Geotechnical Engineering – Rock Quality Classification

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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?

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

Tags: #Classification #Geotechnical Engineer #Geotechnical Engineering #Rock Quality

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