Beishan Granite Research Articles

Evolution of thermal damage and permeability of Beishan granite

Thermal damage in Beishan granite subjected to high temperature treatment (from 100°C to 800°C at different heating rates, ranging from 1 to 15°C/min) is studied in order to assess the thermal effect on physical and mechanical properties. Laboratory tests including acoustic emission (AE) monitoring, ultrasonic velocity and porosity measurements, scanning electron microscope (SEM), tri-axial compression tests with permeability measurement were applied in this study. Two temperature thresholds, 300°C and 573°C, are recognized. Physical and mechanical properties of heated granite weaken with the increase of temperature. Especially when the temperature exceeds 573°C, the weakening becomes more pronounced. Thermally induced cracks in granite are mainly intergranular cracks from 100°C to 573°C. However, once the temperature exceeds 573°C, intra-granular cracks begin to develop due to the phase transition of quartz. Two typical stress-permeability curves of heated granite are observed and their mechanism is analyzed. In addition, the relationship between the number AE hits and initial permeability is discussed. The volumetric strain of cracks is applied to study the permeability evolution during the compressive loading process.

Surface complexation modeling of U(VI) adsorption on granite at ambient/elevated temperature: Experimental and XPS study

The Beishan granitic formation is being investigated as a potential host rock for a high-level radioactive waste repository in China. It is important to understand the retention processes, including influential parameters such as the metal ion concentration, pH, ionic strength (I) and temperature. The present study deals with U(VI) adsorption on Beishan granite using batch-type experiments in a CO2-free atmosphere. U(VI) adsorption on granite is shown to be insensitive to ionic strength. Temperature has a positive effect on U(VI) adsorption indicating that surface reactions are endothermic. Combining X-ray photoelectron spectroscopy (XPS) analysis and adsorption data at 25°C, a Generalized Composite (GC) model with three surface complexes, SOUO2+, SO(UO2)2(OH)2+ and SO(UO2)3(OH)5, was constructed. The experimental data at 40°C and 60°C were fitted by the proposed model to obtain the equilibrium constants (K) of the surface reactions at these two temperatures. The enthalpy changes (ΔH) of the surface reactions were evaluated from the equilibrium constants obtained at three temperatures via the van't Hoff equation. Finally, blind modeling predictions were performed to test the robustness of the proposed model and ΔH. Satisfactory agreement with the literature data confirmed that this GC model with ΔH proving a useful tool to predict U(VI) adsorption on granite samples, especially on Beishan granite at ambient/high temperature.

High-level radioactive waste disposal in China: update 2010

For geological disposal of high-level radioactive waste (HLW), the Chinese policy is that the spent nuclear fuel (SNF) should be reprocessed first, followed by vitrification and final disposal. The preliminary repository concept is a shaft-tunnel model, located in saturated zones in granite, while the final waste form for disposal is vitrified high-level radioactive waste. In 2006, the government published a long-term research and development (R&D) plan for geological disposal of high-level radioactive waste. The program consists of three steps: (1) laboratory studies and site selection for a HLW repository (2006–2020); (2) underground in-situ tests (2021–2040); and (3) repository construction (2041–2050) followed by operation. With the support of China Atomic Energy Authority, comprehensive studies are underway and some progresses are made. The site characterization, including deep borehole drilling, has been performed at the most potential Beishan site in Gansu Province, Northwestern China. The data from geological and hydrogeological investigations, in-situ stress and permeability measurements of rock mass are presented in this paper. Engineered barrier studies are concentrated on the Gaomiaozi bentonite. A mock-up facility, which is used to study the thermo-hydro-mechano-chemical (THMC) properties of the bentonite, is under construction. Several projects on mechanical properties of Beishan granite are also underway. The key scientific challenges faced with HLW disposal are also discussed.

