Search

The validity and rationality of the Brazilian disc test have attracted much attention since it was proposed. To verify the validity of four Brazilian disc tests, red sandstone and yellow rust granite were employed. In these experiments, uniaxial tensile tests and four Brazilian disc tests were performed. Meanwhile, the typical location of the Brazilian discs was arranged with gages to detect the crack initiation location. By analyzing the rationality of the four Brazilian disc tests from two aspects, the crack initiation location and the difference between indirect tensile strength and direct tensile strength, it was found that the Brazilian disc tests without cushion and the Brazilian disc tests with two small-diameter steer rods are suitable for determining the indirect tensile strength of rock materials. However, the Brazilian disc tests with arc loading jaws should be carefully adopted to measure the indirect tensile strength due to their overestimation of the indirect tensile strength. Additionally, flattened Brazilian disc tests were found to be not suitable for measuring the indirect tensile strength of rock materials. [ABSTRACT FROM AUTHOR]

Joints and fissures in natural rocks have a significant influence on the stability of the rock mass, and it is often necessary to evaluate strength failure and crack evolution behavior. In this paper, based on experimental tests and numerical simulation (PFC2D), the macro-mechanical behavior and energy mechanism of jointed rock-like specimens with cross non-persistent joints under uniaxial loading were investigated. The focus was to study the effect of joint dip angle α and intersection angle γ on the characteristic stress, the coalescence modes and the energy release of jointed rock-like specimens. For specimens with γ = 30° and 45°, the UCS (uniaxial compression strength), CIS (crack initiation stress) and CDiS (critical dilatancy stress) increase as α increases from 0° to 75°. When γ = 60° and 75°, the UCS, CIS and CDiS increase as α increases from 0° to 60° and decrease when α is over 60°. Both the inclination angle α and intersection angle γ have great influence on the failure pattern of pre-cracked specimens. With different α and γ, specimens exhibit 4 kinds of failure patterns. Both the experimental and numerical results show that the energy of a specimen has similar trends with characteristic stress as α increases. [ABSTRACT FROM AUTHOR]

This paper investigates the influence of a flaw on crack initiation, the failure mode, deformation field and energy mechanism of the rock-like material under uniaxial compression. The results of laboratory test and numerical simulation demonstrate the flaw inclination effect can be classified into three groups: 0–30°, 30–60° and 75–90°. The characteristic stresses increase as the flaw angle increases. The tensile cracks initiate from gentle flaws ( α ⩽ 30°) and shear cracks appear at tips of steep flaws ( α ⩾ 45°). The input energy, strain energy and dissipation energy of a specimen show approximate increasing trends as the flaw angle increases. [ABSTRACT FROM AUTHOR]

The article cites a study which aims to extend the kinematical element method (KEM) in the analysis of rock slope controlled by major geological discontinuities. Analytical expressions are derived to calculate the factor of safety with inclined discontinuities. An optimization method is adopted to locate the critical failure surface. An extended KEM analysis code for rock slope stability analysis is developed.

Using the Chengmenshan Copper Mine as a case study, a new methodology for open pit slope design in karst-prone ground conditions is presented based on integrated stochastic-limit equilibrium analysis. The numerical modeling and optimization design procedure contain a collection of drill core data, karst cave stochastic model generation, SLIDE simulation and bisection method optimization. Borehole investigations are performed, and the statistical result shows that the length of the karst cave fits a negative exponential distribution model, but the length of carbonatite does not exactly follow any standard distribution. The inverse transform method and acceptance-rejection method are used to reproduce the length of the karst cave and carbonatite, respectively. A code for karst cave stochastic model generation, named KCSMG, is developed. The stability of the rock slope with the karst cave stochastic model is analyzed by combining the KCSMG code and the SLIDE program. This approach is then applied to study the effect of the karst cave on the stability of the open pit slope, and a procedure to optimize the open pit slope angle is presented. [ABSTRACT FROM AUTHOR]

Rock strengths are directly influenced by the open or closed flaws widely distributed in rock masses. Extensive studies have been conducted on the propagations of open flaws in rocks. However, few concerns are paid on the propagation of closed flaws, the influence of the surface friction on the initiation and propagation of closed flaws should be investigated systematically. In present article, the crack initiation and propagation in rock like material subjected to compressive loads have been investigated. The effects of crack surface friction on crack initiation and propagation have been quantified with the help from extended finite element method which is efficient and accurate. Based on the analysis on stress distribution and propagation patterns, following results are obtained: Firstly, minor effects are exerted by crack surface friction on the stress distribution around the flaws when the flaws inclination angle is 30° and 45°. However, as the inclination angle increases to 60°, the effects are much more significant. Secondly, as the inclination angle ranges from 30° to 60°, the most favorable angle for crack propagation is 45°. Thirdly, the initiation location and angle of the wing cracks will not be influenced by the frictions. However, the propagation length will be greatly influenced by the friction and the inclination angle. [ABSTRACT FROM AUTHOR]

