# Influence of Preconditioning and Tunnel Support on Strain Burst Potential
## 1 Introduction  
Deep underground mining and civil tunnels are associated with high stress magnitudes that can lead to serious ground control problems [1, 2]. One of these main problems are the rockbursts, which are explosive failures of rock that occur when high stress concentrations are induced around underground openings in brittle rock masses [3]. Usually, rockbursts are divided into three categories based on their mechanism of occurrence [4, 5, 6]: (1) pillar bursts caused by the complete collapse of support pillars; (2) fault slip bursts caused by the slippage of some pre-existing faults due to stress changes generated by underground excavation; and (3) strain bursts caused by high stress concentrations at the edge of mine openings that exceed the strength of the rock [7, 8]. The most frequent rockburst mechanism in tunneling corresponds to strain bursting [4]. This phenomenon can be preceded by the visible extension of fractures under compressive loading, known as spalling damage [8]. As spalling damage and associated strain bursting present significant issues in underground environments, it is necessary to assess this hazard early by defining the extent and intensity of the event. However, due to the complex nature of the rockburst phenomenon, rockburst prediction is quite difficult. Over the years, various methods have been developed based on empirical knowledge [9, 10, 11], numerical modeling [3, 12, 13, 14] or statistical analysis [15, 16, 17]. A comprehensive review of some of these methods and others can be found in the work of [PERSON] et al. [18]. Rockburst charts, which are simple and straightforward to use, can also be adopted as valuable design tools for the preliminary rockburst potential estimation by practical engineers. Additionally, these charts can be used to help predict the demand of rock support [19]. One of these well-known charts is the oneproposed by [PERSON] et al. [20] and completed by [PERSON] et al. [21], and is based on historical cases. A more rigorous chart has been proposed by [PERSON] [3] using numerical models integrating the damage initiation and spalling limit (DISL) approach [8]. Both of these charts have been shown to successfully estimate the burst potential by predicting the depth of brittle failure around tunnels [22; 23; 24]. Using these charts to assess the rockburst hazard early, it is often possible to reduce the risk by selecting appropriate mining or excavation methods and sequences [4]. Moreover, due to uncertainties in rock mass properties and boundary conditions, mitigation measures are an additional important line of defense to ensure workplace safety. Rock support systems that can resist under rockburst conditions are widely used to mitigate strain burst. They have to meet the following three primary support functions [20; 25]: (1) reinforce the rock mass to strengthen it and control bulking through, for example, rebar or stiff friction bolts; (2) retain broken rock to prevent fractured block failure and unraveling through, for example, mesh, shotcrete or spay-on-liner; and (3) hold fractured blocks and securely tie back the retaining elements to stable ground through, for example, deformable cables and yielding bolts. Classically, shotcrete and rockbolts are the main support systems used in tunneling [26]. In that case, the shotcrete is used to enhance the surface-retaining capacity of the support system while the rockbolts must be able to dissipate the excessive release energy. Preconditioning can also be used to defuse burst activities caused by stress changes during rock excavation. In particular, destress blasting, which is a method of preconditioning consisting in the blasting of highly stressed rock to reduce the local stress and stiffness of the rock [27], is now implemented as a standard rockburst measure across the many mining regions around the world [28; 29; 30; 31].