Mineral resources

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1.INTRODUCTION

Based on great economic significance of mineral resources for a nation, and the view that shallow resources have been gradually exhausted, their exploitation is currently constantly developing to deep, where mineral resources were not exploited at the beginning of mining industry due to the limitation of early mining techniques and suitable equipment. As positively related, the depth of mining increases with the increase of ground stress (Kouame et al., 2017; Liu and Wang, 2018), as show on Fig 1. With this deep deposit mining and high concentration of ground stress, the subsequent frequency of rock bursts occurrence and other dynamic hazards increase consequently, which strongly hinder the safe and efficient mining (He et al.

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, 2012; Kaiser and Cai, 2012; Chen et al., 2012; Cai, 2016; Feng et al., 2017; Sengani and Zvarivadza, 2017; Kouame et al., 2017). Rock bursts are more pronounced in hard rocks and geological features like dykes and faults, of deep underground deposits exploitation. Rock bursts are more often closely related to high extraction ratios and associated with underground deposits exploitation techniques leading to unfavourable ground stress situations (Kaiser and Cai, 2012).

Fig 1. Mining depth and Ground stress Relationship

Maziaira and Konicek, (2015), state that, in order to prevent and control rockburst and realize the safe, efficient and economic recovery of deep-seated mineral resources, the methods and tools that mining engineers can use are to adopt proper mining techniques and construction method, ground preconditioning, and ground support and reinforcement to improve the stress distribution state and properties of rock mass .

According to the previous studies, rock burst in deep mine can be controlled by destressing techniques.

Destress techniques are stress control methods (Serata et al.,1986) employed in deep underground mine to control rock burst hence ensures the safe and efficient mining. Destressing techniques have been widely used in deep underground mines in South Africa, Canada, USA, China and some of the rest of the world (Tang, 2000). Destressing technique is a method to form a destressed area in the surrounding rock of excavation space and a stress bearing area ahead of working face. It has an obvious effect on controlling the stress concentration of surrounding rock and preventing rockburst in the surrounding rock of the stope by the principle of stress transfer.

Destressing technology can be divided into strategic method and tactical method. The strategic method aims at eliminating the conditions of rockburst and other geological disasters. This method mainly includes the proper mining sequences, reasonable development layout, reasonable stope structural parameters (dimension, geometries) and exploitation rates (O'Donnell Sr, 1999). Tactical methods are designed to maintain and control dangerous areas that have become fractured or have potential geological hazards. this method mainly includes destress blasting, destress drilling, reasonable selection of supporting measures (O'Donnell Sr, 1999) and Water infusion. The destress blasting is the focus of tactical method. Table 1, present the destressing method as high stress control techniques in deep mines and other underground civil construction.

Table 1. Destressing techniques as high stress control method in deep mines and other underground civil construction

Destressing techniques Explanations Merits

Destress blasting Destress blasting technique is one of the stress control methods employed explosives (Fig.2) to fracture rock mass intentionally, in deep underground mining and excavation to control and mitigate rock burst occurrence, its damages on face and equipments, and its hazards on workers in the working face, by shifting stress ahead of face Reducing the stress in the area where rock burst may occur; Transferring the possible hypocenter (focal point) to the deep, away from the roadway and stope; Reducing the strain energy of surrounding rock; Reducing the mechanical parameters of rock mass, especially the elastic modulus.

Destress drilling De-stress drilling is a very successful method of coalburst prevention which has been used in Germany and Russia. The holes will burst and fines are removed during drilling until the stress has reduced below bursting levels. It has to be repeated every 5 m or so of advance (Baltz, 2010; Calleja and Porter, 2016). It is a slow process effective method

for low productivity mining; Reducing the stress in the area where rock burst may occur; Transferring the possible hypocenter (focal point) to the deep, away from the roadway and stope; Reducing the strain energy of surrounding rock; Reducing the mechanical parameters of rock mass, especially the elastic modulus.

Water infusion Water infusion is used in coal seams mainly as a standard tool to control rockburst. It was successfully used in the USCB Reducing the stress in the area where rock burst may occur; Transferring the possible hypocenter (focal point) to the deep, away from the roadway and stope; Reducing the strain energy of surrounding rock; Reducing the mechanical parameters of rock mass, especially the elastic modulus.

According to Konicek et al. (2011), the general active rock burst control techniques include destress drilling, destress rock blasting in the wide surroundings of face. Destress blasting technique was and is being applied to control rock burst in most of the deep underground mines and some other underground civil constructions around the world. In this paper, the Authors focus on destress blasting technique to be reviewed as essential underground mine safety tool.

2. LITERATURES ON DESTRESS BLASTING TECHNIQUE

2.1. Meaning discussion of Destress Blasting

Destress blasting technique can also be called stress control method or stress relief method or pre-conditioning method. According to Gregersen (2006), Destressing blasting is a term used when a mass of rock is purposefully damaged, usually by blasting, in order to affect the stress field in the area of mining, with the intent of reducing or eliminating the stress related problems. According to Hedley (1992), Destress blasting is where the blast fractures the rock and reduces its stiffness which in turn transfers stress to the adjacent rock mass for the purpose of overcoming problems of highly-stressed ground and rockbursts. According to Saharan and Mitri, (2011), Destress blasting in one of the tools to improve mines safety and it is one of the most valuable techniques to control the damaging effects of rockbursts. According to Tang (2000), Destress blasting is a rock fracturing technique to stress-relieve potential rockburst zones in deep mines.

From the above literatures, it is seen that, all definitions focus on ground stress control as the major cause of rockburst in deep mines.

2.2. Brief History of Destress Blasting

The conceptual model of destress rock blasting technique was put forward based on the rockburst model. This model was formed since the second half of 28th century (Konicek et al., 2011a; 2011b; Konicek et al., 2012). It is one of the most crucial method for controlling the threats of strain bursts resulting from the higher stress induced by underground excavation (Konicek et al., 2011a; 2011b; Konicek et al., 2012). Destress blasting was first applied in South African gold mine such as at East Rand Proprietary Mines (ERPM), under the guidance of Council for Scientific and Industrial Research (CSIR) in the early 1950's (Hedley, 1992; Tang, 2000; Konicek et al., 2011; Sengani and Zvarivadza, 2017) In Canada, Ontorio, at Kirkland lake mine, destress blasting technique was used on trial and error basis in the 1930s (Saharan and Mitri, 2011), and its first application was done in the early1950s (Hedley, 1992). In Nova Scotia, it was put in use in the 1930s in deep coal mining (Springhill Colliery) In Czech part of the USCB, the technique has been developed since the 1980s. More than two thousand destress rock blasts and more than 3.5 million tons of explosives have been used since the early 1980s (Konicek et al., 2011b; Konicek et al., 2012; Calleja and Porter, 2016).

Even if the main purpose of destress blasting was to improve the stress condition of the surrounding rock of the hanging wall, where the fragmentation zone is formed in front of the working face by blasting, which reduces the high stress concentration around working face and reduce the rock burst occurrence and its associated hazards, however it was not commonly approved by mining industry as essential technique to control and mitigate the occurrence of rockburst in deep mines (Toper, et al., 2000). According this disagreement, the application of technique was stopped for a certain period of time for further research (Hedley, 1992). In 1980s - 1990s, the research for further use of destress blasting was conducted in South African gold mines, The result from which have been used to approve the application of destress blasting as essential technique for rockburst control in deep mines (Adams et al., 1981; Rorke and Brummer, 1988; Hedley, 1992; Adams et al., 1993; Lightfoot et al, 1996), destress blasting is currently being employed by worldwide mining industry as essential technique for rockburst control (konicek et al., 2011a).