ABSTRACT

The assistant tunnel of Jinping Second Stage Hydropower Station, whichiscovered by over 2 000 m-thick rock mass and has maximum principle stress over 60MPa, is used as an example to investigatelaws,characteristic,rangesandvelocityofstressredistributionwithtimealong directionofparallelandperpendiculartunnelaxesandbalancetimeofstress fieldusing numerical analysis method,basedon Kelvin-Voigt-CWFS model whichisaviscoelastoplastic combination modelandisadaptablefordescribingtime-dependantcharacteristicsofhardand brittle rock mass. The result shows that the stress is firstly up and then down gradually at the two side walls and decreases quickly at vault, their principal vectors rotate gradually during the stress redistribution and 90% adjustment of stress is finished at early several days after excavation. At equilibrium state, the stressisfirstly up and then down tostabilityat the two side walls and up gradually to original stress state at vault with increase of distance from free face at the direction ofperpendiculartunnelaxes;thestressisfirstlyassembled,thenfallsdownsuddenlycloseto zero and finally increases to some value as observed profile is firstly close to and then far away from the tunnel face. The laws above mentioned are different from the laws obtained from middle andlowstressenvironment. Knowing these laws, characteristic and mechanism of stress retribution is benefit for design, construction and supporting of tunnel and prevention and cure of disaster.

1 INTRODUCTION

Aseriesof engineeringcatastrophessuchasrock burst, collapse of roof strata and so on broke as redistribution of stress field in progress of cavern excavation and people's life andpropertywerethreatenedbadly. For studying deeply characteristic,lawsandrangesofstressfield redistributionalongdirectionofparalleland perpendicular tunnel axes, many ways, such as testing in laboratoryandinsitu(Liuetal.2005), numerical simulation(Yang et al. 2007) and theoretical analysis(Gao etal.2007)were proposed by scholars at home and abroad and a great deal of interesting result was obtained. Study on evolution law of stress field, which are based on elasto-plastic constitutivewithnumericalmethods,was focused onspatialdistributionat middleandlowstress levelinformer documents, whileredistributionlaw with timewaslessnoticedonthebasisoftime-dependant constitutive,especiallyforhardandbrittlerockathigh stresslevel. Therefore, a numerical algorithm, which is based ontime-dependantconstitutiveanddetermines parameters and initial ground stress field by back analysis method, is proposed to analyze spatial-temporal evolution featureofinitialstressfieldinhighstressexcavation zone. The assistant tunnel of Jinping Second Stage Hydropower Station covered by about 2 500 m-thick rock mass andhavingmaximumprinciplestress60MPa,is used as an example to investigate laws, disturbed ranges

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