SRK has been working for Sierra Metals at the Yauricocha Mine in Peru to help improve ground conditions that have historically resulted in mud rushes. The work has employed risk assessment tools to reduce the risk of fatalities and property damage due to uncontrolled mud rush events.
Published literature of mud rushes is limited to block caving or sub-level caving events in fractured hard rock. Ground conditions at Yauricocha are unique because the mineralised ore is soil-like at depths greater than 800m. The challenge facing the SRK team was to understand the trigger mechanisms associated with fine wet material and the large number of groundwater inflows.
SRK was able to demonstrate that mud rush conditions at the mine are the result of elevated pore pressures in medium permeable materials and the strain-softening effects, due to mining-induced stresses, that result in soil particle mobilisation. The dilated in-situ material increases in water content that results in plastic or liquid flow depending on the fines content and the plasticity index of the ore-material. The conceptual model was tested with UDEC numerical models to define the trigger magnitudes contributing to flow deformations that simulated mud rushes.
The practical results from this work were the development of a risk chart describing the triggers that can cause mud rushes and the laboratory test protocols to be followed. A set of field procedures were developed for sampling and testing so that the engineers could rank the risk potential for each drawpoint. Mining in the highest risk area of the mine had been shut down by the Peruvian mine authority due to the recent mud rush activity, but as a result of this work, the authorities have allowed mining to continue.
SRK has also worked with the mine to improve ground support so that the large strains of these soft material loads underneath are properly transferred to the support elements. Modifications to the steel set installation procedures and application of shotcrete have contributed to the mine’s safety and business sustainability.
