Study On Excavation Disturbed Zone And Ogee Excavation Scheme Of High-stress Dam Foundation At Deep Valley

Focusing on the Excavation Disturbed Zone (EDZ) and the optimizing ogee excavation scheme of High-geostress Dam Foundation at deep valley region, a new nonlinear back analysis method of geostress field of high-stress valley region, the excavation disturbing and anchoring effects of high-stress dam foundation, the optimization design of ogee excavation scheme, as well as the mechanical properties of the dam foundation rock masses are systematically studied. The results are comprehensively applied to the safety assessment and design optimization of Laxiwa dam foundation on Yellow River. The following contents are included in this thesis: (1) According to the back analysis idea using forward calculation, a new nonlinear back analysis of geostress field is developed based on the integration of numerical computation, artificial neural network (ANN) and genetic algorithm (GA), in which both the nonlinear distribution characteristics of geostress with burial depth and the nonlinear mechanical behavior of rock mass are taken into account. Moreover, the new nonlinear back analysis method is applied to the engineering background of Laxiwa hydropower project located on the upper reach of Yellow River. (2) Based on the results of geostress field and a detailed numerical simulation of excavation and anchoring processes, the excavation disturbing and anchoring effects of high-stress dam foundation of Laxiwa hydropower engineering are systematically studied. The excavation disturbing characteristics of stress, deformation, yielding and failure zones of the dam foundation are derived. The method of determining the failure zone is provided according to the results of true triaxial tests on granite in the dam foundation. The reinforcing effects on the aspects of stress, deformation, yielding and failure zones of the dam foundation are revealed. Furthermore, the excavation disturbing and anchoring problems of the high-stress dam foundation are profoundly discussed and a series of important conclusions are drawn. (3) The elevation of foundation boundary of Laxiwa dam is determined based on the analysis of geological conditions and high geostress. Through the comparison of ogee excavation scheme andflat-bottom excavation scheme, at the first time, the optimization design of ogee excavation scheme is provided for the high-stress arch dam foundation. Then, the unloading deformations, stress distributions, and safety factors of dam foundation and the stress distribution characteristics of the dam body under the different excavation schemes are computed and analysed, using 2D and 3D FEM, respectively. And thereby, the rationality of ogee excavation scheme is verified for the high-stress dam foundation. (4) A large-scale comprehensive tests on the engineering and mechanical properties of dam foundation rock masses are conducted, using seismic reflection method, sonic method and borehole photography, etc. the depths of relaxation zone of dam foundation due to excavation are determined. The results demonstrate that the problems induced by excavation of high-stress dam foundation can be solved by the combination of ogee excavation scheme, step-by-step excavation and reinforcement. The achievements presented in this thesis can be expanded into the other similar dam foundation engineering at high-stress valley region.