A mine located in northern Mexico extracts gold, silver, zinc, and lead, processing 3,850 tons of dry tailings daily, transported via pipelines with a water-to-solid ratio of 35% to 65%. The site has three tailings storage facilities (TSFs), with TSF-3 under construction and partially operational during a seepage incident in early 2021. In 2020, SRK contributed to the design of the TSF-3 buttress and raise, carrying out preliminary geotechnical investigations to better understand the dam’s current conditions.
The incident summary begins on February 11, 2021, a seepage issue was observed in Basin 2, a sub-basin within TSF-3. The client promptly reported it to SRK. Over the next five days, the seepage worsened, leading to the discovery of a second-affected zone on the main embankment. SRK’s initial investigation suggested that the cause might be piping, characterized by the erosion of materials due to water flowing through or beneath the structure.
The TSF-3 structure has a mid-level barrier, mainly made up of natural terrain, dividing it into two sections: one active (Basin 1) and another in development (Basin 2). Additionally, the urgent release options for TSF-1 and TSF-2 further contributed to the operational complexity.
In terms of prompt action taken to address the seepage issue, the mine’s team implemented a Trigger Action Response Plan (TARP) designed by SRK. This included a series of coordinated steps aimed at stabilizing the situation.
SRK recommended several measures, including temporarily suspending the disposal of tailings and water into Basin 1 to relieve pressure on TSF-3. Simultaneously, excavation work on Basin 2 stopped, and a buttress was built, changing the affected area’s shape. This modification prevented construction delays and expanded the mine’s storage capacity.
Reclaimed water was rerouted to TSF-1, while TSF-2 underwent emergency cyclonic releases. To prevent water pressure spikes that could endanger the dam’s structural integrity, water was withdrawn from TSF-3.
Alerts were immediately issued to downstream areas near TSF-3 regarding potential risks. Alternative emergency water discharge solutions were explored to ensure operational continuity and safety.
Among key observations during the event, continuous monitoring revealed expanding wet zones on the downstream side of the main embankment, along with altered water flow patterns.
Furthermore, enhanced field inspections identified these changes, as well as an increased expansion of wet zones, indicating infiltration. SRK consultants from various offices supported this finding.
In the process of stabilizing the situation after stopping waste and water discharges on February 21, 2021, no further seepage or wet zone expansion was detected.
Following stabilization, a comprehensive inspection of the basin and geosynthetics was conducted. This led to enhanced quality control measures for geosynthetic placement in future operations.
Finally, lessons learned, proactive monitoring, contingency planning, and quality control are essential for tailings management. Swift TARP activation and coordination prevented escalation, protecting operations and communities. Collaboration among SRK’s multidisciplinary teams was crucial.
