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Observations for the long-term behaviour of rocks based on laboratory
testingChrysothemis Paraskevopoulou, Matthew Perras,
Mark Diederichs, Florian Amann, Simon Löw,Tom Lam, Mark Jensen
Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Canada
Institute of Geology, ETH, Zurich, SwitzerlandNuclear Waste Management Organization, NWMO, Toronto, Canada
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Photo courtesy of Rocker
Photo courtesy of Lyssipos
Long-term Time-dependent processes
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Short-term Time-dependent processesPhoto courtesy of Molinda & Mark
Photo courtesy of NIOSH
Photo courtesy of WalthamPhoto courtesy of Diederichs
Photo courtesy of Barla
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Time-dependent processes
Damage Evolution and Failure of Brittle rocks
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Time-dependent processes
Damage Evolution and Failure of Brittle rocks
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Lab testing series on limestone rock samples• UCS Baseline tests• Static Load (Creep) tests• Relaxation tests
a. Values for Jura Limestone b. Values for Cobourg LimestoneSample
#UCS
(MPa)CD (MPa)
CI (MPa)
E (GPa)Poisson's
ratioSample # UCS (MPa)
CD (MPa)
CI (MPa) E (GPa)Poisson's
ratio
1 65.60 64.88 24.97 102.82 0.26 1 104.14 103.98 31.00 38.20 0.24
2 113.55 113.11 36.10 73.01 0.12 2 145.74 143.35 60.00 37.13 0.18
3 87.80 77.16 35.80 125.15 0.29 3 121.82 121.82 59.00 45.33 0.17
4 111.70 104.57 32.00 86.00 0.26 4 136.49 135.99 55.00 47.29 0.16
5 125.64 100.94 38.00 101.40 0.29 5 131.55 129.52 77.00 45.71 0.15
6 100.76 96.76 38.00 70.87 0.18 6 107.41 103.77 69.00 35.78 0.13
7 67.75 67.07 20.50 94.82 0.22 7 93.71 93.18 17.00 36.97 0.12
8 136.90 125.00 33.00 89.28 0.25 8 148.65 148.61 25.00 40.93 0.11
9 109.53 104.73 20.00 94.31 0.21 9 136.03 133.27 83.00 43.92 0.13
10 111.23 110.87 45.00 92.40 0.22
UCS Baseline testing
• Jura 10 tests
• Cobourg 9 tests
Elastic parameters (E, v)Crack Damage Thresholds (CI, CD, UCS)
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UCS Baseline testing series
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Relaxation testing seriesRock
Formation Jura limestone Cobourg limestone
Type of Test Axial-controlled Radial-controlled Axial-controlled Radial-controlled
Load Level(s)
Single-step
Multi-step
Single-step
Multi-step
Single-step
Multi-step
Single-step
Multi-step
Number of Samples 13 2 -- 3 16 -- -- --
Total 18 16
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Relaxation testing series
Jura vs. Cobourg limestone (single-step)Jura limestone (multi-step)
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Relaxation testing series
Multi-step vs. Single-step_Jura limestone
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Relaxation testing – Data Analysis1 2
43
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Data Analysis_Maximum Stress Relaxation Crack Initiation Stress-ratio = σ0i/CIi
0 < σ0i/CIi < 3.3
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Three Stages of Stress Relaxation
Raw data_Jura limestone
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Three Stages of Stress Relaxation Jura limestone Cobourg limestone
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Three Stages of Stress Relaxation Jura limestone Cobourg limestone
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Three Stages of Stress Relaxation Jura limestone Cobourg limestone
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Three Stages of Stress Relaxation RI
Jura limestone Cobourg limestone
RII RIII
RI
RII
RIII
RI
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Estimating and predicting the relaxation behaviour in rocks
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Estimating and predicting the relaxation behaviour in rocks
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Estimating and predicting the relaxation behaviour in rocks
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Damage evolution and Failure of Brittle rocks
Paraskevopoulou et al. 2015
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Current Study Static Load Tests under Compression
Rock Formation Jura limestoneLoad Level(s) Single-steps Multi-steps
Number of Samples 7 2|2&4 steps
Total 9
Paraskevopoulou et al. 2015
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Long-term strength of various rock types
Paraskevopoulou et al. 2015
Paraskevopoulou et al. 2015
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Concluding RemarksRelaxation is the decrease of applied load at a constant deformation (strain level) . Load relaxation occurs whenever acrack is initiated and it is implied when stress relaxation attains an asymptotic value then the stabilization of crackpropagation is achieved.
Jura limestone relaxes within 6 to 24 hours and Cobourg limestone needs at least 24 to 48 hours, depending on thestress level.
The samples relaxed within the first hour when subjected to a stress level below CI for both rock types and as the stresslevel approached the CD stress level the time needed to relax increased relatively with the stress increase.
The results should be normalized to the crack initiation threshold as it can be determined for each test and providing abetter fit to the data for the single-step tests results by avoiding using an average baseline value.
A better defined approach to estimate the maximum stress relaxation and predict the time of the relaxation of variousrock formations using the CI value and the Young’s Modulus. The latter can be used as input parameters in numericalmodelling and give insight for the rockmass behaviour on the tunnel face during construction period.
The lab test results suggest that a stress threshold below which the rock deformation will be negligible exists below CI.
This study gives more insight into the long-term behavior of brittle rocks and establishes a database which can be usedby researchers and engineers as a preliminary tool to determine the long-term behavior of various rock types.
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Acknowledgements
• Nuclear Waste Management Organization of Canada (NWMO)
• Scientific Equipment Grant Program of ETH Zurich
• Patric Walter, Julian Felder, Linus Villiger and Katrin Wild
• Professor Jean-Pierre Burg
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References
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