Shearing of rock joints is a critical failure mode in rock masses. The shear strength of rock joints must, therefore, be considered in the design of structures in rock masses. Several criteria for prediction of shear strength have been proposed over the years. However, the possible effect of scale on shear strength is an issue. One possible reason for this is that previous experimental studies on the scale effect contain various sources of uncertainties (mixed test methods, multiple testing of same specimen, application of results to other materials than tested, and omitted handling of statistical dispersion). In this paper, the results from a uniquely comprehensive experimental laboratory program, that handles these uncertainties and also extends the range of previously tested conditions, is presented. 46 direct shear tests on two joint types, natural and tensile induced granite rock joints, have been performed under the constant normal stress and the constant normal stiffness boundary condition at 5 MPa initial normal stress applied over three specimen sizes (35 mm × 60 mm, 70 mm × 100 mm and 300 mm × 500 mm). Analysis of variance shows no effect of the specimen size on the shear strength, whereas the joint type and boundary condition has. Quantitative estimates of the influence of the joint type and boundary condition on the shear strength are presented. A consistent approach for determination of shear strength from the point of time associated with a shear stiffness change of the test system is also presented

Rock joints influence the stability of rock mass. Therefore, their shear strength is an important factor in determining the load a structure constructed on or in rock can withstand. Numerical and theoretical models are used to predict the shear mechanical behaviour of rock joints. These models are validated against data from laboratory testing. The data generation process from laboratory testing consists of several parts, each of which is associated with various sources of uncertainties. However, no model is better than the accuracy of the results it is validated against. Therefore, in this work, several approaches have been developed using data from a comprehensive experimental shear testing program, with the overall aim of reducing the uncertainties associated with various parts of the data generation process. Forty-six granite rock specimens containing both natural and artificially tensile induced joints were subjected to direct shear testing. Several tests were carried out for each parameter combination, allowing for statistical evaluations. The tests were conducted under controlled laboratory conditions under both constant normal stress and constant normal stiffness boundary conditions. These tests were performed under previously unexplored conditions, combining stresses occurring at depths of several hundreds of meters with a wide range of joint areas represented across three specimen sizes: 35 mm × 60 mm, 70 mm × 100 mm and 300 mm × 500 mm. In addition, fifteen replicas were manufactured from high-strength concrete based on one of the granite rock joints sized 70 mm × 100 mm for geometrical evaluations. Six of these replicas were subjected to direct shear testing and evaluated against the shear mechanical behaviour of the rock joint. Two quality assurance parameters for replica rock joints have been developed, which together with a method for establishing threshold limits of the parameters, reduces the uncertainties in parametric studies. An approach has been developed in which the effective normal stiffness is calculated and then inserted into the control system in tests under the constant normal stiffness boundary condition. The application of the effective normal stiffness essentially eliminates the error in applied normal load originating from the normal stiffness of the test system. Improved accuracy in displacement measurements has been achieved by applying optical displacement measurements directly over joint traces. Direct displacement measurements exclude errors in conventional measurements, which include undesired but unavoidable displacements originating from gaps and deformations in the test system. Approaches for consistent and physically based determination of both shear strength and shear stiffness have been developed. Analysis of variance shows that the shear strength of the tested rock joint specimens is not influenced by specimen size, whereas shear stiffness is. To sum up, the accuracy of data from laboratory tests is improved, constituting a prerequisite for improved models.  

Experimentella undersökningar av skjuvegenskaperna hos bergsprickor i hårt berg har generellt fokuserat på mindre sprickprover för normalspänningar på upp till 20 MPa, representativa för ett kärnbränsleförvar på ca 400 meters djup. Vid stora sprickprover har skjuvegenskaperna generellt bestämts för låga spänningar, på någon eller några MPa. För att få en förståelse för inverkan av sprickornas storlek för skjuvegenskaperna vid höga normalspänningar är det avgörande att genomföra skjuvförsök på stora bergsprickor (> 200 mm) i hårt berg under höga normalspänningar. I projektet Parameterization of Fractures, POST (2014–2016), studerades skaleffekterna genom in situ-försök, småskaliga skjuvförsök och beräkningssimuleringar. Det konstaterades att laboratorieexperiment under kontrollerade förhållanden och på stora bergprover är nödvändiga för att få tillförlitliga resultat. Det konstaterades också att in-situ-försök är komplexa med stora underliggande osäkerheter och är samtidigt kostsamma. I det pågående projektet POST 2 som startade 2017 har bergsprickor på upp till 500 mm provats i en ny unik laboratorieutrustning, jämte provning av mindre sprickor, vid höga normalspänningar för både CNL och CNS förhållanden och med ny mätteknik. Teknik för att tillverka replikaprover av bergsprickor har utvecklats och provats med syfte att göra lastparameterstudier. Kvaliteten hos geometriavbildningen hos replikaproverna och sprickornas geometrier har uppmätts med högupplöst skanning. I denna artikel presenteras en del av resultaten från projektet. Resultaten från projektet är tillämpbara för andra områden med undermarkskonstruktioner såsom projektering av tunnlar och bergrum för infrastrukturprojekt och gruvor.

