The influence of rock mechanical properties on the electromagnetic radiation (EMR) mechanism of rock fracturing is an important research topic in solid mechanics and earthquake prediction. In this ...study, an EMR model of rock fracturing considering the fracture factor, elastic modulus, Poisson’s ratio, radiation distance and crack length is derived based on the Hertz oscillator array assumption. An experimental system, including an electromagnetic shielding module, an EMR signal induction and transmission module, a signal recording module and a loading module, is developed to understand the EMR characteristics of four different rocks. The validity of the EMR theoretical model is verified and the relationships between the rock cracking morphology and the EMR waveform, amplitude and frequency are revealed. It is found that rock mechanical properties have obvious influences on the EMR waveform, amplitude and frequency during rock fracturing. This study provides a better understanding on the EMR mechanism of rock fracturing and can help to improve the accuracy of rock disaster prediction based on EMR.
A damage plasticity model for different types of intact rock Unteregger, D.; Fuchs, B.; Hofstetter, G.
International journal of rock mechanics and mining sciences (Oxford, England : 1997),
December 2015, 2015-12-00, Volume:
80
Journal Article
Peer reviewed
A constitutive model for describing the nonlinear mechanical behavior of different types of intact rock subjected to complex 3D stress states is presented. It is formulated on the basis of a ...combination of plasticity theory and the theory of damage mechanics along the lines of a damage-plastic model for concrete. Irreversible deformations, associated with strain hardening and strain softening, as well as degradation of stiffness can be modeled. The material parameters and the model parameters are identified by means of an optimization procedure combining an evolutionary and gradient based optimization algorithm with niching strategy. The proposed model is validated by numerical simulations of laboratory experiments conducted on specimens of marble, granite and sandstone. The simulations are performed at integration point level. Thereby, the capability of capturing key features of the constitutive behavior of different types of intact rock is demonstrated. Finally, the application of the model to structural analyses is demonstrated by simulating a boundary value problem, including the formation of shear bands.
•An advanced damage plasticity model for different types of intact rock is proposed.•Strain hardening and strain softening as well as degradation of stiffness are modeled.•A parameter identification scheme adopting an optimization algorithm is presented.•The stress–strain behavior of different types of intact rock can be captured very well.•The capability to handle shear banding is shown by FE-analyses of biaxial tests.
Strainburst occurs during deep hard rock excavations. A series of single-free-face true triaxial compression (TTC) tests were conducted on seven hard rock types to investigate strainburst intensity ...and triggering conditions. Five of the seven rock types failed by strainburst, and two failed by spalling. The differences between the strainburst intensities are attributed to the mineral composition and structure. High-hardness minerals and mosaic-type microstructures facilitate stress conduction, which promotes rock microcrack initiation and propagation, causing severe rock fragmentation and fragment ejection, as observed for the Jinzhou granite. However, if the proportions of high- and low-hardness rock minerals are high and the microstructure is stable, such as in the triangular microstructure of the Chifeng basalt, then microcrack initiation and propagation are inhibited, decreasing the likelihood of rock fragmentation. The triggering conditions for strainburst were further analysed by considering the source of kinetic energy. For intact rock specimens, the kinetic energy for rock fragment ejection mainly originates from the intrinsic potential energy (Wbk) released by the specimen itself; thus, strainburst triggering requires that the applied maximum principal stress exceeds the bearing capacity and that the rock Wbk exceeds zero. However, a high Wbk is needed to trigger strainburst for structural rock specimens. Based on these findings, the possibility of triggering strainburst during excavation is greater when the Wbk (>9.1 kJ/m3), rock quality designation (>75%), and in situ stress of the rock are all high.
•Strainburst and spalling that occur during deep underground excavation were reproduced in the laboratory.•The influences of the mineral composition and mineral connections in rocks on the strainburst intensity were analysed.•The role of rock material properties in strainburst triggering was discussed.
