The joint surface roughness is one of the important parameters which influences the mechanical and hydro-mechanical behavior of rock joints. In most cases, the joint roughness coefficient (JRC) was used to quantitatively express the roughness degree. In this study, we applied the mean Z 2 values of all profiles on the joint surface, rather than picking out one or several typical profiles, as estimated parameter to predict the JRC values. The calculated Z 2 values and JRC values diminish with the increase of sampling intervals. Through analyzing the measured JRC values under three sampling intervals, the 1.0 mm interval is pointed out the most appropriate one to evaluate the joint roughness. In addition, we evaluated and compared the JRC values of joints within granite specimens that have different mechanical properties and weathering state by two different methods (Z 2 method and backward analytical method). From obtained results, The Z 2 method could relatively accurately predict the JRC values of unweathered material at 1.0 sampling interval, while overestimate the JRC values in weathering state. It may be attributed to that the weather process may weaken the mechanical properties of JCS values and basic friction angle which are the controlling mechanical factors of definitional JRC values. Moreover, the Z 2 roughness metric does not take into consideration the mechanical proprieties and topography characteristics of weathering rock mass. In total, Z 2 method should be improved by considering additional parameters related to mechanical properties of rock joint (i.e. distribution of contact area within joint surface). 1. Introduction It has long been recognized that the rock joint is one of the crucial components of a rock mass. The rock joint surface plays a significant role in controlling the mechanical and hydro-mechanical behavior. In previous study, a parameter of joint roughness coefficient (JRC) is proposed to quantitatively express the roughness degree (Barton and Choubey, 1973). Nevertheless, measuring definitional JRC values still needs a tilt test or direct shear test to make a reliable estimation. To overcome this limitation of experiments, ten typical profiling lines are defined and visual comparisons are made to estimate roughness degree (Barton, 1977). However, this method is strongly dependent on the subjectivity and experience level of the researcher. To remove this bias, the root mean square first derivative values (Z 2 ), a statistical numerical parameter, is revealed, which is also easily to be determined.

Abstract Granite is a strong anisotropic rock affected by distribution of pre-existing microcracks. In various rock engineering applications, it has a significant meaning to understand the influence of internal microcrack distribution on the mechanical properties of rock. In this paper, static and dynamic fracture toughness tests were performed with straight notched disc bending (SNDB) specimens of Yougnju granite to investigate the microcrack induced fracture toughness anisotropy. In order to conduct dynamic fracture toughness test, split Hopkinson pressure bar (SHPB) apparatus using pulse shaping technique was adopted. The crack propagating behavior was observed by high speed camera and the crack opening velocity (COV) was also measured with laser gap gauge (LGG) system. The fracture processes of the specimen with different loading rate conditions were analyzed by comparing the images of high speed camera and micro focus X-ray CT. As a result of this study, the fracture toughness of Youngju granite shows obvious loading rate dependency through entire loading rates. It was revealed that the preferred orientation of pre-existing microcracks caused strong anisotropy of fracture toughness under static loading rates which the anisotropy of that became weaker with increasing loading rates. 1. Introduction In the field of rock engineering such as blasting, hydraulic fracturing and drilling, the understanding of fracturing behaviors in rocks under various loading conditions is important. Fracture toughness, as an example, is an indication of resistance against crack growth at the tip of crack. Generally it is well known that granites show strong anisotropy of mechanical properties caused by pre-existing microcrack. Many researchers have reported that fracture toughness of granites is affected by the preferred orientation of pre-existing microcracks and that increases with increasing the loading rate [Nara 2006, Dai 2010, Cho 2015].

