Numerical investigation of seepage dissolution evolution of the bottom expansion cutoff wall in deep overburden

Abstract

The bifurcation defect at the bottom of the cutoff wall in the deep overburden leads to a sharp increase in the local seepage gradient of the adjacent overburden at the bottom of the cutoff wall, which affects the evolution process of seepage dissolution of the cutoff wall. This study presents a novel type of bottom expansion cutoff wall to alleviate the problem that the seepage gradient of adjacent strata at the bottom of the cutoff wall far exceeds its allowed value. Considering the effects of the pore, aggregate, and convection on the calcium ion diffusion coefficient of concrete, we propose a coupling analytical model of seepage dissolution for two cutoff wall schemes, including the bifurcation defect schemes and bottom expansion schemes, to investigate the evolution of calcium ion concentration, porosity, and seepage characteristics, and to reveal the influence mechanism of the bottom bifurcation defect and bottom expansion of the cutoff wall on the seepage dissolution of the cutoff wall in deep overburden. The results indicate that subjected to the bottom bifurcation defect, the calcium ion concentration of the cutoff wall decreases faster, and the porosity increases faster with the increase of service life compared to the conventional cutoff wall when subjected to the bottom bifurcation defect. With a bifurcation height of 24 m at the bottom of the wall, the service life of the cutoff wall is reduced by 6.64%. In contrast, the novel type of bottom expansion cutoff wall is used to reveal that the concrete porosity increases slowly with the increase of the expansion radius over the same service life. The seepage gradient of the adjacent overburden at the bottom of the cutoff wall is significantly reduced. The service life of the cutoff wall is extended by 47.33% due to its 5 m bottom expansion radius. This study can provide a theoretical basis for the evolution of long-term service performance of the cutoff wall of an earth-rock dam on deep overburden.