Lac Du Bonnet Granite Research Articles

The effect of sample disturbance on laboratory properties of Lac du Bonnet granite

Laboratory properties of Lac du Bonnet granite samples collected at depths ranging from near-surface to 1000 m were established. The laboratory properties indicated that either the in situ properties were changing with depth or the samples were changing with depth. Comparison of P-wave velocities in the samples and in situ confirmed the latter, i.e., sample disturbance was affecting the laboratory properties. The strength of the damaged samples indicated that sample disturbance affected the cohesion of the material, not the frictional properties, and that the cohesion loss cannot be accounted for, by applying a confining stress. Sample disturbance started to affect the laboratory properties of Lac du Bonnet granite when the maximum far-field in situ stress exceeded about 10% of the unconfined compression strength. Key words : sample disturbance, core discing, cohesion, brittle rocks, damage, microcracks, stress-dependent Young's modulus.

Qβ estimates from spectral ratios and multiple lapse time window Analysis: Results from an underground research laboratory in granite

An experiment to measure quality factor (Q) in‐situ was carried out at the Underground Research Laboratory in Manitoba, Canada using the Spectral Ratio technique and a form of the Mulitple Lapse Time Window Analysis technique. The volume of rock examined measures approximately 30 × 30 × 30 m at a depth of 420 m in Lac Du Bonnet Granite. The site encompases 16 triaxial accelerometers grouted in the bottom of boreholes drilled from access tunnel walls. The source for the seismic pulses was a mechanical hammer placed at the bottom of specially located boreholes. Results from the Spectral Ratio experiment gave S‐wave quality factor (Qβ) of 114 ± 18. Based upon the short duration of the first arrival pulses and low scattered wave energy in the signal coda, we conclude that the Spectral Ratio technique measured the intrinsic attenuation of the rock mass. Further analysis using the more recently developed Multiple Lapse Time Window Analysis gave results for intrinsic Qβ (Qi) of between 85 and 100. Conversely, values for scattering Qβ (Qs) were greater than 800. The high correlation for Qi values between the two methods help to confirm the nature and absolute values of the attenuation, and also support the use of the later technique where spectral ratios cannot be applied.

Origin of groundwater salinity in the Lac du Bonnet granite, southeastern Manitoba, from 36Cl measurements

The results of analyses of the 36Cl content of groundwaters in the Lac du Bonnet granite batholith are described and compared with previous modelling calculations and 36Cl content of rock matrix solutions obtained from a borehole leaching experiment. The results are used to provide support for the proposal that groundwater salinity is largely derived from dissolution of soluble salts in the rock matrix.

Ultrasonic imaging and acoustic emission monitoring of thermally induced microcracks in Lac du Bonnet granite : D. P. Jansen, S. R. Carlson, R. P. Young & D. A. Hutchins, Journal of Geophysical Research, 98(B12), 1993, pp 22,231–22,243

Ultrasonic imaging and acoustic emission monitoring of thermally induced microcracks in Lac du Bonnet granite : D. P. Jansen, S. R. Carlson, R. P. Young & D. A. Hutchins, Journal of Geophysical Research, 98(B12), 1993, pp 22,231–22,243

Analysis of the migration of instantaneously injected cesium in artificial fractures of Lac du Bonnet granite, Manitoba, Canada : Y. Fujikawa, M. Fukui, D.J. Drew & T.T. Vandergraaf, Journal of Contaminant Hydrology, 14(3–4), 1993, pp 207–232

Analysis of the migration of instantaneously injected cesium in artificial fractures of Lac du Bonnet granite, Manitoba, Canada : Y. Fujikawa, M. Fukui, D.J. Drew & T.T. Vandergraaf, Journal of Contaminant Hydrology, 14(3–4), 1993, pp 207–232

Ultrasonic imaging and acoustic emission monitoring of thermally induced microcracks in Lac du Bonnet granite

Concurrent ultrasonic tomography and acoustic emission monitoring were employed to study thermally induced microfracturing in an unconfined, 15‐cm cube of Lac du Bonnet granite from Atomic Energy of Canada Limited's Underground Research Laboratory. An electrical resistance cartridge heater, placed in a central vertical borehole, was used to cycle the sample to progressively higher peak temperatures between 75°C and 175°C. Tomography data were collected, at room temperature, before and after each thermal cycle. Acoustic emission monitoring proceeded during both heating and cooling phases of each thermal cycle. Microfractures opened above 80°C and eventually coalesced into a macroscopic fracture plane. The macroscopic fracture originated at the outer edges of the sample and then extended inward, parallel to the fast velocity direction, and eventually intersected the borehole. Both acoustic emission locations and slowness difference tomography clearly delineated the fracture plane. We attribute the development of the macroscopic fracture to a thermal gradient cracking mechanism acting upon a brittle, anisotropic medium.

