Effect of time and wear on the basic friction angle of rock discontinuities

Transcription

1 Effect of time and wear on the basic friction angle of rock discontinuities Ignacio Pérez Rey, Leandro R. Alejano, Noelia González Pastoriza, Javier González, Javier Arzúa John P. Harrison Rock Mechanics Laboratory School of Mining Engineering University of Vigo (Vigo, Spain)

2 INDEX 1. Discontinuities 2. The basic friction angle 3. Background 4. Motivation of the study 5. The effect of time on the friction angle 6. The effect of wear on the friction angle 7. Quantifying the effect of wear on the friction angle 8. Conclusions

3 1. Discontinuities Discontinuities determine the behaviour of rock masses. It is fundamental to understand how they behave. a b c e d Alejano, Ramírez Oyanguren (2005)

4 2. The basic friction angle, ϕ b Considerations τ = σn tan φr + JRC log 10 JCS σ n Barton (1976) τ = shear strength of the discontinuity (MPa) σ n = normal applied strength on the discontinuity (MPa) JRC = Joint Roughness Coefficient JCS = Joint Compressive Strength (MPa) ϕ r = residual friction angle ( ) φr = φ smooth and unfilled b discontinuities Estimated in our laboratory by means of tilt or pull test High variability of results while testing same rock specimens González et al. (2014) González et al. (2014)

5 3. Background Alejano et al. (2012) Geometry of samples may affect test results Testing cores along generatrixes, small rock specimens or disc-shaped samples are not recommended Run tilt tests with surfaces > 50 cm 2 and l/h > 2 Hencher (2012a) Friction angles derived from tilt test reflect conditions of surface wear, finish, weathering and presence/nature of debris González et al. (2014) Tilt tests and pull tests produce comparable sliding angle results Friction angle decreases (increases) if dust is (not) removed after each performed test within a series Tests performed at a particular moment and years later yield different results

6 4. Motivation of the study A clear difference of the basic friction angle was observed for the same rock specimens tested in different years TIME? (weathering of rock surfaces) WEAR? (repeated tilt testing) EXPERIMENTAL PROGRAM

7 5. The effect of time on the sliding angle a b 5 cm Pérez Rey et al. (2015) Pérez Rey et al. (2015) Blanco Mera granite (hard rock) Six Observed pairs of friction parallelepipedic angle is largely rock slabs dependent on distance Each pair of of sliding samples were tested once a month with It cleaning has no of dependence surfaces on time after cutting (monthly Temperature scale) (20 24 C) and humidity (45 53%) reasonably constant

8 6. The effect of wear on the sliding angle Blanco A decaymera on the granite friction (hard angle rock) can be Six observed pairs of parallelepipedic rock slabs 100 A logarithmic single tilt fit tests seems withappropriate cleaning of surfaces Dependence of the friction angle on previous sliding Pérez Rey et al. (2015)

9 7. Quantifying the effect of wear on the sliding angle a Extend the study to other rocks Igneous (granite, migmatite, serpentinized dunite), sedimentary (sandstone) and metamorphic (slate, gneiss) rocks were selected About 1000 tests carried out in laboratory b

10 7. Quantifying the effect of wear on the sliding angle Metamorphic rocks: gneiss Gneiss Angle of sliding ( ) Pérez Rey et al. (2015) UCS 250 MPa 10 5 y = -3,194ln(x) + 18,016 R² = 0, ,1 1,0 10,0 Displacement (m)

11 7. Quantifying the effect of wear on the sliding angle Metamorphic rocks: slate Slate Angle of sliding ( ) Pérez Rey et al. (2015) UCS 50 MPa 10 5 y = -2,333ln(x) + 25,648 R² = 0, ,1 1,0 10,0 Displacement (m)

12 7. Quantifying the effect of wear on the sliding angle Igneous rocks: migmatite Migmatite Angle of sliding ( ) Pérez Rey et al. (2015) UCS 100 MPa 10 5 y = -3,469ln(x) + 19,081 R² = 0, ,1 1,0 10,0 Displacement (m)

13 7. Quantifying the effect of wear on the sliding angle Igneous rocks: serpentinized dunite Serpentinized dunite Angle of sliding ( ) Pérez Rey et al. (2015) UCS 190 MPa 5 y = -5,159ln(x) + 20,796 R² = 0, ,1 1,0 10,0 Displacement (m)

14 7. Quantifying the effect of wear on the sliding angle Igneous rocks: granite Granite Angle of sliding ( ) Pérez Rey et al. (2015) UCS 120 MPa 10 5 y = -2,981ln(x) + 24,613 R² = 0, ,1 1,0 10,0 Displacement (m)

15 7. Quantifying the effect of wear on the sliding angle Sedimentary rocks: sandstone Sandstone UCS 45 MPa Angle of sliding ( ) y = -4,595ln(x) + 31,802 5 R² = 0, ,1 1,0 10,0 Displacement (m) Pérez Rey et al. (2015)

16 7. Quantifying the effect of wear on the sliding angle Table 1. Estimated basic friction angle for the studied rocks Rock ϕ b [ ] Gneiss 27.3 Dunite 34.2 Migmatite 29.1 Slate 32.6 Sandstone 41.7 Granite 32.3

17 8. Conclusions A laboratory program has been developed in order to study the effect of time and wear on the basic friction angle of rock discontinuities No method is still suggested to determine the basic friction angle of rock discontinuities Time after cutting of samples does not affect tilt test results at a monthly scale A relevant decrease of the friction angle can be observed after repeated tilt-testing due to wear of the surfaces A logarithmic fit of friction angle values against distance of sliding can reasonably capture testing trends Basic friction angle is proposed as the value of the logarithmic fit evaluated at the corresponding distance for the first test performed for each series

18 It is the existence of discontinuities in a rock mass that makes rock mechanics a unique subject JA Hudson JP Harrison Many thanks for your attention Glückauf!