Granite Rock Specimens Research Articles

Shackleton and Bruce: disentangling Antarctic geological collections at National Museums Scotland

Amongst the Antarctic geological specimens held by National Museums Scotland are those from collections made during early 20th Century expeditions led by William Speirs Bruce and Ernest Shackleton. Historical circumstances and ambiguous labelling led to the Shack- leton material, from his 1907–1909 Nimrod expedition, being incorporated into the Bruce collection and incorrectly assigned to the latter’s 1902–1904 Scotia expedition. The recent identification of the Shackleton specimens, examination of their surviving labels and a re- view of the relevant Polar literature has now allowed the confusion to be resolved and ex- plained. The Shackleton collection proves to comprise ten basaltic rock specimens from Ross Island, from the vicinity of the active Mt Erebus volcano, and two granitic rock specimens from mountains further inland at the western margin of the Ross Sea. The geologists likely to have been involved in the collecting were Tannatt William Edgeworth David, Douglas Mawson and Raymond Edward Priestley. Two additional specimens can be assigned to a subsequent Australian expedition led by Mawson in 1911–1913.

Experimental Investigation of Mechanical and Fracture Behavior of Parallel Double Flawed Granite Material under Impact with Digital Image Correlation

During the excavation of underground projects, the rock masses left as the bearing support system are also subjected to dynamic loads from the excavation activities ahead. These rock masses have been damaged and fractured during the initial exposure (dynamic loads) and are subjected to static loads in the subsequent process as the support system. In this study, granite rock samples and specimens with different angles were produced, preloaded with different confining pressure, and under a combination of dynamic and static loading tests using a modified dynamic and static loading system: split Hopkinson pressure bar (SHPB). The peak strain and dynamic modulus of elasticity are weakened by the inclination angle in a similar way to the strength, with the specimens showing an evolutionary pattern from tensile strain to shear damage. The change in the inclination angle of flaws would weaken the dynamic and combined strengths, and a larger inclination flaw results in a significant decrease in its strength. Fractal analysis revealed that the fractural dimension was closely related to the fissure angle and showed a good linear correlation with the strain rate. This study will provide an important security assurance for deep mining.

Numerical Simulation of Failure Behavior of Brittle Heterogeneous Rock under Uniaxial Compression Test.

Rocks have formed heterogeneous characteristics after experiencing complex natural geological processes. Studying the heterogeneity of rocks is significant for rock mechanics. In this study, a linear parallel bond model with Weibull distribution in two-dimensional particle flow code (PFC2D) is adopted to study the mechanical characteristics and brittle failure mode of granite rock specimens with different heterogeneity. Firstly, we selected several combinations of key micro-parameters of the parallel bond model. Then, we subjected them to a Weibull distribution to satisfy heterogeneity, respectively. Finally, we chose one optimal combination plan after comparing the stress–strain curves of heterogeneous rock specimens. We analyzed the simulated results of heterogeneous rock specimens. The crack distribution of rock specimens under peak stress shows different characteristics: a diagonal shape in rock specimens with low heterogeneity indexes, or a rotated “y” shape in rock specimens with high heterogeneity indexes. As for failure mode, the numerical simulation results show high consistency with the laboratory experiment results. The rock specimen breaks down almost diagonally, and the whole specimen tends to form an x-shaped conjugate shear failure or the well-known “hour-glass” failure mode. With the increase of the homogeneity index of the rock specimen, the shear rupture angle becomes larger and larger. Generally, the crack number increases with time, and when the rock specimen reaches the peak failure point, the number of cracks increases sharply. The development of cracks in numerical rock specimens under compression test is a result of the coalescence of many microscopic cracks. Furthermore, tensile cracks formed initially, followed by shear behavior along the macroscopic crack plane. We also preliminarily study the mechanical characteristics of heterogeneous rock specimens with discontinuous structural planes. The discontinuous structural planes are simulated by the smooth-joint model. We can conclude that the discontinuous structural planes and the microscopic structural planes which contribute to the heterogeneity have a mutual influence on each other.

