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

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The study characterizes soil shear-strength design parameters for deep foundations along two sections of Interstate 64 in Virginia’s Coastal Plain: Area 1, underlain mainly by the St. Marys Formation, and Area 2, primarily by the Yorktown Formation. The analysis also considers Eastover deposits.<br /><br />The Coastal Plain consists of water-deposited sediments arranged in terraces, with geologic units separated by unconformities. Subsurface investigations found varied fine- and coarse-grained soils, with differing layer thicknesses and SPT blow counts across the formations and project areas.<br /><br />Strength estimates were developed from laboratory consolidated-undrained and unconsolidated-undrained triaxial tests, CPT, DMT, SPT, and consolidation-test data. The study distinguishes short-term undrained strength for fine-grained soils from long-term drained strength for both fine- and coarse-grained materials. Reported average drained friction angles range from 30° for Yorktown fine soils to 36° for Eastover coarse soils. Average undrained strengths for the reported fine-grained strata range from 1,700 to 3,200 psf.<br /><br />CPT and DMT-derived parameters were generally comparable with laboratory results. Design parameters were closer to peak strength than to remolded or fully softened strength. Pile driving initially remolds the surrounding soil and reduces shaft resistance; strength and lateral stresses then recover over time, potentially bringing resistance closer to peak conditions. Because construction schedules may not allow full strength recovery, relying on peak strength requires caution. Where recovery time is limited, SPT-based design parameters may be more appropriate, though they can result in longer piles.

Keywords

soil shear strength

deep foundations

Interstate 64

Virginia Coastal Plain

St. Marys Formation

Yorktown Formation

Eastover deposits

CPT and DMT

SPT blow counts

pile driving resistance