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The estimation of the mechanical properties of rock joints is crucial in terms of safety when it comes to design of slopes in open pit mines or caverns used for the storage of hazardous materials, for instance - nuclear waste. Photogrammetry provides a simple, objective method for joints roughness assessment, without the need for expensive and time consuming laboratory tests or subjective empirical methods. In this study, a new photogrammetric method was used to estimate the roughness, shear strength and friction angle of a discontinuity of 2 m by 1 m fresh rock joint. The estimation was done by analyzing the profiles of digital models of joint surface. Surface Length and Slope Measurement methods were used to calculate the values of Joint Roughness Coefficient (JRC) of analyzed surfaces. Next, the shear strength and friction angle of the rock discontinuity were obtained experimentally with multistage pull testing. The results obtained with both methods were analyzed and compared. JRC values from photogrammetrically created digital models of the joint surface were overestimated due to the low density of the models, which resulted in high noise to signal ratio. Shear strength obtained with photogrammetrically created models were overestimates in relation to the results of the pull test by approximately 45 %. The errors made during this research are analyzed in the article and recommendations on how to improve reliability of the results are made. Main error in photogrammetric prediction was low density of the point clouds and in laboratory test too low stiffness of the test arrangement. The alternative methodology for photogrammetric studies used in previous stage of the research project was tested during this study and was proven to give significantly higher accuracy of generated digital models. The stiffness of the testing machine and proper positioning of the sample halves on top of each other were identified as the most sensitive aspects of methodology of big scale pull test when it comes to the reliability of results.
The determination of shear strength of rock discontinuities is an object of research since the middle of the last century, yet the developed models and failure criterions are based on simplifications which are the topic of ongoing discussion in the field of rock mechanics . The reason for that is the multiplicity and complexity of the parameters affecting the value of the shear strength of a joint. Those parameters include joint surface condition (dry, wet, submerged, weathered, unweathered), roughness of the joint surface, matedness (matching) of the opposites of a joint, compressive strength of a joint, normal load which the joint is subjected to and the mineral composition of the jointed rock, which determines its basic friction angle . The parameter which is the most challenging to quantify is the roughness of a joint surface. That is mainly due to the anisotropic character of natural joints. Directional variation in the joint roughness results in the different shear strength of the same joint depending on the direction of shearing . Therefore, most commonly used method of determining the roughness - Joint Roughness Coefficient (JRC) profiling , ISRM suggested method  is considered subjective by significant amount of investigators, since it only quantifies the roughness in one direction and involves a human decision on where to measure the shape of the profile, and then to match the obtained profile with a reference [3, 6, 7].
For low stress mining and civil engineering projects, slope stability is an essential part of safety and financial considerations. While large-scale stability can be modelled using equivalent rock mass properties, at smaller scale the local variations become significant and failure along the fracture planes is possible. The relevant boundary condition for low stress conditions is the Constant Normal Load (CNL), which allows for dilatation to occur. For deep mines the corresponding condition is the Constant Normal Stiffness, which restricts the dilatation. When dilatation is supressed the normal load is increased. This may lead to shearing of the asperities. If both the vertical and horizontal displacement are recorded during CNL testing, the dilatation may be calculated and numerically removed to provide a CNS estimate. In this paper three rock joint samples from the Siilinjärvi open pit mine were tested in a CNL shear box using displacement resetting at three different low normal loads. Profilometer and photogrammetry were used to measure the roughness of the surfaces. The results show poor match between the expected behaviour and observations. One possible cause is the reduced matedness often associated with natural near-surface rock joints. The potential weaknesses of the method are discussed and future research topics are suggested.
Rock mass discontinuities such as rock joints or fractures control the failure of rock mass. The normal stress acting on the discontinuity strongly influences shear strength. In deep excavations the surrounding rock mass resists displacement and limits the dilatation. In near surface excavations, the normal load is constant and there is no normal stiffness. Typically two boundary conditions are used: either the near-surface Constant Normal Load (CNL) or the deep Constant Normal Stiffness (CNS) (Figure 1). Poturovic (2015) examined this effect in his Master's Thesis and a synopsis of the results was published as a conference paper Poturovic et al. (2015). Poturovic et al. (2015) argue that friction angle and dilatation are key factors in determining the shear strength. AI high normal loads of 8 MPa or more the dilatation potential is similar between the methods. The results are consistent with earlier results by Grasselli & Egger 2003, where the dilatation potential was shown to be completely suppressed at normal stress of 20 % of the uniaxial compressive stress.
ABSTRACT: Strength, deformability and fluid flow properties of rock joints are to a great extent controlled by the surface roughness. The hydraulic conductivity of a rock fracture depends on the aperture distribution, the surface roughness and the contact area, each of which depend on the stresses that act on the fracture plane. However, despite much recent work, a complete understanding of the relationship between void space geometry, contact areas, applied stress, and hydraulic conductivity has yet to be achieved. Moreover, although in situ rock joints are subjected to both normal and shear loading, few studies have been conducted to investigate the effect of shear displacements on fluid flow through a single rock fracture, and which have addressed the issue of flow anisotropy induced by shearing. Experimental flow tests show that with increasing shear displacement, the fracture becomes heterogeneous and anisotropic, and becomes more permeable in the direction parallel to the displacement (Yeo et al., 1998). On the other hand, dry shear tests show that the surface fails in areas facing the shear direction (Grasselli and Egger, 2003). Moreover, the experimental results confirm that the shear strength of rock joints is direction-dependent.
