Analysis Of Characteristics Of In Situ Stress And Surrounding Rock Stability In Deeply Buried Tunnel With Large Faults

As one of the important factors of surrounding rock stability of tunnel, in situ stress is affected by geological structures, especially faults, with which the gradients and directions of principle stresses nearby will change because of local stress concentration. Limited by funding, swallow depth of hole and representation, characteristics of the regional in situ stress field in deeply buried tunnel have to be obtained by other means. Predicting the stress of surrounding rock during excavation according to the in situ stress, and then the peripheral rock pressure, can play a useful role in guiding the choice of supporting measures. More accurate rock classification during construction phase is the best evaluating means of the surrounding rock stability, and the correction effects of in situ stress become essential and crucial. Take the highway tunnel of Daping mountain with great length and depth for example, characterizes of in situ stress are analyzed, and then the relationship with peripheral rock pressure, and rock mass sub-classification and the surrounding rock stability evaluation is implemented during tunnel construction with considering the in situ stress correction effects. The main contents and conclusions are as follows:(1)The regional geological structural features of the tunnel site show that the Qingfeng fault, a brittle-ductile thrust nappe shear zone, composed by dozens of thrust faults and brittle-ductile shear zone and thrust nappe structure, has experienced four formation and evolution phases, including crust on bending, protruding structure, nappe and stretching structure, with overall NS trending. Combined with geological survey along Qingfeng fault, a large number of local geological structure features, including "X" conjugate shear joints, multiple faults and folds, exposed along the tunnel site.(2)In situ stress is mainly influenced by the geological structure, topography, surface erosion, lithology, groundwater, temperature, etc. It grows nearly lineally with the increased depth, while it is disturbed by faults evidently, which affect the stress distribution and transfer. Within the same geological units, the magnitudes and directions of in situ stress are consistent during each large cutting faults or other structural surfaces. Near the fractures or other separation surfaces, especially in the corners, at the bifurcations and both ends, where are stress concentration areas, the magnitudes and directions change greatly. Faraway from the faults, the principle directions of the in situ stresses are becoming more and more consistent with regional principal stress directions.(3)Thorough quantitative statistics of characteristics of multiple sets of conjugate shear joints, faults and folds, geomechanical analysis methods are used to analyze the regional ancient tectonic stress field. Combined with stereographic projection software Dips, the plane direction of conjugate shear joint compression zone represents the direction of maximum principal stress, which is NE trending (31°). According to the relationship between fault properties and directions of principle stresses, the directions of the axial planes of the folds, the direction of maximum principal stress in this area is NE-NW trending. The results show that, the directions of regional tectonic stress field are consistent by the three geotechnical analysis means, which can accurately reflect the features of the regional pale tectonic stress field.(4)The hydraulic fracturing method is used in two drillings for in situ stress measurement in engineering district. The distances between ZK10, ZK11and faults, e.g. F8, F10and F11, are nearer whether from the surface to the underground or near the axis, and in situ stresses chang obviously and turn to be local stress concentration, which cannot entirely represent the characteristics of the regional stress. The test results show that horizontal principal stresses and gravitational stresses increase lineally with increasing depth rapidly, and the lateral pressure coefficients of drillings are greater than1within the test depth range, indicating that the tectonic stress is integral part of the stress field in the project area. The orientation of σH is N14°～19°E, showing that in situ stress is mainly formed by squeezing in the NE direction, which is consistent overall in combination with regional geological tectonic evolution, analysis of geological structure characteristics, geomechanical analysis.(5)A3D FE computational model is established to compute in situ stress in four cases, including gravity, tectonic stress field for horizontal extrusion with X、Y direction and shear structure with XY plane. And the regression coefficients are solved using the least square method, and then the calculation values of the measured points with unreasonable point eliminated. The principal stress isolines of tunnel axis are plotted using SUFER software, eventually the regression stress field of the whole area especially important engineering parts. Through comparison between the calculated and measured values of the drilling points, it is found that these two are similar, which suggest that the in situ stress field obtained by regression is reasonable and conforms to the historical background of geological structure. The conclusions are drawn as follows:The values of principle stresses are growing approximately lineally with depth, and the distribution of stress is closely related in topography, which has great influence on stress in the shallow. As the buried depth increases, its influence is relatively minor. When terrain changes greatly, the contour of the stress is relatively close, and the change gradient is great. In addition, the influence of the geological structure is also great, in the fault and its influence in the belt, stress isoline appears abrupt and the contour density increases. In general, tectonic stress is integral part of the stress field in the project area.