Rock is typically a heterogeneous material composed of different types of inherent microstructures. The microstructures of a rock at the grain scale are usually associated with different mineral aggregations and microdefects such as joints, voids, and cleavage planes. In the present manuscript, a discrete element grain-based model featuring three-dimensional random Voronoi tessellations is proposed to study the deformation and fracturing of brittle rocks. Different grains, representing different rock-forming minerals, and the contacts between the grains were assigned different mechanical and physical properties, accounting for the grain scale heterogeneity observed in natural granular rocks. The simulations showed that rock specimens characterized by a wide grain size distribution are weaker than those with a narrow grain size distribution, but that spherical grains do not significantly influence the strength of specimen. Moreover, the simulations show that increasing the pre-existing porosity reduces specimen strength, due to the concentration of stress around the voids, and that intra-granular cracking is the dominant contact damage mechanism during uniaxial compression. 1. Introduction The rock is a heterogeneous polycrystalline material in natural conditions. It composes different mineral compositions and microstructures such as grain boundaries, micro-cracks, cleavages (Eberhardt, Stead, Stimpson, & Read, 1998; Martin & Chandler, 1994). Microstructure and mineral grains of rock is known to control the complex macroscopic mechanical response and fracture pattern. Grain boundaries act as the predominant source of stress concentrating flaws. Once the local stress near the flaws exceeds the local strength of rock, the initiation of microcrack starts from existing flaws. The density of cracks increases as the load increases. Propagation and coalescence of the cracks eventually cause macroscopic failure of rock. This paper aims to simulate crack damage evolution of brittle rock at mesoscale (grain scale). Mesoscale is between macroscale (phenomenological) and microscale (atomic or molecular). We used a three-dimensional discrete element grain-based model (3DEC-GBM), consisting of an assemblage of Voronoi polyhedra grains, to represent rock by a dense packing of bonded mineral grains of non-uniform size and shape. We explored the influence of grain size distribution, grain shape and porosity on the mechanical behavior and strength of granular rock.

Abstract In order to investigate the influence of several factors on the mode-I fracture toughness and fracture behavior of rocks, fracture toughness tests were conducted under various conditions of specimen shape and size, loading rate, confining pressure and water vapor pressure. The influence of not only these extrinsic factors but also an intrinsic one - namely, the microstructure of rocks, was also investigated to understand the fracture toughness and fracture mechanism of rocks, as well as the microscopic fracture toughness of mineral grains. In this lecture, the influence of some factors of them is discussed, based on our various experiments. 1. Introduction It is known that a brittle fracture of rock under a stress state occurs through a following process: pre-existing cracks initiate and propagate, then connect to other cracks and a fracture surface is finally developed. In order to explain this process, fracture mechanics, which was established in the field of metallurgical engineering in the 1970s, was introduced to rock mechanics in the 1980s. Some textbooks for fracture mechanics of rocks were published at that time (Rossmanith, 1983; Atkinson, 1987; Whittaker et al., 1992). In order to understand the fracture behavior of rocks, fracture toughness, which indicates the resistance to pre-existing crack initiation, is an important parameter. Several tests have been developed to estimate fracture toughness of rocks. The International Society for Rock Mechanics (ISRM) suggested the Chevron Bend test, Short Rod test (Ouchterlony, 1998), Cracked Chevron Notched Brazilian Disc test (Fowell,1995), and Semi-Circular Bend (SCB) test (Kuruppu et al., 2014). These testing methods assume that rock materials are of homogeneous elasticity. However, the fracture toughness is influenced by not only microstructure, such as grains, matrix, micro cracks, pores, but also extrinsic factors, such as specimen shape and size, loading rate, confining pressure and water vapor pressure. Kataoka (2015a) estimated fracture toughness of rocks in order to investigate the influence of several factors on it. Then Kataoka et al. (2014) also evaluated microscopic fracture toughness of minerals within granite. In this lecture, the influence of these factors and the microscopic fracture toughness of minerals within granite are discussed, based on our various experiments.

