Prediction of possible causes of sand boil formations using multiple geophysical surveys: A case study at Carroll county dam

Authors: WODAJO, LETI - University Of Mississippi; HICKEY, CRAIG - University Of Mississippi; MOHAMMADI, MOHAMMAD - University Of Mississippi; MACELLONI, LEONARDO - University Of Mississippi; CUMMINGS, ANDREW - Mississippi Department Of Environmental Quality

Submitted to: Fast Times: News for the Near Surface Geophysical Sciences
Publication Type: Other
Publication Acceptance Date: 10/10/2018
Publication Date: 11/1/2018
Citation: Wodajo, L., Hickey, C., Mohammadi, M., Macelloni, L., Cummings, A. 2018. Prediction of possible causes of sand boil formations using multiple geophysical surveys: A case study at Carroll county dam. Environmental and Engineering Geoscience FastTIMES. Vol 23, Number 3, pages 68-74.

Interpretive Summary: The safety and integrity of aging earthen dams is difficult to assess, and geophysical methods, such as seismic refraction, are being adapted for the purpose of testing the dams. Several seismic refraction and electric resistivity surveys were conducted at Carroll County Dam in Northern Mississippi. The as-built plan, historic topographic maps and available borehole information were used to aid the interpretation of the geophysical data. Results from the study indicate two possible causes leading to seepage both related with dam pre-impoundment conditions. A group of geophysical anomalies cluster near an old pre- impoundment stream channel, suggesting that water is seeping through this channel. A second group of geophysical anomalies suggests that water seepage is occurring through a small lens of silty sand (higher porosity) embedded within a clay layer that was not properly sealed during preparation of the base of the dam. The data collected during this study can be used to plan for remediation of Carroll County Dam, and the methods can be used to improve the assessment of other earthen dams in the future.

Technical Abstract: Carroll County Dam (North Mississippi) is an earthen dam approximately 9m high and 229m long, built in 1965 to obstruct an existing little creek and creating a reservoir of approximately 0.12Km2. In 2016, during a routine visual inspection, sand boils were observed downstream of the dam. The dam usually retains very little water; however, exceptional flood events could significantly increase pore water pressure and potentially lead to dam failure. For this reason, during the spring of 2017, the National Center for Physical Acoustics in collaboration with the Mississippi Department of Environmental Quality Dam Safety Division conducted extensive geophysical measurements to investigate the dam conditions. Several seismic refraction and electric resistivity surveys were conducted at the dam. The as-built plan, historic topographic maps and available borehole information were used to aid the interpretation of the geophysical data. Results from the study indicate two possible causes leading to seepage both related with dam pre-impoundment conditions. A group of geophysical anomalies cluster near an old pre impoundment stream channel suggesting water is seeping through this channel. A second group of geophysical anomalies, suggests that water seepage is occurring through a small lens of silty sand (higher porosity) imbedded within a clay layer that was not properly sealed during preparation of the base of the dam.