Rock Strength Anisotropy Research Articles

Measurement of Indirect Tensile Strength of Anisotropic Rocks by the Ring Test

This paper presents a new approach, combined with the Boundary Element Method (BEM) analysis and the diametrical compression on a thin disc with a small central hole, referred to as the ring test, for determining the indirect tensile strength of anisotropic rocks. The stress distribution around the hole can be successfully obtained by the proposed single-domain BEM. The complex variable function method was used for conveniently computing the tractions and displacements of a two-dimensional anisotropic body. If we assume that the tensile strength is given by the maximum absolute value of stress in the direction perpendicular to the loaded diameter at the intersection of loaded diameter and the hole, then from the failure load recorded by laboratory testing of ring (disc), the indirect tensile strength of rocks could be obtained. A marble from Hualien (Taiwan) with clearly black-white foliation, which was assumed to be transversely isotropic, was selected to conduct both ring tests and Brazilian tests for evaluating the tensile strength. The variation of the marble tensile strength with the inclination angle of foliation and with the hole size was also investigated. In general, the tensile strength of anisotropic rocks determined by ring test is not a constant, but depends on the elastic properties of rocks, the angle between the planes of rock anisotropy and the loading direction, the diameter of the central hole, and the contact condition of loading.

How successful are analogue models in addressing the influence of pre-existing fabrics on rift structure?

Sandbox models have been widely used to investigate normal fault geometries, evolution and propagation. As modelling attempts to investigate more aspects of normal faulting, the effects of pre-existing fabrics on normal fault geometry developed within the brittle upper crust have become a topic of interest. Analogue models have been developed for oblique rifting and the influence of pre-existing fabrics on transfer zone geometry. These models use pre-cut geometries in underlying plates to impose `pre-existing' fabrics on the developing normal fault system. However, to really mimic natural systems it is the sand itself that should contain the pre-existing fabrics. The problem is that cohesionless sand has no tensile strength, while the influence of pre-existing fabrics on rift structure in the upper crust attests to the important role played by relatively small variations in rock strength anisotropy caused by pre-existing fabrics. Consequently it is necessary to assess how significant the departures are between model and natural examples and whether a new approach to modelling pre-existing rift structures is required.

Investigating tensile strength and fragmentation of anisotropic rocks in 3D using the Brazilian test

Abstract This paper presents laboratory work where rock strength anisotropy and fines fraction of a gneiss has been determined by means of the Brazilian test of core samples. Samples of a diameter of 42 mm were taken in 3D from rock blocks, cut into 21 mm specimens and subjected to the Brazilian test. Samples of each direction were loaded at two perpendicular directions. The results have been expressed as two factors (a) tensile strength- and (b) fines fraction-anisotropy. The tensile strength anisotropy is defined as Ku = [sgrave]iv/[sgrave]tv where [sgrave]tv and [sgrave]tp are the tensile strength of specimens loaded perpendicular and parallel to foliation respectively. This factor had different values for specimens of different directions and varied between 1.2 to 1.5. The fines fraction anisotropy is defined as the percentage of rock particles smaller than 2 mm in thickness generated due to failure of the specimen. This factor is defined as Kfa = f v/f P where f v , and f p are the fines fraction of ...

Determination of deformability and tensile strength of anisotropic rock using Brazilian tests

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.

