Geophysical Testing of an Embankment Failure

Geophysical seismic refraction and ground penetrating radar measurements were made along a 540 foot section of an Amtrak railroad embankment to evaluate the embankment for soil “loosening” by cracking and or slumped soil conditions. Geophysical data were acquired along a series of parallel lines perpendicular to the axis of the track spaced 10 feet apart. Each line extended from the centerline of the access road at the toe of the embankment to the edge of the ballast at the top of the embankment.

Refraction data were acquired using a 12-channel system with 5 foot spacing between geophones and a hammer impact energy source. Seismic compressional wave energy was generated at the ends and center of each seismic line. Seismic data were used to measure the average compressional wave velocity values. Abrupt velocity changes indicative of cracking were detected approximately half way up the slope on nine consecutive lines. Subsequent excavations identified a crack coincident with the abrupt velocity change.

GPR data were acquired with 300 MHz and 500 MHz antennas. The GPR data were relatively featureless with no noteworthy subsurface anomalies. GPR data were reacquired over the suspect crack, after water was poured into a hole, indicated the crack is approximately 8 to 10 feet deep.