Rock hardness is dependent on the type and quantity of the various mineral constituents of therock and the bond strength that exists between the mineral grains.
As the book "The Complete ISRM Suggested Methods For Rock Characterization, Testing and Monitoring:1974-2006" explains, hardness is a concept of material property. As such, the quantitative measure of hardness depends on the type of the test employed.
Among the three types of tests that have been used to measure the hardness of rocks and minerals: 1) indentation tests; 2) dynamic and rebound tests; 3) scratch tests, the authors of this paper chose indentation tests, more specifically the Vickers test, and complementary verified the correlation between the results of the Schmidt sclerometer tests (rebound tests) and the Vickers microhardness tests in a set of the same 9 (nine) granite rocks.
The objective of this research was to verify the correlation of one created index, based on the Vickers test that can be able to represent the rock material hardness. We know that Vickers tests determine the microhardness of individual rock minerals. A pyramidal shaped diamond is applied to the surface with a specified force. The area of the permanent residual deformation divided by the applied force is a measure of the hardness. Using the relative participation of the mineral components of the rock, we established a hardness index denominated EVM (Equivalent Vickers Microhardness).
The correlation of the EVM with others strength properties and application parameters like UCS (Uniaxial Compressive Strength) to study sawability of granite blocks have presented very good results.
The association of equivalent microhardness with others strength parameters, allowed to find powerful algorithms for analyzes of the rock materials behavior under diversified circumstances, as the sawability of granite blocks, as we can see in Fig. 1.
Other very interesting correlations were found as the EVM values and that were obtained by the use of the Schmidt impact hammer for the superficial hardness determination.
The application of rock bolts for the stabilisation of underground openings needs a good understanding of the response of the bolts during rock excavation. The pull-and-shear performance of two types of rock bolts–that is, the fully encapsulated rebar and the D-Bolt–has been studied both experimentally and numerically. The rock bolts were tested under a pull-and-shear load at five displacing angles (i.e., 0°, 20°, 40°, 60°, and 90°), and at the same time, the strains on the bolts were monitored during testing. The two types of rock bolts have similar loading capacities, while the loading angle has obvious influence on the deformation capacity of the D-Bolt. The ultimate total displacement of the D-Bolt decreases from 140 mm under pure pull loading to 70 mm under shearing. The displacement capacity of the rebar is not significantly influenced by the loading angle but remains at a quite low level from 29 to 53 mm. The D-Bolt yielded in a longer section the rebar bolt under pure shear. A trilinear material model was used to simulate the strain hardening behaviour of the bolt steels in the numerical code FLAC3D. The numerical results show that the axial load in the bolt decreases exponentially with the distance from the loading position for the rebar bolts, but it remains constant in the bolt sections between the anchor positions for the D-Bolts. When the bolt is subjected to a lateral load, shear stresses are created on the D-Bolt surface within a short distance from the loading position.
Rock bolt is one of the most conventional support elements in civil and mining engineering nowadays. Shear movement in underground excavation is a common occurrence in bedded strata or jointed rock masses. The loading condition on rock bolts is often a combination of tension and shear loading in situ. Therefore, it is necessary to know the performance of rock bolts under pull-and-shear loading from the point of view of better understanding the interaction between the bolt and the rock.
Plenty of effort has been made to examine the rock bolt performance in both laboratory and field trials (Bjurstrom, 1974; Dight, 1982; Spang and Egger, 1990; Holmberg, 1991; Ferrero, 1995; Grasselli, 2005; Jalalifar et al., 2006). However, the disadvantages of the existing test methods are the involvement of friction on the joint surfaces and a limitation of the bolt installation when the bolt inclination angle is greater than 45° with respect to the joint surface. In addition, numerical modelling is another way to study the bolt-reinforced rock joint (Ferrero, 1995; Grasselli, 2005; Malmgren & Nordlund, 2008; Jalalifar & Aziz, 2010; Nie et al., 2013; Lin et al., 2014). The objective of this paper is to present the laboratory tests conducted recently on the D-Bolt and the rebar bolt in the Rock Mechanics Laboratory at the Norwegian University of Science and Technology (NTNU). The FLAC3D numerical modelling results on the influence of the displacing angle to the anchorage performance of the rock bolts are also evaluated.
