A description of the Hoek Rock Mass Classification system

1. Rock Mass Classification
Part 1
Hoek’s Rock Engineering Manual

2. Manual Section Outline
 Introduction
 Engineering rock mass classifications
Terzaghi rock mass classification
Rock Quality Designation (RQD)
Rock Structure Rating (RSR)
 Geomechanics Classification
Rock Mass Rating (RMR)
Modifications to the RMR
Rock Tunneling Quality Index, Q

3. Lecture: Friday March 20, 2020
 CalTrans 2010 Manual Review
 When is a design in rock engineering acceptable?
 Introduction: Rock mass classification
 Engineering rock mass classification
Terzaghi (1946)
Stini (1950)
considered as the father of the "Austrian School" of
tunneling and rock mechanics
 Lauffer (1958)
Deere (1976)

4. CalTrans: Soil and
Rock Logging,
Classification,
and Presentation
Manual

5. Rock Core Storage

10. When is a design in rock engineering
acceptable?
 “When is a design in rock engineering acceptable? The aim of the
following text is to demonstrate that there are no simple universal
rules for acceptability nor are there standard factors of safety which
can be used to guarantee that a rock structure will be safe and
that it will perform adequately.
 Each design is unique and the acceptability of the structure has to
be considered in terms of the particular set of circumstances, rock
types, design loads and end uses for which it is intended.
 The responsibility of the geotechnical engineer is to find a safe and
economical solution which is compatible with all the constraints
which apply to the project.
 Such a solution should be based upon engineering judgement
guided by practical and theoretical studies such as stability or
deformation analyses, if and when these analyses are applicable.

11. Introduction: Rock mass classification
1. During the feasibility and preliminary design stages of a
project, when very little detailed information is available
on the rock mass and its stress and hydrologic
characteristics, the use of a rock mass classification
scheme can be of considerable benefit.
2. At its simplest, this may involve using the classification
scheme as a check-list to ensure that all relevant
information has been considered. At the other end of
the spectrum, one or more rock mass classification
schemes can be used to build up a picture of the
composition and characteristics of a rock mass to
provide initial estimates of support requirements, and to
provide estimates of the strength and deformation
properties of the rock mass.

12. Introduction: Rock mass classification
3. It is important to understand the limitations of rock mass
classification schemes (Palmstrom and Broch, 2006) and
that their use does not (and cannot) replace some of
the more elaborate design procedures.
4. However, the use of these design procedures requires
access to relatively detailed information on in situ
stresses, rock mass properties and planned excavation
sequence, none of which may be available at an early
stage in the project.
5. As this information becomes available, the use of the
rock mass classification schemes should be updated
and used in conjunction with site specific analyses.

13. Engineering rock mass classification
 Rock mass classification schemes have been
developing for over 100 years since Ritter (1879)
 Most of the multi-parameter classification schemes
(Wickham et al (1972) Bieniawski (1973, 1989) and Barton
et al (1974)) were developed from civil engineering case
histories in which all of the components of the
engineering geological character of the rock mass were
included

15. Terzaghi's rock mass classification (1946)

17. Terzaghi rock load classification for
steel arch-supported tunnels

18. In Terzaghi’s rock load classification
system assume you have a category 2
“hard stratified or schistose” with B = 20
ft. What is the load on the steel set?
Problem:
Assume unit weight of rock = 165 pcf
Conservative estimate: Hp = 0.5B
Therefore: Hp = (0.5)(20)(165 pcf)
Steel set pressure = 1,650 psf

19. Stini (1950)
 Stini emphasized the importance of structural defects in rock masses.
 Stressed the need to avoid tunneling parallel to the strike of steeply
dipping discontinuities

20. Laufer (1958)
Classifications involving stand-up time
 Both Terzaghi and Stini discussed the time-dependent
instability in tunnels
 Lauffer, however, emphasized the importance of stand-
up time and active span in a tunnel.

