Anthropogenic Effects on River Bank Stability in a Smectitic Terrain, Red River of the North.
Donald P. Schwert, Department of Geosciences, North Dakota State University, Fargo, ND 58105-5515
The Red River Valley of North Dakota/Minnesota is the youngest major land surface in the contiguous United States, having been subaerially exposed upon the final regional drainage of the waters of Glacial Lake Agassiz ≈ 9,200 14C years ago. Underlying the Valley are soils that induce both great agricultural activity and challenging geotechnical conditions. These soils are developed on a wedge of smectitic clays and silty-clays derived predominantly from late-glacial meltwater erosion, which reworked of Cretaceous shales (most notably the Pierre Shale) and dispersed the sediments in suspension into LakeAgassiz. This wedge thickens northward along the axis of the Valley toward Grand Forks. At Fargo, the wedge is 32 m thick, with a predictable stratigraphy of ≈ 6 m of tan-buff laminated silty-clays of the Sherack Formation underlain by ≈ 26 m of gray, slickensided, fat clays of the Brenna/Argusville Formations. Typical soil engineering values for the Sherack Formation (depth: ≈ 1 – 6 m) are: PL = 30, LL = 85, N = 12, and Qu = 3000. For the uppermost Brenna Formation (depth ≈ 8 m), typical values are: PL = 31, LL = 113, N = 6, and Qu = 1370. The high shrink-swell properties of these clays induce foundation shifting, pavement failure, and utility line rupture. Where the sediments are unconfined, their high plasticity leads to slope instability; the valley walls and banks of the Red River of the North and its tributaries are particularly prone to slope failure. Despite the vulnerability, these prone areas have in recent years undergone extensive urbanization, leading both to an artificial acceleration of the clay hydration and an overloading of the clays where they are least confined. Consequent valley wall and river bank instability is resulting in new setback codes specific to this type of smectitic terrain.