Mudrush, mudflow, or mudpush are the most common terms describing uncontrolled ingress of assorted mixture of water and solids. Mudrush is the underground equivalent of surface debris flow. It can have different origins but produce the same results: injury, loss of life, damage to property, excess dilution, and production delays, or -- in extreme cases – mine closure. Mudrush dynamics in underground mining are especially complex due to confinement and stress within the muckpile.
Block caving and sublevel caving operations are inherently susceptible to internal mudrushes because they have the potential to accumulate water, generate fines through comminution process, and through production activities, provide disturbances as well as a discharge point. Block caving operations are also susceptible to external mudrush flows because the broken muckpile connects the surface with the underground excavations.
Although mudrushes are more common in caves than in other mines, any mining activity that enables the accumulation of fine particles and water is susceptible to mudrushes. Cases exist of injuries and fatalities from sudden ore pass discharges, the collapse and subsequent flow of unconsolidated or poorly consolidated backfill, and the failure of tailings and slimes dams. In September 1970, 89 miners were killed at the Mufulira mine in Zambia due to an inrush of 450,000 m3 of muck into the workings. The muck originated from tailings dams, which were located on subsiding ground above the workings. The water, impounded in the depressed crater of the tailings that had subsided, was seen as a major contributor to the inrush.
Over the past two decades, the mining industry developed a comprehensive risk assessment including risk rating and safe operating procedures. Although mudrushes are difficult to predict and impossible to prevent, if taken seriously, the impact on operation can be minimised with proper cave management and draw control.
A mudrush seldom occurs as the result of a single cause or fault; therefore, any risk analysis has to take into account all contributing factors and combinations thereof. A system failure usually results when a combination of failures occurs in such a way that the disturbing forces exceed the capacity of the system to resist those forces.
Before assessing any risks to a mine, however, the following questions need to be answered:
- Is there potential to generate fines?
- Is there potential to accumulate water?
- Is there potential to form mud?
- What disturbance can mobilise and discharge the wet muck?
A number of caving mines operate safely with high rainfall and mud-forming potential using tele-remote equipment and strict mudrush risk mitigation procedures, such as:
- Interception of water flow into the cave (surface and underground)
- Pre-strip of mud-forming overburden waste from the subsidence zone
- Tailing ponds and other sources of water and/or mud located away from the expected subsidence zone
- Inspection of old workings for presence of water and/or mud
- Sealing off all possible access points to the cave other than operating drawpoints
- Strict draw control procedures
- Comprehensive monitoring program and reliable water balance
Pictured above: Examples of very stiff red clays from Northparkes Mine and a typical low-viscosity mudpush (Photos courtesy of Rio Tinto and De Beers)
20210330115542275.webp)