Numerical simulations for describing generation of excavation damaged zone: Important case study at Horonobe underground research laboratory

The aim of the present research was to establish a case study for the prediction of the unknown EDZ (Excavation Damaged Zone) distribution using a numerical analysis calibrated by replicating the trends in the EDZ observed from one of the representative underground research fields in Japan (Horonobe URL). In this study, a 2D numerical analysis using a damage model, which can determine rock deformation and fracturing simultaneously, is presented. It was calibrated to reproduce the excavation of the gallery at the Horonobe URL at a depth of 350 m. Simulated results show an excellent agreement with the extent of the measured EDZ and capture the failure modes of EDZ fractures suggested by the in-situ observations. Finally, the calibrated numerical analysis was used to realistically estimate the EDZ formation for the geological disposal of high-level radioactive waste (HLW) under the same environment as that of the above-mentioned galley at the Horonobe URL. Consequently, it was shown that the tensile/shear hybrid fractures dominantly constituted the EDZ and propagated to a maximum extent of about 0.3 m from the cavity wall during the cavity excavation for the HLW disposal. Overall, the calibrated numerical analysis and resulting estimations, targeted for the environment at the depth of 350 m at the Horonobe URL, where mudstone is located, should be useful for predicting the trends in the EDZ distribution expected in the implementation of HLW disposal projects under deep geological conditions, such as those that exist in Japan, which are dominated by sedimentary rocks, including mudstone

Pengaruh Variasi Ukuran Butiran Tepung Kedelai pada Metode Calcite Precipitation untuk Peningkatan Kekuatan Tanah Pasir

Kedelai merupakan alternatif penggunaan enzim urease murni komersial dalam metode calcite precipitation. Ukuran butiran tepung kedelai merupakan salah satu faktor penting karena berpengaruh terhadap jumlah material yang tidak terlarut dan selanjutnya terhadap reaksi pembentukan kalsit. Tepung kedelai yang digunakan dalam penelitian ini telah diayak dengan saringan No. 300 (50 μm), No. 150 (100 μm), dan No. 75 (200 μm) dan bagian yang tertahan di setiap saringan digunakan dalam penelitian. Larutan tepung kedelai dengan konsentrasi sebanyak 20 g/L digunakan dan disaring melalui saringan No. 400 (40 μm), untuk mendapatkan ekstrak. Pasir Bangka digunakan sebagai sampel tanah pasir dan memiliki koefisien keseragaman sebesar 1.53. Evaluasi kemudian dilakukan dengan mengggunakan tiga parameter yaitu pengendapan kalsit, laju hidrolisis urea, dan evaluasi calcite content menggunakan asam klorida. Berdasarkan uji pengendapan kalsit dengan menggunakan tabung, semakin kecil ukuran biji kedelai, semakin banyak bahan tak terlarut yang dihasilkan. Rasio presipitasi kalsit dan massa kalsit yang diendapkan optimum tercapai pada ukuran butiran kedelai 100 μm. Laju hidrolisis tertinggi terjadi pada ukuran butiran kedelai 50 μm pada 752 U/g. Persentase kalsit tertinggi di dalam sampel terbentuk pada sampel dengan ukuran butiran kedelai 50 μm dengan persentase massa kalsit sebesar 0.60% atau setara dengan 43.81 kPa. Berdasarkan parameter yang dievaluasi, tepung kedelai dengan ukuran 50 μm memiliki hasil terbaik dan penurunan ukuran butiran tepung kedelai sejalan dengan peningkatan kekuatan tanah pasir. &nbsp

The Utilization of Soybean as a Catalyst Material in Enzyme-Mediated Calcite Precipitation (EMCP) for Crack Healing Concrete

