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Characterization, analysis, and remediation of the Cedar Pass landslide complex, Badlands National Park, South Dakota
Radach, Kyle C.
Radach, Kyle C.
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2020
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Abstract
The Cedar Pass Landslide Complex is located in the North Unit of Badlands National Park, South Dakota. The National Park Service has had to regularly maintain the approximately 1.25 km section of Badlands Loop Road (South Dakota State Hwy-240) that travels through the landslide complex. Road surface distress caused by slope movement and other natural processes in the Cedar Pass area have created a financial burden for the park, as the Park Service is responsible for maintenance of the highway. While there has been successful mitigation work to stabilize portions of the road, stability and erosion problems have persisted. Maintenance and mitigation work completed since the 1990s include the construction of two large earth buttresses, roadway resurfacing, regular crack sealing and asphalt patching, grinding to smooth surface offsets, and the installation of a new stormwater collection and conveyance system. This study used a combination of field reconnaissance, sample collection, laboratory testing, and slope stability modeling to estimate and delineate boundaries of several landslides in the Cedar Pass Landslide Complex, assess the current stability, and investigate the sensitivity of these landslides to factors that may increase or decrease stability. These factors include fluctuations in groundwater, reduction in shear strength of landslide materials, and erosion within the landslide mass. Additionally, the effectiveness of mitigation measures was investigated using a probabilistic analysis to identify those methods that result in the greatest increase in stability. Direct shear testing was carried out to measure the drained residual shear strength of soils in the complex, and Atterberg limits and grain-size distributions were measured to characterize soils and to estimate residual strength using an empirical correlation developed by Stark and Eid (1994). Slope stability modeling was conducted using two-dimensional limit equilibrium methods. Results show that highway surface damage in the complex is related to a combination of both movement in smaller, unique areas and movement of much larger landslides. For instance, damage to the Cliff Shelf parking lot is related to destabilized areas above the head scarp of the Prairie Island Landslide located to the southeast. This is compared to highway distress in the Upper and Lower Wedge areas that may relate to settlement and erosion of an embankment fill and continued deformation of the Cliff Shelf Landslide which was thought to be dormant until the late 1990s. The overall slow movement of the landslides observed over the past 30 years may be attributed to dilatant strengthening, which suggests that the landslides present a risk to park infrastructure (mainly the highway), but pose less of a danger due to sudden movement. However, it is prudent to assume that more rapid failure may be possible if climatic and geologic conditions change, specifically, if average groundwater levels across the complex increase or if soils along the landslide slip surfaces reach the critical state density. Mitigation of these landslides may be possible on a localized scale with the construction of earthen buttresses, gravity retaining walls, tieback walls, and/or improved slope drainage. Mitigation of the larger landslides such as the Cliff Shelf Landslide is less feasible due to their size. Therefore, highway distress and deformation may continue and it is recommended that highway improvements include the addition of a flexible pavement or road base that can help distribute deformation and may decrease the frequency of required maintenance in certain areas.
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