MITIGATION OF SLOPE FAILURES IN GREAT LAKES COASTAL ENVIRONMENTS BY REMOVAL OF GROUND WATER

A nine-year study of slope displacements at three perched ground water sites in glacial soils along the Lake Michigan coast shows that mass movements occur largely when bluff faces are frozen and beach ice prevents wave erosion. Slope displacements occur under reduced effective stress conditions when: 1) excess ground water is stored behind frozen bluff faces; and 2) the thaw release of that excess water maintains flow pressures until normal water levels are restored. Erosion from storm waves contributes to the timing and magnitude of displacements, but is not their primary cause. A massive experiment is currently being conducted jointly by WMU and the USACE to test the removal of excess winter/spring ground water as a failure mitigating strategy. During 2003-04, three stratigraphically diverse sites were equipped with 44 in-place inclinometers, 28 VW piezometers, 25 vertical pumping wells, 20 bluff-face horizontal drains, and two weather stations. Displacements during dewatering are being correlated with activities at nearby control sites.. The water removal systems were activated on 17Dec04 and deactivated on 07May05. The following observations apply: 1) slump block permeability is controlled by clay layers smeared along shear planes; 2) ground water flow directions beneath frozen bluff faces periodically changed; 3) displacement magnitudes in the control sites were at least four or more times greater than those in dewatered sites; and 4) at nearly all displacement locations movements were timed on a daily basis with bluff face freezing, or movements occurred during the next-day thaw and seep activity. The dewatering cycle will be repeated periodically for several more years to evaluate water removal versus non-removal under a variety of climate conditions.