oa Time-Lapse Seismic Tomography of a Small Embankment Dam with Possible Zones of Weakness.

Earthen dam or levee failure can occur with little warning. Internal erosion, seepage and piping are among the major causes of failure in earthen embankment dams and levees. these Internal problems can only be detected visually after the damage has progressed significantly. Identifying such problems at an early stage requires a method of imaging the progressive change in the Internal structure of the dam. Seismic velocities depend on the elastic properties of the material; therefore, time-lapse seismic measurements have promise for monitoring changes in the Internal dam structure caused by piping, seepage or erosion. in this paper, results of time-lapse seismic measurements conducted on two small earthen embankments will be discussed. these dams were constructed at the USDA-ARS Hydraulic Engineering Research Unit; Stillwater, OK having two artificially built weak zones susceptible to Internal erosion. Seismic measurements were conducted on these dams at different stages from the start of construction up to failure. Time-lapse surveying of the experimental dams will provide insight on how the changes in the distribution of elastic properties manifest in the seismic Tomography. Comparing seismic Tomography images, an area of increased P-wave or lowered S-wave velocity could be used as an indication of possible seepage or wet area. While the increase of the P-wave velocity is due to the incompressibility of the water, the slight decrease in the S-wave velocity is associated with the increase in total density. A lower P and S wave velocity correlates with the weakening of the surrounding soil structure due to void formation caused by the erosion of fine materials from the dam. [This research was funded by the department of Homeland Security- sponsored Southeast Region Research initiative (SERRI) at the Department of Energy’s Oak Ridge National Laboratory.]