Slope Stability Analysis
The slope stability analysis is crucial for evaluating the Alpha Iota lakefront condition. Through this analysis, the present condition and factor of safety of the slope may be evaluated to determine if the lakefront is in need of revitalization.
Slope Stability Results
Photo from the site investigation. The yellow star is a reference point for Figures 1 and 2.
Left: View from the top of the wooden retaining wall. Right: View from the base of the wooden retaining wall. Both: The yellow star is used as a reference point for Figures 1 and 2.
Geological records at the Alpha Iota Site. Records are from the United States Department of Agriculture (USDA) Natural Resources Conservation Services (NRCS). Alpha Iota is denoted with the yellow star.
The first action taken for performing the slope stability analysis was a site investigation at Alpha Iota. During this investigation, the project team assessed areas of concern along the lakefront. Along the majority of the Alpha Iota lakefront slope there is a stone retaining wall, which from visual inspection appears to be in good condition. This stone retaining wall was not the main area of concern; on the east side of the site there exists a wooden retaining, which appears to be in a questionable to poor condition. The conditions of these retaining structures as of November, 2017 may be viewed in Figures 1 and 2 below.
As shown in the left image of Figure 2, the wooden retaining wall appears to be showing signs of deterioration and approaching failure. It appears to have moved from its original position, as it is not at the same level as the adjacent stone retaining wall. The wooden retaining wall can also be seen starting to slump over the slope, which further implies it is danger of failing.
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The right image of Figure 2 was taken from the base of the slope below the wooden retaining wall. In this image, it may be noticed that the base of the slope appears to be steeper than the rest of the Alpha Iota sight. This part of the slope has exposed soil and does not appear to have any existing vegetation or roots, which implies there have been higher rates of erosion or even minor slope failures.
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For all of the reasons stated in the previous two paragraphs this area on the east side of the Alpha Iota was deemed the most critical and concerning slope at the site. This is why it was chosen for the slope stability analysis model.
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The project team used an inclinometer (as shown in the image at the top of this page) to measure the slope angle, which was found to be approximately 30 degrees. This slope angle, along with an estimated slope height of 20 feet, was used to create the profile in the model.
Geologic data was gathered from the United States Department of Agriculture’s (USDA) Natural Resource Conservation Service (NRCS). An example of this data is shown in Figure 3. The soil abbreviations in Figure 3 correlate to the following USDA soil classifications:
Ap – Silt Loam
2BE – Loam
2Bt1 – Clay Loam
2Bt2 – Sandy Clay Loam
2Bt3 – Sandy Loam
2C – Gravelly Sandy Loam
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These classifications, along with textbook soil property values [5], were used to estimate the soil properties used in this slope stability analysis. The ground water table was assumed to start from the boundary of Lake Mendota and gradually increase as the ground elevation increased.
After all of information previously described was collected, a digital model of the area of concern at Alpha Iota could be produced. The software WinSTABL, a slope stability analysis software, was used to run this model. Results from the model are located on the Slope Stability Results page.