observations for the long-term behaviour of rocks based on

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

Photo courtesy of Rocker

Photo courtesy of Lyssipos

Long-term Time-dependent processes

Short-term Time-dependent processesPhoto courtesy of Molinda & Mark

Photo courtesy of NIOSH

Photo courtesy of WalthamPhoto courtesy of Diederichs

Photo courtesy of Barla

Time-dependent processes

Damage Evolution and Failure of Brittle rocks

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)

UCS Baseline testing series

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

Relaxation testing series

Jura vs. Cobourg limestone (single-step)Jura limestone (multi-step)

Relaxation testing series

Multi-step vs. Single-step_Jura limestone

Relaxation testing – Data Analysis1 2

43

Data Analysis_Maximum Stress Relaxation Crack Initiation Stress-ratio = σ0i/CIi

0 < σ0i/CIi < 3.3

Three Stages of Stress Relaxation

Raw data_Jura limestone

Three Stages of Stress Relaxation Jura limestone Cobourg limestone

Three Stages of Stress Relaxation RI

Jura limestone Cobourg limestone

RII RIII

RI

RII

RIII

RI

Estimating and predicting the relaxation behaviour in rocks

Damage evolution and Failure of Brittle rocks

Paraskevopoulou et al. 2015

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


Long-term strength of various rock types



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.

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

References

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