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Nebraska Specific Slope Design Manual
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2018-12-01
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Edition:Final Report (July, 2016 – Dec. 2018)
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Abstract:This study introduces proper design and retrofitting techniques to mitigate slope failure in Nebraska based on experimental and analytical research. The geological history, unique soil properties, failure mechanisms and potential new design/retrofitting methods are accounted for in this study. The following are the key findings. Nebraska is covered by glacial tills, loess/sands, and shales with expansive clay minerals. Loess and shales are often highly overconsolidated due to the high overburden from Laurentide ice sheet which covered North America between 0.1 M to 0.02 M years ago. Overconsolidated expansive shale clays often exhibit time dependent strength reduction, and collapsible loess often exhibit much lower residual strength than the peak strength. The test results showed the presence of overconsolidated conditions and expansive clays. Test specimens show that the unconfined compression strength and the consolidated undrained strength are higher than the consolidated drained shear strength for clayey soils; typical behavior of overconsolidated conditions. Also, test specimens showed that the residual strength of unconfined compression and the consolidated drained shear strength were substantially lower than that of unconfined compression and consolidated undrained shear strength; typical behavior of clays containing expansive clay minerals. Some test specimens even showed volume expansion during consolidation. XRD (X-ray Diffraction) tests showed montmorillonite clay minerals (as high as 11%) in specimens, explaining the expansive behavior and lower residual drained shear strength. Although unconfined compression tests are easy and inexpensive, this method may overestimate the long term strength of Nebraska soils. This study recommends that the consolidated drained strength be used for the design of new slopes and retrofitting techniques. Among several retrofitting techniques, earth anchor and biopolymer based reinforcement are recommended. Earth anchors are recommended because the resisting force is provided by deep soil layers which are free from weathering and associated strength reduction. Biopolymer based soil treatment is recommended because the technique showed promising weathering resistance in this research. However, biopolymer based soil treatment technique is not thoroughly verified, further verification research may be needed.
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[PDF-10.99 MB]
