LAUSR

Abstract Sandstone (hard rock) and siltstone (relative soft rock) appeared alternately during tunnel boring machine (TBM) excavation of two headrace tunnels at the Neelum–Jhelum hydropower station in Pakistan. Rockbursts occurred frequently near these sandstone-siltstone interfaces, seriously threatening the safety of personnel and equipment. In situ rockburst investigations and microseismic monitoring were conducted to study the characteristics of these rockbursts and their associated microseismic activities around the lithological interfaces under different excavation directions (i.e., the TBM passing though sandstone and siltstone in different sequences). The results show that the rockburst frequency and intensity are greater during excavation from sandstone to siltstone than those during excavation from siltstone to sandstone within 15 m around the lithological interfaces. Moreover, the number of miroseismic events, the amount of microseismic radiated energy and the proportion of high-energy or large microseismic events are higher during excavation from sandstone to siltstone than those during excavation from siltstone to sandstone in the selected zone. In addition, the differences in the rockburst and microseismicity characteristics between sandstone and siltstone are also analysed in the selected zone. Finally, a simple 3D numerical model is employed to assess the rockburst risks in different excavation directions, the results of which agree well with field rockburst results. The formation mechanisms of different rockburst risks under the two excavation directions are discussed and analysed with microseismic data and numerical results. This study may be valuable for predicting rockburst based on microseismic techniques and managing rockburst risks in similar projects.