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Results
Deterioration Of Granite By Cyclic Uniaxial Loading
Chen, Youqing (Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi) | Watanabe, Kota (Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi) | Yamazaki, Arata (Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi) | Kusuda, Hiromu (Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi) | Kusaka, Eishi (Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi) | Mabuchi, Mamoru (Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi)
Abstract: To examine the fatigue process of granite, cylindrical Westerly granite specimens were subjected to a cyclic loading test under uniaxial compression with a maximum of 140–160MPa at room temperature, and crack growth patterns within them were analyzed by microscopic observation and image analysis. The fatigue process is generally divided into three characteristic stages. A series of prefailure specimens were prepared, and their stages in the fatigue process were decided by monitoring the strain behavior during the test. Cross-sectional thin sections were prepared for detailed observation. The fluorescent method was applied to identify microcracks within the specimens. At the initial degradation stage, distinguishing crack growth was identified in quartz grains. It is estimated that the candidate portions of crack growth were damaged at the first or early loading. At the second steady stage, crack development portions were shifted from quartz grains to feldspar grains. It is estimated that a gradual progress of microcracks within feldspar grains was dominant during the second stage. At the final accelerated stage, many intergranular cracks running parallel to the loading direction were observed. Their formation takes the fatigue process from the steady stage to the final stage with a sharp increase in strain. INTRODUCTION The structures constructed in underground spaces are generally used over a prolonged period. It is therefore important to reveal the deterioration characteristics of rocks under repeated stress changes over the long term for the stability evaluation of many rock structures. In recent years, several new proposals for the usage of underground spaces have drawn increasing interest. For example, the base rock surrounding the compressed air energy storage system has to bear cyclic stress changes in the first planning stage. It is well-known that many materials deteriorate due to repeated stress changes over a prolonged period and then finally reach failure even if the change is below their static breaking strength. This phenomenon is generally known as ‘fatigue’. Fatigue occurs in many kinds of materials, and rock is no exception. Many studies on the fatigue characteristics of rocks have been carried out. Attempts to estimate the fatigue life were also conducted (e.g., [3]). In many geological engineering problems, granite is one of the most important materials to investigate. The fatigue characteristics of granite have also been studied. However, the detail process in granite fatigue, i.e., the phenomena occurring in phases from the onset of degradation until the final collapse, has not been clarified. It is necessary to examine the growth of cracks during the fatigue process, since the initiation and elongation of microcracks play an essential role in the granite failure. Therefore, to examine the fatigue process in granite, cylindrical Westerly granite specimens were subjected to the cyclic loading test under uniaxial conditions at room temperature with a maximum applied stress of 140–160 MPa, and the patterns of microcracks at three characteristic stages during the test were observed microscopically, applying the fluorescent approach [4]. In addition, microcrack growth patterns were investigated by the digital image analysis technique.
- Geology > Rock Type > Igneous Rock > Granite (1.00)
- Geology > Mineral > Silicate > Tectosilicate > Feldspar (0.51)
- Geology > Mineral > Silicate > Tectosilicate > Quartz (0.49)
Abstract Slates show typical anisotropic properties and it would change to clay after long term interaction with water, so some experiments were performed to study mechanical properties in different conditions. Microstructures of the minerals forming the rocks and the strength of the rocks in different absorption were measured. Single discontinuity theory was used to analyze the change of rock mass strength with the discontinuity inclinations, and the relationship between them was set up to explain the compression results. Then after analyzed the change of microstructures, water-weakening mechanism was studied. Some conclusions were drawn from the analysis: The deformation forms transferred with the different discontinuity inclinations, and it can be slided from discontinuities, sheared and the complex of the two forms and the deformation form changed from slide to shear when the confining pressure increased. failure strength of slates changed in a paradola line with the discontinuity inclination variation and the least angle is 51.7°; triaxial compression results showed that the slates weakened and the peak strength of the slates declined with the absorption increased in minus logarithm law; the elastic modulus