Slope Stability Analysis and Ground-Water Hydrology in Heterogeneous Glacial Material: Elements for Prediction of Bluff Erosion

摘要

A detailed stratigraphic/geotechnical analysis and three-year monitoring of six hydrostratigraphic scenarios among Lake Michigan shoreline bluffs has been conducted using cross-section balancing and limit equilibrium modeling techniques. Bluffs show no major displacements of glacial materials where perched ground water is absent. Bluffs containing perched water are stable if composed of sand, but unstable where sand and clay are interlayered. Shallow, planar slumps occur where the clay is mostly till, but slumping is more deep- seated and frequent where lacustrine silt/clay layers are present. Displacements are largely by simple shear and by fault-propagation folding. Comparative records of displacements, water table levels, atmospheric temperatures, precipitation, and wave heights shows that: (1) displacements are minimal in the summer and early fall, but accelerate in the late fall and remain rapid through early spring; (2) wave erosion is greatest in the late fall and spring, and nil during the winter; and (3) surface freezing and a rise of perched water levels occur together. Bluff degradation is caused largely of wave action in the fall, freezing of the bluff surface which raises pore pressures and reduces effective stress during the winter, and ground-water release during the early spring thaw.