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Where a mined-out area underlies a slope, it is a direct threat to slope safety and stability. This is of particular concern where a mined-out area underlies the slope of an open-pit mine, and it has a serious impact on the design and safety measures used for the mine. If a mined-out area underlying the final slope of an open-pit mine is not treated adequately and at the appropriate time, it may cause the slip failure of the final slope during the service life of the mine, posing a serious threat to the safety of personnel and equipment during the stripping phase. In light of the potential for such problems, this paper analyzes the instability mode and failure characteristics of an open-pit slope near a mined-out area in China using geological field survey and the polar stereographic projection method. The scale span method, in combination with engineering analogy and consideration of open-pit mining technology, is then used to determine the critical safety thickness at which pretreatment of mined-out areas should be carried out. A pretreatment process to infill the mined-out area during construction of open-pit mine steps is put forward, and its effects on slope stability and reliability are comprehensively evaluated. The results show that circular sliding is the most appropriate instability mode for a slope near a mined-out area. The failure initiates through breakage in the roof of the mined-out area, which induces subduction sliding of the free face of the slope at the left boundary of the mined-out area and subsequent failure of the entire regional slope. Comprehensive analysis methods are used to determine that the critical safety thickness at which a mined-out area under the final open-pit slope should be pretreated is 24 m. The recommended treatment countermeasure is to transfer filling slurry into the mined-out area through drilling holes in benches. This can satisfy the stability and reliability requirements for the slope under different working conditions.

A large proportion of mining in China uses the open-pit method. Slope stability is a core safety issue at such mines. Illegal mining, which has been on the rise since the 1980s, has resulted in numerous undocumented underground goafs around such mines [

If the mined-out area underlying the final slope of an open-pit mine is not treated effectively and at the appropriate time, it may cause the slip failure of the final slope during the service life of the mine, posing a serious threat to the safety of personnel and equipment during the stripping phase. Therefore, an assessment of the critical safe roof thickness is urgently required, so that grouting of mined-out areas can be carried out when this critical thickness is reached to ensure overall slope stability. The present study takes the Niukutou open-pit mine in Qinghai Province as an example. The mode of instability and failure characteristics of the slope overlying mined-out areas at this site are analyzed, and the critical safety thickness at which pretreatment of mined-out areas should be conducted is determined synthetically using a variety of methods. Pretreatment measures for mined-out areas are then proposed, and a comprehensive evaluation of the effects of the treatment on slope stability and reliability is carried out, providing a reference for engineering practice at mines where similar conditions pertain.

Niukutou Mine in Qinghai Province is a polymetallic mine that uses open-pit to underground mining. The service life of the open-pit mine is eight years. The main strata lie in a nearly northeast orientation. Lithologically, it is mainly composed of marble, skarn rock (including the ore layer), and granite. This is covered by quaternary alluvial-diluvial glutenite, generally 30–70m thick. The mining area is located in the 6-degree earthquake zone.

There are a large number of civil mined-out areas beneath the mining area (Figure ^{3} under the open-pit as a whole, and 16,000 m^{3} under the final slope. The mined-out areas are mostly of gallery type. There is a large mined-out area in the northwest of the mine, in the lower part of its final slope. This mined-out area seriously hampers safe production at the mine because it has a major impact on slope stability. On the basis of the results of an existing study [

Distribution and scale of mined-out areas under Niukutou open-pit mine.

Because the depth to mined-out areas below the final slope varies, the mined-out area with the largest scale closest to the final slope and with the greatest influence on the stability of the slope is taken as the study area. The length and width of this mined-out area are 27 m and 18 m, and the area of the roof is approximately 425 m^{2}.

There are many groups of joint fissures in the rocky slope rock mass, and the intersection of many such groups with the excavated slope could potentially give rise to a variety of instability modes. Therefore, before a quantitative calculation of slope stability can be undertaken, it is necessary to make a preliminary analysis of the mode of slope failure so that the appropriate stability analysis method can be selected.

The distribution of joints in the rock mass is the decisive factor controlling the stability of a highly jointed rock mass. A geological field investigation was carried out into the occurrence of structural planes in an inclined shaft in the mined-out area using a scanline method, and a statistical analysis of the structural plane and its features was then conducted using the stereographic projection method. The results of these investigations are shown in Table

Characteristics of structural planes within the study area.

Superior joint group or side slope number | Tendency/° | Inclination /° |
---|---|---|

1 | 286 | 71 |

2 | 248 | 72 |

3 | 321 | 72 |

I - I profile | 92 | 45 |

Geological field investigation in an inclined shaft in the mined-out area.

The results regarding advantageous joint parameters, slope parameters, and the measured shear strength of the structural plane were summarized [

Stereographic projection analysis of slope failure mode.

In light of the mode of slope failure identified, the selection of the mechanical parameters of the rock mass has a serious impact on the reliability of the slope stability analysis. Therefore, it is important to determine reasonable mechanical parameters to be used for this purpose.

The standard recommended methods were used [

Physical and mechanical parameters of rock mass and soil.

Type of rock |
Weight ^{3} |
Cohesion c |
Angle of internal friction |
---|---|---|---|

Quaternary | 18.7 | 27 | 25 |

Marble | 26.5 | 90 | 31 |

Skarn | 37.6 | 170 | 32 |

Granite | 26.4 | 180 | 33 |

Filling | 20.0 | 85 | 27 |

On account of the failure mode identified, Phase 2 7.0 and Slide 6.0 software were used to analyze the process of slope failure when underlain by a mined-out area. The results are shown in Figure

Evolution of the failure of a slope overlying a mined-out area.

