Authors: Castro, LM; Carvalho, J; Sá, G


Cite As:
Castro, LM, Carvalho, J & Sá, G 2013, 'Discussion on how to classify and estimate strength of weak rock masses', in PM Dight (ed.), Proceedings of the 2013 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 205-217,

Download citation as:   ris   bibtex   endnote   text   Zotero

Weak rocks can be found in many mines around the world, such as the weathered (saprolite/saprock) rocks in tropical areas, the (argillic) altered rocks in the Andes and several gold mines in Nevada, and the soft iron ore deposits in Brazil and Africa. However, it is difficult to classify these materials from drilling core and obtain representative strength for these weak rock masses. This paper discusses the rock mass classification and proposes a transition function for estimating their strengths. Current application of the rock mass rating (RMR) – Bieniawski classification system and its subsequent input into the Hoek–Brown strength criterion yields low strength parameters that do not represent high stable slopes excavated within weak rock masses, as observed in many mine operations and road cuts. This paper presents some modifications to the RMR76 system, which somewhat takes into account the Robertson (1988) proposed classification system for weak rock masses, by allowing the collection of ratings for RQD and joint condition to obtain higher RMR values for the upper portion of the R1 (i.e. R1+ or R1/R2) and R2 category rock masses that would be greater than the current minimum value of 18 for dry slopes. The RMR classification should not be applied to R0 type materials (UCS<1 MPa), as they should be treated as soil. It is recognised that at the low end of the rock quality scale, in the transition from inter-block shear failure towards a more matrix controlled rock mass behaviour, a gradual change in the strength curve can be created by considering the reduction in the cohesion component. For the estimation of the weak rock mass strength, a low-end transition Hoek–Brown relationship originally proposed by Carvalho et al. (2007) has been calibrated with additional data and considering the strength range from R1 to R2 materials. Sá (2010) carried out laboratory strength tests and back-analysis of failed slopes for the N4E open pit iron mine in the Vale’s Carajás Mineral Complex, located in the north of Brazil. The calibrated strength parameters were used to assist in defining the lower strength limit, where this transition function should not be applied. Examples for other mines are also included, where weathered/altered rocks exist and were compared with strength parameters estimated from this low-end transition relation.

Barton, N.R., Lien, R. and Lunde, J. (1974) Engineering Classification of Rock Masses for the Design of Tunnel Support, Rock Mechanics, Vol. 6, No. 4, pp. 189–263.
Bieniawski, Z.T. (1976) Rock Mass Classification in Rock Engineering, Exploration for Rock Engineering, Z.T. Bieniawski (ed), A.A. Balkema, Johannesburg, pp. 97–106.
Bieniawski, Z.T. (1989) Engineering Rock Mass Classifications, John Wiley & Sons, New York, 251 p.
Carter, T.G., Diederichs, M.S. and Carvalho, J.L. (2008) Application of modified Hoek-Brown transition relationships for assessing strength and post yield behavious at both ends of the rock competence scale, in Proceedings 6th International Symposium on Ground Support in Mining and Civil Engineering Construction, 30 March – 3 April 2008, Cape Town, South Africa, The Southern African Institute of Mining and Metallurgy, Johannesburg, pp. 37–59.
Carter, T.G., Mierzejewski, J. and Kwong, A.K.L. (1998) Site Investigation for Rock Slope Excavation and Stabilization Adjacent to a Major Highway in Hong Kong, in Proceedings International Conference on Urban Ground Engineering, Session 2, Geotechnics, B. Clarke (ed), 11–12 November, Hong Kong, China, pp. 177–186.
Carvalho, J.L., Carter, T.G. and Diederichs, M.S. (2007) An approach for prediction of strength and post yield behavior for rock masses of low intact strength, in Proceedings 1st Can-US Rock Mechanics Symposium, Meeting Society’s Challenges & Demands, Vancouver, Canada, pp. 249–257.
Hoek, E. and Marinos, P. (2000) Predicting Tunnel Squeezing Problems in Weak Heterogeneous Rock Masses, Tunnels and Tunnelling International, Part 1 – November 2000, Part 2, December 2000, 21 p.
Hoek, E., Carranza-Torres, C. and Corkum, B. (2002) Hoek–Brown Failure Criterion – 2002 Edition, in Proceedings 5th North American Rock Mechanics Symposium, R. Hammah, W. Bawden, J. Curran and M. Telesnicki (eds), 7–10 July, Toronto, Canada, University of Toronto Press, Toronto, Vol. 1, pp. 267–273.
ISRM (1981) International Society of Rock Mechanics. Rock Characterization, Testing and Monitoring – ISRM suggested methods, Brown E.T. (ed), Pergamon, Oxford.
Laubscher, D.H. (1990) A Geomechanics Classification System for the Rating of Rock Mass in Mine Design, Journal – South African Institute of Mining and Metallury, Vol. 90(10), October 1990, pp. 257–273.
Marinos, P. and Hoek, E. (2000) Estimating the mechanical properties of heterogeneous rock masses such as flyash, Bulletin of Engineering Geology and the Environment, London, Vol. 60, pp. 85–92.
Nickmann, M., Spaun, G. and Thuro, K. (2006) Engineering geological classification of weak rocks, IAEG 2006 – Paper number 492.
Robertson, A. (1988) Estimating Weak Rock Strength, AIME Annual General Meeting, January 1988, Tucson, Arizona, 6 p.
Sá, G. (2010) Caracterização litoestrutural e parametrização geomecânica das superfícies de ruptura em taludes da mina de N4E – Carajás, Pará (Litho-Structural Characterization and Geomechanical Assessment of the Slope Failure Surfaces at the N4E Mine, Carajás, Pará), Master thesis in Geotechnical Engineering submitted to the Federal University of Ouro Preto, MG, Brazil, 172 p.
Sharon, R., Rose, N. and Rantapaa, M. (2005). Design and Development of the Northeast layback of the Betze-Post Open Pit, in Proceedings SME Annual Meeting, 28 February – 2 March, Salt Lake City, US, Pre-print 05-09, 10 p.

© Copyright 2020, Australian Centre for Geomechanics (ACG), The University of Western Australia. All rights reserved.
Please direct any queries or error reports to