Soil Testing / Phreatic investigation: Lake Superior Shoreline Stabilization
Lake, Cook and St. Louis Counties,
Minnesota Board of Water and Soil Resources
Basin Program Funds:
There is very little applicable soil testing and phreatic, or groundwater, surface
information available for the red clays and silts that compose a significant
portion of the non-rock Lake Superior shoreline in Lake, Cook and St. Louis
Counties, Minnesota. Without this information, conservative assumptions must
be made for slope stability analyses which, in turn, lead to relatively uncertain
and conservative slope stabilization designs. The goal of this project is to
create a base of information to address these situations, reduce the costs of
shoreline stabilization designs and stretch cost-share funds to more projects.
The North Shore of Lake Superior is often perceived as being entirely rocky.
There are however, approximately 60 miles of Minnesota shoreline on the lake
where the bedrock dips to or below the lake level and red clay and silt deposits
up to 70 feet high. Over geologic time, cobbles and sand veins in the clay eroded
into bolder-strewn and cobbled bays which provide excellent habitat for fish
and other aquatic life.
Nonpoint source pollution is one of the major threats affecting
water quality in Lake Superior. These threats are due to the erosion and sedimentation
of the area's red clay soils. Tourism and development is increasing at a rapid
rate along the north shore of Lake Superior in Minnesota and is imposing increasing
pressures on the resource. Approximately 36 miles of Minnesota's Lake Superior
shoreline have been identified as high erosion hazard areas.
Many of the eroding shoreline areas are landslide prone and must
be stabilized in order to control erosion and maintain water quality. Engineering
solutions to these problems are restricted by a lack of readily or directly
applicable information on soil composition, stability and other relevant characteristics.
This situation makes it more difficult for North Shore landowners to compete
for state-wide cost-share funding for conservation practices. This project is
intended to develop a database of information to help in these situations.
The project team reviewed potential project sites and selected three demonstration
sites with high, eroding clay banks. They installed 18 piezometers at the project
sties. The team reviewed several designs and installation methods and chose
the most effective, low-cost design. During installation, approximately 62 soil
samples were collected. The team reviewed soil logs and initial piezometer performance
in order to determine which samples to send to the lab for plasticity index,
sieve analysis, direct shear and water content. At the same time, the team took
weekly piezometer readings during the project period.
The team held an informal project workshop discussing the results with Dr. Peter
Bosscher, University of Wisconsin. Site design has been revised given the results
of the slope stability analysis. If projects on the three demonstration sites
are implemented, project personnel estimate a total of 16,420 tons of soil saved
over the projects' estimated 20 year life-span. An additional 164,200 pounds
of phosphorus and 32,840 pounds of nitrogen will also be saved. The project
team has met with seven landowners during four site visits during the project
period. They estimate contacting at least an additional 20 landowners whose
property will benefit from the knowledge gained here in the future.
Contact: Gene R. Clark, (218) 723-4752