The green disposal of tailings solid waste is a problem to be solved in mine production. Cemented tailings filling stoping method can realize the dual goals of solid waste resource utilization and mined-out area reduction. However, the volume of the mined-out area of the open-pit method is larger than the filling capacity, resulting in the complex stratification of the underground backfill, and the strength of the backfill cannot meet the requirements. In this paper, according to the delamination situation, the specimens of horizontal and inclination angle layered cemented tailings backfill (LCTB) is made for a uniaxial compression test, and the failure process of delamination backfill is reduced by PFC. The results show that the corresponding reduction factor φ of horizontal LCTB is 0.560–0.932, and the corresponding φ value of inclination angle LCTB is 0.338–0.772. The failure mode of backfill in different layers is mainly manifested as a tensile failure. The PFC numerical simulation results are consistent with laboratory test results, which verifies the correctness of backfill failure. The research results provide a reliable theoretical basis for the strength design of backfill in goaf, which is of great significance for solid waste utilization and environmental protection.

Mineral resources provide basic raw materials for social development, but a large number of tailings wastes are often generated due to technical problems in the process of resource development [

As an important content in the application of extraction steam turbine with stowing, the study of the mechanical strength of backfill has always been attached importance to by scholars [

Although many scholars have studied the influence of water-cement ratio, mass concentration and structural characteristics on the mechanical properties of backfill, the force of backfill under the influence of structural characteristics is more complex and the failure mode cannot be predicted. The rapid development of numerical simulation software solves the complicated mechanical model which cannot be realized by indoor mechanical experiments. PFC particle flow program is a kind of numerical software that can analyze and study macroscopic mechanical problems from a microscopic perspective. The research results are very close to the real situation [

In this paper, the mechanical properties of the LCTB were explored through laboratory experiments, and the failure characteristics of the LCTB were analyzed with particle flow software. The cemented tailings backfill with the cement-tailings ratio of 1:4, filling interval time of 24 h, and concentrations of 65%, 70%, and 75% were designed and prepared. The number of horizontal layers of backfill is 1, 2, 3, and 4, respectively, and the inclination angle between layers is 15°, 30°, and 45°. According to the uniaxial compression experiment, the compressive strength variation and failure mechanism of the backfill were studied to explore the mechanical characteristics of the backfill. At the same time, the uniaxial compression process of the backfill was simulated based on the two-dimensional particle flow software (PFC2D) to analyze the internal crack evolution mechanism.

Aluminum tailings were used as the raw material in the experiment, as shown in

The experiment is carried out from two angles: horizontally and angularly LCTB. The ratio of cement to tailings is set at 1:4, and the three concentrations were 65%, 70%, and 75%, respectively. The number of layers of the horizontal LCTB is successively set as one layer, two layers, three layers, and four layers, and the height of the filling slurry is successively 100, 50, 33.3, and 25 mm.

The uniaxial compression test is the most direct method to obtain mechanical parameters for backfill specimens. WDW-50 microcomputer-controlled electro-hydraulic servo press is used to carry out a uniaxial compression test on backfill specimens with different layer numbers and inclination angles, and the failure characteristics of samples are photographed and recorded, as shown in

To study the failure mode of LCTB from the microscopic point of view, the discrete element model of layered cemented filled body was established in the particle flow program PFC2D according to the results of uniaxial the compression test of filled body samples, and the uniaxial compression simulation test was carried out to analyze the fracture distribution characteristics at the microscopic point of view.

In PFC2D, force or bending moment is mainly transmitted between particles through bonding, and bonding models include the Linear Contact Bond Model, parallel Linear Parallel Bond Model, and Smooth-joint Contact Model. In the Linear Contact Bond Model, the particles have point contact, so the force-torque cannot be transmitted. When the normal or tangential force exceeds the corresponding bond strength, bond destruction occurs. The Linear Parallel Bond Model has a parallel bond bonding between the particles, which can transfer the force and torque between the particles. This model is closer to the real force situation of the filling sample than the contact bonding model. The Smooth-joint Contact Model is the polygonal particles instead of circular particles, which can transfer force and torque while inhibiting the rotation after particle bonding destruction, as shown in

In order to study the strength variation law of backfill under different stratification characteristics, the compressive tests of horizontally stratified backfill and angularly stratified backfill are carried out, and the results are shown in

Mass concentration | Horizontal LCTB | Inclination angle LCTB | |||||
---|---|---|---|---|---|---|---|

