DEFICIENCIES OF GEOTECHNICAL SITE CHARACTERIZATION FOR DESIGNING GROUND WATER MONITORING SYSTEMS IN THE GLACIAL SETTING - A CASE STUDY

摘要

The Unified Soil Classification System (USCS) is the standard for classifying soils for geotechnical and hydrogeologic site characterizations. It has, however, also been mistakenly used as the basis for the identifying and correlating site-stratigraphic units. This case study from northeastern Illinois demonstrates the necessity for recognizing and understanding depositional environments in order to successfully identify and characterize geologic units comprising a typical Midwestern glacial succession. These glacial successions are typically composed predominantly of fine-grained glacial deposits that classify similarly in the USCS, but differ significantly in geotechnical and hydraulic properties. The precise recognition and characterization of the fine-grained glacial units that dominate these successions becomes critical when evaluating the significance of noncohesive sediments that locally occur within the succession. Noncohesive sediments stratigraphically located within the individual fine-grained glacial stratigraphic units tend to be thin, discontinuous lenses of limited lateral extent that are not suitable as groundwater monitoring zones whereas noncohesive sediments occurring between the different fine-grained glacial units tend to be laterally extensive and constitute suitable groundwater monitoring zones. Another result of this study was that the sampling technique proved to be critical for the correct characterization of these glacial deposits. During prior field programs, split spoon samplers had apparently been clogged with stones from within the finegrained diamicton, and the sampling interval was mistakenly interpreted as gravel, resulting in the placement of dry wells in stony lean clay glacial deposits. The large diameter rotasonic samples used for this case study provided nearly complete sample recovery and enabled the correct sample identification to be made.