21. Definition of an active span

22. Active Span and Stand-Up Time

23. Factors affecting rock mass suitability
during tunneling

24. Rock Quality Designation (RQD)

25. Creator of the RQD System: Don Deere

26. RQD Creators:
 Deere, D.U. (RQD)
 Hendron, A.J. (Rock Blasting)
 Patton, F.D. (Rock Shear)
 Cording, E.J. (Gordie Howe
International Bridge)
 Three professors at the
University of Illinois and one
grad student Ed Cording

29. Rock Quality Designation (RQD)
 Rock Quality Designation index (RQD) was developed
by Deere (Deere et al 1967) to provide a quantitative
estimate of rock mass quality from drill core logs.
 RQD is defined as the percentage of intact core pieces
longer than 100 mm (4 inches) in the total length of core.
The core should be at least NW size (54.7 mm or 2.15
inches in diameter) and should be drilled with a double-
tube core barrel.
 The correct procedures for measurement of the length
of core pieces and the calculation of RQD are
summarized in Figure 1.

33. Rock Core Length
Measurement

34. Assessment of Soundness
Assessment of soundness
Pieces of core which are not "hard and sound" (International
Society for Rock Mechanics, 1978, 1981) should not be counted
for the RQD even though they possess the requisite 4-in. (100 mm)
length. The purpose of the soundness requirement is to
downgrade the rock quality where the rock has been altered
and weakened either by agents of surface weathering or by
hydrothermal activity. Obviously, in many instances, a judgment
decision must be made as to whether or not the degree of
chemical alteration is sufficient to reject the core piece.

35. Assessment of Soundness, Continued
One procedure, which the authors have used, is not to count a
piece of core if there is any doubt about is meeting the soundness
requirement (because of discolored or bleached grains, heavy
staining, pitting, or weak grain boundaries). This procedure may
unduly penalize the rock quality, but it errs on the side of
conservatism.
A second procedure which occasionally has been used by the
authors in recent years is to include the altered rock within the
RQD summed percentage but to indicate by means of an asterisk
(RQD*) that the soundness requirement has not been met. The
advantage of the method is that the RQD* will provide some
indication of the rock quality with respect to the degree of
fracturing, while also noting its lack of soundness.

38. Abstract: Rock quality designation (RQD) was introduced by Don Deere in the mid-1960s as a means of
using diamond core to classify rock for engineering purposes. Subsequently, it was incorporated into the
rock mass rating (RMR) and Q-system classification methods that, worldwide, now play substantial roles in
rock mechanics design, whether for tunnels, foundations, rock slopes or rock excavation. It is shown that a
key facet of the definition of RQD is ignored in many parts of the world, and it is noted that there are several
inherent limitations to the use of RQD. Based on mapping of rock formations by 17 independent
professionals at different locations in Australia and South Africa, it is shown that differences in assessed RQD
values result in significant errors in computed RMR and Q ratings, and also in geological strength index (GSI)
and mining rock mass rating (MRMR). The introduction of a look-up chart for assessing GSI has effectively
removed the need to measure, or estimate, RQD. It has been found that GSI values derived from the look-up
chart are as valid as those derived by calculation from the original component parameters, and are
satisfactorily consistent between professionals from diverse backgrounds. The look-up charts provide a
quick and appropriate means of assessing GSI from exposures. GSI is, in turn, a useful rock mass strength
index; one new application is presented for assessing potential erosion of unlined spillways in rock.
Incorporation of RQD within the RMR and Q classification systems was a matter of historical development,
and its incorporation into rock mass classifications is no longer necessary.

47. Rock Structure Rating (RSR)
 Wickham et al (1972) described a quantitative method for
describing the quality of a rock mass and for selecting appropriate
support on the basis of their Rock Structure Rating (RSR)
classification
 Most of the case histories, used in the development of this system,
were for relatively small tunnels supported by means of steel sets,
although historically this system was the first to make reference to
shotcrete support.
 In spite of this limitation, it is worth examining the RSR system in some
detail since it demonstrates the logic involved in developing a
quasi-quantitative rock mass classification system.
 The significance of the RSR system, in the context of this discussion, is
that it introduced the concept of rating each of the components
listed below to arrive at a numerical value of
RSR = A + B + C.

54. Rock Mass Rating Systems
RMR
Q
Use and misuse of rock mass classification
systems with particular reference to the Q-
system