Structural geology (a branch of geology aiming at describing the structures – joints, faults, folds, etc. – at various scales) can be used in the field of rock mechanics and rock engineering, and particularly in underground engineering works (tunnelling and rock caverns) to gather more reliable data for empirical stability analyses and deterministic calculation models. Methods of structural geology are presented and their applications in rock mechanics/rock engineering are highlighted in particular through the observation of faults and joints arrest. Structural geology allows a better understanding of the origin, the chronology and the mechanical behaviour of discontinuities, and therefore a more accurate rock mass characterization and rock mass classification, as well as a validation of the actual stress regime. Examples selected from different countries of using structural models are also given with emphasize on the necessity and the way to build a 3D model at each stage of an underground project, from site selection to investigation and construction, to ensure the quality and validity of rock mechanical data and assumptions.
In the last five years, several publications and oral presentations insisted on the importance of structural geology in the field of rock mechanics (e.g. during Sinorock 2009 in Hong Kong or the ISRM congress in Beijing in 2011). They highlighted the necessity of using structural geology and structural data as input parameters for rock mass characterization and rock mass modelling.
This wish is praiseworthy and corresponds to a real need. The situation is due to the lack of structural geologists on the market: structural geology being less taught in university and often replaced by engineering geology, geologists or engineering geologists are easily involved in big projects (e.g. dams, hydropower plants, tunnelling, etc.) whereas it is nowadays quite uncommon to meet structural geologists in teams in charge of design, modelling or even supervision of site works.
In addition, most structural geologists are academics and very few are full-time practitioners, especially in the field of rock mechanics.
This short paper cannot be exhaustive but set out the various possibilities offered by structural geology. The objective is less ambitious and is limited to the presentation of practical examples in different geological situations in which a structural knowledge can provide a real help for modelling or for characterizing the rock mass through the two most geology-based rock mass classifications, the Q system (Barton et al. 1974) and the RMR (Bieniawski 1989).
Since the vocabulary also acts as a brake, this paper deliberately uses simplified technical vocabulary and concepts, hoping that this will bring rock mechanical engineers to working closer with structural geologists.