| The deep geological disposal is the most efficient solution to insulate high level radioactive waste,underground laboratory of high level radioactive waste is an important part of deep geological disposal.Therefore China has prepared to construct the underground laboratory of high level radioactive waste in Beishan.During the research,it was found that high tectonic stress and high local stress existed in Beishan.As a result,there is possibility that rockburst may occur during the underground laboratory construction.Rockburst has been studied by many scholars for a long time,and many research achievements have been gained.However,rockburst is caused by multiple factors and there are uncertainty factors in the underground engineering.At present,whether it is the mechanism of rockburst or the prediction of rock burst,it is all for specific study.And it has not yet formed a mature and systematic assessment theory and technical methods.Therefore,in order to ensure the safety of the underground laboratory of high level radioactive waste in Beishan,the rockburst characteristics of Beishan granite need to be studied in advance.The basic mechanical parameters of Beishan granite were obtained through laboratory experiments such as uniaxial compression,tension and triaxial compression tests.The rockburst proneness and failure characteristics of Beishan granite is analyzed based on strain energy and acoustic emission(AE).Stress state and characteristics of Beishan granite rockburst were studied through laboratory rockburst experiments,and the mechanism and process of Beishan granite rockburs was further discussed based on rock slab destruction.Based on wavelet analysis and characteristic parameters analysis,the characteristics of microseismic and acoustic emission events in intact rock mass and fractured rock mass during the excavation of the Beishan exploration tunnel are studied separately.Secondary stress distribution of Beishan exploration tunnel was studied through stress relief method.The main researches of this paper are covered in the following five aspects:(1)The rockburst proneness was studied by using brittleness index and elastic energy index.On the basis of ground stress,the relationship between Beishan granite rockburst and buried depth was further studied.Geological conditions of rockburst in Beishan was analyzed by RQD value of deep drilling core and Beishan exploration tunnel condition.Fisher criterion grades for rockburst were established based on rockburst projects at home and abroad,and rockburst grades of Beishan granites was analyzed by the Fisher criterion.(2)The effect of confining pressure on characteristic stress was studied by carrying out triaxial tests,and changes of elastic energy,dissipated energy and total energy during the failure process were analyzed.The new rockburst proneness index was established by the changes in energy during failure,and the influence of confining pressure on the rockburst proneness index of Beishan granite was further analyzed,the results show Beishan granite rockburst proneness increased with the increasing confining pressure.The changes of b value and correlation dimension of acoustic emission during failure process of Beishan granite was studied.In addition,precursor information of Beishan granite destruction was studied based on b value and correlation dimension of acoustic emission.(3)The effect of unloading rate on the deformation,strength and strain energy of Beishan granite was studied by laboratory experiments with loading axial pressure and unloading confining pressure,the influence of unloading rate on the damage of Beishan granite was further analyzed.Hierarchical clustering was used to analyze effects of unloading rate,confining pressure and unloading time on the internal fractures in Beishan granite,the result show ratio of tensile fracture to shear fractures increase with confining pressure unloading rate increasing.The effects of unloading rate and confining pressure on local stress state were analyzed by Maxwell model,and the mechanism of effects of unloading rate and confining pressure on internal fracture form was studied by using Mohr-coulomb model.(4)The stress state and acoustic emission characteristics of immediate rockburst and time delayed rockburst were studied based on laboratory rockburst tests,the results show b value of time delayed rockburst is bigger than b value of immediate rockburst.Geometric features,size distribution and fractal characteristics of fragments generated from the tests were studied,and the differences between immediate rockburst and time delayed rockburst were futher analyzed,the results show fragments of immediate rockburst and time delayed rockburst all were dominated by slab fragments and fragments fractal dimension of immediate rockburst is bigger than time delayed rockburst.The rock slab failure model was established considering the tangential normal stress and radial normal stress of the surrounding rock,and the effects of length-thickness ratio,location,thickness and length of rock slab on the destruction of surrounding rock were studied,and the process of rockburst was further analyzed based on rock slab failure.(5)Arrival time of P and S waves and characteristics of frequency band energy were researched by wavelet decomposition method,and distribution characteristics of microseismic events were studied using characteristic parameters of magnitude and volume potential.The danger events of intact rock and fractured rock wre studied based on energy index,and b value was used to determine stability of surrounding rock,the results show microseismic magnitude of intact rock mass is higher than fractures rock zone,microseismic events of intact rock is mainly low frequency seismic,and microseismic events of broken rock include two types.Differences of acoustic emission characteristics of the intact rock mass and fracture rock zone were further analyzed three dimensional acoustic emission monitoring system,the study showed that the intensity and quantity of acoustic emission events in the intact rock mass were larger than those in the fracture rock zone,b value of acoustic emission in intact rock mass is greater than it in fracture rock zone.The applicability of Pocket AE in tunnel have been analyzed by studying the relation between impact rate of Pocket AE and monitoring time and location.The secondary stress distribution in surrounding rock was analyzed.The applicability of drilling bits in hard rock was further studied.And according to the features of various monitoring methods,the proposal of on-site comprehensive monitoring was put forward. |
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