Ming Tao

Experimental Study of Slabbing and Rockburst Induced by True-Triaxial Unloading and Local Dynamic Disturbance

Slabbing/spalling and rockburst are unconventional types of failure of hard rocks under conditions of unloading and various dynamic loads in environments with high and complex initial stresses. In this study, the failure behaviors of different rock types (granite, red sandstone, and cement mortar) were investigated using a novel testing system coupled to true-triaxial static loads and local dynamic disturbances. An acoustic emission system and a high-speed camera were used to record the real-time fracturing processes. The true-triaxial unloading test results indicate that slabbing occurred in the granite and sandstone, whereas the cement mortar underwent shear failure. Under local dynamically disturbed loading, none of the specimens displayed obvious fracturing at low-amplitude local dynamic loading; however, the degree of rock failure increased as the local dynamic loading amplitude increased. The cement mortar displayed no failure during testing, showing a considerable load-carrying capacity after testing. The sandstone underwent a relatively stable fracturing process, whereas violent rockbursts occurred in the granite specimen. The fracturing process does not appear to depend on the direction of local dynamic loading, and the acoustic emission count rate during rock fragmentation shows that similar crack evolution occurred under the two test scenarios (true-triaxial unloading and local dynamically disturbed loading).

Determination of Dynamic Flexural Tensile Strength of Thermally Treated Laurentian Granite Using Semi-Circular Specimens

AbstractnTo understand the effects of increasing temperature and loading rate on the flexural tensile strength of Laurentian granite, dynamic flexural tensile strength experiments were carried out by means of a semi-circular bend specimen with a modified split Hopkinson pressure bar system. The tests were performed at different loading rates, specimens were treated from room temperature up to 850xa0°C, and a high-speed camera was utilized to monitor the failure process of the specimen. For samples in the same temperature group, a loading rate dependence of the flexural tensile strength was observed; it increased consistently with the increase of loading rate. Temperature effects on rock mechanical properties were investigated from the microscopic viewpoint, and the dynamic flexural tensile strength decreased with the treatment temperature. A formula relating dynamic flexural tensile strength to loading rate and temperature is presented to quantify the results. It was found that the change regulation of the dynamic flexural tensile strength of rock is very similar to that of its crack growth along with the increase of loading rate, which indicates that the essence of rock failure is the initiation and propagation of the internal cracks. Compared with our earlier work on dynamic tensile tests using the Brazilian test, it was observed that the flexural tensile strength is higher than the tensile strength. Non-local failure theory can be adopted to explain this discrepancy at low temperature conditions, but it is no longer effective at high temperatures. Under high loading rates, rock failure is initiated at the centre of the half circular disc, and finally it is separated completely into two equal parts.n

Dynamic Brazilian Splitting Test of Ring-Shaped Specimens with Different Hole Diameters

Keywords: Dynamic loading ; Brazilian splitting ; Ring-shaped specimen ; Full-field strain field ; Digital image correlation ; Split Hopkinson pressure bar Reference EPFL-ARTICLE-223259doi:10.1007/s00603-016-0995-zView record in Web of Science Record created on 2016-11-21, modified on 2017-02-06

Experimental Study of the Triaxial Strength Properties of Hollow Cylindrical Granite Specimens Under Coupled External and Internal Confining Stresses

High geostresses and stress gradients are the predominant stress conditions in deep excavation-disturbed rock masses. The aim of this study is to determine the triaxial compressive strength properties of hollow cylindrical granite specimens under a radially non-uniform confining stress field with different radial stress gradients determined by coupled external and internal confining stresses. Triaxial compression testing of hollow cylindrical rock specimens was performed to investigate the influence of the radial stress gradient, external confining stress and specimen length-to-diameter (L/D) ratio on the triaxial compressive strength. The experimental results and regressed failure criteria indicate that the triaxial compressive strengths of the hollow cylindrical granite specimens increase with the external confining stresses, but decrease with an increase in the radial stress gradients. The calculated goodness of fit (R2) and root-mean-squared error suggest that the nonlinear failure criterion based on the Hoek–Brown model is more accurate than the linear failure criterion based on the Mohr–Coulomb model for determining the influences of the external confining stress and radial stress gradient on the triaxial compressive strength. In addition, the triaxial compressive strength increases with a decreasing L/D ratio due to the strengthening end effect of the hollow cylindrical granite specimens and the change in the failure pattern of these specimens from shear to slabbing.

Influence of Geostress Orientation on Fracture Response of Deep Underground Cavity Subjected to Dynamic Loading

Deep underground cavity is often damaged under the combined actions of high excavating-induced local stresses and dynamic loading. The fracturing zone and failure type are much related to the initial geostress state. To investigate the influence of geostress orientation on fracture behaviours of underground cavity due to dynamic loading, implicit to explicit sequential solution method was performed in the numerical code to realize the calculation of geostress initialization and dynamic loading on deep underground cavity. The results indicate that when the geostress orientation is heterotropic to the roadway’s floor face (e.g., 30° or 60°), high stress and strain energy concentration are presented in the corner and the spandrel of the roadway, where V-shaped rock failure occurs with the release of massive energy in a very short time. When the geostress orientation is orthogonal to the roadway (e.g., 0° or 90°), the tangential stress and strain energy distribute symmetrically around the cavity. In this regard, the stored strain energy is released slowly under the dynamic loading, resulting in mainly parallel fracture along the roadway’s profile. Therefore, to minimize the damage extent of the surrounding rock, it is of great concern to design the best excavation location and direction of new-opened roadway based on the measuring data of in situ geostresses.

Numerical modelling of water-coupling cutting blasting in granite quarry

Abstract To achieve a smooth fracture plane with minimal damage to a rock block or remaining rock mass, a water-coupling cutting blasting technique was applied in a granite quarry. The mechanism of water-coupling cutting blasting is demonstrated with the specification of prerequisites for crack growing and arresting. Using the LS-DYNA program, the maximum principal stress distribution and rock damage evolution process were observed, and the validity of crack extension till coalescence was verified by comparing the calculated and theoretical water pressure needed to induce fracture. The optimum blasting parameters for achieving the best cutting blasting performance were obtained by simulations under different coupling ratios and blasthole spacings. The successful water-coupling cutting blasting practice in the granite quarry has great potential to be applied in other similar engineering.