# Influence of Preconditioning and Tunnel Support on Strain Burst Potential
## 6 Discussion and Concluding Remarks  
The work presented in this paper has allowed the following elements to be highlighted:  
* It is possible to identify damage zone depths in brittle rocks numerically by considering the plasticity degree of the models. The highly damaged zone (HDZ) is related to 100% plasticity and corresponds to the proposed spalling limit. The outer excavation damage zone (EDZ\({}_{\text{o}}\)) is related to the transition between no plasticity and plasticity and corresponds to the proposed damage initiation limit. Both limits present coherent results with in situ measurements [44] in similar conditions (stress ratio, K, and intact rock strength);
* As expected, the use of mitigation measures allows the strain burst occurrence to decrease. This is in agreement with published work to date [7; 63; 64; 65]. However, the strain burst hazard level does not easily decrease even when using mitigation measure. In the case of a serious overbreak hazard, only a combination of system support and destress blasting seems to have an impact on these events, for rock mass presenting high strength (very and extremely strong intact rock presenting a uniaxial compressive strength higher than 220 MPa). For a rock mass presenting a uniaxial compressive strength lower than 220 MPa (with the in situ major stress, \(\sigma\)1, equal to 80 MPa and the in situ minor stress, \(\sigma\)3, equal to 53 MPa), in all cases, none of the mitigation measures could remove the occurrence of the serious event;
* While the DISL method is easy to use and allows the brittle failure to be modeled, it does not consider comprehensively the rock mass discontinuities, and that could be a limitation when assessing the strain burst hazard. Indeed, it is assumed that a GSI higher than 55 is needed for a spall to occur. However, it is understood that the quality of the rock mass could influence the level of the hazard (considering the depth of failure), and it cannot be studied in this study. In this study, the choice was made to only consider shotcrete and rockbolts system support and destress blasting as potential mitigation measures. In reality, it is well known that the management of rockburst is through a multipurpose support system consisting of stiff reinforcement functionality to maintain rock mass integrity and internal strength, dynamic displacement and energy capacity in the event of violent rupture, and integrated  
Figure 10: Influence of the UCS (**a**) and the GSI (**b**) values on the number of events and the occurrence of the strain burst, considering the various mitigation measures studied. The variation of UCS is carried out considering a GSI of 75, and the variation is carried out considering a UCS of 220 MPa. surface retention to resist uncontrolled bulking and to ensure energy transfer [20; 26]. Therefore, there are intrinsic limitations of the presented work as only two of these elements are considered. However, it is also assumed that, in many conditions, some level of dynamic rupture is inevitable and cannot be engineered away [3; 66]. The presented work gives an additional insight toward understanding why, considering the intact rock strength particularly. It aims to give indications when dealing with strain burst hazard management. Support systems and/or preconditioning are strong tools to mitigate low-to-medium strain burst hazards. However, when dealing with high level hazards, a systematic use of these tools is not enough to ensure the non-occurrence of an event. In that case, a specific analysis needs to be realized to define the proper mitigation measures that need to be applied. This highlights the need to assess the level of hazard in a preliminary analysis properly, to be able to not overestimate the support system proposed and optimize the resources in a project. Further work needs to be done to validate these results, considering, for example, other types of support systems. Moreover, the explicit modelling of rockbolt could be realized to validate these preliminary results.