Cyclic Failure (â€œLiquefactionâ€) Mitigation of Loess in the New Madrid Seismic Zone-PPT Presentation

PPT Presentation

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The presentation examines cyclic failure, often called liquefaction, mitigation at Bear Creek Lake Dam near Marianna, Arkansas. Built in 1938, the 50-foot-high, 1,000-foot-long earthen dam is classified as high hazard because roads and structures lie downstream. Its spillway has experienced repeated problems, including settlement and rotation in 1980, repairs in 1993, and slab overturning with extensive erosion in 2019.<br /><br />The dam is founded on loess: wind-deposited, silt-sized soil that is loosely cemented, can stand in steep faces, and may erode or collapse when wet. Its behavior during earthquakes is difficult to classify. Screening of soil samples produced mixed indications of susceptibility, with results not consistently related to depth or location. The loess is best treated as transitional silt rather than clearly sand-like or clay-like material.<br /><br />The analysis considered sand-, clay-, and silt-based methods for estimating cyclic resistance. The silt method, which accounts for plasticity and stress history, may be the most suitable available approach, but such soils remain poorly represented in case histories and are challenging to assess. Limited ground-motion data for the eastern United States adds uncertainty.<br /><br />Deep soil mixing was considered as a mitigation measure. However, the presentation found that existing design methods and guidance were not directly applicable to this loess setting. Analyses explored factors of safety and treatment area replacement ratios, but uncertainty remains about the appropriate target factor of safety.<br /><br />Overall, the project highlights the difficulty of evaluating and mitigating earthquake-induced cyclic failure in loess, particularly given limited regional data and the lack of well-established methods for transitional silts.

Keywords

liquefaction

cyclic failure

Bear Creek Lake Dam

loess

earthquake mitigation

deep soil mixing

cyclic resistance

spillway erosion

transitional silt

high hazard dam