An experimental investigation on thermal damage and failure mechanical behavior of granite after exposure to different high temperature treatments

Modeling of rheological fracture behavior of rock cracks subjected to hydraulic pressure and far field stresses

Evolution of thermal damage and permeability of Beishan granite

With the rapid development of nuclear power in China, the disposal of high-level radioactive waste (HLW) has become an important issue for nuclear safety and environmental protection. Deep geological disposal is internationally accepted as a feasible and safe way to dispose of HLW, and underground research laboratories (URLs) play an important and multi-faceted role in the development of HLW repositories. This paper introduces the overall planning and the latest progress for China's URL. On the basis of the proposed strategy to build an area-specific URL in combination with a comprehensive evaluation of the site selection results obtained during the last 33 years, the Xinchang site in the Beishan area, located in Gansu Province of northwestern China, has been selected as the final site for China's first URL built in granite. In the process of characterizing the Xinchang URL site, a series of investigations, including borehole drilling, geological mapping, geophysical surveying, hydraulic testing and in situ stress measurements, has been conducted. The investigation results indicate that the geological, hydrogeological, engineering geological and geochemical conditions of the Xinchang site are very suitable for URL construction. Meanwhile, to validate and develop construction technologies for the Beishan URL, the Beishan exploration tunnel (BET), which is a 50-m-deep facility in the Jiujing sub-area, has been constructed and several in situ tests, such as drill-and-blast tests, characterization of the excavation damaged zone (EDZ), and long-term deformation monitoring of surrounding rocks, have been performed in the BET. The methodologies and technologies established in the BET will serve for URL construction. According to the achievements of the characterization of the URL site, a preliminary design of the URL with a maximum depth of 560 m is proposed and necessary in situ tests in the URL are planned.

The Beishan underground research laboratory for geological disposal of high-level radioactive waste in China: Planning, site selection, site characterization and in situ tests

\n Uniaxial compression tests with combined acousto-optical monitoring techniques are conducted on flawed granite specimens, with the aim of investigating the fracture-related acoustic emission (AE) event rate characteristics at the unstable cracking phase in flawed rocks. The interevent time (IET) function F(τ) is adopted to interpret the AE time-series from damage stress (σcd) to ultimate failure, and photographic data are used to evaluate unstable cracking behaviours in flawed granite. The results show that a high AE event rate is always registered but intermittently interrupted by macrofracturing at the unstable cracking phase. The reversed U-shaped curve relation between the AE event rate and the loading time is documented in unstable flawed granite for the first time. The acoustic quiescence has a mechanismic linkage and quantitative correlation with stress drop, and this synchronous acousto-mechanical behaviour is a typical result of the initiation, growth and coalescence of macrocracks initiated from the flaw tips. Moreover, the reactivation and intensification of fracture process zones (FPZs) by increasing loads are the dominant mechanism triggering unstable crack growth in flawed granite.

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AE event rate characteristics of flawed granite: from damage stress to ultimate failure

During deep mining, the excavation disturbance stress path is the domination factor for the stability of the surrounding rock mass as well as the ground pressure. One of the important parameters of the stress path is the loading or unloading rate of the disturbed rock or coal, which depended on the mining rate. To achieve a well understanding of the mining rate and its effect on the coal behavior, a preliminary case investigation of the mechanical properties of the coal at the various mining rates in both the laboratory scale and field scale was performed. Based on the uniaxial compression test and the digital image correlation (DIC) method, the mechanical behavior of the coal samples, such as the evolution of the strength, surface deformation, crack propagation, and elastic strain energy of the coal under the various loading rates were analyzed. A threshold range of the loading rate has been observed. The uniaxial compressive strength (UCS) and releasable elastic strain energy (Ue) increase with increasing loading rate when the loading rate is below the threshold. Otherwise, the UCS and Ue may decrease with the loading rate. Under the low loading rate (≤0.05 mm/min), the tensile deformation of the original defects could result in crack coalescence, whereas failure of the coal matrix is the key contributor to the crack coalescence under the high loading rate (greater than0.05 mm/min). Afterwards, with the consideration of the bearing capacity (UCS) and energy release of the mining-disturbed coal mass (Ue), a power exponential relationship between the mining rate (MR) in the field and the critical loading rate (vc) in the laboratory was proposed. The application potential of the formulas was then validated against the field monitored data. Finally, based on the critical loading rate, the released strain energy, and the monitored pressure on the roof supports, a reasonable mining rate MR for the Ji15-31030 working face was determined to be approximately 3 m/d.

Mechanical behavior of coal under different mining rates: A case study from laboratory experiments to field testing

Characterization of damage evolution in granite under compressive stress condition and its effect on permeability

Acoustic emission (AE) is defined as elastic waves associated with a rapid release of localized stress energy that is propagated within a material. It has been proven to be a useful tool for studying rock fractures (e.g., Lavrov 2003; Prikry et al. 2003; Tham et al. 2005; Amann et al. 2011; Shukla et al. 2013; Liu et al. 2015; Moradian et al. 2016; Zhang et al. 2016). Parametric and waveform data are two basic forms of recordable AE data. In the past few decades, parameter analysis has been widely used to observe progressive damage processes resulting from AE, as well as to evaluate the degree of damage (e.g., Butt 1999; Baddari et al. 2011; Xie et al. 2011; Zhao et al. 2013; Zhang et al. 2015). However, a single complicated AE waveform still might conceal important information, or may even present misleading information, since it is discriminated by only a few parameters (Behnia et al. 2014). Thus, it may be difficult to adequately discriminate the characteristics of AE waveforms using such parameter-based approaches. In waveform-based approaches, spectral analysis methods are often used for signal waveforms analyses. Because frequency content in an AE is determined by the wave source, studying the dominant frequency characteristics provides significant controls on investigations of microscopic fractures. Some of the pioneering work that investigated the dominant frequency characteristics of rocks was performed based on spectral analysis (e.g., Chugh et al. 1968; Armstrong 1969). Later, numerous relevant studies were conducted that evaluated the dominant frequency characteristics of AE waveforms (e.g., Stephens and Pollock 1971; Fleischmann et al. 1975; Woodward 1976; Kranz 1979; Sondergeld et al. 1980; Read et al. 1995; Xu et al. 2010). Owing to both hardware and software restrictions, it was impossible to realize continuous sampling, digitization and storage of a large quantity of AE waveforms within a short time. Previous studies did not contain statistical analyses and mainly focused on the change of dominant frequency characteristics since few waveforms could be recorded. With the development of high-performance computational processors and a new generation of sensors, the acquisition of multichannel raw waveforms has become possible (Behnia et al. 2014) and benefits the statistical analysis of the dominant frequency characteristics of AE waveforms. However, some latest studies about the dominant frequency of AE waveforms (Lu et al. 2013; Reiweger et al. 2015) still did not involve the statistical analysis. Ohnaka and Mogi (1981, 1982) expounded on the importance of statistical analysis for investigating the dominant frequency characteristics of AE waveforms. Moreover, statistical characteristics of dominant frequency of AE waveforms may be beneficial to understand the microscopic failure mechanism of rock. & L. Zheng zhenglu@scu.edu.cn

Dominant Frequency Characteristics of Acoustic Emissions in White Marble During Direct Tensile Tests

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.

J. F. Liu

Papers

Mechanical behavior of coal under different mining rates: A case study from laboratory experiments to field testing

Characterization of damage evolution in granite under compressive stress condition and its effect on permeability

Dominant Frequency Characteristics of Acoustic Emissions in White Marble During Direct Tensile Tests

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

A damage-mechanism-based creep model considering temperature effect in granite