| Due to the increasing demand of transportation and the constraints of route selection,topography and environment,some shallow-buried tunnels with large excavation spans have been applied in engineering.The current research cannot meet the needs of shallow-buried exceptionally-large-span tunnel.Key problems of shallow-buried exceptionally-large-span tunnel,such as rockmass pressure calculation,the load-bearing mechanism of support structure and the size effect,are needed to be further studied.The excavation span of Xiabeishan(XBS)tunnel is 26.3 m(the excavation area is over 350 m~2)in the newly-built Hang-Tai high-speed railway,and the overburden scope is 5～57 m).Based on this shallow-buried exceptionally-large-span tunnel,through theoretic analysis,numerical simulation,laboratory test,and field test methods,this dissertation reveals the size effect of shallow-buried exceptionally-large-span tunnel,and proposes the estimation method of surrounding rockmass pressure of shallow-buried exceptionally-large-span tunnel based on the existing experience and theories.Finally,the construction method and support system mechanical response of XBS tunnel are analyzed.The main research results of this dissertation are as follows:(1)For studying the size effect of shallow-buried exceptionally-large-span tunnel,the mechanical properties of surrounding rockmass of XBS tunnel were studied by the modified SRM method.The results showed that the stress-strain relationship and failure mode of rockmass were affected by joint distribution characteristics and size effect.The mechanical parameters of rockmass were accurately estimated by the modified SRM.The elastic modulus and strength parameters estimated by GSI method were close to the minimum estimated results by SRM.The influence of size effect on the shallow-buried exceptionally-large-span tunnel was summarized.The instability of surrounding rockmass caused by the size effect in the shallow-buried exceptionally-large-span tunnel could be reduced by reasonable design of excavation method.(2)The development of surrounding rockmass pressure in shallow-buried exceptionally-large-span tunnel was analyzed by considering the sequential construction process,and an empirical estimation method of surrounding rockmass pressure was put forward.The support parameters of XBS tunnel were initially determined by engineering analogy,and the structural checking result showed that the propsed empirical estimation method of surrounding rockmass pressure was conformed to the currently engineering practice.The development law of surrounding rockmass pressure of XBS tunnel was obtained using the field test method,and a new calculation method of surrounding rockmass pressure considering the sequential excavation effect of the pilot tunnel was also proposed.The comparison with the monitoring results showed the calculation rockmass pressure of the proposed method was more accurate compared with the estimated results from existing theories.(3)The appropriate excavation method of XBS tunnel was studied,and the key excavation technologies of shallow-buried exceptionally-large-span tunnel were summarized,and an experience chart for designing the construction method of shallow-buried exceptionally-large-span tunnel was proposed.The suitable construction method of XBS Tunnel was determined by numerical simulation according to deformation of surrounding rockmass and stress distribution of support structure.The deformation of surrounding rockmass and the stress distribution of support structure were less affected by the construction method under the grade-III rockmass,and the three-step method with the middle heading excavating first.and upper CD method had higher construction efficiency.The surrounding rockmass deformation and the stress distribution of support structure under grade-IV rockmass were closely related to the excavation area and sequence of subsections.The double side drift method should be adopted in grade-IV rockmass when the overburden was greatly changed,and the CRD or three-step method could be adopted in other cases.The double side drift method was generally used under grade-V rockmass.The field test result showed that optimizing the excavation sequence of the upper benches of each heading in double side drift method,removing time of the temporary support and optimizing the number of the lower excavation subsection were used to improve the construction efficiency as a precondition for ensuring the stability of surrounding rockmass.(4)The load-bearing mechanism of support structure in XBS tunnel was revealed,and the optimization measures of support and construction design were also proposed.The three-dimensional excavation effect in the sequential excavation process was beneficial to improving the stress state of support structure.The excavation of the upper benches of the middle heading was the most unfavorable stage in the currently excavation method.The internal force distribution of steel arch and shotcrete had the characteristics of“large in the upper section and small in the lower section”The shotcrete was the main load-bearing structure,and the shotcrete stress was concentrated in arch waist,arch foot and arch shoulder.The rockbolt at arch waist in grade-V rockmass had a significant effect on restraining the surrounding rockmass deformation after the support effect of temporary support failed,and the rockbolt at arch and wall foot should be optimized in future design.The stress adjustment of the initial support caused by removing of the temporary support had been completed after excavating the middle and lower step of the middle polit tunnel.Those stress adjustment phenomenon was not only reflected in the removal construction stage of temporary support,and the research result showed that it was safe to remove the temporary support if the stress of initial support was stable before excavating the lower step of the middle polit tunnel for improving the construction efficiency.When the tunnel overburden was greater than 1.5times of the tunnel excavation span under grade-III and grade-IV surrounding rockmass,the study result showed that the thickness of initial support could be appropriately reduced.The field monitoring result showed that the role of the secondary lining of XBS shallow-buried exceptionally-large-span tunnel could be regarded as emergency capacity. |
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