Seismic Investigation of a Sinkhole on Clearwater Dam

Abstract

A 10 ft wide and 10 ft deep sinkhole that catastrophically formed approximately 120 ft upstream of the crest of Clearwater Dam in southeastern Missouri was the target of a high-resolution seismic imaging program including reflection, surface wave analysis, and crosshole tomography. The primary goals were to determine the general subsidence geometry and integrity of the core and help ascertain the involvement of bedrock and native alluvium beneath this earthen dam. Reflection data from this survey possess excellent frequency content (dominant >150 Hz) and provide high-resolution images of layers within the pervious shell. Using tightly spaced surface wave profiles, an elongated low shear-wave velocity chimney-like feature was delineated and interpreted to represent the root of the sinkhole. Based on seismic, construction, drill, and borehole tracer data, a borehole geophysics program was designed to identify fractures/joints that might provide seepage pathways. Crosshole seismic data detected a large low-velocity zone within the bedrock and an associated low-velocity zone within the pervious fill consistent with the surface seismic interpretations. A comprehensive appraisal of the risk this disturbed zone represents to the overall integrity of the dam and whether it is a symptom of a larger, yet undetermined subsurface leaching problem is being developed.

Investigation of Source of Seeps beneath Earthen Dam, Central Massachusetts

“…Geophysical methods are important tools for investigating seeps at earthen dams, as these have been implicated in numerous dam breaches and levee failures (Carnevale et al, 2004;Hager et al, 2005;Hollema & Olson, 2004;Miller et al, 2004). Geophysics provides a non-intrusive method to identify areas of possible existing seeps and other high permeability zones within or beneath existing dams.…”