Where the Cracks Are: Geophysical Detection of Cracking in and Adjacent to Embankments

Abstract Only - Utilizing a surface seismic method employing a 12-channel seismograph with a sledgehammer energy source, cracking in and adjacent to embankments can be detected. During an investigation to characterize known transverse cracking, it was discovered that embankment cracking could identified in areas with little or no surficial evidence utilizing a method used for the detection of earth fissures described by Rucker and Keaton. This method visually identifies the presence or absence of potential embankment cracking or earth fissuring by identifying sudden decreases in signal amplitude (attenuation) and/or anomalous increase in arrival time (time offset) of the seismic signal between adjacent geophones. The presence of such anomalies in several data sets for each seismic line, such as in both foreshot and backshot trace sets, is considered an indicator of potential cracking. The presence of cracking or earth fissuring is verified by trenching at the location of identified seismic anomalies. This technique has successfully identified embankment cracking at the Buckeye No. 1 Flood Retarding Structure located in western Maricopa County, Arizona, and has successfully identified earth fissures in several locations in Arizona and Nevada, including a series earth fissures that pass beneath McMicken Dam in Maricopa County, Arizona, about 30 miles northwest of Phoenix. Earth fissures were discovered in the vicinity of the southern end of McMicken Dam, a nine-mile long flood retention structure near the White Tank Mountains, in 1981. By 2002, further investigation of impacts of differential ground subsidence revealed that earth fissuring in the area had increased and that a complex of new earth fissures with visible surficial features were trending towards the dam. The authors used this method to trace these new fissures; the fissures were traced well beyond the surficial features and under the dam. The presence of these earth fissures was verified by the excavation of test trenches. Transverse cracks have been visually identified on the crest and slopes of the 7-mile long earthen Buckeye Flood Retarding Structure No. 1. The seismic refraction method developed by Rucker and Keaton for detection of earth fissures was used to identify transverse cracking in the embankment in areas both with and without surficial evidence. This method was successful at identifying previously known and unknown transverse cracks within the embankment. The presence of these transverse cracks was verified by excavation of the embankment.