Synergistic observation and mechanism analysis of time-dependent fracture process of columnar jointed rockmass under strong unloading during excavation
(1. State Key Laboratory of Geomechanics and Geotechnical Engineering，Institute of Rock and Soil Mechanics，Chinese Academy of Sciences，Wuhan，Hubei 430071，China；2. University of Chinese Academy of Sciences，Beijing 100049，China；3. Key Laboratory of Ministry of Education on Safe Mining of Deep Metal Mines，Northeastern University，Shenyang，Liaoning 110819，China)
Abstract：As a kind of dense jointed rockmass with regular structure plane network，columnar jointed rockmass is with complex mechanical response processes under high stress，such as jointed plane opening，rock block rupture and rockmass structural relaxation. Based on the engineering background of the #3 diversion tunnel on the left bank of Baihetan hydropower station，time-dependent fracture process of columnar jointed rockmass under strong unloading during excavation and its mechanism were studied after with a series of synergistic observation. The synergistic observation contains microseismic monitoring，acoustic velocity test and digital borehole camera test. Results show that the period of time-dependent fracture process is more than 10 days. The number of time-dependent fractures decreases with the increasing of days after excavation. It is a logarithmic function and can be divided into three time-dependent stages：weakening period，stable period and weak period. The number of time-dependent fractures decreases with the increasing of distance from the working face，and with the increasing of depth from the side wall. The time-dependent fractures are mainly within 3 m from the side wall，the relaxation depth is about 3 m，and the visible fractures are distributed within 2.5 m. The synergistic observation results have a good spatial consistency. The excavation size and location have significant effect on the time-dependent fractures of columnar jointed rockmass. The number of time-dependent microseismic events generated by step-by-step excavation of half section is less than that of the full section. Tensile fracture is the main fracture type of columnar jointed rockmass and there are a lot of time-dependent tensile fractures beyond 30 m behind the working face. The research results can provide reference for support optimization and safe construction of dense jointed rockmass under high stress.
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2021, Vol. 40 
