San Bernardino sits at roughly 1,050 feet above sea level in the alluvial fan of the Santa Ana River, a setting where soft clays and silty layers are common beneath the surface. When those fine-grained soils can't hold a standpipe or a drive sample, the field vane shear test (VST) becomes the only practical way to measure undrained shear strength in place. We've run dozens of VST profiles across the city's older floodplain deposits, especially near the I-215 corridor where shallow foundations need real numbers rather than correlations. The test follows ASTM D2573 and gives us a direct reading of peak and remolded strength, which is critical for estimating sensitivity in San Bernardino's lacustrine clay pockets.
In San Bernardino's soft alluvial clays, the field vane shear test delivers undrained shear strength directly — no correlations, no guesswork.
Approach and scope
A mistake we see repeatedly in San Bernardino is relying on SPT N-values alone for soft clay design — those blow counts can be 0 or 1, telling you nothing useful. The field vane shear test (VST) fills that gap by measuring torque directly at the vane blades, giving undrained shear strength (Su) in the range of 10 to 80 kPa typical of local alluvial clays. The procedure is straightforward: advance a casing to the test depth, insert the four-bladed vane, rotate at 6°/min, and record peak torque. For deeper soft layers, we often combine VST with a consolidation test to predict settlement rates, and in sensitive clays we check remolded strength right after peak failure to assess quick-clay behavior. The data feeds directly into bearing capacity calculations per IBC 2018 and local geotechnical practice.
Technical reference image — San Bernardino
Site-specific factors
Much of San Bernardino's residential areas sit on Holocene alluvium with clay layers up to 10 m thick, often saturated due to a shallow water table between 3 and 6 m depth. Under seismic loading, these soft clays can lose strength rapidly — a phenomenon called cyclic softening — and the field vane shear test (VST) is the only in-situ method that captures both peak and remolded Su. If the sensitivity ratio exceeds 8, the risk of strength loss during an earthquake becomes significant, and we recommend pairing VST results with a liquefaction assessment to evaluate flow slide potential in the city's soft ground.
A complete VST profile with measurements every 1.5 m depth, including peak and remolded torque readings, sensitivity calculation, and a Su vs. depth plot. Ideal for foundation design in soft clay layers up to 20 m deep.
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VST with Disturbance Assessment
Extended service that adds multiple remolded tests at each depth to evaluate progressive disturbance and sensitivity trends. Recommended for slopes and embankments where post-peak strength loss is a concern.
Relevant standards
ASTM D2573-19 (Standard Test Method for Field Vane Shear Test in Saturated Fine-Grained Soils), IBC 2018 (Chapter 18 – Soils and Foundations), ASCE 7-16 (Seismic Load Requirements for Site Class E/F)
Quick answers
What is the difference between VST and a laboratory triaxial test for soft clays?
VST measures undrained shear strength in situ, preserving the natural fabric, water content, and stress state of the soil. A triaxial test requires undisturbed sampling, which is often impossible in soft, sensitive clays. For San Bernardino's alluvial deposits, VST avoids sample disturbance and gives direct Su values at the actual effective stress.
How deep can the field vane shear test reach in San Bernardino soils?
With standard A-rod extension, we routinely reach 25 to 30 m depth in the city's soft clays. The limiting factor is not the vane itself but the torque capacity of the rods and the head unit. In very soft layers, we use lightweight rods to avoid rod buckling.
What does the field vane shear test cost in San Bernardino?
A typical VST profile with readings every 1.5 m to 15 m depth ranges between US$630 and US$1,580 depending on access, number of test depths, and site conditions. This includes mobilization, equipment, field data, and a report with Su vs. depth chart.
When should I choose VST over a CPT or SPT in soft clays?
Choose VST when you need direct undrained shear strength without empirical correlations. CPT gives continuous profiles but relies on cone factor Nk to estimate Su, which can vary ±30% in San Bernardino clays. SPT is unreliable in soft soils because blow counts are near zero. VST is the direct standard.
Does the VST method work in San Bernardino's sandy or gravelly layers?
No, VST is only valid in saturated fine-grained soils (clays and silts) with sufficient plasticity to hold the vane. In sandy or gravelly layers, the vane rotates freely without mobilizing shear resistance. For those materials, we recommend SPT or CPT instead.
