Liquifaction Potential of the Yukon Prodelta, Bering Sea

ABSTRACT The Yukon prodelta is exposed to large storm waves propagating northward from the southern Bering Sea. Shallow water depths of the prodelta enhance the transfer of energy from the surface waves to the bottom. As the bottom deposits are cyclically loaded by large storm waves, potential decrease in their resistance to shear could ultimately cause liquefaction. A preliminary assessment of the engineering properties of Yukon sandy silt suggests that the prodelta deposits may be susceptible to wave induced liquefaction during severe storm events. In addition, erosion and are suspension of sediment in the prodelta may be intensified because of the liquefaction process. INTRODUCTION The stability of granular sea-floor deposits can be upset by liquefaction of the deposits under cyclic loading and their behavior as a viscous fluid. This liquefaction or fluidization of bottom sediment may pose severe problems to the integrity of offshore installations. The bearing capacity of the sea floor beneath offshore structures (Lee and Focht, 1975; Rahman et al., 1977) may be seriously impaired if the upper few meters of deposits liquefy and mass flows result. Erosion and sediment scouring caused by current-induced bottom shear stresses are other processes significantly related to liquefaction vulnerability. The net effect may be the erosion of foundation-bearing sediment beneath platforms sited on the bottom. The extent of the damage potential would depend on the a real distribution of the liquefiable material and recognition of the liquefaction potential in design considerations. The liquefaction of sea-floor sediment results from repeated loading during either earthquakes or high-intensity storm waves, when pore-water pressures reduce the shearing resistance of the material. In this report we consider the potential of Yukon prodelta deposits (Fig. 1) to liquefy under large amplitude surface water waves. Although Norton Basin does possess several active faults that_ could pose a moderate seismic risk, the liquefaction potential under wave loading is considered particularly destabilizing on the basis of sediment type (borderline sand-silt) in the" prodelta (Fig. 2) and exposure of the prodelta to large storm waves. Typical late-fall storms in Bering Sea generate large amplitude low-frequency waves that propagate into Norton Sound from the southern Bering Sea. Water depths (Fig. 1) throughout Norton Sound are also sufficiently shallow «20 m), that most wave-generated surface energy is imparted to the bottom deposits. Under these conditions, bottom deposits may liquefy during storms. To investigate this possibility, we have made a preliminary assessment of the liquefaction potential of Yukon prodelta deposits. This study was in part based on samples from, and bottom-pressure measurements in, the Yukon prodelta (Fig. 1). At present, however, no direct cyclic strength data have been gathered on undisturbed samples from the area, and thus our conclusions should be considered tentative. This assessment of the liquefaction potential was supplemented by results obtained by other investigators on similar sediment. The dissipation of pore-water pressures for a typical and an extreme storm event was modeled by an isoparametric one-dimensional finite-element method (FEM) of analysis.