decreased either; the grains of the slates bulged and the structure relaxed, which made the porosity increased without confinement and the volume bulge lagged behind the water absorption; As a result, slate rocks are easier to failure following the layer surfaces. Capillary effects and Rhehinder effects played a primary role in slate-water interactions through the results of microstructure and contact angle. Introduction Slates were formed from shale by light metamorphosing and textures, joints, layering and cracks prevailed inside which made the slates show typical anisotropic properties. For the purpose of evaluating the stability of open ditches and underground tunnels constructed in slates in some areas, it's necessary to systematically analyze the characters of slates. Furthermore, it was found that slates may change to clay after long term submerging in underground water in special conditions. Therefore, it was also needed to study the water-weakening properties of slates in different water contents. Recently, many researches were focused mainly on the anisotropic properties of slates in different ways. M. Brooks Clark et al. explained the anisotropic mechanical properties from the micro arrangement of mineral grains. Catalina M. Ltineburg et al. studied the range and distribution characters of pyrite in slate. The orientation of minerals made the magnetic field showed anisotropic and the strains under press had the same features. X ray diffraction analysis, SEM and TEM technology were used by Nei-Che Ho to analyze the array of mica and chlorite in slates from strata of Michigan. T. Heggheim et al. analyzed the change of mechanical characters and microstructures of chalk saturated with different seawater, glycol and brines and he brought a theoretical model to explain the mechanism. S.W.J. Den Brok et al. performed the experiments on natural quartzite at high temperature, different confining pressures and strain rates in the presence of added water to study the microcracks and new minerals.
- North America > United States > Michigan (0.24)
- Asia > India > NCT (0.19)
Microfocus X-ray CT imaging was conducted on a granite core, 50 mm in diameter and 100 mm in length, containing a single fracture in the longitudinal direction. The three-dimensional geometry and the aperture distribution of the fracture were evaluated by analyzing the CT data. A very notable artifact called beam hardening was found in the obtained CT images because of the high X-ray absorption coefficient of the material compared with the source intensity of the X-rays. Therefore, the CT image was corrected using a simulation image of the beam hardening of granite. As a result, the spatially averaged aperture thickness and the contact ratio of the fracture asperities under unconfined conditions were estimated to be 0.39 mm and 2.0%, respectively. The result was compared with the aperture distribution that was measured using a laser beam profiler system. 1. INTRODUCTION Precise measurement of the geometric characteristics of rock fractures such as the elevation distribution of fracture walls, aperture distribution, and contacting asperities within the fracture, is essential because of their significant influence on the mechanical and hydrological behaviors of rock fracture. On a laboratory scale, the elevation distribution of fracture walls can be measured with micrometer accuracy using a system that combines a laser displacement sensor and a high-precision automatic positioning stage (e.g., [1]). However, aperture distribution and contacting area within the fracture are rather difficult to measure experimentally. In previous researches, various techniques have been proposed such as 1) the surface topography approach, in which the topography of a pair of fracture surfaces is measured separately by a laser beam profiler, and the aperture is computed indirectly as the distance between the two fracture surfaces [2,3]; 2) the injection approach, in which the specimen containing the fracture is cut into slices after some resin has been injected, and the aperture is measured as the thickness of the injected resin [4,5]; and 3) the casting approach, in which replicas of the fracture apertures are made by casting [6]. X-ray computed tomography (CT) is a useful technique for visualizing the inner structure of rock samples in a noninvasive and nondestructive manner. It has also been used to measure fracture aperture and to detect contact areas (e.g., [11,13–15]). This study used a microfocus X-ray CT scanner to take CT images of a granite core containing a single fracture in the longitudinal direction. Based on the result of the CT imaging, we evaluated the three-dimensional geometry and the aperture thickness distribution of the fracture. The result was compared with the aperture distribution that was measured using a laser beam profiler system. 2. SPECIMEN The specimen (Fig.1) is a granite core of 50 mm in diameter and 100 mm in length containing a single fracture in a longitudinal direction, which was sampled in Mizunami, Gifu Prefecture, Japan at the depth of 180 to 200 m. The bulk density was 2.58 g/cm3 and the porosity was 1.00%. Based on X-ray diffraction results, the major mineral components were quartz, feldspar and biotite.
