• Title/Summary/Keyword: uniaxial compressive strength mechanical properties

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Comparisons of Brittleness Indices of Shale and Correlation Analysis for the Application of Hydraulic Fracturing (수압파쇄 적용을 위한 셰일의 취성의 개념 고찰 및 취성도 간의 상관관계 분석)

  • Park, Jung-Ah;Park, Bona;Min, Ki-Bok
    • Tunnel and Underground Space
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    • v.24 no.4
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    • pp.325-333
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    • 2014
  • Hydraulic fracturing is the key technology for production of shale gas, which is one of the major unconventional resources. Brittleness index is one of the most important mechanical properties which determine the efficiency of hydraulic fracturing. It was required that the production of shale gas increases with more brittle behavior. Confusingly, there are numerous definitions available for brittleness of rock. This paper summarizes various definitions of brittleness index, and presents correlation analysis of the brittleness indices by using the laboratory experimental results of 48 shale specimens in Korea. Generally, it shows a very weak positive correlation between the brittleness index ($B_1$) which is the ratio of uniaxial compressive strength to tensile strength and the brittleness index ($B_3$) which is calculated by using the Youngs modulus and Poisson's ratio. In addition, the role of Poissons ratio is not clear in defining brittleness indices. In conclusion, standardization of definition for brittleness index is required to apply it to hydraulic fracturing as a parameter for predicting the efficiency.

Ideal body modeling of porous rock by frost-thawing (다공질암의 동결융해 현상에 대한 이상물체 모델의 적용성 연구)

  • Han, Heui-Soo;Back, Yoog
    • Journal of Korean Tunnelling and Underground Space Association
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    • v.12 no.5
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    • pp.399-405
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    • 2010
  • The accumulated displacements and fatigues of rock are increased by the stress-hysteresis, induced from repeated frost-thawing. Also the shear strength is decreased by them continuously. The stress-hysteresis is affected by the atmospheric temperature changes, whose behavior is visco-elasticity, usually. Therefore, to do ideal body analysis, Kelvin model could be used to analyze the frost-thawing behavior in winter. In general, rock slope failure occurs by the deterioration of rocks, which is caused by the repetition of freezing-thawing process. In order to keep the safety of such rock mass structures the deterioration process of rock needs to be described quantitatively using some meaningful parameters. In this work, the deterioration process in freezing-thawing cycle of tuff, which is a famous soft porous rock, is investigated through laboratory tests and successfully described as a differential equation for the change of porosity. And then, the deterioration of the mechanical properties of rock, such as Young's modulus and uniaxial compressive strength, are quantitatively described as a function of the porosity.

The Anisotropic Mechanical Characteristics of the Metamorphic Rocks Distributed in the Samkwang-Mine Area, Cheongyang, Chungnam (충남 청양군 운곡면 일원에 분포하는 편마암의 강도이방성 특성 - 점재하강도지수와 일축압축강도의 관계를 중심으로 -)

  • 배대석;송무영;김경수
    • The Journal of Engineering Geology
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    • v.1 no.1
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    • pp.54-67
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    • 1991
  • In geological media with anisotropic properties, the strength anisotropy in intact rock is the most important issue in engineering aspects. Point-ioad(P/L) strength test designed to estimate the uniaxial compressive strength(${\sigma}_c$) can be used to evaluate the anisotropic strength characteristics of rocks. The relationship between ${\sigma}_c$ and P/L strength indices(I$_s$), obtained from the banded gneisses distributed in the Cheongyang area, varies depend mainly on the dip angle($\beta$) of foliation. The axial P/L strength indicies(I$_{sa}$) decreases with the increment of $\beta$, whereas diametral P/L strength indices(I$_{sa}$) increase with it. However, the ${\sigma}_c$ decreases with the increments of $\beta$ below about 40$^{\circ}$, but it increases with the increments of $\beta$ over about 40$^{\circ}$ in general. The correlation between ${\sigma}_c$ and I$_s$ suggests that ${\sigma}_c$ is related to the Isa withing low angle($\beta$<40$^{\circ}$) and the I$_{sd}$ within high angle ($\beta$>40$^{\circ}$), respectively. The ratios of I$_s$ to ${\sigma}_c$ are obtained as 11 to 14 in the the gneisses in the study area. The ratio of 24, which is generally adopted value, cannot always be truthworth to the gneisses in the study area. The ratio for the dykes, however, show a good correlation as 21 to 24.5 and, the value of 24 can be used for a homogeneous and isotropic materials such as dykes.

