DESIGN OF MONITORING AND DEWATERING INSTRUMENTATION FOR STABILIZATION OF RECEDING LAKE MICHIGAN BLUFFS

Coastal bluffs composed of cohesive glacial sediments along the shorelines of the Great Lake recede at long-term rates of ~0.3 m per year. In a nine-year study of bluff failure processes and displacement rates, perched groundwater was identified as a major factor in bluff recession, independent of wave action and lake level. Perched groundwater is particularly important in stratigraphic settings in which perched aquifer units intersect land surface in mid to upper bluff-face positions. Three bluff sites in Allegan County, Michigan were selected to assess the effectiveness of dewatering in bluff stabilization under the U.S. Army Corps of Engineers Section 227 program. Three types of dewatering systems were installed: (1) passive, bluff-parallel horizontal wells at the top of the bluff to intersect perched water flowing toward the bluff face; (2) passive drains composed of subhorizontal PVC wells of the bluff face in low-permeability units; and (3) an active system composed of vertical wells with submersible pumps in more permeable units on the bluff face. Water produced from all systems was drained by gravity in heat-tape wrapped conduit down the bluff face to seepage cisterns embedded into the beach at the base of the bluff. A network of vibrating-wire piezometers and inclinometers was installed in the bluff face to measure three-dimensional bluff displacements and potentiometric surfaces in previously defined slump blocks. Control sites were established adjacent to the dewatered slopes. All instruments on the unstable, steeply sloping bluff faces (30-40 degree slope) were installed by a drill rig mounted on a steel platform raised into place by a large industrial crane operating from the beach at the base of the ~ 25 m high bluffs. Real-time data including inclinometer displacements and water level fluctuations are monitored via wires leading to an instrument shed and by telemetry to computers in the lab located 60 km from the site. Pump discharge data are manually read frequently from flow meters at each well. Data are also up-loaded to a web site set up for the project. Initial dewatering and monitoring results are discussed in a companion paper (Chase et al.).