Triaxial Shear Testing in Jacksonville, FL

Jacksonville’s skyline didn’t just rise overnight. The consolidation of Duval County in 1968 spurred downtown high-rise development that still demands precise geotechnical assessment, especially where the St. Johns River cuts through ancient marine terraces. Our laboratory runs triaxial shear tests to define the effective stress parameters that govern foundation performance in this region. We see a lot of sandy clay and silty sand from the Hawthorne Group—materials that behave differently under rapid loading compared to slow drained conditions. That’s why we don’t just run one test type and call it done. The CPT test often flags the depth of soft layers, and we follow up with consolidated-undrained triaxial stages to quantify undrained shear strength before the first pier is cast.

Triaxial testing mimics real subsurface stress paths—confining pressure, drainage control, and pore-water measurement—in a way that direct shear simply cannot replicate.

How we work

A recent project near the Southbank had us testing samples from a 45-foot excavation for a mixed-use tower. The contractor needed to know if the foundation silts could hold a mat system without excessive settlement. We set up three CU triaxial stages at confining pressures matching the overburden, plus one unconsolidated-undrained stage for the upper fill. Pore pressure transducers recorded continuous response while strain was applied at 0.05 inches per minute. The failure envelope we built showed a phi’ of 32° and cohesion intercept around 280 psf—enough to proceed with the planned mat foundations but with a drainage layer. Our triaxial cell setup uses internal load cells and on-sample deformation transducers, so the data isn’t contaminated by piston friction or end-platen effects. Each specimen is trimmed to 2.8-inch diameter, back-pressure saturated until Skempton’s B parameter exceeds 0.95, then sheared according to ASTM D4767 for CU or ASTM D2850 for UU. After failure, we run a water content profile across the shear plane to confirm uniform consolidation.

Local geotechnical context

Jacksonville’s subtropical humidity and summer convection storms create a water table that fluctuates several feet between May and October. Samples that arrive in Shelby tubes sealed with wax can still lose moisture during transit from a site like Bartram Park to the lab on the Westside. Our extrusion protocol trims the disturbed ends first and wraps specimens in damp cheesecloth until membrane mounting. If the B-value doesn’t reach 0.95 after back-pressuring, we reject the stage and notify the driller same day. The bigger risk is misjudging drainage conditions—running a UU test on a free-draining sand gives meaningless excess pore pressure, while a CU test on a fat clay at the wrong strain rate overestimates phi’. We’ve seen this mistake cost projects along the Trout River where slope designs failed because the triaxial data didn’t match field loading rates. Our lab pairs every test program with a grain-size distribution and Atterberg limits run, so the drainage decision is data-driven, not assumed.

Typical values

Parameter	Typical value
Test types available	UU (unconsolidated-undrained), CU (consolidated-undrained), CD (consolidated-drained)
Specimen diameter	2.8 in (Shelby tube standard); 1.4 in for remolded
Maximum confining pressure	Up to 300 psi with digital servo control
Saturation method	Back-pressure saturation with B-value check (≥ 0.95 per ASTM D4767)
Shear rate	0.005 to 0.05 in/min depending on drainage condition
Output parameters	c’, φ’, cₜ, φₜ, Af, E₅₀, stress path plots
Reporting standard	ASTM D2850, D4767, D7181; AASHTO T-297

Complementary services

CU triaxial with pore pressure measurement

Consolidated-undrained shear with continuous excess pore pressure monitoring. We report effective stress paths, Skempton’s A at failure, and Mohr-Coulomb envelopes for projects requiring undrained strength parameters in saturated fine-grained soils.

CD triaxial for long-term settlement analysis

Consolidated-drained testing at strain rates slow enough to prevent pore pressure buildup. Volume change is recorded throughout shear; the resulting drained friction angle and cohesion intercept feed directly into embankment and retention structure designs.

Regulatory framework

ASTM D2850-15 (Standard Test Method for Unconsolidated-Undrained Triaxial Compression Test on Cohesive Soils), ASTM D4767-11 (Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils), ASTM D7181-20 (Method for Consolidated Drained Triaxial Compression Test for Soils), AASHTO T-297 (Standard Method of Test for Consolidated Undrained Triaxial Compression Test on Cohesive Soils), ASCE 7-22 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures)

Questions and answers

What does a triaxial test program for a Jacksonville high-rise typically cost?

A three-stage CU program on Shelby tube samples from one borehole, including B-check saturation and ASTM D4767 reporting, runs between US$1,910 and US$2,900. The range depends on whether we’re testing intact specimens or remolded material, and how many confining pressure points the engineer specifies.

How long does the lab need to complete a CU triaxial series?

Saturation and consolidation alone can take 48 to 72 hours for fat clays with low permeability. Shear at the correct strain rate adds another 4 to 6 hours per stage, so a three-stage CU series typically reports within five working days after sample acceptance.

How do you decide between UU, CU, and CD triaxial testing?

The choice depends on the field loading rate and drainage conditions. UU applies to rapid loading where no drainage occurs (embankment failures during construction). CU is for cases where the soil has consolidated under existing stress but will be loaded quickly afterward (foundations on saturated clay). CD suits slow, steady loading with full drainage (long-term slope stability, sand fills). We always run a particle-size analysis first to confirm the soil class before selecting the drainage protocol.