The foundation soils for the Illinois approach to the new Mississippi River Bridge at Cape Girardeau, Missouri, were identified as liquefiable. Ground modification was required for subsurface soils consisting of alluvial deposits of soft silt over loose sand. Vibro-replacement stone columns were used to improve the top silt deposit. Vibro-compaction was used to densify the top portion of the underlying loose sand. The standard penetration test (SPT) was used as an acceptance criterion for vibro-compaction. Liquefaction and slope stability analyses were conducted before and after ground improvement. Significant improvements in soil strength, density, and factor of safety against liquefaction, were achieved for both the soft silt and the loose sand deposits. The short- and long-term slope stabilities were also improved.

¹Geotechnical Engineer, Illinois Department of Transportation, Springfield, Illinois.
²Illinois Department of Transportation, Springfield, Illinois.

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