Inter formational sliding of bedding planes between two different rock types in Badong formation is an important reason that leads to the instability and fracture of rock mass in this stratum. There are no specific research achievements on bedding planes between two different rock types in Badong formation. In order to study the shear mechanical properties of bedding planes between two different rock types. This paper using the software of particle flow code (PFC) and JRC model to establish several typical structures of different roughness coefficient of surface morphology based on the laboratory tests on silty mudstone and marlite in Badong formation. A great deal of numerical shear tests were carried out by ball production, radius expansion and servo infliction. The research results are as follows: First, shear strength of Badong formation is found to vary between that of silty mudstone and marlite isotropic planes by analyzing shear strength of the homogenous bedding planes and inhomogeneous rock types, we can find that the shearing strength in Badong formation is between silty mudstone and marlite isotropic planes and which is closer to the shearing strength of silty mudstone isotropic planes. Second, the basical friction angle of bedding planes between two different rock types in Badong formation is between silty mudstone and marlite isotropic planes and which is closer to the basical friction angle of silty mudstone between isotropic planes. Third, based on the experimental results, an expression is obtained to describe the relationship of the ratio of shearing strength between bedding planes of two different rock types and isotropic planes in soft rock, the rock combination coefficient, and the roughness coefficient. This study has important significance for further research on the shear mechanical properties of bedding planes between two different rock types.
Kolo, I. (Masdar Institute of Science and Technology) | Al-Rub, R. K. Abu (Masdar Institute of Science and Technology) | Sousa, R. L. (Masdar Institute of Science and Technology) | Sassi, Mohamed (Masdar Institute of Science and Technology) | Sirat, Manhal (Abu Dhabi Company for Onshore Oil Operations (ADCO))
Fractures induced by folds are pivotal in hydrocarbon exploration, ground water transport and harnessing of geothermal energy. This is because of the need to predict and understand fracture propagation in reservoirs which have folded rock formations. Despite its prominence, there is a lack of reliable numerical models for simulating fractures resulting from rock folding. This is partially due to the difficulty involved in fracture modelling. As a contribution to fold-fracture modelling, this work applies a coupled plasticity-damage model to rock fracturing and anticlinal folds. Parametric studies on a single folded layer give insight into the fold formation and behaviour. The anisotropic continuum damage model which considers both tension and compression is formulated using the power damage evolution law. For ease in formulation, strain equivalence hypothesis is adopted whereby the strain is the same for both damaged and undamaged configurations. Lubliner plasticity yield criterion is adopted for plastic deformation. The model is coded in Abaqus user subroutine UMAT and is applicable to quasi-brittle materials.
The understanding of fractures in the earth’s crust helps in the prediction, evaluation and characterization of fractured reservoirs, whether they are petroleum, geothermal or underground water reservoirs. This is partly because reservoir permeability and stability are dependent on fracture properties (Nelson, 2001). To achieve this, various approaches have been developed to analyze reservoir rock fractures. These could be grouped into (a) discrete approaches – where the rock mass is represented by a finite number of well-defined components and (b) continuum approaches – where the mathematical assumption of an infinitesimal element is made (Jing, 2003). Ivanova (Ivanova, 1998) comprehensively studied fracture systems in nature and highlighted folds as one of the major geologic settings that produce fractures. Naturally occurring folds in reservoirs sometimes compound fracture analysis. It might not be clear whether the fractures formed before, during or after rock folding. Fractures formed during the folding process are expected to show stress orientations related to folding and are thus of primary concern in rock folding simulation. It could be argued that there are no numerical models that assuredly simulate rock folding and fracturing simultaneously (Jäger, Schmalholz, Schmid, & Kuhl, 2008).
Folds are formed when planar or straight surfaces of the earth become curved or bent due to plastic deformation. The process of folding has two mechanisms: flexure and shear. Flexural folding could be in the form of bending or buckling depending on the compressive force. When the force is parallel to the bedding, buckling occurs; when stresses are applied across layers causing torque, bending occurs. Shear folding occurs due to small displacements along closely spaced planes perpendicular to the bedding. Flexural folding is associated with competent (strong, thick and stiff) rock layers while shear folding is found in incompetent formations (Ivanova, 1998). Fractures resulting from flexural folding are expected to be more pronounced. In this study, fractures due to bending are investigated. Numerical models based on continuum approaches majorly describe material deformation while discrete approaches describe element movement of the system (Bobet et al., 2009). The former will be more appropriate to study the initiation and propagation of fractures due to rock bending. Hence, Finite Element Method (FEM) is adopted.
