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Valley, Benoît

Nom
Valley, Benoît
Affiliation principale
Site web
Fonction
Professeur ordinaire
Email
benoit.valley@unine.ch
Identifiants
https://libra.unine.ch/handle/123456789/943
_
0000-0003-4632-0504

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  • Publication
    Accès libre
    Experimental evaluation of a distributed Brillouin sensing system for detection of relative movement of rock blocks in underground mining
    (2017)
    Madjdabadi, B.
    ;
    Valley, Benoît 
    ;
    Dusseault, M.B.
    ;
    Kaiser, P.K.
    Underground mining in highly stressed, hard rock masses is commonly accompanied by seismic events that are located close to mining, near active stopes and other excavations, most frequently within one diameter of the excavation boundary. However, as the extraction ratio increases or mines progress to greater depths, remote events have been observed at large distances from active mining, where the direct mining related stress changes are negligible. Events in different mining blocks cannot be explained by models involving stress redistribution processes alone.
  • Publication
    Accès libre
    Experimental evaluation of a distributed Brillouin sensing system for measuring extensional and shear deformation in rock
    (2015-9)
    Madjdabadi, B.
    ;
    Valley, Benoît 
    ;
    Dusseault, M.B.
    ;
    Kaiser, P.K.
    Distributed Brillouin sensing systems (DBSs) have growing applications in engineering and are attracting attention in the field of underground structures, including mining. The capability for continuous measurements of strain over large distances makes DBSs a promising monitoring approach for understanding deformation field evolution within a rock mass, particularly when the sensor is installed away from excavation damaged zone (EDZ). A purpose-built fiber optic sensing cable, a vital component of DBSs, was assessed in laboratory conditions. A test program was performed to observe DBSs response to various perturbations including strain and joint movements, including opening and shearing of joints. These tests included assessment of the strain-free cable response and the application of extensional and lateral displacement to various sensing cable lengths (strained lengths), from 1 m down to 1 cm. Furthermore, tests were done to evaluate the time-dependent behavior of the cable and to observe the effect of strain transfer using a soft host material (e.g. a soft grout) under lateral displacement. The noise level of the DBSs range was ±77 le, determined through repeated measurements on an unstrained cable. Stretching test results showed a clear linear correlation between applied strain and Brillouin frequency shift change for all strained lengths above half the spatial resolution of the DBSs. However, for strained lengths shorter than half the spatial resolution, no strain response was measurable and this is due to the applied internal signal processing of the DBSs to detect peak Brillouin gain spectrum and noise level. The stability with time of the measurements was excellent for test periods up to 15 h. Lateral displacement test results showed a less consistent response compared to tension tests for a given applied displacement. Although the Brillouin frequency shift change is correlated linearly with the applied displacement in tension, it shows a parabolic variation with lateral displacement. Moreover, the registered frequency response (correlated with strain) of the system decreased significantly when the sensing cable was embedded in a sand-filled tube compared with direct cable displacement.
  • Publication
    Accès libre
    Numerical simulation of drilling-induced core damage and its influence on mechanical properties of rocks under unconfined condition
    (2015-9-1)
    Bahrani, N.
    ;
    Valley, Benoît 
    ;
    Kaiser, P.K.
    At the early stages of deep underground construction projects, data are primarily obtained from boreholes from which strategic decisions are made for the design of underground infrastructures. For this purpose, the unconfined compressive strength (UCS) and the Young's modulus (E) of intact rock as well as the in situ stress state are obtained as fundamental engineering parameters during the course of geotechnical site characterization. Unfortunately, drilling in relatively high stress environments can induce disturbance, micro-cracking of the cores, which in turn may result in lower rock strength and Young's modulus measured in the laboratory1 compared to their intact values or affect stress measurement results2,3 . This may lead to erroneous estimates of design parameters.
  • Publication
    Accès libre
    Development of connected permeability in massive crystalline rocks through hydraulic fracture propagation and shearing accompanying fluid injection
    (Hoboken, New Jersey, uSA: John Wiley & Sons Ltd, 2014-2)
    Preisig, Giona 
    ;
    Eberhardt, E.
    ;
    Gischig, V.
    ;
    Roche, V.
    ;
    Van der Baan, M.
    ;
    Valley, Benoît 
    ;
    Kaiser, P.K.
