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ABSTRACT The granitoid rocks of Sharm El Sheikh are in south Sinai include alkali feldspar granite. Petrographically, the alkali feldspar granite characterized by the predominance of alkali feldspar, quartz, amphibole, biotite and plagioclase, the secondary minerals are Kaolinite, chlorite and sericite. The hydrothermal solutions are responsible for the formation of muscovite after feldspars and biotite, as well as increasing the cracks in the granite. K-metasomatism changed the physical properties of the granite, increasing their porosity, brittle extent, and providing the storage space for mineralization. Progressive alteration of the rocks has been marked by gradual transformation of the fresh granite to altered rocks. With increasing intensity of alteration, newly formed cracks connect with and intersect preexisting tectonic cracks, providing an isotropic permeability structure for solutions to flow. Polarizing microscope used to elucidate the optical properties and abundance of mica (biotite and muscovite) in the groundmass of granite asserts an important role in the formation of cracks. The influences degree of alteration on the granite consistency is studied numerically with a rock failure process analysis code, RFPA. Microscopic investigation and Numerical simulation results showed that alteration minerals have a negative effect on the strength and elastic modulus of rocks. On the other hand the influence of alteration in disintegrating the rock is greater than any other constituents like quartz and feldspars due to physical, mineralogical, and structural reasons. Therefore the choice of fresh granite (without alteration minerals) for construction projects is a key decision in order to avoid subsequent granite decay. 1. Introduction The Sinai Peninsula is an area of triangle shape, 60,000 Km2 in size, bordered in the southwest by the Suez Gulf and the line of the Suez Canal, in the southeast by the gulf of Aqapa, (Fig. 1A). The alkaline granites under consedration cover about 165 Km2, (Fig. 1B) , and are bounded between longitudes 33ยฐ 3 and 34ยฐ 24 E and latitudes 27ยฐ 47 and 28ยฐ 5 N. Collected samples represented the alkaline rocks of G. Al Att, G. Umm Markha, G. Hedmaiah, G. Al. Khoshby and G. Mdsosss This research aim to study the petrological controls effect on the reduction of mechanical properties, also to gain a better understanding of the influence of the alteration process on the granite failure to apply a numerical model based on mechanics that can be used for analysis. Alkaline rocks are most briefly defined as igneous rocks carrying feldspathoids and /or alkalis/pyroxene/amphiboles, and having a surplus of alkalis when compared to petrographically related rocks. The study of alteration effects, on strength and deformational behavior of rock under uniaxial compression environment is of vital necessary; most engineering works are confined to shallow depths where weathering and alteration have a dominant role to play and affects almost all chemical and physical properties of rocks. In this study, the hydrothermal alteration and weathering events in the Sharm El Sheikh granite have been studied, since they caused the most important solution actions through the fractures and cracks that affected the consistency of the rocks.
- Africa > Middle East > Egypt (0.24)
- Europe > Ukraine (0.15)
- Geology > Rock Type > Igneous Rock > Granite (1.00)
- Geology > Mineral > Silicate > Tectosilicate > Feldspar (1.00)
- Geology > Mineral > Silicate > Phyllosilicate > Biotite (0.64)
Numerical Study On the Dynamic Fractures Evolution Around Cavities In Rocks
Wang, S.Y. (Department of Civil Engineering, Catholic University of America) | Zhu, W.C. (Northeastern University) | Su, L.J. (Xi'an University of Architecture and Technology) | Liu, X.L. (Department of hydraulic and hydropower engineering, Tsinghua University) | Tang, C.A. (Dalian University of TechnologyDalian)
ABSTRACT A numerical code RFPA2D (Rock Failure Process Analysis), was used to simulate the initiation and propagation of fractures around pre-existing cavities in brittle rock. The dynamic loadings were applied to the rock specimens to investigate the mechanism of fractures evolution around single cavity. In addition, the evolution and interact of fractures between multiple cavities was investigated. The numerical simulated results reproduced tensile and remote fractures due to dynamic loading. Moreover, numerical results suggested that both the compressive wave and tensile wave could influence the propagation of tensile cracks. Especially the reflected tensile wave accelerated the propagation of tensile cracks. INTRODUCTION The stability of cavities due to static loading had long been the subject of intensive studies in mining and civil engineering. Extensive research was done on fractures evolution around a single pre-existing cavity (Hoek 1962; Lajtal 1975; Ewy and Cook 1990; Carter et al. 1991). However, few works focused on the fractures evolution surrounding cavities under the condition of dynamic loading. Although many numerical methods, such as finite-element, boundary-element, finite-difference and discrete-element methods did well in simulating non-linear behavior in rock deformation, most of them were not physical modeling of the non-linear behavior of brittle rock, and did not demonstrate the progressive failure due to heterogeneity of rock, which resulted in non-linear behavior. Therefore, a more reasonable numerical code RFPA2D (Rock Failure Process Analysis) was introduced (Tang et al. 1993; Tang and Kou 1998;). In addition, this code had been developed to study the dynamic failure process of rock, considering the effect of strain rate on the strength of rock (Chau et al. 2004; Zhu and Tang 2006). This paper aimed to simulate the evolution of dynamic fracture initiation, propagation around pre-existed cavities in brittle rock. The evolution and interact of fractures between multiple cavities was investigated. 