For several years, SRI has been measuring the dynamic mechanical response of in situ geologic materials near and above the elastic limit. A major achievement has been development of the LASS technique Lagrange Analysis for Stresses and Strains, in which a mathematically complete Lagrange flow analysis is used to calculate dynamic stress-strain trajectories from field measurements of explosive-induced ground motion. Two fields shots were fired to provide data for proof testing the LASS technique the field test data were reduced using LASS and LASS was extended and its accuracy and sensitivity to typical errors in field measurements were evaluated. The two field tests were 116-kg spherical shots fired in a wet clay layer about 3.6 m below the surface at the Pre-Dice Throw II site. Excellent stress and particle velocity records were obtained with peak stress between 0.2 and 1.7 GPa. Plausible stress-strain trajectories were calculated from the field records using LASS. These indicate nearly reversible radial stress-volume strain behavior until release to low stresses, possible bulking at low stresses on release, and about one-third the compressibility observed in laboratory uniaxial strain tests. The calculated principal stress difference amplitude was small, less than the experimental error associated with the calculation. This result was expected because of the high stress level of the tests and the low strength of the site material.