Experimental investigation on the creep behaviour of Beishan granite under different temperature and stress conditions

Based on a series of creep tests performed on MTS815 rock mechanics test system, the influence of temperature and stress condition on the creep properties of Beishan granite is investigated. The experiment data revealed that the creep process of Beishan granite can be divided into three stages: the transient creep, the steady creep, and the accelerated creep, with different strain rate in each creep region. With the employment of 3D acoustic emission monitoring system, a special attention is paid on the cracking process during the creep test. A consistent variation tendency between AE cumulative counts and volumetric creep strain is noticed. Moreover, the variation of recorded AE events is found to be more sensitive to the occurrence of rock failure than rock deformation. Under same confining pressure, it is indicated that the stress level and temperature variation within 90 °C have quite limited influence on the critical state of rock failure, while they may accelerate the cracking process and consequently change the time to failure at constant loading condition.

Experimental characterisation of thermo-mechanical coupling properties of Beishan granite

With the help of MTS815 rock mechanics test system and 3D acoustic emission monitoring system, a series of thermo-mechanical coupling tests at temperatures in the range of 23–90 °C and confining pressures from 5 to 30 MPa are carried out, to investigate the thermo-mechanical properties of Beishan granite. The granite is taken from Beishan site, the most potential area for the China’s high-level radioactive waste repository. The experimental results indicate that the mechanical behaviour and crack characteristics are essential related to the coupling effect of temperature and confining pressure. The mechanical characteristics such as elastic modulus, crack damage stress and rock failure strength are degraded slightly with the increase of temperature. In accordance with the degradation of mechanical properties, high temperature leads to the development of new microcracks or propagation of pre-existing microcracks within the rocks, which can be revealed by the increase of the AE activities. Rock specimen tends to become more stable under high confining pressure. A brittle–ductile mechanical behaviour transition of Beishan granite can be observed, especially under a high confining pressure.

Comparison study on rock crack pattern under a single normal and inclined disc cutter by linear cutting experiments

In order to study the crack pattern of Beishan granite under a single normal and inclined disc cutter, a series of linear cutting experiments were conducted. During the tests, a constant cross-section disc cutter with a diameter of 17in. was used and a block of Beishan granite with the size of 1000×1000×600mm (Length×Width×Height) was adopted. After the experiments, it is found that in the rock sample, there is a broken area beneath the cutting groove with variable degrees of fragmentation. Based on the crack pattern and crack density, the rock sample is divided into the re-compacted zone, the crushed and crack zone, and the intact rock zone. Meanwhile, the size and area of these zones were analyzed and the corresponding ultrasonic velocities were measured. It is shown that the crack patterns for normal and inclined cuttings at the same penetration depth are greatly different due to different included angles and cutting forces in the two scenarios. As the penetration of inclined cutting increases, the size and area of various zones expand significantly. In addition, the ultrasonic test reveals that the ultrasonic velocity of each zone agrees well with the degree of fragmentation and the velocities of corresponding zones for different scenarios are similar.

The sorption interactions of humic acid onto Beishan granite

Abstract To explore the sorption phenomenon and mechanism between humic acid (HA) and Beishan granite (BsG), the influence of factors such as pH, contact time, ionic strength, temperature, HA concentration and solid-to-liquid ratio were investigated using the batch technique. The sorption process was significantly affected by pH and ionic strength. The maximum adsorption capacity of HA on BsG was approximately 0.9493 mg g −1 at pH = 6.7 and T = 305 K, and the thermodynamic parameters indicated that the sorption process was spontaneous and endothermic. The SEM–EDS results indicated the presence of a certain amount of HA on BsG after sorption with HA. The results of FT-IR spectroscopy and the change in pH demonstrated that ligand exchange–surface complexation may be an important mechanism. BET analysis showed that parts of the HA tended to enter the pores formed by close-packing of BsG particles, and albite and muscovite might have played a role in the sorption process, as indicated by the XRD analysis. Overall, it is speculated that the mechanisms governing the sorption behaviors of HA on BsG are precipitation, ligand exchange–surface complexation and electrostatic interaction. Moreover, the colloidal properties of HA under certain conditions may also significantly affect its sorption interactions with BsG.