One layer | Two layers | Three layers | Four layers | 15° | 30° | 45° | |

65% | 3.2 | 3.1 | 2.7 | 2.4 | 1.8 | 1.6 | 1.3 |

3.3 | 3.1 | 2.9 | 2.3 | 1.7 | 1.4 | 1.3 | |

2.9 | 2.4 | 2.5 | 1.9 | 1.5 | 1.1 | ||

The average | 3.25 | 3.03 | 2.67 | 2.4 | 1.8 | 1.5 | 1.23 |

70% | 5.1 | 4.4 | 3.9 | 2.8 | 2.6 | 2.4 | 1.8 |

4.9 | 4.3 | 4.3 | 2.7 | 2.5 | 2.3 | 1.6 | |

4.8 | 4.4 | 3.5 | 2.6 | 2.1 | 1.9 | ||

The average | 4.93 | 4.37 | 3.9 | 2.75 | 2.57 | 2.27 | 1.77 |

75% | 4.6 | 4.2 | 2.7 | 3.8 | 3.6 | 2.9 | |

5 | 4.6 | 4 | 2.4 | 4.1 | 2.4 | ||

5.1 | 4.4 | 4.4 | 3.4 | 3.8 | 3.8 | 2.7 | |

The average | 5.05 | 4.53 | 4.2 | 2.83 | 3.9 | 3.7 | 2.67 |

Note: The unmarked strength in the table is abandoned due to its large discreteness.

According to the analysis of

According to the experimental results of uniaxial compressive strength (UCS), the strength of backfill decreases with the increase of layer number and inclination angle, and the layer number and inclination angle have an obvious weakening effect on the strength of backfill. Strength reduction calculation is performed for backfill with the different number of layers and different inclination angles, as shown in the equation.

The calculation results are shown in

Mass concentrations | Horizontal LCTB | Inclination angle LCTB | ||||
---|---|---|---|---|---|---|

Two layers | Three layers | Four layers | 15° | 30° | 45° | |

65% | 0.932 | 0.822 | 0.738 | 0.585 | 0.462 | 0.338 |

70% | 0.886 | 0.791 | 0.558 | 0.521 | 0.460 | 0.359 |

75% | 0.897 | 0.832 | 0.560 | 0.772 | 0.732 | 0.529 |

Strength analysis of backfill for different LCTB. To analyze the quantitative relationship between the strength of the backfill and the layer number and inclination angle, the test results of four concentrations were fitted by linear, logarithmic, and polynomial methods, and the multiple correlation coefficient R^{2} of each fitting method was calculated.

The fitting results in ^{2} at 65%, 70% and 75% mass concentrations reached 99.38%, 98.84% and 97.2%, respectively. Therefore, the polynomial equation can better reflect the quantitative relationship between compressive strength and layer number. From ^{2} of exponential fitting was the highest, which was 99.49%, 98.52%, and 96.16%, respectively. Therefore, the exponential relationship can better reflect the quantitative relationship between compressive strength and interlayer angle.

The failure specimen of the horizontal LCTB was shown in

Failure specimens of angled LCTB were shown in

A comprehensive analysis of the failure modes of the horizontal LCTB with a different number of layers and the backfill with different inclination angles shows that the upper backfill lags behind the lower backfill and the strength of the upper backfill are less than that of the lower backfill in the 28 d curing period. After loading, different degrees of separation and dislocation occurred in the LCTB specimen, and the impact of separation dislocation caused by angle factors was far greater than that of the number of layers. Delamination factors lead to the formation of low-strength interlayer in layers, which is easy to be destroyed when loading tests are carried out, which reduces the overall strength of the backfill and leads to the separation and dislocation phenomenon in different degrees at the layers of the damaged specimen, which reasonably explains the weakening effect of the compressive strength of the LCTB.

According to the analysis of the results obtained in the above chapters, it can be seen that there is a polynomial function relationship between the strength of the filling body and the number of layers, and the strength gradually decreases with the increase of the number of layers. The strength of backfill decreases exponentially with the increase of stratification angle. The strength reduction coefficient of horizontally layered backfill is 0.560–0.932, and that of inclined layered backfill is 0.338–0.772; Combined with the analysis of the failure characteristics of layered backfill, it can be seen that the failure forms of horizontally layered backfill are mainly semi-penetrating shear failure and the failure of middle weak layer, while the failure of angle layered backfill specimen is mainly the oblique crack penetrating the upper layer. Generally speaking, the weakening effect of horizontally layered backfill is lower than that of inclined layered backfill, and horizontally layered backfill has good integrity when it is destroyed.