Wellbore Failure Mechanisms in Shales: Prediction and Prevention

Summary Shale stability is still one of the most important problems faced during drilling. Until recently, stability problems were most often attributed to shale swelling; however, recent research shows that several mechanisms are involved and that their relative importance can be estimated. This paper presents a review of these mechanisms, including pore-pressure diffusion, plasticity, anisotropy, capillary effects, osmosis, and physicochemical alteration. Pore-pressure diffusion into the rock in the vicinity of the wellbore (transition from undrained to drained behavior) appears to be of major importance in these very-low-permeability rocks. Plasticity is discussed in terms of modeling. Compared with simple elastic models, modeling of plasticity can simulate the actual behavior of wellbore better. Rock anisotropy can influence failure either by its effect on stress redistribution or through rock-strength anisotropy. Analytical studies show that the second effect is more important. Capillary effects can significantly enhance the use of oil-based muds by effectively supporting the borehole wall. On the other hand, the same kind of effect between air and pore fluid can lead to misinterpretation of laboratory results on unsaturated outcrop material. Osmosis is a very controversial topic. Although industry publications do not give a clear idea of the extent of osmosis effects in shales, some guidelines can be established to compare it with other mechanisms. Physicochemical interaction between a shale and the drilling mud can lead to the dissolution of a mineralogical phase of the rock. Subsequent alteration of the rock cohesion can explain shale failure or dispersion. Thermal effects between cooling of the bottom part of the well and heating of the upper part can also be very significant. The behavior of different types of muds is discussed while taking these phenomena into consideration, and the practical use of rock-mechanics models is also addressed. Introduction For a long time, drilling through shales has been one of the most troublesome issues for the drilling industry. Despite the real progress achieved in the area of borehole stability, shale problems are still reported frequently (including long trip times to lost holes and sidetracks, the inability to log, or important washouts leading to poor log quality or poor cementations). Moreover, stability problems become more crucial as wells become deeper and boreholes more deviated. These wells, in which a limited number of casings and mud systems can be used (because of environmental problems with oil-based muds), result in open holes of increasing lengths, especially for larger diameters. Therefore, the open hole is exposed to drilling fluids for longer periods of time, which seriously impairs the resistance of borehole walls. The problems encountered in drilling in shales are caused by the large variety of rocks called shales and to the complexity of the mechanisms involved (mechanical, porosity-related, thermal, and physicochemical). Being able to characterize shales correctly and having a good knowledge of the mechanisms that can actually occur are key points in making an appropriate diagnosis and in solving or reducing problems associated with shales to acceptable levels. In this paper, several mechanisms or particular shale properties are discussed. Pore-pressure diffusion, plasticity, and anisotropy are the mechanical or poromechanical mechanisms, and capillary effects, alteration, and osmosis are the physicochemical mechanisms. Some other mechanisms are also briefly discussed, as well as effects of different mud types and use of rock-mechanics models.

Observations on the use of the hoop test for measuring the tensile strength of anisotropic rock : C. Butenuth, M. H. De Freitas, H.-D. Park, K. Schetelig, P. Van Lent & P. Grill, International Journal of Rock Mechanics & Mining Sciences, 31(6), 1994, pp 733–741

Observations on the use of the hoop test for measuring the tensile strength of anisotropic rock : C. Butenuth, M. H. De Freitas, H.-D. Park, K. Schetelig, P. Van Lent & P. Grill, International Journal of Rock Mechanics & Mining Sciences, 31(6), 1994, pp 733–741

Observations on the use of the hoop test for measuring the tensile strength of anisotropic rock

Observations on the use of the hoop test for measuring the tensile strength of anisotropic rock

Influence of anisotropies in borehole stability

This paper discusses an anisotropic model for assessing the mechanical stability of a deep borehole subjected to an internal wellbore pressure and a far-field stress tensor. The model consists of a three-dimensional stress analysis around an inclined borehole combined with a generalized three-dimensional anisotropic strength criterion for assessing shear fracturing. Parametric studies indicated that the stability of a wellbore is influenced significantly by rock anisotropy, rock strength anisotropy, and in-situ stress differentials when it is highly inclined and oriented in the direction parallel to the maximum horizontal in-situ stress. The influence of anisotropies is more pronounced on passive shear failure caused by wellbore pressurization.

Compressive strength anisotropy of foliated rocks

Compressive strength anisotropy of foliated rocks

Deformation and fracture around cylindrical openings in rock—II. Initiation, growth and interaction of fractures

