explosives.