2 BRIEF DESCRIPTION OF RFPA2D RFPA2D code could be used to model the observed evolution of damage or crack initiation, propagation and coalescence in brittle materials by allowing the linear elastic elements to fail in a brittle manner. The method was used for modeling progressive failure and associated seismicities in brittle rock. Instead of using a fracture mechanics approach where fracture propagation was controlled by the fracture toughness and was related to a stress intensity factor at the advancing crack tip, a failure approach was adopted in the code, RFPA2D, where microfracturing occurred when the stress level in an element satisfied a certain strength criterion (Tang 1993). In the present in investigation, both tensile and shear failures were considered in the analysis. An element was considered to have failed in the tension mode when its minor principal stress exceeded the tensile strength of the element (Eq.1), and to have failed in the shear mode when the shear stress satisfied the MohrโCoulomb failure criterion (Eq.2):(Equation in full paper) 3 MODEL DESCRITPION There were four types of models provided in this study. 1) Single circular cavity with vertical dynamic compression; 2) Single circular cavity with both vertical and horizontal dynamic compression.
ABSTRACT An excavation slope in left abutment trough of Xiluodu arch dam has 380 meters or so. In order to ensure safety of the excavation slope, the designed excavation and reinforcement process of the slope is simulated systematically with self-developed 3D elasto-viscoplastic finite element method (FEM) analysis program based on the model of reinforced jointed rock masses. Distribution patterns of displacements, stresses and point safety factors of the rock slope and reinforcement effects under tectonic initial geostress field are analyzed and the slope stability is evaluated in each excavation step. The simulation results show that displacements of the excavated slope between 470 meter and 400 meter in elevation are relatively bigger and its yield zone extends deeper into the mountain body in the designed excavation and reinforcement scheme. Supplementary reinforcements with some pre-stress cables are suggested for strengthening the excavation slope from 470 meter to 400 meter in elevation. The numerical simulation results show that the new reinforcements help improve the stability of the excavation slope in left abutment trough and ensure the safety of the slope. 1 INTRODUCTION The Xiluodu Hydropower Project is an extreme project in China and its installed capacity is 12.6 MW, which locates on the upper reaches of the Yangtze River. The dam type is double-curvature arch dam and its maximum dam height is 278 meters. Its plan and section X-X in left bank are presented in Figure 1. Figure 1(b) shows lithological characteristics of rock masses and distributions of dominant texture planes in left bank. The rock masses mainly have four rock types according to weathering degrees: II, III1, III2, IV1. The dominant texture planes mainly include three strain-slipzoneinlayers and two joint sets. The strain-slipzoneinlayers are C7,C8,C9. Occurrences of such texture planes are presented in Table 1. After completion of excavations, the final excavation slope in left abutment trough of Xiluodu arch dam has more than 380 meters height, and its spatial shape is very complicated. Because the physical and mechanical parameters of rock masses are usually weakened for exploding construction, failures of the excavation slope are likely to occur. In order to ensure the stability and safety of the excavation slope, many supports should be utilized to reinforce the slope in a construction period, which are pre-stress cables, pre-stress bolts, systematic bolts, etc.. Which position should be reinforced, and how many bolts and cables should be adopted are focuses of engineering design and construction. Unsuitable reinforcement measures and reinforced locations will not prevent slope from instability effectively and only increase engineering investments. Consequently it has a great significance to study excavation slope stability and reinforced effects of corresponding reinforcement measures with computer aided simulation technologies before the slope excavation (FENG Xue-min, WANG Wei-ming, et al., 2004) Finite element method has gained popularity in analyzing geotechnical problems for fewer assumptions and more powerful functions (Chen, S.H., Egger, P.,1999; Chen Sheng-hong, Qing Wei-xin, Shahrour Isam, 2007). The excavation and reinforcement process of rock slope can be simulated conveniently by finite element method with suitable constitutive models of geomaterials.
