Fluid Cushion Truly Triaxial or Multiaxial Testing Device

The derivation of the constitutive relations of soils requires a testing facility that can apply a homogeneous and nearly ideal set of controlled boundary conditions to a specimen. A new multiaxial cubical test apparatus is described. It uses fluid or pneumatically pressurized flexible cushions to transmit a three-dimensional, independently controlled, and compressive stress state to a 102-mm (4-in.) specimen of geologic material. The deformations in three orthogonal directions are detected by a set of linear variable differential transformers. Flexible cushions allow nearly unrestrained deformations in the specimen. The deformations are uniform even at large strains. The specimen preparation and apparatus assembly procedures are straightforward. The simplicity of the multiaxial cubical cell and its easy operating procedures are emphasized. Typical stress-strain curves for straight-line stress paths are discussed. The apparatus appears to operate especially well at low stress levels.