REGIONAL GROUNDWATER CONTROL OF SLOPE STABILITY IN THE GLACIAL LAKE DULUTH BASIN, MINNESOTA

Glacial Lake Duluth occupied the western end of the Lake Superior Basin dammed between a series of moraines on the west and the active ice margin on the east. At least three and possibly four phases of the lake existed between successive ice advances. The resulting stratigraphy consists of thin (4-7 meters) shallow water lacustrine sequences interbedded with thick (up to 40 meters) lacustrine clays or clayey tills. The shallow-water sands are hydraulically connected to sediments of a large moraine complex composed primarily of coarse outwash. Groundwater recharge and resulting high hydraulic head in the moraine result in artesian conditions in the hydraulically-connected sands of the Lake Duluth sedimentary sequences.

The regional topography slopes toward the axis of the Lake Duluth Basin and the potentiometric surface of the confined aquifers exceeds the surface elevation throughout much of the central part of the former lake basin. In this area groundwater seeps are common. Groundwater seeps are concentrated in areas where the potentiometric surface intersects the land surface. One occurrence of groundwater discharge allows analysis of the failure potential of the lacustrine clays. Groundwater discharge is occurring around the perimeter of a drained beaver pond. The discharge is focused at discrete points that are identified by the presence of large sand volcanoes. The sand volcanoes are located along fault scarps that are the surface expression of rotational slumps. A Mohr-Coulomb failure analysis suggests that small reductions in shear strength or small local increases in shear stress lead to failure of the clay by rotational slumping. Our analysis indicates that even the damming or draining of a beaver pond three meters deep can trigger slope instability. Once groundwater has a conduit to the surface along fault scarps, volume loss of sand from the aquifer perpetuates the processes of slumping, dewatering, and further aquifer volume loss.