# Numerical Simulation of Subdam Settlement in Ash Disposal Based on CGSW Optimization
## 3 Modeling and Test Scheme Design  
### Description of the Model  
Before numerical simulation, the modeling process was completed on Rhino 6 (Robert McNeel Associates 2015), which is well known for three-dimensional modeling with powerful surface mesh building capabilities and excellent model structure generation, as shown in Figure 4. This software produces complex geometries and presents high compatibility with other file formats. Griddle 1.0 for Rhino 6, developed by Itasca Ltd., Minneapolis, MN, USA is a specific plugin to re-mesh the original surface mesh to build accurate 3D models. Surface meshes can be used as the boundaries of model meshes to generate high-quality tetrahedral or hexahedral meshes. The numerical model dimensions were approximately 400 m \(\times\) 320 m \(\times\) 200 m. It comprised rock mass, fly ash, two layers of CGSWs, and four subdams. The characteristics of each module are illustrated as follows:  
1. Bedrock: The bedrock of DAD4 is composed of limestone and mudstone, derived from the Yongningzhen Formation and the Feixianguan Formation, respectively, and both are slightly weathered. The bedrock module was divided into 36,061 zones. 2. Fly ash: The fly ash was generated by a power plant 3 km away. It will reach a storage of 26.30 million m\({}^{3}\) when DAD4 meets its ultimate capacity. In the Rhino model, this module was divided into 98,110 zones. 3. Coal gangue solid waste: The primary plan for the CGSW was for it to be dumped in DAD4 temporarily in the year 2016 without a further disposal strategy. So, it was buried eventually due to the continuous delivery of fly ash from the power plant. When they understood its possible impact on the stability of subdams and that it  
Figure 3: Simplified geological engineering cross-section. might even cause dam failure, the management decided to carry out a geological engineering investigation in September 2021 to confirm the shape and distribution of the buried CGSW. The shape of the buried CGSW was constrained by 42 boreholes. The drilling revealed that the CGSW appeared in two horizontal layers, in parallel with each other and wrapped by fly ash. The mechanical parameters of the fly ash and bedrock were obtained through the laboratory geotechnical test, and the CGSW's mechanical parameters were from a large-scale triaxial test, which provided a good data basis for numerical simulation. The CGSW module was divided into 2887 zones. 4. Primary dam: The primary dam is an essential structure for surface ash disposal facilities constructed by the upstream method; as well as retaining fly ash, it also plays an important role in the drainage system to maintain a low phreatic surface. The primary dam in this case is a rolled rockfill dam made of waste limestone, with a slope ratio of 1:2 on the upstream side and a slope ratio of 1:3 on the downstream side. It rises up to the final height of 28 m, with a length of 121 m in axis; the volume of this starter dam is 97,168 m\({}^{3}\). The primary dam module was divided into 31,960 zones. 5. Subdam: The subdam is mainly made up of compacted fly ash by vibratory rolling. The rolling test is carried out before construction to provide the best paving thickness, rolling times, optimum moisture content, and optimum compactness. As shown in Figure 4, the dimensions of these subdams are similar, with 70 m in bottom width, 8m in top width, 12 m in height; only the crest length varies from 124 m to 780 m. The subdam is entirely wrapped by an impervious geotextile, and the downstream slope is protected by dry rubble. 6. Stone column: The stone column is a mixture of breccia, gravel, and sand, with particle sizes of less than 100 mm. The uniformity coefficient (Cu) of the mixture must be greater than 5, with a curvature coefficient (Cc) between 1 and 3. All the column axes are 15 m to ensure the passing through of the coal gangue burial body. This module mesh must be less than one-fifth of the column diameter. 7. Dry ash disposal in the valley contains a perfect drainage system and flood-cutting system.