Abstract Mining-induced fault-slip in deep underground mines is known to cause devastating damage to mine openings. It is thus crucial to be able to estimate its seismic source parameters, such as magnitude and seismically released energy. When slip-weakening laws are employed in fault-slip simulations, the characteristic slip-weakening distance, d c , is one of the most important input parameters. This study proposes a methodology to constrain d c of a large-scale fault in an underground mine, based on seismic efficiency η defined by ER (seismically radiated energy) / ΔW (potential energy change). First, a 3D numerical model that encompasses a fault running parallel to a steeply dipping orebody is constructed with the help of finite difference software FLAC3D. Mining blocks of the orebody are extracted in static conditions. Subsequently, dynamic analysis is performed to simulate fault-slip with a linear slip-weakening law employing Barton's shear strength model. Model parametric studies with respect to d c and mining depth are performed, and seismic efficiency is computed for each model. The results show that when d c ranges from 1.0 cm to 2.5 cm, the analyses yield reasonable seismic efficiencies. The estimated range of d c is reasonable considering the difference in size between rock shear tests, natural earthquakes, and mining-induced fault-slips. Consequently, this study provides a clue to estimating a reasonable range of d c especially for mining-induced fault-slip, thus significantly contributing to more accurate evaluation of the intensity of ground motions induced by mining-induced fault-slip. 1. Introduction The stress re-distribution caused by mining activities at great depths can reactivate pre-existing faults in underground mines, leading to fault-slip with intense seismic waves. As a result, severe damage to mine openings is inflicted. Importantly, seismic waves arising from fault-slip predominantly contribute to the damage initiation. Although previous studies mainly focus on the "static" behaviour of faults in underground mines (Hofmann, et al., 2011), a recent study (Sainoki and Mitri, 2015) has addressed the problem by performing dynamic analysis. It was found that critical slip-weakening distance d c , over which the shear stress acting on a fault decreases during fault-slip, is an important parameter that affects the severity of mining-induced fault-slip, i.e., the intensity of seismic waves and the amount of seismically radiated energy.

Abstract Mechanical properties and failure characteristics of specimens containing a couple of flaws under unloading condition has been studied by particle-flow-code (PFC) simulation. The results show that different unloading rates (Δ t ) and flaw inclination angles (α) result in variations in the unloading strength, deformation and crack effect for the pre-flawed specimens. Compared with intact specimens (without pre-flaws), the unloading strength of the pre-flawed specimens decreases nearly 5.5~20%, and faster unloading rates or smaller flaw inclination angles will lead to lower strength. Furthermore, the crack effect are investigated to understand the failure mechanism. It is observed that unloading rate exerts little effect on the failure modes but can affect the number of cracks, and slower rates will generate more cracks in the specimens. In addition, the changes in flaw inclination angles will influence the failure modes and cracks number. When α = 0° and 30°, the final failure modes of the specimens are similar with each other, while α = 60°specimen is more likely to be inhibited in the generation of cracks. Introduction Excavations in rock mass under high geostress are at great risk, because unloading process would have an important effect on the failure characteristics of rock mass. And the discontinuities such as joints, cracks, fissures, weak surfaces, faults, etc. in rocks play significant roles the mechanical behaviors of rock mass under excavation unloading (Wong and Chau 1998). The mechanical properties of intact rock during unloading under high geo-stress has been widely investigated by laboratory experiments (Bauch et al., 2004; Zhao et al., 2014; He et al., 2010; Huang et al., 2014; Du et al., 2016) and theoretical analyses (Li et al., 2013; Wang et al., 2010). However, numerical simulation of pre-flawed rock specimens, which refer to rock specimens containing pre-existing flaws, under unloading condition have rarely been investigated.