Cracking of granitic rock by high frequency-high voltage-alternating current actuation of piezoelectric properties of quartz mineral: 3D numerical study

This paper presents a numerical study on cracking of granitic rock by high frequency-high voltage-alternating current actuation (HF-HV-AC) of piezoelectric properties of Quartz mineral. For this end, a numerical method based on 3D embedded discontinuity finite elements for rock fracture and an explicit time stepping scheme to solve the coupled piezoelectro-mechanical problem is developed. Rock heterogeneity and anisotropy are accounted for at the mineral mesotructure level. Novel numerical simulations demonstrate that disc-shaped and cylindrical granitic rock specimens (with the tensile strength of about 8 MPa) can be cracked by a sinusoidal excitation with an amplitude of ∼10 kV at a frequency matching one of the resonance frequencies of the specimen (e.g. 125 kHz in the present case of 30 mm radius and 10 mm height). The effects of specimen shape and electrode locations are tested. Various Quartz grain alignment schemes are tested and even the worst case of having a 50%:50% mixture of right- and left-handed Quartz crystals without any preferred orientation show stresses of about 1 MPa at the resonance frequency. The simulation results suggest that HF-HV-AC piezoelectric excitation of Quartz bearing rocks could be a potential pre-treatment technique in comminution.

Residual strength of granitic rocks

When carrying out compressive tests on rock specimens, a stress plateau is typically attained following peak strength within a strain range a few times larger than the strain at peak strength. This stress level is commonly known as residual strength. A large database of these tests on granitic rock specimens has been compiled and analyzed in detail with the aim of improving understanding of the residual strength of this type of rock. It is observed that physical scale does not significantly change this residual strength. This stress threshold is also shown to be independent of the initial level of jointing in the rock volume. The implication of these findings is that residual strength derived from rock specimen testing can be potentially extended to rock mass scale, at least for granitic rocks. Three one-parameter residual strength models have been used to fit laboratory test residual strength data of granitic rocks, and all of them are shown to reasonably approximate the actual data. Additionally, the residual laboratory strength parameters of all the varied studied granitic rocks for varying scale and pre-jointing tend not to be very different, covering a limited range of values. Ultimately, the potential errors in simple excavation analysis that might result from a priori assumptions regarding the residual strength of granitic rock masses are quantified.

Testing the frackability of granite using supercritical carbon dioxide: Insights into geothermal energy systems

Supercritical carbon dioxide (ScCO2) fracturing is an attractive method which has favourable physical and chemical characteristics for the enhancement of underground reservoir permeability, and as a replacement for the current water-based fracturing in reservoir stimulation. However, the behaviour of ScCO2 as a fracturing fluid and the reservoir response to the injection of ScCO2 remain unclear in the geothermal environment. Therefore, in this study, a series of ScCO2 fracturing tests were conducted on Harcourt granite rock specimens under different confining pressures (5–40 MPa) and temperatures (50–300 °C) to better understand the applicability of ScCO2 as a fracturing fluid and its influence on breakdown pressure under geothermal reservoir conditions. According to the results, the breakdown pressure under ScCO2 fracturing is lower than that for water-based fracturing. This is because ScCO2, being a low viscous fluid, has the ability to percolate into the micro-cracks in the reservoir rock, and the pressurisation of these micro-cracks increases the pore-pressure inside the rock matrix, reducing the effective stress required for fracture initiation. Our results reveal that the breakdown pressure increases linearly with increasing confining pressure. High-resolution CT scanning results show that the average density of the induced fracture tends to increase with the increase of confining pressure due to the sudden release of a large amount of energy under higher negative effective stress conditions (26% increase in fracture density with increase of confining pressure from 30 MPa to 40 MPa at 50 °C). In addition, regardless of the confining pressure, the breakdown pressure exhibits a decreasing trend with increasing temperature, which is due to the thermally-induced microstructural and mechanical deterioration of the rock specimen. The temperature increase significantly alters the fracture morphology, resulting in narrow fractures with multiple secondary branches under high temperature conditions. The results of the tortuosities of the induced fractures provide further evidence of this phenomenon. Importantly, ScCO2 has the ability to penetrate into the thermally-induced micro-cracks in the rock and link these micro-cracks to the main diametrical fracture, resulting in interconnected and complex fracture patterns under high temperature conditions. Also, the fractures tend to propagate mainly along the grain boundaries of quartz and feldspar minerals, while biotite grains exhibit a comparative resilience to fracture propagation.

