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- PublicationAccès libreRock strength obtained from core samples and borehole wall - the effect of drilling induced damageinstabilities(2010-6-15)
; ;Bahrani, N.Kaiser, P.K.Borehole breakouts are accepted as one of the best indicators of in-situ principal stress orientation. However, the estimation of the stress magnitude from breakouts is still controversial. One prerequisite to derive stress magnitude from borehole wall failure is to have an independent estimate of the strength of the borehole wall. Zoback et al. (2003) suggest assuming that the Uniaxial Compressive Strength (UCS) from core samples is an acceptable estimate of borehole wall strength, but it has been shown that when drilling in relatively high stress environments core may be damaged, resulting in significantly reduced core strengths (e.g. Martin and Stimpson, 1994). Such core damaging processes are highly probable in stress environments relevant for breakout formation. Thus, an underestimation of UCS due to core damage could lead to an underestimation of in-situ stress magnitude from breakout back-analyses. Preliminary results from the numerical analyses presented here suggest that damage in the core initiates long before any damage occurs in the borehole wall. It is thus suggested that in relatively high-stress situations, strength evaluation from borehole geophysics or from breakouts back-analyses (in situations where the complete stress tensor is independently estimated) delivers a better estimate of the in-situ intact rock strength than laboratory tests. Work is underway to propose solution for the unbiased estimation of in-situ intact rock strength from borehole observations.
- PublicationAccès libreInterpretation of Triaxial Testing Data for Estimation of the Hoek-Brown m i(2011-6)
;Bewick, R. P ;Kaiser, P.K.Triaxial tests are conducted to determine the relationship between confinement and axial compressive strength. Depending on the confining stress applied to a core specimen, the failure process changes depending on the rock type’s internal composition (i.e. porosity, flaws, stiffness heterogeneity, grain shape, etc.). These failure process changes are not typically considered when planning a triaxial testing program or when processing triaxial test data. This paper summarises the changes in failure process that occur depending on the confining stress level for various rock types and outlines a procedure for processing triaxial data depending on the confining stress level for the determination of the Hoek-Brown strength envelop parameter m i and confidence intervals. For this, a triaxial and uniaxial dataset from Bingham Canyon is presented for a quartzite. The dataset is exceptionally complete in terms of the number of tests conducted (total of 217 uniaxial and triaxial tests) and the detail in test data quality and characterization of the specimens. The results show that triaxial data requires m i values outside (m i >50) the typically assumed range (m i ≤50). These high m i values appear to be needed to fit data at high confining stress levels (larger than about UCS/10). Based on the discussion in this paper; (1) the sel ection of confinement levels for testing purposes should include sufficient data in the confinement range of 0 to UCS/10 and UCS/10 to UCS/2; and (2) two sets of strength curves may need to be considered depending on the problem being assessed. One curve is valid for confinements of 0 to UCS/10 (representative of strengths near excavation boundaries) and the other for >UCS/10 to approximately the Mogi line (representative of strengths, for example, in wide pillars or mine abutments), after which a third envelop is needed. These changing envelop requirements are a result of changing failure mechanisms.
- PublicationAccès libreLaboratory-scale strain and temperature response of a distributed optical fiber sensor(American Rock Mechanics Association, 2013-6-23)
;Madjdabadi, B. ; ;Siczkar, L. ;Dusseault, M.B.Kaiser, P.K.Distributed optical fiber sensors (DOFSs), used initially in structural health monitoring for high-rise buildings and bridges, are attracting attention in the field of underground structures, including mining. Designed for long-term study of deformations, DOFSs are more efficient when installed away from excavation damaged zone (EDZ) in a borehole filled with a grout mixture to measure elastic strain field responses to excavations. The DOFS sensing cable, as a component of a complex compliance system, i.e. rockmass and grout, is being assessed through laboratory work. A test program is underway to observe DOFS response to various perturbations including strain and joint displacement. Initially, tests on unstrained sensors are performed in order to assess measurement repeatability and noise-to-signal ratio at both local and global scales. Then, the various lengths of the cable, from 1 m down to 1 cm, will be stretched up to 0.5% strain. In other tests, the same lengths of the cable will be exposed to shear displacement, such as might occur in the vicinity of a joint or fault that experiences shear. The results from these tests will answer uncertainties and questions regarding the scaling factor between straining sections over a full sampling window, i.e. spatial resolution, and a partial sampling window, i.e. validity of calibration factors provided by the supplier, and assessing effects of coating and plastic protective layers of the sensor. Issues such as shear deformation responses of cable and bending direction of the cable are being evaluated. Initial results on unstrained cable to assess measurement repeatability showed variability in length assessment between successive readings. This variability particularly impacts the data interpretation from the strain sensors since these sensors present locally large Brillouin frequency gradients which results in locally large variability in differential readings. Our detailed experimental results will be presented in the paper.
