Understanding and Controlling Coal Damage During Cast Blasting (including Workbook)

The open cut coal mining industry in Australia recognised that cast blasting increases coal damage and loss associated with overburden blasting but very few mines have quantified its impact on overall mine economics. An ACARP project "Understanding and Controlling Coal Damage in Cast Blasting" was undertaken by the Julius Kruttschnitt Mineral Research Centre in close association with the Australian open cut coal mining industry to understand and quantify the impact of cast blasting on coal damage and loss.

An industry survey involving twenty six open cut coal mines in Queensland and New South Wales was conducted in the initial phase of the project. The aim of the survey was to understand current cast blasting practices, to assess industry awareness regarding coal damage and loss and to encourage involvement by the industry in the project fieldwork.

A number of mines participating in the review acknowledged that they have not quantified the impact of coal damage and loss on mine economics in spite of experiencing significant coal loss and coal edge movement during cast blasts. The breakage characteristics of the coal seam (especially near the roof), the hardness of the roof rock, the geological complexity of the seam, identification of the top of coal, implementing appropriate stand-off distances and excessive confinement at the toe are identified as important factors in controlling coal loss and damage. The industry felt that there was a shortage of data and techniques to assess the role of each of these factors on the extent of coal loss. However in some cases appropriate data are routinely captured but not applied to the problem. For example, a number of mines who participated in the review conduct high wall coal edge surveys and drill pre-split holes to touch coal but no one has used these data to either update the coal seam model or to design the stand-off (at least in the front rows). Another example is that many mines survey post blast muckpiles and estimate the casting percentage (prior to coal seam exposure) to assess the blast performance. Even though they observe coal edge movement after the seam exposure, no mine is taking into account the effect of coal edge movement on dragline productivity or re-calculating the casting percentage.

Fieldwork was conducted at three mines within this project. The tests were designed in consultation with the mines to address their key issues. Work at each site consisted of raw data gathering, blast monitoring, assessment of blast results and quantification of coal loss. The sites were Warkworth, Gregory and Mt Thorley.

The understanding of coal loss mechanisms developed during the field studies and numerical modelling were used to develop the following mechanistic models to describe the coal seam roof:

• estimate the roof damage in coal seam
• estimate the toe left in the overburden
• estimate the effect of coal edge movement on casting percentage.

The mechanistic models are incorporated in a computer workbook along with this final report. The computer workbook provides a user-friendly environment to access various chapters of this final report and to run simulations with different cast blast designs. These models are only intended to be indicative and provide general trends and guidelines to the blasting engineers rather than provide prescriptive designs.

This project has not provided any universal guidelines to reduce coal damage during cast blasting because they are necessarily site specific and should be established based on quantitative measurements made at that site. The project findings suggest that the present strategy to maximise the throw in cast blasts without any consideration to the coal loss and edge movement may not be prudent economically. The project also highlighted the need to evaluate alternative cast blast design strategies such as baby decking and buffering the coal edge to determine their impact on the overall economics of each mine. The results from this project clearly show that the extra revenues generated by controlling coal loss and damage are large compared to the extra costs necessary to implement additional control measures.

The associated workbook/interactive software is also available on a CD for $20.00