Third Avenue Extension
To bypass a traffic constriction in the middle of the City of Ketchikan, Alaska Department of Transportation constructed a new roadway bypass. The Third Avenue Extension required a completely new route crossing very steep ground immediately up slope (within 150 feet) of Ketchikan residences. The road alignment traversed a 200-foot tall rock escarpment, requiring rock cut slopes for the new grade of up to 110 feet tall, and steep colluvial slopes with a 400-foot wide landslide.
Landslide Technology, a division of Cornforth Consultants, was retained by Alaska DOT&PF to assist with geotechnical investigation and design of rock cuts, rock slope stability, excavation staging, control methods for rockfall resulting from blasting and other construction activities, structure and foundation recommendations, rock blasting, and stability evaluation of embankments and walls crossing the debris flow area. Landslide Technology performed reconnaissance, rock structure mapping, extensive subsurface investigation and geotechnical instrumentation, rock slope and debris flow stability analysis, rockfall simulation analyses, a constructibility evaluation, and assisted on PS&E.
A key feature of the project included an innovative design for a roller-compacted concrete (RCC) retaining wall, which measured 1,000 feet long and up to 90 feet high. The RCC wall provided the residences with rockfall protection during excavation and blasting; and provides long-term support for an elevated portion of the roadway. Stability of the rock cut was also provided using 20- and 30-foot long, high capacity rock bolts and several rockfall barrier systems. An unstable 5,000-ton rock block immediately uphill of residences required specific design and constraint in order to be safely removed from the escarpment as part of the 110-foot tall rock cut.