This report summarizes the investigations conducted in support of Mixed Company Event III, a 500-ton, high-explosive experiment conducted near Grand Junction, Colorado. The primary purpose of the project was to provide a representative geologic profile of the Mixed Company site along with asociated constitutive properties for use in the preshot two-dimensional ground shock calculations planned under Project LN 312. This report describes results from (1) a field investigation program consisting of a geologic survey, a refraction seismic survey, and an exploration boring and sampling program: (2) a laboratory test program consisting of static and dynamic uniaxial strain tests, isotropic compression tests, triaxial shear tests, and static tension tests; and (3) the analyses applied to the data obtained from both of these programs in order to develop a recommended site profile and matching set of constitutive properties in time to support preshot calculations. (Modified author abstract).