- Geology > Rock Type (0.54)
- Geology > Geological Subdiscipline > Geomechanics (0.47)
ABSTRACT One of the difficulties in describing the rock mass behavior is assigning the appropriate constitutive model. This limitation may be overcome with the progress in discrete element software such as PFC, which does not need the user to prescribe a constitutive model for rock mass. In this paper, the model size of 30m ร 30m was analyzed by using the fracture geometry from two tunnel sites. PFC simulations were carried out to examine the mechanical behavior of rock masses. From the numerical tests, it can be concluded that as the number of joint sets increased, the values of mechanical properties of rock masses were decreased to about 50% of those values of rock mass without joints. And the behavior of the rock mass changed from brittle to perfectly plastic with increase in the number of joints. Also the values of Young's modulus, Poisson's ratio and peak strength are almost similar from PFC model and empirical methods. As expected, the presence of joints had a pronounced effect on mechanical properties of the rock mass. More importantly, the mechanical response of the PFC model was not determined by a user specified constitutive model. So the discrete element model gives very contrasting results compared to the traditional model. 1 INTRODUCTION Although the evaluation of the mechanical properties and behavior of discontinuous rock masses is very important for the design of underground openings, it has always been considered the most difficult problem. The reason is that it is often impossible to carry out large-scale in situ tests and, although widely used, the correlations between strength parameters and quality indexes (for instance GSI or Q index) are still affected by considerable uncertainties (Ribacchi 2000). The evaluation of rock mechanical properties such as deformation and strength properties can be achieved through the application of empirical relationships or by a theoretical approach based on numerical modeling. Both methodologies imply some assumptions and uncertainties that need to be considered. Deformation properties and rock mass strength are not only dependent on the intact rock, but also on the fracture network (number and orientation of fracture sets, intensity, mineralization, and so on) and the presence of deformation zones. Therefore, characterization of both the intact rock and of the fractures is required to define the mechanical behavior of the rock mass. Discontinuous rock masses are usually weaker and more deformable and are highly anisotropic when compared with intact rocks. So constitutive modeling of discontinuous rock masses has long been a subject of interest and numerous models have been developed in attempt to simulate their mechanical responses (Staub et al. 2002). Recent developments in numerical modeling that allow study of the overall response of a synthetic material containing discrete heterogeneities and discontinuities both at the micro (particle) scale and at the larger scale of jointed rock masses can greatly aid the interpretation and application of laboratory test results on these materials (Potyondy & Fairhurst 1999). The methodology for the rock mechanical descriptive model was developed in Sweden.
