Investigation of the anisotropic confinement-dependent brittleness of a Utah coal

Abstract Changes of failure mechanism with increasing confinement, from extensional to shear-dominated failure, are widely observed in the rupture of intact specimens at the laboratory scale and in rock masses. In an analysis published in 2018, both unconfined and triaxial compressive tests were conducted to investigate the strength characteristics of 84 specimens of a Utah coal, including the spalling limits, the ratio of apparent unconfined compressive strength to unconfined compressive strength (UCS), the damage characteristics, and the post-yield dilatancy. These mechanical characteristics were found to be strongly anisotropic as a function of the orientation of the cleats relative to the loading direction, defined as the included angle. A total of four different included angles were used in the work performed in 2018. The authors found that the degree of anisotropic strength differed according to the included angle. However, the transition from extensional to shear failure at the given confinements was not clearly identified. In this study, a total of 20 specimens were additionally prepared from the same coal sample used in the previous study and then tested under both unconfined and triaxial compressive conditions. Because the authors already knew the most contrasting cases of the included angles from the previous work using the four included angles, they chose only two of the included angles (0° and 30°) for this study. For the triaxial compressive tests, a greater confining stress than the mean UCS was applied to the specimens in an attempt to identify the brittle-ductile transition of the coal. The new results have been compiled with the previous results in order to re-evaluate the confinement-dependency of the coal behavior. Additionally, the different confining stresses are used as analogs for different width-to-height (W/H) conditions of pillar strength. Although the W/H ratios of the specimens were not directly considered during testing, the equivalent W/H ratios of a pillar as a function of the confining stresses were estimated using an existing empirical solution. According to this relationship, the W/H at which in situ pillar behavior would be expected to transition from brittle to ductile is identified.

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Method for Predicting In-situ Undrained Strength of Clays Based on the Suction Value and Unconfined Compressive Strength.

Doboku Gakkai Ronbunshu ◽

10.2208/jscej.1996.541_147 ◽

1996 ◽

pp. 147-157 ◽

Cited By ~ 1

Author(s):

Toshiyuki Mitachi ◽

Yutaka Kudoh

Keyword(s):

Compressive Strength ◽

Unconfined Compressive Strength ◽

Undrained Strength

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The influence of soft clay saturation characteristics on unconfined compressive strength in Guangdong-Hong Kong-Macao Greater Bay Area

E3S Web of Conferences ◽

10.1051/e3sconf/202129302011 ◽

2021 ◽

Vol 293 ◽

pp. 02011

Author(s):

DaShu Guan ◽

JiaXi Zheng ◽

WenHao Huang ◽

JunZhi Zhong ◽

WenWen Du ◽

...

Keyword(s):

Compressive Strength ◽

Gas Phase ◽

Unconfined Compressive Strength ◽

Shear Failure ◽

Soft Clay ◽

Plastic Shear ◽

Bay Area ◽

Saturated Clay ◽

Unsaturated Clay ◽

Saturated Soft Clay

In order to study the influence of saturated characteristics of soft clay on unconfined compressive strength, the soft clay of Guangdong-Hong Kong-Macao Bay Area is taken as the research object, comparing and contrasting on unconfined compression test of saturated clay and unsaturated clay in laboratory, studying the variation law of unconfined compressive strength and sensitivity of unsaturated and saturated soft clay. The test results show that: 1. During the failure of unsaturated soft clay samples, oblique fractures appear, showing brittle shear failure, while the saturated clay samples appear constant bulging in the middle, and finally a “cross” is broken in the central bulging part, showing plastic shear failure.2. The unconfined compressive strength of unsaturated clay is about 10kPa higher than that of saturated soft clay, and its corresponding sensitivity is also about 0.4 higher. It can be shown that the soil saturation has a certain influence on the soil strength. The difference between saturated soil and unsaturated soil is the existence of gas phase. To be more precise, the existence of gas phase in unsaturated soil, i.e. the existence of suction, makes the soil stronger and presents the brittle shear failure form, while the saturated soil basically presents the plastic shear failure form.

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Effect of Freeze-Thaw Cycles on the Mechanical Properties of Polyacrylamide- and Lignocellulose-Stabilized Clay in Tibet

Advances in Materials Science and Engineering ◽

10.1155/2021/7723405 ◽

2021 ◽

Vol 2021 ◽

pp. 1-16

Author(s):

Haiping Shi ◽

Zhongyao Li ◽

Wenwei Li ◽

Shaopeng Wang ◽

Baotian Wang ◽

...

