Geotechnical Characterization of the Steeply Dipping Pierre Shale

This study compares alternative approaches for differentiating and characterizing the swelling potential of steeply dipping strata in the Pierre Shale along the Colorado Front Range Urban Corridor. The hazard associated with this geologic setting is commonly referred to as heaving bedrock because adjacent strata expand to different degrees and may cause damaging differential deformation to residential and light commercial structures. This study is part of a larger investigation aimed at determining the feasibility of using reflectance spectroscopy, a remote sensing technology, to identify and characterize the swelling potential of expansive soils and bedrock. A trench in the Upper Pierre Shale was logged and 252 samples were taken horizontally with thin-walled brass tubes. Grain size, Atterberg Limits, filter paper suction, and clod tests provided, for each of the samples, the swell potential indices and rankings according to schemes proposed by Seed, Chen, and MeKeen. Swell-consolidation tests provided percent swell and swell pressure indices under in-situ surcharge pressures. All the indices are consistent in that they suggest the trench wall can be differentiated into two zones of steeply dipping strata overlain by a horizontal and shallow overburden soil. However, the schemes differ in that McKeen's indices vary over a significantly broader range of categories compared with the Seed and Chen indices. McKeen's indices and also the swell-consolidation indices have two advantages compared with Seed's and Chen's indices. They are obtained on relatively undisturbed materials and they are properties involved in quantitative heave prediction methods. These advantages suggest that McKeen's scheme is providing the most sensitive and useful basis for differentiating semi-quantitative categories of swelling potential at this site.