Effective block size scaling for advanced rock modelling in practical applications

A valuable tool in discontinuum modelling is the concept of effective block interactions (size), where predominant structures are modelled explicitly and minor structures implicitly, i.e. as an equivalent pseudo-continuum rock block. This allows practitioners to achieve computationally effective models. However, a frequent question in this modelling approach relates to what “effective block” properties are considered adequate. As the rock mass still contains discontinuities smeared through its volumes, it cannot, in principle, be modelled as an intact block. It should not have rock mass properties estimated at the scale of the geo-structure either, i.e. using the global scale Geological Strength Index (GSI), as this would unrealistically double up the strength and deformability downscaling (reduction). Unfortunately, no detailed guidance can be found in the international technical literature, and decisions are often based solely on the practitioner’s experience in assessing a GSI value at the effective block scale. This paper presents an example of the impact of the GSI value when using the effective block-size approach, varying the scale of the block sizes in the discontinuum model from the actual rock block volumes to larger volumes that are more effective to use in practice. Although the example does not exhaustively explore all aspects associated with the problem at hand, thus not intended as a detailed how-to guide, some preliminary conclusions can be drawn on appropriate GSI values for “effective blocks”, particularly for sub-horizontally bedded rock formations with mostly sub-vertical jointing.