Geotechnical Engineering Online Library

A formulation for the coupled analysis of thermo-hydro-mechanical (THM) problems in joints is first presented. The work involves the establishment of equilibrium and mass and energy balance equations. Balance equations were formulated taking into account two phases: water and air. The joint element developed was implemented in a general purpose finite element computer code for THM analysis of porous media (Code_Bright). The program was then used to study a number of cases ranging from laboratory tests to large scale in situ tests. A numerical simulation of coupled hydraulic shear tests of rough granite joints is first presented. The tests as well as the model show the coupling between permeability and the deformation of the joints. The experimental investigation was focused on the effects of suction on the mechanical behaviour of rock joints. Laboratory tests were performed in a direct shear cell equipped with suction control. Suction was imposed using a vapour forced convection circuit connected to the cell and controlled by an air pump. Artificial joints of Lilla claystone were prepared. Joint roughness of varying intensity was created by carving the surfaces in contact in such a manner that rock ridges of different tip angles were formed. These angles ranged from 0° (smooth joint) to 45° (very rough joint profile). The geometric profiles of the two surfaces in contact were initially positioned in a “matching” situation. Several tests were performed for different values of suctions (200, 100, and 20MPa) and for different values of vertical stresses (30, 60, and 150kPa). A constitutive model including the effects of suction and joint roughness is proposed to simulate the unsaturated behaviour of rock joints. The new constitutive law was incorporated in the code and experimental results were numerically simulated.

Granular mixtures made of high-density pellets of bentonite are being evaluated as an alternative buffer material for waste isolation. Ease of handling is an often-mentioned advantage. The paper describes the experimental program performed to characterize the hydro-mechanical H) behaviour of compacted pellet mixtures. Grain size distribution was adjusted to a maximum pellet size compatible with the specimen’s dimensions. Dry densities of statically compacted specimens varied in most of the cases in the range from 1.3 to 1.5Mg/m3. Pellets had a very high dry density, close to 2Mg/m3. The outstanding characteristic of these mixtures is their discontinuous porosity. Pore sizes of the compacted pellets varied around 10nm. However, the inter-pellet size of the pores was four to five orders of magnitude higher. This double porosity and the highly expansive nature of the pellets controlled all the hydraulic and mechanical properties of the mixture. Performed tests include infiltration tests using different water injection rates and mechanisms of water transfer (in liquid and vapour phases), suction-controlled oedometer tests and swelling pressure tests. The interpretation of some performed tests required back analysis procedures using a hydro-mechanical HM) computer code. Material response was studied within the framework of the elastoplastc constitutive model proposed by Alonso et al. (1990) (Barcelona basic model, BBM). Parameters for the model were identified and also a set of hydraulic laws are necessary to perform coupled HM analysis. A large scale in-situ test (the “EB” test in Mont Terri, Switzerland) was described and analyzed. Rock barrier parameters were adjusted on the basis of available tests. The test excavation, barrier emplacement and forced hydration were simulated by means of the CODE_BRIGHT program. The comparison between measurements and computed results include data on relative humidity in the rock and the buffer, swelling pressures and displacements.