Abstract This study presents the quantitative evaluation of heterogeneity using the characteristic dimension. We propose the modified two-point correlation method to extract the ‘characteristic and representative size’ embedded in rocks applicable to both gray-level and binary images from 3D x-ray computed tomographic images. The method is validated by using a wide range of synthetic images and is applied to 6 rock specimens collected in Korea. Results highlight that the proposed method enables quantitatively defining the representative size in rocks. 1. Introduction Naturally existing heterogeneity of rock often hampers the precise determination of geophysical key parameters (Blair and Cook, 1998; Torquato, 2002; Lan et al., 2010; Schoen, 2011). Heterogeneity exists in multi-scale from micro (ex. mineral grain) to macro (ex. tectonic stratification) (Torquato, 2002). Heterogeneity are originated from pore geometry (ex. porosity and tortuosity), grain geometry (ex. size and shape), and inter-particle contact (ex. spatial configuration and packing) (Torquato, 2002). Inherent heterogeneity of rock governs the stress localization behavior which lead fracture development mechanisms (Blair and Cook, 1998; Lan et al., 2010). In addition, size and spatial configuration of heterogeneity influence the conduction phenomena such thermal, and elastic conduction (Torquato, 2002). Previous studies have relied on qualitative description of microstructures with the lack of deterministic and dimensional analysis. The two-point correlation method is one of the most general method to characterize heterogeneity of materials based on the two phase image (binarized image) (Blair et al. 1996; Chung & Han 2010). To do that, preprocessing is necessary for the acquired raw image. Recent development of 3D X-ray Computed Tomography (XCT) provide 16 bit scaled physical contrast image of materials using CT number. Physical density of materials governs CT number by Beer-Lambert law. Due to the advantage of XCT which is can visualize internal structure based on physical density, XCT image is frequently applied to the characterization of heterogeneity of materials. However, there is limitations of two-point correlation method owing to the preprocessing for the two-phase system. Generating representative two-phase system for various rock composing minerals is difficult. Also, separation of minerals which has similar densities is challenging. So, this study propose modified two-point correlation method to directly evaluate heterogeneity of rocks based on 16 bit gray images from 3D XCT.

ABSTRACT: Based on the theory of three-dimensional elastic-plastic seepage, the prediction of sandstone displacement and stress distribution law of the shaft frozen wall in Xiaozhuang Coal Mine was calculated. The prediction of sandstone displacement of frozen wall is performed by FEM numerical calculation. The results indicate that the biggest radial displacement was 2.036 × 10 −3 m when the thickness of frozen wall and roadway height were 2.5 m and 4 m, respectively, which could meet the requirements of safety regulations that the radial displacement should not exceed 50 mm and would not cause damage to the frozen pipe. The result showed that the design of frozen wall thickness and roadway height was proper, which can ensure the construction smooth. The actual monitoring data were basically consistent with predictive value. The study shows that FEM calculation could well predicate the radial displacement of frozen wall, and it is a quilt effective way of research to predicate stability of surrounding rock. 1 INTRODUCTION Freezing method is the main construction method which can make sure the wellbore across the unstable alluvium and weathering zone of bedrock and nearby the upper aquifer (Chen et al. 2000, Cui et al. 1998), freezing method construction can guarantee the working face anhydrous (Cui 2008). Frozen design must be aiming at the buried characteristics of alluvium and frozen rock mechanics characteristics, to meet the strength of the frozen wall, the deformation and water plugging requirements. At present the main problem of the freezing method construction is the frozen wall displacement is too large which leads to the broken of freezing pipe. Freezing method in general can be divided into the whole deep freezing method and the partial deep freezing method (Yao et al. 2005). Partial deep freezing method construction can avoid the damage, which the in-gate construction can easily produce the annular water channel by the frozen pipe and temperature tube, and the damage caused by the cavern rock (Zhao et al. 2012), and now non whole deep freezing method construction is been widely noticed (Yang et al. 2012). Xiaozhuang coal mine is the first to use the partial deep freezing method construction in northwest. In this paper, based on Xiaozhuang Coal Mine construction, research on the partial deep freezing method construction is carry out. By using of FEM software ABAQUS, following the process of the partial deep freezing method construction, the displacement law of the frozen wall, combined with the actual testing data analyze the stability of surrounding rock of the shaft frozen wall in rich water sandstone is analysed. The reasonable parameters for stability of surrounding rock of the shaft frozen wall in rich water sandstone formation is provided.

