Underground mining operations are more efficient thanks to academic activities of the Chamber of Mines of Peru.
The Chamber of Mines of Peru, in its objective to promote safety in mining operations, promotes training programs on reinforcement elements, such as anchor bolts.
In the development of underground mining operations, it is important to evaluate the geomechanical conditions and the operability of the unstable area in order to select the use of appropriate support systems.
Taking into account that rock fall is one of the major causes of accidents in mining units, the optimization in the design of reinforcement elements, such as anchor bolts, is important to maintain the integrity of workers and equipment, fixing any type of loose rock or improving friction between discontinuities.
Safety is a substantial ingredient in the development of extractive projects. The use of support methods must provide greater stability to the rock, but without altering the resistance capacity of the bedrock in the environment near the excavation.
For this reason, the Chamber of Mines of Peru continues to develop its training programs so that professionals, technicians and operating personnel have the ability to analyze all the variables to use the most appropriate reinforcement elements, depending on what is required inside the mining site.
“Generally, when evaluating the use of a support alternative, including anchor bolts, sometimes the analysis of the geomechanical conditions of the bedrock is chosen and the operational characteristics of the unstable area are left aside”, says Fernando Fernández Lagazio, specialist from the Chamber of Mines of Peru.
“It is necessary to keep in mind that there must be an interaction between these two variables in order to make an adequate selection of support, safeguarding the interests of safety, productivity and costs for the mining company. The selection of anchor bolts to be used will be based on the assessment of the exposure time, associated with the safety degree required in the mining work and the load capacity offered by the anchor device.”
Rocks can be fractured at different degree or level of intensity and depending on that, the principle of action of bolts is different. For example, the bedrock or slightly fractured rock will control the stability of potentially unstable rock blocks and wedges. This is called “wedge effect”. When the blocks or wedges are isolated, only their stabilization using isolated bolts is necessary, which is called isolated or sporadic support. Otherwise, the usual practice will be the use of systematic support on the entire roof and/or walls of the excavation, as required.
In the mining training of anchor bolts, it is considered that, in a type of bedded subhorizontal rock and unbedded rock with a dominant system of subhorizontal discontinuities, the bolts help resist relative displacement between strata. That is, they increase stiffness of the structural beam that forms and creates binding between tabular blocks to minimize roof deflection. This is called the “beam effect”. This concept can be applied to parallel walls or subvertical discontinuities, generating the so-called “column effect”.
Currently, there are several types of rock bolts, whose design differs, but are basically varieties of the same concept. In the training provided by the Chamber of Mines of Peru, students will learn that the anchoring techniques used can be grouped as follows: mechanically anchored bolts, resin or cemented rod bolts and friction anchored bolts.
Mechanically anchored bolts are relatively inexpensive and their reinforcing action on rock is immediate after installation. Their use is limited to moderately hard, massive, blocky or bedded rocks, without the presence of water. Their use is not recommended in very hard, fractured and weak rocks, nor in those that are subject to high stresses. They can only be used for temporary reinforcement; since if they are used for permanent support, they must be protected from corrosion, in case of presence of water. Likewise, their use should be complemented with cement paste between the rod and the wall of the borehole.
As for resin or cemented rod bolts, their effectiveness depends on the adhesion between iron and rock, provided by the cementing agent, which in turn protects against corrosion, increasing the useful life of the bolt. In the presence of acid water, the recommended cementing agent will be resin; in the absence of water, cement will be used. These elements are generally used as permanent reinforcement, but they can also be used as temporary reinforcement in many rock conditions.
Split sets are mostly used for temporary reinforcement. They usually form combined reinforcement systems in soils ranging from fair to poor. They are not recommended in intensely fractured and weak rock. They provide immediate reinforcement action after installation. They are susceptible to corrosion in the presence of water, unless galvanized. These elements are relatively expensive compared to the rest.
While, in the training delivered by the Chamber of Mines of Peru, the students will learn that Swellex bolts provide a split anchorage, since the mechanism is established by friction and mechanical adjustment. They are an alternative system to split sets, but they generate better performance, in terms of quality, as a temporary reinforcement element. Due to the existence of different types in the market, they can be widely applied to the hard to soft rocks, and very fractured soils. Their installation is simple and fast, giving an immediate reinforcement effect. In addition, they are equipped with washers to place the mesh at any time. The main problem is corrosion (although the new models have been supplemented with a protective elastic layer or are made of stainless steel). Finally, they are more expensive than split sets.
As an example of the practical application of anchor bolts, we can mention the case of the Halita Borth salt mine, operated by Esco (European Salt Company), in the city of Rheinberg, Germany. A large area of faults was identified within this deposit that required extensive reinforcement. With the contribution of the Spanish company Dywidag – Sistemas Constructivos, S.A. (DSI), a system was developed that included resin-cemented anchor bolts, which ultimately ensured the stability of the German deposit.
The contribution of specialized companies is important, since with the development of new products, they end up contributing to improve safety conditions in the development of mining operations.
We can mention the friction anchor bolt, Splitbolt, developed by the Peruvian company Corporación Aceros Arequipa and presented at the First International Congress on Mine Design using Empirical Methods, organized by the Peruvian Society of Geoengineering (2014). It is a bolt that is fixed in the rock due to mechanical deformation and generates radial forces along its length inserted in the rock, thus increasing the resistance of the bedrock.
We can also mention the company Prodac Bekaert, which has developed a series of support alternatives. Thus, we have the helical bolt, which acts or transfers its load by adhesion. Its main advantages include its great transfer capacity in the bedrock, as well as its simple and fast installation, which means that it does not require sophisticated equipment for adjustment.
Finally, we can mention the Frirock bolt, which transfers its load by friction. Its most outstanding characteristic is the capacity to drain water coming from the walls and ceilings of the mining works; and it acts on the bedrock immediately after its installation.
The correct selection of reinforcement elements, in this case anchor bolts, contributes to improve stability conditions inside mining operations. To that end, it is necessary that workers have the necessary training to analyze and recognize the conditions of the bedrock and the operational characteristics in order to end up using the best existing systems.