Study On The Risk Forecast And Construction Technique Of Karst Tunnel For Excavation And Support

The complexity and lower level of investigation of hydrogeological conditions in karst area bring about wide discrepancies between documents obtained from investigation and actual conditions after excavation, which leads to frequent emergence of unfavorable geologic bodies during tunnel construction such as loose deposits of soil, fault structural belt, development zone of surrounding rock sandwich, karst cavities and so on. In order to effectively avoid unpredictable accidents as overbreak, roof fall, water inrush, mud inrush, spalling of rock, etc., accurate location of unfavorable geologic bodies in front of the tunnel face, effective risk assessment of geologic hazards under unfavorable geological conditions and reasonable construction types of excavation and support according to the actual conditions of surrounding rock for tunnel face these three aspects are paid more and more attention from engineering world.Taking Ji Changping tunnel as the engineering background, centered on the main system of surrounding rock-supporting-deformation, the paper firstly made risk analysis and assessment of possible geologic hazards in the tunnel based on the fuzzy analytic hierarchy process comprehensive risk analysis (FZ-AHP-RA), and then drew risk lever chart; applying the sub-level classification method of surrounding rock, based on geological analysis, made advance prediction about unfavorable geologic bodies using reflected wave technique to accurately ascertain actual geological conditions of surrounding rock in the tunnel. Then taking such factors as construction safety, surrounding rock conditions and construction progress into consideration synthetically, the paper brought about the reasonable construction method and supporting process flows for Ji-Changping tunnel and drew the overall construction process chart; last but not least, judged the rationality of supporting from site monitoring measurement and numerical simulation. The main contents were listed as follow:(1) Introduced the geological conditions of Ji-Changping tunnel, mainly including project overview, project geological conditions and characteristics of unfavorable geologic bodies;(2) comaring with tunnelling risk acceptance criteria theory, the fuzzy analytic hierarchy process comprehensive risk analysis (FZ-AHP-RA) is presented, which is based on combining the Analytic Hierarchy Process(AHP) and the Fuzzy method, as well as considering other qualitative or quantitative methods. Came up with method of risk assessment of geological hazards and technology roadmap, and made detailed risk assessment of water & mud inrush;(3) On account of geological analysis of tunnel face, combined with synthesis prediction technology of geology, made predictions for unfavorable geological conditions in karst tunnel, comprehensively introduced operating steps, data generation process of TGP206 advance geology prediction system and presented interpretation principles and steps of comprehensive geologic and geophysical map were drew up;(4) According to conditions of surrounding rock after excavation in tunnel faces, concluded the supporting parameters, construction method and supporting process flow corresponded to sub-level classification method of surrounding rock;(5) Combined with site monitoring measurement, offered analysis on rationalization of supporting, and statistical analysis on related deformation, then the favorable phases of different sub-level classification of rock mass was determined, and the convergences calculated.