| Built earth-rock embankments and dams are frequently faced with the seepage erosion problems at different scales,especially in the flood season.As one of the main forms of the seepage erosion,the piping exhibits the widespread,hidden and harmful natures and has been considered to be the major cause of the earth-rock dam accidents and failures.The current strategies against the piping adopted in practice are reactive rather than preventive.To take control of the piping in its early stages and reduce the risk of the dam failures,it is necessary to establish a piping monitoring and early-warning system.The existing methods for detecting the dam piping focus on the the location and size diagnosis of piping leakage channels and therefore fail to realize the real-time monitoring and trend prediction.A piping identification technology with the ability of capturing the evolution state is significant in current research and practice.With the aims of developing an effective method to identify the different stages of the dam piping,this study investigates the electric field characteristics of earth-rock dams in the piping evolution utilizing the combination of model test,theoretical analysis,and numerical simulation.A piping identification method is preliminarily developed according to the obtained electric field characteristics.The research is supported by the National Natural Science Foundation of China(Grant No.51879017).The main research contents and results are as follows:(1)Study on the generalized model of the piping evolution in earth-rock dams.Starting from the basic concept of the piping,this paper sums up the existing definitions of the piping phenomenon.By sorting out the initial inducement and physical evolution process of the dam piping,the occurrence mechanism and development mechanism of the dam foundation piping and dam body piping are analyzed.Then,based on the generalizations of the dam type and piping evolution process,a generalized model of the piping evolution in the dam body is established,which provides a theoretical basis for simulating the piping evolution process of earth-rock dams.(2)Experimental study on the electric field of earth-rock dams during the piping evolution.The high permeability bodies at different length,made of gravel-filled geotextile tubes,are buried in several physical earth-rock dam models,and the piping evolution process is simulated.Under an applied voltage,the response potential differences of the dam models before and after the impoundment are measured by using the Wenner configuration.Based on the potential difference results of the dam models before the impoundment,the differences of the dam models are analyzed,and the reasonability of the used piping simulation method is discussed.Based on the potential difference results of the dam models after the impoundment,the influences of the soil moisture content on the potential difference and the piping channel length on the potential difference are studied.The results show that the piping evolution process and the response potential difference are closely related,which confirms the ability and feasibility of using the electric field characteristics of earth-rock dams to reflect the piping evolution process.(3)Study on the theoretical analysis model of the electric field of earth-rock dams in the piping evolution process.According to the conductive structure characteristics of the earth-rock composite medium,a resistivity theoretical model of the earth-rock composite medium containing the porosity and saturation parameters is derived,and the geoelectric theoretical model for the earth-rock dam piping analysis is established.On this basis,combined with the stable current field theory and the mathematical model describing the piping evolution process,a theoretical model used to analyze the response electric field characteristics of earth-rock dams during the piping evolution process is developed.This theoretical model provides a calculation approach for obtaining the electric field characteristics of earth-rock dams during the piping evolution process.(4)Study on the numerical simulation of the piping evolution process of earth-rock dams.Based on the mathematical model describing the piping evolution process in earth-rock dams,the occurrence and development of the dam piping are simulated by using the finite element method,which is realized by expanding the permeability coefficient of the piping zone and iteratively calculating the seepage field of the dam.The effects of the different piping initial positions on the pore water pressure,phreatic line,seepage velocity and hydraulic gradient of the dam are studied respectively.The expansion characteristics of the piping channel and the changes of the pore water pressure and phreatic line of the dam during the piping development are revealed.On this basis,the reasonability of the piping simulation in this paper is verified by comparing the existing research results.(5)Study on the electric field characteristics of earth-rock dams during the piping evolution process.Based on the numerical simulation results of the piping in earth-rock dams,the electric field characteristics on the crest and downstream slope of the dam under the different power source forms and locations are analyzed respectively.The potential increment distributions of the dam in the piping evolution process and the influence of the power source conditions on the potential increment distribution are studied.The potential increment changes of the dam in the piping evolution process are studied,and the results show that the potential increment first increases and then decreases as the piping development.The aforementioned results lay a foundation for the further research on the identification method of the dam piping based on the electric field characteristics.(6)Study on the identification method and early-warning method of the piping evolution in earth-rock dams based on the electric field characteristics.Using the obtained correlation between the piping evolution process and the response electric field characteristics,a characterization model of the dam piping based on the potential increment is developed.On this basis,a state identification method for the dam piping evolution is preliminarily proposed,including its identification indicators,identification criteria,and field observation system.Through the study of the early-warning system and early-warning criteria,an early-warning method for the dam piping evolution is further proposed.The effectiveness of this proposed method is then verified using the previous numerical simulation results.The aforementioned results provide the technical support for the comprehensive development of the piping monitoring and early-warning technology of earth-rock dams. |
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