| With China's sustained and rapid development of the national economy, highways,high-speed rail and other transportation construction projects have been constructed. China is a mountainous country, in order to shorten traffic mileage,mountain highway construction requires the construction of a large number of tunnels,the construction of highway tunnels will be more and more. If the topography and geological conditions is good, the tunnel is usually designed for the two separation independent tunnel. However, in some special areas and complex terrain conditions, due to various negative factors,the double-arch tunnel should be considered.The application of double-arch tunnel solves the problems that separate tunnels cause, but its excavation section span significantly increased, excavation and supporting are interactional, so the rock stress become very complicated.The force and deformation conditions of different departments of support system, uncertainty surrounding rock pressure distribution, and the mutual influence between tunnel and middle wall , as well as the complex structure of its design and construction to bring tunnel desigen and constrction greater difficulty . Rock mass displacement can visiblely response the stability of the rock mass,through rock deformation monitoring can determine the stability of rock, as well as determine the best rock tunnel Supporting time. Therefore, the displacement monitoring and prediction of rock tunnel are very important.Aapplicatiom Introduction of the new Austrian law to build the tunnel, the adjustment process and deformation monitoring of the rock stress essential. In fact, the determination of excavation methods, the layout of bolt anchorage, supporting parameters of secondary Support depend on the construction monitoring. And the deformation of rock tunnel is the most visible manifestation of the NATM tunnel construction soul. In the NATM construction process, monitoring rock displacement of poor quality of the surrounding rock to make a reasonable analysis and evaluation, through the displacement monitorintg curve to decide the support time. ground settlement monitoring points are arranged in the K123 +284 section of DaChen tunnel. at the same time crown settlement and convergence were emplaced at K123 +270 and K123 +330 section. Tunnel-site monitoring data show that the timing of support is reasonable. Because of the complexity of geotechnical engineering, it involves that engineering geological conditions and parameters of rock mass is not entirely quantitative, it is difficult to determine the mathematical model described. A simple neural network model can reflect on a very complex mapping relations. BP algorithm used in this paper, based on monitoring data,this paper has established a neural network model, the model structure is 7-16-1. Comparing forecast results with the actual monitoring data, the prediction result is suitable for the engineering requirements.Based on the above-mentioned characteristics of neural networks, it is very appropriate that neural network model is used to express the ironline relation between geotechnical engineering parameters of rock and rock mass displacement values. Accordingly, we propose a Genetic-Nerural Network methord of displacement back analysis to resolve the traditional mapping relationship .Becase of the complex of surrounding rock material (non-uniformity, anisotropy, constitutive nonlinear relation , time-dependent, etc.), the complexcity geological structure (jointed, fissures, weak dissection, fault, etc.), the tunnel excavation sequence ,excavation methods, and the distance between two adjacent holes on the rock tunnel face have great impact on the stress and deformation of surrounding rock mass, so reasonable and accurate simulation of the tunnel excavation process is necessary for the choice of a reasonable supporting time, so three-dimensional dynamic finite element numerical modeling analyses is unavoildable. In this paper, we make use of large-scale general-purpose finite element software to simulate the dynamic construction process of DaChen tunnel, drawing some conclutions on the shallow double-arched tunnel in unsymmetrical pressure as follow:the horizontal displacement of the side of the tunnel smaller depth is bigger than the other side. the crown settlement and bottom uplift values of the side of advaced excation is larger than the later side ;disturbance range of rock excavation team working face of the rock mass is 30 meter, the surrounding rock mass displacement has a fluctuating trend, through the displacement of rock mass to reach a new changes balance.In this paper, the main creativity are as follow : First, through the 3-D numerical simulation and Genetic-Nerural Network methord of displacement back analysis to receiverock mechanics index required for the numerical simulation. avoiding the human error and improving the accuracy of the numerical simulation.second,The DaChen tunnel is a shallow double-arched tunnel in unsymmetrical pressure,the construction process is very complex,the application of three-dimensional numerical simulation in such tunnel is not very mature,so this paper make use of the FEM software simulating the tnnnel excavation process,analysising and forecasting the stability of DaChen tunnel.Third,the authors has collected the control point of site topography,scaling the actual topography according to a certain ratio to build a numerical simulation geological model.It can accurately reflect the engineering geological characteristics of rock tunnel.The simulation results can guide excavation process and ensure the tunnel construction efficient, rapid and safe.
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