Having tests that give an idea of the quality of the rock massis essential for engineering purposes.
What are we calling a rock?
Engineering classification of weathered rock
Rock Mass Strength
•Strength depends on the density, nature and extent of the fractureswithin 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
Rock Quality Designation (RQD)
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 ‘GeomechanicsClassification’
Rock Tunnelling Quality Index, Q
(or Norwegian Q system), Barton et al., 1974
RQD = Rock Quality Designation100 -10
Jn= Joint set number1 –20
Jr= Joint roughness factor4 -1
Ja= Joint alteration and clay fillings1 –20
Jw= Joint water inflow or pressure1 –0.1
SRF = stress reduction factor1 –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