Influence of long-term operation of supercritical carbon dioxide based enhanced geothermal system on mineralogical and microstructurally-induced mechanical alteration of surrounding rock mass

Chemico-microstructural and mineralogically-induced time-dependent mechanical alterations of the reservoir rock found in the peripheral zone of a CO2-based EGS is experimentally investigated. A series of uniaxial compressive and triaxial (under 30 MPa constant confining pressure with different temperatures; 25-300 °C) strength tests on granite specimens reacted in ScCO2+water medium for different time periods was performed by coupling acoustic emission (AE) technique. The results were compared with those for intact and water-reacted rock specimens in order to better understand this weakening phenomenon. Scanning electron microscopy (SEM) was also incorporated to evaluate the microstructural and mineralogical alterations.The results indicated that the mineralogical structure of Harcourt granite rock specimens was mainly altered by the dissolution of silicate minerals and plagioclase phase feldspar minerals. In addition, enhanced precipitation of secondary clay minerals including kaolinite, smectite, and illite was observed in the ScCO2+water-reacted rock specimens. The average compressive strength of ScCO2/water-reacted granite specimens found to be reduced with the increase of reaction period. Furthermore, 3.81%, 5.08%, 5.66% and 0.52% reductions in failure strength resulted under 30 MPa confining pressure in 6 months ScCO2/water-reacted granite specimens at 25 °C, 100 °C, 200 °C and 300 °C temperatures, respectively, compared to intact rock values at these temperatures. The post-failure analysis also revealed that the ScCO2/water reaction significantly alters the post-failure behaviour due to micro-mineralogically-induced weakening inside the rock pore structure. It was concluded that enhanced mineral precipitation and dissolution influence in time-dependent degradation of the surrounding rock mass which influence in long-term operation of the geothermal system.

Experimental and Numerical Study on the Damage Evolution Behaviour of Granitic Rock during Loading and Unloading

Theoretical and experimental studies have revealed that the damage evolution plays an important role in stability of rock structures. To investigate the damage characteristics of rocks during loading and unloading, a series of conventional triaxial tests and numerical simulations were conducted on granitic rock specimens under different confining pressures. The stress-strain characteristics and fracture patterns of tested specimens were first analyzed. It was found that the failure strain in unloading is smaller than the failure strain in loading. And the difference between the two strains is growing with increasing confining pressure. The failure patterns of specimens displayed two different failure mechanisms: a single distinct failure and a “X” failure. Based on the law of energy conservation, the energy evolution was analyzed. The results indicated that absorbed strain energy converted into elastic strain energy and dissipation energy. For evaluating and predicting damage, two damage degrees were proposed considering increase of dissipation energy and decrease of tangential modulus, respectively. The results show that before the reversal point of volumetric strain, the damage degrees were almost unchanged. During the process of unloading the damage degrees increases fast. For the same strain, lower confining pressure shows more damage. It indicates that the confining pressure has negative effects on increase of the damage degree. Then, the discrete element model based on elastic and unbreakable voronoi blocks was set-up for tri-axial tests. The energy evolution and damage process were simulated. And the ratio of failed contacts was used to simulate the damage degree. It shows that stress-strain behavior as well as micro- and macro-mechanical damage evolution can be reproduced by the DEM model.

Experimental investigation of hard rock fragmentation using a conical pick on true triaxial test apparatus

This study aims to determine the influences of confining stresses and cutting parameters on hard rock fragmentation using a conical pick. Using a true triaxial testing apparatus, single/double pick forces and static/coupled static-dynamic pick forces were applied to granite rock specimens to break them confined by biaxial, uniaxial and no lateral stress conditions. The corresponding cuttabilities were estimated and compared by the peak pick force, insertion depth and disturbance duration at rock failure and the associated failure patterns. The results showed that excavation-induced unloading and cracking, which can change the biaxial confining stress conditions into the uniaxial and decrease uniaxial confining stress level, respectively, can significantly improve the hard rock cuttability. The experimental, theoretical and regressive results indicated that the hard rock cuttability initially decreases and then increases as the level of uniaxial confining stress increased. A moderate uniaxial confining stress instead improves the rock cuttability, but a high stress level may induce rock burst triggered by conical pick penetration. Therefore, only the hard rocks under stress-free and low uniaxial confining stress conditions can be easily fragmented with high safety and efficiency, as a complete splitting failure occurs. In addition, the hard rock cuttability can be also improved by the dynamic pick disturbance and the artificial defects such as excavation-induced fractures, pre-slits and boreholes in rock mass.

