# Study on the Evolution of a Flooded Tailings Pond and Its Post-Failure Effects

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Overview of the Tailings Pond

^{3}, and the tailing pond is classified as second class according to the Code for the Design of Tailings Facilities (GB 50863-2013). Village No. 1, with a population of 400 people, is 600 m downstream of the tailings pond, and village No. 2 is less than 1 km away. Due to the presence of important towns and industrial and mining enterprises downstream of the tailings dam, the grade of the prototype tailings pond is increased to first class, as shown in Figure 1.

## 3. Results Dam Failure Physical Model Test

#### 3.1. Test Model

#### 3.2. Test Materials

#### 3.3. Dam Failure Process

^{3}, according to the conversion of similar relationships. The amount of water injected was controlled through the water storage system. At the beginning of the test, water was supplied to the model reservoir through the water injection system, and the water level in the reservoir rose slowly, as shown in Figure 5a. When the water level spread over the top of the dam, the dam began to breach, and the change process from this point can be roughly divided into:

- With the slow rise of the water level in the reservoir, a small breach began to appear at the weak part of the dam top under the effect of water infiltration, as shown in Figure 5b;
- The water in the reservoir flowed from the breach to the bottom of the dam, and under the action of water erosion, the tail sand was carried away from the outer slope of the dam, forming an erosion trench, as shown in Figure 5c;
- With the development of the erosion trench, the discharged tail sand gradually transformed from the initial single movement to a group movement, and the tail sand in the erosion trench at the bottom of the dam first started to slip, forming a critical surface after slipping, and then forming a multi-level small steep bump in the lower part of the erosion trench. Subsequently, the multi-level steep cans gradually fused into one large steep can, which continuously expanded upstream until extending into the reservoir. During the migration process, a large amount of tail sand was carried away from the dam, and the depth of the erosion trench further increased, as shown in Figure 5d;
- While the steep moved upwards, the flood erosion rate increased, and when the dam body on both sides of the erosion trench was completely saturated, cracks appeared. When the bond force is weaker than gravity, the dam body collapses along the cracks into the trench, and the width of the erosion trench increases at a faster rate, as shown in Figure 5e;
- When the amount of flood water in the reservoir gradually decreased, the rate of increase in the width and depth of the erosion trench slowed down. When the flood water in the reservoir was fully discharged, the erosion trench stopped developing and the dam tended to a stable state, as shown in Figure 5f.

#### 3.4. Evolution of Dam-Break Full-Field Velocity

#### 3.5. Post-Dam Failure Impacts

^{2}. The larger the size of the tailing sand, the larger the elevation of the foundation in the same section, the smaller the accumulation depth, and the larger the particle size; the maximum accumulation depth of the tailing sand is 14.5 cm.

## 4. Numerical Simulation

#### 4.1. Numerical Model

^{3}, the friction coefficient is set to 0.26, and the Herschel–Bulkley model has five basic parameters: η

_{0}is the shear viscosity at low shear rate, τ

_{0}is the yield shear stress, k is the consistency index, n is the flow characteristics index, and C

_{0}is the temperature. These five basic parameters and the state parameters (using the linear three-parameter USUP equation of state) are usually determined with specific reference to the tailings sand parameters commonly used in the literature [38,39,40], and in conjunction with the field tailings sand conditions. The breaching of the dam occurred in a relatively short period of time; the temperature change can be ignored, and so the effect of temperature was not considered. A sensitivity analysis was performed on the relevant parameters, and the results show that the friction coefficient is the main parameter affecting the flow range and accumulation characteristics. The residual strength parameter obtained from the ring shear test, i.e., 0.26, was used for this calculation, and the other parameters are shown in Table 1, Table 2 and Table 3.

#### 4.2. Analysis of Massflow Calculation Results

- At 20 s, the maximum accumulation height of the tail sand was about 48.8 m, mainly located in the reservoir area, and part of the sand flow advanced to 350 m downstream. The overall speed of the sand flow ahead of the breach was fast—the maximum was 30.2 m/s, and the direction was northeast;
- At 40 s, the maximum accumulation height of the sand flow was about 33.6 m, located in the reservoir area, and the farthest reach of the sand flow was 670 m downstream. The overall speed of sand flow in front of the breach was still fast, with a maximum of 30.0 m/s, and the direction began to shift northward due to the influence of the mountain;
- At 60 s, the maximum accumulation height of the sand flow was about 31.5 m, mainly located in the reservoir area and 250 m in front of the right side of the dam, and the sand flow reached as far as 1060 m downstream. The maximum travel speed of the sand flow was 27.1 m/s, located directly in front of the breach, and the travel speed of the front edge of the sand flow was reduced by ground friction and the blocking effect of the right side of the mountain to about 16.0 m/s. The direction had turned due north at this point;
- At 80 s, the maximum accumulation height of the sand flow was about 26.2 m, mainly located in the reservoir area and 340 m in front of the left side of the dam, and the sand flow reached as far as 1290 m downstream. The maximum travel speed of sand flow was 25.7 m/s, and the travel speed of sand flow within 150 m of the breach exceeded 20.0 m/s. The travel speed of the front edge of the sand flow decreased to 7.0 m/s, while the flow speed in other areas decreased to 1.0 m/s or less;
- At 300 s, the sand flow movement basically stopped. The final evolution distance was about 1.43 km, the tail sand accumulation range reached 603,000 m
^{2}, and the whole was distributed in strips along the downstream gully, with part of the sand flow entering the gully on both sides. The buildings of the village within 1 km downstream of the dam body were completely submerged—only in the area where the accumulation height of the sand flow front edge was less than 3 m were a small number of village buildings partially submerged. The accumulation height along the evolution path was generally decreasing, and the maximum accumulation area was at the left side of the mountain in front of the dam, with a maximum accumulation height of about 31.5 m.

