1. Engineering Classification Systems
for Rocks
INTACT ROCK
ROCK MASS
Rock mass refers to in situ rock with all its inherent
geomechanical anisotropies

2. Engg classification
• Geologists: have a genetic bias, provide little
information relating to engg behaviour
• Tests which are used for engg classification are called
Index Test
• If right index tests are chosen, then rocks having
similar index properties, irrespective of their origin,
will probably exhibit similar engg performance
• First Major classification by Terzaghi (1946)
• After that, there have been many classification =
some are oversimplified, misapplied = led to chaos,
multiplicity
• Last two decades = some effort done, to bring some
order from the chaos

3. Bases of Engg Classification
• Lithology
• Strength
• Modulus of deformation
• Discontinuities, Joint: spacing, inclination,
orientation, roughness, groundwater flow
• In situ stress

4. Terzaghi’s Classification
• Terzaghi : first to attempt classification
• Bases : discontinuity, weathering
Term / Description
Intact = neither joints nor hair cracks
Stratified = individual strata with little or no resistance against
separation
Moderately Jointed = blocks between joints are intimately
interlocked
Blocky and Seamy = rocks consists of chemically unweathered
rock fragments which are entirely separated from each
other
Crushed = chemically unweathered, like crusher run material
Squeezing = slowly advances into tunnel
Swelling = expansion due to swelling capacity

5. Disadvantage of Terzaghi’s Classification
• Overlooked the properties of rocks
• Granite, mudstone may belong to one group
as per Terzaghi
• But their mechanical properties differ
significantly

6. Grade Description Lithology Foundations
VI Soil Some organic content,
no original structure
Unsuitable
V Completely
weathered
Decomposed soil, some
remnant structure
Assess by soil
testing
IV Highly
weathered
Partly changed to soil,
soil > rock
Variable and
unreliable
III Moderately
weathered
Partly changes to soil,
rock > soil
Good for most
small structures
II Slightly
weathered
Increased fractures and
mineral staining
Good for
anything except
large dams
I Fresh rock Clean rock Sound
Engineering classification of weathered rock
Based on Weathering

7. Classification of Intact Rock
• Deere and Miller (1966) Classification of intact rock:
• Bases: unconfined (uniaxial) compressive strength ( 1)
• Young’s Modulus (E)
– Rocks are subdivided into five strength categories on a geometric
progression basis based on unconfined (uniaxial) compressive
strength ( 1)
Description/ Class
Very high A
High B
Medium C
Low D
very low E

8. Based on Young’s Modulus (E)
/Modulus Ratio
Three ratio intervals are employed for the
modulus ratio;
high(H) – medium(M) – low(L).
Rocks are therefore classed as
BH (high strength- high ratio);
CM (medium strength – medium ratio),
etc.

9. C-factor classification (Hansagi, 1974)
• Fissuration factor, C = 1/2S(p x H+k x n)
• S = length of drill hole
• p = number of cylindrical samples which can
be obtained from cores corresponding to
length S
• H = height of the cylindrical samples used for
compression testing
• k = total length of core fragments with
cylindrical heights > the core diameter
• n = number of core samples

10. Coates Classification
1. Uniaxial com strength : Weak, Strong, Very
strong
2. Pre-failure Deformation: Elastic, Viscous
3. Failure Characteristics: Brittle, Plastic
4. Gross Homogeneity: Massive, Layered
5. Continuity: Solid, Blocky, Broken

11. USBM Classification (US Bureau of Mines)
• Competent Rock – excavation requires no
added support
• Massive-Elastic – homogeneous and isotropic
• Bedded- elastic but laminated, little cohesion
b/w beds
• Massive-Plastic – Rocks that will have creep
flow under low stress

12. Rock Quality Designation Index (RQD)
(Deere et al. 1967)
• Aim : to provide a quantitative estimate of rock
mass quality from drill logs
• Equal to the percentage of intact core pieces
longer than 100mm in the total length of core

13. Rock Quality Designation (RQD) and Indices
• Quality and deformability = f (discontinuity)
• Core Recovery = total core recovered/length of drilling
– Expressed in %, good core recovery = good quality rock
– Poor core recovery = poor quality of rocks
– In core recovery, all lengths of cores are counted
• In RQD, only cores with >10cm length are considered
• Quantitative estimate of rock mass quality from drill
core logs
RQD = % intact core pieces >10cm in total length of core
Rock Quality Designation index, or RQD, was introduced by Don Deere in 1963.
It judges rock quality based solely on measurements of recovered rock core

14. Rock Quality Designation (RQD)

15. Natural joint surface vs core broken
during operation
• Mechanical breaking of cores caused by
drilling processes
• Natural joint surface

16. Broken cores
• Cores broken during handling or drilling =
fresh irregular breaks = can be fitted
• Natural Joint surface – broken surface smooth,
pieces cannot be fitted
• For RQD determination, International Society
for Rock Mechanics recommends a core size of
at least 54.7 mm (NX) diameter drilled with
double tube core barrels

