Abstract

Large, curvilinear sand blows, resulting from earthquake-induced liquefaction, commonly occur along the margins of fine-grained channel-fill deposits in the St. Francis Basin of the New Madrid seismic zone. One such example identified on aerial photographs and satellite images was studied in detail by examining relations of sedimentary deposits and liquefaction features in the field and by compiling borehole data previously collected by the Army Corps of Engineers. Ground failure in this example included liquefaction of subsurface sandy deposits, development of en echelon fissures in the overlying channel-fill deposit, forceful venting of sandbearing water to the ground surface resulting in the formation of sand dikes and sand blows, and differential ground subsidence across the sand dikes. A combination of mechanisms, including the development of a water-rich zone below the clayey surficial deposits, may have contributed to the failure.

At the study site, located in southeastern Missouri, the near-surface sedimentary geology can be described as a two-layered system with a sandy channel deposit below and a less permeable clayey overbank deposit above. The contact between the two layers is roughly horizontal, except where embedded by a channel-fill deposit. The basal contact of the channel-fill deposit is inclined along its margin and extends below the base of the overbank deposits. Sand blows are abundant in the overbank deposits, which are about 6-m thick. In contrast, sand blows rarely occur within the 18+ -m-thick channel-fill deposit, except along its margin. Here, feeder dikes of the sand blows are concentrated within 15 m of and arranged in an en echelon pattern subparallel to the margins of the channel-fill deposit. On the side of the feeder dikes toward the edge of the channel-fill deposit, the ground surface is displaced downward as much as 1.2 m.

Large ground displacements often pose the greatest threat to the integrity of engineered structures. As demonstrated in this case study, the spatial relations of sedimentary deposits, as well as their permeability and thickness, appear to influence both the location and mode of ground failure. A better understanding of factors contributing to liquefaction-related ground failures can help to identify sites that may be prone to large ground displacements and thus to mitigate the hazard posed by earthquakes in this region.

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