The effect of using soybean as a catalyst in the Enzyme-Mediated Calcite Precipitation (EMCP) method of crack healing concrete is discussed in this research. The existence of cracks in concrete reduces its quality, therefore, there is the need for repair efforts and one of these is through injection using the EMCP method. This technique employs the plant-derived urease enzyme to catalyze the reaction between calcium chloride (CaCl2) and urea (CH4N2O) towards precipitating calcium carbonate (CaCO3). Its effectiveness was, however, evaluated using a test tube experiment, and the optimum combination of urease, urea, and calcium chloride was selected and used as the preliminary data for the soybean added as a catalyst. Meanwhile, the concrete samples were prepared in the form of 5 cm x 10 cm cylindrical concrete and the crack was made through Universal Testing Machine (UTM) with a controlled area observed to be in the range of ± 0.2-0.3 mm. The injection solution was added to the concrete using the percolation method up to the period the height of the solution was 5 mm above the concrete surface and this was conducted several times with the effect on the concrete cracks evaluated through permeability tests which were used as one of the concrete quality parameters. Moreover, the permeability coefficient value was found to be inversely proportional to the quality and this means a smaller value of the coefficient produced better concrete quality and vice versa. The results showed the total calcite formed in the sample after the 4th injection was 18.3% of the total surface area of the concrete crack and was able to reduce the concrete permeability value by 95.43%. This, therefore, means the use of soybean in the EMCP method is considered feasible to repair concrete cracks

Distribution of grout material within 1-m sand column in insitu calcite precipitation technique

AbstractThis study evaluates the potential of improving an insitu calcite grouting technique. The grout is composed of an equimolar solution of urea–CaCl2 and an enzyme named urease. We examine the distribution of the grout materials and precipitated calcite within sand columns with a diameter of 5cm and a height of 100cm. In the first series of experiments, the concentration distributions of the individual grout materials (i.e., urea, CaCl2 and urease) within the sand specimen are evaluated. In the second series of experiments, an enzyme-reagent mixed solution (i.e., grout) is injected into the sand columns to evaluate the distribution of calcite. Sand samples are collected from various vertical locations within the treated columns and the amount of precipitated calcite is evaluated. Furthermore, attempts are made to achieve the uniform distribution of the injected grout and hence uniform calcite distribution throughout the specimen. The results show that a uniform distribution of the grout materials up to a distance of 1m from the inlet is achievable. It is also observed that a relatively uniform distribution of calcite is achievable as long as the rate of calcite precipitation is well controlled

Optimization of Enzyme-Mediated Calcite Precipitation as a Soil-Improvement Technique : The Effect of Aragonite and Gypsum on the Mechanical Properties of Treated Sand

The effectiveness of magnesium as a substitute material in enzyme-mediated calcite precipitation was evaluated. Magnesium sulfate was added to the injecting solution composed of urea, urease, and calcium chloride. The effect of the substitution on the amount of precipitated materials was evaluated through precipitation tests. X-ray powder diffraction and scanning electron microscopy analyses were conducted to examine the mineralogical morphology of the precipitated minerals and to determine the effect of magnesium on the composition of the precipitated materials. In addition to calcite, aragonite and gypsum were formed as the precipitated materials. The effect of the presence of aragonite and gypsum, in addition to calcite, as a soil-improvement technique was evaluated through unconfined compressive strength tests. Soil specimens were prepared in polyvinyl chloride cylinders and treated with concentration-controlled solutions, which produced calcite, aragonite, and gypsum. The mineralogical analysis revealed that the low and high concentrations of magnesium sulfate effectively promoted the formation of aragonite and gypsum, respectively. The injecting solutions which produced aragonite and calcite brought about a significant improvement in soil strength. The presence of the precipitated materials, comprising 10% of the soil mass within a treated sand, generated a strength of 0.6 MPa

Short- and Long-Term Observations of Fracture Permeability in Granite by Flow-Through Tests and Comparative Observation by X-Ray CT

