A New Suction-Controlled True Triaxial Apparatus for Unsaturated Soil Testing

A new suction-controlled true triaxial apparatus was developed to investigate unsaturated soil behavior under multiaxial stress paths that are not readily achievable in a cylindrical cell. This apparatus is a mixed-boundary type device with a rigid-flexible-flexible boundary that can apply three principal stresses to a cubic specimen via two rigid platens along the vertical direction and four flexible hydraulic bags on the sides. The apparatus was modified based on a previous true triaxial apparatus designed at Xi'an University of Technology, with the addition of four division strips to avoid loading boundary interference. The new apparatus is capable of testing specimens with dimensions of 7 by 7 by 14 cm. The present apparatus was equipped with an independent pore-air pressure control system and pore-water pressure measurement system. Based on the axis-translation technique, a 5-bar high air entry value ceramic disk is seated on the pedestal, and a matric suction state was achieved that remains constant throughout the laboratory tests. Hence, this true triaxial apparatus can be used to conduct suction-controlled experiments. The suitability of this device was validated via two sets of three repeated trial tests and several sets of trial tests performed with remolded loess for different intermediate principal stress parameters under 50, 100, and 200 kPa matric suction values and different net mean pressure conditions. The results presented the stress-strain characteristics and failure loci of remolded loess.