The development of tunnels is one of the main challenges in underground operations. In unstable ground conditions, most fortification methods are applied post-advancement. These traditional methods, such as anchors and mesh, can be limited when the ground is severely altered, and progress cannot be made. The risks of gallery over-excavation and rock bursts are too costly for underground operations. In these conditions, applying a preventive fortification method using resin injection is crucial for safely advancing underground works.
Weber Mining & Tunnelling points out that in tunnel development, preventive fortification with resin injection implemented within the mining cycle can increase safety and efficiency within the operation. The fortification is carried out by drilling holes in the front ceiling parallel to the progress. “A series of four or five holes (depending on the size of the tunnel) are made to place injection lances and packers in each of them. The holes’ length depends on the mine’s progress cycles, generally ranging between 4 and 8 metres. If the resin injection length is 6 metres, the mine can progress 5 metres before repeating the preventive fortification with resin, ensuring that they consistently advance on the consolidated ground.”
Once the injection lances and packers are in place, the injection can begin in each hole. Weber told IM: “The packer in the hole allows it to be pressurised and for the resin to be injected under pressure into the ground. This optimises the migration of the resin within the ground’s cracks. With a setting time of two minutes, the resin seals the cracks close to the area to be consolidated. This reaction time of the resin not only allows for quick action after injection but also to sectorise the injection. This sectorisation ensures the resin is only injected into the area of interest to be consolidated, reducing the amount of product required. In this way, the cost of fortification is reduced, as well as the logistics and time needed to carry it out.”
Weber argues that the mechanical properties of the consolidating resins make them ideal as preventive systems. In particular, most resins have a compressive strength greater than 30 MPa. “With a bonding strength of 6 MPa and very low viscosity, they have a high capacity to bond and consolidate rock packages when migrating into ground cracks. Additionally, regardless of whether they are injected into constantly stressed ground or in the presence of faults, the resins, being a flexible product, maintain cohesion over time with ground movements.”
It adds that they are also very versatile, as they can be injected into grounds with water or moisture without affecting their mechanical properties. In cases of ground with a high percentage of void, resins that expand two to three times their initial size can be used, optimising the filling of ground cracks without greatly increasing the injected amounts or decreasing the fortification’s strength. Finally, once the template of holes is injected, the mine can resume its operation immediately, given the resin’s reaction in just a few minutes, avoiding delays in its operations even in extreme ground situations.
Weber concludes: “Ground consolidating resins present an ideal option in the development of tunnels in severely altered grounds, providing a good tunnel profile and preventing possible over-excavation or cavity generation in advancements.”