A Nightmare On Elm Street: The December 26,2005 Rockslide and Subsequent Rock Slope Stabilization and Repairs, Downtown Montpelier, Vermont

摘要

On December 26, 2005, between 4:15 and 6 p.m., a rock slide occurred in downtown Montpelier, Vermont, starting near the crest of a 100 feet (ft) high rock slope below Cliff Street during a heavy warm rain event following a period of sub-freezing temperatures. About 2,700 cubic yards of rocks, soil and debris slid downslope and onto Elm Street, with several large rocks striking an apartment building. Above ground utility lines were cut by falling rock, and the area lost power and communication services. Damage to Cliff Street included loss of about 5 ft of pavement and 70 ft of guardrail. Both Elm and Cliff Streets were immediately closed to traffic. Mapping, drilling and inspection via rope rappel indicated the failure occurred due to flexural toppling of strata near the crest dipping steeply into the slope, and chevron toppling mid-slope in a weathered bedrock zone about 10 ft deep. The strata consist of interbedded phyllites, calcareous metasiltstones and graphitic slates of the Devonian/Silurian Waits River Formation, complexly folded and foliated, and subjected to differential weathering. The rock mass strength near the crest was decreased by weathering, soil infilling and root jacking, which enhanced dilation of fractures and bedding planes. Slope repairs were completed between mid-January and early May, 2006. The repairs consisted of several slope stabilization construction methods, including mechanical and hand scaling, construction of a shotcrete/rock nail retaining wall, pattern and spot rock bolts, Tecco stabilization systems on steep soil slopes, dental shotcrete, a rockfall drape, and installation of drain holes.