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.  

Bergssprickor påverkar bergmassans stabilitet. Bergssprickors skjuvhållfasthet utgör därför en betydande faktor för vilka laster en berganläggning skall dimensioneras mot. Numeriska och teoretiska modeller används för att prediktera bergssprickors skjuvhållfasthet. Modellerna valideras mot data från laboratorietester. Processen att ta fram data från laboratorietester består av flera delar, vilka var och en innehåller olika typer av osäkerheter. Inga modeller är dock bättre än noggrannheten på resultaten de valideras mot. Med hjälp av data från ett omfattande experimentellt testprogram har därför ett antal tillvägagångssätt utvecklats, vilka syftar till att minska osäkerheterna inom olika delar av dataframtagningsprocessen. 

Direkta skjuvtester utfördes på fyrtiosex bergprov av granit med både naturliga och artificiellt spänningsinducerade sprickor. Flera tester utfördes för varje parameterkombination, vilket möjliggjorde statistiska utvärderingar. Testerna utfördes i kontrollerad laboratoriemiljö under både konstant normalspänning och konstant normalstyvhet. Dessa tester utfördes under förhållanden som inte studerats tidigare genom kombinationen av spänningar motsvarande djup på flera hundra meter och en bred fördelning av sprickareor representerade av tre provkroppsstorlekar: 35 mm × 60 mm, 70 mm × 100 mm and 300 mm × 500 mm. Dessutom tillverkades femton repliker av höghållfast betong från en av granitsprickorna med storleken 70 mm × 100 mm för geometriska utvärderingar. Sex av dessa repliker genomgick direkta skjuvtester och deras skjuvmekaniska beteende utvärderades mot bergssprickans. 

Två kvalitetssäkringsparametrar för replikers sprickgeometri har tagits fram, vilka tillsammans med en metod för framtagning av parametergränsvärden  reducerar osäkerheten i parameterstudier. Ett tillvägagångssätt har utvecklats där den effektiva normalstyvheten först beräknas och därefter anges som styrparameter i tester under randvillkoret konstant normalstyvhet. Den effektiva normalstyvheten eliminerar i princip felet i pålagd normallast orsakad av testsystemets normalstyvhet. Förbättrad noggrannhet vid förskjutningsmätningar har uppnåtts genom tillämpning av optiska förskjutningsmätningar direkt över sprickprofiler. Direkta mätningar eliminerar felen i konventionella mätningar, vilka inkluderar oönskade, men oundvikliga, bidrag härrörande från glapp och deformationer i testsystemet. Tillvägagångssätt som möjliggör konsistent och fysikaliskt baserad framtagning av både skjuvhållfasthet och skjuvstyvhet har tagits fram. Variansanalys visar att skjuvhållfastheten för de testade bergproven inte påverkas av provstorleken, medan skjuvstyvheten gör det. Sammantaget skapas förbättrad noggrannhet i data från laboratorietester av bergssprickor, vilket utgör en grund för förbättrade modeller för att prediktera deras skjuvhållfasthet. 

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, sigma(mf), captures morphological deviations between the replica and the rock joint surfaces. sigma(mf) 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 sigma(mf) <= 0.06 mm. The second parameter is a vector, V-Hp100, 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. vertical bar V-Hp100 vertical bar 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), sigma(mf) and V-Hp100 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, sigma(mf) and V-Hp100, 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 sigma(mf) < 0.06 mm and vertical bar V-Hp100 vertical bar < 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.

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. 

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.

Applying accurate normal load to a specimen in direct shear tests under constant normal stiffness (CNS) is of importance for the quality of the resulting data, which in turn influences the conclusions. However, deficiencies in the test system give rise to a normal stiffness, here designated as system normal stiffness, which results in deviations between the intended and actual applied normal loads. Aiming to reduce these deviations, this paper presents the effective normal stiffness approach applicable to closed-loop control systems. Validation through direct shear tests indicates a clear influence of the system normal stiffness on the applied normal load (13% for the test system used in this work). The ability of the approach to compensate for this influence is confirmed herein. Moreover, it is demonstrated that the differences between the measured and the nominal normal displacements are established by the normal load increment divided by the system normal stiffness. This further demonstrates the existence of the system normal stiffness. To employ the effective normal stiffness approach, the intended normal stiffness (user defined) and the system normal stiffness must be known. The latter is determined from a calibration curve based on normal loading tests using a stiff test dummy. Finally, a procedure is presented to estimate errors originating from the application of an approximate representation of the system normal stiffness. The approach is shown to effectively reduce the deviations between intended normal loads and the actual applied normal loads.

The presence of joints in rock masses influences the structural integrity of geotechnical structures. A critical failure mode is shearing, thus making the shearing process of importance to understand. Historically, studies have been mainly executed on the basis of laboratory experiments, since full-scale in situ tests are seldom performed due to technical and economic considerations. Since each rock joint is unique by nature, the utilization of replicas is applied to carry out controlled experimental parameter studies. However, the manufacturing process of replicas introduces many sources of uncertainty. Therefore, in this work the influence of geometrical variations in replicas on the shear strength characteristics is evaluated, mutually as well as in relation to the mother rock specimen of the replicas. The joint surfaces were 3D scanned and the contact area of the joint was measured using pressure sensitive film before direct shear tests. Deviations in morphology were evaluated by surface comparisons between the joint surfaces of the mother rock and replicas. The initial matching of the joints was evaluated by calibrating the scanning data with respect to the contact area measurements. It could be visualized that geometrical deviations were caused by rock fragments coming off during mould production, positioning of the moulds and pores resulting from replica casting. These factors were found to influence the shear strength characteristics of the replicas. The influence of the deviations originating from morphology on the joint matching is demonstrated. In summary, it is shown that replicas with similar shear strength characteristics as rock can be manufactured, but even small deviations affect the characteristics, in particular the peak strength. Therefore, parameters relevant for geometrical quality assurance should be identified along with required value ranges. Selected introductory results on quantified parameters for geometrical quality assurance are presented, serving as a basis for continued work.