ABSTRACT The objective of this paper is to outline the methodology proposed to determine the in-situ stress field and geomechanical properties of the Bakken Formation in Williston Basin, North Dakota, USA to increase the success rate of horizontal drilling and hydraulic fracturing so as to improve the recovery factor of this unconventional crude oil resource from the current 1% to a higher level. The success of horizontal drilling and hydraulic fracturing depends on knowing local in-situ stress and geomechanical properties of the rocks. We propose a proactive approach to determine the in-situ stress and related geomechanical properties of the Bakken Formation in representative areas through integrated analysis of field and well data, core sample and lab experiments. We plan to use Kaiser Effect technique to estimate in-situ stresses. CDISK method will be used to determined fracture toughness. Geomechanical properties will be measured following ISRM suggested methods. By integrating lab testing, core observation, numerical simulation, well log and seismic image, Intelligent Geomechanical Logging and Imaging methods are proposed to estimate geomechanical properties in locations where no results of lab testing and core observation are available. 1 INTRODUCTION The Williston Basin is a roughly oval-shaped structural down-warp with a surface area between 120,000 and 240,000 square miles. The basin underlies most of North Dakota, western Montana, northwestern South Dakota, southeastern Saskatchewan and a small section of southwestern Manitoba. All sedimentary systems from Cambrian through Quaternary are presented in the basin, with a rock column more than 15,000 ft thick in the deepest section near Williston, North Dakota (Fig. 1). The basin became a major oil province in the 1950s when large oil fields were discovered in North Dakota. (Figure in full paper) The Bakken Formation is a thin (maximum thickness 145 ft), naturally fractured Upper Devonian- Lower Mississippian sedimentary unit. It can be divided into three intervals: the upper shale, the lithologically variable middle member, and the lower shale. The upper and lower shales have rich organic content, and are the source rocks for oil and gas in the Bakken Formation. In North Dakota, the middle member is mainly gray interbedded siltstones and sandstones with a maximum thickness of 85 ft occurring at depths of approximately 9,500 to 10,000 ft (Heck et al., 2002). Although the Bakken Formation is very thin compared to other oil producing horizons, it has recently attracted much attention because its extremely high carbon content places it among the richest hydrocarbon source rocks in the world. Estimates of original oil in place (OOIP) range from 200 to more than 400 billion barrels (Price, 2000). For comparison, excluding these Bakken Formation reserves, so far the total US discovered OOIP is less than 600 billion barrels, of which only less than 200 billion barrels has been produced. With the growth rate of demand outpacing that of new reserves on oil and gas, the importance of these unconventional reserves in the Bakken Formation becomes increasingly important.
- North America > United States > South Dakota (1.00)
- North America > United States > North Dakota (1.00)
- North America > United States > Montana (1.00)
- (2 more...)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.76)
- Geophysics > Seismic Surveying (1.00)
- Geophysics > Borehole Geophysics (1.00)
- North America > United States > South Dakota > Williston Basin > Bakken Shale Formation (0.99)
- North America > United States > North Dakota > Williston Basin > Bakken Shale Formation (0.99)
- North America > United States > Montana > Williston Basin > Bakken Shale Formation (0.99)
Estimation of Deformation Modulus In a Weathered Granite Using the Decrease In Transmissivity With Depth
Jiang, X.W. (China University of Geosciences) | Wang, X.S. (China University of Geosciences) | Wan, L. (China University of Geosciences) | Wu, X. (China University of Geosciences) | Kang, A.B. (China University of Geosciences)
ABSTRACT Both hydraulic and mechanical properties of fractured rock masses are related to the geometry of fractures. From the perspective of hydro-mechanical coupling, the nonlinear decrease in transmissivity with depth can be utilized to calculate fracture normal stiffness of large scale rock masses. In the current study, the degree of weathering and the non-linear decrease in transmissivity with depth are considered simultaneously to estimate fracture normal stiffness of a granite rock mass. The equations of transmissivity-depth correlation in each zone with different degree of weathering are employed to calculate the corresponding fracture normal stiffness; then the equivalent continuum model is utilized to calculate deformation modulus of the rock masses. In the highly weathered zone, the deformation modulus ranges between 2 and 4 GPa; in the moderately weathered zone, the deformation modulus ranges from 15 to 19 GPa; in the slightly weathered zone, the deformation modulus ranges between 24 to 26 GPa. Compared with the values of deformation modulus obtained from measurements or engineering analogy method, which were determined by others, the results in the present study are reasonable. 1 INTRODUCTION The deformation modulus is the most representative parameter describing the pre-failure mechanical behavior of a rock mass. Numerous researchers had reported that deformation modulus, or compressibility, which is defined as the reciprocal of deformation modulus, of rocks is stress dependent (Adams and Williamson, 1923; Fatt, 1958; Zimmerman, 1991). Unfortunately, in situ measurements of the deformation modulus involve difficult test procedures, and are expensive and time-consuming. Moreover, even such in situ tests are still not able to obtain parameters that can represent large scale rock masses. Both hydraulic and mechanical properties of rock masses are related to the geometry of fractures (Chen, 1990), which suggests a new way to estimate mechanical properties from hydraulic information. Rutqvist (1995) utilized hydraulic jacking test to determine normal stiffness of fractures in hard rocks. Jiang et al. (2008) estimated the stress-dependent fracture normal stiffness of large scale rock masses using the permeability data from packer test in a wide range of depths. In Jiang et al. (2008), the most important and sensitive parameter for calculation of normal stiffness is the permeability-depth correlation. It was assumed that the depth dependency of permeability was caused by the nonlinear normal stress-aperture relationship of fractures (Goodman, 1976), which had been observed by Snow (1968) using field permeability measurements. However, the depth-dependent permeability is also influenced by weathering of the rock mass (Rutqvist and Stephansson, 2003). Moreover, it is well known that the mechanical properties, including deformability, differ greatly in rock masses with different intensity of weathering. Therefore, the degree of weathering should be considered while estimating the deformation modulus of large scale rock masses. In this paper, the theory for estimation of deformation modulus using permeability data is presented. This method is then applied to a study area which is composed of weathered granite. 2 THEORY 2.1 The relationship among fracture normal stiffness, transmissivity and stress.
