Time-Lapse Seismic Measeurements on a Small Earthen Embankment During an Internal Erosion Experiment

The failure of earthen embankments is associated with erosion, overtopping, seepage, piping, slope failures, and slides. Precursory evidence for certain failures can be identified by visual inspection; however, others are not so easily detected. Seismic imaging and monitoring may provide unique and valuable precursory information about the onset of piping, seepage, and anomalous pore pressures to assist in the evaluation of the integrity of an earthen embankment. In this paper, we discuss the results of passive and active time-lapse seismic measurements carried out during an internal erosion experiment conducted at the USDA-ARS Hydraulic Engineering Research Unit, Stillwater, OK. The active time-lapse seismic measurement consists of several refraction surveys carried out at different times during the experiment. Images representing a “slice” through the dam were constructed using a finite-frequency seismic refraction tomography code. Clear and significant temporal changes in the seismic response of the earthen embankment were recorded during internal erosion. The zone of weakness in the dam, associated with the flow channel, was clearly represented in the images by an area of lower velocity. An isolated circular zone of low coverage in the ray-based image indicates the development of an air cavity at the top of the water channel. Passive seismic monitoring measures the ambient seismic vibrations at various locations on the embankment. These data indicate that the internal zone of flow was a source of seismic energy and that the magnitude and frequency content of this source were dependent upon the degree of internal erosion. Further studies are underway to assess the use of these techniques to less obvious zones of weakness within embankments. ©2009 The Environmental and Engineering Geophysical Society