Evaluation of straw wattle placement and surficial slope stability Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/mc87pt98z

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  • Straw wattles are common erosion control devices used to trap sediment. This thesis studies the relationship of straw wattles on slope stability through a case study demonstrating their use on steep slopes (1.5H:1V) for the US20 highway realignment project. Several surficial slope failures have occurred on these fill slopes, often bracketed by straw wattles, which were hypothesized to contribute to the slope failures. To date, little is known about straw wattle placement and its effect on surficial slope stability. Prior studies have evaluated slope stability against slope height, slope angle, vegetation, rainfall, and other variables but have not assessed the influence of straw wattle placement on surficial stability. Several laboratory tests were performed to characterize the fill soil and the straw wattles for numerical modeling and evaluation. Straw wattles were shown to quickly absorb a substantial amount of water (a water content of 400% within 15 minutes) and require a substantial amount of time to dry (several days at high temperatures). Several modeling scenarios were run (varying the slope angle, slope height, straw wattle spacing and climate condition) to determine the overall effect of straw wattles on deep and surficial slope stability. Overall, straw wattles were shown to have no significant effect on surficial slope stability, particularly compared to modeling uncertainty and soil variability. Of the 366 models run, 26% showed a change in factor of safety (0.006 on average) against surficial slope failure when straw wattle spacing was increased. Over half of the 26% showed a decrease in factor of safety. Other influencing factors such as slope angle, ground water elevation and environmental conditions have a much more significant impact on slope stability. The slopes themselves were found to have a low factor of safety (≤1, at the limit of equilibrium) against surficial slope failures and a reasonable factor of safety (>1.5) against deeper failures, regardless of straw wattle spacing. Investigations using 3D laser scanning verified that straw wattles were installed along the same slope contours, therefore, not allowing water to pond behind the straw wattle and decrease the factor of safety against surficial slope failure.
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