The Laredo Formation and the younger Quaternary alluvium that runs under the Rio Grande define the subsurface across Webb County. What we typically encounter beyond the first 3 to 5 meters are interbedded clays and silty sands with occasional caliche layers, materials that demand more than a pocket penetrometer reading when vertical loads exceed 150 kips. A triaxial test on an undisturbed Shelby tube sample gives the effective cohesion and friction angle under the exact confining pressure the stratum will see once the footing is loaded. In our experience running consolidated-undrained tests with pore pressure measurement on Laredo clays, the difference between drained and undrained parameters often decides whether the geotechnical report calls for a mat foundation or can stick with isolated footings. The IBC references ASTM D4767 for this procedure, and it is the standard we follow when the site sits within the 100-year floodplain or near the river’s historic meander scars, where saturation is practically permanent. For deep sand lenses we complement the lab data with a CPT sounding to confirm the in-situ state before triaxial interpretation.
A CU triaxial with pore pressure measurement on Laredo clay reveals the true drained friction angle, often 2 to 4 degrees higher than unconsolidated-undrained estimates would suggest.