- Geology > Rock Type > Igneous Rock > Granite (1.00)
- Geology > Geological Subdiscipline > Geomechanics (0.96)
- Geology > Mineral > Silicate (0.88)
Abstract: The purpose of this study is to clarify the relationship between axial point load strength and uniaxial compressive strength in hydrothermally altered rocks, which are typical of the soft and semi-hard rocks found in northeastern Hokkaido, Japan. 1,747 rock specimens were collected for the axial point load strength test along with 326 rock specimens for the uniaxial compressive strength test. These came primarily from the earth's surface in ancient hydrothermal fields. Rock specimens in the form of cores underwent axial point load strength and uniaxial compressive strength tests using a laboratory testing machine with specimens in forced-dry and forced-wet states. An axial point load strength has a strong correlation with a uniaxial compressive strength. The estimated relationship between axial point load strength (Is) and uniaxial compressive strength (qu) is qu = 12.9 Is in soft rocks with axial point load strengths below 1.5 MPa. This combines the relationship between axial point load strength and uniaxial compressive strength in the forced-dry and forced-wet states and might be applied to onsite tests of rock with natural moisture content. Using this relationship, we can calculate the uniaxial compressive strength from only an axial point load strength test for rocks with axial point load strengths below 1.5 MPa. 1. INTRODUCTION The strength of fresh rocks and altered rocks, including hydrothermally altered or weathered rock, is generally evaluated based on uniaxial compressive strength (UCS). However, rock core pieces for the UCS test cannot be always obtained from outcrops of faulted, jointed or cracked rock masses. In these cases, the point load strength (PLS) test is a very convenient and effective alternative to the UCS test because it can be done promptly using on-site testing equipment for small rock specimens having various shapes taken from outcrops or floats. Provided that we can calculate a UCS estimate from an axial PLS value, the PLS test can lead to cost reduction and convenience. Many researchers have already studied the relationship between the PLS and UCS of hard rocks. For example, frequently cited correlations between PLS (Is) and UCS (qu) are qu % 20–25 Is [1] and qu % 24 Is [2]. The purpose of this study is to clarify the relationship between axial PLS and UCS in hydrothermally altered rocks, which are typical of the soft and semi-hard rocks found in northeastern Hokkaido, Japan (Fig. 1). 2. ROCK SAMPLES The geology of the sampling sites consists primarily of the Upper Miocene Oteshikaushinai, Hanakushibe, and Shikerepe Formations, and the Pliocene Shikerepeyama Lava in the Okushunbetsu area of Teshikaga Town; the Upper Miocene Ikutawara Formation in the Ikutahara area of Engaru Town; and the Upper Miocene Komatsuzawa Formation in the Asahi-Nishi area of Rubeshibe Town, Kitami City, northeastern Hokkaido, Japan (Fig. 1). Rock samples, which were collected primarily from the earth's surface in ancient hydrothermal fields, are hydrothermally altered volcaniclastic rocks, including fine tuff, medium tuff, pumice tuff, lapilli tuff, and welded tuff, dacite, tuffaceous mudstone, tuffaceous sandstone, and tuffaceous conglomerate.