Reduction in strength and stiffness in rocks attributed to an increase in water content has been extensively researched on a large variety of rock types over the past decades. Due to the considerable ...variations of texture and lithology, the extent of water-weakening effect is highly varied among different rock types, spanning from nearly negligible in quartzite to 90 % of uniaxial compressive strength reduction in shale. Readers, however, often face difficulties in comparing the data published in different sources due to the discrepancy of experimental procedures of obtaining the water saturation state and how the raw laboratory data is interpreted. In view of this, the present paper first reviews the terminologies commonly used to quantify the amount of water stored in rocks. The second part of the paper reviews the water-weakening effects on rock strengths, particularly focusing on uniaxial compressive strength and modulus, as well as tensile strength, under quasi-static loading and dynamic loading. The correlation relationships established among various parameters, including porosity, density and fabric of rocks, and external factors such as strain rate, surface tension and dielectric constant of the saturating liquid, absorption percentage and suction pressure, are reviewed and presented toward the end of the paper.
Porphyry-type ore deposits are major resources of copper and gold, precipitated from fluids expelled by crustal magma chambers. The metals are typically concentrated in confined ore shells within ...vertically extensive vein networks, formed through hydraulic fracturing of rock by ascending fluids. Numerical modeling shows that dynamic permeability responses to magmatic fluid expulsion can stabilize a front of metal precipitation at the boundary between lithostatically pressured up-flow of hot magmatic fluids and hydrostatically pressured convection of cooler meteoric fluids. The balance between focused heat advection and lateral cooling controls the most important economic characteristics, including size, shape, and ore grade. This self-sustaining process may extend to epithermal gold deposits, venting at active volcanoes, and regions with the potential for geothermal energy production.
To deeply understand the rock failure characteristics under actual engineering condition, in which static geo-stress and dynamic disturbance usually act simultaneously, impact tests were conducted on ...sandstone subjected to axial static pre-stresses varying from 0 to 75 MPa by a modified split Hopkinson pressure bar. The fracturing process of specimens was recorded by a high speed camera. Dynamic parameters of sandstone, such as strain rate, dynamic strength and energy partition were acquired. Fracture mechanisms of pulverized specimens were identified by the method combining the displacement trend line and digital image correlation technique. Moreover, fragments of failed specimens were sieved to obtain the fragment size distribution. Test results revealed that, under the same incident energy, the dynamic compressive strength increases first, then decreases slowly and at last drops rapidly with the increase of pre-stress, and reaches the maximum under 24.4% of uniaxial compressive strength due to the closure of initial defects. Four final patterns were observed, namely intact, axial split, rock burst, and pulverization. The rock burst only occurs when the pre-stress lies in the elastic deformation stage or initial stable crack growth stage and the incident energy is intermediate. For pulverized specimens, the fracture mechanism is transformed into shear/tensile equivalent from tensile-dominated mixed mode as the pre-stress increases. Specimens with 75 MPa pre-stress release strain energy during failure process, contrary to specimens with lower pre-stresses absorbing energy from outside. The crushing degree of pulverized specimens exhibits a positive correlation with the pre-stress as a consequence of higher damage development in rock.
Using a hydraulic servo pressure testing machine, we investigated physical and mechanical properties of granite and sandstone samples after high-temperature treatment. Variations in apparent form, ...density, longitudinal wave velocity, damage, stress–strain curve, compression strength, peak strain, and elastic modulus with temperature are analyzed. We explored the differences in temperature effects on the physical and mechanical characteristics. Granite samples begin to gradually change from taupe to gray above 600°C and sandstone samples start to gradually change from gray to brownish-red above 400°C. 600°C and 800°C are respectively the brittle–ductile transition critical temperatures of granite and sandstone stress–strain curves. Rock density generally shows a gradual decreasing trend with temperature increasing. The influence of high temperature on sandstone density is much greater than granite. The decline rate of longitudinal wave velocity in granite is much greater than sandstone. Overall, damage increases gradually with temperature, but granite appears to suffer negative damage at 100°C. Granite sample compression strength generally decreases with increasing temperature, while sandstone compression strength does not show obvious change between room temperature and 800°C, but begins to decline rapidly above 800°C. Peak strain increases with temperature and the increase rate of granite is similar to that of sandstone. Elastic modulus decreases with increasing temperature and decline rate of granite is much greater than sandstone. In addition, the temperature effect on the physical properties is much more intense than the effect on the mechanical properties of these two rock types.
