A modified empirical criterion for strength of transversely anisotropic rocks with metamorphic origin (original) (raw)
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Engineering Geology, 2014
In this research, different experimental techniques have been used to determine the strength of five types of anisotropic foliated rocks, selected from different parts of the Hamedan Province, west of Iran. For this purpose, after sample preparation and assessment of mineralogical and physical properties, selected samples were subjected to Cylindrical Punch, Point Load and Brazilian tests and their strengths were evaluated with respect to different angles, beta (i.e. β = 0°, 15°, 30°, 45°, 60°, 75°and 90°), between anisotropy planes and the major loading directions. Data analyses show that the types and amounts of minerals have an influence on the physical and mechanical properties of the tested anisotropic rocks. Porosity and water absorption also have a dominant control on the mechanical indices such as cylindrical punch index (CPI), point load index (I s(50)) and Brazilian tensile strength (BTS). On the basis of the test result, two empirical equations (Eqs. (3) and (5)) are proposed for calculating CPI and I s(50). Other empirical equations relating studied indices are also presented.
Determination of Geotechnical Properties of Anisotropic Rocks Using Some Index Tests
Geotechnical Testing Journal, 2014
To determine the geotechnical properties of anisotropic rocks, five types of various metamorphic rocks were selected from different parts of Hamedan province, west of Iran. These samples were subjected to mineralogical, physical, and mechanical laboratory tests and their behaviors were obtained at different angles between loading axis and foliation planes (anisotropy angle or b; 0 , 15 , 30 , 45 , 60 , 75 , and 90). Based on the results, type, and amount of contained minerals have influence on the physical and mechanical properties of anisotropic tested rocks. Also, the Schmidt hardness (H s) and mechanical properties of the samples, such as point load index (I s(50)) and Brazilian tensile strength (BTS) are affected by the anisotropy angle (b). In this research, the empirical equations relating different studied properties of the anisotropic rocks were obtained. In this regard, new equations were proposed for calculating point load index (I s(50)) and Brazilian tensile strength (BTS) in different angles of anisotropy (b).
A review of methods, techniques and approaches on investigation of rock anisotropy
An extensive review on the anisotropy of rock samples has been carried out to characterize the velocity and strength behaviors under a variety of geometrical and mechanical conditions. Primarily, the causes and impacts of anisotropy is discussed to further understand the importance of the effect of such material from an engineering point of view. The strength anisotropy is investigated in laboratory using the standard strength testing practices (UCS, Triaxial, direct shear and etc..) to perceive the directional dependence of strength for anisotropic rocks and the velocity anisotropy using the ultrasonic scanning of the samples under destructive tests to evaluate the cracks propagation, density and orientation. Then, thorough literature review is done to highlight the significant observations that have been previously elicited. Furthermore, the mathematical determination methods of the degree of anisotropy are explored. Finally, this paper summarize that the strength and velocity anisotropy might be influenced by almost the same factors; however, the behavior of each anisotropy may not be the same considering the rock matrix and failure criteria.
Analysis of failure behaviour of the anisotropic rocks in the point load index test
IOP Conference Series Earth and Environmental Science , 2021
The strength and deformational behaviour of rocks under loading conditions are important in underground excavations, mining and every civil engineering constructions as it determines the stability of such structures. This study aims to augment the existing know-how on such behaviour through analysis of point load tests conducted on anisotropic rocks of metamorphic nature. Three types of anisotropic rocks named augen gneiss, granitic gneiss, psammitic schist, and greenschist from different formations of the Lesser and Higher Himalaya of Central Nepal have been investigated. It is observed that the point load index strength of the anisotropic rocks is related to their grain size and the loading angle with respect to the planes of anisotropy. The anisotropic index shows pronounced effects on the surface energy distribution of the samples on the application of stress.