Keyword(s):

Mechanical Properties ◽

Compressive Strength ◽

Internal Friction ◽

Unconfined Compressive Strength ◽

Friction Angle ◽

Internal Friction Angle ◽

Test Results ◽

Freeze Thaw ◽

Original Strength ◽

Compressive Tests

Laboratory freezing experiments were conducted to evaluate the effect of polyacrylamide (PAM) and lignocellulose on the mechanical properties and microstructural characteristics of Tibetan clay. Direct shear and unconfined compressive tests and field emission scanning electron microscopy analyses were performed on clay samples with different contents of stabilizers. The test results show that the addition of PAM can improve the unconfined compressive strength and cohesion of Tibetan clay, but an excessive amount of PAM reduces the internal friction angle. After several freeze-thaw cycles, the unconfined compressive strength and cohesion of samples stabilized by PAM decrease significantly, while the internal friction angle increases. Samples stabilized by PAM and lignocellulose have higher internal friction angles, cohesion, and unconfined compressive strength and can retain about 80% of the original strength after 10 freeze-thaw cycles. PAM fills the pores between soil particles and provides adhesion. The addition of lignocellulose can form a network, restrict the expansion of pores caused by freeze-thaw cycles, and improve the integrity of PAM colloids. It is postulated that the addition of a composite stabilizer with a PAM content of 0.4% and a lignocellulose content of 2% may be a technically feasible method to increase the strength of Tibetan clay.

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Investigation on the Compressive Strength and Time of Setting of Low-Calcium Fly Ash Geopolymer Paste Using Response Surface Methodology

Polymers ◽

10.3390/polym13203461 ◽

2021 ◽

Vol 13 (20) ◽

pp. 3461

Author(s):

Pauline Rose J. Quiatchon ◽

Ithan Jessemar Rebato Dollente ◽

Anabel Balderama Abulencia ◽

Roneh Glenn De Guzman Libre ◽

Ma. Beatrice Diño Villoria ◽

...

Keyword(s):

Compressive Strength ◽

Fly Ash ◽

Unconfined Compressive Strength ◽

Setting Time ◽

Initial Setting Time ◽

Alkali Activator ◽

Setting Times ◽

Confirmatory Tests ◽

Initial Setting

Approximately 2.78 Mt of coal fly ash is produced in the Philippines, with a low utilization rate. Using fly ash-based geopolymer for construction will lessen the load sent to landfills and will result in lower GHG emissions compared to OPC. It is necessary to characterize the fly ash and optimize the geopolymer components to determine if it can replace OPC for in situ applications. The activator-to-precursor ratio, the water-to-solids ratio, and the sodium hydroxide-to-sodium silicate ratio were optimized using a randomized I-optimal design from the experimental results of 21 runs with five replicates, for a total of 105 specimens of 50 mm × 50 mm × 50 mm paste cubes. The engineering properties chosen as the optimization responses were the unconfined compressive strength (UCS), the initial setting time, and the final setting time. The samples were also ambient-cured with the outdoor temperature ranging from 30 °C to 35 °C and relative humidity of 50% ± 10% to simulate the on-site environment. Runs with high unconfined compressive strength (UCS) and short setting times were observed to have a low water-to-solids (W/S) ratio. All runs with a UCS greater than 20 MPa had a W/S ratio of 0.2, and the runs with the lowest UCS had a W/S of 0.4. The initial setting time for design mixes with a W/S ratio of 0.2 ranged from 8 to 105 min. Meanwhile, five out of seven design mixes with a W/S ratio of 0.4 took longer than 1440 min to set. Specimens with an alkali activator ratio (NaOH/WG) of 0.5 (1:2) and 0.4 (1:2.5) also had significantly lower setting times than those with an alkali activator ratio of 1. The RSM model was verified through confirmatory tests. The results of the confirmatory tests are agreeable, with deviations from the expected UCS ranging from 0 to 38.12%. The generated model is a reliable reference to estimate the UCS and setting time of low-calcium FA geopolymer paste for in situ applications.

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Estimation of unconfined compressive strength of cement-stabilized jabre as material upgrade on highway construction

Materiales de Construcción ◽

10.3989/mc.2020.09019 ◽

2020 ◽

Vol 70 (338) ◽

pp. 218

Author(s):

E. Teijón-López-Zuazo ◽

Á. Vega-Zamanillo ◽

M. Á. Calzada-Pérez ◽

L. Juli-Gándara

Keyword(s):

Compressive Strength ◽

Stress Resistance ◽

Unconfined Compressive Strength ◽

Particle Size Analysis ◽

Size Analysis ◽

Dry Density ◽

Practical Engineering ◽

Stabilized Soil

Granite rock has powerful alterations at several meters of depth. The clayed sand resulting is commonly known as jabre. This “in situ” mixture of cement-stabilized soil requires a laboratory formula. Even when the test section is correctly verified, the mechanical properties of the homogeneous mixture of jabre exhibit high degrees of dispersion. The laboratory work undertaken included particle-size analysis and screening, definition of liquid and plastic limits, compressive strength, dry density and moisture content over stabilized samples, modified Proctor, California Bearing Ratio (CBR) and the determination of the workability of the hydraulically bound mixtures. The stress resistance curve was analyzed by means of a multilinear model of unconfined compressive strength (UCS). Since practical engineering only requires UCS for 7 days, in order to gain greater knowledge of the material, other UCS transformations were used at other curing times such as 7, 14 and 28 days.