Strength comparison between cylindrical and prism specimens of Beishan granite under uniaxial compression

Strength comparison between cylindrical and prism specimens of Beishan granite under uniaxial compression

Micromechanical analysis of the failure process of brittle rock

SummaryThe failure process of brittle rock submitted to a compression state of stress with different confining pressures is investigated in this paper based on discrete element method (DEM) simulations. In the DEM model, the rock sample is represented by bonding rigid particles at their contact points. The numerical model is first calibrated by comparing the macroscopic response with the macroscopic response of Beishan granite obtained from laboratory tests. After the validation of numerical model in terms of macroscopic responses, the failure process of the DEM model under unconfined and confined compression is studied in micro‐scale in detail. The contact force network and its relation to the development of micro‐cracks and evolution of major fractures are studied. Confining pressure will prohibit the development of tensile cracks and hence alter the failure patterns. An in‐depth analysis of micro‐scale response is carried out, including the orientation distribution and probability density of stress acting on parallel bonds, the effect of particle size heterogeneity on bond breakage and the evolution of fabric tensor and coordination number of parallel bond. The proposed micromechanical analysis will allow us to extract innovative features emerged from the stresses and crack evolution in brittle rock failure process. Copyright © 2014 John Wiley & Sons, Ltd.

Damage and Plastic Deformation Modeling of Beishan Granite Under Compressive Stress Conditions

Based on experimental investigations, we propose a coupled elastoplastic damage model to simulate the mechanical behavior of granite under compressive stress conditions. The granite is taken from the Beishan area, a preferable region for China’s high-level radioactive waste repository. Using a 3D acoustic emission monitoring system in mechanical tests, we focus on the cracking process and its influence on the macroscopic mechanical behavior of the granite samples. It is verified that the crack propagation coupled with fractional sliding along the cracks is the principal mechanism controlling the failure process and nonlinear mechanical behavior of granite under compressive stress conditions. Based on this understanding, the coupled elastoplastic damage model is formulated in the framework of the thermodynamics theory. In the model, the coupling between damage and plastic deformation is simulated by introducing the independent damage variable in the plastic yield surface. As a preliminary validation of the model, a series of numerical simulations are performed for compressive tests conducted under different confining pressures. Comparisons between the numerical and simulated results show that the proposed model can reproduce the main features of the mechanical behavior of Beishan granite, particularly the damage evolution under compressive stress conditions.

Experimental characterization and micromechanical modeling of damage-induced permeability variation in Beishan granite

Triaxial compression tests with measurements of permeability were performed on core granite samples taken at 450−550m depth from the Beishan area in Gansu Province, a potential site for China׳s high-level radioactive waste (HLW) disposal. Corresponding to the distinct features in the stress–strain behaviors, the permeability of the Beishan granite was found to evolve with a clear permeability decrease in the initial microcrack closure region, a constant permeability value in the elastic region and a dramatic permeability increase in the crack growth region. The permeability increases by up to and over two orders of magnitude as deviatoric stress increases up to sample failure; but at a given deviatoric stress, the permeability reduces remarkably with the increase of confining pressure. An empirical upper bound permeability model was presented by relating the mechanisms involved in the microstructure alteration to the permeability change, and the experimental results were well simulated by the proposed model. Combined with field geological characterization and numerical simulation, the implications of the experimental results for China׳s HLW disposal were discussed.

The adsorption of Eu(III) and Am(III) on Beishan granite: XPS, EPMA, batch and modeling study

Granite has been chosen as a promising host rock for geological repository of high-level radioactive waste in many countries. However, the description of radionuclide adsorption on granite is subjected to its complicated composition and still a challenge. We studied the adsorption of Eu(III) and Am(III) on Beishan granite, a preliminary selection of host rock in China, as a function of pH, adsorbate concentration, ionic strength and the composition of background electrolytes. A surface complexation model was set up using Generalized Composite approach based on Eu(III) adsorption in NaCl electrolyte, supported by X-ray photoelectron spectroscopy and verified by Eu(III) adsorption in the presence of CO2 at PCO2=10-3.58atm and in CaCl2 electrolyte. The model was successfully extended to describe Am(III) adsorption in both NaCl and CaCl2 electrolytes. It was also found that temperature effect on Eu(III) adsorption was negligible at 25–80°C, that the adsorption of Eu(III) on freshly crushed granite with “new” surfaces and aged granite with “old” surfaces were identical to each other, and that the presence of fulvic acid of 2–20mg/L significantly declined the distribution coefficients of Eu(III) and Am(III) in high pH range. Electron probe micro analyses indicated that biotite had higher affinity for Eu(III) than other major minerals, such as oligoclase, quartz and orthoclase.

