The stress state is the fundamental for evaluating the soil strength and stability, playing a crucial role. However, during the stress testing, local damage and other uncertain factors may lead to partial sensor data missing, causing the existing three-dimensional stress calculation method to fail. To accurately restore the soil stress state during data missing, a three-dimensional stress calculation method was developed based on three-dimensional stress testing principles, incorporating axisymmetric and one-dimensional compression characteristics. The three-dimensional stress, principal stress , the first invariant of stress I-1, the second in variant of stress J(2) and stress Lode angle of a sandy soil foundation under one-dimensional compression conditions with different data missing were calculated and compared to results with complete data. The results show that the method is highly accurate; as the load increases, the relative error decreases and converges. The principal stresses, the first invariant of stress I-1, the second invariant of stress J(2) and the stress Lode angle align with one-dimensional compression response, suggesting that this calculation method supports advanced data mining. This study offers a novel approach and a practical method for fully utilizing the test data.
Due to its distinct characteristics of instantaneity and abruptness, the stress variation characteristics of unsaturated soil under impact loads significantly differ from those under static and conventional dynamic loads. To investigate the spatial stress state under impact loads, in-situ testing was conducted on an unsaturated soil roadbed using three-dimensional stress testing technology. The three-dimensional soil pressure cells were set at depths of 0.3 m and 0.6 m below the ground surface. Continuous vertical impact loads were applied at the ground projection of the buried points. Stress testing data was collected in real time, and stress transformation methods were applied to obtain the corresponding three-dimensional stress, principal stresses, and the evolution of principal directions. Based on this, a comparison was made with existing one-dimensional stress testing methods and results, further illustrating the rationality and scientific validity of three-dimensional stress testing. The testing data revealed that under impact loads, the stress component in the impact direction (i.e., the z-axis direction) shows a notable instantaneous increase with a positive increment, whereas the increment of positive stress in the y-direction is negative. The principal stress direction angles alpha, beta, and gamma undergo considerable deviations during the impact. Specifically, alpha varies within a 90 degrees range, while beta and gamma rapidly decrease from their initial values to their supplements. Moreover, all three directional angles experience multiple reciprocating changes within a single impact duration. This research has theoretical significance in deepening the understanding of stress response and evolution processes in unsaturated soils under impact loads, providing valuable references for constitutive models, engineering design, and construction research related to seismic or other impact loadings.
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.
Focusing mainly on the compressive-shear (C-S) stress, the existing soil strength theories fail to well describe the soil strength under tenso-shear (T-S) stress and are not suitable for analyzing the tensile strength and T-S coupling strength. Taking the T-S strength into account, a three-dimensional nonlinear strength model of soil is established. The failure function on the pi plane is a Lade Lade-curved triangle in principal stress space, which reasonably reflects the effect of medium principal stress. The failure function on the triaxial compression meridian plane is divided into the T-S and C-S zones, in which the two failure functions are smoothly connected at the closed stress point. To easily combine it with the specific elastoplastic constitutive theory, the proposed strength model is transformed into the D-P criterion in transformed stress space using the transformation stress method, which provides a theoretical foundation for numerical calculations. Compared with the true triaxial test results and numerical slope examples, the proposed model gives a good description of the nonlinear soil strength under complex stresses and the instability failure mechanism of the slope.