Pillar Reinforcement

Resin injection proves successful in sublevel caving applications

By Anthony Ferrenbach

The use of injectable resins in mining operations has become increasingly versatile due to their ability to consolidate the ground and optimize processes in unstable geological conditions. Resin was recently used successfully to reinforce a pillar that facilitated the sublevel caving of a mineral block next to a collapsed area spanning several levels.

The first approach that was considered, and quickly discarded due to its lack of safety, involved the sublevel caving to extract the block without any treatment of the entire block to be mined. Although this option allowed for the extraction of the entire block, it posed the risk of completely losing the block and the collapsed area spreading to the mining zone. Given there was a massive collapse zone, this risk was even greater.

Another option considered was to leave a natural pillar between the collapsed zone and the mining zone, which would have meant a significant reduction in the size of the block to be mined and the loss of valuable ore. This was mainly due to the required thickness of the natural pillar since the collapsed area spanned several levels. This safer option, however, would have reduced the benefit without sufficiently mitigating the risk. This option did not offer sufficient guarantees of success given the risks involved, and the benefits would be limited.

Instead, Weber Mining proposed reinforcing the pillar with resin injection between the block to be mined and the collapsed ground, creating a consolidated barrier that would allow the mine to extract the pending block safely.

Reinforcing the Pillar

To achieve this objective, a drilling template was made from the upper gallery to the block to be mined, allowing the resin to cover the entire surface of said block. The drilling template consisted of injection lines, located 5 meters (m) apart, with each line having 5 boreholes of various depths. The drilling template covered a depth of 30 m and a length of 20 m, covering the whole length of the block to be mined. A total of 250 self-drilling anchors (SDAs) were used. The use of SDAs proved advantageous, especially since the boreholes tended to collapse, complicating the drilling and placement of injection lances. Using SDAs combined the drilling and injection placement into a single step. Additionally, the traction resistance of the SDAs contributed to ground consolidation.

Given the changing ground mixed with different types of rock, and a collapsed area, this distribution is crucial to ensure the ground is correctly injected. During the injection of each borehole, the resin migrates into the ground’s cracks until the pump increases in pressure, indicating ground saturation, and the injection is stopped. This methodology, with distribution across the entire block, allowed consolidation to be achieved, adapting to the ground’s needs. Between one borehole and another, there was up to a 30% difference in consumption, given the presence of more significant cracks in certain parts of the block.

To bind the entire block, the type of resin to be used also had to be determined based on the type of ground, the percentage of void, the injection distance, the desired consolidation surface, and the injection equipment. With these parameters, it was decided to use a resin that expands to 3 times its initial volume with a setting time of 4 minutes. The 4-minute setting time provides ample opportunity for the resin to migrate into the ground’s cracks before setting, thereby consolidating the ground. Areas with more fractured ground could be effectively consolidated with increased resin injection while still maintaining a sectorized injection process. Finally, the resin selected had a compressive strength of 35 megaPascals (MPa), and bonding strength of 6 MPa, making it ideal to for creating an elastic bond that can retain its integrity even after rock movement has occurred.

Precise Consolidation

Given the collapsed ground and a high void percentage, the resin’s expansion allowed covering the area to be consolidated with less resin, ultimately accelerating the injection process and logistics, while still maintaining excellent compression resistance. The resin’s reaction time, together with the injection equipment used, ensured that the resin was applied only to the pillar’s reinforcement and not beyond, as this would have meant an unnecessary increase in costs and times.

Covering the area with strategically distributed injection points facilitated the precision of the consolidation work. The injection process started with the deepest boreholes to saturate the ground from the pillar’s base and create an upward injection with the other boreholes. It is essential to note that the resin’s compressibility and its adherence to the ground are crucial to ensure block cohesion.

During the sublevel caving process, the ground is under incredible amounts of stress. The resin’s flexibility allows maintaining the ground’s cohesion during these moments, preventing the pillar’s reinforcement from breaking. Ultimately, this makes resin injection ideal for ground consolidation as a preventive fortification method.

Starting with a complex situation, where the most intuitive action for the mine would have been to leave a natural mineral pillar between the collapsed ground and the block to be mined, it was possible to take all available information, adapt the injection parameters, and ensure the consolidation of a resin pillar and optimize long-hole drilling. The entire process took one week to complete, including four days for drilling and three days for resin injection. The injection work was carried out by Weber’s team and hardware, including the pumping unit, mixing gun, and anchors.

Anthony Ferrenbach is gerente general (general manager) for Weber Mining & Tunnelling LATAM. With offices located in France, Poland, Turkey, Australia, and Mexico, Weber Mining is available to assist mines globally with chemical solutions for ground support. www.weber-mining.com