Development of a large triaxial cell apparatus with variable deviatoric and confining stresses

The features of a new large triaxial cell apparatus with variable deviatoric and confining stresses are described. Characteristics of the apparatus are given, as well as a description of the making and instrumentation of samples. The hydraulic and data acquisition systems are also described. The triaxial apparatus is built for testing of railway ballast material. For the time being samples of 300 mm in diameter and 600 mm of height may be tested in the apparatus. A new way of applying the variable confining stress to the specimen has been developed. A hydraulic cylinder works directly on the cell water through a rubber bellows, and this method allows rapid variation in confining pressure. The instrumentation is fixed directly onto stones in the specimen, thus eliminating any effect from the rubber membrane. This technique of direct instrumentation has been developed during this project. The deformation measurements are done with LVDTs that can withstand pressurised water in a sufficient pressure range. Each sample is equipped with (up to) eight LVDTs, four for measuring radial deformations and four for measuring vertical deformations. The problem of LVDT locking, and some countermeasures, are described.