An investigation of the mechanics of rock joints—Part I. Laboratory investigation

Effect of Varying Normal Stiffness on Soft Rock Joints under Cyclic Shear Loads

The evaluation of changes in shear resistance on soft (or weathered) rock joints under cyclic shear loads with constant normal load (CNL) and constant normal stiffness (CNS) significantly contributes to increasing the safety and stability of rock slopes and underground structures. In this study, a series of cyclic shear tests were conducted on simulated soft rock joints with regular (15°-15°, 30°-30°) and irregular (15°-30°) asperities under different normal stiffnesses (k_n). The results indicated that the first peak shear stress increases with the increase in k_n up to the normal stiffness of the joints (k_nj). Beyond k_nj, no significant change was observed in the peak shear stress. The difference in peak shear stress between regular (30°-30°) and irregular joints (15°-30°) increases as k_n increases. The minimum difference of peak shear stress between regular and irregular joints was observed (8.2%) under CNL and the maximum difference was found (64.3%) on k_nj under CNS. The difference in peak shear stress between the first and subsequent cycles significantly increases as both the joint roughness and k_n increases. A new shear strength model is developed to predict peak shear stress of the joints for different k_n and asperity angles under cyclic shear loads.

Shear strength deterioration effect of rock mass joint surface under cyclic shear load

Understanding the shear strength degradation mechanism of a rock mass joint surface under cyclic shear load and determining a corresponding analytical model is an important foundation for accurately evaluating the safety of rock mass engineering under seismic loads. It is worth noting that, to date, there has been a dearth of studies on the strength characteristics of joint surfaces that consider the number of loading cycles, normal load, and initial undulant angle of the structural plane. In this study, focused on the behaviour of sandstone, the particle flow code (PFC) modelling framework was used to simulate a joint surface cyclic shear test considering first- and second-order undulations. Based on the experimental results, the comprehensive effects of the number of cyclic shear cycles, the normal stress, first- and second-order undulation and the dilatancy angle on shear stress during cyclic shear were analysed. Formulas for the joint surface shear basic friction angle and dilatancy angle under cyclic shear were proposed, and a method for calculating the joint surface peak shear strength under cyclic shear considering the deterioration of the dilatancy angle and basic friction angle was established. The peak shear strength of a sample after five cycles of shearing was calculated using the proposed formula and compared with the results of numerical simulations, the Barton method, and the Homand method. The results showed that the calculated values have good consistency with the results of the numerical simulations, demonstrating the effectiveness and accuracy of the proposed formula. However, under a low normal stress, there could be errors in estimating the cyclic shear strength of the joint surface.

Method of calculating shear strength of rock mass joint surface considering cyclic shear degradation

When a rock mass shears along a joint surface, the shear resistance is affected by joint surface undulations and friction between the contact regions. During an earthquake, the seismic load causes dynamic deterioration of the joint surface mechanical properties, mostly reflected as follows. (1) The peak shear strength of the joint surface decreases with an increase in the shear rate. (2) Under a seismic load cyclic shear, the undulant angle α_k decreases. (3) Under a dynamic load, the friction coefficient of the joint surface is reduced. By studying the cyclic shear test of the joint surface, the strength deterioration effect of the joint surface under cyclic shearing is first analysed, and the equations of the dilatation angle and the basic friction angle of the joint surface under the cyclic shearing load are proposed. Then, starting with the effect of cyclic shear deterioration on the joint surface in the rock mass and the reduction in the dynamic friction coefficient between sliding rock blocks caused by relative velocity, an equation for calculating the shear strength of a rock mass joint surface under cyclic shear loading is recommended. Through two case calculations, the shear strength obtained using the proposed method is compared with the experimental results. The results show that the model proposed in this study is in good agreement with the experimental results and can also be used to calculate the structural surface shear strength of the asperity-rich sample. However, when the calculation equation is used to estimate the cyclic shear strength of the joint surface where the sum of the initial undulation angle and the basic friction angle is greater than 70°, there may be some errors in the calculation results.

An Advanced Shear Strength Criterion for Rock Discontinuities Considering Size and Low Shear Rate

The shear strength of the rock discontinuities under different shear rates is of great importance to evaluate the stability of rock mass engineering, which is remarkably influenced by the size effects induced by both the length and the undulated amplitude of discontinuities. An advanced shear strength criterion taking into account the size and the shear rate simultaneously was proposed. There is an advantage of the dimension unity in terms of the new shear strength criterion in comparison to previous related empirical equations. Additionally, it can be degraded into the International Society for Rock Mechanics (ISRM)-suggested Barton shear strength empirical equation on the peak shear strength of the rock discontinuities. Then, based on a new dynamic direct shear testing device on rock joints, the granite discontinuities with various lengths (200 mm to 1000 mm) and undulated amplitudes (3 mm to 23 mm) were designed to conduct direct shear tests under different low shear rates (0 mm/s to 1 mm/s) to verify the involved empirical equations. It was found that the results predicted by the new shear strength criterion agreed well with the experimental results. It was proved that the new shear strength criterion had a better applicability to characterize the shear strength of the rock discontinuities.

Tensile and Shear Mechanical Characteristics of Longmaxi Shale Laminae Dependent on the Mineral Composition and Morphology

Laminae are well developed in shale and generally influence fracture propagation during hydraulic fracturing. Hence, comprehensively understanding the tension and shear behaviors of shale laminae is crucial. There have been limited systematic studies thus far on the tensile and shear strength as well as fracture morphology of shale laminae. In this study, the Lower Silurian Longmaxi Shale (China) was investigated via Brazilian tensile and angle-varied plate shear tests. Five lamina types were tested, i.e., calcite (Cal), pyrite (Py), organic-enriched (Oc), the interface between Cal and Oc (Cal-Oc), and the interface between Py and Oc (Py-Oc) laminae. Results showed that the tensile strength was in the range 0.43–8.22 MPa, mainly in the order of Cal > Py > Cal-Oc > Py-Oc > Oc. The modes of fracture morphology were highly related to the occurrence, continuity, and mineralogy fillings of laminae. Shear strength parameters were within the range 22.50–29.64 MPa for cohesion and 37.29–43.60° for internal friction angle. Fracture surface roughness was strongly related to its cohesion. Calcite laminae considerably influenced the tensile fracturing of shale, suggesting that the geometry and properties of calcite lamina should receive more attention during the design of shale gas exploration.