Abstract Numerical modelling is widely used in the rock block displacement analysis because it has more capabilities than analytical-conventional methods. Numerical modelling is able to solve the case with a complex geometry, precise calculation more parameters and quick calculation time. The use of this method needs to be validated due to a mismatch between the conditions of the numerical model with actual conditions in the field. Numerical modelling is done by distinct element method and one of the numerical validation methods is physical modelling in a laboratory. The geometry of the physical modelling in the laboratory using rock slope model with plane failure type as rock block object which will fail and using cement material. Physical modelling will be given load behind the slope using a pendulum as a seismic load with some variation of angles and be placing vibration test device (minimate III) above physical modelling to record the value of vibration due to a load of a given. Each test will be obtained the value of block displacement and vibration value. Based on this research, the results of numerical modelling approach value of displacement resulting from physical modelling test in the laboratory. 1. Introduction Rock block displacement to be one of problem on rock mechanics and engineering. The analysis of rock block displacement are mainly related to many parameters, such as the geological structure, pore pressure, rock mechanical properties, seismic load, field stress, and other parameters. With the many parameters that are encountered, so it needs a calculation method that can calculate quickly and complex. The numerical method is one that can be used to solve this case because it would be very helpful to rock mechanic engineers in calculating and analyzing a rock block displacement or slope instability.

Abstract Oxidation of lignite with H 2 O 2 solution to produce dissolved organic carbon (DOC) for generating biomethane by methanogen cultivation is an important stage of subsurface cultivation and gasification methods. To obtain more insights into this process, changes of lignite mechanical properties were investigated after its oxidation to produce DOC. Core specimens 30 mm in diameter and 60 mm in height were immersed into 1 wt.% H 2 O 2 to achieve a liquid-to-solid ratio of 5:1. Values of pH and E h were measured at arbitrary time intervals together with concentrations of H 2 O 2 and DOC. P-wave velocity and density were measured before and after immersion. A series of uniaxial compression tests was carried out for both chemically reacted (H 2 O 2 -immersed) and non-reacted (H2O-immersed) specimens. The concentration of DOC, which is the substrate of methanogen cultivation, increased due to the oxidation of lignite at decreased pH by H 2 O 2 . P-wave velocity showed positive correlations with strength, static tangent modulus, and dynamic Young's modulus. The average P-wave velocity ( V p ) decreased by about 1.5% from its initial value due to the average density decrease of 0.6% resulting from the above chemical reaction. This decrease was associated with microcracking caused by swelling and grain boundary change due to leaching. The influence of the above chemical reaction on the mechanical properties of lignite is small, despite the formation of DOC to produce biogenic methane. 1. Introduction Lignite seams of the Tenpoku coal field in Hokkaido, Japan, are considered to be used for biomethane production by subsurface cultivation and gasification (SCG, Aramaki et al., 2015; Tamamura et al., 2016), with the formation of dissolved organic carbon (DOC) from lignite by induced oxidation using hydrogen peroxide (H 2 O 2 ) being the first stage of this method. Subsequent stages feature methanogen cultivation to produce biomethane, using the DOC as a substrate, and the last stage corresponds to gas recovery. The formation of DOC for methanogen cultivation via chemical reactions can change the mechanical properties of lignite, which is the subject of this research.

ABSTRACT: The objective existing in-situ stress field and the physical mechanical properties of rock are closely related to the borehole stability in petroleum engineering. However, in present engineering design, rock mass is simply treated as isotropic material. This method may be acceptable when the well is shallow, but for deep rock engineering, with the increase of drilling depth, the anisotropic properties of rock mass become stronger and need be considered. This paper sets out to further understand the actual situation of the well site taking anisotropy into account. It investigates the effect of the rock anisotropy in the stability and the stress distribution at the wall of various well bores. The closed-form solution for stress distribution in transversely isotropic materials was used on the basis of rock experiments. A program was developed to compute the stress components at the inclined borehole wall whereby sensitivity analyses were performed to analyze the effect of the elastic moduli, stress anisotropy, arbitrary orientation of the borehole, the in-situ stress and the bedding inclination. Tensile and shear failure criteria were considered for the borehole stability analysis. 1 INTRODUCTION Borehole stability means wellbore, drilled to get hydrocarbons from underneath, keeps its regular size and shape during drilling process. It's a core technology guaranteeing drilling safety and high well quality (Jin & Chen 2012). According to statistics, cost on borehole instability worldwide is more than 6 billion dollars annually. Therefore, actively developing study on borehole stability is of significant economic consideration (Chen et al. 2008). Harold (1940) initialed borehole stability mechanics study on wellbore stress distribution. However, Santarelli et al. (1992), Franck et al. (2003), Helstrup et al. (2004), Germanovicha et al. (2000), Chen et al. (2005), Huang RZ et al. (1995), and Jin et al. (2004) have already assumed that surrounding rock and its stress distribution are isotropic, which is acceptable for shallow rock engineering. With the increase of drilling depth, encountered geological body presents various crossed structural planes (joint plane, fracture plane or bedding plane), and surrounding rock shows strong anisotropic characteristics. Generally coal seam gas reservoirs consist of sedimentary rocks with smooth laminar structures. Under normal conditions, the elastic modulus perpendicular to the bedding plane is slightly greater than that in the direction parallel to the bedding plane, and Poisson's ratio as well as other mechanical parameters, also vary between these two orientations.

