LICENTIATE THESIS
Evaluation of rock mass strength criteria
Catrin Edelbro
Department of Civil and Environmental EngineeringDivision of Rock Mechanics
BACKGROUND
BACKGROUND
Correct input data Result
Reliable stability prognoses
Correct design of drifts, tunnels, etc.
Optimised drilling, blasting and reinforcement
Minimise risk of failure
Decreased costs for rock excavators
Minimise environmental disturbances
METHODS TO DETERMINE THE ROCK MASS STRENGTH
Failure criteria
Back analysis of failure
Mathematical modelling
Rock mass classification
Large-scale testing
FAILURE CRITERION
σ1s = σ1 (σ3, parameter 1, parameter 2,…., parameter n)
Rock classification system
OBJECTIVE
Literature review of existing criteria
Selection of criteria based on limitations
Evaluate selected criteria and parameters in a case study
Identify the most applicable criteria and useful parameters
SCOPE
Intact rock Closely jointed rock
Discontinuous ContinuousContinuous
Continuous materialSwedish rock conditionsTypical tunnel dimensionsCompressive failure (spalling, shearing)Exclude rock burst problems, effect of σ2, blasting or creeping
SELECTION OF CRITERIA
Present a result that can be used to estimate the strength
Numerical value
Used after the first publication
Applicable to hard rock masses
REVIEW OF SELECTED CRITERIA
RMRbasic (σc, RQD, water condition, joint condition, and spacing)Hoek-Brown -, Yudhbir - and Sheorey - RMR76
Mining Rock Mass Rating (MRMR)Rock Mass Strength (RMS)
Q (RQD, joint set number (Jn), joint roughness (Jr), joint alteration (Ja), joint water reduction factor (Jw), and stress reduction factor (SRF))
rock mass quality system (Q) rock mass Number (N) Rock Mass index (RMi)
GSI (surface condition and structure of rock)Hoek-Brown – GSI
Intact rock strength
Block size
Block shape
Joint strength
Physical scale
IMPORTANT PARAMETERS
CASE STUDY
1. "Round Robin Test" for the pillar strength test in the Laisvall mine and a fictitious case in hard rock.
11 participants in the Laisvall case7 participants in the fictitious case
2. Rock mass strength determination by the author on the large scale strength test of the Stripa core.
LAISVALL
Full-scale pillar test of 9 pillars (Krauland and Söder,1989)
Stresses measured in pillar number 5 and 9
LAISVALLLAISVALL
The pillar load bearing capacity: 19.8 MPa(Krauland and Söder,1989)
LAISVALL
State of stressCondition class 1 Condition class 3
σpeak
Elastic
σcm
PlasticElastic
Before failure After failure
Initial spalling Pillar load bearing capacity
Condition class 1 Condition class 3
Width
Height
LAISVALL
6
6.7 m
6.0 m
7.8 m
1 2 3
4 5 6
7 8 9
ExamineTAB
6 6.7 m
LAISVALLEstimated average rock mass strength values by 11 participants
0 20 40 60 80Rock mass strength (MPa)
Hoek-Brown - RMR76
RMS
Q
Hoek-Brown - GSI
RMi
N
Sheorey - RMR76
Yudhbir - RMR76
MRMR (DRMS) *
Determined peak strength of pillar surface (30 MPa)
Determined bearing capacity of pillars (19.8MPa)
LAISVALLMinimum, average and maximum value when using Q
0
1
2
3
4
5
6
7
8
9
10
11
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140
Rock mass strength (MPa)
Participant number
Determined peak strength of pillar surface (30 MPa)
Determined bearing capacity of pillars (19.8MPa)
LAISVALL
The effect of the parameters in Q on the rock mass strength
0 10 20 30 40 50 60
Rock mass strength (MPa)
Parameter
Jr
Jn
RQD
Ja
Jw
SRF
