Study On Levee Engineering Risk Analysis Theory And Practice

Safety evaluations on levees are systematic inspections and assessments on the performance of flood prevention of the studied reaches, and also are the main part of job before reinforcement design for levees. Levees are brim over with uncertainty, which are kind of geotechnical structures running under the circumstances of flood water and suffering from hydrological and hydraulic effect. However, risks are inherent in geotechnical engineering as tools to considering and evaluating the uncertainty or things hard to predict which could result in failure of the structure(Casagrande, 1965). As a result, traditional deterministic design approaches due to lack of consideration of existed uncertainty in practice, seems not good enough to represent the safety of levees. Therefore, it will be rather valuable to apply probability-and-reliability-based risk analysis procedure in safety assessment of levees. Based on referring on large numbers of correlative literature at home and abroad, this dissertation presents a groping study on foundations, models, approaches and applications of risk analysis for levees failures. And main conclusions of the study are drawn as follows:(1) Fault tree analysis(FTA) is adopted to analyze the failure types of levee reaches, and failure modes of levee reaches are classified into three levels:Level â… : Hydrological failure mode and structural failure mode.Level â…¡: Hydrological failure mode is classified into overflowing failure mode and overtopping failure mode. Structural failure mode is classified into seepage failure mode, slide instability failure mode and seismic disaster failure mode;Level â…¢: Seismic disaster failure mode is classified into seism-induced slide instability failure mode and seismic liquefaction failure mode. Both slide instability failure mode and seism-induced slide instability failure mode include the riverward slope and landward slope.(2) Calculating models for risk factor of each failure mode of levee reach are established, and accordingly performance function of each failure mode of levee reaches are proposed. Meanwhile an Contributing weight function K(h) is introduced into calculating models of overtopping failure mode and seimic liquefaction failure mode so as to considering of the contribution from risk of the two failure modes to comprehensive failure risk. Then an illustration to flood PDF in calculating model foreach failure mode and risk factor of each failure mode are described in detail. And a points-of-Gauss-Legender-quadrature-based discretized method for integral formula of calculating models is proposed, so it is with nonlinear curve fitting method. Furthermore, methods for estimating value of standard deviation of random variables are also discussed.(3) Analytical solution, bounds estimating and approximate estimating to comprehensive failure risk factor of each levee reach are described. In order to obtain failure risk factor of levee system, a correlative function of failure event between two levee reaches is established, and accordingly the formula and procedure of failure risk factor of levee system is proposed. Meanwhile, economic consequence assessment due to levee failure is described, and then the procedure to failure risk evaluation of levee system is proposed. In addition, over-level flood is defined, and procedure of calculating failure risk of levee system under over-level flood condition is brought forward.(4) One misapplication of Monte Carlo sampling method in practice is pointed out, and procedure of Monte Carlo sampling under correlation conditions is presented. Meanwhile, an extended geometrical optimized method(EGOM) is proposed, procedure of which under conditions of correlation and non-normal distribution variable is presented. Furthermore, EGOM and quadratic polynomial response surface method(RSM) are combined to solve the probabilistic problems with implicit performance function.(5) In allusion to the characteristic of each failure mode of levee reach, calculating approaches for conditional probability of each failure mode of levee reach are presented: EGOM-based RSM for conditional probability of seepage failure mode, a practical method to locate critical probabilistic surface and minimum (3 by Hassan and Wolff for conditional probability of slope instability failure mode. In addition, based on proposed calculating models of failure modes and calculating approaches for conditional probability of failure modes, the code LeveeRisk for failure risk factor of levee reach is developed.(6) Based on Banqiao river left levee reinforcement project, with proposed and presented foundations, approaches and code of risk analysis for levees, the risk factor and risk value of Banqiao river left levee are evaluated, and assessment about the calculating result is also made subsequently. According to its 1995' failure case, critical risk factor and critical risk value for Banqiao river left levee are established. InLevee Engineering;Risk Analysis;Failure Mode;Acceptable Risk;Over-level Flood;External Qinhuai River Levee;Banqiao River Levee;Geomembance...