Several kinds of true triaxial apparatuses have been devised and applied to obtaining the strength and deformability behavior of soils under different stress paths. However, for these true triaxial apparatuses with flexible loading boundaries boundary interference is still not solved. This paper presents a new type of true triaxial apparatus, with rigid-flexible-flexible boundaries, which addresses the boundary interference using four division strip-platens. This apparatus applies three principal stresses on a 70-mm cuboidal specimen from two rigid platens along the vertical direction and four hydraulic rubber bags around four side faces of the cuboidal specimen. The rigid platens and hydraulic rubber bags were assembled in a true triaxial chamber and driven by three servo-hydraulic cylinders. Four division strip-platens are installed along the lateral edges of the confining side wall of the chamber to separate adjacent rubber hydraulic bags and can adjust themselves to accommodate dimensional changes in the deforming specimen. Therefore, this apparatus does not only allow independent control of stresses applied in three dimensions to a cuboidal specimen but also resolve the boundary interferences. The effectiveness of this apparatus was confirmed by tests on remolded loess under five stress paths with the intermediate principal stress parameter b = 0, 0.25, 0.5, 0.75, and 1. The test results show that the stress–strain relationship, volume strain, and failure strength of remolded loess depend on the stress path.

Structure characteristic is the essential property of natural soils. The paper developed an indoor method of artificial structural loess, and studied on the true triaxial tests of artificial structural loess on the base of improving the original true triaxial apparatus. The results show that it is reasonably reliable of this artificial method. At the same time, the variation of failure strength and residual strength of structural loess under complex stress conditions was analysised. When the confining pressure was less than the structural strength of the structural loess, the stress-strain curve was soften, on the contrary the stress-strain curve was harden. In the end the paper analyzed that Mohr-Coulomb strength criterion was accurately to describe the residual strength variation of structural loess but there has much error in describing the peak broken strength, the value was obviously small.

All true tri-axial experimental apparatuses are usually applied to test the mechanics characteristics of soils under complicated stress conditions. Because of the analysis of non-rationality of three kinds of chamber framework developed in past, a new true tri-axial apparatus with a new structural chamber is recently developed by the institute of geotechnical engineering, Xi’an University of Technology. The mechanical description of a new tri-axial apparatus for soil testing is presented herein. The design of new tri-axial apparatus took into the flexibility in adjusting sample size, convenience in assembling sample and easily controlling of three normal stress conditions. The stress-controlled and strain-controlled performance of experiments can be accomplished respectively in the new true tri-axial equipment, which is of a special chamber. The special chamber includes of four cells with bursa filling liquid, being corresponding to four lateral surfaces of cubic sample respectively, separated by steel clapboards. The steel clapboards can turn freely on the horizontal plane and flex in the radial direction so as to adapt to the displacement of sample corner. Through the true tri-axial testing of remolded loess and intact loess were executed on the new true tri-axial apparatus, and the characteristics of stress-strain curves for the true tri-axial tests of remolded loess and intact loess were analyzed under different intermediate principal stresses. The characteristics of the new true tri-axial apparatus are as following: structure is simple, and operation is well-suited, and stress state is real, and strain measurement is accurate, and testing process is controlled easily.