Abstract

A series of events occurred at an earth-filled dam which raised a serious question about the safety of the dam. An integrated geophysical survey, including dc resistivity, controlled source magnetotelluric (CSMT), ground penetrating radar (GPR), and seismic refraction and tomography methods, has been conducted on the dam. The purpose of the survey was to provide basic data through imaging the internal structure of the embankment for a precise safety and stability examination of the dam. One of the essential reasons applying various kinds of geophysical methods is to view the physical properties of the internal structure of the dam and to compare the results each other. The second reason is to accomplish two survey objectives in the safety examination of the dam: imaging of the internal condition of the dam and investigation of the basement structures beneath the dam and its vicinity.

All the applied geophysical investigation showed their own characteristic responses to anomalous zones. The zones delineated by each method agree with each other, confirming that these anomalies are directly related to the weak zones that cause the problems in the dam. The CSMT results indicated the dam was partly located on a fracture zone. The 3-D resistivity imaging provided an overall image and the progressive nature of the weak zone development in the dam. The GPR images implied that some disturbances might take place in the internal part and thus deform the shallow layered structure. Comparing the results of the seismic tomography and 3-D resistivity images led to the conclusion that the clay core not only contains materials other than clay, such as gravels and rock fragments, but also an abnormally large amount of water which have caused a serious dam stability problem.

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