Study On Construction Mechanical Effects Of Double-arch City Tunnel Under Complex Conditions

The construction of city tunnel and the rational use of underground space are important method to solve the growing problem of urban traffic congestion, slowness and other issues. Due to the special location, city tunnels are often subject to the planning of land use, existing buildings and other factors, which leads to difficulty to construct. Under such situation, double-arch tunnel would be an ideal idea. Double-arch tunnel have many advantages, such as a convenient choice of entrance and exit, avoid subdivision of bridge and tunnel line, a smooth line, saving urban lands and so on. However, the span, depth, process and force of double-arch tunnel are very complicated. Therefore there are many unresolved problems in construction and design. Putting efforts on the safety, efficiency and reasonability of building double-arch city tunnel is necessary.The purpose of this paper is to do research on the key issues about the construction methods, design theories, numerical simulation, field measurements and other aspects of tunnel construction by studying a shallow double-arch mountain tunnel in one city, and as same time combining some related research at home and abroad. The conclusion has been drawn as follows:(1) City double-arch tunnels are often have some factors, such as buried deeply, large span, high water levels, complex force structure and difficult construction. It also need an overall consideration of geological conditions, environment, construction technology, duration, and economic factors. Making a correct choice of all those factors is very important to the construction.(2) Using of load-structure model to calculate surrounding rock pressure is a good way to see the height of collapse-arch, which is often between the whole span and half span. Protodyakonov’s theory can be used to calculate this height, while the additional load is distributed on the whole span.(3) Under circumstances of shallow-buried tunnel, using The three pilot drift method is more effective to control the subsidence and the stress of the mid-board than using the middle pilot drift method. If excavated from left half to right half cavern, the left part would be affected more greatly.(4) In the process of double-arch tunnel excavation, the mid-partition will be bent in all directions. Backfilling both sides of the mid-partition can significantly reduce the maximum force and the eccentric compression. However, the soil of backfilling has its own weight, it would be affected the soil settlement.(5) Excavation of each pilot tunnel will have impact on the arch crown settlement, the convergence, and the ground surface settlement of existing tunnel.(6) The data obtained by numerical simulation is lower than obtained by filed measurement, but various mechanical effects about double-arch tunnel excavation can also be reflected, which would be an effective way to study mechanical effects of double-arch tunnel. (7) Introducing information technology to the process of construction would be a good idea to get knowledge of the force of surrounding rock and support. It can verify and adjust support parameters, and take measures immediately when an exception occurs. Meanwhile, it can control the impact of construction on the surrounding environment, which ensure the safety, reliability and economy.