Embankment foundation monitoring using in-place automated inclinometers: Oahe Dam, Pierre, South Dakota

Automation of dam safety instrumentation allows for real-time continuous acquisition and evaluation of data. Automation presents a means to determine cause and effect relationships between key factors influencing a dam structure. Automation also allows for the incorporation of "warning flags" if prescribed parameters are exceeded.

Presented herein is a case history describing the design and installation of an in-place inclinometer system at Oahe Dam located in central South Dakota near the town of Pierre. The Oahe Dam embankment is approximately 9300 feet in length and 245 feet in height. The dam is founded on a layer of alluvial material overlying Pierre Shale bedrock. Near the right abutment of the dam, the structure is founded on a relatively thick and compressible layer of alluvial and slope wash material. Considerable settlement was experienced in this area during and following construction, approximately 15 feet to date. Numerous inclinometers were installed in this area designed to monitor horizontal motion in both the embankment fill and the underlying alluvial / slope wash foundation. The initial inclinometers were installed to a depth slightly into the Pierre Shale bedrock. It was assumed that the shale was competent and would provide a stable reference point for the bottom of the inclinometers. After a few years of monitoring, motion was also detected in the shale formation. A large array of new instruments was installed in the early 1970's designed to monitor not only the embankment fill and alluvial foundation but also the shale foundation. Numerous movement zones were observed in the shale material. Concern developed that the observed motion was the result of a progressive slide. At that time, it was assumed by the Corps that the movements were associated with settlement and would decrease with time. Following a period of a relatively low pool elevation in the late 1980's, a large (20') rapid increase in pool elevation was experienced in 1993. Minor abrupt movement rate changes were observed during and shortly after this rise in elevation. The main concern at this time dealt with the potential relationship between horizontal movement and pool thrust and ultimately the factor of safety for stability at a much higher pool elevation. The movement rates during the early 1990's were relatively small (0.0-0.25 "/yr.) making it extremely difficult, using a manual system, to determine the exact relationship with the pool elevation change if any. A system of automated in-place inclinometers (IPI) was designed and installed to enable the acquisition of more precise data. Dedicated bi-axial electrolitic probes were installed at key movement zones in the foundation. Continuous real-time horizontal movement measurements now allow for a more accurate and definitive evaluation of pool thrust and other outside influences on the safety of the dam. 15 pp., various figures, tables and photos.