## 1 Introduction
Research indicates that the strength estimated under plane strain conditions is generally 10 to 20% higher compared to strength measured under conventional triaxial conditions [27]. If the calculation employs plane strain parameters, a substantial amount of the engineering cost could be reduced. Past studies by [PERSON] (1978), [PERSON] (1997), [PERSON] (2003), and [PERSON] (2001) furnish valuable insights into the impact of confining pressure, compactness, drainage conditions, loading paths, and other factors on the mechanical properties of soil in a plane strain test, and the results of the experimental research are significant [27; 28; 29; 30]. The literature offers limited reports on soil response under both rotations of the principal stress axis and plane strain conditions. Currently, only a few researchers, such as [PERSON] (1978) and [PERSON] and [PERSON] (1986), have conducted anisotropic experiments that consider the effects of changes in the deposition angle (rotation of the principal stress axis relative to the fabric direction) under plane strain conditions [27; 31]. The scarcity of reports on anisotropic testing is due to sample preparation difficulties and considerable artificial effects during the experimental phase. Consequently, to gain a more profound comprehension of the mechanical properties of soil under plane strain conditions with the rotation of the principal stress axis, it is essential to augment our understanding via numerical simulations, constitutive modeling, and other approaches. This paper aims to further investigate this issue from a constitutive modeling viewpoint. The introduction of hypoplastic constitutive models in soil mechanics research has provided a novel perspective for the investigation of soil constitutive models. Hypoplastic models do not involve complex concepts, such as yield functions, hardening rules, and elastic-plastic strain separation, which are present in the theory of elastoplasticity. The fundamental concept of hypoplasticity was first proposed by [PERSON] (1991), who used a rate-independent non-linear tensor function to describe the non-elastic material characteristics [32]. [PERSON] and [PERSON] (1994) presented the first simple and general four-parameter hypoplastic constitutive model [33] based on the general equation of hypoplastic constitutive models proposed by [PERSON] and [PERSON] (1990) [34]. [PERSON] (1996) extended the previous model by incorporating the SMP criterion to propose an eight-parameter model [35]. [PERSON] and [PERSON] (2017) revised the original four-parameter model by introducing new tensor terms to ensure that the constitutive response could still reach the critical state even under large strains [36]. Over the past decade, the studies of hypoplastic theory have undergone exponential development [37; 38; 39; 40]. It can be observed that most of the developments of the aforementioned hypoplastic models have been based on the isotropy assumption of the materials. The isotropic models cannot explain the differences in the mechanical properties of soils with varying directions, while anisotropic elastoplastic models can effectively clarify this phenomenon and accurately predict the constitutive response of soils. Their simulation results are more consistent with the measured results from indoor tests and engineering practice. In geotechnical research, these models can be used to analyze the stability and bearing capacity of soils, and therefore to guide engineering design and construction, while anisotropic models that take into account the effect of principal stress axis rotation have seldom been reported. Considering the simplicity and engineering application of the constitutive model, this paper will integrate the effect of principal stress axis rotation into hypoplastic constitutive models. We will consider the transformational relationship between the principal stress axis and the fabric direction based on fabric tensor.