Particle discrete element was a numerical method to analyze the mechanical behavior of discontinuous media. PFC was an effective tool to simulate complex problems in solid mechanics and particle flow. The size and scheme of the model simulated in this paper were consistent with the size and scheme of the laboratory test specimen. The numerical calculation model was a rectangle with a length of 100 mm and a width of 50 mm. Regardless of slurry mass concentration and curing age, only the number of layers and inclination angle on the mechanical characteristics of backfill were analyzed. Firstly, tailing particles with certain porosity are randomly formed in the rectangle according to the corresponding gradation, and then a certain number of cemented particles were randomly generated in the gap between tailing particles. To simplify the calculation, the particle radius was uniformly enlarged by 10 times. The radius of the tailings particle was 4.1 × 10^{−4} m∼1.5 × 10^{−3} m, and the radius of the cemented particle was 3 × 10^{−4} m, which was slightly smaller than that of the tailings particle. The parallel bond model was adopted for tailings particles and cementing materials. The parallel bond model could simulate the attached cementing substance between two adjacent particles. The contact between different sub-layers of the backfill body adopts smooth joint contact, as shown in

Model | Parallel bond contact | Smooth joint contact | UCS/MPa | |||||
---|---|---|---|---|---|---|---|---|

pb_coh/(N⋅m^{−1}) |
pb_ten/(N⋅m^{−1}) |
pb_coh/(N⋅m^{−1}) |
sj_Kn = sj_Kn/(N⋅m^{−1}) |
fric | large | |||

Horizontal LCTB | 1 | 1.0 × 10^{8} |
2.65 × 10^{6} |
53 × 10^{6} |
200 × 10^{9} |
0.1 | 1 | 4.93 |

2 | 2.33 × 10^{6} |
4.65 × 10^{6} |
4.34 | |||||

3 | 2.56 × 10^{6} |
5.12 × 10^{6} |
3.88 | |||||

4 | 2.25 × 10^{6} |
4.5 × 10^{6} |
2.75 | |||||

Angled LCTB | 15 | 2.45 × 10^{6} |
4.9 × 10^{6} |
2.58 | ||||

30 | 2.13 × 10^{6} |
4.25 × 10^{6} |
2.23 | |||||

45 | 2.75 × 10^{6} |
5.5 × 10^{6} |
1.73 |

By comparing and analyzing

The granular material is mainly densely arranged, and the contact between adjacent particles forms many external load transfer paths, which are usually quasi-linear chain structures, called force chains. The complex mechanical response of the force chain network determines the macroscopic mechanical properties of the granular system. PFC was used to obtain the force chain network after the failure of the backfill model, as shown in

This study is mainly based on the strength of horizontally and angularly LCTB, and the corresponding LCTB discrete element model is established. The reduction law of layered characteristics on compressive strength is analyzed, and the crack distribution characteristics of LCTB at the meso-level are studied. The conclusions are as follows:

Through the uniaxial compression test of cemented tailings backfill and strength reduction, the corresponding reduction factor

There are three failure modes: shear failure, tensile failure, and conjugate shear failure. The formation of low strength interlayer reduces the overall bearing capacity of the backfill, resulting in a degree of mutual dislocation between the upper layer and the lower layer of the damaged specimen, and this phenomenon is more obvious for the angled LCTB.

Based on laboratory tests and particle flow program PFC, the discrete element model is established to analyze the crack evolution law from the microscopic point of view. The bond between particles gradually destroys and forms micro-cracks following the loading, and develops into penetrating and semi-penetrating cracks with the increase of cracks, which verifies the correctness of the gathering mode of crack growth.

The law of strength reduction of backfill under layered characteristics provides a theoretical reference for the strength design of goaf backfill and realizes the green disposal of mine solid waste to some extent. Cemented filling of mine goaf with tailings not only realizes the utilization of mine waste but also ensures the safety of goaf.

This paper mainly studies the influence of layered characteristics on the compressive strength of backfill under static load, but the backfill is also affected by the dynamic load in engineering applications. The next step will be to explore the mechanism of dynamic load on backfill.

Uniaxial compressive strength reduction factor

The compressive strength of layered backfill

The compressive strength of complete backfill

_{2}