Hydro-Mechano-Chemical Analysis of the Reactive Transportation Process Using the Extended DDA Method
Authors: Gao Jingyao , Chen Guangqi , Mitani Yasuhiro 
Published: September 2023

ABSTRACT
The waste repositories or mining activities interfere with the chemical equilibrium of natural underground water, enhancing the hydro-chemical erosion and widening the flow passage, which may lead to serious geohazard. Understanding the evolution of this hydro-mechano-chemical (HMC) coupling process is helpful to the forecast and mitigation of the influence brought by the human activities. This study extends the original DDA method by embedding the discrete seepage network and considering the widening of fracture flow passage by using the finite difference method. After being validated by comparing the numerical results with those of the laboratory test on a single fracture, the extended DDA method is applied to the analysis of a real reactive tailings case. The results agree well with the previous research and laboratory observations. The simulations indicate that the proposed methodology can be applied in the HMC analysis of the reactive transportation in fractured rock masses and promise a wider application in a more comprehensive analysis in the future.

INTRODUCTION
Understanding the reactive transportation of fluid in fractured stratum has been of interest over the last decades in many engineering disciplines for different kinds of applications, including underground radioactive waste repositories, over-pumping and contamination from industry and agriculture, and other energy engineering projects. With increasing attention being paid to the sustainable development of underground space, more advanced tools and methodologies are required for design, operation, and safety assessments of these human activities to mitigate environmental damage (Molson, Aubertin, & Bussiere, 2012).
The reactive transportation in rock mass is a typical multi-field coupling process. Besides moving with the fluid flow in fractured geological materials by convection, the reactive solute or particle in the groundwater system can also be retarded by other physical or chemical mechanisms, such as sorption on fracture surfaces, diffusion in and out of the rock matrix, in-situ stresses, and chemical reactions between the solute and the rock matrix or the fracture walls (Zhao, Jing, Neretnieks, & Moreno, 2011). For instance, the acid mine drainage produces one of the most sever ground water contamination, characterized by low pH and high dissolved concentration of rock minerals, which can erode rock mass and exert significant long-term impact on underground environment. This chemical solution will migrate through different flow paths, including shallow permeable subsurface pathways (for example a porous aquifer), or deeper through bedrock which is usually controlled by flow through fractures. Meanwhile, it is well known that the in-situ stresses must be taken into consideration in underground engineering, especially in the analysis of fractured rock mass. The field stress changes fracture apertures by causing normal closure, opening, or shear dilation, and consequently varies the seepage field (the flow rate and water head in a specific fracture). Combining with the influence of dissolution or precipitation at fracture surface in chemical reaction, makes the reactive transportation in fractured stratum a complicated Hydro-Mechanical-Chemo (HMC) issue.