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