Groundwater Hydraulics and Slope Stability Analysis: Elements for Prediction of Shoreline Recession
Studies documenting the roles that geological materials and groundwater play in Great Lakes bluff recession are rare. This study reports findings concerning relationships between bluff lithology, bluff hydrology, and bluff recession along a 10-mile stretch of eastern Lake Michigan shoreline in heterogeneous Pleistocene deposits. According to a newly-developed, field-tested, GIS-based methodology, bluffs in the study area can be characterized lithologically as sand, clay, or mixed sand/clay, and can be characterized hydrologically as exhibiting either high head or low head.
Geotechnical analysis indicates that Atterberg limits, index properties, consolidation state, and drainage conditions affect the shear strength of bluff materials. Silty grey diamicton, a major constituent of clay-dominated bluffs, exhibits high undrained shear strength, and <ø> = 28o. Laminated, fine-grained clay, a major constitute of mixed sand/clay bluffs, has very low undrained shear strength, and <ø> = 20o. Sandy bluffs in the study area exhibit a range of effective stress failure envelopes angles: 34o < <ø/> 43o.
Comparison of bluff characterization with historical recession patterns indicates that bluff lithology and hydrology influence recession. Mixed sand/clay bluffs with high hydraulic head were found to have receded up to 100 feet (30.0 m) over the long term (1938 to 1996), whereas bluffs characterized as all clay or all sand receded 50 feet (15.0 m) or less during this time. Over the short term (1989 to 1996), a period in which Lake Michigan water levels were not unusually high, mixed sand/clay bluffs were the only ones in the study area to show measurable retreat (> 4 feet, 1.2 m).
Results of Factor of Safety analyses performed upon bluffs at six field sites suggest that effective stress-based analyses that reflect long-term, drained conditions may be most appropriate in modeling recession in sandy bluffs of both high and low head, mixed sand/clay bluffs of both high and low head, and diamicton bluffs with high head. Diamicton bluffs with low head are apparently not destabilized by groundwater; in this case, total stress-based Factor of Safety analysis may be an alternative method for modeling recession.