Experimental investigations of the shear properties of rock fractures in hard rock have generally focused on tests on smaller fracture s for normal stresses of up to 20 MPa, representative of a nuclear waste repository at a depth of about 400 meters. T he shear properties of large fractures determined at experiments have generally been determined at low stresses , of one or more MPa. To gain an understanding of the effect of the size on the shear properties of the fractures at high normal stresses, it is crucial to perform shear experiments on large rock fractures (> 200 mm) in hard rock under high normal stresses. In the project Parameterization of Fractures, POST (2014 2016), the scale effects were studied by in situ experiments, small scale shear experiments and computational simulations. It was found that laboratory experiments under controlled conditions and on large rock samples are necessary to obtain reliable results. It was also found that in situ experiments are complex with large underlying uncertainties and are at the same time costly. In the ongoing POST 2 project, which started in 2017, rock fractures up to 500 mm have been tested in a new unique laboratory equipment, along with test s on smaller fractures , at high normal stress for both CNL and CNS conditions and with new measurement technology. Techniques for producing replica samples of rock fractures have been developed and tested with the aim of making load parameter studies. The quality of the geometry imaging of the replica samples and the geometries of the fractures has been meas ured by high resolution scanning. This article presents some of the results from the project. The results from the project are applicable to other areas with underground construction, such as the design of tunnels and rock caverns for infrastructure projects and mines.

Att kunna prediktera en bergssprickas skjuvhållfasthet är svårt i de fall där hela sprickytan inte är tillgänglig, såsom exempelvis en bergsspricka belägen under en befintlig betongdamm eller i projekteringsskedet för en tunnel. Denna artikel presenterar en metodik som undersöker möjligheten att utnyttja information från uppmätt sprickvidd och sprickans råhet i 3D med optisk scanning i mindre storlek, såsom borrkärnor, för att därefter prediktera skjuvhållfastheten i större skalor. Den framtagna metodiken har verifierats med två storskaliga skjuvförsök genomförda i laboratorium med konstant normallast med sprickprover tagna vid Krångede kraftstation. Den främsta nyttan med denna metodik är att den kan utgöra en möjlig väg framåt för att prediktera skjuvhållfastheten för bergssprickor i fall där sprickytan inte är helt tillgänglig.

The prediction of a rock joint’s peak shear strength becomes complex when its joint surfaces are not fully accessible, such as the rock foundation under an existing concrete dam or the design stage for a tunnel. This paper presents a methodology that investigates the possibility of using information from measured aperture and 3D roughness with optical scanning at smaller sizes, such us drill cores, to predict the peak shear strength of large natural, unfilled rock joints. The presented methodology has been tested in the laboratory under constant normal load conditions on two natural, unfilled rock joint samples obtained from existing rock joints in the foundation of the Krångede concrete dam. The main benefit of this approach is that it may enable the prediction of the peak shear strength in the field under conditions of difficult access.

Since each rock joint is unique by nature, the utilization of replicas in direct shear testing is required to carry out experimental parameter studies. However, information about the ability of the replicas to simulate the shear mechanical behavior of the rock joint and their dispersion in direct shear testing is lacking. With the aim to facilitate generation of high-quality direct shear test data from replicas, a novel component in the testing procedure is introduced by presenting two parameters for geometric quality assurance. The parameters are derived from surface comparisons of three-dimensional (3D) scanning data of the rock joint and its replicas. The first parameter, σmf, captures morphological deviations between the replica and the rock joint surfaces. σmf is derived as the standard deviation of the deviations between the coordinate points of the replica and the rock joint. Four sources of errors introduced in the replica manufacturing process employed in this study could be identified. These errors could be minimized, yielding replicas with σmf ≤ 0.06 mm. The second parameter is a vector, VHp100, which describes deviations with respect to the shear direction. It is the projection of the 100 mm long normal vector of the best-fit plane of the replica joint surface to the corresponding plane of the rock joint. |VHp100| was found to be less than or equal to 0.36 mm in this study. Application of these two geometric quality assurance parameters demonstrates that it is possible to manufacture replicas with high geometric similarity to the rock joint. In a subsequent paper (part 2), σmf and VHp100 are incorporated in a novel quality assurance method, in which the parameters shall be evaluated prior to direct shear testing. Replicas having parameter values below established thresholds shall have a known and narrow dispersion and imitate the shear mechanical behavior of the rock joint.