- Asia > Japan > Hokkaidō (1.00)
- North America > United States > Kansas > Butler County (0.24)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.55)
- Geology > Mineral > Silicate > Phyllosilicate (0.51)
Effects Of Loading History On Mechanical Properties Of An Artificial Rock In Multiplestep Loading Test
Taheri, A. (Civil Engineering Department, Tokyo University of Science) | Sasaki, Y. (Civil Engineering Department, Tokyo University of Science) | Tatsuoka, F. (Civil Engineering Department, Tokyo University of Science)
ABSTRACT: The effects of loading histories on the stress-strain behaviour of an artificially soft rock, compacted cement-mixed well-graded gravelly soil, in multiple-step loading (ML) triaxial compression (TC) tests were evaluated. Associated with an increase in the strain until the peak stress state with an increase in the confining pressure (σh) in single-step loading (SL) TC tests, all the loading steps in ML tests increasing σh, loading was terminated consistently during strain-hardening approaching the peak stress state with volume contraction, On the other hand, associated with a decrease in the strain until the peak stress state with a decrease in σh in SL TC tests, in ML tests decreasing σh, at the first and second loading steps, loading had to be terminated far before the peak stress state during strain-hardening. At the third and subsequent steps, in particular at the last step, the stress-strain relation quickly entered into the strain-softening regime exhibiting large dilation while significant effects of damage by preceding loading steps were observed. A comparison of the pre-peak stress-strain relation at respective steps of the ML tests with those during SL TC tests revealed that the stress-strain behaviour in the ML tests is affected by; 1) stiffening by cyclic loading due to elastoviscoplastic properties; 2) effects of σh on the stress-strain behaviour; and 3) damage effects that have taken place during preceding loading history, in particular those by previous shearing into the strain softening regime and unloading causing large negative shear strains. The shear strengths at different σhs could be determined based on results from relevant ML tests using a single specimen. INTRODUCTION To evaluate the Mohr-Coulomb failure criterion (the M-C failure criterion) by using a single undisturbed sample of soil or rock, multiple-step loading triaxial compression test (ML TC test) consisting of a series of consolidation and shearing steps has been employed [1–2]. A series of ML TC tests on undisturbed samples from a given site can provide a spatial variability of soil or rock strength represented by the M-C failure criterion. Despite the above, details of loading histories comprising a sequence of consolidation and TC loading/unloading at different confining pressures are usually poorly stated by different researchers. For example, Kim and Ko (1979) [5] reported only the relevance of a ML test method on the stress-strain property of a certain rock type. Akai et al. (1981) [6] found that, in a number of ML TC tests on siltstone and tuff, it often became difficult to continue TC loading, because, soon after the specimen state became near failure, the specimen exhibited abrupt post-peak strain-softening before increasing the confining pressure. They controlled the lateral strain during TC loading to avoid abrupt failure. However, effects of the details of the test procedure were not studied. In the studies by Cain et al. (1986) [7] and Crawford and Wylie (1987) [8], volumetric strain was used to detect imminent failure at respective TC loading steps in ML test. However, the effects of precedent loading histories were not reported.
- Research Report > New Finding (0.68)
- Research Report > Experimental Study (0.54)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.34)
Synopsis: The appropriate assessment of the strength of a jointed rock mass is a fundamental requirement for the successful design of structures built in or on rock. Due to the complexity of the rock mass, a large number of empirical methods are developed, for its estimation. Its analytical treatment has been tackled mainly by the plane of weakness theory. In this study, an extended plane of weakness theory is applied to a well documented jointed rock physical model. For the failure mechanisms observed in the experiment, the non linear strength envelope provided by this extended plane of weakness theory fits well to the experimental data. 1 INTRODUCTION Rock mass strength estimation belongs to the problems that are too complex to be tackled easily with analytical methods. Complexity is mainly due to fracturing, anisotropy inhomogeneity, and the variety of pertinent modes of failure. Therefore, empirical correlations, that do not need theoretical treatment, are usually employed. They come from the systematic observation of the factors that affect the strength. Analytical methods for the estimation of jointed rock strength have been developed, for the case of sparsely jointed rock masses. The most widely known analytical method for such a purpose is the plane of weakness theory, presented by Jaeger [1]. This method has been extended in order to take into account the roughness of joint surfaces. This extended method is presented and then, both the original and extended theories of weakness plane are applied to a jointed rock physical model. 2 THE EXTENDED THEORY OF WEAKNESS PLANE The original theory of weakness plane (Jaeger, [1]) allows for the analytical evaluation of the anisotropic or equivalent isotropic strength of jointed rock. This theory uses the Mohr - Coulomb failure criterion for the joints. However, this linear criterion is unable to describe adequately the shearing behaviour of rock joints, which are rarely smooth, and their strength is a non linear function of the existing normal stress. Experiments ondiscontinuities have shown that for low normal stresses, the surface roughness causes expansion with shear movement, while for higher normal stresses there is failure of asperities and suppression of any expansion. 3 EXPERIMENTAL EVIDENCE ON DISCONTINUOUSSPECIMENS Various researchers, such as for example, Goldstein et al. [5], Hayashi [6], Lama [7], Brown [8], Einstein and Hirschfield [9], experimented on artificial specimens made by blocks, usually of plaster, arranged in such a way to form a jointed structure. By this procedure, the jointed structure of the specimen is considered to represent the rock mass. For low values of confining stress, shear failures of joints or in intact material, were observed. For higher lateral stresses, the failure was due to the formation of many almost parallel shear planes, mainly within the intact material. The strength of the specimens depended on joint dip, except from the case of high lateral pressures, where the strength of the specimen was nearly equal to that of the intact material. The behaviour of specimens was brittle for low lateral pressures and ductile for higher.
