In this paper, we introduce a three-dimensional triaxial apparatus with rigid walls. Its pressure chamber comprises four sliding rigid plates, a rigid specimen cap, and a rigid bottom plate. It has a three-dimensional servo hydraulic load control system, an intelligent control and data storage system, and a water-air suction control system. Considering a cuboid soil specimen as a true triaxial shear layer and a vertical principal stress transfer layer, the vertical principal stress is transferred from the transfer layer to the shear layer, and the orthogonal horizontal principal stress is applied by the horizontal slip rigid plates. That solves the technical problem of mutual interference observed in conventional three-dimensional rigid plate loading. The L-shaped loading plate is improved, which reduces the deflection and friction between them. Linear guides ensure that the horizontal stress is applied synchronously and the specimen is always centered during a test. True triaxial testing of Xi'an loess is reported, and the results confirm the applicability of the apparatus in soil testing.

Limited laboratory studies have investigated the cyclic behavior of sands under plane strain state, despite the current extensive applications of the plane strain hypothesis in modeling the behavior of subgrade soils beneath long road embankments. This study aims to explore the traffic-induced deformation behavior of sand under plane strain state and compare it to the conventional triaxial stress state. A series of one-way high-cyclic tests were performed on Fujian sand under both states using a true triaxial apparatus, considering different cyclic stress levels, consolidation stresses, consolidation anisotropies, and relative densities. In the plane strain scenario, the deformation of the specimen in the direction of intermediate principal stress was restricted when the cyclic major principal stress was applied. The test results indicate that during long-term cyclic loading, the sand exhibits substantially lower accumulated axial and volumetric strains when subjected to plane strain state as opposed to the conventional triaxial state. The reduction effect of plane strain state on the accumulated axial strain was found to be distinctively correlated with the strain levels, regardless of the cyclic stress amplitude and relative density. A practical formula was developed to estimate the difference in accumulated axial strain between the plane strain and triaxial states. Additionally, the intermediate principal stress of specimens under plane strain state was observed to oscillate cyclically in accordance with the one-way vertical cyclic stress. The intermediate principal stress coefficient, triggered by vertical cyclic loading, is more pronounced under high deformation, with its magnitude dependent on the specific loading conditions.

Unbound granular materials (UGMs) are extensively used in pavements mostly as subgrade and subbase materials. Excessive permanent settlement or rutting is the main damage mechanism encountered in UGMs. Rutting is a result of accumulated gradual plastic strain in the subbase and subgrade layers subjected to repetitive traffic loadings. Axisymmetric triaxial apparatus or repeated lateral triaxial (RLT) devices are commonly used to explore the rutting of UGMs. However, these devices are not able to capture the actual stress state generated in traffic. A soil element in pavement layers is in a three-dimensional (3D) stress state and includes all three components of cyclic principal stresses. A typical pavement also can be considered geometrically as a plane strain structure. Accordingly, aim of this study is to carry out experiments to determine the long-term deformation of a silty sand in plane strain and in a 3D stress state using a multistage true triaxial apparatus (TTA). It is found that the permanent deformation of UGMs under plane strain and 3D anisotropic stress state differs significantly from that under axisymmetric stress. An increase in the intermediate principal stress was observed to decrease the total and permanent deformation. An increase in cyclic stress level was also found to increase the rutting in UGMs. The deformation of soil under the plane strain state was found to be less than that in the axisymmetric stress state but falls into an intermediate range when compared to tests involving 3D cyclic loading.