Classification Systems
Published on March 12 2011A. General
Classification systems are probably among the most frequently used tools in the design of underground excavations in rock. From a set of parameters on the rock, rockmass, groundwater and stresses quality of rockmass and ground with respect to stability can be estimated.
Definitions:
rockmass = rocks penetrated by discontinuities (joints), representing the material in which the excavation is excavated
ground = rockmass subjected to stresses and groundwater, representing the conditions at site
Some of the more known classification systems are presented here and some comments on such systems here.
To view a paper on the ability and use of classification systems, see the paper Classification as a tool in rock engineering
B. RMR classification system
The RMR (or Geomechanics) system was launched by Bieniawski in 1973. It was a further development of the RSR system by Wickham, Tiedemann and Skinner (1972), see references. Later, the system has been revised/updated by Bieniawski in 1976 and 1989.
Numerous papers have been presented and published on the RMR system and the system is currently being used by many practitioners.
The input parameters to RMR and the RMR support table are shown here.
The RMR-values can be estimated using a computer spreadsheet together with Q-values and RMi-values.
C. Q classification system
The Q system was launched in 1974 by Nick Barton, Reidar Lien and Johny Lunde of the Norwegian Geotechnical Institute (NGI). Over the years, some developments have been introduced by; first a new design of the support diagram (Grimstad and Barton, 1988, see list of published paper here, then adjustments of the input parameters SRF (Grimstad and Barton, 1993), inclusion of the rock strength in (Barton, 1995), inclusion of shotcrete ribs as a support element (Barton and Grimstad, 2004).
A paper presenting Q and NATM systems gives information on the two systems.
From late 1980s, the originator of the Q system has increased the limitation of the original Q to also incorporate excavation by TBM (tunnel boring machines) introducing QTBM (Barton,1999) and to also use Q-values for estimates of the effect of grouting (Barton et al, 2001) estimate Q-values from sound velocities of the ground (found from refraction seismic measurements) (1993).
Some comments of the limitations of the use of classification systems are presented in the paper Use and misuse of classification systems with special reference to the Q system. This paper was first presented at the annual Norwegian conference on tunneling in 2002. A summary in English of this paper can be read here.
The Q input parameters and the Q support chart can be studied.
The Q-values can be estimated using a computer spreadsheet together with RMR-values and RMi-values.
D. Other classification/characterization systems
Several classification systems have been developed over time. Some have been developed specifically for a project.
The first one to present a practical classification system for use in rock engineering was Dr. Karl Terzaghi when he in 1946 wrote the famous Introduction to tunnel geology as a chapter in the book Rock tunneling with steel supports by Proctor and White after visits to several tunneling projects in America and in Europe. Many of these projects experienced tunneling problems.
In the 1940s the rock support was often performed by steel arches, on which the support evaluation in the Terzaghi system was developed. Today, the Terzaghi classification system has lost much of its interest. However, the engineering geological part of the book is very interesting and pinpointing important geological features with respect to tunnel construction.
The GSI system was developed in the 1980s and in first part of the 1990s by Evert Hoek et al. The GSI value expresses an approximate of the compressive strength of rockmasses. The following references may be of interest:
- Hoek E. and Brown E.T. (1994): Practical estimates of rock mass strength. Int J Rock Mech Min Sci 34, pp. 1165-1186
- Hoek E., Marinos, P. and Benissi M. (1998): Applicability of the geological strength index (GSI) classification for very weak and sheared rock masses. The case of the Athens schist formation. Bull. Eng. Geol. Env. No 57, pp. 151 - 160.
The GSI value can be further used in the Hoek-Brown failure criterion in various rock engineering assessments.
E. Combination of RMR, Q and RMi systems
In two papers Comparing the RMR, Q and RMi classification systems, a Comparison on the RMR, Q and RMi systems is presented in Part-1 and Part-2.
Shorter versions are presented in the papers Combining the RMR, Q and RMi classification systems and in a Technical note on the same theme.
A computer spreadsheet has been worked out where the RMR, Q and RMi values are calculated independently by combining the input parameters as described in the papers mentioned above.