Studies of a Riverbed Scour Monitor System Using a Pier-Interior Cased Borehole

Abstract

One approach to seismic-based riverbed scour measurement utilizes a seismic source together with detectors deployed in a cased borehole adjacent to the bridge pier (the Parallel Seismic System). This approach can have disadvantages as, during river floods, cased boreholes near a pier could be damaged by the action of floodwaters. Detectors deployed in a cased borehole located within the pier would, however, be protected from the action of floodwaters. This paper describes simulated field experiments designed to determine if detectors deployed within a pier-interior cased borehole could record useful seismic data. If so, then the same data analysis will be applicable that successfully identifies the depth of scour around a bridge pier from seismic data obtained in a cased borehole adjacent to the pier. The test site contained a replica of a bridge in a water-filled pond. A cased borehole was located within one of the bridge piers. Tests were carried out with the detectors located in the pier-interior cased boreholes. Both hammer strikes applied to the top of the pier and an air gun located in the water layer were used as seismic sources. Comparative tests were carried out where the detectors were deployed in a cased borehole exterior to the pier. Analysis of the test results indicated that valid seismic events could be detected within the pier and that data derived from these could be used to provide information about scour conditions adjacent to the pier. Thus, a pier-interior cased borehole could be a useful component of a seismic based scour measurement system.