The tailings pond project is a large system, including subsystems such as tailings treatment in the concentrator, tailings slurry thickening and transportation, tailings dam construction, tailings discharge, anti-seepage and drainage, flood control and drainage, water circulation, wastewater treatment and pollution control, land restoration and vegetation in the reservoir area, and tailings pond monitoring and management. It covers the complex physical, chemical, and biogeochemical reactions and solute migration processes within the tailings pond system (tailings and tailings wastewater) and between the tailings pond system and the environment (seepage water-foundation soil-groundwater or surface water). It involves engineering issues such as tailings pond design, infrastructure and operation(xingaonai), closure and land restoration, and later pollution control. It reflects the interweaving, penetration, integration, and vast time and space of geotechnical engineering and environmental engineering issues. The main safety and stability issues of tailings ponds include: dam breach, flood overtopping, slope instability, seepage damage, structural damage, and geological disasters in the reservoir area.

1. Dam break

During the whole process of tailings pond survey, design, construction, operation and management, problems in each link may cause the tailings pond to be unable to be used normally or even to break. Once a tailings pond breaks, the tailings often liquefy immediately, expanding the gap of the tailings dam, causing a large amount of tailings silt to pour along the valley, which will cause serious disasters such as flooding of downstream farmland, casualties, and environmental pollution. The reasons that may cause dam break hazards are as follows:

1) The tailings pond has low drainage capacity, the drainage system is blocked or there is no drainage system, the form, layout and size of the drainage system are changed at will, and the construction quality does not meet the specification requirements.

2) The ore placement is improper, or the ore is not placed alternately and evenly, resulting in excessive water level, fan-shaped slope, etc., and the minimum safety superelevation and the minimum dry beach length of the tailings pond do not meet the design and specification requirements.

3) Tailings, waste or wastewater outside the design enter the reservoir; the slurry flows horizontally along the inner slope toe of the sub-dam and scours the inner slope of the sub-dam.

4) Poor management, such as local concentrated ore discharge causing slurry to wash away the outer slope of the dam, resulting in dam collapse and dam failure; there is no intercepting ditch at the dam end, and rainwater from the hillside washes away the dam shoulder, etc.

5) Deformation, damage, fracture and abrasion of drainage pipes or drainage chutes, drainage wells, etc., the maximum crack width exceeds the allowable value, the expansion joints, water stops and fillings fail, and the pipes are blocked.

6) Deformation, damage, fracture and abrasion of the dam surface drainage ditch and the dam end intercepting ditch masonry, and siltation in the ditch.

7) Without technical demonstration, conventional sub-dams are used to intercept floods, and flood discharge outlets are set on the tailings beach or dam shoulders.

8) Atmospheric precipitation increases suddenly in a short period of time, large-scale landslides and collapses occur in the mountains around the reservoir, and heavy rainstorms, landslides and collapses in the mountains around the reservoir cause the reservoir water level to rise sharply and overflow accidents occur.

9) Without technical demonstration and approval, illegal mining, illegal construction and illegal operations are carried out around the tailings dam or reservoir area. 10) An earthquake with a higher intensity than the design intensity occurs in the reservoir area, and the earthquake causes the tailings in the bearing area to liquefy.

11) The dam slope is too steep, there is a local collapse or uplift, and there are scouring and collapse pits on the dam surface.

12) There are soft strata or karst under the dam foundation, and the dam body is loose and not dense.

2. Flood overtopping

The main causes of tailings pond flood overtopping accidents are:

1) The flood discharge structure is damaged or blocked, resulting in insufficient flood discharge capacity and the water level in the reservoir area exceeding the dam top.

2) Insufficient safety height leads to flood overtopping.

3) In the months with concentrated precipitation, rainfall of super-standard intensity occurs, and flood overtopping accidents occur.

4) Mudslides or bank slope collapses occur in the reservoir area, causing water to impact the dam body at high speed, causing the dam slope to become unstable and collapse.

When a flood overtopping accident occurs in a tailings pond, the tailings often liquefy immediately, widening the gap in the tailings dam, causing a large amount of tailings to pour down the valley. The degree of harm is much more serious than when the reservoir bursts, and the powerful mudslide endangers the personal and property safety of downstream residents.

3. Slope instability

Slope instability refers to the phenomenon that the dam slope is partially or completely sheared and moves down along the sliding surface or slope under the load of deposited tailings (Hou Yongli et al., 2020). The main causes of slope instability are:

1) The tailings dam construction materials have not been tested, the designed cohesion, internal friction angle and other parameters are inconsistent with the actual situation, the stability calculation is wrong, the slope is too steep, and the dam body is unstable.

2) The upstream tailings dam construction materials are too fine and do not meet the design specifications. During the construction of the rolled earth dam, the layered soil is too thick, the rolling is not solid, or the water content and dry weight do not meet the design standards, and the dam body is unstable.

