Near Surface Geophysical Survey at a Collapsed Site of a Highway Embankment Caused by a Heavy Rainfall

Abstract

We conducted a detailed near surface geophysical survey at a site where an 8 m high road embankment had been partly collapsed caused by a heavy rainfall. The failure broke down a half of the 4 lane road body embanked on the very soft alluvial sediments even though it had been reinforced with geotextile-anchored concrete wall and with a group of ground improvement columns. The collapse spread out debris at most 4 m but heaved the surrounding soft sediments up to 30 m in lateral distance. The field survey purposed to clarify deformation structure and failure process of the road embankment and to evaluate the geophysical properties of the soft sediments damaged by the collapse. Employed methods or tools were GPR, DC resistivity tomography, capacitively coupled resistivity using OhmMapper and hybrid surface wave survey. In addition, detailed DSM (Digital Surface Model) of the road surface was reconstructed from densely taken photo images aided by SfM (Structure from Motion) technique and it clearly highlighted several hair cracks and small bulges generated in association with the collapse. GPR clearly detected the boundary between subgrade and underlying roadbed, and the amplitudes of the reflection were relatively stronger at a specific zone. This indicated de-coupling of the boundary between subgrade and roadbed. Namely, GPR was useful to estimate the extension of hidden deformation associated with the collapse, undetectable by surface visual inspection. Hybrid surface wave survey delineated characteristic S-wave velocity profiles. S-wave velocities of soft sediments were as very low as 50m/s, and decreased to about 30m/s in and around the heaved area. ERT along a cross line at the collapsed part successfully imaged the foot and frontal thrust and was helpful to interpret a possible sliding surface. In conclusion, detailed geophysical survey was quite capable to delineate and interpret the disturbed near surface structure caused by rainfall collapse.