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Relationship of box counting of fractured rock mass with Hoek-Brown parameters using particle flow simulation

  • Ning, Jianguo;Liu, Xuesheng;Tan, Yunliang;Wang, Jun;Tian, Chenglin
    • Geomechanics and Engineering
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    • v.9 no.5
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    • pp.619-629
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    • 2015
  • Influenced by various mining activities, fractures in rock masses have different densities, set numbers and lengths, which induce different mechanical properties and failure modes of rock masses. Therefore, precisely expressing the failure criterion of the fractured rock influenced by coal mining is significant for the support design, safety assessment and disaster prevention of underground mining engineering subjected to multiple mining activities. By adopting PFC2D particle flow simulation software, this study investigated the propagation and fractal evolution laws of the micro cracks occurring in two typical kinds of rocks under uniaxial compressive condition. Furthermore, it calculated compressive strengths of the rocks with different confining pressures and box-counting dimensions. Moreover, the quantitative relation between the box-counting dimension of the rocks and the empirical parameters m and s in Hoek-Brown strength criterion was established. Results showed that with the increase of the strain, the box-counting dimension of the rocks first increased slowly at the beginning and then exhibited an exponential increase approximately. In the case of small strains of same value, the box-counting dimensions of hard rocks were smaller than those of weak rocks, while the former increased rapidly and were larger than the latter under large strain. The results also presented that there was a negative correlation between the parameters m and s in Hoek-Brown strength criterion and the box-counting dimension of the rocks suffering from variable mining activities. In other words, as the box-counting dimensions increased, the parameters m and s decreased linearly, and their relationship could be described using first order polynomial function.

A Proposal of Stress-Strain Relations Model for Recycled-PET Polymer Concrete under Uniaxial Stress (일축 하중을 받는 PET 재활용 폴리머콘크리트의 응력-변형률 모델의 제안)

  • Jo Byung-Wan;Moon Rin-Gon;Park Seung-Kook
    • Journal of the Korea Concrete Institute
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    • v.16 no.6 s.84
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    • pp.767-776
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    • 2004
  • Polymer concrete shows excellent mechanical properties and chemical resistance compared with conventional normal cement concrete. The polymer concrete is drawing a strong interest as high-performance materials in the construction industry. Resins using recycled PET offer the possibility of a lower source cost of materials for making useful polymer concrete products. Also the recycling of PET in polymer concrete would help solve some of the solid waste problems posed by plastics and save energy. The purposed of this paper is to propose the model for the stress-strain relation of recycled-PET polymer concrete at monotonic uniaxial compression and is to investigate for the stress-strain behavior characteristics of recycled-PET polymer concrete with different variables(strength, resin contents, curing conditions, addition of silane and ages). The maximum stress and strain of recycled-PET polymer concrete was found to increase with an increase in resin content, however, it decreased beyond a particular level of resin content. A ascending and descending branch of stress-strain curve represented more sharply at high temperature curing more than normal temperature curing. Addition of silane increases compressive strength and postpeak ductility. In addition, results show that the proposed model accurately predicts the stress-strain relation of recycled-PET polymer concrete

Mechanical behavior of rock-coal-rock specimens with different coal thicknesses

  • Guo, Wei-Yao;Tan, Yun-Liang;Yu, Feng-Hai;Zhao, Tong-Bin;Hu, Shan-Chao;Huang, Dong-Mei;Qin, Zhe
    • Geomechanics and Engineering
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    • v.15 no.4
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    • pp.1017-1027
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    • 2018
  • To explore the influence of coal thickness on the mechanical behavior and the failure characteristics of rock-coal-rock (RCR) mass, the experimental investigation of uniaxial compressive tests was conducted first and then a systematic numerical simulation by particle flow code (PFC2D) was performed to deeply analyze the failure mechanical behavior of RCR specimens with different coal thicknesses in conventional compression tests. The overall elastic modulus and peak stress of RCR specimens lie between the rock and the coal. Inter-particle properties were calibrated to match the physical sample strength and the stiffness response. Numerical simulation results show that the deformation and strength behaviors of RCR specimens depend not only on the coal thickness, but also on the confining pressure. Under low confining pressures, the overall failure mechanism of RCR specimen is the serious damage of coal section when the coal thickness is smaller than 30 mm, but it is shear failure of coal section when the coal thickness is larger than 30 mm. Whereas under high confining pressures, obvious shear bands exist in both the coal section and the rock section when the coal thickness is larger than 30 mm, but when the coal thickness is smaller than 30mm, the failure mechanism is serious damage of coal section and shear failure of rock section.

Mechanical Anisotropy of Pocheon Granite under Uniaxial Compression (일축압축하에서 포천화강암의 역학적 이방성)