- The tailing sand accumulation on the left side of sections MS1 and MS2 is significantly higher than on the right side, the accumulation at MS3 is low in the middle and high on both sides, and the accumulation on the right side of MS4 is higher than on the left side due to the sand flow being diverted by the mountain. The presence of village buildings will increase the accumulation height in the area;
- The tailing sand accumulates rapidly in the downstream channel after the breach. Before 50 s, a large amount of tailing sand is discharged per unit of time, the potential energy is large, the tailing sand accumulation distance is great, and the tailing sand accumulation height at each measurement point increases rapidly. After 50 s, a small amount of tailing sand is discharged within a unit of time, the potential energy is small, the tailing sand accumulation distance is small, and the tailing sand accumulation height at locations far from the initial dam no longer increases, while the accumulation height near to the dam body continues to increase. The tailing sand accumulation height curve at monitoring points near the dam is bimodal, and can be divided into four stages, i.e., sharp rise–significant decline–continuing to rise–gradually stabilizing, and the tailing sand accumulation height curve at monitoring points farther away from the dam is largely unimodal, and can be divided into three stages, i.e., sharp rise–small decline–gradually stabilizing;
- The change in the velocity curve at the monitoring points near the breach is more complicated—the sand flow is faster and the movement lasts longer. The velocity curve at other monitoring points essentially shows a steep rising and steep falling triangular shape—the sand flow velocity is reduced and the movement lasts for less time.

#### 4.3. Analysis of SPH Calculation Results

- At 20 s, the maximum displacement of the breached tailing sand was about 320 m, and the tailing sand movement speed was fast, with a maximum of about 33.0 m/s;
- At 40 s, the maximum displacement of the breached tailing sand was about 820 m, and the sand flow movement speed was more evenly distributed, with a maximum of about 29.9 m/s;
- At 60 s, the maximum displacement of the breached tailing sand was about 1120 m, and the sand flow front and the tailing sand movement speed in front of the breached opening were at their maximum;
- At 80 s, the maximum displacement of the tail sand of the breached dam was about 1250 m, and the maximum velocity of the tail sand movement was about 18.0 m/s in front of the breached mouth and the front edge of the sand flow, while the velocity of the tail sand movement in other areas had dropped to less than 6.0 m/s;
- At 300 s, the maximum displacement of the tail sand of the breached dam was about 1350 m, and the tail sand movement had basically stopped. The sand accumulated in strips along the gully, with some of the tail sand flowing downstream. At both sides of the ditch, the accumulation range reached 527,000 m
^{2}, and village buildings within this range were completely submerged.

## 5. Results and Discussion

- There is a lag in the physical model test when the maximum flow velocity of the discharged tailings sand appears during the dam breach process, reaching the peak flow velocity of 120 s after the breach appears at the top of the dam, after which it remains relatively stable for a period of time and then starts to decrease rapidly. On the other hand, the numerical simulation reaches the peak flow velocity of the discharged tailings sand at the beginning of the dam breach and then rapidly decreases to close to 0 m/s, creeping for a longer period of time before the end of the dam breach;
- The tailing sand flow velocity, evolution distance and depth obtained from the numerical simulation are large. This is because (i) the breach pattern in the numerical simulation is set with reference to the physical test, but in order to consider more unfavorable conditions, the breach is set larger and deeper, and the total volume of the breached tail sand is slightly higher, and further, (ii) the numerical simulation does not truly reflect the model test conditions and processes. In the test, rainfall continues to wash the breach, carrying the tail sand continuously downstream, and there is obvious mud–water stratification in the tail sand flow, while the numerical simulation is instantaneous. In the full-break mode, the potential energy of the tailing sand body is released instantaneously, while the tailing sand and water are completely mixed, and the flowability is good in all directions.

