Modified triaxial apparatus for nonuniform loading beyond frictional ends

Available laboratory testing devices are designed to generate uniform stress and strain states within a tested specimen. Accordingly, these devices provide information on material behavior within a narrow range of stress-strain paths and do not cover the loading conditions that occur in field problems. This limited information generally results in a constitutive model that may not be justifiable to represent loading conditions that differ substantially from the ones in laboratory tests. This paper presents the design of a newly developed next generation laboratory testing device that imposes nonuniform loading on a soil specimen beyond frictional ends. This device is integrated with an evolutionary inverse analysis computational engine to develop a soil constitutive model that reliably captures the soil behavior under general loading conditions. The design inherits features of the conventional triaxial test and adds frictional ends and lateral displacement restraint clamps. Numerical simulation showed that the sheared specimen with clamps includes shear modes that cannot currently be mobilized with available testing devices. The employed close-range digital photogrammetry system measures 3D lateral deformations at 0.5% axial strain or greater with a maximum error of about 2.5%.