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Excavation‑induced structural deterioration of rock masses at different depths

Zhang R., Xie H. P., Ren Li, Deng H. I., Gao M. Z., Feng G., Zhang Z. T., LI X. P., Tan Q.

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 2

art. no. e81, 1--15

The excavation-induced deterioration of rock mass quality and integrity may significantly affect the stability of deep underground spaces. However, the influence of the burial depth on excavation-induced rock mass structural deterioration remains unclear. To address this issue, in the Jinping II auxiliary tunnels, borehole acoustic wave and digital panoramic borehole imaging tests were conducted at five depths, and the surrounding rock mass structure deterioration was comprehensively quantified at these depths. The results show that the wave curve significantly varies with increasing depth, and the deeper the tunnel section is, the more severe the rock damage is, the larger the excavation damage zone is, and the more complex the generated fracture network is. An excavation-damaged zone and excavation-disturbed zone (EdZ) were found in the rock masses at shallow depths, while a highly damaged zone (HDZ) and EdZ were observed at greater depths. Further investigation demonstrates that the formation pattern of excavation-damaged zones (EDZs) at deep depths follows a stress-concentration controlled mode, and at very shallow depths, horizontal unloading plays a dominant role in the EDZ formation process (yielding a pure unloading-controlled mode), while at intermediate depths, a mixed EDZ formation mode is observed.

Tracking the development of seismic fracture network from The Geysers geothermal field

Orlecka-Sikora Beata, Cielesta Szymon, Lasocki Stanisław

Acta Geophysica

2019

Vol. 67, no. 1

341--350

Underground fluid injections result in rock mass fracturing. The associated environmental hazards in a significant part stem from a possibility for linking these fractures. The resultant crevices may allow for an undesired and hazardous fluid migration. We studied the fracture linking problem on data from a part of The Geysers geothermal field in California, USA. We parameterized seismic events by the distance between hypocenter and injecting well, by the angle between the position vector of hypocentre and the maximum horizontal stress direction and by the angle of rotation required to turn the event’s doublecouple mechanism into the prevailing in this area faults’ orientation. To make these parameters comparable, we transformed them to equivalent dimensions. Based on distances between events in the transformed parameter space, we divided the seismic events into clusters. The percentage of potentially linked fractures in clusters was greater at low than at high injection rate.