## 6 Rock Burst
### Rock Burst Prediction Criteria for Neelum-Inelum Hydroelectric Project  
The traditional rock burst prediction approaches are based on the rock strength in practice, while the actual occurrence of rock bursts depends more on the structure and strength of the rock mass (\(\sigma_{\rm cm}\)). [PERSON] and his co-researchers used different parameters, i.e., stress reduction factor (SRF), \(\sigma_{0}/\sigma_{c}\), \(\sigma_{\rm t}/\sigma_{1}\), and \(\sigma_{c}/\sigma_{1}\) for rock burst estimation using the conventional rock burst criteria [59; 60; 61], for five dangerous chainages along the headrace tunnel of a project with predicted heavy rock burst [18]. For design engineers, the traditional rock burst criteria are appropriate for understanding the causes; however, it is too simple and conservative to forecast rock burst occurrence in complex geological environments. The prediction criteria, which are based on the strength-to-stress ratio, measure the strength of the rock, which is attained from laboratory tests; however, they ignore the consequence of the in situ stress effect on \(\sigma_{\rm cm}\)[31]. When considering the influencing factors and limitations of traditional rock burst criteria, a comprehensive study was conducted based on the data that were related to the characteristics of the rock bursts that occurred during NJHEP construction, and a novel rock burst criterion was proposed [23]. This criterion utilizes the rock mass strength-stress ratio. This ratio is between \(\sigma_{\rm cm}\), based on the Hoek-Brown strength criterion, and \(\sigma_{\rm H}\), which is perpendicular to the longitudinal axis of the tunnel. The criterion also considers other factors that influence rock burst, i.e., rock mass characteristics, construction disturbances in the vicinity, and the geologic structure. A close relation exists between the rock burst and the stored energy release rate during excavation; moreover, the unloading-confining-pressure-based energy criteria is another approach for the evaluation of rock bursts in the NJHEP, which is defined in Equation (2) [62]. Before excavation, the elastic energy storage in the rock mass is due  
Figure 8: Over-excavation profile of the headrace tunnel after the rock burst on 31 May. to in situ stresses, which is followed by excavation-induced redistributed stresses in the excavation surroundings. \[\frac{U^{e}}{U^{e-\max}}=\left| \begin{array}{ll}\geq 0.7&(severe\,rockurst)\\ 0.5&(strong\,rockburst)\\ 0.4&(medium\,rockburst)\\ 0.3&(weak\,rockurst)\end{array} \right. \tag{2}\]  
Before failure is attained, \(U^{e-\max}\) is the maximum storage elastic energy of the rock. This energy is before the occurrence of failure and it is achieved from the unloading confining pressure tests. The storage elastic energy of the rock mass is represented through \(U^{e}\), owing to in-situ stresses and excavation. From the NJHEP sandstone stress-strain curve, the three parts of the store elastic energy are as shown below [31]:  
\[U^{e}=U^{e}_{h}+U^{e}_{l}+U^{e}_{u}, \tag{3}\]  
where \(U^{e}_{h}\) is the storage elastic energy during the application of the hydrostatic pressure, \(U^{e}_{l}\) is the storage elastic energy during the application of the initial axial stress, and \(U^{e}_{u}\) is the storage elastic energy during unloading confining pressure. \(U^{e}_{h}\) can be calculated, as follows:  
\[U^{e}_{h}=\frac{3(1-2\times v)\times(\sigma^{0}_{3})^{2}}{2\times E}, \tag{4}\]  
where \(v\) is [PERSON]'s ratio, \(E\) is the elastic modulus of the rock specimen, and \(\sigma^{0}_{3}\) is the initial confining pressure. The storage elastic energy during the application of the initial axial stress can be calculated, as follows, where \(\sigma^{u}_{1}\) is the maximum axial stress at the maximum confining pressure. \[U^{e}_{l}=\frac{1}{2\times E}\Big{[}(\sigma^{u}_{1})^{2}-(\sigma^{0}_{3})^{2}- 4\times v\times((\sigma^{u}_{1}\times\sigma^{0}_{3}+{(\sigma^{0}_{3})}^{2}) \Big{]} \tag{5}\]  
During the unloading process, the storage elastic energy \(U^{e}_{u}\) is:  
\[