LICENTIATE THESIS Evaluation of rock mass strength criteria Catrin Edelbro Department of Civil and Environmental Engineering Division of Rock Mechanics
BACKGROUND
BACKGROUND Correct input data Reliable stability prognoses Correct design of drifts, tunnels, etc. Optimised drilling, blasting and reinforcement Result Minimise risk of failure Decreased costs for rock excavators Minimise environmental disturbances
METHODS TO DETERMINE THE ROCK MASS STRENGTH Back analysis of failure Failure criteria Large-scale testing Mathematical modelling Rock mass classification
FAILURE CRITERION σ 1s = σ 1 (σ 3, 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 Continuous Discontinuous Continuous Intact rock Continuous material Closely jointed rock Swedish rock conditions Typical tunnel dimensions Compressive failure (spalling, shearing) Exclude rock burst problems, effect of σ 2, 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 RMR basic (σ c, RQD, water condition, joint condition, and spacing) Hoek-Brown -, Yudhbir - and Sheorey - RMR 76 Mining Rock Mass Rating (MRMR) Rock Mass Strength (RMS) Q (RQD, joint set number (J n ), joint roughness (J r ), joint alteration (J a ), joint water reduction factor (J w ), 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
IMPORTANT PARAMETERS Intact rock strength Block size Block shape Joint strength Physical scale
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 case 7 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
LAISVALL LAISVALL The pillar load bearing capacity: 19.8 MPa (Krauland and Söder,1989)
LAISVALL Condition class 1 Condition class 3 Height State of stress Width Initial spalling Pillar load bearing capacity Condition class 1 Condition class 3 σ peak σ cm Elastic Elastic Plastic Before failure After failure
LAISVALL Examine TAB 1 2 3 4 5 6 6.0 m 6 6.7 m 6.7 m 7 8 9 7.8 m
LAISVALL Estimated average rock mass strength values by 11 participants Hoek-Brown - RMR76 RMS Q Hoek-Brown - GSI RMi N Sheorey - RMR76 Yudhbir - RMR76 MRMR (DRMS) * 0 20 40 60 80 Determined bearing capacity of pillars (19.8MPa) Rock mass strength (MPa) Determined peak strength of pillar surface (30 MPa)
LAISVALL Minimum, average and maximum value when using Q Participant number 11 10 9 8 7 6 5 4 3 2 1 0 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 Determined bearing capacity of pillars (19.8MPa) Determined peak strength of pillar surface (30 MPa) Rock mass strength (MPa)
LAISVALL The effect of the parameters in Q on the rock mass strength Parameter Jr Jn RQD Ja Jw SRF 0 10 20 30 40 50 60 Rock mass strength (MPa)
FICTITIOUS CASE Estimated average rock mass strength values by 7 participants Hoek-Brown - RMR76 RMS Q Hoek-Brown - GSI RMi* N Sheorey - RMR76 Yudhbir - RMR76 MRMR* 0 20 40 60 80 Rock mass strength(mpa)
STRIPA CASE Uniaxial compressive test Core diameter 1 m and length 2 m σ cm = 7.4 MPa
STRIPA CASE Estimated rock mass strength values by the author Hoek-Brown - RMR76 RMS Q Hoek-Brown-GSI RMi N Sheorey - RMR76 Yudhbir - RMR76 MRMR 0 20 40 60 80 Measured rock strength (7.4 MPa) Rock mass strength [MPa]
RESULTS FROM THE CASE STUDY Conformance - criteria and measured strength Criterion N Q (2002) RMi Laisvall case Number of values between 19.8-30 [MPa] 7 4 3 Stripa case Deviation from 7.4 [MPa] + 8 + 12-3 Hoek-Brown -GSI(2002) 3 + 45 Yudhbir - RMR 76 2 +20 Hoek-Brown -RMR 76 2 + 27 Sheorey - RMR 76 1 + 32 MRMR RMS 1 0 + 29-4
RESULTS FROM THE CASE STUDY Scatter of criteria Laisvall Case Fictitious Case Stripa Case Criterion Scatter [MPa] Criterion Scatter [MPa] Criterion Scatter [MPa] Q (2002) 138.8 Sheorey - RMR 76 101.5 Sheorey - RMR 76 40.8 Sheorey - RMR 76 65.5 Hoek-Brown -RMR 76 100.5 Hoek-Brown -RMR 76 38.0 Hoek-Brown -RMR 76 62.0 Yudhbir - RMR 76 91.8 Hoek-Brown -GSI(2002) 37.5 Yudhbir - RMR 76 48.7 RMi 58.0 Yudhbir - RMR 76 27.5 RMi 48.0 Hoek-Brown -GSI (2002) 56.0 RMi 8.0 Hoek-Brown -GSI (2002) 47.5 MRMR(DRMS) 46.0 Q-system (2002) 5.5 N 38.0 Q-system (2002) 29.0 MRMR(DRMS) 3.5 MRMR(DRMS) 33.5 N-system 14.0 N-system 3.5 RMS 9.5 RMS 9.5 RMS 2.5
RESULTS FROM THE CASE STUDY Scatter of criteria Laisvall Case Fictitious Case Stripa Case Criterion Scatter [MPa] Criterion Scatter [MPa] Criterion Scatter [MPa] Sheorey - RMR 76 65.5 Sheorey - RMR 76 101.5 Sheorey - RMR 76 40.8 Hoek-Brown -RMR 76 62.0 Hoek-Brown -RMR 76 100.5 Hoek-Brown -RMR 76 38.0 Yudhbir - RMR 76 48.7 Yudhbir - RMR 76 91.8 Hoek-Brown -GSI(2002) 37.5 RMi 48.0 RMi 58.0 Yudhbir - RMR 76 27.5 Hoek-Brown -GSI (2002) 47.5 Hoek-Brown -GSI (2002) 56.0 RMi 8.0 Q (2002) 41 MRMR(DRMS) 46.0 Q-system (2002) 5.5 N 38.0 Q-system (2002) 29.0 MRMR(DRMS) 3.5 MRMR(DRMS) 33.5 N-system 14.0 N-system 3.5 RMS 9.5 RMS 9.5 RMS 2.5
Parameters RESULTS FROM THE CASE STUDY Criterion Major scatter Laisvall case Minor scatter Major scatter Fictitious case Minor scatter RMR 76 Joint condition and joint spacing RQD Joint condition and joint spacing RQD MRMR Joint condition and joint orientation RQD Joint condition and joint spacing RQD RMS Joint condition and joint spacing RQD Joint condition and joint spacing RQD Q SRF and J a RQD Ja RQD and Jn N Ja RQD Ja RQD and Jn Hoek-Brown - GSI GSI D GSI D RMi ja and Vb jr ja and Vb jr and jl
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 Criteria N Yudhbir - RMR 76 RMi Q Hoek-Brown - GSI Parameters Size of construction (B) Rock mass quality (A) Block volume (Vb, jl) Joint strength parameters (J r, J a ) Rock properties and surface condition (m, s, GSI)
FUTURE RESEARCH Develop a method to estimate the rock mass strength Hard rock mass strength = f (σ 3 ; σ c ; 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 Erling Nordlund, at Jonny Sjöberg, at Per-Ivar Marklund, at Lars Malmgren and Christina Dahnér-Lindqvist at
FINANCIAL SUPPORT Research council of Norrbotten LKAB LTU LKAB foundation