Mechanical Behavior And Stability Control Of Deeply Buried Closely Spaced Triple Tunnels

Due to the growing traffic flow and special building needs,the closely spaced triple tunnels are increasingly constructed in field engineering.Compared with the single or twin tunnels,triple tunnels' mechanical transformation is more complex,leading to the significant tunnel-group effect and pressure arch effect.The triple tunnels are excavated one after another,causing severe mutual disturbances,resulting in considerable damages,destruction,and the rock pillar sliding.Thus the safety issue is more prominent.Domestic and foreign scholars have currently accumulated mature construction experience and research results of closely spaced twin tunnels.However,triple tunnels' research is limited to case reports,and very few studies are related to the interaction law and arching behavior among triple tunnels.Besides,there is no mature stability control measure for reference.Based on the Badaling triple tunnel project of Beijing Zhangjiakou high-speed railway,this paper mainly focuses on the calculation method of rock pressure,evolution law of pressure arch,influencing law of adjacent excavations,the control mechanism of rock stability,field monitoring of mechanical response,and engineering application of stability control measures of deeply buried closely spaced triple tunnels.The research methods mainly include theoretical analysis,numerical simulation,and field monitoring.The main conclusions and innovations are as follows:(1)The load model of extended Protodyakonov's arch of deeply buried closely spaced triple tunnels is established,and the calculation method of surrounding rock pressure is put forward.Based on Protodyakonov's arch theory,two different load models of deeply buried closely spaced triple tunnels are established by considering the excavation sequence and pillar function.They can be divided into three bearing arch state cases,the calculation method of surrounding rock pressure is then put forward,and the current calculation method is improved.The influencing law of rock classification,excavation span,pillar strength,and pillar width on rock load is analyzed.The significant load bias characteristic of deeply buried closely spaced triple tunnels is revealed.The results show that the surrounding rock pressure at the inner side is significantly larger than the outer side,and the surrounding rock pressure of the middle tunnel is much larger than the side tunnels.The theoretical method is verified by field measurement,and it is found that the Class-V surrounding rock section tends to form an ultimate large bearing arch,while the Class-III surrounding rock section tends to form three independent small balanced arches.The load-bearing ratio of the primary lining and the second lining is defined,and the main bearing function of the primary lining is clarified,which provides quantitative guidance for support design.(2)The evolution law of pressure arch of deeply buried closely spaced triple tunnels is expounded,and the control principle of rock stability is clarified.The mechanical indexes of surrounding rock stability are put forward,including rock deformation,plastic zone,and pressure arch boundary.The progressive evolution characteristics of the pressure arch are studied.The transformation mechanism from small independent pressure arches to the large combined pressure arch is clarified.The formation mechanism of the double arch effect and the leading arch effect is revealed.The different safety state of triple tunnels is analyzed: the lagging middle tunnel's safety state is the worst,and the leading left tunnel's safety state is the second-worst.The influencing rules of rock grade,clearance,lateral pressure coefficient,buried depth,and support thickness on the pressure arch's stability are analyzed.The criteria of reasonable clearance and critical depth for distinguishing deeply or shallowly buried are put forward.Longitudinally,pre-control measures are proposed to solve the large pre-deformation and severe pre-failure of surrounding rock;transversely,pillar support measures are proposed to solve rock pillar deterioration and instability.(3)The pre-control mechanism of pipe roof and curtain grouting is revealed longitudinally,and the control mechanism of pillar-reinforcing rock bolt is revealed laterally.Longitudinally,the three function modes of pipe roof pre-support are put forward,including circumferential micro arch effect,longitudinal long beam effect,and spatial shed frame effect.The corresponding mechanical model of pipe roof is established,and the quantitative evaluation method of support effect is put forward.According to the poor geological section's safety requirements,the combined pre-control scheme of pipe roof and curtain grouting is proposed.The combined pre-control scheme of pipe roof curtain grouting is proposed,and the synergy effect of them in time,space,stiffness,and strength is analyzed.The pipe diameter,pipe layout angle,and grouting thickness are optimized,and they provide the scientific basis for engineering application.The pillar-reinforcing bolt's two function modes are proposed,including compression reinforcement effect and bearing and transferring effect.The pretension load at both ends and excavation sequence are considered,and the load transfer mechanical model of the pillar-reinforcing bolt is established.The internal force's distribution curve along the bolt is deduced,and field data verified the theoretical method.The influencing factors of the pillar-reinforcing bolt's supporting effect are analyzed.It is pointed out that the pillar-reinforcing bolt is more suitable for the poor and broken surrounding rock.(4)The mechanical response of closely spaced triple tunnels was systematically monitored and analyzed,and the stability control measures are successfully applied in the field project.Based on the Badaling triple tunnel project of Beijing Zhangjiakou high-speed railway,the field test was carried out to monitor support stress and rock deformation systematically.The field excavation mechanical behavior is reproduced,the monitoring data was analyzed and fed back to the tunnel design.Based on preliminary monitoring in the test section,it is found that triple tunnels' interaction behavior was significantly severe,and the original supports failed to meet the safety requirement.Therefore,pipe roof and curtain grouting are simultaneously used as the pre-control measures,the supporting parameter of the pillar-reinforcing bolt is also improved.The results show that the support safety state is significantly improved,the rock stability is guaranteed,and the triple tunnels' safety construction is ensured.