The application of multi-point FBG bolt in measuring the excavation damaged zone of underground caverns

The excavation damaged zone (EDZ) is crucial for support designs and the stability analysis of underground caverns. A new method of determining the thickness of the EDZ in underground caverns is proposed in the study, which is built based on the “neutral point” theory of a fully grouted bolt that the shear stress between the bolt and the grout bond surface is zero and the axial stress reaches a maximum at the neutral point. This finding was verified through fine simulations, laboratory pull-out tests, and in-situ monitoring. The simulation results show that the EDZ’s thickness, characterized by fragments, is heavily related to the neutral points of the fully grouted bolt simulated using the hybrid bolt technique in the 3DEC. The laboratory pull-out test proves that peak axial stress along the fully grouted bolt can be accurately measured by multi-point Fiber Bragg Grating (FBG) sensors. The in-situ monitoring results indicate that for single-peak curves with one neutral point, the position of peak axial stress in each bolt corresponds to the EDZ’s thickness. For bimodal curves with several neutral points, if the axial stress at the last neutral point and the previous one is close, the position of the last one can be viewed as the EDZ’s thickness; otherwise, the previous one. Based on the above findings, the EDZs’ thickness in different places of the underground research cavern in Changyang Pumped Storage Power Station measured by multi-point FBG bolts is within the range of 0.11–1.89 m, which is similar to the results (0.54–2.09 m) measured by borehole television and single-hole wave velocity.