When applying numerical techniques to problems of deformation of rocks, the body to be analyzed is divided into small portions and is represented by a system of points linked together with the neighboring points. In the so-called distinct element modeling, the particles in the system are directly connected by springs and a contact law is used to specify forces on the particles to simulate the mechanical behavior of the body. The relation between the spring stiffness and commonly measured material properties, such as the Young’s modulus and the Poisson’s ratio, should be known a priori. For the general case of a random packing of particles, the relation is found by means of a process in which a system of particles and set of parameters are prepared to simulate a set of specimen-size test and then the input parameters are chosen to match the measured material properties. The validity of the numerical result is only demonstrated by comparison model behaviors with the measured responses. The process includes trial-and-error manner, therefore, no model is complete to reproduce the mechanical properties. In this paper, a relation between the spring stiffness and the elastic constants; the Young’s modulus and the Poisson’s ratio, is introduced to a three-dimensional distinct lattice spring method. Two simple numerical examples will be presented to demonstrate the performance of the method with the procedure for the determination of the spring stiffness.
In numerical modeling of engineering, problems are replaced by the behavior of an adequate model using a finite number of components. Such modeling can be divided into two categories; continuous and discontinuous. The numerical modeling based on the classical elastic theory could provide an adequate description of the mechanical behavior of most materials, although they are heterogeneous. The FEM is one of the continuous modeling and is perhaps the most widely used method in engineering today. However, the behavior of some materials, such as rock and concreate, cannot be simulated realistically without directly introducing the inherent discontinuous nature of the materials.
In discontinuous modeling, the medium is represented as assemblies of particles or rigid blocks which are directly connected by springs and a contact law is used to specify forces on the particles (Cundall, P. A., 1971, Shi, G. H. & Goodman, R. E., 1989). The essence of the discontinuous modeling is to solve the equation of motion and is able to simulate large displacements including fracture openings and complete detachment. To formulate the methods to simulate the mechanical behavior of rock mechanics applications, the followings must be solved: (i) the determination of constitutive relations for blocks, (ii) integration of the equation of motion for dynamic relaxation.
As a powerhouse, a transformer or a valve chamber, large underground caverns are a major element of hydropower plant. On the contrary to galleries and shafts which are linear and long underground structures, large caverns are designed with often complex geometries and large spans. Moreover, as these caverns are generally located on the foot of the hydraulic scheme, the high overburden is often responsible of a high stress state. Thanks to current experiences in France and around the world, the Hydro Engineering Centre (CIH) of EDF has defined great principles in the design of caverns leading to a more general methodology. The project of Tehri in India is an example of difficult ground, with in the same time a high overburden, a ductile material and an important schistosity. These physical realities are attempted to be approached in numerical modelling thanks to a constitutive law with softening behaviour and an oriented criterion integrated in the continuum model. As a result, a flexible support is advised, based on a high density bolt system and a thin lay of shotcrete. This methodology has been developed in the case of Gilboa in Israel, except the schistosity which has not been observed in situ. At last, the example of Gavet (France) illustrates a discontinuous case of numerical modelling for a good quality but fractured rock mass.
Principes Et Methodes De Conception De Grandes Cavernes Hydroelectriques Souterraines
Les grandes cavernes souterraines abritant les turbines, les transformateurs et certaines vannes sont des ouvrages majeurs dans les amé nagements hydroé lectriques. A la diffé rence des galeries et des puits qui pré sentent de longs liné aires, les grandes cavernes sont des structures à la gé omé trie souvent complexe et avec des porté es consé quentes. De plus, ces cavernes é tant gé né ralement situé es en pied du circuit hydraulique, la grande couverture rocheuse peut se traduire par un é tat de contraintes important. Forte de ses expé riences ré centes en France et dans le monde, le Centre d’Ingé nierie Hydraulique d’EDF a identifié de grands principes de conception des cavernes permettant l’é laboration d’une mé thodologie plus gé né rale. Le projet de Tehri en Inde est un exemple de terrain difficile, pré sentant à la fois une forte couverture, un maté riau ductile et une schistosité marqué e. Ces ré alité s physiques sont approché es au mieux grâ ce à la prise en compte dans les modé lisations numé riques d’une loi de comportement radoucissante avec un critè re orienté inté gré dans un modè le continu. Les ré sultats ont conduit à privilé gier un soutè nement souple basé sur un boulonnage systé matique relativement dense associé à une fine couche de bé ton projeté . Cette mé thodologie a ensuite é té reprise pour le projet de Gilboa en Israël, exception faite de la schistosité puisque non observé e sur site. L’exemple de Gavet (France) enfin permet d’illustrer un cas de modé lisation numé rique discontinue d’un massif rocheux de bonne qualité mais fracturé .
