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Rock quality classification

Having tests that give an idea of

the quality of the rock mass isessential for engineering purposes

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What are we calling a rock?

Grade Description Lithology Excavation Foundations

VISoil Some organic content,

no original structure

May need to

save and re-use

Unsuitable

V Completelyweathered

Decomposed soil, someremnant structure

Scrape Assess by soiltesting

IV Highlyweathered Partly changed to soil,soil > rock Scrape NBcorestones Variable andunreliable

III Moderatelyweathered

Partly changes to soil,rock > soil

Rip Good for mostsmall structures

II Slightlyweathered

Increased fractures andmineral staining

Blast Good foranything exceptlarge dams

I Fresh rock Clean rock Blast Sound

Engineering classification of weathered rock

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Rock Mass Strength

Strength depends on the density, nature and

extent of the fractures within it

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Rock fractures and their

characterization

Typically carried out using 3 orthogonal scanlines

orientation

spacing length

roughness

aperture filling

block size

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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

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Rock Quality Designation (RQD)

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RQD

RQD

A. Very poor 0 25B. Poor 25 50C.Fair 50 75

D.Good 75 90E. Excellent 90 - 100

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RMR and Q Rock classification

systems

Primary use of RQD is as a parameter inmore widely used

RMR (Bieniawski, 1976) and Q Rock (Barton et al., 1974)

classification systems

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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

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RMR or Geomechanics

Classification

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Rock Tunnelling Quality Index, Q

(or Norwegian Q system), Barton et al., 1974

=

SRF

Jw

Ja

Jr

Jn

RQDQ

RQD = Rock Quality Designation 100 - 10Jn = Joint set number 1 20Jr = Joint roughness factor 4 -1Ja = Joint alteration and clay fillings 1 20

Jw = Joint water inflow or pressure 1 0.1SRF = stress reduction factor 1 20

Typically: 0.01 < Q

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Q system

=

SRFJw

JaJr

JnRQDQ

(RQD/Jn) = crude measure of block size

(Jr/Ja) = roughness/friction of surfaces

(Jw/SRF) = ratio of two stress parameters (active stress)

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Guideline properties of Rock Mass

Classes

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Using Rock Mass Classification

Systems

RMR and Q most widely used

Both use similar parameters; difference in

weighting

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Using Rock Mass Classification

Systems

Good practice toassign a range ofvalues

Field example

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CN Tower, Toronto

Worlds tallest free-standingstructure

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

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