True-Triaxial Experimental Study of the Evolutionary Features of the Acoustic Emissions and Sounds of Rockburst Processes

Rockbursts are markedly characterized by the ejection of rock fragments from host rocks at certain speeds. The rockburst process is always accompanied by acoustic signals that include acoustic emissions (AE) and sounds. A deep insight into the evolutionary features of AE and sound signals is important to improve the accuracy of rockburst prediction. To investigate the evolutionary features of AE and sound signals, rockburst tests on granite rock specimens under true-triaxial loading conditions were performed using an improved rockburst testing system, and the AE and sounds during rockburst development were recorded and analyzed. The results show that the evolutionary features of the AE and sound signals were obvious and similar. On the eve of a rockburst, a ‘quiescent period’ could be observed in both the evolutionary process of the AE hits and the sound waveform. Furthermore, the time-dependent fractal dimensions of the AE hits and sound amplitude both showed a tendency to continuously decrease on the eve of the rockbursts. In addition, on the eve of the rockbursts, the main frequency of the AE and sound signals both showed decreasing trends, and the frequency spectrum distributions were both characterized by low amplitudes, wide frequency bands and multiple peak shapes. Thus, the evolutionary features of sound signals on the eve of rockbursts, as well as that of AE signals, can be used as beneficial information for rockburst prediction.

Experimental Study of Remotely Triggered Rockburst Induced by a Tunnel Axial Dynamic Disturbance Under True-Triaxial Conditions

During deep underground excavation, dynamic ejection failure of a highly stressed rock mass near an excavated boundary is easily triggered by a dynamic disturbance in the tunnel axial direction, induced by blasting on the tunnel face. Such a dynamic ejection failure is usually called remotely triggered rockburst, and it poses a threat to underground construction. To clarify the characteristics of remotely triggered rockburst, the development of remotely triggered rockbursts of granite rock specimens was investigated using an improved true-triaxial test system. Experimental results show that with increasing static Z-direction stress (i.e., in situ tangential stress on the cross section of the tunnel), rockburst is triggered more easily and the kinetic energy of ejected fragments increases. Under other constant static stresses and dynamic disturbance, with increasing horizontal stress including X-direction stress (i.e., in situ axial stress) or Y-direction stress (i.e., in situ radial stress on the cross section of the tunnel), rockburst is more difficult to trigger and the kinetic energy of the ejected fragments decreases. Under constant static stresses, once the amplitude and frequency of the dynamic loading exceed their thresholds, the rockburst intensity increases rapidly and the rockburst can be triggered much more easily with small increments of the amplitude and frequency. Furthermore, Z-direction strain increases during the dynamic disturbance process, indicating that the ultimate energy-storage capacity of the specimen decreases with increasing damage. When the elastic strain energy is greater than the ultimate energy-storage capacity of the damaged specimen, part of the residual elastic energy is converted into kinetic energy of the ejected fragments.

Experimental investigations on the thermal conductivity characteristics of Beishan granitic rocks for China's HLW disposal

Crystalline rocks are potential host rock types for the construction of high-level radioactive waste (HLW) repositories. A better understanding of thermal conductivity of rocks is essential to safe evaluation and engineering optimization of a HLW disposal system in the rock at depth. In the present study, experimental investigations on the thermal conductivity characteristics of 47 pairs of granitic rock specimens were conducted using the Transient Plane Source (TPS) method. The specimens were collected from borehole cores in the Beishan area, which is being considered as the most potential candidate area for China's HLW repository. To evaluate geological nature of the rocks, mineralogical compositions of the rocks were identified, and porosity of the specimens was measured. The thermal conductivities of the specimens under dry and water-saturated conditions were determined, and the effect of water saturation on the thermal conductivity was investigated. In addition, the influence of temperature and axial compression stress on the thermal conductivity of dry specimens was studied. The results revealed that the thermal conductivity of tested rocks was dependent on water saturation, temperature and compression stress. Based on the obtained data, some models considering porosity were established for describing the thermal conductivity characteristics of the tested rocks. Furthermore, when the rocks have a similar porosity, the quartz content dominates the thermal conductivity, and there exists an obvious increase of the thermal conductivity with increasing quartz content. The test results constitute the first systematic measurements on the Beishan granitic rocks and can further be used for the development of thermal models for predicting thermal response near the underground excavations for HLW disposal.

