Abstract: During the construction of long-distance large-section water conveyance tunnels across watersheds, small-diameter drilling and blasting pilot tunnels or tunnel boring machine (TBM) pilot tunnels are often used for pilot excavation to explore unknown geological conditions and manage risks associated with the surrounding rock. However, due to the constraints of the construction period and conditions, these pilot tunnels will exist for a relatively long period of time. Therefore, it is necessary to clarify the adjustment and failure laws of strain energy in the surrounding rock of the tunnel under the conditions of pilot tunnels, providing technical support for the excavation of pilot tunnels and subsequent secondary expansion excavation. Initially, this study utilizes numerical simulation to elucidate the adjustment mechanisms of strain energy triggered by pilot tunnel excavation, subsequently corroborating these findings through laboratory and field experiments. The results indicate that during the excavation of the pilot tunnel, the strain energy near the secondary expansion excavation face is initially released, then re-accumulated deep within surrounding rock. The peak value of the re-accumulated strain energy significantly decreases, and the distance between the peak value of the strain energy and the tunnel surface gradually increases and remain stable. Therefore, when the pilot tunnel exists, the degree of rock failure and the amplitude of acoustic emission decrease during the continuous loading experiments of the surrounding rock, and the location of rock failure is far away from the expansion excavation face. Based on the field acoustic monitoring test results, it can be concluded that the stress release depth of the stable section of the pilot tunnel excavation is relatively shallow. As the distance to the tunnel face decreases, the stress-release depth of the surrounding rock increases and may extend beyond the contour line of the secondary enlargement excavation, thereby reducing the disturbance caused by blasting and TBM excavation.

Key words: pilot tunnels, surrounding rock failure, strain energy release, sonic wave testing