Influence of stress concentration at rock sampling on the in-situ rock stress value estimated by tangent modulus method, which is one of the oriented core methods for in-situ rock stress measurement, was experimentally investigated. Cylindrical specimens of Kimachi sandstone were preloaded at 30% UCS for 24 hour simulating in-situ stress and then a triangular shape-loading up to 40% UCS simulating stress concentration which took approx. 1 min. were carried out. Loading up to 50% UCS was applied to the specimen twice after certain delay time. Bending points were observed at 40% UCS for 0 to 1 hour delay time, at 30% UCS and 40% UCS for the 3 hour delay time, and at 30% UCS for 1 and 3 day delay time. The bending point was not observed for 1 week delay time. It is considered that the memory of concentrated stress acted at rock sampling for a short time can be lost and in-situ rock stress which acted for geological long-term can be accurately estimated if the test is carried out after an appropriate delay time. INTRODUCTION Tangent modulus method (TMM) is one of the oriented core methods for in-situ rock stress measurement. The following procedure will be used to determine in-situ rock stress.Oriented rock cores are sampled from the site. Cylindrical rock specimens are made. The specimens are uniaxially or triaxially compressed twice to a certain stress level. The stress value of the bending point in the stress-tangent modulus curve in the first loading cycle or the point where the first and the second stress-tangent modulus curves begin to separate (this point is also called bending point later for convenience) is regarded as the normal component of the in-situ rock stress in the direction of the specimen. In order to check the validity of the tangent modulus method, rock specimens were compressed to a certain stress level and the stress was kept for certain time (preloading). The specimens were cyclically compressed to a higher stress level twice after certain delay times. Comparison between the stress value at the bending point and the preloading stress value was made. It was confirmed that bending point appeared at the preloading stress level for dry Shirahama sandstone (Fig. 1), Shikotsu weleded tuff, Inada granite and Kimachi sandstone. Bending points became vague with delay time. However, bending points were observed at the preloading stress level even the delay time was 6 weeks for 17 hour preloading (Fig. 2). The mechanism of tangent modulus method can be explained by nearly irrecoverable closures of such voids as microcracks and pores in rock. Let's assume that A in Fig. 3a is the in-situ condition. Some voids which are tabular enough and large enough are partly closed at the in-situ stress level. A few voids would slightly open with stress relief by rock sampling (B). Rock is stiff during the first cyclic loading up to in-situ stress level (B to C) because nearly no more void closure.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.69)
AE Monitoring And X-Ray Ct Observation For Failure Of Berea Sandstone With Pore Pressure Increase
Niwa, T. (Kyoto University) | Ishida, T. (Kyoto University) | Fukahori, D. (Kyoto University) | Ishida, M. (Kyoto University) | Sato, R. (Kyoto University) | Murata, S. (Kyoto University) | Onozuka, S. (Japan Oil, Gas and Metals National Corporation) | Oseto, K. (Japan Oil, Gas and Metals National Corporation) | Yamamoto, K. (Japan Oil, Gas and Metals National Corporation)
We monitored AE events accompanied with a failure of Berea sandstone specimen induced by pore pressure increase in a tri-axial test. After the test, several located AE events were compared with X-ray CT images obtained on the fractured specimen. The comparison elucidated that most of the sources are closed to or exactly on the locations of macroscopic fracture planes. From the stress and strain data, we also found that the failure can be explained and predicted by Mohr-Coulomb's criterion and the effective stress theory. 1. INTRODUCTION AE (Acoustic Emission) monitoring has been recently often applied to detect behavior of petroleum reservoirs. For example, at oil fields in Ekofisk and Valhall in North Sea and in Clinton County, Kentucky in U.S., AE events have been monitored with operation of EOR (enhanced oil recovery) [1]. EOR is the technology to recover oil and gas from depleted reservoirs by injecting fluid such as water, natural gas, and carbon dioxide. Since AE events are most likely caused with increase of pore pressure by the injected fluid, the AE monitoring helps to understand how the injected fluid penetrates and permeates. In this research, we monitored AE events accompanied with a failure of Berea sandstone specimen induced by pore pressure increase in a tri-axial experiment. Moreover, we compared location of AE sources to fracture planes on X-ray CT (computed tomography) images obtained after the experiment. 