Characterizing the mechanical tensile behavior of Beishan granite with different experimental methods

This work investigates with different test methods the mechanical tensile behavior of Beishan granite, especially the damage evolution process. The granite is taken from the Beishan site, a potential Chinese area for high-level radioactive waste repository. In the work, three widely used tensile testing methods are employed, namely the Brazilian test with simplified ISRM standard (Type I), the Brazilian test with the China standard (Type II), and the direct tensile test. An acoustic emission test system is also used to capture the microcracking process under different loading conditions. Even though the tensile strength obtained by the three testing methods is similar, some difference can still be noticed. The highest value of average tensile strength (11.24MPa) is obtained by the indirect method proposed by IRSM, which is about 22% higher than the indirect method with China standard (9.15MPa) and about 18% higher than the direct tensile test (9.53MPa). The damage evolution process is further analyzed based on the recorded AE events. It is revealed that in direct tensile testing, the AE events accumulated mainly along the failure surface, whereas in indirect tests the location and variation of AE events are much more complex. Moreover, due to the stress concentration induced by the steel bars placed between the flat plate and the specimen, the accumulation of AE events around the loading surface is more significant in the indirect test with the China standard than the accumulation recorded in the IRSM method. The recorded AE events give rise to a reasonable explanation of the macroscopic mechanical behavior of granite, and are also valuable for understanding the damage evolution process of granite induced by tensile stress.

The adsorption behavior of U(vi) on granite

The effects of pH, counter ions and temperature on the adsorption of U(VI) on Beishan granite (BsG) were investigated in the presence and absence of fulvic acid (FA) and humic acid (HA). The adsorption edge of U(VI) on BsG suggested that U(VI) adsorption was mainly controlled by ion exchange and outer-sphere complexation at low pH, whereas inner-sphere complex was the dominant adsorption species in the pH range of 4.0-9.0. Above pH 9.0, Na2U2O7 might play an important role in the rise of U(VI) adsorption again. Counter ions such as Cl(-), SO4(2-) and PO4(3-) can provoke U(VI) adsorption on BsG to some extent, which was directly correlated to the complexing ability of U(VI)-ligand. More noticeably, the large enhancement of U(VI) adsorption in the presence of phosphate can be attributed to the ternary complex formation (BsG-PO4-UO2), precipitation ((UO2)3(PO4)2(s)) and secondary phase (Na-autunite). Both FA and HA can slightly increase U(VI) adsorption at low pH, whereas they strongly inhibited U(VI) adsorption at high pH range. Artificial synthesized granite (AsG) prepared in the laboratory is impossible to use as an analogue of natural granite because of the large difference in the adsorption and surface properties.

Damage stress and acoustic emission characteristics of the Beishan granite

• Crack initiation stress ( σ ci ) and crack damage stress ( σ cd ) of the Beishan granite under uniaxial and triaxial compressions are determined.

Influence of Unloading Rate on the Strainburst Characteristics of Beishan Granite Under True-Triaxial Unloading Conditions

