# Numerical Analysis of an Upstream Tailings Dam Subjected to Pond Filling Rates
## 3 Simulation Results
### Effects of the Pond Filling Rates on the Stability and Excess Pore Water Pressure  
Figure 8 shows the effect of the pond filling rates (_PFR_) on the stability of the upstream tailings dam with the rockfill width of 20 m during the pond filling, comprising raising phases (R) and consolidation phases (C). As shown, the _FoS_ decreased very slightly when the _PFR_ increased from 2.5 m/year to 5.0 m/year, regardless of construction phases. At the pond filling rate of 5.0 m/year, the factor of safety of the dam at every stage of construction fulfilled the adopted code requirement of the Swedish dam safety guidelines, except for the raising phase R5 (_FoS_ at R5 was 1.472). However, when the _PFR_ increased from 5.0 m/year to 10 m/year, the _FoS_ suddenly dropped to 1.40 at the raising phase R5. Thus, the _FoS_ did not satisfy the abovementioned requirement, which might be a sign of potential instability issues for the dam. Figure 9 shows excess pore water pressure after the last consolidation phase C5 with respect to the pond filling rates. For the ease of interpretation, the shading views of excess pore water pressure in Figure 9 are fixed at the same color intervals, except for the model with the pond filling rate of 10 m/year. As shown, an effect of the pond filling rate on excess pore water pressure was discovered: an increase in pond filling rate can lead to an increase in excess pore water pressure in the model. In particular, the maximum excess pore water pressure in the model with the pond filling rate of 2.5 m/year was 29 kPa. This value rose up to 43 kPa, 68 kPa, and 138 kPa in the models with the pond filling rates of 3.3 m/year, 5.0 m/year, and 10 m/year, respectively. In cases of high pond filling rates, there was less time for the excess pore pressure to dissip  
Figure 6: Rockfill width optimization based on slope stability analysis (filling rate of 5.0 m/year). Figure 8: Effect of the pond filling rates (_PFR_) on the stability of the upstream tailings dam (rockfill width of 20 m). Figure 7: Excess pore water pressure at the last construction stage with the pond filling rate of 2.5 m/year and rockfill width of 20 m: (**a**) after the raising phase R5 and (**b**) after the consolidation phase C5. Figure 9: Excess pore water pressure after the last consolidation phase with respect to the pond filling rates: (**a**) 2.5 m/year, (**b**) 3.3 m/year, (**c**) 5.0 m/year, and (**d**) 10 m/year. In addition, excess pore water pressure which developed along cross sections A-A\({}^{\prime}\) and B-B\({}^{\prime}\) (defined in Figure 1b) are plotted in Figure 10. Excess pore water pressure increased with an increase in the \(P\!F\!R\), regardless of the cross sections analyzed. Take the elevation of 5 m from the initial ground level as an example: at the section A-A\({}^{\prime}\), excess pore water pressure in the model with the pond filling rate of 2.5 m/year had the lowest value of 4.65 kPa, followed by 7.50 kPa (\(P\!F\!R\) = 3.3 m/year), 12.25 kPa (\(P\!F\!R\) = 5.0 m/year), and 27.25 kPa (\(P\!F\!R\) = 10 m/year). Similarly, at the cross section B-B\({}^{\prime}\) at 5 m height, excess pore water pressure in the models with the pond filling rates of 2.5 m/year, 3.3 m/year, 5.0 m/year, and 10 m/year were 7.5 kPa, 12.1 kPa, 19.6 kPa, and 43.1 kPa, respectively. Interestingly, the same trend was observed at both cross sections, even though excess pore water pressure at the cross section B-B\({}^{\prime}\) was higher than the corresponding excess pore water pressure at the cross section A-A\({}^{\prime}\). It should be noted that cross section A-A\({}^{\prime}\) had a draining distance shorter than cross section B-B\({}^{\prime}\). It was also found from Figure 10 that excess pore water pressure decreased with increased heights from the initial ground level, regardless of the \(P\!F\!R\) and sections. This is because of the presence of the impermeable base in the finite element model. Finally, the build-up of excess pore pressure (\(E\!P\!P\)) was also considered in this study.