| With the construction of domestic highway transportation gradually inclining to the mountainous areas,the occurrence for tunnel engineering encountering shallow-buried unsymmetrical surrounding rock load has become much more frequent due to the acute topographic undulation of those areas.Excavating tunnel sections in shallow-buried unsymmetrical surrounding rock is prone to face geotechnical hazards such as collapse,rock fall and subsidence,conventional displacement measuring measures,however,are insufficient to directly indicate the stress state of the primary lining.Therefore,in order to characterize the stability of the primary lining so as to instruct the change of construction scheme for tunneling,it is extremely important to calculate the key load responsible for the primary lining's behavior via the understanding of structure and rock masses as well as the data obtained in the field.Based on the Longsheng tunnel project located on Bai-Yong freeway,Inner Mongolia,this paper carefully studies and combines the theories of back analysis and influence lines,and then applies inverse back analysis and normal back analysis to calculate the unknown shallow-buried and unsymmetrical surrounding rock load,respectively,the main research works and conclusions are as follows.(1)The influence line theory is introduced to associate the “load-structure” FEA model with the inverse calculation for the unknown structural load,and its validity is verified by examples.The application outcome shows that the complicated formula derivation is avoided so that the computational methodology for the load back analysis is largely simplified.(2)With the understanding of the tunneling technique used in Longsheng tunnel and the processing of the field measured pressure data between rock masses and the newly installed primary lining,the reason for the phenomena that the construction of partial lining rib wall at the lower bench dose little effect to the surrounding rock masses,is that i)the distance between the working face and lower bench is too far away and ii)the shoot concrete nearby might provide with strong longitudinal bonding effect,all of which indicating that the object chosen for the stability analysisin tunnel excavated in shallow-buried unsymmetrical surrounding rock load should be the semicircular arch-shaped primary lining close to the working face.(3)According to the analysis for stress and displacement data obtained in chainage YK86+630,the uniformly distributed radial load and the trapezoidal distributed tangential load are determined to characterize the interaction between the rock masses and the primary lining,the forward analysis model is established and then the load back analysis is conducted according to the influence lines calculated from the model.By comparing the back analyzed load with the field measured one,the results show that the assumptions used for model establishment and the characterization for rock-lining interaction is verified to be reliable.(4)The complicated unsymmetrical surrounding rock load is further characterized by multiple parameters and measuring error.By introducing the methodology for solving programming problems,the original much inequation constrained multiple parameter unsymmetrical load back analysis problem is transformed into multivariate quadratic programming problem.The four-variate and sixteen-variate examples' results show that the interior points method based fmincon function in the Matlab software can realize the fast and accurate calculation for the multiple parameter unsymmetrical load back analysis.Based on the load back analysis conducted for semicircular arch-shaped primary lining in tunnel sections with shallow-buried unsymmetrical rock masses,this paper puts forward a convenient method to use conventional displacement measurement to indicate the state of the rock masses,which can provide reference to similar engineering in the dynamic design of lining structure. |
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