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Schwab, W.C., 1985

Slope instability on the Alsek prodelta, Gulf of Alaska

Bibliographic Reference

Schwab, W.C., 1985, Slope instability on the Alsek prodelta, Gulf of Alaska: Durham, North Carolina, Duke University, Ph.D. dissertation, 388 p., illust.


A variety of slope failure types in Holocene glacial-marine sediment, ranging from slides and slumps to sediment gravity flows, have been identified on declivities less than 1.3 degrees on the Gulf of Alaska continental shelf. One area of slope failure, the Alsek prodelta failure, is particularly interesting because of the variety and complexity of sediment gravity flow deposits. Laboratory and in-place geotechnical tests identify clayey silt with a water content between 35% and 45% as the sediment most susceptible to earthquake loading. Cores containing a majority of this susceptible sediment roughly correlate with the location of slope failures. It is proposed that the failure type is related to the nature of the failure load. Remolding and incorporation of water, which probably occurs more easily during wave-loading from a long storm in comparison to the limited number of loading cycles generated by an earthquake, reduces the strength and increases the fluidity of the failed sediment mass, thereby causing it to flow rather than slide or slump. Alsek prodelta sediment gravity flows are morphologically similar to subaerial quickclay flowslides. Sediment analyses, including microstructural investigation using scanning electron microscopy, reveal that the Alsek prodelta clayey silt is similar to some quickclay deposits with respect to grain size, clay mineralogy, liquidity index, activity, and other physical properties. However, the Alsek prodelta clayey silt is much less sensitive and is consolidated to a denser state than quickclays. Therefore, the engineering behavior of quickclay and Alsek prodelta clayey silt differ; quickclay displays contractive behavior and associated generation of excess pore-water pressure under a critical static load, while Alsek prodelta clayey silt exhibits dilatant strain-hardening behavior. The explosive generation of excess pore-water pressure in quickclay reduces the sediment's strength to that of a viscous liquid, and allows unlimited flow. For the Alsek prodelta clayey silt to undergo unlimited flow, there must be a water content increase during storm-loading episodes to bring it to a critical state (where dilation is negligible). Thus, the Alsek prodelta clayey silt is deposited in too dense a state to be quick, but the depositional environment is conducive to the formation of flow deposits due to wave-induced cyclic mobility in water depths less than ~40 m. (Abstract shortened with permission of author.)

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