Comparison of spatio-temporal characteristic of microseismic events in deep-buried tunnels with two excavation methods

Two main methods are commonly used in tunnel excavation, drilling and blasting (DB) and tunnel boring machine (TBM) tunnelling method. To better understand the differences and predict rockbursts, the characteristics of microseismic (MS) events in two parallel tunnels excavated by DB and TBM methods were compared, including temporal distribution, spatial distribution, and intensity distribution. The results indicated that the characteristics of MS events induced by two excavation methods had some differences. In the DB excavated tunnel, the number of MS events decreased exponentially as the time to a blasting cycle increased. The MS events predominantly occurred within 15 min to a blasting cycle. On the other hand, in the TBM excavated tunnel, the MS events primarily occurred while the TBM was working. Similarly, when a tunnelling cycle was completed, there was an exponential decrease in the number of MS events, which mainly occurred within 15 min. The distribution of MS events in relation to the tunnelling face exhibited similarities for both excavation methods, presenting a normal distribution. The range of the TBM excavation (approximately 2.3 times of tunnel diameter) was generally larger compared to the DB excavation (approximately a time of tunnel height). In terms of energy levels, both the DB and TBM excavations predominantly produced low-level energy MS events (less than 30 kJ). For moderate and high-level energy events, the percentage of TBM excavation induced events was higher than that of DB excavation induced. In the TBM excavation, surrounding rocks maintain good integrity and high bearing capacity, and a large tangential stress gradient is near the tunnelling face. It is more likely to induce a large rupture scale of seismic sources and lead to a more prolonged and widespread active period. On the other hand, the fracturing zone induced by the DB excavation reduces bearing capacity of surrounding rocks, and strain energy within surrounding rocks is released to a certain extent. The tangential stress gradient is relatively small, so it is associated with more localized and short-lived seismic activity in the DB excavated tunnel.