Larson Design Group (LDG) provided geotechnical services related to the investigation of a landslide at Dorsey Knob Park for the city of Morgantown, Monongalia County, West Virginia. The project’s goal was to assess the subsurface conditions, determine the cause of the slope failure, and provide remediation recommendations. Work included a subsurface exploration using 14 test borings located throughout the project site. Laboratory testing was conducted on select samples obtained from the test borings and data from these tests was used to classify the soils based on the Unified Soil Classification System (USCS). LDG also provided design recommendations for possible retaining structures that could be used to remediate the slide.
The slide occurred along the eastern edge of the parking lot for the Dorsey Knob Lodge, about 50 feet north of lodge. The slide was approximately 60 feet long and appeared to have originated due to top loading that occurred when fill was placed in an attempt to widen the existing roadway and parking area. The weak underlying silts were not capable of supporting the weight of the fill soils and the ground movement started. The lack of drainage accommodations beneath the existing roadway and parking areas, allowing stormwater to flow between the fill and the natural ground surface, compounded the problem.
The design recommendations for stabilizing the roadway and parking area impacted by the slide took into account the abrupt bedrock changes on the steeply sloping hillside and the need for adequate drainage of the project area. LDG evaluated three options for stabilizing the site; a fill embankment, soldier piles and a lagging wall, and a mechanically stabilized earth wall.
As the most cost-effective of the three options, LDG recommended the construction of a fill embankment along the eastern edge of the impacted parking area. This would involve removing the existing fill soils, which were prone to failure, and backfilling the area with structural fill keyed into soils capable of bearing the weight of the fill, the roadway, and parking area. Based on LDG’s preliminary calculations, if a 2H:1V fill embankment were keyed into the hillside at the approximate elevation of the bench where the water line is present, about 25 feet of additional space could be provided from the edge of the existing parking area pavement. The recommendation also included consideration for toe keyway drains, bonding bench drains, as well as a diversion ditch or French drain near the base of the stone wall.