Shallow Large Span Of Surrounding Rock Stability Of Engineering Geology

The Baise Multipurpose Dam Project is located on the Youjiang river, 22 km upstream of Baise city, Guangxi Zhuang Autonomous Region, P.R. of China. It is a large scale development privileged for flood control, incorporated with power generation and navigation, as well as combined with irrigation and water supply. The Project is composed of a main dam, two saddle dams, an underground powerhouse and a navigation system.The underground powerhouse complex consist of powerhouse cavern, transformer cavern, tailrace main tunnel and other caverns, are most successfully excavated in diabase rock mass with the small rock pillar of only about 1.0x the cavern width and the thickness of the rock cover is 0.8~1.5x the span of the cavern. Where the diabase is only about 150m wide, in blocky to broken structure, and with sloping ground surface.Based on the abundant complete information available in different stage of reconnaissance, construction and operation, the thesis topic of "Engineering Geological Study on the Stability of Rock Mass Surrounding the Shallow-Seated Large-Span Underground Caverns" was selected to study why the shallow-seated large span underground powerhouse complex could be successfully carried out in blocky to broken rock mass. From the analyzing on the original stress, the original structure of rock mass and the original rock mass quality, the original environment rock mechanical model was built, then the stability analyzing of the caverns was conducted, the original rock mass quality was calculated in back on the monitoring data. After all the discussion on the factors of successful construction of the shall-seated caverns, such as the geologic conditions, rock mass engineering properties, the proper support measurement and so on, were done in detail. The main contents of this dissertation are:(1) Rock mass original structure analysis-analyzing compared with the joint discontinuities, longitudinal wave, water pressure test value and other test data from the adits and the boreholes in study area, the differences of rock mass structure in disturbed condition and in original condition were studied. Based on the unanimity rule in geometric integrity and in mechanical integrity, the original structure and its ratio of the surrounding rock mass were analyzed.(2) Rock mass original quality analysis-analyzing compared with the deformation modulus tested in the boreholes, adits and the open air, the mathematic model between the longitudinal wave and deformation modulus was derived. After the longitudinal wave input, the deformation modulus of the surrounding rock mass were calculated. From that the original quality of the surrounding rock mass in the underground powerhouse complex area was reached.(3) Origin rock stress analysis- Based on the geological analyses, the FEM simulation, as well as the measured rock stress data, the direction and the magnitude of principal stress in underground openings area were derived, and the spatial distribution features of original geostress were discussed.(4) Surrounding rock mass classification-Engineering geological classification for surrounding rock mass by RMR, Q-system and T were carried out successively. And then the correlations between the assessment results of two different methods were discussed.