RESUME: La rugosite des joints en roche joue un rôle primaire sur la reponse hydromecanique du massif fracture. En effet, la conductivite hydraulique des discontinuites en roche est influencee par la rugosite, et par la distribution des ouvertures et des zones de contact, dont chacun depend de l'effort normal et du cisaillement qui agissent sur la surface du joint. Les essays d'ecoulement en laboratoire montrent que si cisaillee, la fracture devient anisotrope, et plus permeable dans la direction parallèle au deplacement (Yeo et al., 1998). D'autre part, les essais de cisaillement indiquent que seules les zones de la surface qui font face à la direction de cisaillement sont impliquees dans le cisaillement (Grasselli et Egger, 2003). D'ailleurs, les resultats experimentaux confirment que la resistance au cisaillement des joints de roche est dependante de la direction. Dans cet article, plusieurs fractures en roche ont ete analysees en utilisant un système de mesure topographique à l'Universite de Bale. La methode d'analyse de donnees proposee par Grasselli (Grasselli et al., 2002a) a ete employee pour calculer les paramètres de la surface qui determinent la resistance au cisaillement.
ZUSAMMENFASSUMG: Die Festigkeit, Verformbarkeit und Hydromechanik von geklueftetem Fels wird hauptsachlich von der Oberflachenrauhigkeit der Bruchflachen bestimmt. Die hydraulische Leitfahigkeit einer Kluft hangt von der Verteilung der Rissbreiten, der Oberflachenrauhigkeit und den Kontaktflachen ab, die ihrerseits von dem auf die Bruchflache wirkenden Druck beeinflusst werden. Obgleich in letzter Zeit viel in dieser Richtung geforscht wurde, kann der Zusammenhang zwischen Porenraumgeometrie, Kontaktflachen, wirkendem Druck und hydraulischer Leitfahigkeit noch nicht vollstandig erklart werden. Obwohl Kluefte in situ sowohl Normal- als auch Schubspannungen ausgesetzt sind, wurde der Einfluss von Scherverschiebungen auf den Kluftwasserfluss und die daraus resultierende Fliessanisotropie in einer einzelnen Kluft bisher kaum untersucht. Experimentelle Untersuchungen des Durchflusses zeigen, dass die Kluft fuer größer werdende Scherverschiebungen heterogen und anisotrop wird wobei die Permeabilitat parallel zur Verschiebungsrichtung ansteigt (Yeo et al., 1998). Andererseits zeigen trockene Scherversuche, dass die Oberflache an Stellen versagt, die gegen die Verschiebungsrichtung anstehen (Grasselli und Egger, 2003). Des weiteren bestatigen die experimentellen Ergebnisse eine Richtungsabhangigkeit der Kluftscherfestigkeit. In diesem Artikel wird eine dreidimensionale morphologische Beschreibung mehrerer natuerlicher, durch Scherspannungen verschobener Kluefte vorgestellt, die vom Automated Topographic System der Universitat von Basel erstellt wurde.
The assessment and management of natural resources, as well as of risks associated with natural hazards, play a crucial role for the sustainable development of a liveable and safe environment. One of the major problems for efficient management is the current lack of a thorough understanding of the physical processes involved in most kinds of natural phenomena.
In rock mechanics and petroleum engineering, it is known that the macroscopic behaviour of rock masses and reservoirs are to a great extent controlled by the presence of joints, bedding planes, faults, and other recurrent planar features. The hydro-mechanical behaviour of the rock mass on a large scale is controlled by the density and orientation of these features, and also by physical properties of the individual joints and fractures themselves. In turn, the properties of the individual joints and fractures, such as their strength, deformability, and permeability, are greatly influenced by the surface roughness of the joint surfaces.
Among all factors that influence the behaviour and the stability of rock masses, fractures play a major role. Therefore, their full understanding is important in all fields of rock mechanics (i.e., tunnelling, rock slope stability, foundations in rock, etc.).
ABSTRACT: Joint roughness directly affects the behaviour of discontinuities at the laboratory scale and the rockmass at excavation scales. As large infrastructure projects (e.g. deep base tunnels for transportation, large open pit and underground mines with complex geometries, and deep geological repositories for the storage of nuclear waste with unprecedented design life spans) are being constructed in more challenging environments, the need for sound understanding of the geomechanical behaviour of rock joints and in turn the roughness of joint surfaces is paramount. This paper presents practical methods for the analysis of joint surface roughness from three-dimensional Structure-from-Motion (SfM) photogrammetry models and two-dimensional cylindrical core scan photographs. In order to understand the variability of joint roughness on the surface of a discontinuity through drill core, hundreds of profiles were measured with JRC values calculated for each profile. Through this process, the variability of JRC values based on two-dimensional profiles across a surface is highlighted. Finally the effectiveness of cylindrical core scans is evaluated by comparing the roughness estimates to scanlines taken across the surface of the discontinuity.
Discontinuities, such as joints and bedding, are a critical geomechanical component of a rockmass and govern behaviour around rock engineering projects such as natural slopes, surface excavations and underground infrastructure such as tunnels or caverns. The topology or degree of roughness of a discontinuity directly affects the shear strength characteristic of the discontinuity and ultimately the rockmass. Degrees of roughness and angles of asperities have been incorporated into geomechanical numerical models by means of constitutive models that quantify discontinuity mechanics (e.g. Patton, 1966; Barton and Bandis, 1990). One of the most widely used parameters to describe the roughness of a discontinuity is the Joint Roughness Coefficient (JRC) (Barton and Choubey, 1977). At its inception, JRC was visually compared to standardized profiles published by Barton and Choubey (1977). Visual comparisons can be subjective based on a user's level of experience (Beer et al., 2002; Stigsson, 2018). As a measure to remove this bias, several approaches have been made to quantify roughness through statistical (Myers, 1962; Tatone and Grasselli, 2010), fractal (Kulatilake et al., 2006) and signal analysis methods (Pickering and Aydin, 2016; Wang et al., 2019).