Investigation for cause of sinkhole and assessment of core integrity in Clearwater Dam

The Clearwater Lake Dam of southeast Missouri has had a history of seepage through the left abutment dating from initial impoundment in 1949. The seepage distress appeared to be reasonably stable and the dam was believed to be safely functioning; however, the discovery of a sinkhole on 14 Jan 2003 in the upstream slope of the dam, approximately 7 months after a new Lake of Record, prompted a major re-assessment of the dam's safety. This paper details the ensuing investigation to determine the sinkhole's cause and inference as to the dam's water retention capability or risk of failure, specifically addressing efforts to determine the integrity of the impervious soil core which was identified as the most seriously "at risk feature'' of the dam embankment. Overviews are presented of the site karst geology and history relating to design and construction. Initial emergency response and repair for the sinkhole is discussed. Post-sinkhole investigations by geophysical seismic refraction method, and by sonic drilling/sampling are detailed. Sinkhole causing mechanisms are proposed which relate to 1) progressively developing internal erosion within the dam core and foundation, and 2) limited developing defects that do not progress to failure. A quantified risk to the dam of catastrophic failure is presented via a simplified Risk Assessment using a logic based "event tree" developed from characterizing the piping characteristics of embankment materials and foundation conditions relative to possible sinkhole causes. Interim actions and preliminary conclusions from an on-going Foundation Drilling and Grouting and Investigation Project are presented to the extent results are available.