| High arch dam resists huge load which will be born by DFRM (dam foundation rock mass) through the transmission of the dam body. Because the upper DFRM will relax after the overlying strata being excavated, researches on engineering geological properties of DFRM of high arch dam are very important to the theory and practice, which can provide basis for the engineering treatment and thus make the construction progress shortened and cut down expenses.Some monographic studies on the relaxation properties of DFRM of the arch dam for the Laxiwa hydropower station, located in the upper stream of the Yellow River, have been systematically finished through collecting abundant geological data and using many techniques and methods. The purposes of the thesis include: (1) systematically generalizes the engineering geological properties of unload-relaxed DFRM; (2) combine existed results of various tests and survey from Xiaowan and Jinping hydropower station, some attempts have been tried to get some common image, mechanism and regularity existed in the excavating process of dam foundation in order to provide some useful guides to the programmed similar projects in the future. The main conclusions include:(1) On the basis of study on the regional and valley ground stress field, the thesis achieves some advances in the following aspects: 1) relaxation mechanism of the DFRM, 2) deformation and failure characteristics due to unloading, 3) time effect for rock mass relaxation, 4) unloading-relaxation genetic classification, 5) testing method and rock mass quality classification of the relaxation zone, and 6) grouting treatment and effect.(2) Based on regional geological background, neotectonic movement, bank slope and valley terrace feature, the evolution process of the valley is devided into several stages on which the ground stress field is analyzed through the numerical method, this approach is closely near the practical situation.(3) The relaxation mechanism of unloading rock mass is discussed and the tectonic background under which the relaxation zone is created due to excavation is analyzed through thinking different scale structural surface existed in the rock mass. The analysis of the whole process of stress release shows that the rock mass with the origin nature stress field {σ0,ε0} near the dam foundation of high arch dam will experience processes of unloading (brittleness), rebounding (elasticity,plasticity) and relaxation (rheologicity) while undergoing high-intensity blast wave, which is completely different from the simulation experiment result that the stress is loaded from zero.(4) It is analyzed for the deformation and failure phenomena of the DFRM of high arch dam by taking the examples of several important hydropower stations. Some common images are generalized and some mechanical modes are analyzed(include tension, shear and tension-shear) for the deformation and failure of DFRM in different depth. The phasic characteristics and corresponding deformation mode are summarized for rock mass relaxation in each stage, and it is thought that deformation duration will become longer along with the increase of the slope height being excavated.(5) Based on analysis of relaxation characteristics of DFRM of many high arch dams, it can be divided into four types for the relaxation of rock mass in view of mechanical genesis, namely unloading-rebounding type, structure relaxation type, excavating-explosion type and epigenetic time-dependent structure type and each type is related to its location and formation mechanism. Furthermore, the influence of the bore-hole blast to DFRM (granite) is studied and the effect zones are divided into failure zone, intense unloading zone, weak unloading zone and no-effect zone,while the relationship has been made between dynamic load of bore-hole blast and bear depth of DFRM.(6) The thickness of unloading-relaxation rock mass achieved by field estimation, numerical simulation, explosion analysis, geoexploration and analogism is near closely.(7) The concept of excavating-unloading-relaxation of DFRM is presented and the methods to decide the thickness of unloading-relaxation rock mass are systematically summarized. Numerical simulation, field tests and field measures are used to identify the relaxation zone of Laxiwa dam foundation, some valuable achievements origined from numerical simulation is found, such as the stress of a certain point in DFRM will continually change in the process of excavating. The research technique and process of using wave velocity to classify unloaded-relaxed rock mass quality and its mechanical properties can be used by analogous engineering projects in the future.(8) The results of grouting treatment for the DFRM show that consolidation grouting for the superficial rock mass can make rock mass quality better and thus enhance its mechanical strength. But for the deep rock mass under the dam foundation surface, the grouting treatment will have less effect. So it should be considered whether or not using consolidation grouting according to the ground stress field and rock mass integrity for analogous projects.
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