1、1Numerical simulation method for Surrounding rock stability analysis of surge chamber under seismic cAbstract: Underground engineering has broad prospects for development, but its surrounding rock stability is related to the project construction security as well as operation security. In this paper,
2、 quasi-static method is adopted, and based on the surge chamber at the Liuping Hydropower Station, the overall three-dimensional model is established to analyses the surrounding rock stability of Surge chamber in complex geological areas under seismic conditions. The compute results show that, the o
3、verall stability of the surge chamber surrounding rock and the lining structure is good, but the safety factor near the left side wall and right spandrel is small. So it is necessary to take the key reinforcement measure and pay the key attention in the construction. Key words: Numerical simulation;
4、 Surge chamber; Stability; Surrounding rock; Seismic 1 Introduction The Osaka Kobe earthquake of Japanese in 1995 and the 1999 2chi-chi earthquake and so on had caused heavy damage to part underground engineering. In 71 tunnel projects with rock grounds that are affected by the earthquake fluctuatio
5、n in places such as American California, Alaska, Japan and so on, there are cracking until collapsing seal happened in 42 examples. Its shown that underground structure earthquake stability is directly related to underground chamber deformation and failure evolution under seismic load 1. Chinas wate
6、r resources are mainly concentrated in the southwest, and a lot of Hydropower Station are constructed. There are all located in mountain gorge areas with strong new tectonic movement, extremely unstable geological environment, in performance of the strong seismic activity and frequent landslides, av
7、alanches, and other geological disasters. Diversion development method is adopted in many large-scale Hydropower projects, and a large number of surge chamber have been built. Surrounding rock stability of surge chamber in complex geological areas under seismic conditions has become one of the key i
8、ssues and key technologies in hydropower project construction. In this paper, numerical simulation method is adopted to study the dynamic properties and response characteristics of 3surge chamber at the Liuping Hydropower Station. The compute results are valuable for underground engineering design o
9、r construction, and recommendations provide a basis and reference for similar projects. 2 Numerical simulation method Seismic analysis method. Static method in the seismic design and seismic stability checking in, the structure of the gravity, the design earthquake acceleration and the ratio of grav
10、itational acceleration, coefficient of dynamic distribution of a given product as the design seismic force among the static analysis. In determining the earthquake will be followed as a static load imposed on building structure, and static loading, the load will increase as the construction of stati
11、c structures, and static loads for structural analysis as in the case. Static method used when acting on the particle along the building height of horizontal earthquake inertia force Fi can be calculated as the representative, where, is the design level of intensity corresponding to the representati
12、ve value; is the earthquake response reduction factor; is the representative value of gravity; is the dynamic distribution coefficient of point i; g is the acceleration of gravity. 4Constitutive model for rock mass. The tensile strength Mohr-Coulomb constitutive model is adopted for rock masses to c
13、arry out the numerical simulation, the shear yield criterion and the tensile yield criteria are as follow, where, and are the first and the third principal stress; c is the cohesion; is a function of friction angle ( ); is the tensile strength of rock mass. 3 Numerical simulation model and parameter
14、s Engineering geology. The Liuping Hydropower Station is developed by diversion, a diversion tunnel is arranged in upstream, surge chamber at its end, and three pressure pipes are arranged surge chamber downstream. According to the results of geological survey and exploration data, upper part and do
15、me of surge chamber locate in rock of weak weathered and weak unloading; the stability of surrounding rock is poor. Three-dimensional simulation model. According to geological data, establish three-dimensional numerical model of Liu Ping hydropower Station surge chamber region. In order to ensure th
16、e accuracy of numerical calculation, the compute element of surge chamber is used hexahedral element. Fig. 1 shows the overall three-dimensional compute mesh, Fig. 2 shows the mesh of surge chamber. 5In the numerical simulation process, two section are selected to analyzed, their coordinates are x =
17、 130 (section I-I) and y = 204 (section II-II), receptively. Rock masses mechanics parameters. Mechanics parameters of rock masses and concrete are referring to the criterion suggestion and design value, as shown in Table 1. Support form of Surge chamber is: hanging mesh anchor bolt-spray support, h
18、anging mesh steel bar 12, inter-row spacing of steel bar 20 20cm; anchor bolt L = 4.5 (28) and L = 6.5 m cross-arranged, inter-row spacing is 1.5 m, sprayed C20 concrete thickness of 12 cm. The support method for cracks, faults and fracture zones and other special parts of poor geological conditions
19、 are: timely hanging mesh anchor bolt-spray support, while to reduce inter-row spacing of anchor bole from 1.51.5m to 1.01.0m. Seismic loading condition. The stability analysis for surge chamber under seismic loading is mainly concerned about stress, deformation and safety factor after surge chamber
20、 excavated and the support method is carried out. The seismic intensity for Liuping Hydropower Station surge chamber is VIII, seismic peak acceleration is 0.25g. 4 Numerical simulation results 6Displacement analysis. Fig. 3 shows the displacement vector at section II-II under seismic loading conditi