The process of failure calculated using the strength reduction method of Phase2 7.0 and the simplified Bishop method of Slide 6.0 is consistent. Namely, failure occurs by fracturing of the roof of the mined-out area, which causes subduction sliding of the free face of the slope at the left side of the mined-out area, leading to slope failure across the whole area. Therefore, the existence of the mined-out area seriously affects the stability of the slope, and relevant pretreatment measures must be taken to deal with the mined-out area.

Treatment of the mined-out area must be carefully timed so that it does not affect production but guarantees the stability of the slope during the production process. Therefore, it is necessary to determine the critical thickness at which the mined-out area should be pretreated.

There have been studies into the safe thickness for the roof (crown pillar) of a mined-out area, in both China and elsewhere, using approaches such as the load transfer line intersection method, span-to-depth ratio method, broken arch theory method, Rubienie Yite theory estimation method, and Bogoliubov theory calculation method. These methods are mainly based on single geometric parameters relating to the span of the mined-out area and so have certain limitations. On the basis of data from more than 300 boundary crown pillars, Carter et al. proposed the scaled span method, which determines the critical safety thickness for the roof of a mined-out area based on its geometrical parameters and the dip angle of the orebody in the mined-out area [

The key to using the scaled span method to determine a safe thickness for the roof is to determine the scaled span of the crown pillar. The calculation formula is as follows:^{3};

The probability

The relationship between the thickness of the crown pillar, the safety coefficient of the crown pillar for pretreatment at Niukutou Mine, and the failure probability as determined using the above formula is shown in Figure

The relationships between the thickness of the crown pillar and both the safety factor and probability of failure.

As shown in Figure

Engineering analogy has frequently been used to determine the safe thickness of a mined-out area roof in China and abroad, and a large number of engineering examples have therefore become available. Mines with a similar span and extent of mined-out area, slope scale, and rock lithology as Niukutou Mine were selected for statistical analogy and are listed in Table

Statistics of boundary pillar thickness in comparable mines.

Name of mines | Span of mined-out |
Area of mined-out ^{2} |
Safety thickness of crown |
---|---|---|---|

Krivoy Rog Mine | 15–25 | 200–600 | 20–30 |

Haydargangski Mine | 25–30 | 100–500 | 15–20 |

Nuosisiji Chai Mine | — | 200–2100 | 14–16 |

Niki Torfs Ki Mine | 20–25 | — | 15–30 |

Datang Mine | 20 | — | 15 |

Chuanyandong Mine | 20–25 | — | 24 |

Jinchangyu Mine | >20 | — | 25 |

Shirengou iron Mine | 20 | — | 20 |

The safe thickness of the roof determined by the scaled span method and engineering analogy is essentially the same. To reduce the impact that pretreatment of the mined-out area has on production and considering that strip mining at Niukutou Mine uses a step height of 12 m, the critical safety thickness for pretreatment of the mined-out area under the slope is 24 m. This means that pretreatment should be conducted two steps ahead.

The selection of pretreatment strategy has a decisive effect on the stability of the open-pit slope. After a comprehensive comparison of a variety of treatment methods, the method that has been adopted is to fill the mined-out area is to discharge filling slurry into drill holes during step construction (Figure

Treatment scheme for the mined-out area under the final slope of the open-pit mine.

The pretreatment measures directly affect the stability of the slope near the mined-out area. The failure mode near the mined-out area was preliminarily determined to be circular sliding. On the basis of a combination of related standards [

The slope stability safety factor and probability of slope failure under different working conditions.

Working conditions | Calculation method | ||
---|---|---|---|

Safety coefficient | Failure probability | ||

Simplified Bishop method | Strength reduction method | ||

Unfilled mined-out area | |||

No earthquake force | 0.756 | 0.49 | 100% |

Earthquake force | 0.747 | 0.49 | 100% |

Filled mined-out area | |||

No earthquake force | 1.169 | 1.13 | 0.2% |

Earthquake force | 1.145 | 1.10 | 0.4% |

The two methods both indicate that the safety coefficient of the slope when the mined-out area is not treated is much less than 1.0: the slope cannot be kept stable. When pretreatment filling of the mined-out area is carried out, the slope stability is greatly improved, meeting the requirements that the safety coefficient of the slope over the service life is not less than 1.15 and the safety coefficient of slope under earthquake force is not less than 1.10 [

Due to the specific characteristics of the rock itself, the shear strength index has a certain variability. Therefore, the influence of the random distribution of the shear strength index on the analysis of slope stability must be considered. A large number of studies have shown that the probability density function of the shear strength index, which plays a key role in slope stability analysis, has a normal distribution or an approximately normal distribution (Figure

Typical probability density functions of shear strength index.

Cohesion

Internal friction angle

The analysis of slope reliability was carried out with Slide 6.0 slope stability analysis software and the Monte Carlo method, running 1,000 cycles. As shown in Table

The authors declare that they have no conflicts of interest.

Dr. Zhu proposed the idea of this article, and Dr. Tao and Dr. Zhu completed the first draft of this article together. Zhigang Tao and Chun Zhu contributed equally to this work. Since the later revision work was mainly completed by Chun Zhu, all authors agreed that Dr. Tao and Dr. Zhu can serve as co-first authors.

This work is supported by the National Natural Science Foundation of China (no. 41502323 and no. 41602365) and Beijing Natural Science Foundation of China (8142032).