Scarp Occuring During Construction
Wrangell Airport is located along the Inland Passage waterway in southeastern Alaska on northwest side of Wrangell Island. The project required extension of the runway to improve airport safety and aircraft operations in accordance with Federal Aviation Administration (FAA) standards. Plans included lengthening the runway on both ends to expand the Runway Safety Area (RSA), which included construction of embankment fills into the waterway at both ends. The project required placement of approximately 1.5 million cubic yards of fill material to extend the runway approximately 400 feet to the northwest and 800 feet to the southeast. The fill placement was to occur over extremely soft marine sediment slopes, in water depths of up to 40 feet.
Landslide Technology was retained by the State of Alaska Department of Transportation and Public Facilities (AKDOT&PF) to perform geotechnical investigation and preliminary design for the runway extension. During early stages of the investigation a layer of very soft, sensitive marine sediment with thicknesses up to 23 feet was encountered at the northwest end of the runway. In-situ testing of these materials indicated that they experienced significant strength loss when disturbed or remolded. Based on these results, AKDOT&PF determined to shorten the length of the runway extension in the northwest direction to minimize slope stability and constructability concerns on the soft sediments.
Evaluations included an off-shore exploration program with borings, in-situ vane shear testing, and geophysical testing. Swift currents in the Inland Passage and tidal swings of up to 20 feet complicated offshore geotechnical investigation procedures. A barge-mounted drill rig was used to perform investigations through unconsolidated marine sediments into underlying alluvial sands, gravels and hard bedrock materials. Vane shear tests were performed to determine undrained shear strength and residual strength of in-situ materials. An extensive laboratory testing program was implemented to develop settlement, strength and hydraulic parameters critical to engineering evaluations. Recommendations were provided for marine sediment sensitivity, treatment methods for marine sediment, slope stability evaluations, settlement analysis, and construction. Evaluations considered multiple options to treat stability concerns related to the sensitive marine sediments, including an excavate and replace approach, ground improvement techniques, a retaining wall founded on denser materials, and staged-fill construction.
The contractor’s plan for treating the soft marine sediment utilized a “rolling surcharge” method, where rockfill was progressively pushed from the shoreline causing the marine sediment to fail and be displaced. This approach resulted in significant safety concerns for contractor and AKDOT&PF personnel. A comprehensive safety plan was developed to reduce risk to personnel, aspects of which included a test fill program, allowable levels of fill placement, restricted work during extreme tidal fluctuations, analysis of impacts to the existing runway embankment and testing to verify that soft marine sediment had been adequately displaced. Landslide Technology provided technical support to the owner to review the contractor's plans and submittals, observation of construction approach, and technical review of verification testing of marine sediment removal/densification.