FICTITIOUS CASEEstimated average rock mass strength values by 7 participants
0 20 40 60 80Rock mass strength(MPa)
Hoek-Brown - RMR76
RMS
Q
Hoek-Brown - GSI
RMi*
N
Sheorey - RMR76
Yudhbir - RMR76
MRMR*
STRIPA CASE
Uniaxial compressive test
Core diameter 1 m and length 2 m
σcm = 7.4 MPa
STRIPA CASEEstimated rock mass strength values by the author
0 20 40 60 80
Rock mass strength [MPa]
Hoek-Brown - RMR76
RMS
Q
Hoek-Brown-GSI
RMi
N
Sheorey - RMR76
Yudhbir - RMR76
MRMR
Measured rock strength (7.4 MPa)
RESULTS FROM THE CASE STUDYConformance - criteria and measured strength
- 40RMS
+ 291MRMR
+ 321Sheorey - RMR76
+ 272Hoek-Brown - RMR76
+202Yudhbir - RMR76
+ 453Hoek-Brown - GSI (2002)
- 33RMi
+ 124Q (2002)+ 87N
Deviation from 7.4 [MPa]
Number of values between 19.8-30 [MPa]
Stripa caseLaisvall caseCriterion
RESULTS FROM THE CASE STUDYScatter of criteria
2.5RMS9.5RMS9.5RMS3.5N-system14.0N-system33.5MRMR(DRMS)3.5MRMR(DRMS)29.0Q-system (2002)38.0N
5.5Q-system (2002)46.0MRMR(DRMS)47.5Hoek-Brown - GSI(2002)
8.0RMi56.0Hoek-Brown - GSI(2002)
48.0RMi27.5Yudhbir - RMR7658.0RMi48.7Yudhbir - RMR76
37.5Hoek-Brown - GSI (2002)91.8Yudhbir - RMR7662.0Hoek-Brown - RMR76
38.0Hoek-Brown - RMR76100.5Hoek-Brown - RMR7665.5Sheorey - RMR76
40.8Sheorey - RMR76101.5Sheorey - RMR76138.8Q (2002)
Scatter [MPa]
CriterionScatter [MPa]
CriterionScatter [MPa]
CriterionStripa CaseFictitious CaseLaisvall Case
RESULTS FROM THE CASE STUDYScatter of criteria
2.5RMS9.5RMS9.5RMS3.5N-system14.0N-system33.5MRMR(DRMS)3.5MRMR(DRMS)29.0Q-system (2002)38.0N
5.5Q-system (2002)46.0MRMR(DRMS)41Q (2002)
8.0RMi56.0Hoek-Brown - GSI(2002)
47.5Hoek-Brown - GSI(2002)
27.5Yudhbir - RMR7658.0RMi48.0RMi
37.5Hoek-Brown - GSI (2002)91.8Yudhbir - RMR7648.7Yudhbir - RMR76
38.0Hoek-Brown - RMR76100.5Hoek-Brown - RMR7662.0Hoek-Brown - RMR76
40.8Sheorey - RMR76101.5Sheorey - RMR7665.5Sheorey - RMR76
Scatter [MPa]
CriterionScatter [MPa]
CriterionScatter [MPa]
CriterionStripa CaseFictitious CaseLaisvall Case
RESULTS FROM THE CASE STUDYParameters
jR and jLjA and VbjRjA and VbRMi
DGSIDGSIHoek-Brown - GSI
RQD and JnJaRQDJaN
RQD and JnJaRQDSRF and JaQ
RQDJoint condition and joint spacing
RQDJoint condition and joint spacing
RMS
RQDJoint condition and joint spacing
RQDJoint condition and joint orientation
MRMR
RQDJoint condition and joint spacing
RQDJoint condition and joint spacing
RMR76
Minor scatterMajor scatterMinor scatterMajor scatterFictitious caseLaisvall caseCriterion
COMMENTS TO THE CASE STUDY
Information lost “on the way”…
Uncertainty of rock stress measurements
No systematic errors in the estimated strength values by the participants
CONCLUSIONS
Most applicable criteria and useful parameters
Rock properties and surface condition (m, s, GSI)
Hoek-Brown - GSI
Joint strength parameters (Jr, Ja)Q
Block volume (Vb, jL)RMi
Rock mass quality (A)Yudhbir - RMR76
Size of construction (B)N
ParametersCriteria
FUTURE RESEARCHDevelop a method to estimate the rock mass strength
Hard rock mass strength =
f (σ3; σc; block size and shape; joint strength; physical scale)
New case studies where measured rock mass strength can be compared to determined strength from a method.
REFERENCE GROUP
• Lars Malmgren and Christina Dahnér-Lindqvist at
• Erling Nordlund, at
• Per-Ivar Marklund, at
• Jonny Sjöberg, at
FINANCIAL SUPPORT
Research council of Norrbotten
LKAB LTU
LKAB – foundation
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