Tensile Strength of Geological Discontinuities Including Incipient Bedding, Rock Joints and Mineral Veins

Geological discontinuities have a controlling influence for many rock-engineering projects in terms of strength, deformability and permeability, but their characterisation is often very difficult. Whilst discontinuities are often modelled as lacking any strength, in many rock masses visible rock discontinuities are only incipient and have tensile strength that may approach and can even exceed that of the parent rock. This fact is of high importance for realistic rock mass characterisation but is generally ignored. It is argued that current ISRM and other standards for rock mass characterisation, as well as rock mass classification schemes such as RMR and Q, do not allow adequately for the incipient nature of many rock fractures or their geological variability and need to be revised, at least conceptually. This paper addresses the issue of the tensile strength of incipient discontinuities in rock and presents results from a laboratory test programme to quantify this parameter. Rock samples containing visible, natural incipient discontinuities including joints, bedding, and mineral veins have been tested in direct tension. It has been confirmed that such discontinuities can have high tensile strength, approaching that of the parent rock. Others are, of course, far weaker. The tested geological discontinuities all exhibited brittle failure at axial strain less than 0.5 % under direct tension conditions. Three factors contributing to the tensile strength of incipient rock discontinuities have been investigated and characterised. A distinction is made between sections of discontinuity that are only partially developed, sections of discontinuity that have been locally weathered leaving localised residual rock bridges and sections that have been ‘healed’ through secondary cementation. Tests on bedding surfaces within sandstone showed that tensile strength of adjacent incipient bedding can vary considerably. In this particular series of tests, values of tensile strength for bedding planes ranged from 32 to 88 % of the parent rock strength (intact without visible discontinuities), and this variability could be attributed to geological factors. Tests on incipient mineral veins also showed considerable scatter, the strength depending upon the geological nature of vein development as well as the presence of rock bridges. As might be anticipated, tensile strength of incipient rock joints decreases with degree of weathering as expressed in colour changes adjacent to rock bridges. Tensile strengths of rock bridges (lacking marked discolouration) were found to be similar to that of the parent rock. It is concluded that the degree of incipiency of rock discontinuities needs to be differentiated in the process of rock mass classification and engineering design and that this can best be done with reference to the tensile strength relative to that of the parent rock. It is argued that the science of rock mass characterisation may be advanced through better appreciation of geological history at a site thereby improving the process of prediction and extrapolating properties

Fracturing and indirect tensile strength of brittle and ductile rocks

Rock failure can either be referred to as brittle, ductile, or at the brittle-ductile transition. Brittle failure is said to occur when the ability of the rock resist load decreases with increasing deformation. Brittle failure is associated with the materials that undergo little to no permanent deformation before failure and, depending on the test conditions, may occur suddenly and catastrophically. Ductile failure is said to occur when the material can sustain permanent deformation without losing its ability to resist loading (without failing). Brittleness may also be defined in terms of the ratio of specific elastic strain energy at the fracture to total specific strain energy at the fracture when a fracture is accompanied by plastic deformation, it is called a ductile fracture and when plastic deformation is absent it is called a brittle fracture. There are different definitions of rock brittleness in rock mechanics literature; however, Uniaxial Compressive Strength (UCS) and BrazilianTensile Strength (BTS) values are very important parameters to determine brittleness and ductility of rocks. Thus, the study presents the obtained BTS values of rocks by using standard Brazilian Jaws. As the Brazilian test has been criticized since it was initially proposed by ISRM, the aim of this study to investigate the accuracy of Brazilian jaws to determine the BTS values of brittle and ductile rocks separately. FRANC2D software was used to analyse the stress distribution at the centre of disk specimens by considering ductility and brittleness of rocks. Both experimental and numerical results showed that both BTS values and fracturing of rocks were found depended on the brittleness and ductility of rocks and accuracy of using the proposed standard Brazilian jaws may change depending on ductility and brittleness of rocks

The effect of cyclic loading amplitude and notch crack inclination angle on the fracture toughness test on Brisbane tuff-multiple factorial analyses

