FACING A GRAVE ISSUE: ASSESSMENT OF THE MT. CALVARY CEMETERY SHOPYARD LANDSLIDE, PORTLAND, OREGON JANUARY 13, 2006

On Friday, January 13, 2006, a large landslide event took place on the north facing slope of Mt. Calvary Cemetery near the facility maintenance shop yard. This landslide had an average scarp height of 6 meters, a scarp width of 70 meters, and a total length of 220 meters. The approximate volume of material displaced by the landslide was calculated to be 6100 cubic meters. Common the to the West Hills of Portland, this landslide is classified as an earthflow. While onsite, faster moving liquefied soils characteristic of debris flows were observed. Four major elements combined to allow this earthflow to occur including steep slopes associated with the position on the crest of the Tualatin Mountains, geologic factors associated with the weak strength loess soils, climatic factors associated with rainfall volume and intensity of late December 2005 to early January 2006, and human factors associated with placed excavated fill on the upper slopes near the shop yard. The primary contributing factor seems to have been the increased water in the soil system, although the surcharging of the slope with placed fill also had a significant contribution. Landslides in the West Hills constitute the majority of slides occurring in the Portland area (Burns et al., 1998). Of major concern regarding landslides in the West Hills, are the hazards to people and property located in their paths. Fortunately, no homes or structures were directly impacted by the landslide. Mitigation included tarps covering the scarp and upper slide, revegetation of the whole slide with grasses and planting conifer trees on the lower part of the slide. The toe is buttressed by woody vegetation and should remain stable.