(6)Qualitative analysis, and quantitative discriminate are used comprehensively to identify the in situ stress state of the design elevation of Daping mountain tunnel, with the results that overall are low to moderate stress levels, locally are high to very high stress levels. The large deformation of shale has close relationship with its high stress state, which verifies the accuracy of regression analysis of the in situ stress field. To the problems induced by high in situ stress follow-up, there is the need to strengthen the security monitoring and protecting, strengthening the temporary support and advanced geological forecast of the surrounding rock.(7)Take the partly monitoring sections of rock classification with IV and below level of Daping mountain tunnel for example, the excavation stress release rate model is used to analyze the residual stress values of the tunnel wall. Vault sink and horizontal convergence displacement values are fitted by exponential fitting method, which has proved to be a feasible and relatively accurate method after error analysis. It is used in five monitoring sections, and displacements including missing displacement before buried and ahead the tunnel face are obtained. According to the horizontal and vertical in situ stress component values, the structural loads are calculated in different release rate conditions. The results showed that:K, the integrated stress residual rate, is greater significantly when the value of Kl0is0.25than when that is0.35.(8) Based on in situ stress and radial displacement (the stable value of displacement fitted using a cubic polynomial), the peripheral rock pressure is obtained with modified Fenner formula. The theoretical values of horizontal and vertical peripheral rock pressure is obtained respectively.(9)The peripheral rock pressure has been tested using embedding pressure box, and was transferred to the total values according to load ratio conversion, and the vertical and horizontal values. The measurements of peripheral rock pressure are compared with the theoretical calculation results, and the results show that, because of the values of pressure are influenced and constrained by many factors, while the calculation theory methods consider only limited factors, all these lead that there are several differences between calculated and measured values. Therefore, the obtaining of more precise values of the peripheral rock pressure should not only rely on measurements or theoretical results, but also combine a variety of methods, and to compare and verify the accuracy of the values comprehensively.(10)The values of the coefficient of lateral pressure of in situ stress affect the displacements, the distribution characteristics of stress and the failure modes of surrounding rock of tunnel. With λ increasing, the vertical displacements of the arch and bottom of the tunnel decreases, whereas the horizontal displacement increases, and its impact increases evidently with the depth increasing. The principle stresses of surrounding rock increas with λ increasing, and the maximum principal stresses of the arch and bottom of the tunnel are in compressive states, and its magnitudes decrease with increasing lateral pressure and the depth. When the value of λ is small, cracks appear to be mainly in the vertical direction, while they turn to be in a horizontal direction mainly with λ increasing.According to the relationships among the values of the three principal stresses, the in situ stress is divided into three types, namelyσH、σHV、σV. For a different type of in situ stress, the angle between direction of maximum principal stress and the hole axis is different, with varying degrees of surrounding rock stability of tunnel, which are divided into three states, that is favorable, general and unfavorable.(11)Combined with category of surrounding rock stability with quantitative indicators values of the state of in situ stress, the correction coefficients of in situ stress are adjusted for the surrounding rock sub-classification, and it is used in tunnel site. The results show that the rock classification of the construction deviated with the designed classification in some degree, mostly are higher or lower half degree. However, the difference of classification in local surrounding rock of shale is nearly two levels, which are closely related with the high to very high in situ stress state.(12)Analyzing the deformation and failure characteristics of rock sections with great difference of surrounding rock classification along the Daping mountain tunnel, and combined with geological survey and construction methods, the stability of the surrounding rock is evaluated integratedly and some conclusions are drawn as follows. The surrounding rock in the imported limestone and dolomite segments is relatively stable, while the export shale segments are affected severely, with large deformation, collapse and local rock breakout in some sections. All these have a great relationship with high in situ stress state. It is proposed that the excavation segment, where may appear deformed or destruction phenomenon, should be strengthened monitoring and advanced geological forecast at the follow-up work.