Each rock joint is unique by nature which means that utilization of replicas in direct shear tests is required in experimental parameter studies. However, a method to acquire knowledge about the ability of the replicas to imitate the shear mechanical behavior of the rock joint and their dispersion in direct shear testing is lacking. In this study, a novel method is presented for geometric quality assurance of replicas. The aim is to facilitate generation of high-quality direct shear testing data as a prerequisite for reliable subsequent analyses of the results. In Part 1 of this study, two quality assurance parameters, σmf and VHp100, are derived and their usefulness for evaluation of geometric deviations, i.e. geometric reproducibility, is shown. In Part 2, the parameters are validated by showing a correlation between the parameters and the shear mechanical behavior, which qualifies the parameters for usage in the quality assurance method. Unique results from direct shear tests presenting comparisons between replicas and the rock joint show that replicas fulfilling proposed threshold values of σmf < 0.06 mm and < 0.2 mm have a narrow dispersion and imitate the shear mechanical behavior of the rock joint in all aspects apart from having a slightly lower peak shear strength. The wear in these replicas, which have similar morphology as the rock joint, is in the same areas as in the rock joint. The wear is slightly larger in the rock joint and therefore the discrepancy in peak shear strength derives from differences in material properties, possibly from differences in toughness. It is shown by application of the suggested method that the quality assured replicas manufactured following the process employed in this study phenomenologically capture the shear strength characteristics, which makes them useful in parameter studies.

A direct shear equipment for testing rock fractures up to 400×600 mm size, and up to 5 MN force in both normal and shear loading directions, was developed. Normal loading and direct shear tests under constant normal stiffness (CNS) and constant normal load (CNL) conditions were conducted on 300×500 mm specimens, one planar steel joint and two natural and two tensile induced rock fractures. Design targets, e.g. system to maintain undisturbed fractures up to testing and high system stiffnesses to achieve well-controlled shear tests, were verified by the experiments. A new optical system for local deformation measurements was used to accurately determine fracture displacements besides conventional non-local deformation measurements. The determined normal stiffnesses were similar previous results from the literature on smaller fractures, whereas the shear stiffness data are novel. The results provide a new insight into processes at the onset of fracture slip.

The impact of shear displacement under different mechanical boundary conditions on fluid flow and advective transport in a single fracture at the laboratory scale is demonstrated in the present study. The shear-induced changes of fracture aperture structures are determined by using the measured normal displacements and digitalized fracture surfaces from laboratory shear tests. Five shear tests on concrete replicas of the same fracture under different mechanical boundary conditions, including constant normal loading (CNL) and constant normal stiffness (CNS), are conducted to analyse the influence of mechanical boundary conditions on the shear-flow-transport processes. Fluid flow in the fracture with different shear displacements are simulated by solving the Reynolds equation. The Lagrangian particle tracking method is applied to model the advective transport in the fracture after shearing. The results generally show that the shear displacements and the normal loading conditions can significantly affect flow patterns and advective travel time distributions in the fracture. For mated fractures, the flow and transport will be enhanced by the increasing shear displacement because of shear dilation. For cases with the same shear displacement, the median advective travel time increases with the increasing boundary normal stiffness. The median advective travel time under the CNS boundary condition is generally longer than that under the CNL boundary condition. The results from this study can help to improve our understanding of stress-dependent solute transport processes in natural rock fractures. © 2021 The Author(s)

Significant uncertainties remain regarding the field assessment of the peak shear strength of rock joints. These uncertainties mainly originate from the lack of a verified methodology that would permit prediction of rock joints’ peak shear strength accounting for their surface area, while using information available from smaller samples. This paper investigates a methodology that uses objective observations of the 3D roughness and joint aperture from drill cores to predict the peak shear strength of large natural, unfilled rock joints in the field. The presented methodology has been tested in the laboratory on two natural, unfilled rock joint samples of granite. The joint surface area of the tested samples was of approximately 500 × 300 mm. In this study, the drill cores utilised to predict the peak shear strength of the rock joint samples are simulated based on a subdivision of their digitised surfaces obtained through high-resolution laser scanning. The peak shear strength of the tested samples based on the digitised surfaces of the simulated drill cores is predicted by applying a peak shear strength criterion that accounts for 3D roughness, matedness, and specimen size. The results of the performed analysis and laboratory experiments show that data from the simulated drill cores contain the necessary information to predict the peak shear strength of the tested rock joint samples. The main benefit of this approach is that it may enable the prediction of the peak shear strength in the field under conditions of difficult access.

Experiments at constant normal stiffness (CNS) are normally carried out to understand underground shear processes of rock joints. However, in many test setups the available space around the joint is limited implying it is not possible to measure the dilatancy directly over the joint. Therefore, the displacement transducers must be in locations where the risk is that additional displacements originating from deficiencies in the test system will be measured causing too low normal loads to be applied. Herein, this issue is investigated in a new 5 MN direct shear test setup. The system normal stiffness was found to be about 11 300 kN/mm derived from normal loading up to 4.5 MN using a steel specimen. The direct shear testing performance under the CNS configuration was evaluated using the steel specimen, which had a joint with a known angle of inclination. The normal load error at 3.9 MN (28 MPa) was 11%, but by application of the effective normal stiffness approach using the system normal stiffness as input the error basically could be eliminated. The results demonstrate the robustness of the setup designed for joint areas up to 400 × 600 mm with normal and shear loads up to 5 MN.