Inversion Analysis of Initial Geo-Stress in Tunnel of Daping Mountain
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Inversion Analysis of Initial Geo-stress in Tunnel of DaPing Mountain Zhemin You PhD candidate, Faculty of Engineering, China university of Geosciences; No.388, Lumo Road, Wuhan, 430074, [email protected] Jianping Chen Professor, Faculty of Engineering, China university of Geosciences; No.388, Lumo Road, Wuhan 430074, China. [email protected] Yongsong Li PhD candidate, Rock Foundation Division, Yangtze River Scientific Research Institute, Hubei Wuhan 430010, China, song90397@sohu. com Ying Xu PhD candidate, Faculty of Engineering, China university of Geosciences; No.388, Lumo Road, Wuhan 430074; [email protected] ABSTRACT Based on the geological conditions and measured data of in-situ stress in DaPing mountain tunnel, the initial geo-stress field of tunnel area is analyzed by 3D multivariate FE regression. A 3D FE computational model is established to compute geo-stress in four cases, including gravity, tectonic stress field for horizontal extrusion and shear. And the regression coefficients are solved using the least square method , and then the calculation value of the measured points , eventually the regression stress field of the whole area especially important engineering parts .Through comparison between the calculated and measured geo-stresses of measuring points, it is found that these two are similar, which suggests that the geo-stress field obtained by regression is reasonable and conforms to the historical background of geological structure. The results of the geo-stress field have important reference value of the dynamic design and construction of projects. KEYWORDS: DaPing Mountain; deep buried tunnel; initial geo-stress field; FEM; regression analysis INTRODUCTION Guzhu (Gucheng-Zhuxi) expressway in Hubei province of China is an important section of the skeleton road network planned which includes six longitudinal and five horizontal and one - 2255 - Vol. 17 [2012], Bund. P 2256 toroidal part, connected Fuzhou-Yinchuan highway with Pingli-Ankang highway in Shanxi province. Once completed, it is of great strategic significance on the implementing of the rise of the central and western regions, accelerating social and economic development along the regions. DaPing mountain tunnel is one of the separate tunnels with great depth and super-length in Guzhu highway(the length 8252m*2, maximum depth 900m), which lies in the Zijin and Siping town of Xiangyang city, with the hole axis 218°. The tunneling lie in area with low mountains and terrain large ups and downs, effected by structure eroding. Along the route the strata exposed are as follows: shale(S1x), biological clastic limestone mixed up with a few carbon shale and powder sand shale(O1n-O3s1l), muddy limestone, carbonaceous limestone with shale partly(Є1s-sl), dolostone and limestone(Z2-Є1dn). Qingfeng fault, stretching EW nearly, passed through the tunnel axis with small angle, formed more than 10 secondary faults, caused surrounding rock broken, local groundwater rich, overall stability poor. Because of the geological activity and great depth, the tunnels may pass through high geo-stress area, and come across a series of deep engineering geology and “weak rock” mechanical problems inevitably. The distribution of regional initial stress field has great importance on the stability of surrounding rock. Therefore, it is necessary to calculate the initial stress field by limited measured data, provide the good premise for the safety of the tunnel construction. The initial stress field of rock mass is the primary factors which need to be considered in geotechnical engineering design and construction, and the important research subject in rock mechanics and engineering. However, it is always a problem of reflecting the initial stress field accurately in underground engineering1~2, whether the initial stress field reliable, parameter of rock mass selected reasonable, they will influence the reliability and safety of the design and construction directly. Complex stress cause and influence factors, less measured data due to measurement condition and the limited funds, measurement error, all these make the results a certain degree of discrete. Therefore, it is necessary to analysis the initial stress field through the proper calculation method, in order to obtain stress field more accurately, widely applicable scope3. At present there are two kinds of inversion and regression analyzing methods of stress field widely-used: the one is displacement back analysis. That is, inverse the regional stress field according to the tectonic deformation observation or the displacement monitoring during engineering construction and excavation. It is an indirect method, which is suitable for the situation as