A four-year record (2002–2005) of river stages and very small displacements measured deep in the foundation soil of the interstate bridge at Vicksburg, Mississippi, explains movements that were detected within the bridge. Most foundation-soil displacements occurred within a horizontal zone about 6 ft (2 m) thick in Tertiary strata at elevation −62 ft (−19 m) relative to mean sea level. The stratigraphic zone of weakness consists of stiff, CH clay and subordinate thin beds of fine sand. The zone owes much of its vulnerability to prehistoric landsliding. Lesser movements, however, have extended along the same horizon into the bluff and ground not previously affected by the landslide. During the study interval, displacement events within the bridge foundation soil ranged up to about 0.1 in. (0.3 cm) and lasted a few days or less, coinciding with relatively rapid drops of the river to low levels. This coincidence seems to result from a lag in the response of groundwater pressure to dropping river level, as well as from vulnerability at a time of rapid drawdown.

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