Study On Construction Mechanical Behavior Of Highway Tunnel With Large Section And Small Spacing

Following the strategic direction of insisting sustainable development and protecting ecological environment and limited land resources, and to satisfy the rapid development of economy and transportation, small spacing highway tunnel with large section, such as three-lane road tunnel, seems be of great demand. This type tunnel has the advantage of optimizing the whole road allocation plan under special geologic and topographic conditions; however, owning to its particularity and infrequence, the interaction between two tunnels is fairly serious. Thus, there are still several key problems needed to be solved during the design and construction procedure, and it's necessary to make a systematic and considerate analysis on it.Supported by the project of transit construction technology R&D program from Fujian Provincial Communications Department" Key Technology Research on the Construction of Large Section Highway Tunnel with Three Lane and Small Spacing", this paper focus on the study of construction mechanics of tunnels with the method of theoretical analysis, numerical simulation and field monitoring. The main researches are as follows:1. According to the code for design of road tunnel and Protodrakonov rock pressure, theory, an analysis method of rock pressure and loads mode for highway tunnels with large section and small spacing has been put forward in this paper, which takes the interaction of two tunnels construction successively into account. And then, the calculation methods are established for shallow and deep tunnel, respectively. Result shows that the rock pressure at inter-space rock side is larger than that at lateral side of tunnel, and the unsymmetrical degree of rock pressure distribution on the tunnel constructed firstly is more serious than that of tunnel constructed secondly. Based on the calculation methods, the design of tunnel structure will be more safety and reasonable, which can be of reference for the code for design of road tunnel.2. With numerical simulation and field monitoring, the influence degree and range of tunnel construction on the deformation and stability of rock mass has been concluded, and the proposal of construction method in different geologic and topographic conditions is also presented. The main influence range is about 0.5～0.75B(B stands for span of tunnel) from working face in front, and about 1.0～2.0B from working face behind. It's appropriate and feasible to keep the distance between working faces in two tunnels about 1.0B～1.5B. The double-side pilot tunnel method is preferential In Grade V surrounding rock, and center diagram method for Grade IV surrounding rock, and bench method for Grade III surrounding rock. For tunnels with unsymmetrical rock pressure, which mainly caused by topographic conditions, it's favorable to excavate shallow tunnel firstly.3. Following the research findings of two lane tunnels with small spacing, a criterion of reasonable clearance for tunnels with large section and small spacing was advanced. That is, with reasonable construction method and conventional preliminary support only, the yield region ofη= 0.7 (degree of approaching yield) at inter-space rock shouldn't be conjunctive, and the construction of latter tunnel shouldn't cause excessive influence on the deformation and inner force of former tunnel. With numerical analysis and field monitoring, the interaction principle of tunnels with various clearances was analyzed, and the reasonable clearance value of tunnels in different surrounding rock was put forward. They are: D≥0.6B in Grade V surrounding rock, D≥0AB for Grade IV rock, D≥0.3B for Grade III rock, D≥0.25B for Grade II rock, and the minimal clearance of tunnels shouldn't be less than 4.0m in any grade surrounding rock.4. Based on the researches of reasonable tunnel clearance value, the reinforcement principle and spacing range of inter-space rock were presented. Comparing with two-lane small-space tunnels, the reinforcement characteristic of large section tunnel with three lane and small spacing was analyzed, and the proposal of reinforcement area and construction measures was put forward. They are: if tunnel spacing is larger than minimal reasonable clearance value, there is no special need to reinforce the inter-space rock except routine systemic support, and just taking some appropriate construction method is enough. While if tunnel spacing is less than minimal reasonable clearance value, it's necessary to take relevant reinforcement measures, and make some adjustment according to field monitoring datum. In the weak surrounding rock, the upper part of inter-space rock (Yanxing Part) is one of the worst places of stress condition, where the reinforcement should be put emphasis on. In the hard surrounding rock, it's enough to generally reinforce the middle part of inter-space rock only, and there is no need to make reinforcement at the upper part of the inter-space rock.