KCGM manages one of the most complex mining operations in the world, facing challenges found in few other regions. The vast underground network of historical workings throughout the Golden Mile has shaped the way KCGM has mined the Pit during the past 25 years. Mining methods have also been governed by the proximity of the Fimiston Open Pit to Kalgoorlie-Boulder.
Mining the Pit is a carefully planned sequence involving cooperation across a wide range of departments, specialist teams and partner contractors. Each has their part to play, from geology, environment, mine planning and survey teams to drillers, blasting engineers and the giant shovels and haul trucks which transport waste for disposal and ore for processing.
Voids: An ever present hazard
No other open pit operation in the world has as many voids as the Fimiston Open Pit. A century of mining on the Golden Mile left over 3,500 kilometres of historical underground workings stretching the length of the Mile and extending more than 1,200 metres below the surface.
Voids officers use historical plans and drilling data as well as Cavity Autonomous Laser Survey (CALS) technology to survey the position and size of the old workings. It is precise work involving checking the location of the voids they find against the recorded data and nothing is taken for granted as the voids may have changed in size or shape since the last survey. Safety zones are designed around the voids and flagged with red and white striped tapes to denote the higher risk voids, or black and white tape for lower risk voids.
Voids significantly complicate mining in the Pit. If they are close to the surface of the Pit floor, their location has to be identified and confirmed before a blast pattern can be laid. They influence the direction and magnitude of a blast, sometimes resulting in loss or dilution of ore. If large voids are detected further underground they may be collapsed by deep blasting or backfilled to reduce the risk and allow continued mining.
Download Fimiston Open Pit Voids Models
Once the old workings have been marked, they are drilled to confirm their boundaries. The risk of working near voids has prompted KCGM, with partner contractor Ausdrill, to develop the long-reach drill and a remote drill to better and more safely navigate voids.
The long-reach drill allows the rig and operator to remain on firm ground while the extended drilling arm probes the void. The remote drill, allows the operator to remain even further away from the target area. Probe drilling identifies the area of the voids more precisely and the hazard zones may be extended or reduced. This depends on whether they have been satisfactorily backfilled or whether the old workings have expanded through “self mining”. Once the exact positions of the old workings are known they are mapped into the mining plan.
The Mining Plan
Mine planning at KCGM is based on the location of the ore, its grade and sulphur content, the current gold price, whether there are old workings present and the quantity and grade of ore required by the Fimiston Processing Plant.
In order to design the key component of the mining plan – the blast – planning engineers work closely with geologists, drill and blast engineers and voids officers. Production geologists define the position of the ore on the bench to be mined, using technology known as Conditional Simulation to estimate the grade.
After the Voids Team has identified and marked out the voids, the geologists instigate grade control drilling using Reverse Circulation (RC) drill rigs. RC drills were developed onsite in 1999 for greater accuracy on the Golden Mile’s sharply sloping ore body. RC rigs drill longer holes at an angle to the ore body and produce a more accurate sample of ore. RC drills are able to penetrate up to 80 metres deep, they are also useful for detecting voids further underground.
After the drill samples are assayed for gold and sulphur content, the Geology Team generates a computer model of the ore body and designs the ore blocks on the bench. This dictates the blast pattern, which is laid down by production drill rigs. The 11.5 metre deep blast holes are 165 millimetres in diameter and evenly spaced about six metres apart in rows five metres apart. The blast holes are checked for depth and the presence of water, and handed over to the Blast Crew for charging.
Blasting is usually conducted three or four times a week between 7am and 6pm. Blast times are planned around production requirements, which usually require blasting at 1pm or 5pm, but this can vary. A range of factors influence the decision to blast, including wind conditions and the type, size and location of the blast. The afternoon blast is a feature of life in Kalgoorlie-Boulder, a reminder that gold is still coming out of the ground.
KCGM’s close proximity to Kalgoorlie-Boulder has shaped the production blasting methods used. Every effort is made to reduce the effects of blasting – vibration, dust and blasting noise, known as “airblast” – with blasts designed to minimise these. This requires significant monitoring both before and after blasts: weather stations provide information on wind, humidity and atmospheric pressure, and a network of monitoring stations measure airblast, vibration and dust levels.