21、on. Fig. 4 shows the displacement distribution of surrounding rock mass at the section II-II. Figure4. Displacement contours of the section II-II: (a) horizontaldisplacement and (b) vertical displacement (unit: mm) The compute results show that, both sides of the surge chamber performed to be horizo
22、ntal deformation to the inside. Maximum horizontal displacement of the left wall is about 60 mm, maximum horizontal displacement of the right wall is about 50 mm, and both sides of the wall displacement are not completely symmetrical; maximum vertical displacement can reach 50 mm, appears in the sur
23、ge chamber bottom, mainly for bottom rebound deformation. The vertical displacement of surge chamber crown is not large; the maximum value is about 20mm, which is mainly for the main chamber crown structure design conducive to the chamber stability. Stress analysis. Fig. 5 shows the stress calculati
24、on result at the section I-I under seismic loading condition. The compute results show that, there is only compressive stress appeared with the surge chamber surrounding rock and the maximum principal stress of not more than 10MPa. The minimum 7principal stress are likely to be local tensile stress
25、appeared at the two sides of surge chamber wall, but not great, the strained condition of surge chamber in a relatively secure state. The maximum stress and safety factor are analyses for the various parts of surge chamber, and stability of surge chamber under seismic conditions is evaluated. Table
26、2 shows the maximum first principal stress and safety factor at different parts of surge chamber. The compute results show that, the first principal stress range between 1.7-8.4 MPa, The safety factor of all parts are greater than 1.0 witch meet the seismic requirements, and the maximum of the crown
27、 and the bottom are between 1.8 to 2.0, both greater than that of the left and right side. Axial displacement discipline of surge chamber. The displacement discipline of surge chamber at the Liuping Hydropower Station under seismic load is analyses, consideration on the displacement distribution cha
28、nge of the crown, the lower with the horizontal coordinates, and the left and right side, at the vertical section (section I-I) of the surge chamber. The maximum displacement absolute value of the left side is distributed in X direction, Y direction, that 8is, the maximum of the left wall distribute
29、d in the horizontal direction. The maximum displacement absolute value of the bottom is distributed in Z direction, behaved as a rebound deformation. Fig. 6 shows the vertical displacement distribution discipline with the coordinates at the bottom of surge chamber under seismic conditions. Fig. 7 sh
30、ows the horizontal displacements distribution discipline at the right side of surge chamber. The compute results show that, maximum displacement change mainly focus in the left side and bottom of surge chamber; the maximum horizontal displacement is about 45mm, larger than that of the right side (22
31、mm); the maximum displacement of the bottom is about 43mm, larger than that of the crown (30mm).The main reasons are: left side is closer to the valley and the rock mechanics parameters is decreased under seismic loading conditions. Meanwhile, the direct impact on the bottom of vertical seismic wave
32、s from deep underground make the displacement deformation of the bottom much greater than that of the crown. For the crown and bottom, the displacement absolute values is Z Y X direction; for the side surrounding rock, the 9displacement absolute values is YX Z direction. The vertical displacement of
33、 the surge chamber bottom appeared to be rebound deformation, and the displacement of the middle much larger than that of both sides. Stability analysis. Fig. 8 shows the stability compute results of the surrounding rock of surge chamber (section II-II) under seismic conditions. The compute result s
34、hows that, the minimum safety factor of surrounding rock can be maintained above 1.0 under seismic loading conditions, surrounding rock is stability. Fig.9 shows the safety factor compute results at the left and right side (series 1 is the left side; series 2 is the right side). The compute results
35、show that, safety factor of crown changed greatly with the coordinates change, while that of bottom changed little, the safety factor of that near the corner of wall relatively large, close to 1.5. Safety factor of the left side and crown are all just over 1.0, it is necessary to take the key reinfo
36、rcement measure and pay more attention in the construction process. 5 Conclusions (1) Both side walls displacement of surge chamber is not 10completely symmetrical under seismic loading; vertical displacement of crown is not very large, maximum vertical displacement occurs in the surge chamber botto
37、m, which is mainly the bottom bounce deformation; (2) Surrounding rock of surge chamber is only compressive stress and the maximum principal stress is not exceeding 10MPa. (3) Left side wall displacement absolute value in the horizontal direction is maximum, mainly because the left side near the val
38、ley and there is overhead free surface in the inner side (right side); bottom absolute value in the z direction is maximum displacement of Z direction, and its middle part is much bigger than both sides, mainly because inside bottom (Z direction) has overhead free surface and longitudinal seismic wa
39、ves rebound. (4) Displacement absolute value of crow and bottom in the X, Y, Z directions is Z Y X; side wall is Y X Z. (5) If minimum safety factor value of surrounding rock and lining structure of surge chamber maintains at above 1.0, it indicates that surrounding rock and lining structure stability of surge chamber is good, but the safety factor near the left side and crown is small. So it is necessary to take the key reinforcement measure and pay more attention in the