## 6. Risk Assessment and Recommendations

^{2}, and the maximum accumulation depth can reach 31.50 m. Based on the maximum flow velocity and the accumulation depth of tailing sand, the river downstream of the tailing pond can be divided into risk areas. In this way, relocation measures can be formulated. In high-risk area, the flood flow velocity is fast—the maximum flow velocity is above 14.85 m/s—and the impact force is high, the accumulation height is over 6 m, and the destructiveness is strong. In the medium-risk area, most of the kinetic energy is consumed and the travel speed is greatly reduced, so the maximum flow velocity is below 14.85 m/s. The accumulation height is also reduced—the accumulation height is below 6 m, and the destructiveness is further reduced. In low-risk areas, the tailings accumulation area is outside the inhabited area. The low-risk area is outside the tailings accumulation area; this area includes a large amount of farmland, villages and industrial facilities. This area is not affected by the dam failure, and the warning and personnel evacuation times will be sufficient—see Figure 19.

## 7. Conclusions

- During the breaching process, after the tailings dam forms an erosion trench, the lower part of the erosion trench is the first to slip, and after the formation of a steep can, the upper part of it causes slippage in the tailings, such that the erosion trench first develops vertically and then laterally. The final evolution of the breach is determined by the amount of water stored in the reservoir;
- When the top of the tailings dam is breached, the downstream tailings sand flow rate will quickly reach a peak value of 33.00 m/s in a short period of time, after which the downstream tailings sand flow rate reduces to a creeping state. After creeping for a long period of time, the front edge of the sand flow is the first to stop moving, while the trailing edge of the tailings sand accumulation depth continues moving until the end of the breach, at which point the tailings sand flow rate of the initial downstream dam bottom area is the largest. The impact force is the most significant factor use to form prevention and control measures;
- The discharged tailings eventually accumulate in the downstream channel, showing a pattern whereby at points further away from the initial dam, the accumulation depth will be smaller and the particle size will be larger, while the larger the elevation of the foundation in the same section, the smaller the accumulation depth and the larger the particle size. The maximum accumulation depth is 31.50 m, at which point the presence of shade will cause the local tailings accumulation depth to increase;
- There are small differences between the results of the numerical simulation and physical model tests, and the bias value should be used as the basis when carrying out engineering prevention and control measures. The final evolution distance of tailing sand after the collapse can reach 1.43 km, and the maximum accumulation depth can reach 31.50 m. Based on the flow velocity, downstream tailing sand accumulation distance and depth, the risk area of the river downstream of the tailing pond can be categorized, such that relocation measures can be formulated.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 2.**Indoor test modeling. (