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: With the rapid development of the construction of new wells in western of China, it's necessary to solve these technical problems that are about drilling wells through the cretaceous water-rich stratum by the technique of artificial frozen. Based on wind shaft freezing engineering in Gansu xin-zhuang mine, the physical mechanical characteristics and thermal properties of cretaceous rocks are studied in frozen conditions by laboratory physical mechanics testing, field measurement and finite element numerical simulation. The stress mechanism of frozen wall is discussed by field measurement during shaft wall sinking period, and the change rule and causes of freezing pressure with the cretaceous frozen wall are analyzed. The actual measurement results and simulated results about the radial temperature of thermometer hole and frozen wall are compared, mainly to study the influence of concrete hydration heat on the frozen wall by fitting. The results show that the influence depth is 440 mm-480 mm. The study not only plays a positive role in enriching the basic theory of rock mechanics in China, but also provides the basis for the optimization design and the security and stability study of shaft sinking by freezing method in the western region. 1 INTRODUCTION With the rapid development of China's coal industry, well construction encounters a lot of the Cretaceous strata which has the characteristics of weak cementation, low strength and rich in water. At present, the achievements of freezing method in China are more concentrated in the eastern alluvial area (Ma & Cheng 2007, Hu et al. 2010, Yang & Xi 2010), and there are seldom reports about the study of the Cretaceous strata (Yue & Wang 2009, Yao et al. 2010). The design and construction of the wall still follow the theory of experience in the East (Yang et al. 2010, Hua et al. 2010). Because of the difference in physical and mechanical properties between rock and soil (Yang et al. 2010, Sun et al. 2010), the frozen wall and shaft lining design are conservative. There are still problems about high temperature stress and even the rupture of the outer shaft wall (Chen 2007). The physical mechanical characteristics and thermal properties of cretaceous rocks are studied in frozen conditions by laboratory physical mechanics testing, field measurement and finite element numerical simulation, and will provides the basis for the optimization design and the security and stability study of shaft sinking by freezing method in the western regions.

ABSTRACT: The safety of coal seam mining is affected by the evolution of vertical fissure in the overburden strata with ascending extraction of lower coal seam. In this paper, similar material simulation method was used in the experiment to research the overburden rock fissure about the ascending mining at Xichagou coal mine district of Northern Shaanxi. The result shows that: after the mining of the lower 5 −1 seam at this coal, overburden presents the typical characteristics of "three zones"; after the sixth periodic weighting, ascending crack through strata, which is located above 3 −1 seam at about 60 m, is formed in the process of later periodic weighting with the different pressure degree in the direction of caving angle nearly coal wall on one side of working face, the evolution process is that the crack opens when pressure comes and gradually closes when pressure has gone, the issue which crack through strata connects the surface can affect safety of the coal seam mining and its upper seam; Ascending mining of 3 −1 seam working face haven't the sliding instability and bench convergence phenomena in the vertical crack through strata, the working face can finish the continuous advance. The study of ascending safety mining has significant reference to other similar coal mine. 1 INTRODUCTION Coal seam group mining usually adopts descending mining method. However, in some historical and specific conditions, it is likely to mine the lower coal seam firstly and mining its upper seam later, which is named ascending mining method. Ascending mining method, which began in the 20 or 30's of last century abroad, was studied in West Virginia and Pennsylvania in the USA, Silesia coalfield in Poland, Donbas coal in the former Soviet Union. By these studies, we accumulated much successful experience (Dunham & Stace 1978, Wang & Li 1995). Domestic practice of ascending mining began in 1970s, the domestic scholars also obtained some research results about ascending mining theories (Yuan 2006, Zhang et al. 2007, Ma et al. 2007). After years of research and practice, for ascending mining, domestic scholars preliminarily think that (Liu 2007): when only one seam is mined and its mining influence factor K > 7.5, the upper seam can be mined normally.

