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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
    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.
  • Publication
    Accès libre
    Fractal characteristics of fractures in crystalline basement rocks: Insights from depth-dependent correlation analyses to 5 km depth
    (2022-5-27)
    Afshari Moein, Mohammad Javad
    ;
    Evans, Keith F.
    ;
    Valley, Benoît 
    ;
    Bär, Kristian
    ;
    Genter, Albert
    The scaling laws describing the spatial arrangement of fractures along six deep boreholes penetrating the crystalline rocks in the Rhine Graben were derived using a correlation analysis. Five of the wells, two to 5 km depth, were located at the Soultz geothermal site and one well to 5 km depth was located at Basel, some 150 km from Soultz. Five datasets were derived from borehole imaging logs, whilst one stemmed from the analysis of 810 m of continuous core at Soultz. The two differed inasmuch as the core dataset included essentially all fractures, whereas the image log dataset had few fractures narrower than 1–3 mm. The results of the analysis for all image datasets showed that the spatial arrangement of fractures followed fractal behavior at all scales from meters to several hundred meters, the largest scale amenable to assessment, and that the fractal dimensions were confined to the narrow range 0.85–0.9. However, the core dataset showed significant deviation from fractal behavior, the best-fit fractal dimension of 0.8 being somewhat lower than values obtained from imaging logs in neighboring wells. Eliminating fractures with apertures less than 1 mm from the core dataset to improve comparability led to even lower fractal dimension estimates, indicating the discrepancy was not due to imaging log resolution. Analysis of successive depth sections of the core log suggested the discrepancy was due to the presence of a localized zone between 1750 and 2070 m where the fractal organization is disturbed or takes a lower dimension than elsewhere. Aside from this zone, no systematic variation of fractal dimension with depth was observed in any dataset, implying that a single exponent together with intensity adequately describes the arrangement of fractures along the entire length of the boreholes. The results are relevant to the parameterization of DFN models of deep rock masses.
  • Publication
    Métadonnées seulement
  • Publication
    Accès libre
    On the link between stress field and small-scale hydraulic fracture growth in anisotropic rock derived from microseismicity
    (2018-7-1)
    Gischig, Valentin
    ;
    Doetsch, J.
    ;
    Maurer, Hansruedi
    ;
    Krietsch, Hannes
    ;
    Amann, Florian
    ;
    Evans, Keith F.
    ;
    Nejati, M.
    ;
    Jalali, Mohammadreza
    ;
    Valley, Benoît 
    ;
    Obermann, A.
    ;
    Wiemer, Stefan
    ;
    Giardini, Domenico
  • Publication
    Métadonnées seulement
  • Publication
    Accès libre
    Fault anatomy of the La Sarraz strike-slip fault system
    (2015-10-26)
    Schmitt, Nicole
    ;
    Grassi, R.
    ;
    Miller, Stephen Andrew 
    ;
    Perrochet, L.
    ;
    Valley, Benoît 
    ;
    Mosar, Jon
  • Publication
    Métadonnées seulement
  • Publication
    Accès libre
    A comparison of FBG- and Brillouin-strain sensing in the framework of a decameter-scale hydraulic stimulation experiment
    (: American Rock Mechanics Association, 2018-6-18)
    Krietsch, H.
    ;
    Gischig, V.
    ;
    Jalali, R.
    ;
    Doetsch, J.
    ;
    Valley, Benoît 
    ;
    Amann, F.
    In the framework of the In-situ Stimulation and Circulation (ISC) experiment Fiber-Bragg-Grating (FBG) and Brillouin strain sensing systems were installed to monitor deformation during six hydraulic shearing and six hydraulic fracturing experiments. Three boreholes were dedicated to strain monitoring. Both systems are installed in the same boreholes, offering a unique opportunity to compare these systems with respect to their applicability in hydraulic stimulation tests. A total of 60 FBG sensors with 1 m base length were installed across fractures, shear zones and intact rock. Along the entire borehole length, pre-stressed optical cables for Brillouin distributed strain (DBS) sensing were embedded in grout with two installation methods: a bare cable and a cable packed and fixed with glue every 0.65 m. The strain signals were compared as time series for a given borehole depth and as profiles along the borehole axis. The study reveals that the FBG system gives a high accuracy (0.04 µ-strain) and temporal resolution (>1s) with pointwise measurements. The bare DBS leg yield good quantitative strain data with poorer strain accuracy (>500 times poorer than FBG) and poorer temporal resolution (factor of >100). The packed DBS leg provide no meaningful information about the strain field.
  • Publication
    Métadonnées seulement
    Numerical Modeling of Strain Transfer From Rock Mass to a Fibre Optic Sensor Installed Inside a Grouted Borehole
    (: American Rock Mechanics Association, 2012)
    Madjdabadi, Behrad M.
    ;
    Valley, Benoît 
    ;
    Dusseault, Maurice B.
    ;
    Kaiser, Peter K.