Ground‐Water Flow Systems and Stability of a Slope

Abstract Shoreline recession on the Great Lakes has caused millions of dollars worth of property damage in the lake bordering States and Canadian provinces. A variety of projects have been funded to study the erosion problem. The general conclusion of these studies is that wave action at the base of the bluff is the most important basic cause of bluff‐top retreat. The purpose of this study was to investigate the role of various physical processes in causing bluff erosion. One part of the study was to ascertain the role of ground water in bluff stability. In this study, along a certain stretch of the Lake Michigan shoreline, 9 to 10 m (27 to 30 ft) per year of bluff‐top recession that is not correlated to toe erosion was recorded. Heads were measured in 25 piezometers and a complex ground‐water flow system was defined at this site. A glacial sand unit that is under artesian pressure was found 5 m (15 ft) above the toe of the bluff. Another “perched” system was observed in a fractured till unit at the top of the bluff. Water‐table fluctuations have been recorded over a year and fluctuations of up to 13 m (39 ft) in the lower sand unit were measured. These fluctuations were found to have a significant influence on bluff stability. Pore pressures calculated from field measurements, along with the effective strength parameters of the soil units, were then used in the slope stability analysis to determine safety factors. Ground‐water flow systems are found to be highly complex at the land‐lake interface due to the inhomogeneities of the glacial materials that compose the bluffs. Furthermore, these complex ground‐water flow systems influence the stability of the upper part of the bluffs. Because of the complexities, extrapolation of results to other stretches of the shoreline is difficult. However, methodologies and the awareness of how water‐table fluctuations and multi‐layered ground‐water flow systems affect slope stability can be of help in other field situations.