This paper presents an experimental study in which molded gypsum specimens with different crack geometries (T-shaped and X-shaped cross crack) were tested in biaxial compression. Crack propagation and failure behaviors were investigated and nine crack types were detected, namely, wing crack, antiwing crack, secondary crack, horizontal wing crack, quasi-coplanar wing crack, quasi-coplanar secondary crack, far-field crack, surface spalling, and lateral split crack. Moreover, horizontal wing crack and quasi-coplanar wing crack were first observed in the rocklike specimens, which was caused by the added confining pressure. Besides, the cracks and failure modes under uniaxial and biaxial compression were compared according to proposed nine types of new cracks. With an increase in confining pressure, surface spalling and lateral split crack appeared. Lateral split cracks were observed only in T04 (${\sigma }_2=1.5\mathrm{MPa}$ ), X03 (${\sigma }_2=1.0\mathrm{MPa}$ ), and X04 (${\sigma }_2=1.5\mathrm{MPa}$ ), which indicate that lateral split crack is hardly induced under lower confining pressure. The reason is that confining pressure prevents the lateral deformation, therefore, failure can appear only along the free face such as surface spalling and lateral split crack. In practice, surface spalling is the common failure near excavation faces in deep tunnels, which validates the experimental results here.

A moment tensor analysis of acoustic emission was applied to quantitatively study the microcracking mechanism and damage evolution process in cylindrical specimens of granite and sandstone under diametral compression. It was found that the localized damage zone tended to originate and propagate from the vicinity of the loading ends. During the damage accumulation process, the tensile cracks played a dominant role in number, but the acoustic emission (AE) energy release of microcracks of different types presented an inconsistent trend for the two types of rock. Scanning electron microscope (SEM) observations on the fractured surfaces were conducted, which indicated that during the rock failure process there coexisted microcracks of three types—tensile, shear, and mixed-mode—due to the heterogeneity of the mechanical properties of the rock. The microscopic observations using a polarized light microscope technique revealed that the mineralogical composition and microstructure of rock had a significant effect on its microcracking mechanism.

This paper develops a novel empirical strength criterion and proposes a manifold method for fracture initiation and propagation. First, to investigate the influencing factors of fractured rock mass strength, a set of uniaxial and biaxial compression tests were performed on gypsum specimens. According to the experimental results, rock mass strength is attributable to nonlinear relationships with fracture inclination angle, length, and lateral pressure. Then, on the basis of strength theory and tests, an improved empirical strength criterion, which can describe the multi-influence of fracture inclination angle, fragmentation degree, and lateral pressure on strength of the rock mass and the path of fracture propagation, was established. Moreover, the critical conditions of tensile and shear fracture for fracture propagation are proposed and used with a numerical manifold method to simulate fracture propagation. Furthermore, in the modeling, the manifold element is regarded as the basic failure elemen...

Prediction of rock mass strength is important for estimating stability in rock engineering projects. In this paper, a novel strength criterion was proposed to determine the strength of a jointed rock mass subjected to a complex stress state. First, a series of tests were carried out on specimens with different joint geometries and under different confining pressures. From these tests, the factors affecting the strength of the jointed rock mass were obtained. The experimental results showed that normalized strength decreases initially and then increases as joint inclination angle increases, and it increases consistently and nonlinearly with increasing normalized confining pressure. Based on the experimental results, an improved nonlinear strength criterion was presented to describe the combined influence of the two aforementioned factors on jointed rock mass strength, and methods for determining the involved parameters were also introduced. Furthermore, multiaxial test data from the literature and biaxial test data from this paper were used to validate the feasibility and reliability of the new criterion. The results showed that the new criterion can predict the strength of jointed rock with high accuracy.

Mudstone material in a deep roadway is under the coupled stress-seepage condition. To investigate the permeability change and damage development during rock excavation in roadways, a stress-seepage damage coupling model has been proposed. In this model, damage capacity expansion of mudstone material is considered as the initiation and propagation of micro-cracks and the fracture penetration. A damage variable is introduced into the proposed model based on the principle of minimum energy consumption. As a result, an elastoplastic damage constitutive equation is established. Then, the permeability evolution equation describing the micro-macro hydraulic behavior of mudstone is deduced via percolation theory, which can describe the characteristics of sudden permeability change after rock capacity expansion. Furthermore, a finite element model is established based on commercial finite element software-ABAQUS. The numerical model was firstly verified by comparison between experimental and simulation results. On the basis of it, numerical investigation of the temporal and spatial evolution law of pore pressure, damage and permeability coefficient during roadway excavation is undertaken. The numerical results indicate that with increase of construction time, pore pressure first increases and then decreases, while the damage zone and permeability coefficient increase gradually and finally nearly keep constant. The proposed coupling model and finite element method can describe damage and permeability evolution for mudstone material under coupled stress-seepage well.