Rock Mass Characterization

Rock mass is a matrix consisting of rock material and rock discontinuities. Its characterization and classification aim to determine the rock mass characteristics by assigning values to a set of rock parameters. The behavior of intact rock material can be determined by continuum mechanics but rock masses are usually highly fractured. The fact that fractures control the mechanical response of rock masses makes the determination of their mechanical behavior difficult. Consequently, simplified classification systems capable of dealing with the geological and geotechnical uncertainties evolved for various engineering design purposes. Those systems were based on empirical correlations between rock mass parameters and practical engineering projects including foundations, tunnels, slope stability, and mining. According to Bieniawski (1993), the rock mass classification systems were designed to act as an engineering design aid and were not intended to substitute field observations, analytical considerations, measurements, and engineering judgment.

Parameters for Rock Mass Classification

To derive the overall behavior of a rock mass, both the engineering properties of the rock material and the fractures should be taken into consideration. The most significant parameters that are used in classification systems are the following:

• Strength and deformability of intact rock
• Rock Quality Designation (RQD) which considers the intensity of fracturing in a drill core
• Rock fractures parameters (spacing, orientation, width, roughness, weathering, etc.)
• Groundwater pressure and flow
• In-situ stress
• Geological structures such as faults and folds

Common rock mass classification systems

Most of the multi-parameter rock mass classification systems were developed from civil engineering case histories. Those that are commonly used in rock mechanics to assist designing in infrastructure are the following:

• Rock Load Classification: Introduced by Terzaghi in 1946, the system evaluates the impact of rock load on steel supported tunnels based on a descriptive classification of rock classes.
• Rock Mass Rating (RMR) System: The RMR system was developed by Bieniawski in 1973 in order to derive the required support measures for tunnels. The system is practical as it uses a few basic parameters. Since its conception, it has been tested and improved by many case studies.
• Q-system: The Q-system was introduced by Barton, Lien, and Lunde in 1974 and its purpose was to determine the rock mass characteristics and tunnel support measures based on 212 hard rock tunnel case histories from Scandinavia. Q-rating is derived by assigning values to six parameters that are grouped into three quotients.
• Geological Strength Index (GSI): GSI was introduced by Hoek in 1994. Its purpose was to derive the rock mass properties by using in-situ observations of the rock mass conditions along with correlations developed from the RMR-system. GSI utilizes 2 main parameters, the rock mass structure and the discontinuity surface quality.

List of References

• Barton N., Lien R. and Lunde J., (1974). Engineering classification of rock masses for the design of tunnel support. Rock Mechanics, Vol. 6, No. 4, pp. 189-236.
• Bieniawski, Z.T. (1973). Engineering classification of jointed rock masses. Transactions of the South African Institution of Civil Engineers, 15(12), 335–344.
• Bieniawski, Z.T. (1993). Classification of rock masses for engineering: The RMR system and future trends, In: Hudson, J.A., ed., Comprehensive Rock Engineering, Volume 3: Oxford; New York, Pergamon Press, p. 553-573.
• Cook, N.G.W. et al. (1966). Rock Mechanics applied to Rockbursts – a synthesis of the results of rockburst research in South Africa up to 1965. J. S. African Inst. Min. Metall. Vol. 66, No. 10, 435-528.
• Hoek, E. (1994). Strength of rock and rock masses. News J ISRM 2(2):4–16.
• Terzaghi, K. (1946). Rock defects and loads on tunnel supports, in Proctor, R.V., and White, T.L., eds., Rock tunneling with steel support, Volume 1: Youngstown, Ohio, Commercial Shearing and Stamping Company p. 17-99.