lacking measured data or even exists but is disturbed stress, and often used in local scope of stress field of rock mass inversion. For some large projects, generally the second method are used, the stress regression analysis method, that is, geo-stress field is obtained through fitting the 3D numerical simulation and measured stress data4. Take DaPing mountain tunnel of Guzhu highway as an example, the stress regression analysis is used, and a 3D geological model is set along the area, calculate based on the measured data, and gain the initial stress field about the whole area. Vol. 17 [2012], Bund. P 2257 MATERIAL AND METHODS Theory of regression analysis Generally, the shallow rock stress field is mainly composed of horizontal tectonic stress field and gravity field related to terrain5.This paper will set up according to the view of the stress field mathematical model, using multiple regression method fitting to the analysis. Based on the above statement, mathematics calculation model is established. According to the theory of the multiple regression method6, geo-stress calculation value is the dependent variable, and its Components the independent variable, such as gravity stress and tectonic stress field, calculate based on the limited measured data. And in accordance with the least square method the equation is solved with the sum of squared residuals minimum value, and the regression coefficients corresponding, making the calculation stress and the measured stress to achieve optimal fitting, then the initial stress field of engineering area7. σ σ σ σ σ ++++Δ= aaaa σ 44332211 (1) σ Δσ σ σ σ where is The calculation value of the initial stress field, is random error, 1 、 2 、σ 3 、 4 is the initial stress components respectively such as gravity, the extrusion structure along X, Y, XY plane, a1、a2、a3、a4is the Corresponding regression coefficient. Geo-stress regression analysis in DaPing mountain tunnel The geological model is the basis of the calculation model, in-depth understanding of the engineering geological conditions and the reasonable generalization is an important prerequisite for the establishment of geological generalization model8. Comprehensive considering the factors of the landform features and rock mechanics properties, take the tunnel axis calculation model Gucheng-Zhuxi as the x direction (218°), the GuCheng import as a starting point. Coordinate z- axis vertical up, the y-axis accord with the right hand screw rule. The total range of the Calculation model is 8375m×3000m, with the bottom elevation -1000m (Fig 1). It includes the tunnel site primarily, with drilling point near the middle of the field (Fig 2). The computational domain is divided into 340 277units, 63 079nodes. On the basis of the flexibility assumption of the geo-stress field, linear elastic secant modulus is used to compute. Vol. 17 [2012], Bund. P 2258 Figure 1: Computation model Figure 2: Calculation zone and drill hole position The axis geological profile of DaPing mountain tunnel is shown in Figure 3, and the physical and mechanical parameters of rock mass in Table 1. 218° Zhuxi Gucheng F9 F10 F8 F7 F26 F4+F5 F11 O1n-O 3S1l s-sl F3 1 Z2 1 dn F6 Z 2 dn Z dn bioclastic 2 1 limestone dolostone O1n-O 3S1l limestone dolostone bioclastic dolostone 1s-sl S1x shale limestone limestone s-sl 1 limestone limestone limestone limestone tunnel axis Figure 3: Geological section of tunnel axis of DaPing Mountain Vol. 17 [2012], Bund. P 2259 Table 1: Mechanical parameters of rock masses Number Lithology Level E/GPa μ ρ/g·cm-3 ① Dolostone limestone Ⅴ 2.0 0.35 2.0 ② Dolostone limestone Ⅳ 5.0 0.31 2.3 ③ Dolostone limestone Ⅲ 18.0 0.26 2.4 ④ shale Ⅳ 3.0 0.32 2.2 ⑤ shale Ⅴ 1.2 0.35 2.0 ⑥ F7、F 8、F 9 Ⅳ 1.5 0.32 2.2 ⑦ F3-6、10-11、26 Ⅴ 0.8 0.36 2.0 Stress regression model imposes the boundary conditions of displacement in the gravity field and the three boundary horizontal tectonic stress field respectively. The density measured of rock mass is used to calculate the gravity stress field under its weight, displacement constraints on the side and bottom of the model; all is limited only to the normal direction of displacement. Apply uniformly normal displacement with different direction in the two sides of the model, to simulate the horizontal direction of tectonic stress field, the same constraints of lateral and bottom boundary constraint conditions as gravity stress field simulation. Shear stress in the horizontal plane, is simulated via imposing boundary displacements9. The hydraulic fracturing method is used to test in situ stress in two drillings, which located nearby the middle of the tunnel site. Results are shown beelow in table Ⅱ. To identify the stress distribution of the area, geo-stress value of 15 points from two sets drilling are obtained using hydraulic fracturing10 (Fig 4). (a)ZK10 (b)ZK11 Figure