- PublicationAccès libreEffect of grain scale geometric heterogeneity on tensile stress generation in rock loaded in compression(American Rock Mechanics Association, 2012-6-24)
;Bewick, R.P. ;Kaiser, P.K.Brittle failure of rock is dominated by tensile mechanisms even in an overall compressive stress field. Heterogeneities play a key role in the development of the localized tensile conditions. However, details on how heterogeneities affect brittle rock failure processes are still open for debate. Through the use of regular honeycomb grain arrangements progressing to highly irregular Voronoi arrangements, the impact of grain geometric heterogeneity through finite element tools and discrete element methods was assessed and it is shown that non-uniformity of grain size distribution is not a critical parameter to evaluate crack initiation, peak strength, or micromechanical behaviour. The results demonstrate that grain boundary orientation and grain system arrangements control tensile stress generation inside a brittle rock specimen under compression and thus impact the crack-initiation stress level. This suggests that crack interaction and peak strength is then affected by the kinematic and allowable degrees of freedom in the grain assembly of the damaged rock. At this stage, grain deformability and more importantly grain breakage is needed to increase the degrees of freedom required for the linkage and formation of a macroscopic rupture. Based on this it is suggested that if it is possible to characterize grain boundary orientations or arrangements, various micro-mechanical behaviour could potentially be forecast.
- PublicationAccès libreEvaluation of PFC2D Grain-Based Model for Simulation of Confinement-Dependent Rock Strength Degradation and Failure Processes(2011-6)
;Bahrani, N. ; ;Kaiser, P.K.Pierce, M.Estimation of rock mass strength has become more critical in recent years due to the increase in the number of mining and civil projects at greater depths (> 2 km). Empirical approaches for the estimation of rock mass strength (e.g., Hoek-Brown/GSI) are primarily based on experiences at shallow depths and low confinement problems (e.g., tunnel wall failure), and therefore may not be representative for the strength in highly confined rock masses (e.g., for the core of pillars or abutments at depth). In this study an attempt is made to investigate the strength of rock masses, using the discrete element code PFC2D. For this purpose, the recently developed PFC2D Grain-Based Model (GBM) was used to match the laboratory response of intact and granulated Wombeyan marble. The term “granulated” refers to a heat treated sample where the grains have been completely separated at their boundaries due to anisotropy and contrast of their thermo-elastic properties. This material is considered to represent an analogue for a randomly jointed rock mass. It is shown that the PFC2D-GBM calibrated to unconfined and confined intact marble strengths and then to the unconfined granulated marble strength, underestimates the strength of the confined granulated marble. This problem was resolved by increasing the grain boundary friction angle in the granulated model to account for micro-scale roughness of the grain boundaries as observed in the microscopic image of the granulated marble. The calibration methodology taken to obtain micro-properties for both intact and granulated marble as well as implications for the determination of rock mass strength at various confinement levels are discussed.
- PublicationAccès libreInfluence of confinement dependent failure processes on rock mass strength at depth(2011-10-16)
; ;Kim, B. ;Suorineni, F. ;Bahrani, N. ;Bewick, R.P.Kaiser, P.K.Changes of failure mechanism with increasing confinement, from tensile to shear dominated failure, is widely observed in the rupture of samples in laboratory and in rock masses in situ. However, common failure criteria typically consider only shear mechanisms. A hybrid criteria based on a sigmoid function is introduced to account for a transition from tensile to shear dominated failure with increasing confinement. When evaluated by fitting to an extensive laboratory database the sigmoid criteria does not provide a better fit compared to the Hoek-Brown failure envelope, but provides insight into rock strength controlling factors that have significant consequences with respect to the interpretation of laboratory test results. It also leads to a differentiated approach for design by considering two types of behaviour process: 1) in the inner shell, i.e. the direct vicinity of openings, the failure mode is dominated by tensile cracking leading to spalling and related geometric dilation processes and 2) in the outer shell, i.e. remote from excavations, where confinement promotes interlock, we suggest that rock masses could be significantly stronger than predicted by standard approaches.
- PublicationAccès libreMonitoring mining-induced rock mass deformation using distributed strain monitoring based on fiber optics(2012-5-28)
; ;Madjdabadi, B. ;Kaiser, P.K.Dusseault, M.B.Triggered seismicity remains one of the main geomechanics hazards that affect safety in deep mining. Mobilization or propagation of existing faults can potentially release large amounts of seismic energy that in turn may trigger rockbursts and falls of ground, threatening worker safety and generating production delays. Cur-rent modeling approaches, typically based on stress analyses, do not fully succeed in capturing such seismically triggered mechanisms as the main seismic event location may be beyond the volume of rock affected by the mining induced stress perturbation. However, the displacement field generated by the excavation process may explain the triggering of far-field seismicity. Deeper understanding of the mining-induced defor-mation field is the motivation for the development of mine-scale deformation monitor-ing techniques. Innovative deformation sensors developed for structural monitoring based on fibre optic technology allows distributed measurement of strain at high spa-tial sampling rates over large distances. This paper presents the results of the testing of one of these systems in an active mining context. The system selected is based on Brillouin Optical Time Domain Ana-lyses (BOTDA) and allows for a spatial resolution of 10 cm. It has been tested in the laboratory and installed in five boreholes piercing through an actively mined, 25 m thick, 1000 m deep, sill pillar. Benchmarking of the system against extensometer re-sults was successful in a qualitative manner. The high spatial resolution of the fibre optic system brings valuable additional insights to rock mass deformation processes.