Elastic and inelastic deformation, fracture and failure around underground openings were investigated through experiments on thick-walled hollow cylinders of Berea sandstone and Indiana limestone, incorporating plane strain loading, the application of different stress paths, transference of the external pressure to infinity, and freezing of the fracture geometry under stress through metal saturation. This second of two parts investigates fracture development and failure progression on both an experimental and theoretical level. Failure is oriented and occurs on the two opposite sides of the hole with the highest ratio of tangential stress to tangential strength (determined by rock strength anisotropy). The size of the failed zones may be influenced not only by the final stress state but also by stress path, strain rate and test boundary conditions. Microscopic observation reveals that the fundamental fracture mechanism is the growth of small, opening-mode, splitting cracks oriented parallel to the tangential stress, starting very close to the hole wall and occurring deeper in the rock with increasing stress. Numerical models confirm that splitting cracks preferentially grow close to the surface of a stressed hole. These models also predict a size effect on strength. Around unsupported holes these cracks coalesce to form macroscopic splitting fractures subparallel to the hole wall, and also form en echelon patterns that meet the wall. These two types of features combine to define the new surface of a spalled piece, which moves radially into the hole. Slabs of fairly uniform thickness progressively detach from the surrounding rock, resulting in a triangular failure zone with a pointed tip. Significant dilation occurs in the failing material. With 5-10 MPa support pressure in the hole, the rock is greatly strengthened and stabilized. Macroscopic splitting fractures, and therefore detached spalled pieces, do not form. With high enough stress, microcrack interaction eventually results in the formation of what appear to be bands of shear localization, but the rock still maintains significant cohesion. (Author/TRRL)

The influence of foliation on point load strength anisotropy of foliated rocks

The engineering behaviour of rocks depends upon whether the fabric is isotropic or anisotropic, which is a reflection of its genetic environment. Even though anisotropy in fabric can occur in nearly all natural environments, metamorphic terrains usually offer the main source of anisotropic fabric elements. Metamorphic anisotropic fabrics, collectively known as foliation, may be quantified by a foliation index determined from optical microscopic or X-ray diffraction analysis depending upon the degree of coarseness or fineness of the rock. On the basis of this index, the role of foliation in conditioning anisotropy in point load strength has been quantified. The practical significance of the resultant classification scheme lies in the fact that reliable predictions of point load strength anisotropy of foliated rocks can be made on the basis of a critical analysis of their primary anisotropic fabrics—foliation.

Experimental and analytical studies on compressive strength of anisotropic rocks : Borecki, M; Kwasniewski, M In: Application of Analytical Methods to Mining Geomechanics (paper to the Proceedings of the 7th Plenary Session of the International Bureau of Rock Mechanics, World Mining Congress, Katowice, 24–26 June 1981)P23–49. Publ Rotterdam: A. A. Balkema, 1982

Experimental and analytical studies on compressive strength of anisotropic rocks : Borecki, M; Kwasniewski, M In: Application of Analytical Methods to Mining Geomechanics (paper to the Proceedings of the 7th Plenary Session of the International Bureau of Rock Mechanics, World Mining Congress, Katowice, 24–26 June 1981)P23–49. Publ Rotterdam: A. A. Balkema, 1982

Elastic constants and tensile strength of anisotropic rocks : Amadei, B; Rogers, J D; Goodman, R E Proc 5th Congress of the International Society for Rock Mechanics, Melbourne, 10–15 April 1983V1, PA189–A196. Publ Rotterdam: A. A. Balkema, 1983

A closed-form solution is proposed for the distribution of stresses and strains within a thin disc of anisotropic material loaded diametrically by a line or a strip load. This solution is then incorporated into a procedure to measure the elastic constants and the tensile strength of rocks that can be idealized as linearly elastic, homogeneous, orthotropic or transversely isotropic continua. The number of the covering abstract of the congress is TRIS no. 385148. (Author/TRRL)

Experimental studies of the influence of rock anisotropy on size and shape effects in point-load testing

Size and shape effects in point-load testing of anisotropic rock were experimentally shown to be independent of the degree of rock anisotropy and independent of the loading direction. Standard size and shape correction factors could therefore be applied to the four rock types tested (augen gneiss, ruhr sandstone, chiandone gneiss and nuttlar slate). The obtained accuracy is sufficient for an index value. Because of the wide range of the degree of anisotropy of the point-load strength examined in this study, the results should apply to all other rocks considering even the small number of investigated rock types. Specimens of standard size (diametrally compressed cores of 50 mm dia.) should be tested whenever possible. The existence of size and shape correction factors permits, however, the testing of specimens of arbitrary geometries, within certain limits. Maximum specimen size is determined by the load capacity of the testing machine. Specimens of less than 30 mm dia should not be used, since there the loaded areas can no more be considered to be point-shaped in relation to the specimen size. Estimation of uniaxial compressive strength of anisotropic rock by means of point load strength index can lead to significant errors. The point-load strength index should therefore be used as an independent parameter and additional unconfined compression tests should be performed whenever required. (TRRL)