**a**) Cut off panel. (

**b**) Standing plate. (

**c**) Fill sandy soil. (

**d**) Conservation. (

**e**) Stacking of sub-dams. (

**f**) Final model.

**Figure 5.**Dam breach process. (

**a**) Water injection in the tailings pond. (

**b**) Ulcer formation. (

**c**) Erosion trench formation. (

**d**) Steep can formation. (

**e**) Erosion trench horizontal development. (

**f**) End of dam failure.

**Figure 6.**Variation in the depth of the erosion trench on the dam body with time. (

**a**) Variation in erosion trench depth with time. (

**b**) Variation in erosion trench width with time.

**Figure 7.**Flood topping dam breach full-field flow velocity change curve with time. (

**a**) Instantaneous flow rate. (

**b**) Mean flow rate.

**Figure 8.**Impact of downstream tailing after dam failure. (

**a**) Depth of tailing sand accumulation in each section. (

**b**) Range of tailing sand accumulation.

**Figure 10.**The process of tailing sand discharge from the breached dam. (

**a**) Height distribution of tailing sand accumulation at 20 s. (

**b**) Tail sand flow rate distribution at 20 s. (

**c**) Height distribution of tailing sand accumulation at 40 s. (

**d**) Tail sand flow rate distribution at 40 s. (

**e**) Height distribution of tailing sand accumulation at 60 s. (

**f**) Tail sand flow rate distribution at 60 s. (

**g**) Height distribution of tailing sand accumulation at 80 s. (

**h**) Tail sand flow rate distribution at 80 s. (

**i**) Height distribution of tailing sand accumulation at 300 s. (

**j**) Tail sand flow velocity distribution at 300 s.

**Figure 12.**Tailing sand accumulation pattern at each monitoring section. (

**a**) MS1 monitoring cross-section; (

**b**) MS2 monitoring cross-section; (

**c**) MS3 monitoring cross-section; (

**d**) MS4 monitoring cross-section.

**Figure 13.**Variation curves of tailing sand accumulation height and flow rate with time at each monitoring point. (

**a**) Height–time curve of tailing sand accumulation at each monitoring point; (

**b**) velocity–time curve of sand flow at each monitoring point.

**Figure 14.**Process of tailing sand discharge from the breached dam. (

**a**) Cloud map of tailing sand displacement distribution at 20 s. (

**b**) Cloud plot of tailing sand velocity distribution at 20 s. (

**c**) Cloud map of tail sand displacement distribution at 40 s. (

**d**) Cloud map of tailing sand velocity distribution at 40 s. (

**e**) Cloud map of tail sand displacement distribution at 60 s. (

**f**) Cloud map of tailing sand velocity distribution at 60 s. (

**g**) Cloud map of tail sand displacement distribution at 80 s. (

**h**) Cloud plot of tailing sand velocity distribution at 80 s. (

**i**) Cloud map of tail sand displacement distribution at 300 s. (

**j**) Cloud plot of tailing sand velocity distribution at 300 s.

Ratios Name | Geometric Ratios | Flow Rate Ratios | Flow Ratios | Time Ratios | Roughness Ratios | Area Ratios | Volumetric Ratios |
---|---|---|---|---|---|---|---|

Formula | ${\mathsf{\lambda}}_{\mathrm{L}}=\frac{{\mathrm{L}}_{\mathrm{P}}}{{\mathrm{L}}_{\mathrm{M}}}$ | ${\mathsf{\lambda}}_{\mathrm{v}}=\sqrt{{\mathsf{\lambda}}_{\mathrm{L}}}$ | ${\mathsf{\lambda}}_{\mathrm{Q}}={\mathsf{\lambda}}_{\mathrm{L}}{}^{5/2}$ | ${\mathsf{\lambda}}_{\mathrm{t}}=\sqrt{{\mathsf{\lambda}}_{\mathrm{L}}}$ | ${\mathsf{\lambda}}_{\mathrm{n}}={\mathsf{\lambda}}_{\mathrm{L}}{}^{1/6}$ | ${\mathsf{\lambda}}_{\mathrm{A}}=\frac{{\mathrm{L}}_{\mathrm{P}}{}^{2}}{{\mathrm{L}}_{\mathrm{M}}{}^{2}}={\mathsf{\lambda}}_{\mathrm{L}}{}^{2}$ | ${\mathsf{\lambda}}_{\mathrm{V}}=\frac{{\mathrm{L}}_{\mathrm{P}}{}^{3}}{{\mathrm{L}}_{\mathrm{M}}{}^{3}}={\mathsf{\lambda}}_{\mathrm{L}}{}^{3}$ |

Numerical Values | 200 | 14.14 | 565,685.4249 | 14.14 | 2.42 | 40,000 | 8,000,000 |

Specific Gravity | Water Content | Gravity | Porosity Ratio | Saturation | Peak Strength of Ring Shear Test | Residual Strength of Ring Shear Test | ||
---|---|---|---|---|---|---|---|---|

G_{s} | ω(%) | γ/(kN/m^{3}) | e_{0} | S_{r} | c/kPa | tan φ | c/kPa | tan φ |

2.9 | 16.2 | 16.86 | 0.958 | 49 | 15.9 | 0.2643 | 8.6 | 0.2622 |

Parameters | ρ kg/m ^{3} | Basic Parameters | USUP Status Parameters | ||||||
---|---|---|---|---|---|---|---|---|---|

η_{0}MPa/s | τ_{0}MPa | n | k MPa/sn | C_{0} | s | γ_{0} | Friction Coefficient | ||

Takes values | 1950 | 0.2 | 2 | 0.4 | 2.2 | 1480 | 2.0 | 0.9 | 0.26 |

Simulation Method | Maximum Travel Speed/m∙s^{−1} | Final Evolution Distance/km | Stacking Range /10,000 m ^{2} | Maximum Accumulation Depth/m | |
---|---|---|---|---|---|

Model test | 26.68 | 1.19 | 47.60 | 29.00 | |

Numerical simulations | Massflow | 30.20 | 1.43 | 60.30 | 31.50 |

SPH | 33.00 | 1.35 | 48.70 | - |

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## Share and Cite

**MDPI and ACS Style**

Chang, M.; Qin, W.; Wang, H.; Wang, H.; Wang, C.; Zhang, X. Study on the Evolution of a Flooded Tailings Pond and Its Post-Failure Effects. *Water* **2023**, *15*, 173.
https://doi.org/10.3390/w15010173

**AMA Style**

Chang M, Qin W, Wang H, Wang H, Wang C, Zhang X. Study on the Evolution of a Flooded Tailings Pond and Its Post-Failure Effects. *Water*. 2023; 15(1):173.
https://doi.org/10.3390/w15010173

**Chicago/Turabian Style**

Chang, Mengchao, Weimin Qin, Hao Wang, Haibin Wang, Chengtang Wang, and Xiuli Zhang. 2023. "Study on the Evolution of a Flooded Tailings Pond and Its Post-Failure Effects" *Water* 15, no. 1: 173.
https://doi.org/10.3390/w15010173