How it works: Coal mine drilling and blasting
Glencore is one of Australia’s largest coal producers, with 13 active mining operations across New South Wales and Queensland.
One important step in open cut coal mining is drilling and blasting. This carefully planned process helps break up layers of rock and overburden so mining teams can safely access the coal seams below.
To explain how it works, Ravensworth Mine Manager Oliver Wallace takes us through the key stages of drilling and blasting at an open cut coal mine.
1. Planning and preparation
Before any drilling begins, detailed planning takes place.
A geological model of the area is developed using exploration data, log holes and geophysical information. This helps the team understand what lies beneath the surface, including the depth of the coal seams, the type and hardness of the surrounding rock, and any other ground conditions that need to be considered.
Once the area has been assessed, the surface is prepared. This includes clearing the area and removing vegetation and topsoil so drilling operations can begin.
2. Drilling
The drilling stage involves creating a pattern of holes in the rock above the coal seam.
The number, size, depth and direction of these holes depend on several factors like the depth of the targeted blasting horizon(thickness), the hardness of the rock and the outcome the team is trying to achieve.
At Ravensworth, drill hole depths can vary significantly because coal seams are not at consistent depths between seams.
“Inter-burden depth can vary significantly,” says Oliver. “One seam could require a two metre deep blast, and the next could be 25.”
3. Loading
Once the holes are drilled, a blast pattern is finalised and loading instructions are developed. These instructions inform the type and quantity of blasting product required for each hole.
“The amount of explosives is determined by the number of holes, burden and spacing, depth and the targeted powder factor, which means the amount of explosives per cubic metre of rock,” says Oliver.
Different ground conditions require different product choices.
“We use variations of the same base compounds,” Oliver explains. “It’s about matching the right product for the application.”
For example, holes containing groundwater require a different product to dry holes, while reactive ground requires a specialised inhibited product.
4. Stemming and preparation
After the blasting product is loaded, the holes are stemmed. This means inert material is placed on top of the explosives to help contain the blast energy and reduce risks such as flyrock and overpressure.
The initiating equipment is then connected, and the blast is prepared for firing. Throughout this stage, strict safety procedures are followed.
Oliver says some of the common challenges can include broken ground, fallback from drilled material, limited working areas and changing weather conditions.
“Weather conditions can also complicate the process at times, particularly wind speed and direction at the time of firing” he says.
5. Communication and safety controls
Before a blast takes place, communication is critical.
Notifications are provided to relevant stakeholders, including neighbouring operations and members of the public where required. On the day of the blast, exclusion zones are established, guards are positioned and affected areas are checked to ensure no personnel remain within the exclusion zone before the blast is fired.
These controls are an important part of ensuring the process is carried out safely.
6. Mining the exposed coal
Once the blast has been fired, the broken waste material is mined and taken to spoil dumps. This exposes the targeted coal seam so it can then be mined and transported to the Coal Handling and Preparation Plant.
The process is then repeated for the next waste horizon, continuing the mining cycle.
“The drilling and blasting process has been risk-assessed in detail,” says Oliver. “These assessments underpin the procedures and management plans developed to undertake the process safely.”