Laboratory study of borehole breakouts in Lac du Bonnet granite: a case of extensile failure mechanism

We subjected boreholes drilled vertically in the center of cubical specimens of Lac du Bonnet granite to constant vertical (S v) and least horizontal (S h) farfield stresses, and increased the maximum horizontal stress (S H) until breakouts occurred. A thin section study revealed that extensile cracking behind the borehole in two zones aligned with S h preceded the development of breakouts. Progressive detachment of thin long flakes bounded by these densely spaced subparallel cracks led to ‘dog ear’ shaped breakouts. Breakout mechanism and appearance match those found in the field. Results also suggest the potential for using breakout occurrence to estimate S H if S h is known or vice versa.

Analysis of the migration of instantaneously injected cesium in artificial fractures of Lac du Bonnet granite, Manitoba, Canada

Breakthrough curves of 137Cs and tritiated water injected instantaneously into artificial fractures in Lac du Bonnet granite were analyzed using the analytical solution for a single rock-fracture system and assuming the linear sorption isotherm of the solute. Parameters of nuclide diffusion and sorption in rock matrices, obtained by fitting, varied depending on the flow velocity in the fractures. According to theoretical calculations, different fracture flow velocities lead to different diffusion distances of nuclides in matrices at the same injection volume. As microscopic inhomogeneity is considered to exist in the rock matrix, the average diffusion-sorption characteristics of the matrix within the diffusion distance may have varied with the fracture flow velocity. Surface sorption was marked in fractures that had relatively high matrix sorption-diffusion capacities. The phenomenon was interpreted using the theoretical relationships developed between the surface sorption, matrix sorption and pore diffusion coefficient, and the porosity of matrices. The effect of the nonlinear sorption of solute was examined by numerically solving model equations that incorporate the nonlinear isotherm. This incorporation may contribute to the reduction of deviations between theoretical and experimental BTC's.

Mapping the state of fracture around cavities

The state of stress around cavities is heterogeneous. Consequently, the state of fracture may also vary from point to point. Under compressive loading, cavity may be in one of four possible states: pre-fracture (pre-microfracture initiation), microfracture propagation (initiation to the onset of dilata dilatancy to failure) and post-failure. These four states are separated by the crack initiation stress, the crack damage stress and the failure stress. fracture events with confining pressure is examined with reference to three intact rocks: a brittle granite, a semi-brittle limestone and a ductile sal The maximum principal stress (σ1) at crack initiation, crack damage (onset of dilatancy), yielding and failure are established as a function stress (σ3). For a single intact rock, all four fracture events can be represented using one function (the Rocker function) with a single fac various fracture states. The proposed fracture criteria, based on the experimental data, are combined with the existing state of stress to prepare a fracture map around an elli intact Lac du Bonnet granite. The state of stability is expressed through a newly defined stability factor, the unconfined strength ratio (USR), wh to the traditional safety factor. In contrast to the conventional safety factor in rock mechanics (SFstrength/σ1), which is σ3 <tensile strength space, the USR is defined for any point in the σ3–σ1, space of the stability diagram.