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Simulation Study on Strength and Failure Characteristics for Granite with a Set of Cross-Joints of Different Lengths

Advances in Civil Engineering ◽

10.1155/2018/2384579 ◽

2018 ◽

Vol 2018 ◽

pp. 1-10 ◽

Cited By ~ 9

Author(s):

Dawei Yin ◽

Shaojie Chen ◽

Xingquan Liu ◽

Hongfa Ma

Keyword(s):

Compressive Strength ◽

Crack Propagation ◽

Uniaxial Compressive Strength ◽

Shear Failure ◽

Tensile Failure ◽

Failure Characteristics ◽

Included Angle ◽

Loading Direction ◽

Joint Plane ◽

Plane Direction

The strength and failure characteristics for granite specimen with a set of cross-joints of different lengths were studied using PFC2D software. The results show that when the included angle of α between the main joint and loading direction is 30° or 45°, no matter what the included angle of β between main and secondary joints is, the main joint controls crack propagation and failure of granite specimen, which occurs the shear failure propagating from main joint tips, and the corresponding uniaxial compressive strength is low. Meanwhile, the secondary joint is the key joint for crack propagation and failure at α of 0° and 90° except when β is 90°. The granite specimen occurs the shear failure propagating from secondary joint tips. And, the shear failure crossing upper tips of main and secondary joints is found at α of 0° or 90° and β of 90°. Their uniaxial compressive strengths are large. Also, the combined actions of main and secondary joints determine crack propagation and failure at α of 60° except when β is 90°. The granite specimen occurs the hybrid failure, including shear failure propagating from main joint tips and tensile failure propagating from main and secondary joints center or secondary joint tips. And, when α is 60° and β is 90°, the granite specimen occurs the shear failure along secondary joint plane direction, and its uniaxial compressive strength is small. Generally, when α or β is a fixed value, the uniaxial compressive strength firstly decreases and then increases with the increase of β or α. Additionally, when α is 60° and β>45°, the uniaxial compressive strength represents a decreasing trend. The uniaxial compressive strength at α and β between 30° and 60° is generally small. Finally, the microdisplacement field distributions of granite specimen were discussed.

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Study on Mechanical Characteristics and Failure Modes of Coal–Mudstone Combined Body with Prefabricated Crack

Mathematics ◽

10.3390/math10020177 ◽

2022 ◽

Vol 10 (2) ◽

pp. 177

Author(s):

Huayong Lv ◽

Defeng Wang ◽

Zhanbo Cheng ◽

Yaning Zhang ◽

Tao Zhou

Keyword(s):

Compressive Strength ◽

Unconfined Compressive Strength ◽

Mechanical Characteristics ◽

Failure Modes ◽

Shear Failure ◽

Acoustic Emissions ◽

Body Part ◽

Unconfined Compression Test ◽

Crack Angle ◽

Continuous Loading

There are normally pre-existing cracks that can be observed in the coal seam and immediate roof that influences the stability of the rib spalling and the movement law of overlying strata. In this study, comprehensive research methods (e.g., theory analysis, experimental tests and numerical simulations) were adopted to reveal the mechanical characteristics, acoustic emission behaviors and failure modes of a coal–mudstone combined body with a single prefabricated non-penetrating crack. The results show that the influence of the crack angle on the elastic modulus of the coal–mudstone combined body samples was limited. With the increase in the crack angle, the unconfined compressive strength of samples decreased first and then increased in a V-shaped trend. In addition, the minimum unconfined compressive strength could be observed at a crack angle of 45°. Moreover, the number of acoustic emissions significantly increased with the process of continuous loading. In addition, the stress reduction zone could be observed in both ends of the prefabricated cracks at the initial stage of loading. The high- and low-stress zones were transformed with the process of continuous loading. Under an unconfined compression test, the failure models of the coal body part in the samples were mainly caused by shear failure, and only a few cracks occurred in the upper tip of the prefabricated cracks of the mudstone part. Therefore, airfoil cracks could be observed in the samples due to the strength difference of the coal mass and mudstone.

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