The influence of confining stress on rock breakage by a tunnel boring machine cutter was investigated by conducting sequential indentation tests in a biaxial state. Combined with morphology measurements of breaking grooves and an analysis of surface and internal crack propagation between nicks, the effects of maximum confining stress and minimum stress on indentation efficiency, crack propagation and chip formation were investigated. Indentation tests and morphology measurements show that increasing a maximum confining stress will result in increased consumed energy in indentations, enlarged groove volumes and promoted indentation efficiency when the corresponding minimum confining stress is fixed. The energy consumed in indentations will increase with increase in minimum confining stress, however, because of the decreased groove volumes as the minimum confining stress increases, the efficiency will decrease. Observations of surface crack propagation show that more intensive fractures will be induced as the maximum confining stress increases, whereas the opposite occurs for an increase of minimum confining stress. An observation of the middle section, cracks and chips shows that as the maximum confining stress increases, chips tend to form in deeper parts when the minimum confining stress is fixed, whereas they tend to formed in shallower parts as the minimum confining stress increases when the maximum confining stress is fixed. [ABSTRACT FROM AUTHOR]

Joints and fissures with similar orientation or characteristics are common in natural rocks; the inclination and density of the fissures affect the mechanical properties and failure mechanism of the rock mass. However, the strength, crack coalescence pattern, and failure mode of rock specimens containing multi-fissures have not been studied comprehensively. In this paper, combining similar material testing and discrete element numerical method (PFC2D), the peak strength and failure characteristics of rock-like materials with multi-fissures are explored. Rock-like specimens were made of cement and sand and pre-existing fissures created by inserting steel shims into cement mortar paste and removing them during curing. The peak strength of multi-fissure specimens depends on the fissure angle α (which is measured counterclockwise from horizontal) and fissure number ( N). Under uniaxial compressional loading, the peak strength increased with increasing α. The material strength was lowest for α = 25°, and highest for α = 90°. The influence of N on the peak strength depended on α. For α = 25° and 45°, N had a strong effect on the peak strength, while for higher α values, especially for the 90° sample, there were no obvious changes in peak strength with different N. Under uniaxial compression, the coalescence modes between the fissures can be classified into three categories: S-mode, T-mode, and M-mode. Moreover, the failure mode can be classified into four categories: mixed failure, shear failure, stepped path failure, and intact failure. The failure mode of the specimen depends on α and N. The peak strength and failure modes in the numerically simulated and experimental results are in good agreement. [ABSTRACT FROM AUTHOR]

Theoretical investigation on the rock breaking and chipping characteristics subjected to TBM cutters under varying confining stresses is conducted. Theoretical study presents that the deflection angle of the lateral crack will increase as the increase of confining stress. Combining previous conclusions on initiation angle with above conclusion on deflection angle, theoretical study shows that in the low confining stress conditions, the chip composed of a triangle shaped part in deep part and a rectangle shaped part in shallow part will be formed; secondly, the triangle shaped part tends to diminish as the confining stress increases. Thirdly, as the confining stress increases in further, the chips are nearly rectangle shaped, and the height of chips will be decreased. Laboratory and numerical studies are also conducted for verification. In both laboratory and numerical tests, the shape evolution mentioned above is verified. And then, due to the varying shapes of chips under different confining stresses, when the spacing is fixed, experimental and numerical tests show that cutting efficiency will be increased as the confining stress increases in a certain degree, while the efficiency will be restrained by the increasing confining stress as the confining stress exceeds a critical value. [ABSTRACT FROM AUTHOR]

Taking sandstone as a research object, this study investigated the damage evolution law of rock mass with different macroscopic and microscopic defects under freeze–thaw cycles. For the first time, macroscopic defects were simulated by prefabricating 0°, 45°, and 90° fractures in real rock materials. Initial microscopic damage was caused by applying prestressing forces equal to 0%, 30%, 50%, and 70% of the ultimate compressive strength of the specimen. The microstructure changes of samples during freeze–thaw cycles were tracked by nuclear magnetic resonance (NMR) technology, and the characteristic parameters such as porosity, aperture distribution, and T 2 spectral distribution were obtained. Through freeze–thaw cycle tests and uniaxial compression tests on specimens undergoing 0, 20, 40, and 60 freeze–thaw cycles, the influence of different damage levels on the changes in the surface, mass, volume, compressive strength, and elastic modulus of rock mass was investigated. The experimental results show that the porosity of the intact specimen is greater than that of the fractured specimen during freeze-thaw cycles. The mechanical properties of the intact specimen and the 90° fractured specimen are controlled by the pore distribution, and the strength drop increases first and then decreases during freeze-thaw cycles, while the mechanical properties of the 0° fractured specimen and the 45° fractured specimen are affected by the prefabricated fracture, and the strength drop increases with the increase of the fracture damage. The initial damage can accelerate the damage degradation of the rock mass, and when the damage level exceeds a certain threshold, which is between 30% and 50%, the deterioration effect will be more significant. • Freeze-thaw damage evolution of rock mass was studied with macro and micro defects considered. • The microstructure changes of samples during freeze-thaw cycles were tracked by NMR technology. • The influence of crack inclination on the mechanical properties of the specimen was analyzed. • The initial damage has an increasing effect on the degradation of the rock mass with damage level. [ABSTRACT FROM AUTHOR]