Prediction of strength and deformability of an interlocked blocky rock mass using UDEC

The accurate prediction of strength and deformability characteristics of a rock mass is very challenging. In practice, properties of a rock mass are often estimated from available empirical relationships based on the uniaxial compressive strength (UCS). However, UCS does not always give a good indication of in-situ rock mass strength and deformability. The aim of this paper is to present a methodology to predict the strength and deformability of a jointed rock mass using UDEC (universal distinct element code). In the study, the rock mass is modelled as an assemblage of deformable blocks that can yield as an intact material and/or slide along pre-defined joints within the rock mass. A range of numerical simulations of UCS and triaxial tests were conducted on rock mass samples in order to predict the equivalent mechanical properties for the rock mass under different loading directions. Results are compared against the deformability parameters obtained by analytical methods

Chalk-steel Interface testing for marine energy foundations

The Energy Technology Partnership (ETP) and Lloyd’s Register EMEA are gratefully acknowledged for the funding of this project. The authors would also like to acknowledge the support of the European Regional Development Fund (ERDF) SMART Centre at the University of Dundee that allowed purchase of the equipment used during this study. The views expressed are those of the authors alone, and do not necessarily represent the views of their respective companies or employing organizations.Peer reviewedPostprin

Photogrammetry for the Characterization of Rock Masses: Two Case Histories for Slopes and Caverns

1noCharacterization of rock masses may benefit from photogrammetry, which is the science of turning photographs into 3D models. Two case histories (one for a slope, and one for a cavern) show the data that may be extracted from the models, and how a much clearer understanding of the fracture sets and of the geologic controls in general may be achieved within a permanent, unbiased documentation of a rock mass.reservedmixedFulvio TononTonon, Fulvi

Developing a damage model to simulate multiple-step loding triaxial compression tests in rocks

Multiple-step loading triaxial compression test (ML-TCT) method is a useful tool to evaluate strength parameters of rock samples using a single specimen applying several loading/unloading. However, because of accumulated damages in the specimen with repeated cycles of axial loading/unloading, the shear strength is prone to be underestimated. A multiple-step loading damage (MLD) model was proposed to simulate ML-TCT results. Two series of ML-TCTs were carried out on a sedimentary soft rock of mudstone. The first series was to determine the geotechnical parameters to describe the MLD model, and the second series was to verify the model. The results demonstrated that the proposed MLD model was powerful to simulate ML-TCTs on the mudstone and modify the results of carried out tests to generate more reliable results. Moreover, a generalized MLD model was constructed. This model allows prediction of peak deviator stresses and the relevant excess pore water pressures in a ML-TCT for rocks having different strength which generally are affected by the previous loading history. The generalized MLD model indicates that the margin between shear strength parameters obtained by single-step loading triaxial compression tests and ML-TCTs, increases with an increase in the rock strength. Moreover, upper bound values for effective cohesion, c′, and lower bound values for, effective friction angle, φ′, was obtained in a ML-TCT with increasing effective confining pressure, σ′c. Whereas, upper bound values for φ′ and lower bound values for c′ predicted in a ML-TCT with decreasing σ′c. It was concluded that, ML-TCT increasing σ′c is preferable to ML-TCT decreasing σ′c. © 2012 Springer Science+Business Media Dordrecht.A. Taheri, K. Tan

Fracture toughness anisotropy in shale

The use of hydraulic fracturing to recover shale-gas has focused attention on the fundamental fracture properties of gas-bearing shales, but there remains a paucity of available experimental data on their mechanical and physical properties. Such shales are strongly anisotropic, so that their fracture propagation trajectories depend on the interaction between their anisotropic mechanical properties and the anisotropic in-situ stress field in the shallow crust. Here we report fracture toughness measurements on Mancos shale determined in all three principal fracture orientations; Divider, Short-Transverse and Arrester, using a modified Short-Rod methodology. Experimental results for a range of other sedimentary and carbonate rocks are also reported for comparison purposes. Significant anisotropy is observed in shale fracture toughness measurements at ambient conditions, with values, as high as 0.72MPam1/2 where the crack plane is normal to the bedding, and values as low as 0.21MPam1/2 where the crack plane is parallel to the bedding. For cracks propagating non-parallel to bedding, we observe a tendency for deviation towards the bedding-parallel orientation. Applying a maximum energy release rate criterion, we determined the conditions under which such deviations are more or less likely to occur under more generalized mixed-mode loading conditions. We find for Mancos shale that the fracture should deviate towards the plane with lowest toughness regardless of the loading conditions