Experimental Study on Radon motion during Rock Compression

A mathematical model to describe the radon motion outside the surface of rocks during the sustained stress on the rocks has been established and an experimental device for monitoring changes of radon concentration has been designed. The theoretic and experimental results show that there is a negative correlation between radon concentration emitted from granite and sandstone rock specimens and the axial pressure loaded on the rock specimens. The maximum error of the radon concentration between theoretic and experimental results is about 10.74%.

Strength and dilation of jointed granite specimens in servo-controlled triaxial tests

We investigated the complete stress–strain response of artificially jointed granitic rock specimens with two joint sets. A series of confined compressive tests on these jointed rock specimens were performed in a fully servo-controlled press. Elastic, strength and post-failure parameters were calculated and analysed, with special attention paid to the dilation angle. The results were compared to those for corresponding intact specimens investigated in a previous study. Although the peak strength of the jointed cores was much lower than that for the intact cores, the residual strength envelopes were practically equal. The apparent elastic and drop moduli tended to be lower for jointed specimens than that for intact specimens. At low confining stress levels, the jointed rock tended to dilate less than the intact rock, but at higher confinement dilation behaviour converged. Another finding with respect to post-yield behaviour was that the dilation angle of the jointed specimens could adequately be represented by an existing plastic-shear–strain and confinement-dependent model developed for intact specimens.

연마재 워터젯 암석절삭을 위한 결합 노즐의 기하학적 변수 영향

연마재 워터젯을 이용하여 암반을 굴착하기 위해서는 노즐이 삽입되고 절삭하는 연속적 공정이 요구된다. 본 연구에서는 노즐이 삽입되기 위한 충분한 절삭 폭을 확보하기 위해 한 쌍의 연마재 워터젯 노즐을 이용하여 암석 절삭실험을 실시하였다. 한 쌍의 노즐형태와 위치에 따른 기하학적 변수에 따라 암석 절삭형상과 절삭 폭이 달라지는 것을 확인하였다. 정의된 기하학적 변수에 따라 절삭 깊이 및 폭을 측정하여 암반에 형성되는 절삭단면 형상을 분석하였다. 그 중 노즐 삽입이 가능한 기하학적 변수를 제시하고 현장적용 가능성에 대해 검증하였다. Inserting a nozzle assembly into a removed cutting space during a continuous cutting operation is necessary in rock excavation using an abrasive waterjet. In this study, a combined two nozzle assembly is used to secure enough removal width. The shape of the cut space is affected by the geometric parameters (standoff distance, nozzle angle, and vertical distance between the nozzle tips) of the combined nozzle assembly. Abrasive waterjet cutting tests are performed with various geometric parameters for granite rock specimens. Optimized geometric parameters for the nozzle inserting process are determined and verified through the experimental tests.

A computer program (TSecSoft) to determine mineral percentages using photographs obtained from thin sections

The geomechanical behavior of rocks is controlled mainly by their mineral content and texture. For this reason, determining the mineral content of rocks is highly important for their genetic classification and understanding their geomechanical behavior. Conventionally, the mineral content of rocks has been determined by point counting on thin sections. However, this process is exhaustive and time-consuming. This study presents a computer program, TSecSoft, that determines the mineral content of rocks. TSecSoft is developed with MATLAB10a, and the MATLAB scripts (m-files) are converted to a standalone application using MATLAB Deployment Toolbox. After automatically obtaining an initial segmentation, the user can correct segments to produce perfect segmentation. When correcting segments, the user can merge segments or divide one segment into several using the TSecSoft pen function. To assess the TSecSoft's performance, point counting and TSecSoft are used on six different thin sections prepared from granitic rock specimens, and their results are compared. The correlation coefficients of the mineral percentage values obtained from point counting and TSecSoft are considerably high. All results indicate that a useful and time-saving tool has been produced for determining the mineral percentages of rocks.

