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Edge-to-edge is the most common form of contact in a two-dimensional joint block system. Discontinuous deformation analysis (DDA) is a powerful tool for handling block systems and treats an edge-to-edge contact as two vertex-to-edge contacts with a couple of normal and shear springs at the extremes of the contact segment. However, this method is unsuitable for evaluating the tensile failure of a joint under asymmetrical tension conditions, even though it can simplify the contact type and increase computational efficiency. This is mainly because the distributed forces at an edge-to-edge contact are simplified as two couples of concentrated forces. To solve this problem, this paper proposes incorporating a multi-spring edge-to-edge contact model with DDA. In the model, several contact pairs with normal and shear springs on an edge-to-edge contact are set to simulate the distributed forces. Several benchmark tests were performed to demonstrate the discrepancy between the original and improved DDAs. The results revealed that the latter more accurately models the stress distribution across the edge-to-edge contact, providing a more reasonable acceleration required to topple a series of blocks resting on an inclined stepped base. [ABSTRACT FROM AUTHOR]

In slope stability evaluation, it is common that a potential tensile sliding surface is not accurately positioned. In this paper, the strength reduction method considering shear and tensile failure with a strain-softening model is adopted to simulate the deterioration of the strength parameters of geomaterials during failure. Based on the initial stress method, a new quantitative display index of the full sliding surface is proposed. This method was applied to the stability analysis of a heterogeneous slope and a homogeneous slope. The results show that the plastic zone determined with the strain-softening model is much smaller than that determined with the Mohr-Coulomb model, and the difference is 34–40%. The safety factor obtained without considering tensile failure is 3.1% larger than that obtained considering tensile failure. The index can be used to quantitatively display the shear failure zone and tensile failure zone of a sliding surface. [ABSTRACT FROM AUTHOR]

Tensile failure, hybrid failure (combined tension and shear) and shear failure are the three basic failure types of brittle rock. For a long time, little attention has been paid to the first two failure types except for the failure strength of direct/indirect tension, and the deformation characteristics and failure criterion of rock in triaxial extension (herein "triaxial extension" denotes axial tension/unloading-tension in confining pressure P c) are still unclear. This paper proposes a simple auxiliary apparatus to implement triaxial extension tests of cylindrical rock specimens combined with a servo-controlled MTS machine. Subsequently, the mechanics behaviors of sandstone in triaxial extension tests under different constant confining pressures (P c = 0, 5, 10, 15, 20, 30, 40, 50 and 60 MPa) are experimented and analyzed in detail, including the fracture angle, the failure surface morphology, the deformation and strength. With the increase of confining pressure, the failure type of sandstone presents a continuous transition from tensile failure to hybrid failure and to shear failure as a whole. The changes of failure morphology, elastic modulus, Poisson's ratio, maximum strain and failure strength with increase in P c show significant difference among three failure types. For the tested sandstone, the partitions of confining pressure between tensile failure and hybrid failure and between hybrid failure and shear failure are approximately at 20 MPa and 40 MPa, respectively. Hoek-Brown criterion with a tension cut-off and Fairhurst criterion are applicable to fitting the failure strength of triaxial extension test, and the latter presents a satisfying continuous transition from tensile failure to shear failure. [ABSTRACT FROM AUTHOR]

A fundamental study on the tensile failure of rock is conducted using the three-dimensional lattice spring model. The model covers three aspects: (1) the relationship between the mesoscopic tensile/shear failure and the corresponding macroscopic tensile failure; (2) the effects of the size, shape, and location of the initial defect on the macroscopic tensile failure; and (3) the effects of the porosity, heterogeneity, crystal structure, mesoscopic constitutive model, and model scale on its macroscopic tensile responses. Through investigation, this study reveals that the mesoscopic strength heterogeneity affects the macroscopic pre-peak response of rock, and the initial defect could control its macroscopic post-peak response. The post-peak characteristics of the mesoscopic constitutive model influence both the macroscopic pre-peak and post-peak responses, which are scale independent and scale dependent, respectively. Based on these investigations, a parameter-selection method for the mesoscopic constitutive model is established to fully utilize the macroscopic tensile experimental data. [ABSTRACT FROM AUTHOR]

Abstract Knowledge of the in situ stress state of rock mass is fundamental for engineering, geological and geophysical applications. In situ stress state determination requires in principle the evaluation of the three principal stresses and the related principal directions, but it is widely recognized in the literature that the maximum horizontal stress is the most difficult component to accurately estimate. In the context of borehole methods, this paper proposes a step-by-step analytical procedure to estimate some bounds to the maximum horizontal stress, starting from a geomechanical description of the rock and relying on information generally available in the engineering practice. The procedure is divided in substeps, each one requiring additional information about the mechanical properties of the rock and on the geometrical properties of the failed portion of rock: more information available implies a lower uncertainty on in situ stress estimate. Furthermore, since the proposed procedure is analytical, it allows a complete and very easy implementation in a spreadsheet. The aim of the work is thus to provide a rigourous but simple analytical tool that can be used in engineering practicte to estimate some bounds to the maximum horizontal in situ stress state. The approach is finally validated by means of both numerical simulations, performed with a sophisticated numerical tool, and experimental field data coming from the literature. Highlights • Determination of in situ stress state is crucial for any underground application in many fields of engineering. • Borehole methods have been accepted to be one of the most reliable method to estimate in situ stress state. • A step-by-step analytical procedure is presented to estimate the in-situ stress state relying on drilled borehole information. • Any step of the procedure reduces of the bounds of the maximum horizontal stress, at the cost of more information required. • The procedure is validated by means of both simulations with advanced numerical methods and in situ literature data. [ABSTRACT FROM AUTHOR]

