Controlling Block Movement of Coal During Overburden Blasting

Several open pit coal mines in Australia recognise that overburden blasting especially cast blasting, increases the likelihood of coal damage and loss resulting in significant negative economic impact on their operations. Earlier ACARP Projects C3017, C5005 and C8040 investigated this problem in detail (Wedmaier and Scott (1995), Kanchibotla et al. (1999)) and identified that block movement of coal during overburden cast blasting to be an issue at several mine sites for further investigation.

In December 2002 DynoConsult, the consulting arm of Dyno Nobel Explosives was awarded the Australian Coal Research Association Program (ACARP) Project C11051 - Controlling Block Movement of Coal During Overburden Blasting. The main objective of this project is to understand the mechanisms of block movement of coal during overburden blasting and develop practical solutions to minimise this movement. The project has been implemented over two and half years in five phases including industry reviewing, benchmarking, modelling, validation and final reporting.

The project started in February 2002 with an industry review, during which a number of medium to large scale open pit coal mines in the Hunter Valley and the Bowen Basin were visited. The main aim was to understand the current industry practices with respect to coal loss problems. An important component of the review was the collection of site-specific data on geological conditions, blast designs and practices to understand the conditions that enhance the block movement of coal during overburden blasting. The industry review provided good background information on coal losses resulting from overburden blasting and in particular cast blasting. It is generally understood that cast or throw blasting tends to increase the likelihood of coal losses and negatively impact subsequent ROM recoveries. During the industry review Goonyella Riverside mine of BMA and Rix's Creek Mine of Bloomfield Colliery agreed to participate in the field studies.

The industry review was followed with a benchmarking exercise at the two mine sites. The benchmarking involved a drill and blast audit, high speed video photography and pre and post blast coal roof survey. Two full-scale production blasts at Goonyella Riverside mine and one full-scale production blast at Rix's Creek were monitored as a part of the benchmarking exercise. These studies were designed in consultation with the site personnel with the central objective of understanding the site specific conditions and predominant mechanisms promoting coal loss and block movement during blasting.

The drill and blast audit results from the benchmark blasts at both the sites showed significant variation in the hole depths between design and actual, resulting in inconsistent standoff distances. At Goonyella Riverside, ragged high-wall conditions also resulted in inconsistent front row burdens. Current industry practice identifying the top of coal is based on geological models or by drilling one in every fourth or fifth hole to the coal seam. This however can lead to inappropriate standoff design and hence coal loss.

The benchmarking study results from Goonyella Riverside indicated that the dominant type of blast induced coal loss was edge loss with block movement localised and controlled by the presence of a geological structure. The predominant coal loss mechanism was due to the over-confinement of the explosive in the toe region or top of coal seam. A combination of excessive front row burdens and insufficient standoff distance resulted in excessive confinement at the toe region of the bench promoting edge losses.

The benchmarking results from Rix's Creek indicated a combination of roof, edge and block movement type losses especially in the Arties 25 seam. The results identified block movement or 'dragging' of the seam towards the low-wall especially along the parting band between A25 and A24 seams. The predominant coal loss mechanism at this mine was found to be a combination of excess energy from the insufficient standoff distances and high drag forces resulting from high overburden velocities at the toe. Presence of major geological discontinuities in the overburden and parting bands between the A25 and A24 seams aggravated the intensity of coal loss and damage.

A two-dimensional discrete element numerical code was developed to simulate overburden bench blasting in an open-pit coal mining operation. This model was based on the Particle Flow Code (PFC) developed by Itasca Consultancy Group, USA. The main objective of this model is to understand the impact of different blast design parameters under different geological conditions, such that it can be used with proper engineering judgment to control and manage coal loss.

The model was calibrated to the site specific parameters of the two participating mines and a number of simulations were conducted for each site. The purpose was to understand the predominant coal loss mechanisms and to develop alternate solutions. The mechanisms of coal loss observed in the model simulations and the overall trends in the results are found to be consistent with the field observations. Based on the model simulations and benchmarking results a number of site specific recommendations were made for each site.

The cost benefit analysis from this project clearly indicates that the potential value generated from the additional revenues far outweighs the cost of preventative measures. Such measures will vary from site to site as they are dependent on geological characteristics, drill and blast practices and mine economics. A systematic process based on quantitative measurement is necessary to understand the mechanisms of coal loss and develop appropriate preventive measures to minimise loss. Such a process was developed and successfully implemented in this project.