18. RQD
RQD%
A.Very poor 0 – 25
B.Poor 25 – 50
C.Fair 50 – 75
D.Good 75 – 90
E.Excellent 90 - 100

19. RQD : when core is unavailable
• RQD = 115 – 3.3 Jv, for Jv between 4.5 and 30.
• For Jv < 4.5, RQD is 100%, and for Jv > 30, RQD
is 0%.
Jv = number of joints per m3 volume of rock mass

20. Outcrop Measurements of RQD

21. RQD
• Directionally dependant parameter
• Intended to indicate rock mass quality in-situ
• Adapted for surface exposures as ‘Jv’ number of
discontinuities per unit volume
• Used as a component in the RMR and Q systems
• Palmstrom (1982)
• Priestai Hudsona (1976)
l - number of joints per unit length
v
J
RQD 3
.
3
115 

  l
l 1
.
0
1
.
0
1
100 

 e
RQD

22. Mass factor = j
• j = ratio of deformation modulus of rock mass
(in situ, outcrop)/ that of intact rock core
comprising the same lithology
• j = reflects effects of discontinuities

23. Velocity ratio
• Vcf/Vcl
• Vcf = in situ (field f) compressional (c) wave
velocity
• Vcl = compressional (c) wave velocity in lab (l),
intact rock core
• Difference in Vcf and Vcl caused by structural
discon

24. Quality
classification
/ Rock
Quality
Description
RQD% Mass
Factor, j
Velocity
Ratio,
Vcf/Vcl
Fracture
frequenc
y
C-factor
Very Poor 0-25 - 0-0.2 >15 0-0.15
Poor 25-50 <0.2 0.2-0.4 15-8 0.15-
0.30
Fair 50-75 0.2-0.5 0.4-0.6 8-5 0.30-
0.45
Good 75-90 0.5-0.8 0.6-0.8 5-1 0.45-
0.65
Excellent 90-100 0.8-1.0 0.8-1.0 <1 0.65-
1.0

25. Multi parameter Rock Mass Classification Schemes
• Rock Mass Structure Rating (RSR)
• Rock Mass Rating (RMR)
• Rock Tunnelling Quality Index (Q)
• Geological Strength Index (GSI)

26. Rock Mass Rating (RMR) /
Geomechanics Claasification
• Rating Concept
• Weightage
• Total Rating

27. Rock Mass Rating (RMR), Geocmechanics
Classification (Bieniawski,1976, 1989)
• Classifies rock mass according to 6 rated
parameters:
– UCS (unconfined com strength)
– RQD
– Spacing of discontinuities
– Condition of discontinuities
– Groundwater conditions
– Discontinuity orientation

28. RMR or ‘Geomechanics Classification’
RMR emphasizes STRUCTURAL consideration and Orientation

29. Rock Mass Rating System
1976 to 1989 Bieniawski
• System refined by greater data
• Ratings for parameters changed
• Adapted by other workers for
different situations
• PROJECT SPECIFIC SYSTEMS

30. Development of Rock Mass Rating System

31. Rock Mass Rating System
Rating Class Description
81-100 I Very Good Rock
61-80 II Good Rock
41-60 III Fair Rock
12-40 IV Poor Rock
Less than 20 V Very Poor Rock

32. Rock Tunnelling Quality Index, Q
(or Norwegian Q system), Barton et al., 1974
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
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
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


SRF
Jw
Ja
Jr
Jn
RQD
Q
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
Good Bad
Q Range 0.001 to 1000
Exceptionally poor squeezing : 0.001
Exceptionally good unjointed Rock : 1000

33. Q system
• (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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


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





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




SRF
Jw
Ja
Jr
Jn
RQD
Q
•Q Does NOT include joint orientation
Mistake in Bell New Volume/Indian Ed

34. Guideline properties of Rock Mass Classes

35. Rock Mass Structure Rating (RSR) (1972)
• Introduced the concept of rating components to arrive
at a numerical value
• Demonstrates the logic in a quasi-quantitative rock
mass classification
• Has limitations as based on small tunnels supported
by steel sets only
RSR = A + B + C
Maximum value of RSR = 100

36. Rock Structure Rating
Parameter A: General area geology
Considers (a) rock type origin
(b) rock ‘hardness’
(c) geotechnical structure

37. Considers (a) joint spacing
(b) joint orientation (strike and dip)
(c) direction of tunnel drive
Rock Structure Rating
Parameter B: Geometry : Effect of discontinuity pattern

38. Considers (a) overall rock mass quality (on the basis of A + B)
(b) joint condition
(c) water inflow
Rock Structure Rating
Parameter C: Groundwater, joint condition

39. Using Rock Mass Classification Systems
• RMR and Q most widely used
• Both use similar parameters; difference in
weighting
• Q Does NOT include joint orientation
• RMR emphasizes STRUCTURAL consideration and
Orientation