# Numerical Simulation of Subdam Settlement in Ash Disposal Based on CGSW Optimization
## 4 Results
### Subsidence Analysis  
Figure 6 (up) showed the vertical displacement of the section when subdam-XIV was loaded. The dotted line sketches the original surface, fly ash, CGSW, and subdam-XIV. Vertical displacement on the top surface was increased as the thickness of the fly ash beneath increased. The maximum subsidence value was 0.358m near monitoring point P29. Figure 6 (down) plots the fitted curve of the settlement around all the monitoring points. It is noted that a double-valley shape appears (the CGSW is distributed in the left valley), resulting in a fitting curve that is similar to the original ground surface, which indicates a strong correlation between them. The analysis revealed that the displacement value of the monitoring point was mainly affected by the thickness of the fly ash below it. The greater the thickness of the fly ash, the larger the displacement value of the monitoring point. The results of the predicted section subsidence of the 10 schemes when subdam-XIV was loaded are shown in Figure 9. Table 2 summarizes the maximum subsidence of subdam-XIV's top surface in the 10 schemes according to the MC failure criterion. It is remarkable that the series number of subplots in Figure 9 is consistent with that of the scheme in Table 2. The results indicate that all the construction schemes will reduce the subsidence value of subdam-XIV. Among them, the displacement method performed the worst with a maximum subsidence of 320 mm, a reduction of 10.615% compared to the original condition (scheme 1, subplot 1 of Figure 9). The composite foundation method performed the best; scheme 8 ended up with a maximum subsidence of 277 mm, a reduction of 22.625% (see Figure 9). Figure 10 demonstrates the comparison of the fitted settlement curves of all the schemes. The numbering of the curves corresponds to the numbering of the schemes in Table 2. Among them, the solid black line (line 0) represents the settlement of the original condition, and the solid black line (line 1) represents the replacement method. Meanwhile, the colored line (line 2-10) charts the settlement of the composite foundation method, where the green line, blue line, and red line, respectively, express the stone column diameters of 0.5 m, 1 m, and 1.5 m; the solid, dotted, and dashed lines represent the column distances of 3m, 6m, and 9m, respectively. These curves formed by the fitted settlement values of the monitoring points show almost no difference in the right valley segment (x > 560). A significant difference exists between the 0 m-560 m segment in the x-axis, where various sizes of stone columns are plugged. Thus, the variation of curves is mainly observed in this segment. The contrast of curve 0 and curve 1 demonstrates a similar variation tendency where the curves show a downward trend first and reach the maximum settlement at X = 320, then both turn upward. These two curves have different decline amplitudes; the peak of curve 1 is 10.61% less than  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Serial Numbers**} & \multicolumn{2}{c}{**Parameter of Stone Column**} & \multicolumn{2}{c}{**Subsidence Results**} & \multicolumn{2}{c}{**Plastic Zone Information**} & \\ \cline{2-9}  & **Center** & **Diameters/m** & **Maximum** & **Reduction** & **Percentage** & **Percentage** & **Reduction** & **Reduction** \\  & **Distances/m** & **Subsidence/mm** & **Degree of MS** & **of SFZ** & **of TFZ** & **Degree of SFZ** & **Degree of TFZ** \\ \hline
0 & - & - & 358 mm & - & 3.60\% & 0.16\% & - & - \\ \hline
1 & - & - & 320 mm & 10.615\% & 2.68\% & 0.13\% & 25.63\% & 18.93\% \\ \hline
2 & 3 m & 0.5 m & 283 mm & 20.950\% & 2.80\% & 0.08\% & 22.24\% & 50.62\% \\ \hline
3 & 6 m & 0.5 m & 285 mm & 20.391\% & 1.42\% & 0.02\% & 60.5 that of curve 0. The contrast of curve 0 and curves 2-10 indicates that both diameter and spacing can affect the uneven settlement of the subdam. Figure 10: Comparison of fitted subsidence curves for all schemes. Figure 9: Comparison of numerically predicted section subsidence for 10 schemes.