Use of Ambient Vibration Measurements to Infer Dynamic Properties of Poorly Characterized Old Earth Dams—A Case History from Puerto Rico

This paper presents a case history involving the seismic evaluation of the Patillas earth dam constructed in 1914 in southeast Puerto Rico. The dam is located in a region of high seismicity and like many old structures there was little or no information regarding the properties and compaction levels of the different materials used for the construction of this dam. This paper presents a description of the procedure used to estimate the material properties for the seismic analyses of Patillas Dam, with emphasis on the maximum shear modulus (G_max). The paper presents a comparison of the predominant vibration period of the dam in the transverse direction estimated using three different equivalent linear models for the dynamic finite element analyses and the values obtained from ambient vibration measurements using accelerometers placed at the crest of the dam. The resulting predominant periods for the finite element models (T = 0.42, 0.49 and 0.56 s) had very good agreement with the field tests (T = 0.43 to 0.45 s). The proposed methodology of using ambient vibration testing to help calibrate dynamic models of poorly characterized old dams proves to be promising.