Mapping the state of fracture around cavities

The state of stress around cavities is heterogeneous. Consequently, the state of fracture may also vary from point to point. Under compressive loading, cavity may be in one of four possible states: pre-fracture (pre-microfracture initiation), microfracture propagation (initiation to the onset of dilata dilatancy to failure) and post-failure. These four states are separated by the crack initiation stress, the crack damage stress and the failure stress. fracture events with confining pressure is examined with reference to three intact rocks: a brittle granite, a semi-brittle limestone and a ductile sal The maximum principal stress (σ 1) at crack initiation, crack damage (onset of dilatancy), yielding and failure are established as a function stress (σ 3). For a single intact rock, all four fracture events can be represented using one function (the Rocker function) with a single fac various fracture states. The proposed fracture criteria, based on the experimental data, are combined with the existing state of stress to prepare a fracture map around an elli intact Lac du Bonnet granite. The state of stability is expressed through a newly defined stability factor, the unconfined strength ratio (USR), wh to the traditional safety factor. In contrast to the conventional safety factor in rock mechanics (SF strength/σ 1), which is σ 3 <tensile strength space, the USR is defined for any point in the σ 3–σ 1, space of the stability diagram.

Overcoring in highly stressed granite: Comparison of USBM and modified CSIR devices

An overcoring program in three nearly orthogonal boreholes, intersecting the same rock volume, was carried out in highly stressed Lac du Bonnet granite at the 240-m level of the Underground Research Laboratory (URL). The program was designed to determine if the more commonly used United States Bureau of Mines borehole deformation gauge (USBM gauge) gave similar in situ stress results as a modified Council of Scientific and Industrial Research (CSIR) triaxial strain cell. A total of 15 successful USBM tests and 17 successful CSIR tests were carried out. The measured mean stress tensor for the USBM results fell within the 90% confidence limits for the CSIR results.

High levels of uranium and radium in groundwaters at Canada's Underground Research Laboratory, Lac du Bonnet, Manitoba, Canada

High concentrations of U and 226Ra, and elevated 234U/ 238U activity ratios have been measured in groundwater samples collected from water supply wells and exploratory boreholes in the area surrounding the Underground Research Laboratory (URL) of Atomic Energy of Canada Limited, in southeastern Manitoba. All groundwaters come from the Lac du Bonnet granite batholith or sediments overlying the batholith. Uranium concentrations attain almost 1 mg/l in some shallow, low-salinity groundwaters, whereas 226Ra tends to be high (up to 38 Bq/l) in deeper, saline waters. The U concentrations are some of the highest observed in global groundwaters, yet no significant ore body or mineralization is known in the area. Analyses of unaltered rock samples of the Lac du Bonnet granite show slight U enrichment over average Canadian Shield granites (6.5 μg/g vs 4 μg/g), and altered wall rock in fracture zones is enriched in U by up to an order of magnitude compared to adjacent bedrock. Low 234U/ 238U activity ratios in this altered rock indicate active and recent leaching of U by groundwater. The key control on U concentration appears to be redox potential. Concentrations of U in rock, residence time and groundwater composition are of lesser importance. Geochemical modelling of the shallower, oxidized waters indicates that U speciation consists mainly of anionic carbonate complexes of the uranyl ion. This is supported by the remarkable efficiency of an anionic filter developed to remove high levels of U from drinking water in the area. In more reducing groundwaters, U concentrations are similar to those determined in recent experimental work on uraninite solubility in the pH range 7–8.5. Colloidal U is <10% of total U and organic complexation is unlikely to be significant because of low dissolved organic concentrations. The results emphasize the significance of redox potential in controlling U mobility in both oxidizing and reducing environments and indicate the usefulness of U concentration in estimating groundwater Eh.

The effect of water on the time-dependent deformation and fracture of a granite

The effect of water on the deformation, fracture and strength of Lac du Bonnet granite has been investigated through a variety of experimental techniques: standard short-term tests for compressive strength and fracture toughness, and longer-term, time dependent experiments that measure creep, static fatigue and slow crack velocity. When specimens of this granite are tested while in direct contact with water, such conventional measures of the short-term strength as the uniaxial compressive strength and the fracture toughness decrease by about 5% from the levels measured at room temperature and humidity. Because short-term tests require a few minutes to complete, part or all of this decrease in strength may only be due to time-dependent processes. When results of other, longer-term tests are extrapolated to zero elapsed time, the indication is that at very rapid loading rates (instantaneous or dynamic loading condition), the effect of water on strength may be negligible. In all time-dependent tests for creep strain, static fatigue and slow crack velocity, the effect of water is substantial. The creep strain and slow crack velocity increase and the long-term strength decreases when water is introduced into the previously dry testing environment. The size of the change, however, depends on the stress level (applied stress relative to the instantaneous strength). It is small at high stress levels becoming more substantial as the stress level is decreased. For example, at a stress level of 65%, the steady state creep rate of the lateral strain increased by 300% when water was allowed to flood the previously dry environmental chamber.

