ASTM D2573 governs the field vane shear test (VST) for measuring undrained shear strength of saturated clays. In Charlotte, where Piedmont residual soils transition into deep alluvial deposits along the Catawba River valley, soft clays and silts can reach thicknesses of 15 to 20 feet. These low-strength layers demand In-Situ because disturbed sampling underestimates true strength. The VST is run by advancing a four-bladed vane into undisturbed soil at the bottom of a borehole and rotating it at a standard rate. Peak torque is recorded, then remolded torque after rapid rotation, giving sensitivity. The test is efficient: a complete profile at one depth takes about 12 minutes. Charlotte's geotechnical practice relies on VST data alongside compression simple testing to calibrate undrained strength for foundation design in soft zones.
In Charlotte, VST is the only reliable method to measure undrained shear strength of soft alluvial clays without sample disturbance.
Scope of work
Area-specific notes
Consider the difference between the sandy Piedmont soils in southeast Charlotte near Matthews and the soft alluvial clays along the Catawba River floodplain in northwest Mecklenburg. In the floodplain, undrained shear strengths can drop below 15 kPa. A foundation designed using only SPT data would underestimate settlement and bearing failure risk. The VST captures the actual in-place strength of these soft layers. Ignoring this distinction has led to differential settlement in several warehouse slabs near Interstate 485. The VST provides direct measurement of the strain-softening behavior that governs progressive failure in these clays. For embankment stability, we also use ensayo triaxial to model drained conditions once strength gain from consolidation is considered.
Standards used
ASTM D2573-18 (Standard Test Method for Field Vane Shear Test in Saturated Fine-Grained Soils), ASTM D2487-17 (Standard Practice for Classification of Soils for Engineering Purposes), IBC 2021 Section 1803 (Geotechnical Investigations), FHWA NHI-06-088 (Soils and Foundations Reference Manual)
Linked services
Borehole Vane Shear Testing
VST performed at the bottom of cased boreholes in soft to firm clays and silts. Includes peak and remolded readings at 1.5 m intervals or as specified. Data plotted as su vs. depth with sensitivity ratios.
Hand Vane Shear Testing (Field Screening)
Lightweight hand-held vane for rapid su estimates in test pits or exposed cuts. Suitable for preliminary site characterization and excavation stability checks during Charlotte utility projects.
Typical parameters
Top questions
How does the VST differ from the torvane test?
The VST is a borehole test run at depth on undisturbed soil, following ASTM D2573 with controlled rotation rate and vane geometry. The torvane is a hand-held device used on exposed surfaces or split-spoon samples and does not meet ASTM D2573 requirements. VST data are used for design; torvane is a screening tool only.
What is the typical cost of a field vane shear test in Charlotte?
Cost for a borehole VST in Charlotte ranges between US$560 and US$1,470 per test depth, including mobilization, vane insertion, peak and remolded readings, and a summary report. Volume discounts apply for multiple test points on the same project.
When should I request VST instead of a triaxial test?
VST is preferred for soft, saturated clays where sample disturbance would invalidate triaxial results. It measures in-situ undrained strength directly. Triaxial tests are better for stiff clays and for obtaining effective stress parameters (c', phi') needed for long-term stability analysis.