Search

Rockbursts are a complex phenomenon characterized by a violent ejection of rock fragments from the free face of deep underground mines. Due to the sudden, disruptive, and complex nature of these events, accurate predictions of rockbursts are difficult. Therefore, geotechnical intervention is imperative to prevent the occurrence of a rockburst. The scenarios of microwave destressing is introduced here as an application method to potentially prevent their occurrence. To understand the influences of microwave destressing, such as microwave exposure duration, spatial layout, and geometric parameters of the microwave preconditioned zone (MPZ), on the occurrence of a rockburst, numerical simulation was conducted based on an impact-induced rockburst model. The feasibility of microwave destressing technology was validated numerically. This paper also provides guidance for the utilization of microwave destressing in practice. Discontinuous MPZ is suitable for a known blasting source with a fixed position, and continuous MPZ can be applied for a working condition with multiple blasting sources with variable positions. A closer distance of MPZ from the free face results in a better microwave destressing effect. However, the closer distance of MPZ from the free face also introduces more negative influences on the static stability of surrounding rock.

AbstractTo analyse the different mechanism of polycarboxylate superplasticizer and polyester fibre and its effect on cement mortar during freeze-thaw cycles, by taking cement mortar as main research object and the proportion of additive as variables, the varying pattern of the mechanical properties, size and distribution of the pores of the cement mortar during freeze-thaw cycles were studied by the NMR technology for the first time. The influence of different proportion of additive on the changes of the surface scaling, mass, compressive strength, splitting tensile strength and elastic modulus of cement mortar after 0, 25, 50, 75 and 100 times freeze-thaw cycles was studied. Meanwhile, the T2spectra change law of the cement mortar under different freeze-thaw environment was analysed and the change rule between freeze-thaw cycle, additive proportion and the pore size within the cement mortar was obtained. Also, the relationship between the mechanical parameters and pore properties was studied. This study on the damage evolution mechanism of additive cement mortar during freeze-thaw process is of great theoretical and practical significance to the analysis and prevention of freeze-thaw disasters in cold area.

To study the physical properties and dynamic mechanical response characteristics of rock materials in seasonally frozen regions, density, P-wave velocity, nuclear magnetic resonance, and dynamics Brazilian disc tests were carried out on sandstones treated with different numbers of freeze-thaw cycles. The experimental results show that the water-saturated density and P-wave velocity of sandstone decrease and the Nuclear magnetic resonance porosity increases as the number of freeze-thaw cycles increases. Under a certain loading rate, the dynamic tensile strength of water-saturated sandstone gradually decreases and that of frozen sandstone gradually increases as the number of freeze-thaw cycles increases; at the same time, the dynamic tensile strength of both water-saturated and frozen sandstone increases as the loading rate increases under the same number of freeze-thaw cycles. The reason for the variability of dynamic tensile strength between water-saturated and frozen sandstones with the increase in the number of freeze-thaw cycles can be attributed to the change in porosity. On this basis, the concept of relative dynamic tensile strength of sandstones was introduced and regression analysis was carried out with the increase ratio of porosity, and it was found that the correlation between the relative dynamic tensile strength and the increase ratio of porosity was strong. The relative dynamic tensile strength of water-saturated sandstone decreases exponentially with the increase ratio of porosity; the relative dynamic tensile strength of frozen sandstone increases exponentially with the increase ratio of porosity.

Cutter wear or damage is a significant issue during tunnel boring machine (TBM) tunneling in hard rock. Microwave preconditioning has been verified as a promising approach to reduce cutter wear and enhance the TBM excavation efficiency. Thus, understanding the TBM cutting performance for microwave-treated hard rock is necessary. First, numerical verification of a cutting model was performed to examine the universality and reliability of the model. Then, the rock mechanical parameters of microwave-treated basalt were calibrated using linear Mohr–Coulomb theory. Finally, linear cutting simulations were conducted with an unrelieved rock model by considering the variables of the disc cutter penetration depth and microwave irradiation conditions. The numerical results indicated that the maximum reduction in the rolling and normal forces reached 38.38% and 44.95% (under a 5-kW microwave power and 3-mm penetration depth), respectively. A novel indicator of the linear friction energy was proposed to assess disc cutter wear, and the maximum reduction reached 36.81% under a microwave power of 5 kW and penetration depth of 4 mm. Considering the microwave weakening efficiency, TBM tunneling efficiency and cutter wear, microwave parameters including a high microwave power and short irradiation time were suggested for future practice.

In high-altitude cold areas, freeze-thaw (F-T) cycles induced by day-night and seasonal temperature changes cause numerous rock mass slope engineering disasters. To investigate the dynamic properties of rock in the natural environment of a high-altitude cold area, standard specimens were drilled from the slope of the Jiama copper mine in Tibet, and dynamic compression tests were performed on water-saturated and frozen sandstone with different numbers of F-T cycles (0, 10, 20, 30, and 40) by the split Hopkinson pressure bar (SHPB) system with a cryogenic control system. The influence of water-saturated and frozen conditions on the dynamic performance of sandstone was investigated. The following conclusions are drawn: (1) With increasing strain rate, the attenuation factor (λa) of water-saturated sandstone and the intensifying factor (λi) of frozen sandstone linearly increase. As the number of F-T cycles increases, the dependence factor (λd) of water-saturated sandstone linearly decreases, whereas the λdof frozen sandstone linearly increases. (2) The prediction equation of the dynamic compressive strength of water-saturated and frozen sandstone is obtained, which can be used to predict the dynamic compressive strength of sandstone after various F-T cycles based on the strain rate. (3) The mesoscopic mechanism of water-saturated and frozen sandstone's dynamic compressive strength evolution is investigated. The water softening effect causes the dynamic compressive strength of water-saturated sandstone to decrease, whereas the strengthening effect of pore ice causes it to increase. (4) The decrease in the relative dynamic compressive strength of water-saturated sandstone and the increase in the relative dynamic compressive strength of frozen sandstone can be attributed to the increased porosity.