Mine operators must consider rock mechanics when designing mines and exploiting resources. Coal is still a vital resource for nations such as China, where coal consumption meets 57% of its energy needs. However, deep excavation brings an increased risk of catastrophic failures. Advanced knowledge of rock mechanics will help inform the future of mine planning and resource exploitation. This article will discuss this subject, exploring current, new, and future developments in the field.
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How Rock Mechanics Affect Mining
Engineering roadways and stopes in mining requires intimate knowledge of rock mechanics to avoid potential disasters. Mines are exploiting deeper resources due to increasing energy demands and exhaustion of shallower reserves.
As mines increase in depth, rocks encountered may display characteristics such as strong rheology and large degrees of deformation. Deep mining disasters such as cave-ins, critical deformation of surrounding rocks, and gas explosions are increasing in number. This presents dangers to life and material assets as well as increased costs for mining operations. Due to this, the focus of research into rock mechanics in mining is increasing amongst companies and scientists.
Recent research advances in the field have provided pertinent information on several factors in rock mechanics. This includes the theory of deep high-stress induction, failure and strength characteristics of rocks found in deep mining, fracture induction mechanisms, theories of multiple seepages under strong disturbance, and energy regulation. Laboratory experiments, numerical, and theoretical studies all provide information that aids safer deep resource mining.
However, there is no unified research that tackles the multivariate properties and issues facing safe resource exploitation in the 21st century. There are also very few advances and innovations in mining theory. Due to this, challenges exist in solving the problems of resource exploitation. Reform of mining methods based on rock mechanics is needed to solve the critical issues facing the mining industry.
Current Methods of Longwall Mining
The longwall mining method has been used since the early 18th century when it was developed in England. This method involves undercutting coal along the coal face’s width, removing the coal as it falls. Props are used to support the roof of the coal face to prevent cave-ins. There has been considerable change in technology over the intervening centuries, but the basic idea is the same. Longwall mining removes a panel of coal, exploiting all the available resources as miners proceed.
Historically, the 121 method of longwall mining has been the most widely used method worldwide since its invention. This method works a coal face by digging two roadways and reserving one pillar. The roadways tend to be destroyed in this process due to considerable excavation. Stress is concentrated in the reserved pillar, causing severe deformation on surrounding rocks, leading to potential disasters. The coal pillar also contains exploitable resources that must be left in situ for safety reasons.
Due to these issues, alternative methods have been proposed and implemented. These include the 111 method first proposed in the Soviet Union, which reduces roadway excavation and improves resource extraction but still has problems with pillar stress, and the cutting cantilever beam theory.
Analyzing Characteristics of the Traditional 121 Longwall Method
Several characteristics of the traditional 121 method have been studied over several years. They affect the safe operation of longwall mining and understanding them informs developments in methods that improve upon this traditional mining technique.
Mining pressure law of the working face is divided into five general stages: (Before immediate roof collapse, immediate roof collapse, main roof collapse, fracture and subsidence of overlying strata, and ground surface subsidence.
Characteristics of the roadway and pillar are analyzed to understand the overlying strata movement and its mechanical state at different positions. The analysis process includes the coal pillar’s force characteristics, the movement law of the overlying strata, and the roadway support system’s force state.
Using Rock Mechanics for Proposed Longwall Excavation Methods
A study published online in 2021 has proposed methods for longwall mining based on advanced studies of rock mechanics. Updating the 121 and 111 methods currently in use will provide solutions to common problems. A key concept is to use rock mechanics characteristics to avoid roadway excavation and wasted resources. By utilizing mining pressure and the collapsed rock mass’s expansion characteristics, roadways can be automatically formed.
Proposed advances include the short cantilever beam model and the equilibrium mining theory. Furthermore, the 110 and N00 methods are proposed. Roof cutting and pressure releasing without the need for pillars are proposed for automatic roadway formation. Key proposed technologies include roof directional presplitting technology, bidirectional energy cavity tension blasting techniques, constant-resistance support technologies, and gangue blocking supports.
New mining methods have been applied in Chinese mines with different characteristics such as overburdens, thicknesses of coal seams, roof types, and gases. These field studies have realized an integrated mode of both roadway retaining and seam exploitation. Recent studies have informed the basis of an intelligent and unmanned development direction for mining.
This equilibrium mining model takes account of both the intelligent mining demands of the modern industry and the complex geometries of deep mines.
The Future of Mining Rock Mechanics
The proposed methods to realize equilibrium mining have already been implemented in mines, including the Ningtiaota coal mine in western China. There are future prospects for longwall mining that exploits rock mechanics to achieve automated roadway excavation without the need for resource-rich pillars. These methods will allow the mining industry to achieve its goal of intelligent, automated, and worker-free coal face mining that significantly improves miner and asset safety whilst reducing costs associated with catastrophic mine failures.
Further Reading and More Information
Hualei, Z, Chen, Y & Zhije, W (2021) Special Issue: Rock Mechanics in Deep Mining Engineering [online] Advances in Civil Engineering | Hindawi.com. Available at: https://www.hindawi.com/journals/ace/si/389502/
He, M et al. (2021) Innovation and future of mining rock mechanics [online] Journal of Rock Mechanics and Geotechnical Engineering 13(1) pp. 1-21 | sciencedirect.com. Available at: https://doi.org/10.1016/j.jrmge.2015.07.002