Foundation Soil Strength Characterization of Coastal Plain Deposits along I-64 Corridor in Virginia-Paper

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The study characterizes the strength and compressibility of Coastal Plain deposits along the I-64 widening corridor near Williamsburg, Virginia, with emphasis on the Yorktown, St. Mary’s, and Eastover Formations. Investigations included SPT, CPT, DMT, vane shear, laboratory strength and consolidation tests, and pile-driving analyses.<br /><br />SPT blow counts were often low in the Yorktown and Eastover Formations, likely because testing generated excess pore-water pressures and localized soil disturbance. Consequently, SPT-based correlations can underestimate soil strength and lead to overly conservative pile designs. CPT- and DMT-based estimates generally agreed better with laboratory results and measured pile performance.<br /><br />The authors compiled strength estimates and assessed their variability. Most-likely undrained shear strengths were approximately 1,700 psf for fine-grained Yorktown and Eastover soils and 3,200 psf for fine-grained St. Mary’s soils. Drained friction-angle estimates varied by formation and soil type, with most-likely values ranging from 30° to 36°.<br /><br />Pile tests showed that shaft resistance increased after installation as disturbed soils regained strength and lateral stresses recovered—a process known as setup. In the Yorktown Formation, a pile developed 640 kips of resistance after five days, substantially exceeding the SPT-based estimate. In St. Mary’s soil, resistance developed more slowly. The results indicate that in-situ and laboratory-based parameters can support more realistic, potentially shorter pile designs when sufficient setup time is available. However, peak-strength parameters may not be achieved within tight construction schedules; in such cases, conservative SPT-based parameters may be more appropriate.

Keywords

Coastal Plain deposits

I-64 widening corridor

Williamsburg Virginia

Yorktown Formation

St. Mary's Formation

Eastover Formation

SPT

CPT

DMT

pile setup