Analysis for Microseismic Energy of Immediate Rockbursts in Deep Tunnels with Different Excavation Methods

Abstract

This study integrates microseismic data and data from hundreds of rockbursts of different intensities that occurred in a water drainage tunnel and four deep headrace tunnels at Jinping II Hydropower Station in Sichuan Province, China. The tunnels with overburden depths between 1,900 and 2,525 m, the maximum principal stress of which reaches 63 MPa in a rock mass, is composed mainly of marble. The tunnels have a total length of 12.4 km. The microseismic energy produced during the development of immediate rockbursts induced by the excavation by the drill-and-blast method (DBM) and a tunnel-boring machine (TBM) were studied. The results indicate that the daily maximum microseismic energy can be used as a basis for estimating rockburst intensity. The daily maximum microseismic energy (logarithm) corresponds to intense rockbursts (>6), moderate rockbursts (between 5 and 6), weak rockbursts (between 4 and 5), and no rockbursts (<4). Before the occurrence of intense rockbursts, there were numerous weak and moderate rockbursts during TBM excavation, whereas such a phenomenon was not observed during DBM excavation. The microseismic energy (logarithm) primarily concentrated in the range of 1 to 5 under TBM excavation, which is concentrated in the range of –1 to 4 under DBM excavation. For rockbursts of the same intensity, the range of microseismic energy remained the same for either type of excavation method. The distribution range of the microseismic energy moved in the direction of high energy as the level of rockburst intensity rose (intense rockbursts > moderate rockbursts > weak rockbursts > no rockburst). Microseismic energy can be used as a guideline for building a warning system and reduce the risk of rockbursts during construction.