ABSTRACT: A series of consolidation tests and tri-axial tests under high compression are carried out to obtain suitable constitutive model for Gaomiaozi bentonite and the regarding parameters. The elastic-plastic mechanical features of Gaomiaozi bentonite are studied based on the constitutive model and the relative experiments. In addition, a numerical simulation using ABAQUS is carried out to verify the validity of the suitable constitutive model, i.e. Cam-Clay model, and the rationality of the values of the parameters. Thus this research contributes a lot to the further study about the mechanical features of Gaomiaozi bentonite. 1 INTRODUCTION Deep geological repository is admitted internationally as high-level radioactive nuclear waste disposal method, while bentonite, due to its high adsorption, expansibility, low permeability and good thermal conductivity, is chosen as buffer/ backfill material for the repository internationally (Liu Yue-miao et al., 2001 and Ye Wei-min et al., 2008). Scholars at home and abroad have made quite a few experimental researches on bentonite, however their searches mainly concentrated on the basic physical and chemical properties such as microstructure characteristics, cation exchange, expansion deformation and also expansion force, heat conduction characteristics and penetration performance. Research results on physical and mechanical properties and features of bentonite under different soil-water conditions are rare. Shen Zhen-yao et al. (1998) researches on Xinjiang sodium base bentonite. Through uniaxial and triaxial compression tests on bentonite under high pressure with different moisture content, he gets the stress-strain curve. Through triaxial shear experiments, Zhu Guo-ping et al. (2008) gain stress-strain relationship of Gaomiaozi bentonite under different moisture content and dry density. They summarize curve and laws on changes between cohesive force, internal friction angel and moisture content as well as dry density.

ABSTRACT: Nuclear power is a highly efficient clean energy. With the widely applying of nuclear energy, nuclear waste disposal is increasingly paid attention. Bentonite is selected as the buffer material of high-level radioactive waste repository which is because that it has a very low water conductivity, great capability of thermal insulation, excellent expansion and good adsorption properties of radionuclides. While bentonite as a buffer material needs to be compacted and remodeling. Its special properties are not fully applicable to existing norms bentonite laboratory tests. In this paper, bentonite, the buffer material for nuclear waste disposal, is studied by axial compression tests. According to associated test codes and references, specimen preparation method is designed, and the related strength of buffer material specimens in laboratory experiments analyzed. I INTRODUCTION Disposal of highly radioactive nuclear waste is to divide high-level radioactive waste from the human living environment. In order to prevent the migration of radioactive substances to the biosphere, a lot of methods was raised to solve the problem, including "deep geological disposal", "glacier disposal", "space disposal", "molten rock disposal" and so on. After years of research around the world and practice, now it is acceptable that the most feasible solution is deep geological disposal of nuclear waste. According to the method of deep geological disposal of nuclear waste, a metal can will be built in the underground repository with 500–1000 m deep, and the buffer and backfill material will be filled between the metal cans and rock mass to assure the safe disposal of high level radioactive nuclear waste and a permanent and isolated environment from human survival. Solidified nuclear waste, the waste canister, buffer and backfill materials, repository and rock together constitute a "multiple barrier system". Buffer and backfill material is filled between the waste cans and the rock mass. Nearly three decades' research shows that bentonite containing montmorillonite is ideal buffer material because of low permeability, high absorption, good thermal conductivity, excellent expansion performance and suitable mechanical properties.

