Studies On Seepage Control Of Levee And Piping Mechanism

Studies on seepage failure mechanism and seepage control technique of levee can prevent seepage failure of levee, predict the developement of seepage failure and suggest the scientific instruction to levee reinforcement, they are of great theoretical and practical significance. This paper is based on seepage control of Dongting Lake region. The main works are as follows:(1) A comprehensive and systemic summary on the status and hotspot of vertical cutoff wall technique, downstream berm technique and piping was done, and some shortages were put forward, which provided a fundamental guide for the following researches of this paper.(2) The anti-seepage effect of suspended cutoff wall and piping control effect in sand-cobble levee foundation under the case of penetrating and partially penetrating piping channel were studied systemically. The application effect of the suspended cutoff wall in sand-cobble levee foundation was appraised comprehensively and objectively, it provided a theoretical basis for adopting reasonable seepage control technique of Dongting Lake region levee including sand-cobble levee foundation broadly.(3) According to the geological characteristics of levee foundation in Dongting Lake region, a semi-closure cutoff wall model in multi-structure levee foundation was established, the thickness and permeability of less permeable covering layer, strong permeable layer, anti-seepage supporting layer affecting the anti-seepage effect of semi-closure cutoff wall were studied systemically. A concept of optimal penetration ratio was proposed, and an empirical formula of optimal penetration ratio was fitted by least square method, then the empirical formula was validated by numerical examples and Zhoujiawan Levee in-situ test. This formula indicates the relationship between the thickness, permeability of anti-seepage supporting layer and optimal penetration ratio, the independent variables are only the thickness of anti-seepage supporting layer, the permeability ratio between anti-seepage supporting layer and strong permeable layer, and it is very convenient for engineering, it provides a theoretical basis for designing of semi-closure cutoff wall in multi-structure levee foundation.(4) The phenomenon of two or more infiltrations in downstream berm seepage calculation and its reason were analyzed, a downstream berm seepage finite element program was developed based on seepage FEM and the imaginary elements fixed mesh method. The second free surface boundary was converted to the free surface in the whole region, and it converted the problem with judging the second free surface boundary complicatedly to the problem with solving the free surface in the whole region simply by adding a lot of strong permeable air elements on top of downstream berm. Analysis indicates that air element integrating imaginary elements fixed mesh method solves the flux superposition problem of free surface elements and the convergence problem brought by the two infiltrations (or more infiltrations) easily, and it improves the efficiency of downstream berm seepage calculation.(5) A downstream berm model was established according to the geology characteristics of levee foundation in Dongting Lake region. The width and thickness of downstream berm were analyzed optimally, and the concepts of optimization width and thickness of downstream berm were put forward, then the critical failure point method was proposed, it is defined as follows:the intersection point between the extending influence curve of downstream berm width on the maximal seepage gradient inside levee and the allowable seepage gradient of covering strata is called critical failure point, the left region of the critical failure point is danger and needs to cover, whereas the right region is safe and needs not to solve. The application of critical failure point method reduces the size of finite element model to a great extent, avoids repeating finite element calculations and improves the calculation efficiency remarkably. The critical failure point method is preponderant obviously compared with the traditional methods including Code for Design of Levee Project. This method provides a simple and convenient method determining the optimal width of downstream berm.(6) Soil is divided into three phases (solid particles phase, fluid phase and fluidized particles phase) based on continuous medium mechanics theory, and a seepage-erosion coupled mechanism of piping development was proposed. Based on this coupled mechanism, three phases mass conservation differential equations were constructed and the constitution equation describing the mass transformation between solid particles phase and fluidized particles phase was introduced simultaneously, then a three phases seepage-erosion coupled piping mathematical model was established based on the standard Galerkin, a relative FE program was designed and validated, and the main model parameters were analyzed systemically. The new model proposed here considers the coupling effect of solid particles phase erosion and fluid phase seepage, and realizes the coupling solution of soil and fluid phases, it could simulate the unsteady and highly nonlinear course of piping, it also could show the progressive failure characteristics of piping, and it provides an effective tool probing into the development mechanism of piping.(7) The numerical oscillation essence of the piping program based on the standard Galerkin was analyzed, and it is believed that the nonlinear convection item in governing differential equations induces the non-self-adjoint equations and unsymmetrical total matrix, which destroys the optimal approximation characteristics of the standard Galerkin. In order to eliminate numerical oscillation, the stabilized finite element technique Streamline Upwind Petrov/Galerkin (SUPG) method was adopted to analyze the coupled piping mathematical model based on comparing all kinds of stabilized finite element technique. Investigations indicate that the SUPG method can eliminate the numerical oscillation of pure convection equations effectively, and gain more satisfactory results under the condition of fewer elements compared with the standard Galerkin, but this method is impacted by the perturbation term seriously.