3) The dam body filling quality is poor, and there are adverse phenomena such as scouring and pit collapse on the dam slope. The loose dam body causes the seepage damage to continue to expand, resulting in cracks in the dam body, pipe bursts or soil flow, causing landslides and collapse of the dam body.

4) The dam body foundation is improperly treated or the shear strength of water immersion is reduced. When the shear strength between the tailings dam and the base contact surface is less than the shear strength of the tailings pond material itself, landslides along the base contact surface are likely to occur.

5) The tailings pond is located on a weak base, or there are cracks and fissures in the bedrock. Due to the low bearing capacity of the base, slippage occurs, and the tailings pond causes landslides along the weak surface of the base.

6) The tailings particles are small, and most of them are in a loose to slightly dense state due to the short consolidation time. Under the action of earthquake loads, saturated tailings sand is easy to liquefy and cause dam instability.

7) After reaching the final design dam height, no demonstration and design for heightening and expansion were conducted, no dam stability analysis and flood control capacity verification were performed, and the dam was operated beyond the deadline blindly, resulting in the sliding force of the dam body being greater than the ultimate anti-sliding strength, leading to the instability of the dam body.

4. Seepage damage

Seepage damage is one of the main causes of tailings dam accidents. Because the tailings dam infiltration line is too high, the infiltration line escapes, forming seepage and pipe bursts on the dam slope, causing the slope of the tailings dam to become saturated and soft until the dam body collapses. The main causes of seepage damage are:

1) Seepage at the junction of the initial dam and the accumulation dam. When the seepage volume increases and a large amount of seepage cannot seep out from the bottom of the dam, the infiltration line moves up along the inner slope of the initial dam, and the escape point escapes from the junction of the initial dam and the accumulation dam. Possible causes include: ① arbitrarily changing the setting of the dam body's anti-seepage, drainage and anti-filtration layers; ② the selection of materials for drainage facilities in the drainage facilities does not meet the quality requirements or is not constructed according to technical specifications; ③ improper relaxation leads to irregular changes in the dam body's infiltration line, and local infiltration line escape occurs; ④ the dam body's anti-seepage, drainage and anti-filtration layers are destroyed or damaged, fail or have reduced capacity; ⑤ improper or too high reservoir water level control.

2) Seepage in the accumulation dam body. The centralized discharge of tailings and unreasonable sub-dam construction methods cause the concentration of sludge, making the accumulation dam an uneven body. The complex distribution of tailings particles, the formation of sludge interlayers and lens-shaped water-blocking layers, lead to irregular changes in the dam body's infiltration line, local infiltration line escape or local concentrated seepage.

3) Seepage in the contact section between the tailings dam and the hillside. The shoulder of the tailings dam is located on an untreated weathered hillside. Before each dam construction operation, the grass, tree roots, waste pipes and other debris on the slope that endanger the safety of the dam body were not removed. The slope was not treated when there were springs, wells and caves, causing tailings water to leak from the natural slope. As the water level in the reservoir increases, the intensity of the leakage water also increases, resulting in seepage on the outer slope of the tailings dam, causing tailings loss and the dam surface to become soft and slippery.

4) A large amount of concentrated seepage occurs on the outer slope of the initial dam and auxiliary dam. The filter layer is damaged during construction or production; the surrounding filling protective layer of the drainage in the initial dam is improper.

5) Concentrated seepage occurs along the outer wall of the drainage pipe. The construction quality of the drainage pipe does not meet the standards and the foundation is improperly treated. The drainage pipe sinks unevenly, causing the pipe joints to move and the pipe cracks to leak water, causing the tailings water to leak along the pipe wall, scouring the dam body around the pipe and causing the dam body to collapse.

5. Structural damage

The tailings pond was not constructed according to the design, and the structural parameters of the tailings pond were changed arbitrarily, causing structural damage to the tailings pond, bringing unsafe factors, and even causing landslides, dam breaches and other accidents. The main reasons for structural damage are:

1) The dam crest width is less than the specification requirements.

2) The upstream slope ratio of the rockfill body of the permeable rockfill dam is steeper than 1:1.6.

3) There is no horse trail on the downstream slope of the dam body.

4) There is no intercepting ditch on the slope where the downstream slope of the tailings dam meets the slopes on both sides.

5) The dam body is not equipped with a filter layer, causing pipe bursts.

6) The tailings dam is not equipped with observation facilities such as dam displacement.

6. Geological disasters in the reservoir area

Most tailings ponds are built in valleys and surrounded by steep mountains. Due to mining activities, rock weathering, and flooding of the tailings pond, landslides or collapses may occur on the slopes of the reservoir bank, and mudslides may occur upstream. When there are geological disasters such as goafs or karst collapses under the dam foundation, the tailings dam may also cause dam breaches.