Anisotropy effect on strengths of metamorphic rocks
Journal of Rock Mechanics and Geotechnical Engineering, 2018
This paper aims to study the effect of anisotropy on strengths of several metamorphic rocks of southern (Çine) submassif of Menderes metamorphic massif in southwest Turkey. Four different metamorphic rocks including foliated phyllite, schist, gneiss and marble (calcschist) were selected and examined. Discontinuity surveys were made along lines for each rock and evaluated with DIPS program. L-type Schmidt hammer was applied in the directions parallel and perpendicular to foliation during the field study. Several hand samples and rock blocks were collected during the field study for measurements of dry and saturated densities, dry and saturated unit weights and porosity, and for petrographic analysis and strength determination in laboratory. Land N-type Schmidt hammers were applied in the directions perpendicular (anisotropy angle of 0°) and parallel (anisotropy angle of 90°) to the foliation on selected blocks of phyllite, schist, gneiss and marble (calcschist). The phyllite and schist have higher porosity and lower density values than the other rocks. However, coarse crystalline gneiss and marble (calcschist) have higher rebound values and strengths, and they are classified as strong-very strong rocks. Generally, the rebound values in the direction perpendicular to the foliation are slightly higher than that in the direction parallel to foliation. Rebound values of N-type Schmidt hammer are higher than the L-type values except for phyllite. Sometimes, the rebound values of laboratory and field applications gave different results. This may result from variable local conditions such as minerals differentiation, discontinuities, water content, weathering degree and thickness of foliated structure.
Brazilian test: stress field and tensile strength of anisotropic rocks using an analytical solution
International Journal of Rock Mechanics and Mining Sciences, 2002
Tensile strength of rock is among the most important parameters influencing rock deformability, rock crushing and blasting results. To calculate the tensile strength from the indirect tensile (Brazilian) test, one must know the principal tensile stress, in particular at the rock disc center, where a crack initiates. This stress can be assessed by an analytical solution. A study of this solution for anisotropic (transversely isotropic) rock is presented.
Determination of deformability and tensile strength of anisotropic rock using Brazilian tests
International Journal of Rock Mechanics and Mining Sciences, 1998
This paper is the first of a series of two papers dealing with the determination of the deformability, tensile strength and fracturing of anisotropic rocks by diametral compression (Brazilian test) of discs of rock. It presents a combination of analytical and experimental methods for determining in the laboratory the elastic constants and the indirect (Brazilian) tensile strength of transversely isotropic rocks, i.e. rocks with one dominant direction of planar anisotropy. A computer program based on the complex variable function method and the generalized reduced gradient method was developed to determine the elastic constants of idealized linearly elastic, homogeneous, transversely isotropic media from the strains measured at the center of discs subjected to diametral loading. The complex variable function method was also used to construct charts for determining the indirect tensile strength of anisotropic media from the failure loads measured during diametral loading. Brazilian tests were conducted on four types of bedded sandstones assumed to be transversely isotropic. Based on strain measurements obtained with 45 ° strain gage rosettes glued at the center of the discs, the five independent elastic constants of the tested rocks could be determined. The elastic constants determined with the Brazilian tests were compared with those obtained from conventional uniaxial compression tests. The indirect (Brazilian) tensile strength of the tested sandstones was found to depend on the angle between the apparent planes of rock anisotropy and the direction of diametral loading.
On Direct Tensile Strength Measuring of Anisotropic Rocks
2021
DOI:10.22044/jme.2021.10601.2013 The tensile strength of the anisotropic rock-like material specimens is meastred directly in the laboratory using a new device converting the compressive loading to that of the tensile before the rock breakage. The specially prepared concrete slabs of dimensions 19 cm * 15 cm * 15 cm with a central hole of 7.5 cm in diameter are tested experimentaly. The specimens are located in the compressive-to-tensile load converting device, and tested under a compressive loading rate of 0.02 MPa/s by the universal testing machine. The cubic slab samples are made in three different configurations to have the directions of 0°, 45°, and -45° with respect to the applied loading direction. In order to compare the direct tensile strength of the concrete samples with that of the indirect measuring tests, some Brazilian tests are also carried out on the concrete disc specimens prepared in the laboratory. By comparing the direct and indirect testing results of the concre...
A simplified approach to directly consider intact rock anisotropy in Hoek-Brown failure criterion
Many rock types have naturally occurring inherent anisotropic planes, such as bedding planes, foliation, or flow structures. Such characteristic induces directional features and anisotropy in rocks' strength and deformational properties. The Hoek-Brown (H-B) failure criterion is an empirical strength criterion widely applied to rock mechanics and engineering. A direct modification to H-B failure criterion to account for rock anisotropy is considered as the base of the research. Such modification introduced a new definition of the anisotropy as direct parameter named the anisotropic parameter (Kβ). However, the computation of this parameter takes much experimental work and cannot be calculated in a simple way. The aim of this paper is to study the trend of the relation between the degree of anisotropy (Rc) and the minimum value of anisotropic parameter (Kmin), and to predict the Kmin directly from the uniaxial compression tests instead of triaxial tests, and also to decrease the ...