Study On The Structural Behavior Of Deep Water High Earth-rock Cofferdam Over The Thick Overburden Layer

According to hydroelectric scheme of China, most of large hydropower projects under construction and planning building engineering will be built on the thick overburden layer in the Jinshajiang River, Yalona River, Dadu River and Wu River, etc. The thickness of the thick overburden layer is up to tens meters to hundreds meters. For instance, the thickness at Wudongde dam sites is up to60m, Baihetan thickness is59m, Xiangjiaba thickness is generally30～60m and Yele hydropower is420m. To ensure successful operation of the whole project construction, especially for the flood protection of the project and the security of the large-scale construction instrument equipments, safety and stability of the cofferdam in the hydraulic and hydroelectric engineering is very important. Once the cofferdam has an accident, it will directly affect the construction of the main project and the life and property safety of the lower reaches residents. The loss is huge.The structural behavior of deep water high earth-rock cofferdam over the thick overburden layer is studied in this thesis from several aspects such as the cofferdam slope stability, the contact force element model of the coarse-grained materials and cutoff wall, the interaction effects of the thick overburden layer-cofferdam body-impervious body and so on. The main research contents and conclusions are as follows.(1) Based on the basic principle of the finite element method, the application of the preconditioned conjugate gradient method for the finite element method solution, the AutoCAD-based system development of the preprocessing and postprocessing platform and the finite element method application on the geotechnical engineering are discussed and studied. And the finite element method programming system considering the filling and excavation construction process is established. The results show that finite element mesh mobile technology based on the known topology is a convenient and effective mesh generation and mesh mobile method. It is applicable to the adaptive computation and the complex topographical and geological condition modeling in the hydraulic engineering.(2) From the shear mechanism of the coarse-grained structural plane, according to size between the self shear strength of the coarse-grained and the shear strength of the contact surface, the tangential stress-strain constitutive relation of the contact surface of the coarse-grained structure is divided into two sizes:rigid-plastic model and strain-softening model. When the shear strength of the contact surface is relative small, the rigid-plastic deformation phenomenon appears. But when the shear strength of the contact surface is relative large, the presentation is the strain softening phenomenon.(3) Based on the damage mechanics principle, contact surface constitutive relation on the damage model is established and verified by the experimental data. It is shown that damage model can be used to describe strain softening phenomenon and the consequent is good. And the damage model is applied to case of the contact plane of the coarse-grained-cutoff wall with mud layer. The contact plane element calculation model considering mud layer normal deformation and tangential damage model is derived and verified by the numerical example. The results show that the element can be used to analyze the structure contact problem.(4) Combing with the upstream earth-rock cofferdam of the Wudongde hydropower station, considering different factors combination (the depth and density of the overburden layer, the mechanical properties of the cutoff wall, the properties of the contact plane, various water level condition et. al), stress-strain of the cofferdam structure is analyzed. The all computation results show that the deformation law of the cutoff wall is mainly controlled by the deformation parameters of the overburden layer and upstream side gravel sand. The denser the overburden layer and upstream side gravel sand, the larger the rigidity, the smaller the cutoff wall deformation. The stress of the cut-off wall is more sensitive to the overburden layer. When the rigidity of the overburden layer increases, the horizontal displacement and stress level of the cutoff wall decrease and safety margin increases. It is suggested to compact overburden layer near the cutoff wall as possible, to properly enhance filler density near the cutoff wall for enhancing confining ability of the cutoff wall, to improve the stress state of the wall body. The adapt deformation ability when adopting the plastic concrete anti-seepage wall is better than that of rigid concrete anti-seepage wall. The plastic concrete anti-seepage wall should be adopted because its stress state is better and safety margin is larger.(5) The interval analysis application, which is a non-probabilistic reliability analysis method, on the dam slope stability is discussed. When the interval variable of the structural performance function Z is assumed to be uniformly distributed, the non-probabilistic failure probability and the non-probabilistic reliability of the interval model are proposed. The safety evaluation of the slope stability based on interval analysis is improved. By applying the fictitious variable method to the probabilistic reliability analysis of dam slope stability, dam safety evaluation is perfected and promoted based on the probability reliability. A probabilistic and non-probabilistic hybrid reliability model is proposed and applied to the dam slope stability evaluation of the upstream earth-rock cofferdam of the Wudongde hydropower station. The results show that under the case of data deficiency or information incomplete, the boundary or range of the failure probability is higher reliability than that of giving uncertain single value.