Reactive ground is a term used in the mining industry to describe the condition where an exothermic chemical reaction can occur between sulphide bearing rock and blast holes loaded with ammonium nitrate- based explosives. Reactive ground conditions increase the risk of explosives deflagration and/or unplanned detonations. Unexpected and highly reactive ground conditions were recently experienced at a large open-pit zinc mine in the Northern Cape of South Africa.
The mine was found to have geologically bounded reactive zones within its rich zinc deposits. Due to the natural process of weathering/leaching the upper benches of the transition zone are more prone to reactivity as these benches contain more exposed sulphide or sulphide bearing rock/soils. Reactive zone mapping of the geology of the mine is used to mark out potential reactive ground areas in the current and future mining blocks. An unexpected reactive ground zone was encountered at a deeper level of the mine. This level had been mined in the past and without a history of ammonium nitrate explosives and ground reactivity. This unknown reactive area was loaded with uninhibited bulk ammonium nitrate explosives and stemmed. After the loading process was completed and before blast firing an unexpected detonation of one hole occurred. There were no injuries associated with this event.
The blast block was evacuated and barricaded. For two days other blast holes showed signs of reaction including smoke emission and yellow-orange reacted emulsion froth ejected from the blast holes. After signs of reaction ceased, and the pit was declared safe, an in-pit inspection was conducted. Ground samples were collected from the reactive areas and sent for ammonium nitrate and ground reactivity analysis. Exiting this event, additional reactive ground testing and screening procedures have been enacted to reduce the risk associated with blasting in reactive ground.
Isothermal lab testing and bucket sample field screening methodologies are described in the 2012 AESIG Code of Practice and have been widely adopted as global standards for the identification and predictive behaviour of reactive ground. Other testing methods have been developed and/or modified to determine whether a specific rock sample may be classified as reactive or not. Isothermal testing more fully characterizes a suspect reactive ground sample however results require days to weeks to properly conduct. Some mines implement bucket sample screening to quickly identify areas with ground reactivity potential. In South Africa and neighbouring countries, the PVJ bucket method is widely known and employed in the field to screen for reactive ground before blast loading.
When reactive areas are identified the correct procedures can be put in place can greatly reduce the potential of causing harm.
