GEOTECHNICAL INVESTIGATION OF PORE PRESSURE BEHAVIOR OF MUDDY SEAFLOOR SEDIMENTS IN AN ARCTIC PERMAFROST ENVIRONMENT

摘要

Herschel Island, Yukon, Canada, is made of ice-rich permafrost and is affected by high rates of coastal erosion, likely to increase with decreasing summer sea ice extent. During an inter-disciplinary expedition to Herschel Island in July 2014, geotechnical investigations were carried out in shallow water environments of up to 20 m water depth and at different beaches. The free-fall penetrometer BlueDrop was deployed at 299 positions. Apart from obtaining vertical profiles of sediment strength and the pore pressure response upon impact, the pore pressure evolution over a period of one hour after deployment was investigated. The focus area for these tests was Pauline Cove, located at the south-eastern side of the island, being sheltered by a spit from the open Beaufort Sea and affected by a number of old and young retrogressive thaw slumps, delivering large amounts of mud. The sediment resistance profiles revealed up to three distinct layers of sediment strength, expressing different consolidation states, or possibly changes in sediment composition. This stratification was supported by the pore pressure results, including pore pressure evolution "on-the-flight" during penetrometer penetration as well as pore pressure evolution at maximum penetration depth with the penetrometer being at rest. The sediment surface layer 1 was characterized by a thickness of 5-20 cm depending on the respective location, low sediment resistance and predominantly hydrostatic pressure. It most likely has frequently been reworked by wave action, and exhibited similar geotechnical signatures as fluid mud. Layer 2 reached sediment depths of 30-60 cm, showed an increase in sediment resistance and distinct subhydrostatic pore pressures during penetration, while pore pressures increased in an asymptotic manner to suprahydrostatic (160-180% of hydrostatic pressure) over an observation period of 30-50 minutes. Based on comparison to other examples from the literature, it was hypothesized that layer 2 was composed of overconsolidated mud. Layer 3 featured a significant increase in sediment resistance as well as pore pressure during penetration. As soon as the probe came to rest, the pressure decreased significantly to subhydrostatic conditions, before swinging back to being suprahydrostatic and then slowly dissipating. A similar behavior has been associated to silty sands and high bulk densities. Here, it may suggest a change in sediment composition, likely influenced by coarser nearshore and beach sediments, representing also a denser sediment matrix. The pore pressure results will complement the geological and geotechnical characterization of the coastal zone of Hershel Island, and contribute to the investigation of erosion and deposition processes.