The engineering performance of a vast majority of geotechnical infrastructure, including earth slopes, embankments, pavements, tunnels, and earth retaining systems, may be more suitably modeled using plane strain analyses, given the particular geometries, stress paths, and boundary conditions such geosystems normally feature or experience in the field. Biaxial devices allow for direct and reliable testing of soils under truly plane strain conditions, facilitating accurate assessments of shear banding phenomena and stress-strain-strength parameters under these conditions. The majority of biaxial devices developed to date, however, allow for soil testing mostly under dried or fully saturated conditions. This paper introduces a suction-controlled biaxial apparatus that is suitable for soil testing under controlled-suction states via the axis-translation technique. The design of its core system is based upon the original Vardoulakis type of biaxial apparatus. In this work, biaxial specimens are prepared by uniaxial consolidation of a slurry mixture, made of 75% silty sand and 25% kaolin, into a custom-made biaxial consolidation mold. A thorough performance verification of the newly implemented apparatus was first accomplished, followed by a preliminary series of constant-suction plane strain tests. Results reflect the important role played by matric suction in the stress-strain-strength response of intermediate unsaturated soils under plane strain conditions.
