Numerical analysis of the effects of chemistry on the THM behaviour of rocks

The chemical characteristics of water present in the pores and fractures of the rock mass and the mineralogical rock composition determine the chemical processes that may take place in a geological formation. The chemical reactions occurring in a geotechnical problem play an important role because their development can influence on the mechanical and hydraulic behaviour of rock masses. For example, a change in the concentration of dissolved salts in groundwater can lead to a modification of the swelling potential or trigger and expansive or shrinkage response of clayey formations (Yustres et al., 2017). Therefore, a good knowledge of the chemistry involved in the rock mass is relevant both to understand and to predict the response of the geological media. This is of great importance in the case of the construction of critical civil infrastructures like energy and nuclear waste storage facilities.

Chemical effects are expected to occur in soils and rocks, and several variables may affect the intensity at which they take place. The literature describes several case histories in rocks where the effects of chemistry are more intense than in soils. Tunnels and foundations may respond hydraulically and mechanically under the influence of the chemical processes of the dissolution/precipitation of soluble minerals present in the rock. These phenomena are capable of leading to severe and rapid expansions that can result in infrastructure and building damage. The extreme expansions observed in anhydritic rocks are one example in which dissolution and precipitation processes affect the geotechnical behaviour (Alonso et al., 2013, Ramon & Alonso, 2018).

A coupled numerical model has been developed to address the chemical processes coupled to thermo-hydro-mechanical (THM) problems in geological media. The numerical model is capable to simulate general chemical processes and the associated hydraulic and mechanical effects. The formulation of the chemical interactions is implemented in a coupled manner within a Finite Element code for THM analysis in geological media (CODE_BRIGHT). The presentation will describe the details of the equations and hypothesis considered in the model. The simulation of a real case will be also analysed.

Alonso, E.E., Berdugo, I.R. and Ramon, A. (2013). Extreme expansive phenomena in anhydritic-gypsiferous claystone: the case of Lilla tunnel. Géotechnique 63 No. 7, 584 – 612

Ramon, A. and Alonso, E. E. (2018) Heave of a Building Induced by Swelling of an Anhydritic Triassic Claystone. Rock Mech. Rock Engng.: 51, Issue 9, pp 2881–2894.

Yustres, A., Jenni, A., Asensio, L., Pintado, X., Koskinen, K., Navarro, V., Wersin, P. (2017). Comparison of the hydrogeochemical and mechanical behaviours of compacted bentonite using different conceptual approaches. Applied Clay Science, 141: 280-291.