Experimental investigation on the mechanical properties of thawed deep permafrost from the Kuparuk River Delta of the North Slope of Alaska

Abstract

Thawing permafrost around oil wells in the circum-Arctic region incurs local subsidence and causes damage to the energy extraction infrastructure. This study aims to investigate the mechanical properties of thawed permafrost for analyzing the interaction between the oil wells and surrounding permafrost during thaw settlement. This article describes a series of drained triaxial tests on permafrost samples extracted from boreholes up to 170 m deep from the Kuparuk River Delta of the North Slope of Alaska. Factors, including soil type, ice content, depth, and orientation, were considered during specimen preparation. Testing results, including the stress-strain relationship, initial modulus, secant stiffness modulus at 50% of the peak stress and its corresponding strain, were obtained. The shear strength parameters, including the total-stress cohesion and friction angle, were analyzed, and their variation with depth was discussed. The impacts of confining pressure, ice content, and dry density on the initial modulus, and secant stiffness modulus at 50% of the peak stress and the corresponding strain were analyzed, and their prediction models were proposed. It is found that both the elastic moduli and shear strength parameters such as the friction angle of thawed permafrost do not follow the trend observed at typical unfrozen soil sites, where soil stiffness and shear strength parameters generally increase with increasing depth. The friction angle of silt and sand specimens at deeper depths is relatively low, indicating that the deep permafrost is not consolidated. Strong anisotropy exists in the failure stress and elastic moduli of clayey permafrost due to its fabric and geological history. The results shed light on the mechanical properties of thawed unconsolidated deep permafrost. They could be used to analyze soil-oil well casing or soil-foundation interaction during thaw subsidence in permafrost.