Mississinewa Dam Settlement Investigation and Remediation

The Mississinewa Lake Dam is located in northern Indiana on the Mississinewa River, 7.1 miles above its confluence with the Wabash River, approximately 65 air miles northeast of Indianapolis, Indiana. The Mississinewa Lake project consists of an 8,100- foot long embankment, consisting of compacted impervious earth fill, having a maximum height of 140 feet, and a 36-foot crest at elevation 797. The embankment was completed in 1967, and the dam was placed in full operation in 1968. During construction of the outlet works and left abutment, karstic limestone was found to be prevalent. As a result, impervious cut-off trench with dental treatment into competent limestone was required in these areas. Unfortunately, the right abutment was nearly complete when the left abutment foundation was characterized. Thus, the right embankment was founded on glacial outwash materials overlying karstic limestone. In 1988, project personnel noticed an anomalous “depression” in the guardrail on the right embankment. Re-evaluation of the data from the surface displacement monuments confirmed that 300-400 feet of the dam was continuing to settle about 0.035-foot per year. By 1999, the crest elevation in the settlement zone is currently 0.9-feet lower than the initial crest elevation after construction and showed no signs of stopping. In fact two aluminum slope inclinometers, in the area of distress, were destroyed as a result of the dam settlement. Subsurface investigations revealed a 25-foot deep, clay-filled, solution feature beneath the settlement zone. As a result, operational restrictions were placed on the structure until foundation remediation could be performed. The remediation consists of a 2,600-foot long, concrete cutoff wall extending down at least 150-feet into competent limestone. The cut-off wall is being installed with the use of clam-shells and hydromills. During construction, several problems arose, including sudden slurry loss and solution features which were much deeper than expected. A grouting program was initiated in advance of the cutoff wall installation to explore conditions in front of the hydromills and to help reduce the risk of sudden slurry loss. The construction history, settlement investigation, and foundation conditions will be reviewed to set the stage for current remediation effort discussion. 15 pp., 9 figures.