Novel rockburst criterion based on the TBM tunnel construction of the Neelum-Jhelum (NJ) hydroelectric project in Pakistan

The assessment of rockburst proneness has become a major technical bottleneck in brittle, and high-strength hard rock under high local stresses. Various approaches or criteria have been used over the years to predict rockburst, many of them were within an acceptable range, while some too conservative, simple and missing comprehensive consideration of rockmass quality and excavation characteristics, particularly during the tunnel construction phase. Aiming at the shortage, a novel rockburst criterion was put forward, which is defined as the ratio between the rockmass strength and the horizontal stress perpendicular to the tunnel axis as determined. First, the rockmass strength based on the Hoek-Brown strength criterion was estimated by accounting for important parameters such as rock strength, brittleness coefficient, the quantitative geological strength index (GSI), the TBM construction disturbance, and the in situ stress. Furthermore, in practical application at the NJ-TBM tunnel, the quantitative models of the geological strength index (GSI) and rock uniaxial compressive strength were proposed based on the boring/specific energy (SE) information and the field penetration index (FPI) recorded in the TBM performance database, respectively. The observations and classification of 26 rockbursts cases in different geological units indicate that the novel criterion greatly enhanced the accuracy and applicability of rockburst prediction during the construction phase, through comparative analysis with the traditional criteria.