The first thing we set up on a Hampton soft-ground tunnel job is the triaxial cell and the incremental consolidation frame. You need to know how much pore pressure that silty clay can hold and how fast it will settle once you start the TBM or cut open the face. Hampton sits on the Atlantic Coastal Plain, barely 3 meters above mean sea level, with the water table often less than 2.5 meters below grade. That means every sample we receive is fully saturated and needs back-pressure saturation in the lab before we run a consolidated-undrained test. Our team handles the full sequence: extruding Shelby tubes, trimming specimens, running grain-size analysis to verify the fines fraction, and then loading the triaxial cell for effective stress paths. In a city where the Yorktown Formation clay dominates the upper 10 to 15 meters, getting the undrained shear strength right is the difference between a stable face and a blowout.
In Hampton, the undrained shear strength of the Yorktown clay often dictates face pressure on the TBM; we run CU triaxials at in-situ confining stress to give the contractor a number they can trust.
