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Improving Blasting Outcomes & Reducing Geotechnical Risks

Open Cut » Geology

Published: September 21Project Number: C28041

Get ReportAuthor: Italo Onederra, Felipe Chacon, Sarma Kanchibotla | University of Queensland

This project has been the first attempt to integrate developments in blast modelling with geotechnical analysis to improve blasting outcomes and minimise geotechnical risks.

The project's three main objectives include:

  • The development of preliminary documentation for floor disruption blasting, by configuring, applying, and testing advanced three-dimensional blasting models with existing mine based data. The aim was to understand how floor disruption can be consistently and successfully applied to reduce the risk of low wall instabilities.
  • Demonstrating via a proof of concept how blast modelling and slope stability analysis could be integrated to evaluate low wall stability risks.
  • Demonstrating through the use of modelling capabilities the impact of blast direction and geological structures on rock mass damage.

One of the key outcomes of this project was the development of a workflow that adapted a suite of models to integrate blast modelling outcomes with geotechnical analysis. The focus was primarily on understanding floor disruption practices to minimise low wall stability risks, however modelling techniques were also used to explore the impact of blast direction and geological structures on rock mass damage.

HSBM blast modelling results suggested that when blasting weak bands, surface cratering effects are reduced, and this may have an impact on disruption when drilling perpendicular to these bands. It is however important to note that HSBM modelling does not consider gas flow so the ultimate surface heave may be underestimated.

Peak particle velocity modelling using the HSBM indicated that rock mass breakage can also concentrate between layers, and particularly at layer boundaries due to stress wave reflections. This would have an impact on the overall floor fragmentation.

Using advanced blast movement modelling (SMIBLAST), disruption and continuity indices were developed to rate the extent of layer disruption, based on material movement profiles. Analysis from layer disruption models indicated that, for practical purposes, a disruption index greater than 50% can be used as a target to accept that the continuity of a layer has been disrupted. This also translates to a continuity index of less than 50%. We refer to this as the 50/50 rule.

Modelling and analysis using existing mine site data showed that timing direction and confinement condition may also affect the extent of disruption. Firing from the High wall to the Low wall side can produce relaxation effects (power through) on the low wall toe side (i.e. dynamic void formation) which appears to increase the risk of potential instabilities; having an immediate impact on the current low wall, however its needs to be noted that this low wall will be covered by the next strip and may not cause any operational risks.

Floor disruption designs modelled and analysed in this study suggest that disruption of lower (deeper) layers is not as effective as the less confined upper layers. However, whether this will affect overall stability needs to be further investigated as those lower layers may not be playing a significant role on the formation of the final failure envelope.

The interaction of firing direction and major structures is complex and requires further work to be fully understood. Evidence of localised damage shows the importance of effective pre-split practices. Focus may need to be directed to auditing pre-split outcomes in a systematic and practical way.

A preliminary integration between blast modelling output and FLAC 3D was successfully demonstrated. The first phase of work involved building a data transfer protocol to achieve the required geometry and meshing configuration.

Following the strength reduction method used in FLAC3D, factors of safety (FoS) for undisrupted and disrupted cases were calculated. Results indicated that the data transfer and modelling approach is sensitive to the simulated disruption outcomes. The analysis also showed the importance of assumed material properties, location of disrupted layers and ground water conditions on overall stability.

The analysis using FLAC 3D also confirmed that disrupted surfaces could be incorporated realistically into a geomechanical code; and that the potential benefits or otherwise of floor disruption could be evaluated in a quantitative manner; this may also include testing the optimum location of disruption zones to minimise risks to low wall stability during construction and extraction of the next coal seam.

Learnings from the research conducted indicated that in order to improve the frictional properties of the floor and minimise the risk of low wall instabilities, it is important to re-configure (disrupt) the orientation of the existing bedding planes (including weak bands) without excessively fragmenting the material. The objective of the blast is to promote more heave than fragmentation. A localised zig-zag disruption effect would be ideal. Further analysis has shown that this “ideal” disruption could be achieved by changing the configuration of the floor disruption pattern.

Design details will be site specific but may involve some simple modifications to the current pattern configuration. Note that this is only conceptual; and it is not expected that angled holes will be exact; but to enhance bedding plane rotation and surface heave, some level of angularity will be needed. Parameters can be compared to box-cut trench blasting with lower powder factors and with the use of low-shock water resistant products. Modelling of surface heave for this configuration showed improvements that can enhance the overall disruption and rotational effects required in both upper and lower layers. Lower powder factors will also help in maintaining coarse fragmentation outcomes.

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