Study On The Conditions And Hazard Prediction Of Rockburst On The Deep-Buried Overlong Tunnel Through Niba Mountains

Niba mountains tunnel is 10007m long, buried 1701m deep, with complicated geologic condition, located in deep-sheared high-middle mountain areas with relative hight difference of 2100m, in southeast of 'Y' tectonic zone in west part of Sichuan with active tectonic activities, complicated fault zone and high in-situ stress. Tunnel wall rock is strong and brittle, with the properties of rockburst. While excavating the tunnel, the stress of wall rock will be re-distributed, by which the stress centralization will be increased. Thus, tunnel wall rock has the internal and external conditions for rockburst, which is one of the main potential engineering hazards in this tunnel.Study on regional geologic enviornmental condition, rock joint and engineering property, tunnel area in-situ stress site, rock physical-mechanical property, and wall rock stress property in Niba mountain tunnel area, this assey forcasts the rockburst property and danger, and concludes as follows:(1)Fault dip of anticlinal east limb fault zone in tunnel area is mainly west-dipping, however, dip of anticlial west limb fault zone is mainly east-dipping, which reflects that fault zone and fold is deeply contacted in the formation and control of tectonic framework.Tunnel area has suffered at least four-term bigger geogolical tectonism in different direction, roak-layer is fold and deformed, fault is developed. With higher current tectonic stress, it conforms to high in-situ enviormental condition for rockburst and is good for rockburst development.(2) In tunnelr area, the rock mass is more complete ~ extremely fractured , the rock mass structure is divided into block structure, inlaid cataclastic structure, cataclastic structure, scattered structure, and incompact structure. Except the rock in fractured zone is softer, the rock in other part is much stronger. The average value of uniaxial compressive strength of complete rhyolite reaches 98.2MPa, the value is bigger while buried deeper. It shows that Niba mountain tunnel wall rock conforms to rock condition of rockburst, but the rock structure will influence rockburst intensity.(3) Numerical calculation shows that: intermediate principal stress is vertical. Max.and min. principal stress is generally horizontal. Direction of max. principal stress is N50°～60°W, max. principal stress in deeper-buried tunnel is 20MPa～55MPa, intermediate principal stress is 10Mpa～35MPa, min. principal stress is 0～15MPa. The tunnel area is of high in-situ stress enviorment, stress centralization os wall rock caused by excavation is bigger. Thus, stress condition for rockburst is conformed, the rockburst may occur.(4) Combining the calculation results of wall rock stress theory and rockburst forecasting analyzing result of tunnel wall rock quality-grade distribution, it shows that: the length that rockburst may occur in Niba mountain tunnel is 51% of total tunnel, including: weak rockburst tunnel is about 28%, medium rockburst tunnel is about 5%, strong rockburst is about 18%. Because of fault zone and difference of rock joint, the tunnel that rockburst may occur is un-continuous, and belt-distributed along the tunnel. With finite element analysis, the possibility of rockburst in complete wall rock tunnel has been analyzed and forecasted. The result shows that: complete strong wall rock is possible to rockburst, weak or mediea rockburst may occur in lower-buried tunnel.