| Bifurcation tunnel with super-large cross-section is the construction trend of widening underground structure of highway tunnel.As an important section connecting large cross-section tunnel and small clearance tunnel,the variable tunnel structure and complicated excavation process of super large cross-section branching-out tunnels bring challenges to the safe construction of drill and blast method.To prevent safety accidents caused by blasting excavation,it has great theoretical significance and application value to systematically study excavation methods and controlled blasting techniques of the branching-out tunnel.This paper takes the 428.5 m2 oversized section and 0.5 m ultra-small clearance section of the branching-out tunnel of the Shenzhen Liantang Tunnel,one of the three landmark tunnel projects in China in 2018,as the background,it focuses on the three major objectives of forming a safe excavation method for the oversized section,spatial and temporal partitioning of the excavation of the small clearances tunnel back tunnel,and precise control blasting technology for different types of sections in the branching-out tunnel.In this paper,numerical simulation,field monitoring,theoretical analysis,field experiment and other methods are used to study the safety controlled-blasting techniques of the bifurcation tunnel with superlarge cross-section.For the formation method of the super-large cross-section in the bifurcation tunnel,the safe excavation method of advance climbing,lateral expansion and reverse boring support from small section tunnel is proposed for the blasting safety problems brought by the large span and height of oversized section tunnel.MIDAS GTS program is used to analyze the stress and deformation characteristics of surrounding rock of different excavation schemes,and the correctness of the recommended method is verified by field displacement and pressure monitoring of supporting structure.The technical problems of blasting implementation of super-large crosssection excavation method are solved.The electronic detonator initiation technology is used in large section tunnel blasting,and the design method of electronic detonator blasting parameters is studied and formed.This method is based on Anderson’s theory of delayed blasting vibration synthesis.The measured waveform of single-hole blasting experiment is marked as the calculation vibration source.Different charge number and delay interval are designed and programmed into the program,and various delay vibration synthesis curves between holes are calculated.The delay time between holes of the maximum charge amount at safe vibration speed is selected as the optimization parameter.The method is applied to blasting of the large-section in Liantang tunnel.The peak particle vibration(PPV)calculated and predicted before blasting is highly consistent with the measured value,and the PPV of blasting is reduced by 69.1%compared with that of conventional blasting,which ensures the safety of super-large-section construction.To solve the safety problem caused by the blast-induced vibration in the following section in the small clearance tunnel,which include rock slump and collapse,the four-part excavation method of reserved seismic mitigation layer was proposed in the following tunnel.The equivalent circle method and Schwarz alternating method are used to study the influence of different cushioning layer thickness on the stress of surrounding rock,and the optimal thickness of cushioning layer of Liantang tunnel is calculated to be 3m.According to this,different zoning geometric sizes are divided,and the excavation scheme of space-time zoning for the back tunnel with small clear distance is established.On the basis of the abovementioned space-time partition excavation scheme of the following tunnel,the blasting technology of different partitions is studied pertinently,specifically as follows:(1).The technical problem of safe blasting,which takes into account vibration control and cost control,is solved in the first excavation area(Area I)of the following tunnel.To eliminate the potential safety hazards of serial section,misfire and open circuit of the mixed initiation network,the time parameter calculation model of the mixed initiation network during the extension in the hole is constructed.The formula of key time parameters is derived based on safe initiation constraints.The field experiments showed that the mixed initiation network can save 60.3%of the number of electronic detonators compared with the electronic detonator initiation network,and reduce the PPV by 81.5%compared with the conventional blasting,which reduced the damage of the middle interval rock by the blasting for Area Ⅰ of the following tunnel.(2).For the controlled-blasting of cushioning layer in the key blasting area(Area Ⅱ)closely connected with the middle interval rock,the damage mechanism of blasting load on the middle interval rock in cushioning layer was analyzed by the LS-DYNA program.Base on the gradual change of the thickness of the middle interval rock,the combined technology with multi-parameter controlled-blasting was put forward.The field monitoring results of blast-induced vibration showed that the PPV of the advance tunnel nearby the middle interval rock was reduced by 73.8%compared with the conventional blasting,and the middle interval rock with 0.5 m thickness is intact after blasting,which verifies the safety of the combined technology with multi-parameter controlled-blasting.The research method and blasting technique proposed in the paper provides a scheme for the safe excavation of the branching-out tunnel with super-large crosssection.The potential safety hazards and technical problems of Liantang tunnel project have been solved in field practice.Meanwhile,the stability of surrounding rock in super-large cross-section and the middle interval rock in small clear distance section have been guaranteed.It provided to the theoretical support and practical guidance for efficient and safe blasting excavation of branching-out tunnels with super-large cross-section above 400 m2. |
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