Having a grasp of the variation in the fracture contact area is a kernel in the understanding of the permeability evolution of fractured rocks. However, the number of studies that focus on measuring the long-term variation in the fracture contact area under different conditions is insufficient. In this study, a series of short- and long-term permeability tests under coupled conditions is performed to check the performance of permeability. The results reveal that the permeability measured in the short-term tests shows reversible behavior and a dependence on the applied confining pressures and temperature. In contrast, the permeability in the long-term tests displays irreversible behavior and an irregular change under the constant confining pressure. In order to verify the evolution of permeability, microfocus X-ray computed tomography (CT) is developed to observe the changes in the internal fracture structure under the same conditions as those in long-term permeability tests by assembling a triaxial cell with heating capability. The fracture aperture and the fracture contact-area ratio are calculated by a CT image analysis technique. The image analysis results show that the estimated aperture is seen to decrease with an increase in the confining pressure and to also decrease with time under a constant confining pressure. Moreover, the increase in the fracture contact area under the constant confining pressure observed by X-ray CT is confirmed. This also corresponds to a decrease in permeability in long-term tests. The hydraulic aperture calculated from the permeability tests and that evaluated from the CT observation have a similar decreasing trend. Therefore, the CT observation can better capture the evolution of the internal fracture contact area. These experiments underscore the importance of mechanical compaction and/or mineral dissolution at contacts in determining the rates and the magnitude of permeability evolution within rock fractures

Multi-physics numerical analyses for predicting the alterations in permeability and reactive transport behavior within single rock fractures depending on temperature, stress, and fluid pH conditions

The aim of the current study was to establish a validated numerical model for addressing the changes in permeability and reactive transport behavior within rock fractures based on the fluid pH under coupled thermal-hydraulic-mechanical-chemical (THMC) conditions. Firstly, a multi-physics reactive transport model was proposed, considering the geochemical reactions that depend on the temperature, stress, and fluid chemistry conditions (e.g., fluid pH and solute concentrations), as well as the changes in permeability in the rock fractures driven by these reactions, after which the correctness of the model implementation was verified by solving the 1D reactive transport problem as a fundamental benchmark. Secondly, the validity of the model against actual rock fractures was investigated by utilizing the model to replicate the measurements of the evolving permeability and the effluent element concentrations in single granite fractures obtained by means of two flow-through experiments using deionized water (pH ∼ 6) and a NaOH aqueous solution (pH ∼ 11) as permeants under stressed, temperature-elevated conditions. The model predictions efficiently followed the changes in fracture permeability over time measured by both experiments. Additionally, the observed difference in the changing rates, which may contribute to the difference in the fluid pH between the two experiments, was also captured exactly by the predictions. Moreover, in terms of the effluent element concentrations, among all the elements targeted for measurement, the concentrations of most elements were replicated by the model within one order of discrepancy. Overall, it can be concluded that the developed model should be valid for estimating the changes in permeability and reactive transport behavior within rock fractures induced by geochemical reactions which depend on the fluid pH under coupled THMC conditions

Coupled thermo-hydro-mechanical-chemical modeling by incorporating pressure solution for estimating the evolution of rock permeability

A coupled THMC numerical model has been developed to examine the long-term change in permeability of the porous sedimentary rocks that are assumed to be composed purely of quartz. Specifically, the chemo-mechanical process of the pressure solution was incorporated into the model. The developed model was validated by replicating the existing experimental measurements of the porosity reduction and the evolving silica concentration. Subsequently, by simulating the burial of high-level radioactive wastes in the deep subsurface, namely, by applying the simulated confining pressure and temperature conditions, the long-term evolution of the rock permeability was predicted. The model predictions clearly showed a significant influence of the pressure dissolution on the change in permeability with time. The predicted permeability of the rocks close to the wastes decreased by one order of magnitude in 104 years when considering the pressure dissolution, while the permeability changed little during the same period when the pressure dissolution was not considered. This reduction should delay the dispersion of the radioactive materials dissolved in the groundwater