# Influence of Preconditioning and Tunnel Support on Strain Burst Potential
## 4 Results
### Influence of Mitigation Measures on the Depth of Failure and Strain Burst Potential  
Figure 8 presents the depths of failure for the spalling and damage initiation limits considering the rock mass without any mitigation measures, with a support system, considering distress blasting, and considering a combination of destress blasting and a support system. It highlights that, when only considering destress blasting, the depth of failure does not decrease as could be expected. This can be explained by the defined destressed zone itself, as it corresponds to a ratio \(\mathrm{Rf}/\mathrm{a}\) of 2. In other words, the destress blasting could debilitate the rock mass and therefore generate major brittle failure. When considering a support system or a combination of both studied mitigations measures, the depth of failure decreases drastically. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Rock Type** & **Stress Ratio K** & **Mean UCS (MPa)** & **Zone** & \(B\) & \(D\) & \(\mathbf{R^{2}}\) \\ \hline \multirow{3}{*}{Andesite (from this work)} & \multirow{3}{*}{1.5} & \multirow{3}{*}{220} & Damage & \multirow{3}{*}{1.07} & \multirow{3}{*}{0.43} & \multirow{3}{*}{0.83} \\  & & & & & & Spalling limit & 0.55 & 0.42 & 0.79 \\ \multirow{3}{*}{Mudstone [62]} & \multirow{3}{*}{1.5} & \multirow{3}{*}{48} & EDZ\({}_{0}\) & 0.71 & 0.59 & 0.93 \\  & & & & HDZ & 0.2 & 0.52 & 0.67 \\ \multirow{3}{*}{Granite [62]} & \multirow{3}{*}{1.5} & \multirow{3}{*}{246} & EDZ\({}_{0}\) & 0.62 & 0.58 & 0.81 \\  & & & HDZ & 0.09 & 0.62 & 0.48 \\ \multirow{3}{*}{Limestone [62]} & \multirow{3}{*}{1.5} & \multirow{3}{*}{113} & EDZ\({}_{0}\) & 0.66 & 0.63 & 0.95 \\  & & & HDZ & 0.18 & 0.34 & 0.42 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Multiplier (B) and exponent (D) for the best fit damage zones, damage initiation and spalling limit for the various cases modeled, compared with published work. Figure 8: Influence of mitigation measures on the depth of failure. Table 5 presents the best fit for each studied case, based on Equation (1). In all cases, the spalling and damage initiation limits present a low scatter with a high R2. Based on Table 1 presented in this paper, and the work of [PERSON] and [PERSON] [49], the level of rockburst potential can be defined. Three levels are defined: a low hazard corresponds to a minor spalling (\(\sigma_{max}\)/UCS between 0.4 and 0.6), a medium hazard corresponds to a moderate overbreak (\(\sigma_{max}\)/UCS between 0.6 and 0.8) and a high hazard corresponds to a serious overbreak (\(\sigma_{max}\)/UCS higher than 0.8). Figure 9 presents the influence of the mitigation measures on these levels. It highlights, similarly to previously, that, when considering a support system combined with distress blasting, the occurrence of strain burst, regardless of the level of the hazard, decreases by more than half. When considering only a support system, minor spalling and moderate overbreak can be controlled. Indeed, the five low and five medium hazard cases do not occur when the use of a support system is proposed. However, the high hazard cannot be mitigated. The use of only destress blasting does not have an impact on a medium hazard (the five cases occur at the same hazard level as when no mitigation measure is proposed), nor on the high hazard level. However, destress blasting allow the occurrence of a low hazard to be mitigated. Finally, when a combination of a support system and destress blasting is used, minor spalling and moderate overbreak can be controlled (the 10 low and medium cases are mitigated). Moreover, on some occasions, the combination of mitigation measures has an impact when facing a serious overbreak hazard. Indeed, from the initial high-level cases that occur when considering no mitigation measures, only a support system and only destress blasting, a third of them can be mitigated using a combination of mitigation measures. However, there are still two-thirds of the serious overbreak that occur even when using all mitigation measures.