  • Park Deok-Won
    • The Journal of Engineering Geology
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    • v.15 no.3
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    • pp.337-348
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    • 2005
  • Jurassic granite from Pocheon area were tested to investigate the effect of microcracks on mechanical properties of the granite. Three oriented core specimens were used for uniaxial compressive tests and each core specimen are perpendicular to the axes'R'(rift plane),'c'(grain plane) and'H'(hardway plane), respectively Among vacious elastic constants, the variation of Poisson's ratio as function of the directions was examined. From the related chart between ratio of failure strength and Poisson's ratio, H-specimen shows the highest range in Poisson's ratio and Poisson's ratio decreases in the order of C-specimen and R-specimen. The curve pattern is nearly linear in stage $I\simIII$ but the slope increases abruptly in stage H-3. As shown in the related chart, diverging point of a curve is formed when ratio of failure strength is $0.92\sim0.96$ Stage IV -3 is out of elastic region. The behaviour of rock in the four fracturing stages was analyzed in term of the stress-volumetric strain me. From the stress increment-volumetric strain equations governing the behaviour of rock, characteristic material constants, a, n, Q, m and $\varepsilon_v^{mcf}$, were determined. Among these, inherent microcrack porosity$(a, 10^{-3})$ and compaction exponent(n) in the microcrack closure region(stage I ) show an order of $a^R(3.82)>a^G(3.38)>a^H(2.32)\;and\;n^R(3.69)>n^G(2.79)>n^H(1.99)4, respectively. Especially, critical volumetric microcrack strain($\varepsilon_v^{mcf}$) in the stage W is highest in the H-specimen, normal to the hardway plane. These results indicate a strong correlation between two major sets of microcracks and mechanical properties such as Poisson's ratio and material constants. Correlation of strength anisotropy with microcrack orientation can have important application in rock fracture studies.

Statistical analysis of NTNU test results to predict rock TBM performance (TBM 굴진성능 예측을 위한 NTNU 시험결과의 분석)

  • Choi, Soon-Wook;Chang, Soo-Ho;Lee, Gyu-Phil;Bae, Gyu-Jin
    • Journal of Korean Tunnelling and Underground Space Association
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    • v.13 no.3
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    • pp.243-260
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    • 2011
  • To predict TBM performance in design stage is indispensable for its successful application. The NTNU model, one of the representative TBM performance prediction models uses two distinct parameters such as DRI and CLI obtained from three different tests on bored rock cores. Based on DRI and CLI, it is possible to predict TBM advance rate and cutter life in the NTNU model. In this study, NTNU testing methods and their related testing equipments were introduced to measure DRl and CLI for the NTNU model. Then, in order to derive their relationships, the two key parameters measured for 39 domestic rocks were compared with physico-mechanical properties of rock such as uniaxial compressive strength and quartz content. Lastly, the experimental results were also compared with NTNU database to verify their reliability.

Dynamic failure features and brittleness evaluation of coal under different confining pressure

  • Liu, Xiaohui;Zheng, Yu;Hao, Qijun;Zhao, Rui;Xue, Yang;Zhang, Zhaopeng
    • Geomechanics and Engineering
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    • v.30 no.5
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    • pp.401-411
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    • 2022
  • To obtain the dynamic mechanical properties, fracture modes, energy and brittleness characteristics of Furong Baijiao coal rock, the dynamic impact compression tests under 0, 4, 8 and 12 MPa confining pressure were carried out using the split Hopkinson pressure bar. The results show that failure mode of coal rock in uniaxial state is axial splitting failure, while it is mainly compression-shear failure with tensile failure in triaxial state. With strain rate and confining pressure increasing, compressive strength and peak strain increase, average fragmentation increases and fractal dimension decreases. Based on energy dissipation theory, the dissipated energy density of coal rock increases gradually with growing confining pressure, but it has little correlation with strain rate. Considering progressive destruction process of coal rock, damage variable was defined as the ratio of dissipated energy density to total absorbed energy density. The maximum damage rate was obtained by deriving damage variable to reflect its maximum failure severity, then a brittleness index BD was established based on the maximum damage rate. BD value declined gradually as confining pressure and strain rate increase, indicating the decrease of brittleness and destruction degree. When confining pressure rises to 12 MPa, brittleness index and average fragmentation gradually stabilize, which shows confining pressure growing cannot cause continuous damage. Finally, integrating dynamic deformation and destruction process of coal rock and according to its final failure characteristics under different confining pressures, BD value is used to classify the brittleness into four grades.

The structural safety diagnosis of Dabo Pagoda of Bulkuk Temple using analyses of ultrasonic wave velocity (초음파 속도 분석을 통한 불국사 다보탑 구조 안전 진단)

  • Suh, Man-Cheol;Song, In-Sun;Choi, Hui-Soo
    • Journal of the Korean Geophysical Society
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    • v.5 no.3
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    • pp.199-209
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    • 2002
  • We have carried out a nondestructive close examination for the purpose of the structural safety diagnosis of the Dabo Pagoda of Bulkuk temple located in Kyungju, Kyungbuk Korea. For estimating the mechanical properties of each rock block of the pagoda, ultrasonic measurements were conducted at 641 points of 255 blocks. The P-wave velocity ranges from 584m/sec through 5,169m/sec, and averages 2,901m/sec Based on this result, the uniaxial compressive strength was estimated to be $93{\sim}1,943kg/cm^2\;with\;396kg/cm^2$ of average, and the index of weathering is $0.07{\sim}0.88$ with 0.43 of average, which means the moderate degree of weathering. The comparison of the rock strength of each block with the overburden acting on the block reveals that the rock blocks related to the structure of the pagoda are relatively sound for uniform stress, but it is highly possible for a concentrated stress to lead to a partial failure. We suggest a monitoring of cracks due to the concentrated stress. The parapets of 1st and 2nd floors composed of small rock pieces are severely weathered. However, this is not directly related to the structural safety of the pagoda.

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