MINERAL AND CHEMICAL CHARACTERISTICS OF SOME QUATERNARY CLAYS AND THEIR RELATION TO STANDARD ENGINEERING PROPERTIES

Construction of a water treatment plant in Alexandria, Kentucky, encountered clay-bearing materials including Kope (Ordovician) shale and Quaternary glaciolacustrine and alluvial sediments. Shelby tube samples were studied to investigate the relationship between mineralogy and engineering properties. Kope shale consists primarily of illite and chlorite while the Quaternary clays contain various amounts of expandable mixed-layer clays. The engineering tests measured moisture content, liquid and plastic limits, standard proctor, and unconfined compressive strength. The unconfined compression strengths of the in-situ Shelby tube samples ranged from 2,700 to 5,200 pounds per square feet (psf), thereby suggesting approximate shear strengths of 1,350 to 2,600 psf. Soils with such shear strengths can be classified as stiff to very stiff soils based on standard classification systems. Furthermore, it was observed that when the same samples were remolded, their respective compressive strengths decreased by 50%, implying a 50% decrease in shear strength for the remolded samples with respect to the in-situ samples. Thus, these remolded soils can be classified as medium stiff to stiff soils based on standard classification systems. For two tested samples, liquid limits were determined to be 69% and 82% with respective plastic limits of 25% and 31%. Hence, the plasticity indices for the two tested samples were 44% and 51%, respectively. These values suggest that the soil samples are fat clays (CH) in a highly plastic state based on the Unified Soil Classification System. In-situ moisture contents were approximately 10% to 20% higher than the plastic limits obtained for the corresponding samples, leading to the conclusion that such soils will deform plastically under confining compressive loads. It was also noticed that the moisture contents obtained from the standard proctor tests were 10% to 20% above their respective optimum water content. Such results imply highly plastic behavior, and clays with these properties have typical cohesion values of about 2,000 psf when compacted. However, these clays have very low strength characteristics for long-term conditions with a very low effective friction angle of about 19º. Consequently, these soils are poor for foundation fills and pavement subbases.