Study On Long Term Hydro-mechanical Coupled Behavior Of Belgium Boom Clay

Argillaceous rock is considered as a kind of alternative medium for underground nuclear waste repository storage in Europe. Boom Clay is studied as a reasonable geological barrier in geological disposal of high level radioactive waste because of its swelling, low permeability and self-healing, et al. In Belgium, a series of in situ tests have been investigated in HADES underground laboratory since 1980. The main research work of the dissertation has been focused on long-term mechanical behavior, hydro-mechanical coupled behavior, creep damage behavior of Boom Clay. The main achievements are stated as follows:(1) The mineral composition and microstructure of Boom Clay is obtained by X diffraction test and scanning electron microscopic (SEM) test, respectively. As the average contents of 60%, clay mineral containing illite, smectite, illite-smectite mixed layer and kaolinite is the main composition of Boom Clay, and quartz and feldspar is the main composition of non-clay minerals for Boom Clay. The SEM test further conform the high contents of clay particles of Boom Clay, and those fine clay particles make up of larger particle sets, which present flocculated structure.(2) Oedometer tests containing standard oedometer test and hydrated oedometer test are carried out. Basic physical and mechanical parameters such as preconsolidation stress (about 5MPa) and compressibility coefficient (about 0.19) are obtained from standard oedometer test of Boom Clay. The hydrated oedometer test results show that Boom clay has significant swelling behavior under synthetic Boom Clay (SBC) water.(3) Triaxial consolidated undrained shearing tests have been conducted in order to further study the hydro-mechanical coupled behavior of Boom Clay. After the sample saturated by back pressure in both ends under hydrostatic pressure, the triaxial consolidated shearing test is processed under very low shearing rate (20μm/min) with deformation and pore pressure measured. Test results show that the swelling deformation and the excess pore pressure are gradually increasing with time during shearing process. The shear strength under effective confining stress 2.5MPa is about 3.0MPa. The variation of micro structure before and after the shearing tests are studied by SEM. It shows that the micro structure arrangement of Boom Clay after test is more directional than undisturbed Boom Clay. (4) Transversely isotropic elasto-plastic damage constitutive model and the damage evolution equation are established considering modified Mohr-Coulomb criterion. The model can describe the hardening and softening behavior of Boom Clay. Parameters for the developed constitutive model are obtained from triaxial test results by inverse analysis and analytical method.(5) Long-term hydro-mechanical coupled behavior of Boom Clay is studied by one dimensional consolidation creep tests (more than 1 year). The test results highlighted the creep potential of Boom Clay. After the creep test, the variations of micro structure before and after the tests are studied through SEM. It shows that the porosity of Boom Clay is decreased and the microstructure directionality is more ordered. Based on the test results, a creep constitutive model is established to describe consolidated creep behavior of Boom Clay.(6) Long-term triaxial creep tests (more than 1 year) are conducted to study hydro-mechanical coupled creep behavior of Boom Clay. Based on the triaxial creep test results, a nonlinear creep damage constitutive model is established for Boom Clay. At the same time, the relationship between creep damage and creep strain is developed. The parameters for creep damage constitutive model are obtained from the triaxial creep test results. Taking ABAQUS as a platform, programming UMAT subroutine, the subroutine was verified by simulating the triaxial creep test. The results show that the developed creep damage constitutive model can describe the nonlinear creep damage process very well.(7) Based on the achievements of long-term behavior of Boom Clay,3D numerical analysis on stability of tunnels in HADES has been conducted. Comparison between the numerical results and the field measurements has been done, which shows that the variation law of pore water pressure and convergence of the tunnel after excavation is nearly the same as what we obtained from monitoring.