Wind direction and speed are deciding factors in whether a surface blast will go ahead as planned. Surface blasts are known to generate more dust and, to minimise the impact of this on the city, wind conditions are closely monitored in the lead up to a blast. For this reason, surface blasts are often scheduled “within one hour’s notice”, when favourable weather is predicted to allow for safety preparations ahead of the blast. However, unexpected changes in weather and wind direction can result in surface blasts being cancelled or rescheduled at the last moment.
To prepare a blast pattern, detonators and primers are lowered into the 11.5 metre drill holes and liquid explosive, a mixture of ammonium nitrate and diesel called “bomb”, is pumped in to within four metres of the surface. The hole is then filled with stemming – screened gravel – to act as a plug and force the explosive energy down and into the surrounding rock.
Once all holes have been charged they are tied in – connected to detonate in a predetermined sequence – and the blast is ready for firing by remote control after all personnel have left the area. To minimise vibration and airblast, delayed detonation sequencing is used, meaning no two holes are fired at exactly the same time.
Download KCGM Blasting Information Sheet
Where blasting is over a void, the blast site is left for at least 12 hours, giving any old workings underneath time to settle. The blast crew will clear the site, ensuring there is no unexploded bomb in any of the holes, then the surveyors will ensure any voids are marked and mapped.
Before the ore can be dug, geologists mark out the shot, or blast site, pegging and flagging the blocks of ore and waste rock. From this they produce a “dig plan” which is sent electronically to the onboard navigation systems of the shovels which will load the ore and waste rock into haul trucks.
Load and Haul
The shovels and trucks in Load and Haul operate 24 hours a day, 365 days of the year, moving the ore and waste rock from the Pit floor to the next step in the gold recovery process – the Primary Crusher. The WENCO Production Monitoring Control System coordinates the dig plan of the blast with the schedule supplied by the mining planners in the Mine Technical Services Department to create an operating manual for the Pit for that day.
When a 750-tonne Komatsu PC8000 hydraulic face shovel moves in to scoop 66 tonnes at a time into its bucket, the dig plan in the cab’s Computer Aided Earthmoving System (CAES) navigation programme shows the operator what type of material the shovel is mining. This may be ore, marginal subgrade material or waste. The screen also shows the void areas.
The shovel operator notifies the dispatch operator in the WENCO hut, who then updates the computer with the material type and its eventual destination. As a Caterpillar 793 haul truck pulls up to the shovel, its onboard GPS notifies the dispatch system which shovel it is being loaded by. The dispatch system then uploads information on the type of load the shovel is digging and where the truck driver is to take its load. This information is displayed on a screen on the cab’s dashboard for the driver.
Once the 240-tonne capacity truck is full, the dispatch system directs the driver to its destination. If the load is clean high grade ore, it will either be directly taken to the ROM (run of mine) pad at the Primary Crusher, or unloaded to a stockpile on Pad 12 for feed into the CSI Crusher. Waste rock is taken to a dump for storage, while low grade ore is taken to a stockpile for potential processing at the end of mine life if economic.
The round trip for the haul truck may take anything from 20 to 40 minutes, depending on the load’s destination, before the truck is under the shovel bucket, ready to be filled again. Once digging starts on the blast site, the ancillary equipment moves in to assist the shovels and clean up the site once it is fully dug. Excavators, bulldozers and graders clean up the pit walls, keep the haul roads clear of debris and remove any material from old workings the shovels cannot reach.
Where the shovels are mining through historical workings the rock may be interspersed with anything from timbers, rail tracks and side-tipping wagons to picks, scrapers and even old bottles and newspapers from a long-past crib room. High grade ore contaminated with this type of debris is first spread out by machinery and then cleaned by the Stick Picking Crew. The clean material is then transferred to the crusher feed for processing, while some of the better preserved artefacts are earmarked to become part of the mining display at the Hannans North Tourist Mine.
Download KCGM Fleet List