2. EXPERIMENTAL METHOD In this experiment, we used a cylindrical specimen of Berea sandstone, measuring 38mm in diameter and 76mm in height, as shown in Figure 1. P-wave velocities of the specimen measured along X-, Y-, and Z-directions were 2.46, 2.48, 2.46 km/s, respectively. This indicates that the specimen is almost isotropic. Since a permeability test conducted on the specimen indicated a large permeability, 92.1md, the specimen was expected to be completely saturated in a few seconds after applying pore pressure in the tri-axial test. To monitor AE events, twelve cylindrical piezoelectric elements, 5 mm in diameter and 6.7 mm long, having resonance frequency of 300 kHz, were placed on the surface of the specimen. After above preparations, the specimen covered by silicon rubber with pedestals was set in a pressure cell, and tri-axial test was conducted. After the tri-axial test, X-ray CT images were obtained on the fractured specimen with a medical CT scanner (TOSHIBA Aquilion16). Through using the CT scanner, we can observe cracks inside of the specimen with leaving silicon rubber covering the specimen, only with removing piezoelectric elements and strain gauges that interrupt X-ray due to their high density. 3. RESULTS Fig.2 shows change of axial pressure (σ1), confining pressure (σ3), pore pressure (P), and strains (ε1, ε3 and εvol) along the elapsed time in this experiment. The procedure of this experiment is shown as follows. 3.1. Procedure of Loading At the first step from the beginning to the point (1) in Fig.2, the specimen was set in a pressure cell, and then the axial pressure was applied slightly.
- North America > United States > West Virginia (1.00)
- North America > United States > Pennsylvania (1.00)
- North America > United States > Ohio (1.00)
- North America > United States > Kentucky > Clinton County (0.24)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Tor Formation (0.97)
- Europe > Norway > North Sea > Central North Sea > Central Graben > PL 018 > Block 2/4 > Greater Ekofisk Field > Ekofisk Field > Ekofisk Formation (0.97)
- Europe > Norway > North Sea > Central North Sea > Central Graben > Block 2/8 > Valhall Field > Tor Formation (0.97)
- (3 more...)
Abstract: The behaviour of rock mass is largely influenced by the in-situ anisotropy and is different from other engineering materials. The assessment of the strength and deformation of rocks is essential for engineering design and analysis. The dynamic modulus of elasticity is very important when dealing with the problems like blasting. Field tests to determine these parameters directly are time consuming and expensive. Therefore, several authors have proposed empirical relations for estimating the rock mass deformation modulus on the basis of classification schemes. This paper presents the study on the strength and deformation characteristics of jointed rock by conducting laboratory tests on cylindrical specimens of plaster of Paris (POP) by introducing artificial joints, under static and dynamic conditions. Cylindrical specimens of POP mixed with Portland cement to simulate rock of higher strength were also tested. The specimens having one to four joints at inclinations varying from 0o to 90o were tested for both static and dynamic properties. The laboratory results were presented as stress-strain plots and were examined to understand the effect of joint frequency and joint inclination on the strength and deformation behaviour of jointed rock mass. In this paper the compressive strength/elastic modulus of the jointed rock mass was estimated as a function of intact rock strength/modulus and joint factor. The joint factor reflects the combined effect of joint frequency, joint inclination and joint strength. Therefore, having known the intact rock properties and the joint factor, jointed rock properties can be estimated. The test results indicated that the rock mass strength decreases with an increase in the joint frequency and a sharp transition was observed from brittle to ductile behaviour with an increase in the number of joints. It was also found that the rocks with planar anisotropy exhibit the highest strength in the direction perpendicular to the anisotropy and the lowest at an inclination of 30o-45o in jointed samples. The anisotropy of the specimen influences the dynamic elastic modulus more than the static elastic modulus. The results were also compared well with the published works of different authors for different type of rocks. INTRODUCTION The behaviour of rock mass is largely influenced by the in-situ anisotropy and is different from other engineering materials. The assessment of the strength and deformation of rocks is essential for engineering design and analysis. The deformation modulus of a rock mass is an important input parameter in analysis of rock mass behaviour. Field tests to determine this parameter directly are time consuming and expensive. Therefore, several authors (references) have proposed empirical relations for estimating the rock mass deformation modulus on the basis of classification schemes. There are two elastic moduli, namely, static and dynamic. According to Ciccitti and Mulargia (2004) the values of the static modulus, in general, are 5–10% lower than those of dynamic moduli. The dynamic modulus of elasticity is very important when dealing with the problems like blasting. Cylindrical specimens of POP mixed with Portland cement to simulate rock of higher strength (wall strength) were also tested.