- Asia > China (0.29)
- North America > United States (0.28)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Igneous Rock > Granite (0.82)
Numerical Simulations of Crack Growth From Surface Flaw Under Uniaxial Compression
Wong, R.H.C. (Department of Civil and Structural engineering, The Hong Kong Polytechnic University) | Tang, S.B. (Department of Civil and Structural engineering, The Hong Kong Polytechnic University) | Chau, K.T. (Department of Civil and Structural engineering, The Hong Kong Polytechnic University) | Tang, C.A. (Dalian University of Technology,)
ABSTRACT The purpose of this study is to quantify the relationships between the grain size characteristics and physicomechanical properties of selected marble rocks from Ma On Shan, Hong Kong. The rock specimens can be subdivided into white, light grey and dark grey, depending on the amount of impurities of the original limestones before metamorphism takes place. A series of laboratory tests, including the point load test, Schmidh hardness test, and uniaxial compression test, have been carried out on the same set of core samples in accordance with the procedures given by ISRM. Also microscopic observations based on the double replica method have been made to quantify the microphysical characteristics, in particular the grain size. The results are statistically analyzed and some selected physical and mechanical properties of the marble rocks are plotted against each other in order to explore possible relationships. The study in particular examines the influence of gain size characteristics on the engineering properties of the chosen marble rocks. 1 INTRODUCTION The properties of rocks are influenced by the mineral composition, texture (grain size and shape), fabric (arrangement of minerals and voids) and the weathering state (Irfan, 1996). A number of petrographic techniques have been developed to document and quantify the mineralogical and textural characteristics of various rocks using an optical microscope (e.g., Mendes et al., 1966; Onodera and Asoka Kumara, 1980), scanning electron microscope (SEM) (e.g., Clelland & Fens 1991), or other tools. Among the typical petrographic characteristics that affect the mechanical properties of rocks, grain size has long been recognized to be closely related that in general the strength of rocks is greater for finer grained rock. Hugman and Friedman (1979) suggested that the peak uniaxial compressive strength decreases linearly with the mean grain size in carbonate rocks. However, it has been more frequently found that the peak strength decrease inversely with the square root of the grain size by Olsson (1974) for marble, Brace (1961) for quartzite, Brace (1964) for dolomite and limestone, and Fredrich et al. (1990) for calcite marbleand limestone. For Yuen Long marbles in Hong Kong, Wong et al. (1996) also found experimentally similar conclusion for fine-grain and coarse-grain Yuen Long marbles. The purpose of this study is to apply correlation analysis to investigation the relationships between grain size and engineering properties of local marbles. A variety of marble rock samples from Ma On Shan area in Hong Kong were subjected to microscopic investigation. The rocks can be categorized into white, light grey and dark grey type, depending on the amount of impurities in the original limestone before metamorphism takes place. The white marble is typically coarse-grain, and the light grey and dark grey marbles have relatively smaller grain size. The same samples were then tested to determine the specific gravity, Schmidt hardness, point load strength index, uniaxial compressive strength, modulus of elasticity and the Poisson's ratio. The relationships between these properties and average grain size are described by simple regression analysis.
- Research Report > New Finding (0.67)
- Research Report > Experimental Study (0.48)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock > Limestone (0.84)