Geotechnical investigation of sand flow slides, Haines, Alaska

Abstract

The Lutak Spur (LS) - a prominent glacio-deltaic landform located near Haines, Alaska - formed during the retreat of the Cordilleran Ice Sheet during the Last Glacial Maximum. This study evaluates its geologic evolution, soil development, and hydrologic-slope stability response to a December 2020 atmospheric river (AR) event, which triggered multiple slope failures. Geologic mapping and interpretation of previous work suggest that the LS formed as an ice­ contact kame delta at the margin of a retreating valley glacier. Following deglaciation, post­ glacial isostatic adjustment and sea-level changes exposed the LS surface, allowing for the development of Spodosols with iron-cemented (Fe-cemented) horizons beneath forest vegetation. We used field observations, laboratory testing, and surface drainage mapping to assess slope behavior. Laboratory tests indicated that the Fe-cemented layers contribute cohesion to the strength of near-surface soils with a cohesion of 80 kPa, an average friction angle of 30.9°, and a hydraulic conductivity of 7.6x10-3 cm/s, while the underlying stratified sand exhibited an average friction angle of 36.4°, and an average hydraulic conductivity of 9x10-3 cm/s. Based on measured precipitation and NOAA Atlas 14 precipitation frequency estimates (PFE), the December 2020 AR event approached the intensity of a 1,000-year storm. Hydrologic modeling using HEC-HMS indicated that peak discharges during this extreme precipitation event were approximately three times greater than those produced by a modeled 100-year storm. Seepage modeling in SEEP/W demonstrated that infiltration during the December 2020 AR event elevated groundwater levels and pore-water pressures. Slope stability modeling in SLOPE/W indicated dry slopes were stable, but removal of the organic mat and Fe-cemented layers reduced the factor of safety (FS). Under saturated conditions, the FS dropped below 1.0, consistent with the occurrence of slope failures in 2020. Field evidence, residents’ observations, and modeling results support our hypothesis that tree throw at the slope crest disrupted the Fe-cemented sand stabilizing layer, triggering failures during the AR event. These results suggest that extreme precipitation, in combination with tree throw and surface disturbance, reduced slope stability through transient increases in pore-water pressure and loss of near-surface strength.