Study On Construction Method Of Very Large Section, Shallow-buried, Large-span City Tunnel Under Complex Conditions

The maximum excavation span of Tunnel excavation Daping of Chongqing Light Rail Da-ping Station is 26.3m, with flat arch structure, the maximum excavation of an area of 430m2, the largest excavation height of 20.6m and the minimum depth of 4.0m. It is the classⅢmudstone and the hole-span ratio is 0.15 ~ 0.5. The most important thing is that this project is located in a downtown area, passing through sections of the surface humanities complex, mostly poor seismic performance of buildings or traffic trunk road surface.So it needs very strict requirements for blasting vibration. The study of large section under complicated conditions and ultra-shallow large-span multi-city tunnel construction focuses on tunnel construction safety, but how to ensure the security and economic aspects of the unity is rarely considered. In this paper, we study the largest segment excavation span of tunnel excavation Daping of Chongqing Light Rail Da-ping Station under complicated conditions and the excavation methods, excavation parameters, blasting program and parameter selection and optimization of blasting conducted. Considering both of the control surface settlement and blasting vibration velocity to ensure the safety of tunnel construction, we also achieve rapid tunnel excavation and creat a good economic benefit. From the choice of construction methods to the development and application of related technologies embody the modern tunnel vision and innovation, enrich and improve the shallow urban tunnel construction technology. It provides an effective way to construction control of large section under complicated conditions and ultra-shallow large-span city tunnel:①Combined with shallow tunnel arch characteristics of long-span flat, we focused on the tunnel surrounding rock stress distribution basic characteristics. We put forward five kinds of excavation methods according to the Chongqing Light Rail Station Daping tunnel engineering to analsied various excavating conditions of the construction process. A new method of tunnel construction is propsed in the subway and urban light rail construction which is the half-section side wall pit method, the next section in the groove after the first side wall excavation, the first arch the back wall lining.②Attempts in the complex conditions of the tunnel construction process of adopting a numerical simulation as the leading role and optimize the program to simulate the construction process simulation, choose a reasonable step size of excavation, and mutual staggered distance, advance support excavation methods, and to determine the optimal parameters, calculation results and the actual measurements close to each other and the realizing the effective control of the surface subsidence.③Used FLAC3D to simulate tunnel blasting, the blasting excavation dynamic load applied to the boundary, result show that attenuation law of numerical explosion seismic waves better agree with measured seismic wave attenuation, studies have shown that the greatest surface particle velocity is feasible; used FLAC3D to simulate the ground vibrations of Daping station tunnel blasting excavation of different methods, from the surface settlement displacement, vibration speed and force of bolt to carrie out numerical simulation of calculated show that the design of blasting under an relatively stable state, the proof of adoption of the blasting program is reasonable, and thus prove that from the perspective of blasting in the subway and urban light rail construction in a "lead on the half-section side wall pit method, the next section in the groove after the first side wall excavation, first arch lining the back wall, "new methods of tunnel excavation is feasible; the use of FLAC3D simulation Daping station tunnel blasting excavation of the left guide-hole lead on the right holes have been lining the impact of changes from the vibration velocity and displacement analysis of the circumstances come design of blasting in the same dose of blasting excavation right to do under the guide hole has little effect a description of the construction parameters and blasting dose is feasible, thereby complex under the conditions of a large cross-city tunnel construction parameters and blasting parameters for determination of an important basis.④According to span the characteristics of ultra-shallow tunnel, propose a complete layout of the measurement points and measurement methods in ultra-large shallow cross-city subway tunnel, and the analysis of measuring results show that the large cross-shallow tunnel construction need not only to carry out regular projects measurements but also for the early strength of bracing measurements. So it can bring us an accurate, intuitive understanding of rock stress changes and have a very important practical significance for guiding tunnel construction and preventing dangerous situations.⑤Used A variety of field measurements showed that routine measurement have small absolute value of each deformation, it is easy to overlook that the large cross shallow tunnel in addition to routine items to be measured, the importance of selecting can reflect surrounding changes, the absolute value of large deformation measurement project (such as the initial support I-beam to Measure), to guide the tunnel construction, to prevent dangerous situations have very important practical significance.