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logo LGM logo LGM UNSAT unsat Rock mechanics under partial saturation conditions
Contact Bertrand François
Keywords Partial saturation, poromechanics, triaxials tests, Biot coefficient
Description
The presence of water may have appreciable consequences on the mechanical properties of soils and rocks. In soil, the effect of partial saturation on the mechanical behaviour of soil has been studied for many years. However, only few studies have addressed this topic for rock materials. New research fields, such as risk analyses related to the closure of mines or underground disposal of nuclear waste now require better knowledge of these aspects.

This experimental study therefore aims at a better understanding of the hydromechanical behaviour of partially saturated rock materials. The approach followed here consists in adapting to rock materials the techniques and concepts successfully developed for soils. In this context, a first study was carried out on an argillaceous rock material named Beringen shale. The influence of suction was evaluated for unconfined compression stress states.

Following this first study, other stress states were considered, mostly using the triaxial test. A new suction-controlled triaxial system using the axis translation technique was designed (Figures 1 and 2). A second series of experiments were conducted on a porous and very homogeneous rock called Sorcy limestone.

Triaxial and polyaxial tests performed on dry rock samples allowed to build three dimensional yield envelopes. In the meridian plane an increase in strength with confining pressure associated with brittle failure is noticed at low confining pressures, while the behaviour becomes ductile at higher confining pressures. Also, it has been observed that the shape of the failure envelops in the octahedral plane is strongly affected by the confining pressure (Figure 3).

A consistent set of poroelastic parameters was measured for saturated rock samples, among which the Biot coefficient. The variability of these parameters between samples and the influence of the stress state were also discussed. In the plastic range, Terzaghi's effective stress was found to control the mechanical behaviour, despite the Biot coefficient being lower than 1.

The yield surface obtained for the saturated material was found to be quite close to the dry one when the yield points were normalized with respect to the horizontal length of the surface. This suggests that the partially saturated states may be accounted for by adding an axis for suction, as is commonly done for soils.

 

figure 1

Figure 1: Scheme of the triaxial cell, able to control the rock suction through the axis translation technique.

 

figure 2

Figure 2: Triaxial cell for unsaturated soil.

 

figure 3

Figure 3: Failure locus in the octahedral plane as a function of the confining pressure.

   
Support
  • FRIA Grant: "Etude théorique et expérimentale du comportement poromécanique des roches non saturées », PhD Thesis of Mikael Ramos Da Silva
   
Selected publications M Ramos da Silva (2010). Etude théorique et expérimentale du comportement poromécanique des roches non saturées. PhD Thesis, ULB.

M. Ramos da Silva, C. Schroeder, J.C. Verbrugge, Poroelastic behaviour of a water-saturated limestone, International Journal for Rock Mechanics and Mining Sciences, 47, 797-807 (2010).

Ramos Da Silva, Mikaël, Schroeder, Christian, & Verbrugge, Jean-Claude (2008). Unsaturated rock mechanics applied to a low porosity shale. Engineering geology, 97, 42-52.

Ramos Da Silva, Mikaël, Schroeder, Christian, & Verbrugge, Jean-Claude (2009). Poroelastic behaviour of a limestone. Proceeding of 16th International Symposium on Plasticity and its Current Applications (pp. 271-273).

Descamps, F, Ramos Da Silva, Mikaël, Schroeder, Christian, Tshibangu, J.P., & Verbrugge, Jean-Claude (2009). Limiting envelopes of a limestone under true triaxial stress state. Procoodings of 16th International Symposium on Plasticity and its Current Applications (pp. 274-276).

Ramos Da Silva, Mikaël, Schroeder, Christian, & Verbrugge, Jean-Claude (2008). Poroelastic constants of a limestone: an experimental study. Proceedings Poromechanics IV (pp. 240-245).