Rockburst is a sudden and violent failure of rocks and it often occurs in hard rocks in highly stressed ground. Strainburst is classified as one type of rockburst and it often occurs in rocks near or at the excavation boundary. Deep insight into the strainburst phenomenon is essential for safe underground construction at depth. In this paper, an experimental laboratory study on the strainburst behavior of Beishan granite is presented. Based on in-situ stress measurement data from the Beishan area in China, a series of tests under different unloading rates were performed to investigate the strainburst process using a true-triaxial strainburst test system which was equipped with an acoustic emission (AE) monitoring system. In addition, a high-speed video camera was used to record and visualize the initiation and ejection of rock fragments as well as the sudden dynamic failure (strainburst) of the test samples. AE characteristics associated with the cumulative energy and frequency–amplitude distributions were analyzed. Characteristics of the microscopic structure of a fragment generated from one test were observed using a scanning electron microscope. The experimental results indicate that the degree of violence during failure and the associated AE energy release in the strainburst process are dependent on the unloading rate. When the unloading rate is high, the rock is prone to strainburst. On the other hand, as the unloading rate decreases, the failure mode changes from strainburst to spalling. In addition, the cumulative AE energy is not sensitive to unloading rates greater than 0.05 MPa/s. When the unloading rate is less than 0.05 MPa/s, the cumulative AE energy shows a marked decreasing trend during rock failure.

Migration study of iodine in Beishan granite by a column method

The fate and migration behavior of radionuclides in environment are influenced by a series of physical and chemical processes such as advection, hydrodynamic dispersion (including mechanical dispersion and molecular diffusion), retention, chemical reaction and so on. In this study, the migration of 125I− in Beishan granite and the potential retention of iodine by silver halide additives were investigated by a pulse injection column method. All breakthrough curves were analyzed according to the analytical solution of transport equation and the dispersion coefficient (D), and first-order sorption constant (k) were obtained. For conservative nuclide, the dispersion behavior is only related to the dispersion medium. Silver halides were proved having sorption ability for 125I− in the order of AgCl > AgBr > AgI. The transport of iodine in the crushed granite column can be adequately described by the advection–dispersion equation with a first-order, irreversible sorption term. The pulse injection column method can be used as a fast method to evaluate the sorption or retention ability of solid phase.

Adsorption of Co(Ⅱ) and Ni(Ⅱ) on Beishan Granite：Surface Complexation Model and Linear Free Energy Relationship

The adsorption of Co(II) and Ni(II) on crushed Beishan granite (BS03, 600 m) was studied by a batch experimental method. The distribution coefficient (K-d) was found to vary as a function of the pH, ionic strength, and the initial concentrations of Co(II) and Ni(II). In the low pH range, the K-d values of Co(II) and Ni(II) decreased significantly as the ionic strength increased, whereas the effect of the ionic strength was weak in the high pH range. The adsorption of Co(II) and Ni(II) on granite was quantitatively interpreted by a model with one cation exchange reaction and two inner-sphere surface complexation reactions. A linear free energy relationship (LFER) between the equilibrium constants (K) of the surface complexation reactions and the hydrolysis stability constants of the divalent transition metals (K-OH) was established. Predictions based on the LFER are in good agreement with the experimental results for the adsorption of Pb(II) and Cu(II) on granite.

The Diffusion of Tc-99 in Beishan Granite-Temperature Effect

In the safety assessment of a potential site for high-level radioactive wastes (HLW) disposal, the investigation on the geochemical behaviors of key radionuclides with the possibility for releasing from the potential repository is an important aspect. Due to the high mobility of technetium under most repository conditions, lots of research works were performed to investigate the diffusion of technetium in different potential rocks. In spite of these studies, there remains a lack of data addressing temperature effects. In this paper, the diffusion of 99Tc in Beishan granite at temperatures from 25℃ to 55℃ was studied with laboratory small scale diffusion devices. The experimental data were fitted with a finite difference scheme to get the effective diffusion coefficient (De) of . The results indicated that the relationship of De with temperatures could be described as the modified Stokes-Einstein equation, and the formation factor of Beishan granite was constant in the temperature range of 25℃ - 55℃ with the value of (3.91 ± 1.77) × 10-4.

Study on Triaxial Creep Experiment of Beishan Granite under Low Confining Pressures

Time-dependent brittle deformation is a foundational process operating in the underground engineering. So, the study of its characterization is essential to the design and construction of excavations in the rocks for high level radioactive waste disposal repositories. In this study, three constant stress tests under different confining pressures were performed on specimens of Beishan granite. Based on the laboratory results, we investigated mechanical creep behavior of Beishan granite under low confining pressures and compared the results with previous investigations.