- Asia > India (0.71)
- North America > Canada > Ontario (0.28)
Fracture Toughness Measurement Of Marble Specimens From Ma On Shan, Hong Kong By Chevron-Notched Bend Test: Some Discussions Based On Nonlinear Fracture Mechanics Approach
Zhou, Y.D. (Department of Civil Engineering) | Tang, X.W. (Department of Civil Engineering) | Hou, T. (Department of Civil Engineering) | Tham, L.G. (Pokfulam Road)
Abstract: A series of fracture tests have been carried out to determine the mode-I fracture toughness of marble rock samples from Ma On Shan area in Hong Kong. The rock specimens can be subdivided into white, light grey and dark grey specimens, and present notable difference in the mean grain size. The ISRM suggested chevronnotched three-point bend test method is chosen in this study. The fracture toughness parameter and some relevant mechanical properties are summarized in this paper. Noticeable difference is shown in the test results between the three groups of marble specimens. Moreover, nonlinear fracture mechanics approach, capable of incorporating the effect of FPZ, is applied in this study. The cohesive crack model is chosen for describing the mode-I FPZ in rock, and a three-dimensional finite element model has been established. A series of numerical study has been carried out to simulate the fracture test. Typical simulation results of load versus load point displacement are shown and compared with test measurements. Some preliminary discussions are given in the final part, mainly focused on the influence of strain softening property on the nonlinearity correction factor and the predetermined crack length at failure load in the suggested method. INTRODUCTION To date, a wide variety of testing methods have been employed to determine the plane strain fracture toughness of rock material, including the ASTM Fracture toughness Test Method developed for metals [1]. In the Blue Book "The Complete ISRM Suggested Methods for Rock Characterization, Testing and Monitoring: 1974–2006"[2], three suggested methods have been included for measuring the fracture toughness of rocks. In 1988, the ISRM Testing Commission recommended two suggested methods using two types of specimens that can be machined directly from pieces of rock core, the Chevron Bend specimen and the Short Rod specimen. In 1995, the cracked chevron notched Brazilian disc (CCNBD) specimen was introduced into the suggested methods, which would form a complete set of specimens for a full rock anisotropic fracture toughness investigation since the crack orientations of these three suggested specimen geometries can be easily arranged to be orthogonal to each other if they are machined from the same rock core. Similar as other quasi-brittle materials (such as ceramic and concrete), the crack development and growth in rocks would be influenced by the grain size due to the presence of a sizable microcrack zone formed ahead of the crack tip, called the fracture process zone. Hence the conventional test methods based on linear elastic fracture mechanics cannot be used directly, and correction arisen from the influence of nonlinear material behaviour was usually considered in the above suggested methods. Also it should be noted that some approximations or assumptions made in the above methods may require further consideration. This paper describes the results of mode-I fracture toughness tests conducted on chevron-notched three-point bend specimens. The rock specimens are marble rock samples from Ma On Shan area in Hong Kong, which can be subdivided into three groups by the difference in grain size.
- Well Completion > Hydraulic Fracturing (1.00)
- Well Drilling > Wellbore Design > Wellbore integrity (0.34)