In this research, a novel, unique systematic procedure was implemented to investigate the influence of the fracture networks and confining stresses on the jointed coal mass strength (JCMS). Both a laboratory experimental scheme and a numerical modeling scheme were carried out at the 3-D level. The laboratory experiments were performed to achieve the following three goals. Firstly, the geomechanical properties for the intact coal and coal discontinuities were estimated through the laboratory geomechanical property tests. Secondly, naturally existing fracture networks in the cubic coal blocks were first detected by the industrial Computed Tomography (CT) scanning technique and then quantified by the fracture tensor based methodology. Thirdly, polyaxial tests were conducted on the same cubic coal blocks to obtain the JCMS values under different confining stresses. With respect to the numerical modeling, PFC^3D and 3DEC software packages were used to simulate the polyaxial compression tests for intact and jointed cubic coal blocks, respectively. From more than twenty intact rock strength criteria, nine criteria were selected for this research. The intact coal strength data bank obtained from PFC^3D modeling was used to evaluate the applicability of nine different intact rock strength criteria. A modified grid search (MGS) procedure is proposed and used to find the best fitting parameter values and calculate the coefficient of determination (R²) values for each criterion. These criteria are compared in detail using the following features: R² values, σ₁ - σ₂ plots for different σ₃, shapes on the deviatoric planes, linearity or nonlinearity on the meridian planes. The regression analysis and the MGS procedure were found to be equivalent in finding the best fitting parameter values for a certain intact rock strength criterion. Through the comparisons, the modified Wiebols-Cook and modified Lade criteria were found to provide the highest R² values and fit the intact coal strength data best on the σ₁ - σ₂ coordinate plane and meridian planes. Based on the appearances on the deviatoric plane, the nine intact rock strength criteria are categorized into three types: the single shear stress criteria, the octahedral shear stress criteria and the criteria incorporating the maximum principal shear stress and partial intermediate principal shear stress. The relative positions of the different criteria on two specific meridian planes are also discussed. The geometric model of the jointed coal block was first set up by incorporating the fracture network constructed from the CT scanning into the intact coal block using a modified fictitious joint procedure. The numerical parameter values of intact coal and coal discontinuities were then calibrated and validated through a trial and error procedure using the laboratory test results of some selected samples. Next the JCMS data bank was consummated by performing a four-phase numerical investigation on several jointed coal blocks having selected fracture networks and five additional artificial fracture networks under different confining stress combinations. Finally, a new empirical coal mass strength criterion was developed to estimate the JCMS values at the 3-D level. The developed new model is capable of capturing the scale effect and anisotropic strength behaviors. It can also be applied to rock masses having approximately orthogonal fracture systems or for masses where fracture system can be reduced to an equivalent orthogonal fracture system.The following new contributions were made in this dissertation to advance the existing state-of-art on the dissertation topic: (a) A new, unique methodology as shown in Fig. 1.1 incorporating the following aspects was used to develop a new 3-D coal mass strength criterion: a complete set of geomechanical property tests, fracture network detection and quantification, polyaxial compression tests, numerical decomposition techniques; (b) A new procedure was developed to construct the fracture network in the coal cubes starting from CT scans to perform numerical modeling using 3DEC. In this procedure, a modified fictitious joint framework was also proposed to extend the applicability of the original fictitious joint framework, which allows incorporating a large quantity of non-persistent joints with acceptable numerical calculation effort; (c) A new 3-D coal mass strength criterion was developed to incorporate the fracture network and 3-D confining stress system to capture the anisotropy and scale effect of coal mass strength. The proposed criterion not only includes the influence of the intermediate principal stress, which is ignored by some existing strength criteria, but also includes the intensity and orientation and size probability distributions of the fracture system explicitly by a fracture tensor based methodology, which is far more advanced than most of the current criteria that are based on rock mass classification systems having only scalar indices; (d) A modified grid search procedure was proposed and used to evaluate the applicability of nine different intact rock strength criteria. The best intact rock strength criteria applicable for the intact coal data obtained through PFC^3D modeling were found by performing the most detailed intact rock strength criteria evaluation incorporating σ₁ - σ₂ - σ₃ plots and behaviors on the deviatoric and meridian planes, which improves the understanding of the available intact rock strength criteria.
| Identifer | oai:union.ndltd.org:arizona.edu/oai:arizona.openrepository.com:10150/612083 |
| Date | January 2016 |
| Creators | He, Pengfei |
| Contributors | Kulatilake, Pinnaduwa H. S. W., Zhang, Jinhong, Sternberg, Ben K., Momayez, Moe, Kulatilake, Pinnaduwa H. S. W. |
| Publisher | The University of Arizona. |
| Source Sets | University of Arizona |
| Language | en_US |
| Detected Language | English |
| Type | text, Electronic Dissertation |
| Rights | Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. |
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