The non-stationary trend of rock joints significantly affects the shear strength of rock joints. Taking the least square fitting plane of a rock joint as a reference plane, two trend direction parameters α and β of the rock joint are proposed to characterize the non-stationary trend of rock joints quantitatively. α is the inclination angle of the rock joint along the shearing direction, and β is the deflection angle of the rock joint perpendicular to the shearing direction. Based on the trend direction parameters, the external normal and tangential stresses are decomposed into the normal and tangential stresses on the reference plane. Consequently, a rock joint shear strength model is established. Next, samples of different sizes are obtained from a large rock joint. Based on the established shear strength model, the shear strengths of the rock joint samples in different sizes under the influence of non-stationary trends are statistically analyzed.
The shear strength of the rock joints is one of the most important mechanical parameters for the stability evaluation of engineering rock masses (Barton et al. 2023; Wang et al. 2021, 2023). In actual engineering, it is difficult to directly obtain the shear strength parameters of rock joints through in-situ tests and laboratory tests. Commonly, the rock joint of rock mass is cut into rock joint specimens in the same size, and the shear strength of the rock joint is estimated by evaluating the shear strength of each rock joint specimen (Yong et al. 2019; Kulatilake et al. 2021; Wang et al. 2022). According to the direct shear test method suggested by ISRM (Muralha et al. 2013), the shear region of the rock joint specimen should be guaranteed to be parallel to the shear plane. For rough and undulating rock joints, the least squares fitting plane can be taken as the reference plane of the rock joint (Tatone & Grasselli 2010).
As shown in Figure.1, the dotted line is the least square fitting plane of the rock joint, and i1∼i5 are the angles between the least square fitting plane and the shear plane of the rock joint specimens. Figure 1 shows that when the reference plane of the rock mass structural plane is parallel to the shear plane, there is an angle between the cut rock joint specimen and the shear plane; that is, there is a non-stationary trend. When evaluating the shear strength of rock joint specimens, due to the non-stationary trend, the normal stress and shear stress acting on the actual shear plane is different from the externally applied stress values. If the existing shear strength formula that did not consider the influence of the non-stationary trend of the rock joint is used to calculate the shear strength of the rock joint specimen, the calculated shear strength will be inconsistent with the actual shear strength, which will eventually affect the accuracy of the shear strength estimation of rock mass.
