Effects of Grain Scale Heterogeneity on Rock Strength and the Chipping Process

Villeneuve, Marlène; Diederichs, Mark S.; Kaiser, Peter K.

doi:10.1061/(asce)gm.1943-5622.0000194

Cited by 53 publications

(17 citation statements)

References 22 publications

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“…Preconditioning is low to moderate in isotropic rock, is concentrated in mica grains, and increases slightly with mica content, but does not vary greatly with grain size (Villeneuve 2008). Rocks with fabric and higher mica content tend to improve chipping performance (Villeneuve et al 2012). When stress is taken into consideration preconditioning leads to even greater chipping performance.…”

Section: Relaxation Preconditioningmentioning

confidence: 98%

“…Numerical modeling of the cutting process (using FLAC, as in Villeneuve et al 2012) was used to refine the field observations. The models simulated the stress changes at the face arising from the excavation process, and leading to preconditioning (Fig.…”

Section: Investigation Of Stress Impacts On Face Instability and Chipmentioning

confidence: 99%

“…Numerical modelling of the chipping process (Villeneuve et al 2012) has shown that geomechanical characteristics, which enhance fracture initiation and propagation will favour chipping over grinding. In particular, the results showed that increased mica content and decreased quartz to feldspar ratio will promote fracture initiation and propagation, and that fractures will propagate easiest along fabric.…”

Section: Introductionmentioning

confidence: 99%

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Geomechanical Characterisation of Hard Rocks for Disc Cutting in Deep Tunnels

Villeneuve

2014

Engineering Geology for Society and Territory - Volume 6

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The disc cutting process for TBM excavation is dependent on the ability of the discs to initiate and propagate fractures into the tunnel face. At any depth, the geomechanical characteristics of the rock will determine how efficiently the fracture initiation and propagation processes occur. In deep tunnels the stresses induced at the tunnel boundary can lead to stress-induced failure mechanisms such as spalling and bursting. This paper examines the impact of geomechanical characteristics in combination with induced stresses at the tunnel face on the disc cutting process. TBM performance data and tunnel face maps were combined with mineralogy, grain size and fabric for deep tunnels in granitic and foliated massive rocks to determine how induced stresses enhance or hinder the fracture initiation and propagation processes. The impact of the induced stress varies with different geomechanical characteristics depending on the orientation and relative magnitudes of the stresses. In addition, stress rotation and relaxation ahead of the face can lead to stress-induced fracture creation at the face, which acts to precondition the rock prior to the cutters excavating. These results show that the sensitivity of different rock types to stress-related enhancement or hindrance of disc cutting must be taken into account for deep tunneling projects and are used to propose a geomechanical characterisation approach to identify potential for increased or reduced disc cutting efficiency in deep tunnels. KeywordsTunnel boring machine Á Deep tunnels Á Stress IntroductionThe excavation process for a hard rock TBM involves fragmentation occurring between disc cutters, which apply cyclical pressure on concentric rings, or kerfs, in the tunnel face (Roxborough and Phillips 1975). This fragmentation can comprise the creation of chips or fines, the former being the more efficient fragmentation process. The chipping process is the generation of chips when tensile fractures are induced into the rock, and then propagate parallel to the tunnel face. Grinding is the generation of fines when fractures do not propagate through the rock and only comminution occurs at the cutter-rock interface.Numerical modelling of the chipping process (Villeneuve et al. 2012) has shown that geomechanical characteristics, which enhance fracture initiation and propagation will favour chipping over grinding. In particular, the results showed that increased mica content and decreased quartz to feldspar ratio will promote fracture initiation and propagation, and that fractures will propagate easiest along fabric.Observations in deep TBM tunnels show that spalling can affect the tunnel face, however, this does not necessarily correspond to spalling at the tunnel wall. The two photos in Fig. 76.1 show two examples of tunnel faces in deep Alpine tunnels. While both tunnels have smooth walls without spalling, the tunnel on the right has spalling in the face. The stress-rock interaction at the tunnel face is different than at the walls, leading to this difference in b...

