# Numerical Simulation of Subdam Settlement in Ash Disposal Based on CGSW Optimization
## 1 Introduction  
China is the world's largest coal producer and consumer, and the proportion of China's coal production and consumption has risen to 50.69% and 54.33% of global capacity, respectively [1]. Over half of the coal consumption is applied to power generation. Fossil-fired power plants provide 60.80% of China's electricity capacity [2]. Significant quantities of mine tailing materials and post-production ash are generated in the processes of coal fire production and are mainly disposed of at surface ash disposal facilities near the power plants [3]. Leaching tests reveal that fly ash can have an extensive environmental impact and that it commonly releases heavy metals like arsenic, copper, cadmium, manganese, zinc, lead, and nickel into the surrounding ecosystem [4; 5; 6; 7]. Surface ash disposal facilities are mainly composed of ash retention dams, drainage and flood discharge systems, and flood control dams. Ash retention dams are basically classified as retention dams or raised dams according to their construction methods. The height of retention dams remains constant once disposal begins, whereas raised dams are raised by the construction of a new subdam on the surface of under-consolidated tailing materials. Raised dams can be built by upstream, downstream, or centerline methods [8; 9; 10]. With the enhancement of the government's environmental awareness, ensuring the stability of fly ash disposal is increasingly crucial. There are two main methods for ash disposal. The first one is the wet ash disposal method, in which ash is mixed with proportional water and pumped as slurry to the ash pond. In the second one, the dry ash disposal method, the fly ash is transported to theconstruction industry, such as that for brick making or cement making, or it is carried by trucks to a dry ash disposal facility for temporary or permanent storage. The No.4 dry ash disposal (DAD4) is the fourth and latest surface ash disposal facility of the Panjiang power plant; the facility is at a distance of 3.5 km from the main power plant. DAD4 is located in the province of Guizhou in southwestern China. This facility was constructed in phases based on the need for additional volume by the upstream method in 2014. One initial dam and fourteen subdams were designed with a final storage capacity of 26.30 million m\({}^{3}\) (Figure 1). The project design documents show that the initial dam was a rockfill dam with a height of 28 m and a crest axis length of 78 m, whereas the subsequent subdams were constructed with rolled fly ash, with crest lengths ranging from 124 m to 780 m, as illustrated in Figure 1. The initial dam building used a slope ratio of 1:2 on the upstream side and a slope ratio of 1:3 on the downstream side. The subdam building, however, used a slope ratio of 1:3 on the upstream side and a slope ratio of 1:4 on the downstream side. A geotextile was applied as an inverted filter on the inside slope while the outside slope was protected by a geotextile and a drystone wall. Ultimately, the crest of the last subdam will reach an elevation of 1720 m when the whole disposal area is fully filled. The crest of the latest subdam (subdam-VII) had reached an elevation of 1640 m by the end of September 2021, with a fill volume of 9.24 million m\({}^{3}\). Figure 1: Geological property mapping of studied area. Coal gangue solid waste (CGSW) is an inevitable solid waste formed by syngenetic sedimentation with a coal seam; it is produced in tunneling/mining/washing processes, with an emission amount of approximately 10-15% of domestic coal production [11]. The accumulated amount of piled-up CGSWs is about 6 billion tons so far; subsequently, it is anticipated to increase by 10-15% of annual coal production. The improper management of CGSW disposal could generate persistent air, agricultural soil, and groundwater pollution due to its content of heavy metal elements, thereby putting the nearby residents' health at risk [12; 13; 14; 15].