    ;
    Duff, D.
    ;
    Lowther, R.
    The ability to generate deep flow in massive crystalline rocks is governed by the interconnectivity of the fracture network and its permeability, which in turn is largely dependent on the in situ stress field. The increase of stress with depth reduces fracture aperture, leading to a decrease in rock mass permeability. The frequency of natural fractures also decreases with depth, resulting in less connectivity. The permeability of crystalline rocks is typically reduced to about 1017–1015 m2 at targeted depths for enhanced geothermal systems (EGS) applications, that is, >3 km. Therefore, fluid injection methods are required to hydraulically fracture the rock and increase its permeability. In the mining sector, fluid injection methods are being investigated to increase rock fragmentation and mitigate high-stress hazards due to operations moving to unprecedented depths. Here as well, detailed understanding of permeability and its enhancement is required. This paper reports findings from a series of hydromechanically coupled distinct-element models developed in support of a hydraulic fracture experiment testing hypotheses related to enhanced permeability, increased fragmentation, and modified stress fields. Two principal injection designs are tested as follows: injection of a high flow rate through a narrow-packed interval and injection of a low flow rate across a wider packed interval. Results show that the development of connected permeability is almost exclusively orthogonal to the minimum principal stress, leading to strongly anisotropic flow. This is because of the stress transfer associated with opening of tensile fractures, which increases the confining stress acting across neighboring natural fractures. This limits the hydraulic response of fractures and the capacity to create symmetric isotropic permeability relative to the injection wellbore. These findings suggest that the development of permeability at depth can be improved by targeting a set of fluid injections through smaller packed intervals instead of a single longer injection in open boreholes.
  • Publication
    Accès libre
    Hydraulic Fracturing Mine Back Trials — Design Rationale and Project Status
    (2013-10-23)
    Kaiser, P.K.
    ;
    Valley, Benoît 
    ;
    Dusseault, M.B.
    ;
    Duff, D.
    Last year, a joint Mining and Oil & Gas industry consortium was established in Canada to conduct hydraulic fracturing (HF) tests accompanied by a mine-back of fractured regions to assess HF models and microseismic monitoring data during controlled experiments. Details about the displacement field, fracture aperture and extent, and micro-seismic parameters could then be verified and used as calibration data for modeling of HF processes in igneous and dense sedimentary rocks. Various injection experiments are planned and they will include pre-fracturing rock mass characterisation using best available current techniques, dense arrays of multi-parameter wall and borehole-mounted instruments, and the treated volume will be mined through to assess fracturing effectiveness, existing fractures and new fracture interactions, and to determine if pathways can be identified for improving currently available numerical and fracture network modeling tools. In this paper we present the results of the experimental design and planning phase, outlining objectives and justifications for planned experimental layouts. Preliminary plans for a first mine-through trial at Newcrest Mining’s Cadia East mine in New South Wales, Austral‐ ia are described. The hypotheses advanced in this experimental design, supported by evidence from the literature, are that activation and development of a fracture network by hydraulic stimulation is possible if the injection procedure is designed such that injection pressures and rates are maintained within an optimal window, thereby producing conditions under which effective stress management for risk mitigation in deep mining can best be achieved. The evaluation of these hypotheses is the focus of the current high level experimental plan presented in the paper.
  • Publication
    Accès libre
    Laboratory-scale strain and temperature response of a distributed optical fiber sensor
    (American Rock Mechanics Association, 2013-6-23)
    Madjdabadi, B.
    ;
    Valley, Benoît 
    ;
    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.
  • Publication
    Accès libre
    Numerical 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.
    ;
    Valley, Benoît 
    ;
    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.
  • Publication
    Accès libre
    Effect of grain scale geometric heterogeneity on tensile stress generation in rock loaded in compression
    (American Rock Mechanics Association, 2012-6-24)
    Bewick, R.P.
    ;
    Valley, Benoît 
    ;
    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.
  • Publication
    Accès libre
    Monitoring mining-induced rock mass deformation using distributed strain monitoring based on fiber optics
    (2012-5-28)
    Valley, Benoît 
    ;
    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.
  • Publication
    Accès libre
    Numerical Investigation of the Influence of Borehole Orientation on Drilling-Induced Core Damage
    (2012-5-28)
    Bahrani, N.
    ;
    Valley, Benoît 
    ;
    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.