# Numerical Simulation of Subdam Settlement in Ash Disposal Based on CGSW Optimization
## 1 Introduction
As a governing agency, the National Energy Administration is supposed to guide relevant academic research institutions in CGSW reutilization by the building material industry, metallurgy industry, chemical industry, and light industry so that CGSW can have a positive environmental effect [16]. According to the design specification, only ash and desulfurization gypsum can be disposed in the dump area [17]. However, 121,186 m\({}^{3}\) of CGSW was improperly dumped between Chimney No.2 and Chimney No.3 by mistake (as shown in Figure 2) and then covered by fly ash. This buried CGSW is spatially located beneath the unbuilt subdam-XII, subdam-XIII, and subdam-XIV, where it faces a complex mechanical environment [18; 19; 20; 21]. CGSW weathering produces clay minerals such as illite, kaolinite, and montmorillonite, and these clay minerals expand when they meet water. On the one hand, this expansion characteristic of clay minerals resulted in the blocking of the void channels inside the CGSW [22], which blocked the pores of underlying fly ash after migrating downward to the junction between the CGSW and fly ash through seepage flow; on the other hand, this formed an aquitard. Ignoring the CGSW would cause the uneven settlement of future subdams, leading to dam failure. The fly ash is continually delivered to DAD4 from the power plant, which makes the buried CGSW go deeper. However, previous endeavors have not evaluated the effects of buried CGSW for subdam differential subsidence. Now, it is urgent to assess the hazard of the buried CGSW so that the management can preliminarily understand its impact on  
Figure 2: Plan view of CGSW and borehole distribution. the subdams, as well as on the safe operation of the entire ash dam, and can then make a proper decision. Although the stability of the tailing dam in the valley context has been researched for more than 100 years, controlling the risk of a deep foundation to ensure dam stability is still challenging. Tailing dams collapse every few years. The international dam committee had recorded 366 global tailing dam failures spanning 1905-2020, and the rate of tailing dam failure is increasing [23; 24; 25]. The statistics show that upstream dams are much more likely to fail than centerline dams and downstream dams [26; 27]. Generally, the reasons why these collapses occur can be summed up in one principal cause: a lack of a thorough comprehension of the key factors related to adequate design. The finite difference method is one of the oldest methods for solving equation sets and differential equations. The FLAC finite difference program developed by ITASCA in the late 1980s has allowed this method to be widely used in geotechnical engineering numerical calculations. The FLAC uses the Lagrangian difference method, which, unlike finite elements, does not require the formation of an overall stiffness matrix; so, it is easy to correct for large deformations at each calculation time step. The coordinates and displacement increments are applied to the grids then move and deform along with the material. This overcomes the limitations of the traditional Eulerian method with relatively fixed meshes for material movement and deformation. Other papers have reported the effect of mining on surface settlement [28; 29] and have conducted stability analyses of tailing ponds [30] using Flac3D simulation. [PERSON] et al. used static analysis with the Duncan tensor model to study the effect of different stages of subdam raising on the stress and deformation of ash dams [31]. [PERSON] et al. calculated the dam safety values and failure probabilities from the kinetic point of view to analyze the stability of ash dams. Previous studies have analyzed the stability of dams from the seepage perspective. In this paper, rainfall and seismic factors are not considered for the time being.