LARGE CAVITY FILLING INJECTION REPORT

The mine needed to connect two galleries (access A and access B) to improve traffic. This new roadway is critical as on access A, the mine has its underground offices and mechanical shops. Direct access to this area would accommodate the newly developed area in access B.

The first collapse

The mine started its gallery development from access A toward access B; however, 30 meters before connecting, they had a rock burst, generating a 600m3 cavity. Since the rockfall was ongoing, removing the fallen rock from the ground was impossible to facilitate the installation of sets. Therefore, an alternative method had to be considered to stabilize the cavity while minimizing personal exposure, thereby improving operator safety.

Weber Mining phenolic foam was chosen and injected from afar of the cavity. This technique has been used to great effect worldwide in Hard Rock mines and was brought to Mexico by Weber Mining's team in 2017. To ensure nothing was left to chance and guarantee the project's success, the Weber Mining procedure was employed, with its own hardware and technical team.

Immediately after the beginning of the collapse, the area was evacuated safely and promptly without incident or injury. The Weber Mining team initiated the injection from a safe location, 10 meters from the rock burst, installing a set of four injection tubs with different lengths inside the cavity to guarantee a complete filling. The injection started from the roadway floor, with a first injection lance of 4 meters. Due to the pump pressure, the injection lance has a 4 to 5 meters flush, enabling the technicians to steer the foam where needed. As the filling continues and the foam goes up inside the cavity, the previously installed injection lances are used, with a better reach higher up inside the hole. Weber Mining's pumps are equipped with manometers that indicate any increase in pressure, indicative of voids getting filled. The expansion of the Marifoam? between the surrounding strata and the containment backing attached to the confined area allows for the pressurization of the cavity filling. This pressurization exerts a resultant force on the surrounding strata and provides an active support function.

Load distribution of a foamed product like Marifoam? typically allows for a better load transfer under the weight of collapsing rock.

Finally, the 600m3 cavity was filled in only 8 hours, from a single place of injection, without exposing any of the mining personnel to an unsafe area. From there, the mine had to follow a strict excavation process consisting of excavating 1 meter of gallery with fallen rocks and foams, installing sets and shotcrete before excavating again. This process allows the mine to recover the gallery quickly without ever being exposed to further rock bursts, as the roof is now composed of foam.

The second collapse:

As the roadway recovery on access A pursued, the mine continued development from access B. Unfortunately, the roof on access B collapsed as well. This time, the collapse was so big that the rock burst reached the cavity previously filled by Weber Mining on access A, leading to a completely collapsed area between the two access to be connected. Again, the gallery had to be evacuated by the mining personnel, and the Weber Mining team was commissioned for the roadway recovery. At this point, there was no way for the mine to give Weber Mining an indication of the size of the void, except that it was more than twice the first one, that the recorded height was 25 meters, and that it had propagated to the right. Therefore, due to the voids’ sheer size and difficult access, an iterative process of cavity fills and excavation had to be implemented instead of a one-event cavity filling.

In normal circumstances, a cavity can easily be filled with foam, even with a sole injection tub. This is because the foam sets and expands in two minutes, giving it time to migrate inside the cavity and fill any existing voids as the cavity is getting filled. However, due to the shape and size of this cavity, we placed two sets of injection tubs at different locations. Each injection tube was intentionally placed to ensure the optimum foam distribution inside the cavity.

The first set of tubes was placed at the center of the cavity, close to the top, while the second set was placed on the right side. Once placed, the first tube was connected to the hoses and the mixing gun. The mixing gun was then connected to the pump, and pumping started. This system allowed for a safe and simple transfer from one injection tube to the next.

Foam calculations:

Due to the cavity's size and the ongoing rock fall, it was impossible to calculate the exact volume of foam required to ensure stabilization. Given that Weber Mining Mexico has a local stock, they first came to the mine with an initial conservative quantity of resins. As it was clear that the cavity was bigger than assumed, more resin was supplied to the mine in less than 12 hours. With a 40 times expansion rate, the required resin quantity is reduced, as is the logistic requirement, ultimately easing the completion of the work.

Roadway recovery, first eight sets:

Once the first part of the cavity was filled, the excavation process began, following the same method used in access A. The mine excavated through the fallen rocks and foam, placing sets every meter, with shotcrete, before pursuing. After 8 meters of excavation, the Weber Mining teams continued filling the remaining void using the same strategy. Given the resin consumption, a void volume of nearly 1,500m3 was estimated at the finish. The first injection lasted 5 hours, while the second had to take place over two shifts for 11 hours.

Initial evidence of the effectiveness of the stabilization came from direct observation and measurements underground by the mine operation teams when the mine started excavating the fallen rock above which Weber Mining injected the foam and could place sets without difficulty and a stable "foam" roof. Another aspect is the cavity filling secures the new sets in place by absorbing the energy of falling rock in case of further collapse from the top of the cavity. That means even if the cavity is not completely filled the support of the gallery built by the sets is secured by the above foam canapee.

Conclusion:

The two galleries could be connected in approximately two months without incident, accident, or injury of any king. The 30-meter length of collapsed area between the two galleries is now protected by a foam roof, providing a safe and long-term roadway for heavy traffic between the mine’s underground offices and its newly developed mining area.

It serves as the most fitting testament and legacy of the efficacy of the foam as an alternative to traditional cavity-filling products. Success was achieved in the face of technical adversity. Even though the rock fall was ongoing, and the size of the cavity could have blocked the access for months and delayed the mining operation, adherence to the procedure and hardware was successfully used not only worked but did so better than expected.