Open Cut » Drilling & Blasting
ACARP Project C5004 "Assessment and Optimisation of Pre-Split Blasting in Open Cut Mines" ran from March 1996 to May 1997. Its aims were:-
- to review and assess current pre-split practice in the Australian open cut coal industry;
- to investigate pre-split mechanisms and the effect of rock mass structure on pre-split results; and
- to develop a computer tool which would aid the pre-split design process and provide a means of assessing and archiving pre-split results.
The project began with an industry review of pre-splitting techniques at mines in New South Wales and Queensland. The purpose of this review was to assess the status quo in pre-splitting, to brief the industry at large about the C5004 project, to obtain industry guidance for the next project phases and to identify suitable sites for field trials.
The review covered 26 mines in Queensland and New South Wales. While not exhaustive, the survey did represent over 90% of Australian open cut coal mines by ROM tonnage and was very successful in providing a clear picture of the technical status of the open cut coal industry in pre-splitting.
The review information was used to establish a typical approach to pre-splitting for Australian open cut coal mines. The elements of the typical approach were then examined in turn and any site-specific variations from the norm were detailed. In this way, each mine's response to it's own site problems was highlighted without losing sight of the broad picture.
Observations made during the review trip clearly showed the need for a model of pre-splitting which incorporates geological structure. The review trip confirmed that the initial model concept was sound though the need to reassess some details was demonstrated.
Field work was conducted at two mine sites during the Project - BHPAC Gregory Mine and CRA's Mount Thorley Mine in the Hunter Valley. Gregory provided a full data set for validation of the empirical model developed during the project, while Mount Thorley provided valuable data about pre-split mechanisms.
The investigation of pre-split mechanisms included detailed numerical work using the FLAC package. Cases for charged and uncharged holes, and joints at varying angles were modelled successfully. In general, numerical modelling of joints validated the conceptual empirical model which was developed during the project and is contained in the project software.
Numerical modelling of empty holes in the pre-split line ran in conjunction with field work at Mount Thorley mine to investigate a 'dynamic notch' phenomenon. This suggests that an uncharged hole in the line can provide an adequate focus for the tensile stresses, leading to the development of a successful split in many circumstances. This gives rise to a number of cost-saving possibilities which are explored in this report.
The numerical modelling of dynamic notching was backed up with model-scale experiments. Although not totally successful, these experiments reinforced the results from modelling.
The project software was developed according to plan, the conceptual 'joint influence model' modified in response to the industry review observations and subsequent field work. An easy means of assessing pre-splits through image analysis was incorporated in the software.
The program runs a database which can store essential pre-split data and relevant production blast information on a case-by-case basis. It also stores pictures and contains unlimited comment fields to extend its archiving role.
The first comprehensive data set was obtained from BHPAC' Gregory Mine. It was one of two sites which agreed to host C5004 fieldwork. The fieldwork at Gregory was used to validate the model and develop the program. Gregory Mine uses a typical approach to pre-splitting and had linked a deterioration in wall quality to changes in joint characteristics. In fact, Gregory was one of the few mines to report that it adjusted hole spacing in the pre-split line to counter unfavourable jointing.
Gregory is a typical mid-Bowen Basic open cut coal mine, situated close to Emerald in Central Queensland. The overburden is a mixture of hard sandstone, weak shales and variable mudstone. Although jointing is not generally such a problem as to cause major wall failures, face quality had been linked to joint characteristics in the past.
Geology & Structure
The work was performed in the Ramp 4 West area. Here, the field team ran a 50 metre scan line along the exposed highwall in the Ramp 4 West area. A cherry picker was used to elevate the survey tape above the top of coal and to pick out the joints in the face. Twenty-five significant joints were logged in the face. It was determined that the joints for this face fell into two sets, one major and one minor.
The corresponding lithology fell into three zones:-
A lower sequence of interbedded sandstones, shales and siltones.
A thin coal seam (the Corvus) bounded by weak shales and mudstones at mid-bench level.
An upper unit consisting of a massive, fine sandstone.
Uniaxial Compressive Strength (UCS) and tensile strength measurements of overburden at Gregory were made available. Measurements for the weakest rocks to be exposed in the wall averaged 20 Mpa compressive strength and 2 Mpa tensile strength.
There is a broad agreement between numerical modelling and field observation concerning the mechanisms behind a successful pre-split. Once the shock wave from a detonating hole reaches a neighbouring hole (or zone of damage created by that hole) the geometry of the pre-split line ensures that succeeding events will co-operate to produce the required split. Both modelling and field observation also confirm that an empty hole between two charged ones can direct the split successfully in many situations. The modification of stress distribution caused by the presence of an uncharged hole can produce a fracture in its walls which will propagate towards the detonating hole. This crack has been termed a 'dynamic notch'.
Clearly, this situation raises the possibility of leaving uncharged holes in the line. However, numerical modelling shows that this effect is constrained by the effects of rock mass structure. It appears that joints striking perpendicular to the line are virtually ignored by the split. Therefore, it is proposed that leaving empty holes in the line should only be attempted in a massive rock mass or in one containing favourably orientated joints.
Since the industry review showed that few mines have a detailed description of their joint domains, careful field trials should be undertaken even where joints appear to be favourably distributed. The effect of jointing is considered further in Section 4 of this report.
It may be possible to improve pre-split performance for a given hole spacing and charge distribution by introducing timing into the line. If holes are tied in alternatively with a small amount of timing, it should be possible to generate the dynamic notch in a charged hole before it detonates. The timing must be sufficient to allow the shock wave to reach the next hole and develop the crack (in the region of 10 milliseconds), but not long enough to risk cut-off.
When the delayed hole does detonate, the notch would be present to guide the split in the correct direction. In the case of cord surface ties, this is easy to achieve. Two lines would be run, each going to alternate holes. A 9 millisecond delay would be inserted between the two lines.
Alternatively, where the pre-split program is rated successful for a given site, there is an option to trial increased hole spacing in conjunction with the dynamic notch effect. This would save drill capacity for a slight increase in accessories cost and tying-in time on the bench.
It should be stressed that the success of these measures will be very site-dependent and there are no guarantees. Numerical modelling and model-scale experiments have an important role to play in advancing our understanding of the mechanisms involved but they are no substitute for carefully designed field trials.
When the face was exposed, approximately 5 months after the pre-split blast, the field team returned to the mine in order to take assessment photographs. The photos were digitised and the program used to assess the hole trace reaction as described above. Over forty holes were processed to give an overall hole trace ratio of 0.45.
While this is less than half, the result was rated successful. In many cases, a change in face lithology obscured the hole trace and it could not be logged with any confidence. In virtually all cases, a clear hole trace could be seen exposed at a number of points along the hole position in the face.