A cohesive fracture model considering tension, compression and shear material behaviour is implemented into the hybrid continuum–discrete element method, i.e. Universal Distinct Element Code (UDEC), to simulate possible multiple fracture in soft rocks. The fracture model considers both elastic and inelastic (decomposed to fracture and plastic) displacements. The norm of the effective inelastic displacement is used to control the fracture behaviour. Three numerical examples, including Mode-I, Mode-II and Mixed-mode tests, are conducted to verify the model and its implementation in UDEC. The model is subsequently applied to simulate uniaxial compression and Brazilian disc tests on soft rocks and the results are compared with experimental results. The results indicate that the cohesive fracture model is capable of realistically simulating the combined tensile, shear and mixed-mode failure behaviour applicable to geomaterials. [ABSTRACT FROM AUTHOR]

The behavior of transversely isotropic rock material was studied under Brazilian test conditions for nine different rocks (two sandstones, one shale, two slates, one schist, and three gneiss). Both the variation of the strength and the final fracture patterns induced by testing were examined as a function of the inclination angle of the weak planes. The combination of the observations for both parameters illustrated clearly the complexity of the failure of such rocks, which was summarized by assuming four different trends. The four trends for the variation of the strength as a function of the inclination angles range from little or no variation (trend 1, i.e., isotropic behavior for strength) to a sharp decrease of the strength from very small angles onwards, followed by a leveling off (trend 4). Trend 2 is characterized by a constant value between 0° and about 45°, followed by a linear decrease, while trend 3 corresponds to a decrease of the strength over the entire interval, but a rather systematic decrease, approximating a linear variation. Different variations were observed among these four trends in terms of the relative lengths of the respective fractures along the weak planes and in any other direction. For the rocks investigated, there is a cross-over from dominant fractures in directions other than the weak planes to dominant fractures along the weak planes. For the four trends, there are systematic changes in the position of this cross-over point, i.e., on average, at about an inclination angle of 75° for trend 1 and at about 15° for trend 4. For inclination angles larger than the cross-over point, the rock specimens failed primarily by splitting between both loading lines. [ABSTRACT FROM AUTHOR]

Abstract: There has been a debate in the rock mechanics community regarding the mechanisms causing what is known as core discing. This phenomenon occurs when diamond drill cores are retrieved from rock masses in which high in situ stresses relative to rock strength are present. The interest in that phenomenon is to use it to estimate the in situ stresses from the shape and frequency of the failures along the core axis. In the present paper we argue that discing is only an indicator of high stresses, and that estimating in situ stresses from fracture observations is much too inaccurate. As most of the literature found on the subject has tackled the problem using elastic numerical models, it is shown that the stress distribution in the core being formed obtained from such models does not exist once failure has been reached. Numerical analyses using Flac2D with an elasto-plastic cohesion softening friction hardening model show that for a given stress state, discing or core damage may involve tensile failure, a combination of shear and tensile failure, or only shear failure, depending on the stress state and ratio of tensile to shear strength of the rock. The numerical model used is validated by replicating core discing observed under controlled laboratory conditions. Parametric analyses involving changes in mesh density, deformability parameters, dilatancy, drill bit pressure, drilling fluid pressure and applied stress states are also performed. Finally, it is shown that drilling-induced core discing or damage store important residual stresses in the core which may explain why recovered core samples tend to show a deterioration of their mechanical properties with time. [Copyright &y& Elsevier]

To bridge the gap between elemental concentrations and rock mechanics, this paper proposed to predict a tensile failure using the weight concentrations of various elements. In this research, the Brazilian test was used to measure the indirect tensile strength of sandstones under different environmental conditions, i.e., oven drying, water immersion, and mud filtration. According to the mechanical test results of 37 sandstone specimens, the fracture patterns can be quite complicated due to an interaction of tensile fracture with shear fracture or bending fracture. To improve the prediction accuracy of the statistical analysis, a digital image correlation (DIC) technique was used to capture the dynamic fracturing process and to verify the validity of the Brazilian tests. Finally, only 15 specimens met the principle of the Brazilian test with a valid tensile failure. Using the energy-dispersive system (EDS), a distinct disparity of elemental concentrations was identified between those two sides of a fracture, such as the content contrasts of C, Mg, Al, Si, Ca, Ti, Fe, and Zn. These key elements can be used to identify the fracture positions and predict the rock mechanical properties, e.g., Brazilian test strength, Young's modulus, and Poisson's ratio. The excellent correlation performance, between rock strength parameters and elemental concentrations, proved it applicable to provide another potential function of the Elemental Capture Spectroscopy log in the evaluation of wellbore integrity. [ABSTRACT FROM AUTHOR]