In Canada and other countries, several types of rock formations are being considered for the geological disposal of radioactive wastes. In order to better understand the ability of these rocks to contain and isolate the wastes, the Canadian Nuclear Safety Commission collaborates with different international research organizations and has access to experimental data from Underground Research Laboratories (URLs) in the world. Such an URL is situated in Opalinus Clay, at Mont Terri, Switzerland. An experiment, consisting of the excavation of a tunnel followed by water and gas injections in a test section of the tunnel, was performed at the URL. The authors have developed a mathematical model, based on the poromechanics theoretical framework, to simulate those phases of the experiment. The constitutive relationship for the stress-strain behaviour of Opalinus Clay takes into account its inherent anisotropy due to bedding. The model was able to capture the main features of the experiment such as the prediction of the shape and extent of the excavation damage zone and its influence on water and gas migration.
In Canada and many other countries, several types of rock formations such as crystalline rocks and sedimentary argillaceous rocks, are being studied for the disposal of used nuclear fuel from nuclear power plants. Geological disposal relies on multiple natural and engineered barriers for the long-term containment and isolation of the wastes. The host rock is a major natural barrier and is the subject of extensive research. The Canadian Nuclear Safety Commission (CNSC), Canada’s nuclear regulator, will be responsible for the licensing of future deep geological repositories. In order to evaluate the safety of deep geological repositories, CNSC staff keep abreast of scientific developments by collaborating with national and international partners on experimental and theoretical research on the long term performance of those engineered and natural barriers. Through that collaboration, the CNSC has access to data resulting from experiments performed at different Underground Research Laboratories (URLs).
In June 1985, Chinese Society for Rock Mechanics and Engineering (CSRME) was established. During the past 30 years, various major achievements are made in the discipline of rock mechanics and rock engineering field, especially in the last ten years. This paper will discuss the main rock engineering progress in association with China National Science and Technology Awards, “973 Program”, and “Tan Tjongkie Lecture”. The current status of rock engineering in China and some hot problems are summarized. Furthermore, some suggestions are given for addressing those problems.
The modern concept of rock mechanics and engineering was initiated and developed after the Second World War. In this regard, the International Society for Rock Mechanics (ISRM) was established in 1962, subsequently further promotes its development. In June 1985, the Chinese Society for Rock Mechanics and Engineering was established. Various chief scientists, including professors Tan Tjangkie, Pan Jiazheng, Sun Jun, Wang Sijing, Qian Qihu, have made their huge efforts to the development of Rock Mechanics and Engineering in terms of the large-scale rock engineering projects in China.
The achievements in the last 30 years significantly promote the development of China in different fields, such as energy, transportation, national defense and other national economy and major infrastructure facilities (Wang et al. 2004; Qian 2010, 2011). In other words, these projects also promoted the science and technology progress in rock mechanics and engineering across the world. Reviewing and analyses of the experiences on those key rock projects are beneficial to its technological development. She and Dong (2013), in view of literature statistical analysis, discussed the status and research progress in rock mechanics field associated with the last 30-year China's developments, including the rock strength and deformation theory, rock fracture and damage mechanics, the constitutive relation of rock dynamics, rock field coupling, rock reinforcement and stability analysis, etc. The challenging issues in rock mechanics were also proposed, which are the main concerns in rock engineering practices, engineering exploration, design, construction and operation.
In this paper, on the basis of summarizing the main achievements of Chinese rock engineering projects, main rock engineering progress in association with China National Science and Technology Awards (http://www.most.gov.cn/kjjh/kjcg/), “973 Program” (http://www.973.gov.cn/onstudy/973_1.htm), and “TAN Tjongkie Lecture”(She and Lin, 2014) was given. Finally, some suggestions and prospects for the development trends of rock engineering were provided.