Anisotropy of Rocks and Effect of Water Absorption

The deformation and movement developed by the geological structure in some regions yield faults, fractures, folds and so on. Teref ore approach to the strength properties of rock mass is led by the understanding of the orientations of strength anisotropy of rocks due to these geological structures.In this paper the authors described the result of an experimental study on the change of strength and deformation characteristics with water absorption, taking their anisotropy into account. On the one hand tensile strength of rocks has been determined by the point load test on untrimmed test specimens to know the influence of water absorption to strength. On the other hand elastic wave velocities of specimens have been measured and the dynamic moduli Ed have been calculated in view of their deformation anisotropy.The tensile strength is denoted “σta” and “σtc” respectively in the direction normal and parallel to the discontinuity planes such as joint surfaces. Similarly the dynamic modulus in the direction normal to the discontinuity planes is called “Eda” and that parallel to them “Ede”.The results obtained as to the dynamic modulus and point load test are summarized as follows:-(1) Degree of strength anisotropy shown by point load test, “σta/σtc” and that by dynamic modulus, “Eda/Edo” correlate nearly with a straight line.(2) While degree of anisotropy of dynamic modulus “Eda/Edc” is decreased, that of strength anisotropy “σta/σtc” is increased when water is taken into discontinuity planes.

The Role of Rock Strength Anisotropy in Natural Hole Deviation

The influence exerted by the interaction of rock and bit is a major factor preventing a comprehensive solution to the problem of natural hole preventing a comprehensive solution to the problem of natural hole deviation. Described here is an analysis of the bit-tooth chipping mechanism in dipping laminated anisotropic rock. There are encouraging indications that the tendency of a bit to deviate in such rock can be diminished by proper bit-tooth design. Introduction The inability to drill a straight vertical hole economically in dipping bedded rock constitutes a major problem in the drilling industry today. The crooked problem in the drilling industry today. The crooked hole frequently impedes drilling progress and raises drilling costs when light bit weights are needed to control hole angle. Often remedial action must be taken, such as runs with the Dynadrill, the Bit Boss, or a whipstock to overcome natural hole deviation tendencies, so that the hole will penetrate the target area. Since the early 1950's many theories have been advanced to explain hole deviation. The significant work to date has been in drillstring mechanics and has resulted in the design of drillstrings to control the rate of change of hole angle. The major factor yet to be fully understood with regard to natural hole deviation is the interaction of the bit and the rock. Once this factor can be put in analytical form, it should be possible to design bits and drillstring hookups for straight hole drilling. It has been shown that one of the causes of hole deviation is the nature of the rock failure under the bit. When a conventionally shaped wedge penetrates a dipping laminated anisotropic rock the chips formed on each side of the wedge are not equal in size. This is in contrast with the behavior of the homogeneous and isotropic rocks as revealed in studies by various authors. This phenomenon, referred to here as "preferred chip formation," has been correlated with natural hole deviation tendencies as observed in the Canadian foothills. In this paper, a theory is presented that relates this phenomenon to hole deviation. presented that relates this phenomenon to hole deviation. Theoretical expressions have been derived that predict the occurrence of preferred chip formation in terms of the anisotropic strength characteristic of the rock, the dip of the beds, and the included angle of the bit tooth. The theory has been used to design a wedge geometry for preventing preferred chip formation in dipping laminated rock. The tooth design has been incorporated into a bit, and to date, 27 field tests have been run. The results of the tests are encouraging. Discussion To illustrate in a meaningful manner the role of rock strength anisotropy in natural hole deviation, a number of individual concepts must be discussed. In the most logical sequence, these concepts are: the nature of preferred chip formation, the role of preferred chip formation in natural hole deviation, preferred chip formation in natural hole deviation, the strength characteristics of anisotropic rock, the proposed wedge penetration model for anisotropic rock, and the effect of wedge geometry on preferred chip formation. Then the results of tile preferred chip formation. Then the results of tile initial field test of the experimental deviation-control rock bit will be discussed. JPT P. 1313