The Cracked Chevron Notch Brazilian Disc (CCNBD) method has been used to determine rock fracture toughness value of rocks in rock mechanics since many years. The CCNBD method has advantages over the other proposed fracture toughness tests in terms of the simplicity of sample preparation and less material requirement for testing. In this study, the specifications of Brisbane tuff sample geometry have been selected according to the suggestions of International Society of Rock Mechanics (ISRM). The main aim of this research is to evaluate the effect of amplitude variations and change of chevron notch angles on the fracture toughness of rocks under both static and cyclic loading. The cyclic loading was applied in three different levels of amplitudes, 10%, 20% and 30% of the static ultimate loading (SUL). In addition, three different chevron crack angles 30°, 45° and 60° have been chosen to investigate the effect of initial crack angle on the rock fracture toughness value. A series of multiple variation analyses by using Analysis of Variance (ANOVA) were carried out in this study to evaluate the effect of amplitude and inclination angle of chevron crack on the rock fracture toughness value of rocks. Statistical results demonstrated that rock fracture toughness values are very sensitive with the change of amplitudes less than 20% of SUL whereas rock fracture toughness is less sensitive with the change of amplitudes between 20% and 40% of SUL. Moreover, 45? notch crack inclination angle used to investigate the mixed Mode I-II fracturing behavior of rocks was found the most critical inclined pre-existing crack under various amplitude cyclic loading in other crack inclination angles. These outcomes are believed very important findings for many rock mechanics applications such as investigations of behavior of bedded rocks, anisotropic rocks and discontinuities in rock masses encountered with dynamic loads and fatigue

A numerical study on Oscillating Disc Cutting (ODC) technology for hard rock cutting

Rock cutting includes many unknown parameters which result in different consequences of the different cutting conditions. However, based on the current suggested models for rock cutting, there are some critical parameters that control the fracture initiation and propagation in the rock domain. In general, there are two major cutting tools are used in rock cutting: drag bits and indenters. Different mechanical attacks of cutters result in different fracture models. For example drag bits break the rock parallel to the rock surface, which is called 'undercutting' while indenters cut rocks by pressing normal to the rock surface. Both tools cause rock failure by inducing tensile cracks in the rock. However, drag bits induce more tensile cracks compared with the indenters. The full benefits of improvement in rock cutting efficiency will arise from a clear understanding of rock fracture mechanics principles. Therefore, fracture mechanics principles will be applied to the rock cutting as the rock fragmentation process is based on the rock fracturing. The Oscillating Disc Cutting (ODC) has more advantages than drag bits and disc cutters. The forces transmitted back through the cutting head are much lower with ODC. Although ODC attacks rock in much the same way as drag bits, a combination of the oscillating action of the cutter and the undercutting mode produces the lower cutting forces and causes rock fatigue. Fatigue phenomenon in rock has advantages for the rock breaking by applying less effort and energy due to the cyclic loading. The aim of this study is using the Extended Finite Elements Analyses (XEFM) numerical modelling for simulating the ODC cutting with different cyclic loading conditions. An accurate prediction of numerical investigation requires detail laboratory experiments. A series of laboratory tests were performed to obtain the fracture toughness values of our Monsonite rock samples under various cyclic amplitude and frequency values. The numerical results showed different types fatigue damage mechanisms due to the various cyclic loading conditions. Thus, modelling and understanding the crack propagation behaviour depending on different amplitude and frequencies are believed very important for the dynamic rock cutting researches and rock cutting machine manufacturers

An innovative and effective approach to hard rock cutting

One of the most important problems of hard rock cutting machines is machines are able to cut the rocks having strength up to a certain value (e.g. 250MPa) with very high energy costs. This study will find applications in many engineering areas such as mining, tunnelling and drilling etc. This research focuses on ‘fatigue of rock/granular material’ and its technical application in hard rock cutting machinery and mining. Oscillating Disc Cutting (ODC) – Australia technology has been developed with the latest technology in this field in the world and is capable of breaking very hard rock, with strength up to 350MPa, at acceptable-to-good excavation rates by using 70% less cutting forces and energy compared with the standard hard rock cutting machines. Of course there are many rock cutting parameters including machine parameters such as attack angle of cutters and torque values etc. and rock parameters such as rock strength and brittleness etc. However, this research just focuses on the behaviour of the rock-tool interaction zone under various cyclic loading types. Research results showed the developed Fracture Process Zone (FPZ) under cyclic loading is much bigger than the FPZ developed under monotonic loading even with the use of lower forces

