Research On The Deformation Mechanism And Dynamic Response Of Typical Landslides In Three Gorges Reservoir In Case Of Frequent Microseisms

The impoundment of Three Gorges Reservoir begins from2003, and now the water level has achieved to175m successfully. Compared with the original situation, the water level raised about100m in the head area of the reservoir (from the Three Gorges Dam site to Badong Town in Hubei Province), huge additional water pressure load is applied to the rock mass under the reservoir. Meanwhile, the water level will fluctuate cyclically from145m to175m each year, and changes the original hydrogeoiogy and engineering geology conditions greatly. The possibility of induced earthquakes also increased greatly.Professional researches and media reports showed that:the seismic activity trend increased significantly after the impoundment of reservoir, especially in the head area of the reservoir. Based on incomplete statistics:in total,541times of seismic occurred in2003, and compared with1996-2002, the frequency of seismic increased by3-13times; in2004, the frequency of seismic is1062, nearly one time more than that of in2003, the seismic intensity also increased; from2008to the present, frequency of seismic still maintains at a high level.As reservoir induced earthquake was of shallow seismic focus, decay rapidly and relative concentration of energy release, so, compared with the tectonic earthquake which has similar scale, it can cause a higher seismic intensity on the ground surface. Even for a M3induced earthquake, the seismic intensity can reach Ⅴ-Ⅵ. Although the intensity of induced earthquake much lower than the designed seismic performance of the Three Gorges Dam and the other important hydraulic structures, as the areas of high geological disasters (eg. landslide, collapse) in China, the frequency of seismic, combine with the cyclical fluctuation of water level, which also promotes the induced earthquake, whether this kind of dynamic-seepage multi-field coupling activities will cause an adverse effect on the landslides, especially for the landslides with loose structure? According to the research result in last several years, after impounding, signs of revival appeared on many landslides. Whether this phenomenon is closely related to cyclical fluctuation of water level and sudden increase of seismic, and how it works in this process? Until now, there's nearly no systematic research in this area.In this paper,"the deformation mechanism and dynamic response of typical landslides in Three Gorges Reservoir in case of frequent microseisms" was selected as the research issue. Landslides with loose structure which locate in the head area of the Three Gorges reservoir are summarized as two categories:tectonic origin and debris origin.Huangtupo riverside landslide and Qiaotou landslide, both of which still show the deformation trend are selected as the representatives respectively for the two types of loose structure landslide. Huangtupo riverside landslide is the key research subject, the research begin from statistical analysis of the actual seismic data in the head area of the Three Gorges reservoir(Chapter2), then along the clue of microcosmic (Chapter3:the water-rock (soil) interaction mechanism),mesoscopic (Chapter4to Chapter6:static and dynamic testing for unfavorable geologic elements and typical rock mass) and macroscopic (Chapter7:3D numerical simulation for landslide based on multi-field coupling theories), and the deformation mechanism and dynamic response feature of loose structure landslide with tectonic origin are finally got.Qiaotou landslide formed in multi-stage debris flow, it has a relative simpler geological formation and is treated as the sub-key research subject. According to the analysis of geology background, field investigation and statistical analysis of actual seismic data (Chapter2),3D numerical simulation for landslide based on multi-field coupling theories will be done (Chapter7) and get the deformation mechanism and dynamic response feature of loose structure landslide with loose debris origin.In addition, detailed analysis for the differences of deformation mechanism and dynamic response between two categories of loose structure landslides is carried out, and targeted prevention programs are proposed.Throughout the study, the author obtained the following conclusions and innovative achievement:1. The actual water level fluctuating data and the detailed data and description of induced seismic with the magnitude being greater than2.0in the latest3years are collected comprehensively. Using these actual data, the author analyzed and summarized the induced seismic most likely occur under the following conditions:(1) Shortly after the impoundment process has finished and reaching high water level, the additional load increases rapidly and induces the seismic (rapid loading type);(2) shortly after the sluicing process has finished and reaching low water level, the additional load decrease rapidly and induces the seismic(sudden unloading type);(3) the water level fluctuating extraordinarily during the flood season (loading-unloading transformation type). The induced seismic has the hysteresis characteristics, it often occurs shortly after the load transforming process has finished. Since now, the