Rockburst and microseismic activity in a lagging tunnel as the spacing between twin TBM excavated tunnels changes: A case from the Neelum-Jhelum hydropower project

Abstract

A reasonable spacing between tunnels is very important in deeply buried tunnels, as it may mitigate the risk of rockburst occurrence. This spacing is particularly important for tunnels excavated using a tunnel boring machine (TBM), as the methods available for controlling rockburst occurrence are inflexible and limited using these machines (compared to tunnels excavated using the drilling and blasting (D&B) method). In this paper, a rare example was studied in situ in which the spacing between twin tunnels was changed to mitigate the rockburst risk during TBM excavation. First, the characteristics of the rockburst and microseismic activity were investigated in three sections of the lagging tunnel before, during, and after the spacing between the tunnels was changed (we concentrated on the lagging TBM696 tunnel, which is part of the Neelum-Jhelum hydropower project). We observed a significant occurrence of rockburst and microseismic activity in the lagging tunnel at the original spacing of the twin tunnels in both the sandstone and siltstone lithologies, and thus a consideration of enlarging the spacing between the twin tunnels is reasonable. A more detailed investigation was then conducted to explore the effects of tunnel spacing on rockburst occurrence and microseismicity. The rockburst frequency and microseismic radiated energy due to excavation unloading generally decrease as the spacing between the twin tunnels increases (substantially at first and then more slowly as separation increases). In particular, the microseismic radiated energy exhibits a power-decreasing type in both the sandstone and siltstone. Increasing the spacing beyond a certain point, therefore, exerts a very limited effect on the microseismic radiated energy and occurrence of rockbursts (increments of 16–21 m and 15–20 m in sandstone and siltstone, respectively). Consequently, in practice, a very reasonable approach would be to enlarge the spacing of twin tunnels by 22.48 m. Thus, microseismic monitoring is a useful technique for assessing the rockburst risk and deriving a reasonable value for tunnel spacing. This study can be used as a guideline to derive reasonable tunnel spacings when using not only TBMs but also D&B.