Abstract: An instrument for testing earthen samples under triaxial load conditions includes an upwardly extending cylindrical housing with two access windows formed in the side thereof, and a transparent cylindrical sleeve adapted to be slidably received about the housing to seal the chamber within. A lower stage is supported on a laterally translatable bearing block, and connected to a double-acting horizontal hydraulic ram. An upper stage is disposed in superjacent opposition to the lower stage, and is connected to the lower end of a vertical hydraulic ram supported atop the housing. The stages include shallow sockets adapted to receive sample platens in vertical alignment with the vertical ram. The platens each include peripheral grooves adapted to be engaged by swing clamps extending from each stage. Each platen includes a filter stone portion adapted to contact a core sample disposed between the platens, with a fluid channel extending radially through each platen to the filter stone.