Experimental and numerical studies on development of fracture process zone (FPZ) in rocks under cyclic and static loadings

The development of fracture process zones (FPZ) in the Cracked Chevron Notched Brazilian Disc (CCNBD) monsonite and Brisbane tuff specimens was investigated to evaluate the mechanical behaviour of brittle rocks under static and various cyclic loadings. An FPZ is a region that involves different types of damage around the pre-existing and/or stress-induced crack tips in engineering materials. This highly damaged area includes micro- and meso-cracks, which emerge prior to the main fracture growth or extension and ultimately coalescence to macrofractures, leading to the failure. The experiments and numerical simulations were designed for this study to investigate the following features of FPZ in rocks: (1) ligament connections and (2) microcracking and its coalescence in FPZ. A Computed Tomography (CT) scan technique was also used to investigate the FPZ behaviour in selected rock specimens. The CT scan results showed that the fracturing velocity is entirely dependent on the appropriate amount of fracture energy absorbed in rock specimens due to the change of frequency and amplitudes of the dynamic loading. Extended Finite Element Method (XFEM) was used to compute the displacements, tensile stress distribution and plastic energy dissipation around the propagating crack tip in FPZ. One of the most important observations, the shape of FPZ and its extension around the crack tip, was made using numerical and experimental results, which supported the CT scan results. When the static rupture and the cyclic rupture were compared, the main differences are twofold: (1) the number of fragments produced is much greater under cyclic loading than under static loading, and (2) intergranular cracks are formed due to particle breakage under cyclic loading compared with smooth and bright cracks along cleavage planes under static loading

A simulative model for optimum open pit design

Previous research in the area of open pit mine optimization has focused on profit maximization by optimizing pit limits or optimizing production schedules. However, these two approaches are interrelated and dependent on each other. Recent studies have recognized the limitation of this optimization approach, Consequently, there is a need for a more comprehensive modelling strategy. Such an optimization model should incorporate mining activities that are simulated (as they are expected to occur) as much as possible. Considerations should be given to such parameters as the cut-off ore grade, the starting location of mining, equipment selection, annual production rates, excavation sequence, and the synchronization of the mining plant operations during the planning stage of the mine. A change in any of these parameters will cause a concomitant change in other parameters. Therefore an efficient optimization system is one that attempts to simulate the mine in a comprehensive manner and thereby provides more realistic results

Determination of Indirect Tensile Strength (ITS) of rocks

Being brittle materials, rocks exhibit different behaviours under tensile and compressive loading. The tensile strength of rock is orders of magnitude less than its compressive strength. Hence, the determination of the tensile strength of rocks is of crucial important for both civil and rock mechanics applications, such as tunnelling, underground mining, underground repositories, etc. The difficulties associated with performing a direct uniaxial tensile test on a rock specimen have led to a number of indirect methods for assessing the tensile strength. In 1978, the Indirect Brazilian Test (IBT) was officially proposed by the International Society of Rock Mechanics (ISRM) as a method for determining the indirect tensile strength of rocks. However, the Brazilian test has been criticised by many researchers since its introduction. The standard Brazilian indirect tensile test causes catastrophic crushing failure of the disc specimens, rather than the expected tensile splitting failure initiated by a central crack. This finding led to the current investigation of the effect of loading conditions on the failure of Brazilian disc specimens using three steel loading arcs of different angle applied to rock disc specimens. With no international standard for determining the indirect tensile strength of rocks under diametral loading, numerical modelling and analytical calculations were undertaken. Numerical simulations using the RFPA2D software were conducted with a heterogeneous material model. The experimental and numerical results showed that 20° and 30° loading arcs result in diametral splitting fractures starting at the disc centre