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PALEOLIQUEFACTION STUDIES IN MED-AMERICA
The New Madrid Seismic Zone (NMSZ) is considered to be a highly-seismic region hi the United
States. During 1811 and 1812, more than 200 separate earthquake events occurred hi the region, with
the three largest ones estimated to have moment magnitudes of 7.9, 7.6, and 8.0 (Johnston & Schweig,
1996). Due to rapid development in the late twentieth century, the NMSZ has become a highly
populated area and hosts large cities, including Memphis, Tennessee and St. Louis, Missouri. The
inevitable recurrence of the large seismic events would cause widespread loss of life and damage to
structures, because of the ground shaking and soil liquefaction. In order to map the seismic ground
hazards and dynamic soil properties in the NMSZ, series of SCPTu tests were conducted in this region.
A representative SCPTu sounding performed at the Walker paleoliquefaction site near Marked Tree,
Arkansas is shown in Figure 6. The small-strain shear modulus and the soil stratigraphy for this
sounding are also shown. The site mainly consists of sandy soils, with a capping layer of clay between
the depths of 2 to 5 meters. This site was selected for study because two large sand dikes were found on
the property. Recent research hi Japan and western USA, where earthquakes occur more frequently,
notes the important finding that sites that experience liquefaction will likely liquefy again during
earthquake events of similar magnitude (Youd, 1988; Yasuda & Tohno, 1988). Therefore, the Walker
site has the potential to liquefy again during large earthquakes hi the future.
Tip Resistance Sleeve Friction Pore Pressure
qT(MPa) f, (kPa) u2, uB(kPa)
0 20 40 0 200 400 -200 0 200 400
SW Velocity Shear Modulus
V,(m/s)
GmM(MPa)
0 200 400 0
1 °0 200
Figure 6. Seismic Piezocone Results at Walker Paleoliquefaction Site in Eastern Arkansas
The procedure of liquefaction evaluation by SCPTu is depicted in Figure 7, indicating the use of q t , f s ,
and Ub for soil delineation, V s for obtaining G ma x hi site amplification analyses (a raax ), and normalized
q t i and V sl for evaluating liquefaction potential. The results of liquefaction analysis from the SCPTu
sounding at the Walker paleoliquefaction site corresponding to a magnitude M w =8.0 earthquake are
given in Figure 8. At a factor of safety of Fs = 1.0 for liquefaction, the critical values of tip resistance
and shear wave velocity are superimposed with the measured data. This critical value, which changes
with depth, is shown in the same graph as the measured value in the figure. Liquefaction is likely
whenever the measured value is less than the critical value. The gaps in the critical value plots
represent soil layers that are not susceptible to liquefaction due to their classification (e.g., clayey soils
for which liquefaction analyses are not relevant).