The long-term slaking behavior of clay-bearing rocks in the field can be quite different than the short-term slaking behavior in the laboratory. To assess this difference, 12 replicate samples of each of 20 clay-bearing rocks, including 5 claystones, 5 mudstones, 5 siltstones, and 5 shales, were exposed to natural climatic conditions for 12 months. After each month of exposure, one replicate sample of each rock was removed from natural exposure and its grain size distribution was determined. Disintegration ratio (DR), defined as the ratio of the area under the grain size distribution curve of the slaked material for a given rock to the total area encompassing all grain size distribution curves of the tested samples, was used to quantify the amount of disintegration under natural climatic conditions. DR values of naturally exposed samples were then compared with the corresponding values of second-cycle slake durability index (Id2) determined in the laboratory. The relationship between Id2 and DR after, 1 month of exposure, exhibits an R2 value of 0.65 but the relationship deteriorates when DR values for 6 and 12 months of exposure are used, with R2 values of 0.63 and 0.25, respectively. However, when DR values for laboratory samples, computed from grain size distributions of slaked material after second cycle, are compared with DR values of naturally exposed samples for the three periods of exposure, the relationship improves with the corresponding R2 values being 0.61, 0.71, and 0.41. These results suggest that DR may be a better parameter for assessing the long-term disintegration behavior of clay-bearing rocks than Id2.
Clay-bearing rocks disintegrate easily upon weathering. When exposed to natural climatic conditions for a long time, changes in moisture content weaken the particle bonding and degrade these rocks to varying extents (Sarman, Shakoor, & Palmer, 1994; Gökçeoğlu, Ulusay, & Sonmez, 2000; Nicholson & Nicholson, 2000; Wuddivira, Ekwue, & Stone, 2010; Jamei, Guiras, Chtourou, Kallel, Romero, & Georgopoulos, 2011; Kodoma, Goto, Fujii, & Hagan, 2013; Miščevič & Vlastelica, 2014).
An anisotropic constitutive model is proposed in this paper accounting for both structural anisotropy and induced anisotropic plasticity. It is assumed that the rock is composed of a matrix and of potential planes of weakness. The matrix is assumed to be linear, transversely isotropic and the plasticity is described by a non linear yield function where the parameters are deduced from the nonlinear Hoek-Brown envelopes in pre- and post-peak, and derived from the laboratory characterization. A non-associated flow rule is used with a distinction between compression and extensional stress paths, as well as the absence of volumetric strain beyond large plastic distortion.
The planes of weakness are considered as known a priori or assumed to be oriented perpendicular to the direction of the current minor principal stress. An elastic-perfectly plastic behavior according to the Mohr-Coulomb criterion is assumed in the planes of weakness; while the elastic part is considered as linear and transversely isotropic.
Finally, the proposed model was implemented in FLAC3D and used to simulate triaxial compressions to provide a verification of the implementation. The applicability of the implemented model to reproduce damage (pre-peak) and/or failure developments around a circular opening is checked. The GCS drift, one of the mine-by experiments set up at the main level of the Meuse/Haute-Marne Underground Research Laboratory, is selected for this first application.
Numerous experimental results available in the literature indicate that most sedimentary and metamorphic rocks, such as shales and slates, display a strong anisotropy of behavior and strength. These types of rocks usually exhibit some preferentially oriented structures or possess distinct bedding planes, which results in transversely isotropic behavior at the macroscopic scale: this is particularly the case of Tournemire shale (Niandou et al., 1997) or the Callovo-Oxfordian claystone (David et al., 2005).
Based on the experimental results, various failure criteria for anisotropic materials have been proposed where a large review can be found in Duveau et al. (1998). Furthermore, various constitutive models were developed in order to approach the anisotropic mechanical behavior. These are mainly (a) the empirical models based on the theory of variational cohesion and / or friction (e.g., McLamore & Gray 1967, Saroglou et Tsiambaos 2008, Wang et Yu 2014); (b) the models built on the concept of ubiquitous joint with several planes of weakness, (c) the models where damage and/or plasticity are incorporated and formulated in the framework of irreversible thermodynamics (Pietruszczak et al., 2002, Yu et al., 2013).
Since 2000, the French National Radioactive Waste Management Agency (Andra) has been constructing an Underground Research Laboratory (URL) at Bure with intent to demonstrate the feasibility of a geological repository in the Callovo-Oxfordian claystone (COx) formation by collecting in situ experimental data. The excavation of galleries at the main level of the laboratory showed a significant fracturing induced by the excavation (Armand et al., 2014) in addition to structural or inherent anisotropy observed during the laboratory tests.