RELATIONSHIP OF A SHEAR-WAVE VELOCITY PROFILE WITH THE PHYSICAL PROPERTIES OF ALLUVIAL SEDIMENT IN THE MISSISSIPPI/ MISSOURI RIVER FLOODPLAIN

Loosely consolidated alluvial sediment with a high water table is a weak substrate when exposed to ground acceleration during an earthquake. Thus, floodplain alluvium would likely experience severe ground effects during a great earthquake, including liquefaction, sand blows, ground failure, lateral spreading, differential settlement, and a loss of bearing strength. Rupture of pipelines, rail lines, and roads and structural damage to buildings are likely. This makes a floodplain hazardous during earthquakes and at an exceptionally high risk if urbanized. However, alluvial deposits are quite variable so that the severity of ground effects is likely to vary across the floodplain. The study area, St Louis, Missouri, is a densely populated transportation hub within a seismic risk area. Immediately north of the city, the Mississippi/Missouri river valley is filled with a mixture of sediment from the two rivers that share a single floodplain. Approximately 35-45 m of alluvial sediment, much of which is saturated, fills the valley. At the present time the area is dominantly agricultural, but is the locus of much infrastructure, including gas and petroleum pipelines, rail lines, power plants, airports, and highways which may be used as alternate evacuation and emergency vehicle transportation routes in a seismic emergency.

Detailed lithology and geotechnical data, including texture and moisture content of the strata beneath the backswamp between the Mississippi and Missouri rivers correlates to a shear-wave velocity profile at the same location. This allowed an evaluation of the physical properties of the stratigraphic units present in the core and determination of the shear-wave velocity profile resolution of those units. The backswamp and point-bar deposits are more variable than resolution of the shear wave velocity profile. Geomorphic and subsurface mapping of the floodplain strata provides a 3D spatial distribution of those units and a more realistic distribution of likely ground failure than site-specific data commonly collected for construction purposes.