Discussion on how to classify and estimate strength of weak rock masses

Abstract

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. Sa (2010) carried out laboratory strength tests and back-analysis of failed slopes for the N4E open pit iron mine in the Vale’s Carajas 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.