Improvement and strength testing of true tri-axial apparatus with one chamber and four cells and rigid-flexible-flexible loading boundary mechanism—True triaxial apparatus developed in Xi'an University of Technology

The true triaxial apparatus is an important instrument, which is usually emplyed to investigate the mechanical characteristics of soils under complicated stress conditions. The new true triaxial apparatus, developed in Xi’an University of Technology, is of the rigid-flexible-flexible loading boundary system, which is different from the rigid, flexible and rigid-rigid-flexible complex loading systems at home and abroad. The chamber of true triaxial apparatus includes four cells, in which the liquid bags face two groups of orthogonally lateral surfaces of cubic specimen and top and bottom rigid plates. In the vertical direction the normal stress acts on the ends of the cubic specimen. In the two orthogonal directions, two normal stresses are applied to the lateral surfaces of the cubic specimen. To eliminate the restraint of the top and bottom plates, the height of sample increases. Accordingly, the ratio of height to width of specimen is 2∶1. To measure directly the lateral deformation of the sample in orthogonal biaxial directions, a measurement element is developed by a smooth connect pole passing through the liquid bags. To reduce the fluctuation of loading signals, an automatic control system including automatic control circuit board and control program is developed by the developing software. The strength laws of saturated sand, unsaturated soil and intact loess are investigated by the true triaxial tests. The improved true triaxial apparatus is verified by the experiments of different soils.