biggest seismic intensity caused by induced seismic between intensity Ⅳ～Ⅴ.2. Relying on the large-scale test tunnel which cross the whole Huangtupo landslide, the author made the follow-up geological record in the field for a long time, comprehensive collected10kinds of typical rock and soil specimens continuously, including slip soil, soft interlayer, typical rock mass and so on. Mineral chemical composition, microstructure and size distribution tests are carried out for each specimen. Then based on the theory of water and rock (soil) interaction and the multi-stage transformation of landslide activity, the homology characteristics of the formation, development and evolution of the unfavorable geologic elements are presented. The research shows the formation process of unfavorable geologic bodies including4stages:the original structure of relative soft rock was damaged under the activity of water chemistry, the gradual loss of the original rock structure, formation of soft interlayer and at last, formed the slip soil or argillization interlayer accompany with the deformation adjustment of landslide. Using these homology regular and clues, spatial distribution of unfavorable geologic bodies are gotten obviously.3. The slip soil, also including the argillization interlayer, is widely distribution in the loose structure landslides with tectonic origin, and they usually control the deformation trend of the whole landslide. These kinds of samples always contain gravels in it and not distribute continuous, so the collecting a large number of these samples is very difficult; and referring the existing laboratory test methods, it has a very strict limit to the particle size of the test samples, the particles of which the size beyond the limited granule should be picked out, this defect makes the test result can't reflect the actual mechanical properties of coarse soil very good. In this paper, a new method which combines with CT scanning technology, indoor direct shear test and numerical simulation tests was put forward. Only using a small quantity of soil, the particle size distribution, particle distribution characteristics and the arrangement mode of the intact coarse soil keep undisturbed, so as to get the actual and reliable mechanical indices. The result shows:even the proportion of coarse particle is about30%, it can influence the deformation mechanism of the whole soil structure, especially for the increasing of cohesion. Relying on the advantage of CT scanning and numerical test, a large number of tests can be carried out, and makes it possible to summarize the coupling mechanism between granule and course. This research method can provide a new method to obtain reliable shear strength parameters of intact coarse soil.4. With the conditions of squeezing and stretching stress stem from tectonic fold, interbedded shear zones are very developed inner the rock layer, especially the rock layers which have different competency. The shear strength of this special geological element can be treated as the residual strength, so generally speaking, it's also can be defined as a typical unfavorable geologic element. The shape of rock discontinuities is mostly very complex and anisotropic. In present, it's not easy to get the shear parameters of rock discontinuities by test directly, because for one rock discontinuity, the shear test is not easy to repeat. But the shear strength results from different rock discontinuity are usually very discrete. JCR index for statistics the feature of rock discontinuities also has some defects as follow:it only can reflect the2D feature of rock discontinuities; during the quantification process, the measurement data should be compared with the typical shape of rock discontinuities, and need a lot of experience. In this paper, a new method which combines with shear tests for rock discontinuities,3D spatial analysis for the morphology of rock discontinuities and numerical simulation tests was put forward. Using this method, the comprehensive frictional angle of rock discontinuities can be separated into internal frictional angle and the frictional angle caused by climbing effect from mechanism. Take the factors of slope, aspect and shear direction into account,3D spatial analysis was done by ARCGIS software, and establish the model of rock discontinuities with3D attributes. Then numerical simulations are carried out for the shear test of rock discontinuities. By comparison with mechanical test results, the numerical experiments are very reliable. This research method can provide a new method to obtain reliable shear strength parameters of rock discontinuities.5. GDS dynamic soil triaxial apparatus is used to test the dynamic modulus, damping ratio for slip soil, argillization interlayer and soft interlayer. Due to high content of coarse particles in the samples, the program of using mid-size samples (Φ100mm×h200mm) is adopted for testing. It can overcome size effect and avoid picking out too many coarse particles, and avoid big different between test results and the actual indicators at utmost. Both confining pressure and consolidation ratio in the test program are obtained by actual stress conditions. Meanwhile, cyclic stress ratio is used to estimate the strength of each sample, and it can be useful to