History of actinide and minor element mobility in an Archean granitic batholith in Manitoba, Canada

Surface and borehole core samples from the Lac du Bonnet granite, Manitoba, Canada, have been analysed for major element concentrations,Fe 3+/Fe (total) ratios, rare earth element (REE) content and actinide isotopic abundances. This work forms part of the geological investigations of the Canadian Nuclear Fuel Waste Management Program, performed by Atomic Energy of Canada Limited (AECL). The study attempts to understand the history of, and processes governing, mobilisation of elements and naturally occurring radionuclides during high- and low-temperature alteration events in fluid-bearing fractures in the granite. One surface sample and two core samples (from ∼ 150 m and 730 m) are each in contact with fractures in the granite and show evidence of alteration events that penetrated the rock matrix over distances of at least 3 cm. Loss of Ca and Na is seen in cores from a depth of ∼ 150 m from the highly altered, hematite-rich rock adjacent to sub-horizontal fracture zones at the Underground Research Laboratory (URL) of AECL, near Lac du Bonnet. In contrast, K, Fe,Fe 3+/Fe and U concentrations increase towards the fracture surface due to formation of illite and association of U with hematite and the illite. At the fracture surface, U continues to increase, but Fe and theFe 3+/Fe ratio decrease indicating Fe removal by reduction. The REE also show some enrichment in more altered rock at intermediate depths, but the total REE concentration is lower than in the surface and deep core samples. No clear trends are visible for parent and fracture-surface REE in surface and deep core samples, however. Disequilibrium values of 234U/ 238U and 230Th/ 234U ratios in surface and intermediate depth core samples indicate that U has been mobilised in recent geological time (the last Ma), but Th has remained relatively immobile. High Th/U and 230Th/ 234U ratios in surface samples are indicative of rapid leaching of U but little isotopic fractionation, probably within the last 10 5 a. Apparently unaltered rock, several centimetres distant from the fracture in surface and intermediate- depth samples, has lost appreciable U, but evidence from U-series disequilibrium studies suggests that this process occurred more than one million years ago, perhaps during deuteric or hydrothermal alteration. Core from a fracture at depth in the granite shows little hematite or clay formation and lacks evidence of REE and recent or ancient actinide mobilisation. The U-series results are correlated with the observed concentrations and isotope activity ratios of U in groundwaters sampled from the same or adjacent fractures. Analyses of samples of highly altered rubble recovered from centre portions of fracture zones at the URL show both excesses and deficiencies of 234U and 230Th in neighbouring locations, possibly due to the presence of a redox front whose position is controlled by modern groundwater composition. The implications of these results are discussed for the concept of disposal of nuclear fuel waste at depth in plutonic rock on the Canadian Shield.

Stress corrosion cracking of Lac du Bonnet granite in tension and compression

Rocks subjected to long-term loading have been known to suffer microcracking. The rate of cracking is sensitive to the type of the applied stress (tensile or compressive), and the magnitude of the stress relative to the instantaneous strength. In addition, crack growth is influenced by the environment (pressure and temperature) including the presence or absence of moisture. For tensile loading, the sensitivity of granite to time-dependent cracking is demonstrated through a fracture mechanics test known as double torsion. The crack velocity versus stress intensity function is established for two environments, room temperature and humidity and room temperature and 100 percent humidity. For compressive loading, time dependent cracking is evaluated from creep tests conducted in uniaxial compression in the same two environments. The rate of cracking is defined by finding the functional relationship between the rate of crack growth, expressed as the rate of crack volume strain, and uniaxial compressive stress. A variety of mathematical functions has been fitted to the obtained data. The traditionally-used power and exponential relationships give good correlation for both crack velocity and crack volume strain rate. The crack volume strain rate versus stress function can be integrated to obtain a lifetime estimate for Lac du Bonnet granite. After 1 000 years of loading in uniaxial compression at room temperature and 100 percent humidity, the strength of this granite could reduce from 225 MPa to 90–100 MPa.