Study On Vertical Earth Pressure Characteristic And Calculation Mehtod Of Unloading Structure For High Fill Loess Open Cut Tunnel

The loess plateau region has become increasingly accelerated urbanization, the urban land tension becomes the main bottlenecks that restrict the course of urbanization. Considering the high mountains and deep trench in loess plateau, cutting and filling has become an important way to satisfy the need of the urbanization development land. The high back fill is inevitably for the open cut tunnel of existing and new highway(railway).At present, it is mainly changed the design of open cut tunnel for bearing the load of high back fill in existing and new open cut tunnel, through using the unloading measures to make the structure performance meet the requirements are very rarely. Lack of study on unloading structure mechanical analysis and application measures of high fill loess open cut tunnel.Therefore in this paper, the vertical earth pressure(hereinafter referred to as earth pressure)characteristics on the top of high fill open cut tunnel and unloading structure are studied through the numerical simulation and the indoor model test, it is verified in the Lanzhou-Chongqing Railway Chang Shou-shan tunnel,The earth pressure change rule on the top of the open cut tunnel and unloading structure is revealed which is a nonlinear variation with the increase of filling height,The unloading mode of high fill loess open cut tunnel is proposed which is the EPS combined with geogrid mode. Finally, based on the basic principle of rock and soil mechanics,the mechanical calculation model of high fill loess open cut tunnel and unloading structure are established,the earth pressure calculation method of high fill loess open cut tunnel and unloading structure are further deduced which is consider the influence of slope. the results were compared with field test and numerical simulation, the correctness of the calculation method and the effectiveness of the unloading measures are verified. The main research contents and innovation are as follows:(1) Through the experimental study on the mechanical properties of the loess filling element and the EPS, it can get loess compression modulus and shear strength index in different compaction coefficient, and also get the change rule of compression feature with different density and thickness for EPS, a basis for parameter selection of numerical simulation, indoor and outdoor test can be provided.(2) The plane strain numerical model of high fill loess open cut tunnel and unloading structure are established, the earth pressure on the top of the open cut tunnel and the soil settlement in different filling height are analyzed, the influence of earth pressure on top of high fill open cut tunnel are further discussed,such as filling characteristic,slope characteristic, section characteristic and unloading material characteristic with EPS and geogrid.The results show that the earth pressure concentration factor on the top of open cut tunnel is greater than one because of the differential settlement of inside and outside soil, the slope angle, slope friction factor, section form of open cut tunnel, groove width ratio of open cut tunnel and the filling constrained modulus on both sides are the most significant effect. The laid of EPS and geogrid reduced the earth pressure on top of open cut tunnel effectively. The thickness and density of EPS for unloading effect is most affected, The choice of the geogrid stiffness and layers depend on the EPS which can provide the size of deformation.(3) For the earth pressure change rule of high fill loess open cut tunnel and unloading structure, seven kinds of test working condition is established under the combination of three factors such as the slope, geogrid and deformation layer form, and compared with the results of numerical calculation. The results show that the earth pressure on the top of open cut tunnel is not changed linearly as the filling height. the differential settlement of inside and outside soil column is changed through setting deformation layer, combined with geogrid can effectively reduce the earth pressure on the top of the open cut tunnel. Reasonable geogrid layers and the setting mode of deformation layer are more advantageous to increase the unloading rate.(4) In order to further illustrate the applicability of the earth pressure change rule for high fill loess open cut tunnel and unloading structure in actual engineering. Relying on the back filling practical engineering, two kinds of working condition are used, the change rule of earth pressure on top of the open cut tunnel as the filling height are studied, and compared with the results of numerical calculation. The results show that the earth pressure on the top of open cut tunnel is a nonlinear variation with the increase of filling height, and the effectiveness of unloading measure which combine with setting deformation layer and geogrid, at the same time, the research of numerical simulation method for earth pressure on top of high fill open cut tunnel is feasible.(5) Based on the earth pressures characteristics of high fill loess open cut tunnel, using the basis of rock and soil mechanics theory, the mechanical calculation model of high fill loess open cut tunnel and unloading structure are established. the earth pressure computational formula of high fill open cut tunnel and unloading structure are deduced which is consider the influence of slope. and compared with the results of numerical simulation and outdoor test. The correctness of the calculation method is verified, it can provide theoretical and technical reference for design and construction on high fill loess open cut tunnel and unloading structure.