Comparing Geotechnical to Geologic Estimates for Past Overburden in the Pierre-Hayes, South Dakota Area: An Argument for In-situ Pressuremeter Determination

A knowledge of past overburden thickness is useful for designing underground structures such as waste repositories. This study attempts to determine d a correlation can be made between a geologic estimate and two types of geotechnical calculations of past overburden thickness. i%r the PierreHayes, South Dakota area, we have found that of the hw geotechnical determinations, the in-situ pressuremeter is better than the laboratory consolidation determination as an indicator of past overburden thickness. In the PierreHayes area, Late Cretaceous Pierre Shale is the only bedrock present, but clasts of the Miocene Ogallala Formation were found in the Pleistocene deposits, suggesting that rocks of the Ogallala Formation once covered this area Based on the geologic estimate, the Ogallala surface was 1,100 fl higher than the present surface. Of the two types of geotechnical data acquired for the Hayes site, the laboratory over consolidation ratios indicate a past overburden thickness value of 2,300 fl, whereas the in-situ pressuremeter over consolidation ratios indicate 1,318 IY. We, therefore, believe that in-situ determination is a better indicator of past overburden that the laboratory results. However why the two test results dir7er to this degree is unknown at present.