Millimeter-Wave Heating, Radiometry, and Calorimetry of Granite Rock to Vaporization

Millimeter-wave (MMW) technologies can provide unique heating and diagnostic capabilities to research the thermal dynamics of materials to extreme temperatures. The MMW properties of rocks in the molten state up to their vaporization temperatures are not well known. Using a 28 GHz gyrotron beam collinear with a 130 GHz radiometry view in a calorimetric chamber, the transitions of granite rock specimens through solid phases, melting, and vaporization were observed, including release of trapped trace gas (<0.07%). The 28 GHz emissivity of molten granite was observed to be approximately constant at 0.66 ± 0.03 up to vaporization where it increased to 0.70 ± 0.03 at an equilibrated temperature of 2710 ± 120°C. An analysis of the thermal power balance during a 76 s steady state vaporization time period indicates that the MMW emissivity of the molten granite is larger than in the infrared. The observations support the possibility that MMW thermal ablative penetration into hot crystalline rock formations could be a more practical approach than infrared laser drilling to access deep resources.

Mechanical and hydraulic coupling of injection-induced slip along pre-existing fractures

A series of injection-induced slip experiments using pre-fractured granitic rock specimens was conducted to investigate the dynamic hydraulic responses of existing fractures during hydraulic stimulation. The results show a characteristic stepwise slip and temporal increase in fluid flow in the fractures during slip. This increase is considered to result from significant shear dilation that is sufficient to change the flow properties of rough fractures with large asperities. Our laboratory data show that the stepwise slip is caused by temporal drops in pore fluid pressure in the fractures associated with shear dilation, which in turn indicates that the geometry of fracture surfaces plays a significant role in terms of both slip behavior and changes in fluid flow during induced slip. These results raise the possibility that stepwise slip and temporal permeability changes occur in actual rough fractures. This hydro-mechanical coupling behavior of such fractures during fluid injection suggests that dynamic (temporary) permeability changes may occur in geothermal reservoirs subjected to hydraulic stimulations.

Acoustic emission monitoring of hydraulic fracturing in laboratory and field

In the hydraulic fracturing tests, water was injected into four kinds of granitic rock specimens with different grain sizes. The fault plane solutions of acoustic emission (AE) indicated that the shear fracturing is dominant in the specimens with larger grains whereas the tensile fracturing is dominant in those of smaller grains. In addition, the experiments were conducted in granitic rock specimens of the same kind, under three different conditions: injection of water, injection of viscous oil and pressurization via a urethane sleeve. The shear fracturing was dominant due to water injection and the pressurization whereas the tensile fracturing was dominant due to viscous oil injection. Further in a field, AE was monitored by a borehole sonde in the hydraulic fracturing to measure rock stress. The AE count rates increased with closing and reopening hydraulically induced cracks. The result suggested that AE monitoring could improve the reliability and the accuracy of hydraulic fracturing stress measurements.

The experimental research of apparent resistivity, elastic wave and acoustic emission in the rupture process of large scale specimens and rock blocks in-situ

Abstract This paper deals with observations and features of experiments of apparent resistivity (denoted by ϱ s ), elastic wave and acoustic emission in large scale concrete blocks (1.0 × 1.0 × 1.5 m 3 ) under the condition of triaxial stress, large scale granite rock specimens (1.0 × 1.0 × 0.85 m 3 ) in the process of slowly expansional rupture, and in rock blocks in-situ (in a mine tunnel) under the condition of local stress disturbance due to the mine excavation. The changes in apparent resistivity, elastic wave and acoustic emission and stress-strain measurements both in the stressed large scale specimen and in the rock block in-situ , could be divided into several stages which are associated with different features of deformation inside the specimens or rock blocks. All these results show the good prospects for applying these measurements to synthetic research on determining the mechanical properties of construction material and detecting the stability of concrete blocks in-situ such as in mine engineering. It is especially pointed out that the application of a specific loading source—the static burst agent in the experiment, makes it possible to do the loading test on large scale rock specimens and to understand its complex rupturing process of large scale rock specimens, which is very significant for research on earthquake mechanics and earthquake precursory mechanism as well as rock mechanics itself.