Surface tension acts at the interface of pore water and pore air inside any unsaturated rock, independently of the gravity of the Earth.' This paper will focus on capillary forces caused by surface tension because they are considered to play an important role in the Rock Mechanics field, and typically tend to be ignored. Drying-induced deformation of porous materials is a typical and a good example used to show the importance of surface tension. As a rock sample dries gradually, the pore water is evaporated from its surfaces, which causes the creation and evolution of menisci inside the pores, producing capillary pressure. Rocks shrink in the initial stage of drying. As the rock dries up, it may expand by forming liquid islands and by failing to transfer the capillary force to the skeleton of a rock. In light of considering these processes, we can say that drying-induced deformation is a coupled behavior consisting of rock deformation and moisture movement/distribution inside the rock.
In this paper, the drying-induced deformation behaviors of sedimentary rocks will first be introduced. These behaviors have been observed through laboratory experimental procedures, and are explained by using the results obtained from hand specimen-sized samples of Opalinus Clay, Shirahama sandstone, Tage tuff and pumiceous tuff. The goal is to understand their general and characteristic deformation behaviors during drying. Secondly, an example of in-situ evaporation measurements and strain measurements in the Mont Terri Underground Rock Laboratory is provided to show similar phenomena occurring on site. Finally, the applicability of the drying-induced deformation of rocks to the real field is described in place of the Summary.