ABSTRACT: A Thin Spray-on Liner (TSL) is defined as a thin chemical based coating or layer that is applied onto the mining excavations with a thickness of 3 to 5 mm. Compared with traditional support systems, TSL applications can bring significant benefits to the operation such as low volume, rapid application and rapid curing. These properties will ease logistics, improve on cycle times, increase mechanization, and improve safety for underground support. Over the past 20 years, extensive research has been carried out to study the supporting capabilities and mechanisms of TSLs, mostly in hard rock applications; however, inadequate research has been performed for underground coal mine applications. This paper aims to examine the support mechanism of TSLs in underground coal mines through a laboratory approach. Three types of laboratory tests were carried out to study the performance of TSLs using coal samples: direct pull adhesion tests, coated core tests and indirect tensile strength (Brazilian) tests. 1 INTRODUCTION The effectiveness of ground support systems is important to the safety of personnel and equipment in underground coal mines. Over the years, various means have been applied to ensure the stability of roadways and excavations to meet the complex ground conditions in underground coal mines. Mesh, shotcrete, mesh and shotcrete in combination, and shotcrete reinforced with various fibres are usually used for mining excavations as surface support (Stacey 2001, Archibald 2001). But the installation of mesh is labour intensive, expensive and time consuming (Tannant 2001), and it is difficult to automate to meet the rapid roadway development requirement (Nemcik et al. 2009). Shotcrete is chemically bonded to the rock surface and is able to achieve automation. However, shotcrete once sprayed onto the excavations, requires time to reach an effective supporting strength (Gilbert et al. 2010). The required thickness of shotcrete results in logistical problems for underground support (Tannant 2001, Laurence 2004). Besides, shotcrete may be considered too brittle in tension and shear in substantial ground deformations (Spearing 2001).

ABSTRACT: Taking square red sandstone samples as study objects, both destruction properties and destruction development model were analyzed with self-designed shear creep experimental device of coal or weak rock and monitor software of coal or rock micromechanics properties under restrictive shear creep. The results show: development process of red sandstone destruction under restrictive shear creep was show in process that first formed main rupture plane, then formed micro-cracks area near the main rupture plane, rupture plane was formed and burst destruction occurred in finally, and accompanying with a large crack voice. Development process of destruction model was show in progressive model but destruction model was shown in burst model, the rupture plane was non-flat and non-smooth wavy plane. 1 INTRODUCTION Obvious slip plane appeared and accompanied with significant slip trace after most of landslide slip instability occurred, shown in Figure 1. Obvious slip plane and significant slip trace were formed in the process of growth and development of slope slip through continuing overcoming cohesion in rock or soil and friction on slip plane. And all of state of stress, properties of rock or soil and change of external conditions near slip plane can affect growth, development and forming of slope slip plane. Therefore, analyzing and obtaining state of rock stress near slip plane, and researching mechanical evolution properties of rock under above state of stress, which were key important fact for obtaining the growth process properties of slope slip and preventing of slope destruction occur. Based on comprehensive analysis of many slopes, the conclusion that these rock near slip plane can be taken as in state of restrictive shear creep is obtained, restrictive shear creep load comes from the weight of overlying rock or soil of slip plane, direction of shear creep is slip direction of slip plane, restrictive effect is shown in addition of friction on slip plane because of the weight of overlying rock or soil of slip plane, simplified model is shown in Figure 2.