The Giovanni Grasselli (GG) shear strength criterion is an empirical formula which estimates the peak shear strength of a rock joint or discontinuity feature from its measured 3D surface roughness characteristics. This criterion is highly dependant on empirical correlation to the laboratory direct shear testing data from which it was derived. Therefore one of the main objectives of this body of work, was to perform additional testing and analysis to validate and update the criterion. Using the new lab results, modifications were proposed to the GG-shear strength criterion. Modifications were also proposed to the well known Ladanyi-Archambault shear strength criterion to utilize the same roughness parameters used in the GG-criterion. To asses the validity of these modified criteria, the peak shear strength of 35 tensile rock fractures, previously used for the development of the original GG-criterion, was predicted and compared to their tested shear strength.
Since the 1960's, rock mechanics researchers have developed empirical shear strength criteria which attempt to estimate the shear strength of a rock joint or discontinuity based on prescriptive parameters associated with the joint wall material strength (also related to mineral alteration) and geometry (roughness). The difficulty in objectively estimating these properties/parameters is the main limitation in correctly assessing the peak shear strength of rock joints. In the past two decades, the advent of modern digital and computational technologies has given researchers new avenues for measuring joint surfaces. The Giovanni Grasselli (GG) shear strength criterion (Grasselli & Egger 2003) is one such empirical approach to estimating shear strength. The criterion was initially developed from the empirical correlation between surface roughness characteristics and laboratory shear testing data. Unique to the criterion was the characterization of 3D roughness based on the analysis of high resolution 3D measurements of rock joint surfaces.