Research Of Parameter Characterization Of Rock Fracturing And Optimizing Of Locating Methods Of AE Monitoring In Underground Engineering

As a passive nondestructive monitoring technology,acoustic emission is commonly used in underground engineering but still has some limitations especially in in-situ monitoring.In this case,the optimizing of acoustic emission monitoring methods in underground engineering were studied.And the research is mainly focused on the aspects of parameter characterization of rock fracturing and source locating in this thesis.In the aspect of parameter characterization of rock fracturing,Brazilian disk splitting tests were conducted on the granite disk specimens to verify that shear cracks would increase near the damage stage even in the tensile failure process of rock.Based on this phenomenon,an index,CV(r),which is based on RA(Risetime/Amplitude)and AF(Average Frequency),was proposed to illustrate the fracturing intensity of rock failure.And a suggested threshold,CV(r)=1,was proposed in this thesis.According to the AE monitoring results during Brazilian disk splitting tests and in-situ construction excavation blasting of Huayingshan Tunnel,the validity and advantage of index CV(r)were well verified.The results show that evaluation based on the index CV(r)would lead to safer results,which means that the index shows a good property in resisting attenuation so that it is more appropriate than regular indexes during in-situ AE monitoring in underground engineering.Based on the results of in-situ AE monitoring in tunnels,influence of factors such as the distance between the event source and sensor,the attaching methods of sensors,the selection of parameter indexes and the statistical methods of parameters Attaching methods in Huayingshan Tunnel contains attaching sensors on exposed rock,primary lining and the tip of bolts.In Wulaofeng Tunnel,sensors were attached on exposed rock and in drill hole.According to the result,attaching sensors on the exposed rock or primary lining is relatively easy to conduct and the corresponding parameter analysis results are reliable.Sensors attached in drill hole would be little affected by the noises,which means a better monitoring results,but the attaching and coupling process are significantly difficult.Attaching sensors on the tip of bolts shows a moderate difficulty and would amplify some of the parameters.However,some other parameter indexes such as CV(r)were not suggested when the sensors were attached on the tip of bolts.However,in in-situ acoustic emission monitoring,only parameter analysis results may not meet the requirement of space density of cracks,so that a further event source locating would also be needed.As for event source locating,an optimized grid search method combining the database technics was proposed,which is Time-difference-field search method.Hydraulic fracturing tests were conducted to verify the feasibility of the method and the locating results were compared with which located by Simplex method.According to the statistical results of correlation coeff-icient,the Time-difference-field search method showed a better property in this experiment.Furthermore,a regional velocity model was proposed to verify the applicability of the method when dealing with problems with complex velocity models.Parameters densities were compared according to the locating results and a great difference was shown in different parameters.So that when analyze the parameter density,different kinds of parameters should be chosen to reflect different information to get a more comprehensive result.Based on the Time-difference-field search method,an equivalent locating method was suggested for in-situ monitoring in underground engineering to reduce the requirements of conditions in event source locating and simplify the AE events locating process during in-situ monitoring.The method was used in Wulaofeng Tunnel to locate the slight rock burst events on the working face.Different velocity models were compared according to the locating results during drilling process and a build-up velocity model was chosen to locate the events after drilling.The locating results were around the right region of the working face and the appearance time and the corresponding maximum of r all verified that the events are related to the macroscopic on the work face.According to the results of parameter analysis and equivalent locating,the events would be some slight rock bursts on the working face.