Coastal Slope Stability Analysis for Hampton Roads Infrastructure

We retrieve undisturbed thin-walled tube samples from the critical depth range—typically the interface between the Columbia Group sands and the underlying Yorktown Formation clays—and subject them to consolidated-undrained triaxial compression with pore pressure measurement (ASTM D4767). For low-plasticity silts that tend to dilate during shear, we supplement with drained direct shear tests at rates slow enough to prevent excess pore pressure buildup. The envelope is then corrected for the field stress path using the SHANSEP procedure when the clay is normally consolidated, or direct peak-to-residual strength correlations when we are analyzing a pre-existing failure surface.

Groundwater is the dominant destabilizing force in Hampton's coastal slopes, where the phreatic surface can rise several feet in response to a single hurricane rainfall event. We construct transient seepage models in SEEP/W calibrated against site-specific CPT dissipation test data and long-term piezometer records, then couple them with SLOPE/W to compute the factor of safety as a function of time during the critical drawdown period. This approach captures the lag between falling creek levels and the slower drainage of the embankment, which is precisely when the most unfavorable hydraulic gradients develop.

Although the Central Virginia Seismic Zone produces only moderate events, ASCE 7-22 still requires a pseudo-static slope stability check for Site Classes E and F, which are common in Hampton due to the thick soft clay profiles. We apply a horizontal seismic coefficient kh of 0.5 × PGA (approximately 0.04g–0.06g for the 2475-year return period) and require a minimum factor of safety of 1.1. For critical facilities, we go beyond pseudo-static methods and perform Newmark sliding block displacement analyses to demonstrate that the calculated permanent deformation remains within tolerable limits for the structure.

Temporary cut slopes in Hampton's silty sands require a different analytical approach than permanent embankments. We model the short-term, undrained condition using total stress parameters from unconsolidated-undrained triaxial tests on the cohesive layers, and apply a factor of safety of 1.3 for cuts that will remain open less than six weeks. The analysis must also consider the surcharge from adjacent roadways—particularly along Mercury Boulevard and the I-64 corridor—and the potential for tension cracks to develop behind the crest during dry weather, which can fill with rainwater and trigger a wedge failure.

A complete slope stability analysis for a single cross-section in Hampton, including the necessary site investigation (SPT borings or CPT soundings), laboratory shear strength testing on representative samples, and the limit equilibrium modeling with a written report, generally falls between US$1,240 and US$4,550. The range depends on the slope height, the complexity of the stratigraphy, and whether a transient groundwater analysis is required. Projects involving multiple cross-sections or probabilistic sensitivity analyses will be at the upper end of this range.