determine the load step. The result shows, the test program is available, nearly10steps of dynamical load are applied to the samples successfully and achieve the test purpose. In the researching for constitutive model of soil dynamics, the maximum dynamic shear modulus (Gmax) is a very important parameter. However, there is no uniform calculation method for each kind of soil so far. The existing methods always require many parameters and engineering experience to calculate Gmax semi-quantitatively, so it's not easy to be used in actual engineering. But in fact, under a determine condition, Gmax is a unique parameter. Combine with the modified Davidenkov model, which is wide applicability in constitutive model of soil dynamics, the author multiple regression the three parameters in modified Davidenkov model, the adaptive algorithm is also carried out to back analysis, iterative, then to approach each Gmax come from different load steps, which is a variable in the function of the model. The calculation can fitting the curve of Davidenkov three parameters model well, and at the same time approach the true value of Gmax comes from different load step, it can reduce the calculated residual to the lowest level. The method is verified available in this research, and the biggest advantage is avoid reaching the Gmax directly, and obtains the dynamic constitutive model for each sample.6. The terrane under Huangtupo riverside landslide belong to T2b3(Triassic Badong Formation, third member), and it has the characteristic of hard rock interbedded with soft rock. The spread regular of seismic waves and dynamical response characteristics in the terrane are worthy of researching.5kinds of typical rock specimens (including marlite and clay limestone) are collected from the test tunnel, frequent loading-unloading test were carried out on MTS815Flex Test GT rock mechanics test system, the test wave for cyclic loading test was sine wave, and its frequency was1.0Hz. The dynamic modulus of elasticity, dynamical possion ratio and damping ratio were gotten. By changing the frequency of cyclic loading, for each sample, the variation trend of strength is summarized, and it can be reflected by the change of uniaxial compressive strength. Then, based on observing the microstructure of mechanical traces from failure surface, some phenomenon show, the activity of frequent loading-unloading may destroy the structure of marlite and corrosive clay limestone. This research fully quantitative the different dynamical response features for each sample under the condition of frequent loading-unloading, and it also provide many necessary parameters which can be useful in multi-field coupling numerical simulation in the final part.7. Based on detailed field investigation and typical geological profiles, the accuracy3D geological model for Huangtupo riverside landslide and Qiaotou landslide are established separately. Accordance with the foregoing summarized regular of induced seismic, two kinds of seismic intensity (including gradeIV and V) and three kinds of adverse seepage conditions (including145m-175m in30d,175m-145m(60d) and145m-165m-145m(30d)) are determined to make a multi-field coupling numerical simulation for both landslide. The dynamic response feature and deformation mechanism of Huangtupo riverside landslide and Qiaotou landslide in the above conditions are gotten, conclusions are as follows:1, these seepage conditions play adverse effect to the deformation of landslides and the seepage activity can't be' ignored, the unbalance seepage vectors will intensify the deformation under the activity of seismic load, especially for Qiaotou landslide (the represent of debris origin landslide);2, the dynamic response characteristics for two kinds of loose structure landslides are quite different. For Huangtupo riverside landslide (with tectonic origin), the damping ratio of slip soil and soft (argillization) interlayer are very big, and the hysteresis damping characteristics is also very obvious. The energy of seismic waves can be amplified in these geological elements, and this effect will aggravate the deformation of landslide;3, By analysis the deformation mechanism for both landslides, the deformation of Huangtupo riverside landslide is mainly controlled by dynamical load, and for Qiaotou landslide, unbalance seepage factor plays a more important role in the deformation process;4, with intensity Ⅳ, the deformation tendency for both landslides is stable and convergence under frequent seismic, but with intensity V, the deformation tendency for Huangtupo riverside landslide shows the tendency of non-convergence under frequent seismic. Then, compared with monitoring data, the process and conclusion of numerical simulation is verified.The research process is close relates to the actual project from beginning to end. Integrated technology methods are carried out to make a systematic study on the dynamic response and deformation mechanism for two kinds of loose structure landslide under frequent microseisms. The conclusions and regulars which are summarized can be very useful in developing the reasonable geological disaster prevention programs, keep the people's lives and property safety in reservoir area and protect the long-term safe operation of the Three Gorges reservoir.