Joints are the main features encountered in rock and sliding of rock blocks on joints is classified as the principal source of instability in underground excavations. In this regard, joints’ peak shear strength is the controlling parameter. However, given the difficulty in estimating it, shear tests are often performed. These are often quite expensive and also time consuming and, therefore, it would be valuable if shear tests could be artificially performed using numerical models. The objective of this study is to prove the possibility to perform virtual numerical shear tests in a PCF2D environment that resemble the laboratory ones. A numerical model of a granite rock joint has been created by means of a calibration process. Both the intact rock microparameters and the smooth joint scale have been calibrated against macroparameters derived from shear tests performed in laboratory. A new parameter, the length ratio, is introduced which takes into account the effective length of the smooth joint compared to the theoretical one. The normal and shear stiffnesses, the cohesion and the tensile force ought to be scaled against the length ratio. Four simple regular joint profiles have been tested in the PFC2D environment. The analysis shows good results both from a qualitative and from a quantitative point of view. The difference in peak shear strength with respect to the one computed with Patton´s formula is in the order of 1% which indicates a good accuracy of the model. In addition, four profiles of one real rough mated joint have been tested. From the scanned surface data, a two-dimensional profile has been extracted with four different resolutions. In this case, however, interlocking of particles along the smooth joint occurs, giving rise to an unrealistic distribution of normal and shear forces. A possible explanation to the problem is discussed based on recent developments in the study of numerical shear tests with PFC2D.

In Sweden, the rock mass is commonly used for construction of tunnels and caverns. The rockmass is also used as a foundation for large structures such as bridge abutments and dams. Forthese structures, the understanding of the rock mechanical properties play a key role for reachingan acceptable safety level and minimizing cost. One of the properties that has a high uncertaintyis the shear strength of rock joints. These rock joints constitute the weakest link in the rock massand often govern it´s strength. The uncertainty lies in the amount of factors that affect the shearstrength such as the degree of weathering, the matedness, the roughness of the surface and thescale. Various authors have tried to develop a failure criterion that can predict the peak shearstrength of rock joints and takes into account the influence of the various factors.The aim of this thesis is to evaluate the ability of the newly developed Casagrande et al.criterion to determine the peak shear strength for perfectly mated and natural rock joints withdifferent degrees of matedness. All samples analyzed in this thesis have been scanned andcustomized to run in the programmed version of the Casagrande et al. criterion. This iterativeprocess will stop as the application reach the apparent dip angle where the total shearing force issmaller than the total sliding force. This angle combined with the basic friction angle gives thepeak friction angle for calculations of the peak shear strength.The result show that the Casagrande et al. criterion can predict the peak shear strength forperfectly mated joint. However, for the natural rock joint, as the degree of matedness decreases,the accuracy of the prediction of the peak shear strength decreases. The conclusion of this studyis that the Casagrande’s criterion cannot determine the peak shear strength of natural rock jointsand that further development of the Casagrande et al. criterion is needed taking this parameterinto account. / Sverige är berg ett vanligt material för byggande av tunnlar och bergrum. För dessakonstruktioner spelar bergegenskaperna en nyckelroll för att nå en acceptabel säkerhetsnivåoch minimera kostnaden. En av de egenskaper som har stor osäkerhet är skjuvhållfastheten förbergsprickor. Osäkerheten ligger i de många faktorer som påverkar skjuvhållfastheten, såsomgraden av vittring, passning, ytans råhet och skala. Olika författare har försökt att anpassa ettbrottkriterium för bergsprickor som tar hänsyn till faktorernas inflytande och som kan användastill att uppskatta den maximala skjuvhållfastheten.Syftet med detta examensarbete är att utvärdera förmågan hos det nyligen utveckladebrottkriteriet av Casagrande et al. att bestämma den maximala skjuvhållfastheten för perfektpassade sprickor och naturliga sprickor med olika grad av passning. Alla prover i detta arbetehar skannats in och anpassats för att köras i den programmerade algoritmen som beräknar denmaximala skjuvhållfastheten enligt kriteriet av Casagrande et al.. Kriteriet använder sig av eniterativ process som pågår tills algoritmen når den vinkel där den totala skjuvkraften är mindreän den totala glidkraften. Denna vinkel kombinerad med sprickans basfriktionsvinkeln ger denmaximala friktionsvinkeln för beräkning av skjuvhållfastheten.Resultaten visar att Casagrande et al. kan förutspå den maximala skjuvhållfastheten förperfekt passade sprickor. När passningsgraden minskar för naturliga bergsprickor minskar kriterietsförmåga att prediktera den maximala skjuvhållfastheten. Slutsatsen från detta arbete äratt kriteriet av Casagrande et al. kan prediktera skjuvhållfastheten för perfekt passade sprickormen saknar förmågan att beakta inverkan från passning, vilket leder till att skjuvhållfasthetenöverskattas om kriteriet användas på naturliga sprickor som inte är perfekt passade. Fortsattforskning krävs för att vidareutveckla kriteriet så att graden av passning kan beaktas.