Site Characterization of Paleoliquefaction Features in Missouri

In the central United States, paleoliquefaction features (clastic sand dikes intruding clayey silt alluvium) have recently been discovered and documented at sites in Missouri and Illinois considerably north of the Mississippi Embayment area where the New Madrid Seismic Zone (NMSZ) is located. These northern paleoliquefaction features are thought to have formed during at least two earthquake events in the past 6,500 years. The implications of having moderate to large magnitude earthquakes centered much closer to St. Louis than in the NMSZ is very significant, given the very densely populated area. Paleoliquefaction sites discovered in drainage ditch banks near Dexter, MO (in southeast Missouri near the NMSZ) and as far north as the banks of the Meramec River near St. Louis were characterized by a team of geologists and engineers. The objectives of the site characterization plan were to define the soil column and paleoliquefaction features in engineering parameters, so a credible earthquake magnitude could be back calculated using an effective stress approach with development of pore water pressure and soil liquefaction. The site characterization program included bank cleaning and logging, typical borehole exploration, testing and sampling, and seismic piezocone probing. The site characterization techniques defined the source materials of the clastic dikes, provided shear wave velocity profiles, values of resistance to penetration via CPT and obtained SPT values via standard exploration drilling methods. The combination of methods allowed for both low strain and large strain soil parameters for use in liquefaction modeling