ABSTRACT: There is a large burning area in Xinjiang Laojunmiao open pit coal mine. To ensure the slope safety, characteristics of surrounding rock after baked and burnt were understood through the geological investigations and the rock mechanics tests. The typical profiles of burning area were selected according to the engineering geological characteristics. The safety factors of the profiles in the different final slope angles and the different states were computed through the limit equilibrium method, then the stabilities were analyzed to determine the reasonable final slope angle. The results show that strengths of burnt rock decrease compared with original rock. When the final slope angle of Laojunmiao mine is 36°, the slope safety can be ensured. 1 INTRODUCTION Due to the dry climate and shallow coal seam in Xinjiang, coal-field fire are very serious. According to statistics (Cai & Wei 2008), there are currently 50 coal-field fire areas in Xinjiang, and the total area is about 570m 2 , which lead to the loss of more than 15 million t coal. A large burning area is formed after the coal spontaneous combustion, causing a serious impact on safety production of mine. Many scholars have studied the coal spontaneous combustion, burning area and burnt rock and achieved certain results. Ide et al. (2011) delineated the range of coal fires in underground area by magnetic method, and the geomagnetic characteristics were studied and reported. Heffern & Coates (2004) made a detailed study on distribution and cause of the geologic history coal fires in the Powder River Basin of the United States and used a variety of dating methods to date burnt rocks. Hoffman et al. (2004) used differential interferometric synthetic aperture radar to detect the coal fires in northern China. Christian et al. (2009) detected the range of coalfield fires using thermal infrared remote sensing technique. Voigt et al. (2004) conducted a survey to detected and analyzed coal seam fires in north China using integrating satellite remote sensing techniques.

ABSTRACT: The cemented filling body is very important structural unit in cut-and-fill stoping. Therefore, it is of important significance to forecast the catastrophe by researching the response characteristic of cemented filling body at critical state. The damage characteristics and response ratio characteristics of cemented filling body with different cement-sand ratio have been studied by acoustic emission under uniaxial compression test. The results showed that the damage threshold of cemented filling body of cement-sand ratio 1:4 is 33%~41% of uniaxial compressive strength, as for cement-sand ratio 1:8; threshold is 37%~49%. As the loading stress is over 75% of uniaxial compressive strength, the filling body with cement-sand ratio 1:4 was damaged aggressively. By contrast, the filling body is damaged aggressively for cement-sand ratio 1:8 as the loading stress over 85% of uniaxial compressive strength. The response ratio for AE accumulative events, accumulative ringing count, and accumulative energy appeared to be very small at the low stress level. However, the response ratio increased greatly at critical state for all AE parameters. Therefore, this characteristic can serve as failure omen. 1 INTRODUCTION With the increase of the mining exploitation depth, the underground pressure activity becomes more and more frequent. Therefore, it is important to predict and forecast the failure of the underground stratum and the main structural unit. The essence of the rock material failure is the process of energy dissipation and release. On the one hand, the dissipation of energy is the power of rock material failure. On the other hand, the release of energy is the immanent cause of rock material failure. And the phenomenon of acoustic emission is nothing but the quick energy release of the rock material failure. Chen (2001) studies the acoustic emission characteristics of the sandstone during the uniaxial compression process, and the results show that: the main parameters of the acoustic emission characteristics are apparently increasing when the stress is close to the peak stress. Sun (2012) concludes that the corresponding stress of the acoustic emission uprush point of the deep layered sandstone matches to the critical point of rock specimen. Miao (2009) conducts the uniaxial circulation of loading and unloading destabilization experiment, the study finds that the response ratio of loading and unloading will dramatically increase when the rock sample approaches to the critical state.