- PublicationAccès libreNumerical Investigation of the Influence of Borehole Orientation on Drilling-Induced Core Damage(2012-5-28)
;Bahrani, N. ; ;Maloney, S.Kaiser, P.K.Damaged and disked core from boreholes are indicators of high stress relative to the intact rock strength at the drilling location. While core disking is mainly used as a means to estimate the in situ stress state, core damage (micro-fractures) and its level need to be identified to allow for the accurate estimation of the in situ intact rock strength and therefore proper design of underground infrastructure such as tunnels and pillars. It is generally recommended that laboratory testing be done on samples from boreholes drilled parallel to the major principal stress. In this study, results are reported from an investigation on the influence of borehole orientation on the drilling-induced core damage and associated strength and modulus reductions. The drilling-induced coring stress paths, for boreholes drilled parallel to σ1 and σ3 within a stress state representative for the 420 Level at the Underground Research Laboratory, were first obtained from an elastic three-dimensional finite element model. The coring-induced stress path for the borehole drilled parallel to σ3 was then applied to a two-dimensional discrete element model previously calibrated to the undamaged Lac du Bonnet granite to create damage in the form of micro-cracks. Once the model was calibrated to both undamaged and damaged LdB granite, it was used to predict the damage level and the unloading-induced micro-crack characteristics of the core from the borehole drilled parallel to σ1. The results confirm the effect of sample disturbance on rock strength and modulus measured in the laboratory and potentially offer a mean to model this process and quantify drilling-induced core damage.
- PublicationAccès libreNumerical modeling of strain transfer from rock mass to a fibre optic sensor installed inside a grouted borehole(: American Rock Mechanics Association, 2012-6-24)
;Madjdabadi, B. ; ;Dusseault, M.B.Kaiser, P.K.Strain measurements in underground excavation are usually done locally, with extensometers or similar devices placed within 10-15 meters of adit or stope faces, mainly to gage development of the EDZ (excavation damaged zone) and assess its evolution and impact on local safety (rock falls, rapid deterioration of wall condition...). However, the calibration of three-dimensional stress analysis models used to assess excavation geometry and sequencing requires strain (displacement) measurements in those parts of the rock mass that are in the elastic domain for some or all of their active design life. Recently developed distributed fibre optic sensors are now being used to measure local linear displacements continuously in a large rock mass volume in real mining conditions in Canada. Grouted inside a borehole and therefore encased in a material of far lower stiffness that the rock mass, an optical fibre may register strains different from those actually occurring in the rock mass. A number of factors affect the process of rock mass strain conveyance through the grout to the fibre. This paper reports a study that simulates the borehole-grout-fibre interaction to find how the strain transfer takes place and whether there are any issues serious enough to warrant alterations in installation procedures and grout materials.
- PublicationAccès libreNumerical analyses of the effect of heterogeneities on rock failure process(American Rock Mechanics Association, 2010-6-27)
; ;Suorineni, F.T.Kaiser, P.K.While heterogeneities in strength and deformation properties are thought to play an important role in the failure processes of rocks and rock masses, they are rarely explicitly introduced in numerical models. This paper present the results obtained by introducing heterogeneities in a Finite Element Modeling tool (Phase2TM). Particularly the effect of heterogeneities in rock modulus and strength are investigated at the laboratory test sample scale. Limited modulus variability (coefficient of variation smaller than 1.5%) is sufficient to generate rock behaviour that is highly affected by induced tensile stress conditions. This variability in modulus reduces the peak strength and the post-peak strength drop: with increasing heterogeneity in deformability, the rocks become less brittle (i.e., more strain-softening). However, the brittleness in the low confinement range, leading to spalling behaviour, is enhanced by this heterogeneity. Various loading case and geometries are investigated highlights the influence of rock and rock mass heterogeneities. Modulus heterogeneities generate tensile conditions and damage when taking core samples from relatively high stress conditions. This may influence the samples strength and lead to an underestimation of the in-situ rock strength at depth. Heterogeneities influence the failure pattern around opening, with practical implication on ground support requirements. An equivalent homogeneous properties concept, as used for example by the GSI system, doesn't properly capture the failure pattern generated by the presences of heterogeneities, suggesting that the approach of ” equivalent” homogenous material could be inadequate and that heterogeneities should be introduced explicitly in numerical analyses of geomechanics problems.