Geotechnical Engineering – Rock Quality Classification, Having tests that give an idea of the quality of the rock mass is essential for engineering purposes.
What are we calling a rock?
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Geotechnical Engineering classification of weathered rock
Rock Mass Strength
- Strength depends on the density, nature and extent of the fractures within it
Rock fractures and their characterization
Typically carried out using 3 orthogonal scanlines
- • orientation
• spacing
• length
• roughness
• aperture
• filling
• block size
Rock Quality Designation (RQD)
• Quantitative estimate of rock mass quality from drill core logs
– % intact core pieces >10cm in total length of core
• Deere et al., 1967
RQD
A. Very poor = 0 – 25
B. Poor = 25 – 50
C.Fair = 50 – 75
D.Good = 75 – 90
E. Excellent = 90 – 100
RMR and Q Rock classification systems
• Primary use of RQD is as a parameter in more widely used
– RMR (Bieniawski, 1976) and
– Q Rock (Barton et al., 1974)
classification systems
Rock Mass Rating (RMR), Bieniawski (1976, 1989)
• Classifies rock according to 6 parameters:
– UCS
– RQD
– Spacing of discontinuities
– Condition of discontinuities
– Groundwater conditions
– Discontinuity orientation
RMR or ‘Geomechanics Classification’
Rock Tunnelling Quality Index, Q (or Norwegian Q system), Barton et al., 1974
RQD = Rock Quality Designation 100 – 10
Jn = Joint set number 1 – 20
Jr = Joint roughness factor 4 -1
Ja = Joint alteration and clay fillings 1 – 20
Jw = Joint water inflow or pressure 1 – 0.1
SRF = stress reduction factor 1 – 20
Typically: 0.01 < Q <100
Q system
• (RQD/Jn) = crude measure of block size
• (Jr/Ja) = roughness/friction of surfaces
• (Jw/SRF) = ratio of two stress parameters (active stress)
Guideline properties of Rock Mass Classes
Using Rock Mass Classification Systems
• RMR and Q most widely used
• Both use similar parameters; difference in weighting
Using Rock Mass Classification Systems
• Good practice to assign a range of values
Field example
CN Tower, Toronto
• World’s tallest free-standing structure
• 550m high, 110,000 tonnes
• Foundations:
– shale
– UCS = 10 – 25 MPa
– E = 3.7 GPa
– RQD = 50 – 70 %
• Mean load = 580 kPa
– supported by slab foundation