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Lecture21_Rock Classification_06_feb2024.pptx
- 1. Rock Mechanics SubjectCode: CE6130 Lecture Rock Mass Classification
- 2. Intact Rock Classification •Geological Classification • Engineering Classification • Deer-Miller Classification Rock Mass Classification • Terzaghi rock load classification (1946) • Stand up time classification (1958) • Rock Quality Designation (1964) Engineering classification for rock masses • Rock Structure Rating, 1972 (RSR) • Rock Mass rating (RMR) • Q-System • Classification System for design of tunnels • Geological Strength Index (GSI)
- 3. Rock Tunneling QualityIndex Q-System • was proposed on the basis of an analysis of 212 hard rock tunnel case histories. • numerical value of Q ranges from 0.001 to a maximum of 1,000 on a logarithmic scale. where: RQD = Rock quality designation Jn = Joint set number Jr = Joint roughness number Ja = Joint alteration number Jw = Joint water reduction number SRF = Stress reduction factor RQD/Jn is a measure of block size Jr/Ja is a measure of joint frictional strength Jw/SRF is a measure of joint stress SRF J J J J RQD Q w a r n
- 12. Parameter Description Value RQD75 to 90 RQD = 80 Joint Sets Two joint sets plus random joints Jn = 6 Joint Roughness Smooth, undulating Jr = 2 Joint alteration Slightly altered joint walls, non-softening mineral coatings, sandy particles, clay-free disintegrated rock, etc. Ja = 2 Joint water reduction factor Medium inflow with occasional outwash of joint fillings Jw = 0.66 Stress reduction factor Medium stress, favorable stress condition SRF = 1.0
- 13. Application of Q-System •Empirical correlations = ultimate roof pressure (MPa) = ultimate wall support pressure (MPa) where: Qw = wall factor = 5 Q --------Q > 10 = 2.5 Q --------0.1 < Q < 10 = Q --------Q < 0.1 3 1 2 . 0 Q J P r v 3 1 2 . 0 w r h Q J P
- 15. Q- related totunnel support requirements through equivalent dimensions = ESR diameter height span De / /
- 17. Correlation between theRMR and Q-System Bieniawski (1976) RMR = 9 logQ + A where A varies between 26 and 62, and the average of A is 44 Abad et al. (1983) RMR = 10.5 logQ + 42 Limitations of Q system • It is difficult to obtain the Stress Reduction Factor SRF. • Use of open logarithmic scale of Q varying from 0.001 to 1000 as compared to the • linear scale of up to 100 induces difficulty in using the Q-system (Bieniawski 1989). • The ratio RQD/Jn does not provide a meaningful measure of relative block size and the ratio Jw/SRF is not a meaningful measure of the stresses acting on the rock mass to be supported. • Q-system is not suitable for soft rocks; their best application is with drill and blast tunnels (mining origins).
- 18. Geological Strength Index(GSI) • Hoek and Brown (1997) introduced the GSI, both for hard and weak rock masses. • There are five main qualitative classifications of rock mass structures: 1. Intact or massive 2. Blocky 3. Very blocky 4. Blocky/folded 5. Crushed 6. Laminated/sheared • Discontinuities are classified into five surface conditions which are similar to discontinuity conditions in RMR as described earlier: 1. Very good 2. Good 3. Fair 4. Poor 5. Very poor • Based on the actual rock structure classification and the discontinuity surface condition, a block in the 6x5 matrix can be picked up and the corresponding GSI value can then be read from the figure.
- 26. Generalized Hoek-Brown strengthcriterion for undisturbed rock masses: mj = reduced value of the material constant mi D is a disturbance factor that depends upon the degree of disturbance to which the rock mass has been subjected by blast damage and stress relaxation. It varies from 0 for undisturbed in situ rock masses to 1 for very disturbed rock masses. n ci j ci s m 3 3 1 D GSI m m i j 14 28 100 exp D GSI s 3 9 100 exp 3 / 20 15 / 6 1 2 1 e e n GSI b ci tj n ci cj m s s ;