Soutenance de Jian Zheng

Numerical, analytical and experimental studies of the hydro-geotechnical behaviors of slurried materials

Mines produce not only valuable minerals, but also a large amount of solid wastes including tailings and waste rocks. For most cases, the tailings are transported by pipes and confined in tailings dams while the waste rocks are disposed as waste rock piles. These traditional mine waste management methods can pose some environmental and geotechnical problems. Thus, more and more mines use the mine wastes to fill underground mine stopes. This practice allows improved ground stability, increased ore recovery and reduced surface disposal of mine wastes.

However, tailings dams and barricades can fail. In most cases,  the consequences associated with a tailing dam or barricade failure are very serious and catastraphique. It is thus very important to correctly evaluate the pore water pressure (PWP) in the tailings dams and in mine backfilled stopes during and after the deposition operation to ensure stable tailings dams and barricades of backfilled stopes. This is the main objective of the thesis.

For the case of ongoing deposition of a slurry, Gibson proposed a model in 1958 to evaluate the excess PWP. His solution contains an integral part, which can only be evaluated numerically. Analytical solutions are thus proposed in the context of this thesis based on the Gibson (1958) model. The proposed analytical solutions can be solved by commonly available tools of calculation (e.g. Excel of Microsoft). The proposed solutions have been validated by numerical results obtained by SIGMA/W. The good agreement between the analytical and numerical results indicates that the proposed solutions can be a useful tool to evalaute the PWP during the slurry deposition, especialliy in the preliminary stage of a project. For the case of after the end of deposition operation, Gibson only proposed a governing equation in 1958. The differential equations cannot directly be applied to evaluate the PWP or excess PWP. New development has been done, resulting in a new solution that can be used to evaluate the PWP with some commercial calculation tools like MATLAB. The proposed solutions have also been validated by numerical results obtained by SIGMA/W. They can be used to evalaute the PWP after the slurry deposition, in the preliminary stage of a project.

For the case of barriade design in backfilled stopes, the most critical factor is the total stresses during and shortly after the slurry depostion.  During theses processes, self-weight consolidation can take place, accompanied with the generation of effective stress and arching effect along the fill-wall contact interfaces. Until now, the self-weight consolidation and the arching effect have never been considered simultaneously in the evaluation of stresses in backfilled stopes. In this thesis, new solutions are proposed to evaluate the total and effective stresses in backfilled stopes by considering the self-weight consolidation and arching effect. For backfill with very high permeability, the proposed solution for the case of impervious base can reduce to the solution proposed by Li and Aubertin in 2009 for backfilled stopes with hydrostatic pressure. The proposed solutions are also validated by numerical modeling conducted with Plaxis2D. Sample application of the proposed solutions shows that high consolidation coefficient and low filling rate are favorable to the drainage, resulting in high effective stress and low total stress in the backfilled stope.