Scale-effect Study Of Tunnels On Quality Evaluation And Stability Of Surrounding Rock

With rapid social development,two-lane and three-lane highways can’t satisfy ever-growing demands for transportation any longer,so construction of super-large cross-section tunnels is gradually increasing.However,there are few super-large cross-section tunnel engineering projects both home and abroad,the void of normative standards remains to be filled,and there is no mature engineering experience which can be taken for reference Rock mass quality evaluation is a key issue in the tunnel engineering construction.Faced with larger and larger underground engineering such as super-large cross-section tunnels,common evaluation methods have many problems,e.g.the information quantity reflected by a single evaluation method is small,engineering features are not taken into consideration in the classification process,accuracies of some parameters are poor,etc.,which have to be settled urgently.Geological conditions encountered by super-large cross-section tunnel engineering are more complicated with high construction risks and high difficulty level,and its key construction technologies have to be sumnlarizied and perfected urgently.Under the engineering background of construction of multiple highway tunnels and mountain ridge pipe racks,theoretical analysis,laboratory test,field monitoring and numerical simulation were integrated in this paper to conduct a profound study of current surrounding rock quality evaluation methods,correlations between the stability of surrounding rock and evaluation indexes,the influence of structural plane characteristics on the stability of surrounding rock as well as design and construction of super-large cross-section tunnels,and the following study results were obtained:(1)A deep analysis of four important evaluation indexes,which were used foreign and domestic rock mass quality evaluation methods in commonly,was carried out,including rock mechanical properties,rock mass integrity,underground water conditions and initial ground stress.Meanwhile qualitative evaluation and quantitative characterization means of various indexes were summarized.Based on BQ method,Hydropower Classifying method,RMR classification method and Q system classification method,a normalized method of different evaluation methods was proposed.Index weight analysis of the four evaluation methods was implemented.it’s confirmed that the BQ method took rock mechanical indexes as the main evaluation indexes,but other three methods took properties of the structural plane as the main evaluation indexes.Based on 187 sets of rock mass classification results,function fitting analysis was performed on the correlation of the four evaluation methods.The results showed that BQ method presented a relatively obvious linear correlation with HC method,and a relatively obvious logarithmic correlation with Q system,but the correlation coefficient was low between other methods.In consideration of limitations existing in single evaluation methods,a combinational evaluation method based on a support function was put forward,which could reflect actual rock mass conditions from multiple aspects.Proper rock mass quality evaluation methods were selected according to different projects so that results could be more approximate to the true situation.(2)Based on universal distinct element code software(UDEC),changes of rock mass stress field after the tunnel excavation were simulated.And Spearman correlation coefficients between surrounding rock stability and 7 factors(rock elasticity modulus,rock Poisson’s ratio,rock cohesion,rock internal friction angle and spacing,cohesion and friction angle of structural planes)were obtained under four different tunnel spans(20m,15m,10m,5m).Calculation results indicated that mechanical properties of the structural plane had a stronger influence on surrounding rock stability than rock mechanical properties.The factors had different influence degrees on surrounding rock stability under different excavation spans of the tunnel chamber.As the tunnel span increased,the influence of the structural plane was reduced while that of rock physical and mechanical parameters was enhanced.Based on this result,a correlation coefficient-based scale effect correction method of the national standard corrected BQ method was established.This correction method mainly corrected weight coefficients and reflected different influence degrees of evaluation indexes on surrounding rock stability under different chamber spans.(3)Deformation mechanisms and instability patterns of monolithic blocky structure,blocky structure and cataclastic structure were summarized.Based on discrete element numerical calculation methods under 72 groups of different conditions,instability proportions under different combinations were calculated,and it could be deemed that whether the surrounding rock experienced instability was decided jointly by tunnel span and structural plane type to a great extent.Problems existing in the conversion method of rock mass integrity coefficient and volumetric of joints in rock mass as specified in the national standard were analyzed.A correction method of the conversion formula between the two was proposed through the analysis of 120 groups of field data.(4)Based on BQ method presented in the national standard,[BQ]S-P classification method is established,which is the comprehensive correction of scale-effect and structural plane classification for BQ method.Meanwhile,a quick determination procedure of rock mass classification is established to guarantee the objectivity,accuracy and authenticity of the classification results and ensure the smooth advancement of dynamic design and construction.The necessity of setting a reasonable transition section is clarified when the surrounding rock condition is suddenly changing.The FEM numerical simulation and monitoring results show that it is reasonable to set a 9-10m transition section in the super-large cross-section tunnel for excavation method from CD method to bench cut method.By improving the layout of the pressure cells,the whole section monitoring of the surrounding rock pressure is realized,which overcomes the disadvantages of the traditional monitoring method of the surrounding rock pressure.And this technique was applied to the rock pressure monitoring in the collapse section of the super-large cross-section tunnel.Collapse accidents of super-large cross-section tunnels were studied,collapse causes were obtained by combining the study result of surrounding rock instability mechanism and laboratory test result of samples on the field,the collapsed chamber filling method using bubble mixed with light soil was adopted,the improved full-cross-section surrounding rock pressure monitoring technology was used to monitor surrounding rock pressure in the collapsed section,and monitoring results indicated that controlling value of surrounding rock pressure in the collapsed section was only 47.8%of theoretically calculated value.