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Section: Relaxation Preconditioningmentioning

confidence: 98%

Section: Investigation Of Stress Impacts On Face Instability and Chipmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Geomechanical Characterisation of Hard Rocks for Disc Cutting in Deep Tunnels

Villeneuve

2014

Engineering Geology for Society and Territory - Volume 6

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“…Martin et al (1999) considered tunnel instability and brittle failure as functions of RMR and the ratio of the maximum far-field stress to the unconfined compressive strength. Villeneuve et al (2012) considered the effect of grain scale heterogeneity on rock strength and the chipping process which is conceived as fragmentation of joints in several studies. As per Terzaghi (1946), the joints are among the most important causes of overbreak and of troubles induced by water, and they always deserve careful consideration.…”

Section: Introductionmentioning

confidence: 99%

Computing in-situ strength of rock masses based upon RQD and modified joint factor: Using pressure and damage sensitive constitutive relationship

Trivedi

2015

Journal of Rock Mechanics and Geotechnical Engineering

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a b s t r a c tIn this study, a new model was presented for computing strength of rock masses based upon in-situ observations of RQD popularly known as rock quality designation. This model links up the rock mass parameters from in-situ investigations with the strength parameters of jointed rocks obtained from laboratory scale experimental observations. Using the constitutive relation, the author derived a pressure and damage sensitive plastic parameter to determine strength of rock masses for varied extents of discontinuity and pressure induced damage. The test results show that plasticity characterized by hardening and softening inclusive of damage invariably depends upon mean pressure and extent of deformations already experienced by rock masses. The present work explores the test data that reveal the dependence of in-situ strength on incremental joint parameters obtained from the joint number, joint orientation, joint roughness, gouge parameters and water pressure. Substituting the relationship between the RQD and modified joint factor with that between modulus ratio and strength ratio, the model shows successfully that using damage inclusive plastic parameter and RQD provides a relationship for estimating the strength of rock masses. One of the main objectives of this work is to illustrate that the present model is sensitive to plasticity and damage together in estimating in-situ strength of rock masses in foundations, underground excavation and tunnels.

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“…This parameter is multiplied to the mineralogy and porosity portions of the equation to reduce the strength index value based on the severity of fracturing. Primary and secondary mineralogy both influence the strength of rocks, where rocks with predominately weak minerals lead to a lower strength compared to rocks containing predominantly strong minerals (Vutukuri et al, 1974;Rigopoulos et al, 2010;Li et al, 2012;Villeneuve et al, 2012). We developed a method for deriving a representative percentage of the primary and secondary minerals to address this.…”

Section: Proposed Equation -Alteration Strength Index (Asi)mentioning

confidence: 99%

The development and application of the alteration strength index equation

Wyering

Villeneuve

Wallis

et al. 2015

Engineering Geology

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We have developed an alteration strength index (ASI) equation to address the effect of hydrothermal alteration on mechanical rock properties. This equation can be used to estimate a range of rock strengths, comparable to uniaxial compressive strength (UCS), based on rapid analysis of mineralogy and microstructure. We used rock samples from three geothermal fields in the Taupo Volcanic Zone (TVZ) to represent a range of alteration types. These are sedimentary, intrusive and extrusive rocks, typical of geothermal systems, from shallow and deep boreholes (72 measured Depth (mD) to 3280 mD). The parameters used in ASI were selected based on literature relating these aspects of mineralogy and microstructure to rock strength. The parameters in ASI define the geological characteristics of the rock, such as proportions of primary and secondary mineralogy, individual mineral hardness, porosity and fracture number. We calibrated the ASI against measured UCS for our samples from the TVZ to produce a strong correlation (R 2 of 0.86), and from this correlation we were able to derive an equation to convert ASI to UCS. Because the ASI-UCS relationship is based on an empirical fit, the UCS value that is obtained from conversion of the ASI includes an error of 7 MPa for the 50th percentile and 25 MPa for the 90th percentile with a mean error of 11 MPa. A sensitivity analysis showed that the mineralogy parameter is the dominant characteristic in this equation, and the ASI equation using only mineralogy can be used to provide an estimated UCS range, although the error (or uncertainty) becomes greater. This provides the ability to estimate strength even when either fracture or porosity information are not available, for example in the case of logging drill cuttings. This research has also allowed us to provide ranges of rock strengths based solely on the alteration zones, mineralogy, and depth of lithol-ogies found in a typical geothermal field that can be used to update conceptual models of geothermal fields.

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Effects of Grain Scale Heterogeneity on Rock Strength and the Chipping Process

Cited by 53 publications

References 22 publications

Geomechanical Characterisation of Hard Rocks for Disc Cutting in Deep Tunnels

Geomechanical Characterisation of Hard Rocks for Disc Cutting in Deep Tunnels

Computing in-situ strength of rock masses based upon RQD and modified joint factor: Using pressure and damage sensitive constitutive relationship

The development and application of the alteration strength index equation

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