Moment tensor analysis of transversely isotropic shale based on the discrete element method

In this study,the moment tensor of transversely isotropic shale was analyzed using a discrete element method-acoustic emission model(DEM-AE model).Firstly,the failure modes of the shale obtained from the acoustic emission(AE) events and physical experiments were compared.Secondly,the relationships between AE events and seismic magnitudes,and AE events and the resulting cracks were analyzed.Finally,a moment tensor T-k chart describing the seismic source was introduced to demonstrate the differences in the transversely isotropic shale.The results showed that,for different anisotropy angles,a linear logarithmic relationship existed between the cumulative AE events and the seismic magnitude in the concentration area of the AE events.A normal distribution was observed for the number of AE events as the seismic magnitude changed from small to large.The moment tensor T-k chart indicated that the number and proportion of linear tension cracks in the shale were highest.When θ = 30°,the peak seismic magnitude was at a minimum.The average seismic magnitude in the concentration area of the AE events was also relatively small.Points close to the U=-1/3V line and the number of cracks included in a single AE event were at a minimum,and the corresponding peak stress also reached its lowest level.In contrast,when θ=90°,all related parameters were contrary to the above θ = 30° case.The DEM-AE model and the moment tensor T-k chart are suitable for analyzing the distribution of shale cracks appearing during the loading process.This study can provide constructive references for future research on the fracturing treatment of shale.

Acoustic emission monitoring of rockbursts during TBM-excavated headrace tunneling at Jinping Ⅱ hydropower station

To better understand the mechanical properties of marble at Jinping II hydropower station, this paper examines the changes of brittle rocks in excavation damaged zones(EDZs) before and after excavation of tunnel with the tunnel boring machine(TBM). The paper attempts to employ the acoustic emission(AE) to study the AE characteristics and distribution of rockburst before and after TBM-excavated tunnel. It is known that the headrace tunnel #2, excavated by the drill-and-blast(D&B) method, is ahead of the headrace tunnel #3 that is excavated by TBM method. The experimental sub-tunnel #2–1, about 2000 m in depth and 13 m in diameter, between the two tunnels is scheduled. In the experimental sub-tunnel #2–1, a large number of experimental boreholes are arranged, and AE sensors are installed within 10 m apart from the wall of the headrace tunnel #3. By tracking the microseismic signals in rocks, the location, frequency, quantity, scope and intensity of the microseismic signals are basically identifed. It is observed that the AE signals mainly occur within 5 m around the rock wall, basically lasting for one day before tunnel excavation and a week after excavation. Monitoring results indicate that the rockburst signals are closely related to rock stress adjustment. The rock structure has a rapid self-adjustment capacity before and after a certain period of time during tunneling. The variations of rock stresses would last for a long time before reaching a fnal steady state. Based on this, the site-specifc support parameters for the deep tunnels can be accordingly optimized.