Research On Evaluation Method Of Seepage Failure For High Embankment Dams Based On Risk Concept

Dams are the main engineering projects for reducing natural disasters such as flood.Due to the importance of dam safety,a failure accident may cause a great threat to the safety of lives,properties and environment downstream of reservoir dams.Furthermore,people's requirements for the safety and reliability of existing dams are gradually increasing,and they gradually change from the concept of "engineering safety" to "engineering risk".This paper studies the evaluation method of seepage failure for high embankment dams based on the concept of risk.The following research results have been obtained.1.The construction and management of resevior dams in the USA started earlier than China,and the safety management background and model of resevior dams in China and the United States have certain reference values.This paper takes the dam safety management system in the United States as an example.Stating from the risk management of reservoir dams,the paper summarizes the dam safety management system and the development process of risk management in the United States.At the same time,in order to help China to transform the concept of dam risk management,this paper discusses the importance of risk management for resevior dams in reducing the risk based on the spillway accident happened in Oroville Project.The author believes that it is necessary to strengthen the concept of risk in the next stage of dam safety management process.People should face the risk directly and understand the risk correctly.What's more,China should develop related risk management techniques to control risk better and safeguard lives and property donwstream.2.Internal erosion is the most common reason which induces failure of embankment dams besides overtopping,and there are no indicators and methods for rapid evaluation for the internal erosion risk of embankment dams.Relatively large leakage is frequently concentrated at defects of impervious element of embankment dams,and this will lead to eventual failure.The amount of leakage depends not only on integrity of impervious element,but also on dam height,shape of valley,shape of impervious element and water level in reservoir.The integrity of impervious element,which represents the relative level of seepage safety,is not easy to be determined quantitatively.A simple method for generalization of steady seepage state of embankment dams with thin impervious element which is proposed in this paper.The apparent overall value of permeability coefficient for impervious element can be obtained by this method with reasonable accuracy and efficiency.Furthermore,an evaluation parameter of impervious element is defined in this paper as an index to characterize seepage safety of embankment dams.Subsequently,seepage safety of several embankment dams with compressible and incompressible impervious element is evaluated and the evolution of defect level of impervious element of embankment dams is investigated.It is proved that the newly proposed method and the definition of evaluation parameter of impervious element in this paper are feasible in the evaluation of relative seepage safety level of embankment dams with thin impervious element.3.Based on the generalized model of steady seepage state of embankment dams with thin impervious element,it is well known that seepage failure risk of a dam changes greatly with reservoir water level and time.An analogical approach to dynamically estimate seepage failure frequency based on the steady state seepage generalization model and finite numbers of concrete faced rockfill dam cases is proposed in this paper.What's more,a method based on Bayesian analysis is proposed in this paper to overcome the lack of failure data.In order to synthesize the advantages of both methods,a combined method is proposed in this study to estimate seepage failure frequency for concrete faced rockfill dams by introducing the analogical approach into traditional Bayesian approach.The time after the dam was put into operation is divided into several time segments,and the seepage failure frequency is calculated segment by segment according to Bayesian method.At the same time,the analogical approach is introduced to adjust the parameter of prior distribution in the Bayesian approach in which only the individual operation history of analyzed dam is considered.For the combined method,dynamic seepage failure frequency at each time segment equals weighted logarithmic average of annual seepage failure frequency values both by Bayesian approach and by analogical approach.Hence,the relationship between seepage failure frequency and evaluation parameter can be established.It can be proved that this method can make a better effect of estimating seepage failure frequency dynamically and distinguish the risk status of embankment dam with impervious concrete face slab after applied to three different projects,thus providing an new idea for risk assessment for high embankment dams.