Triaxial Testing in Little Rock — Shear Strength for Foundation Design

A full three-specimen CU triaxial set with pore pressure measurement runs between US$1,810 and US$2,840, depending on whether we are working with undisturbed Shelby tube samples from the site or remolded specimens at a specified density. The price includes saturation, consolidation, shear at three confining pressures, and the complete report with Mohr-Coulomb envelopes. If the engineer only needs a single UU test for a temporary excavation design, the cost is lower — we can quote that once we know the loading case.

Figure on two to three weeks for a complete three-specimen consolidated-undrained set with pore pressure measurement. Most of that time is consolidation and shear at the slow rate required to equalize pore pressures in Little Rock's low-permeability clays. The actual shearing stage alone can take three to five days per specimen. We do not rush this — if the rate is too fast, the measured φ' comes out unconservatively high.

Triaxial testing is limited by specimen size. For gravels with particles larger than about 0.2 inches, the 2.8-inch diameter specimen gives unreliable results because the gravel-to-specimen ratio is too high. In those cases we recommend large-scale direct shear or a field plate load test instead. But the silty and clayey matrix from the terrace deposits tests very well in the triaxial cell once we trim out the occasional pebble.

A single specimen gives you one point on the Mohr diagram — one shear strength at one confining pressure. You cannot draw a failure envelope from a single point. Three specimens tested at three different cell pressures (typically 15, 30, and 45 psi for Little Rock foundations) define the line. From that line we measure the friction angle as the slope and the cohesion intercept as the y-axis crossing. Without the envelope, the engineer is guessing the strength at confining pressures different from the test condition.