•600°C and 800°C are critical temperatures of granite and sandstone stress–strain curves.•Damage of granite appears negative at 100°C.•UCS of sandstone does not obviously change from room temperature to 800°C.•Temperature effect on physical property is more intense than mechanical property.
Serpentinized rocks closely associated with Paleoproterozoic eclogitic metabasites were recently discovered at Eseka area in the northwestern edge of the Congo craton in southern Cameroon. Here, we ...present new field data, petrography, and first comprehensible whole-rock geochemistry data and discuss the protolith and tectonic significance of these serpentinites in the region. The studied rock samples are characterized by pseudomorphic textures, including mesh microstructure formed by serpentine intergrowths with cores of olivine, bastites after pyroxene. Antigorite constitutes almost the whole bulk of the rocks and is associated (to the less amount) with tremolite, talc, spinel, and magnetite. Whole-rock chemistry of the Eseka serpentinites led to the distinction of two types. Type 1 has high MgO (> 40 wt%) content and high Mg# values (88.80) whereas Type 2 serpentinite samples display relatively low MgO concentration and Mg# values (< 40 and 82.88 wt%, respectively). Both types have low Al/Si and high Mg/Si ratios than the primitive mantle, reflecting a refractory abyssal mantle peridotite protolith. Partial melting modeling indicates that these rocks were derived from melting of spinel peridotite before serpentinization. Bulk rock high-Ti content is similar to the values of subducted serpentinites (> 50 ppm). This similarity, associated with the high Cr contents, spinel-peridotite protolith compositions and Mg/Si and Al/Si ratios imply that the studied serpentinites were formed in a subduction-related environment. The U-shaped chondrite normalized-REE patterns of serpentinized peridotites, coupled with similar enrichments in LREE and HFSE, suggest the refertilized nature due to melt/rock interaction prior to serpentinization. Based on the results, we suggest that the Eseka serpentinized peridotites are mantle residues that suffered a high degree of partial melting in a subduction-related environment, especially in Supra Subduction Zone setting. These new findings suggest that the Nyong series in Cameroon represents an uncontested Paleoproterozoic suture zone between the Congo craton and the São Francisco craton in Brazil.
A new, high-efficiency technology for fracturing and breaking rocks is required. Due to various advantages including high efficiency, energy-saving, and having no secondary pollution, the technology ...of microwave-induced fracturing of hard rock has been considered as a potential method for rock fracturing and breaking. Aiming at the realisation of two engineering applications: microwave-assisted mechanical rock breaking and stress release from rock masses in deep underground engineering works to prevent geological disasters caused by high-stress concentrations such as rockbursts, a novel (open-type) microwave-induced fracturing apparatus (OMWFA) for fracturing hard rocks was developed. On this basis, the two modes of microwave-induced subsurface fracturing and microwave-induced borehole fracturing of hard rocks were proposed. Due to removal of the restraint of the microwave cavity, OMWFA can be used to fracture large-size rock samples and engineering-scale rock masses. Using the apparatus, the fracturing effects of the two fracturing modes on different dimensions of cuboidal basalt samples were investigated. By combining the microwave-induced fracturing apparatus with a press machine to explore the influence of unidirectional stress on the fracturing effect of microwave treatment on basalt. Moreover, field tests were carried out on rock masses encountered in underground engineering works at Baihetan Hydropower Station in Sichuan Province, China, and the fracturing effects were evaluated by applying a digital borehole televiewer and conducting acoustic wave testing. The results show that the apparatus had favourable fracturing effects on the subsurface and borehole samples of basalt. When no stress was applied, the cracks radially expanded from the approximate centre of the radiant surface and unidirectional stress promoted fracturing. The number and depth of cracks increased with prolonged microwave exposure. After microwave treatment, the P-wave velocity of the samples declined, and the longer the microwave exposure, the more significant the reduction in P-wave velocity was. The results of field test reveal that borehole fracturing can exhibit a favourable effect around boreholes. The sound velocity around the borehole and between the boreholes both declined to some extent. Microwave-induced hard rock fracturing offers guiding significance to those exploring and developing new rock breaking and tunnelling methods, and generally enhances construction safety in deep underground engineering works.
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