J. Cent. South Univ. (2016) 23: 2669-2675

DOI: 10.1007/s11771-016-3328-1

Mass ratio design based on compaction properties of backfill materials

LI Meng(李猛), ZHANG Ji-xiong(张吉雄), HUANG Peng(黄鹏), GAO Rui(高瑞)

Key Laboratory of Deep Coal Resource Mining of Ministry of Education (China University of Mining & Technology), Xuzhou 221116, China

Central South University Press and Springer-Verlag Berlin Heidelberg 2016

Abstract: The backfill-mining mass ratio is the ratio of the mass of the backfill materials in the goaf to the mass of the produced raw coal during solid backfill mining and it is regarded as a direct control index of the backfill effect in solid backfill mining. To design the backfill-mining mass ratio in a solid backfill mining panel, the backfill-mining mass ratio was defined on the basis of the basic principle of solid backfill mining. In addition, the density-stress relationship of backfill materials under compaction was obtained for five types of materials to derive a design formula for backfill-mining mass ratio. Moreover, the 6304-1 backfill panel under the large-scale dam of Ji′ning No. 3 coal mine was taken as an engineering case to design the backfill-mining mass ratio. In this way, it is found that the designed backfill-mining mass ratio is 1.22, while the mean value of the measured backfill-mining mass ratio is 1.245. Besides, the maximum roof subsidence is only 340 mm which effectively guarantees the backfill effect in the panel and control of strata movement and surface subsidence.

Key words: solid backfill mining; backfill-mining mass ratio; backfill materials; in-situ monitoring

1 Introduction

Solid backfill mining is a green mining method aimed at solving problems arising coal mining, such as mining under buildings, water bodies and railways, and gangue piles on the surface [1-3]. Solid backfill mining aims to backfill solid waste directly, for instance, gangue, fly ash, and slag into the goaf [4]. The materials are then compacted to a dense backfill body by using the compactor at the back of the backfill mining hydraulic support. Owing to the roof being managed by backfill materials, overlying strata movement can be decelerated to some extent [5-7], the strata weakens significantly compared with traditional caving mining methods [8]. The backfill materials should be timeously backfilled into the goaf when the raw coal is mined [9-11]. If the mass of backfill materials is insufficient, the backfill effect in the goaf is impaired and problems such as strata caving and surface subsidence arise, while if the mass of backfill materials is adequate, the control effect on strata movement and surface subsidence are ensured. Therefore, the relationship between the mass of the backfill materials and the mass of the exploited coal is a direct control indicator of the backfill effect.

So far, scholars have researched the relationship between the mass of backfill materials and raw coal. FLAC^3D numerical simulation software is adopted by HUANG et al [12] to conduct a numerical simulation analysis of the overlying strata movement in backfill mining stope under different backfill ratios. LI [13] proposed the concept of backfill-mining mass ratio, and also selected it as an indicator to measure the backfill effect. WANG et al [14] explored the backfill mass control of solid dense backfill mining by combining similar material simulations, the analysis of actual measurements, and theoretical research. ZHANG et al [15] studied several control measures, such as the use of reinforcing support to the panel, which controls the roof pre-subsidence, with monitoring of the backfill-mining mass ratio, etc. to control the implementation effects of backfill mining. With Pingdingshan No. 12 mine as an example, ZHOU et al [16] considered the ratio of the amount of backfill in the panel to the amount of coal produced coal as a measure of the backfill effect. The results of all of this research promote the development of the theory of backfill mining. Whereas, how to design backfill-mining mass ratios requires further research.

Based on the basic principle of solid backfill mining, the design method for the backfill-mining mass ratio is proposed by using the results of compaction property test. Moreover, the proposed design method for the backfill-mining mass ratio is verified and applied to a practical engineering case study.

2 Basic principle of solid backfill mining

As shown in Fig. 1, gangue, loess, slag, aeolian sand, fly ash, etc., as backfill materials, are conveyed to an underground storage silo by a vertical feeding system. Then the backfill materials were transported to the backfill mining panel [17]. After that the backfill materials were backfilled into the goaf by the backfill conveyor hanging on the back top beam of the backfill mining hydraulic support. Finally, the compactor behind the backfill mining hydraulic support was used to compact the backfill materials to ensure better support of the roof, reduce overlying strata movement, and control surface subsidence [18-20]. The backfill materials should be backfilled into the goaf as soon as the coal has been mined. If the mass of backfill materials is inadequate, the backfill effect was reduced. So it is necessary to establish the relationship between the mass of backfill materials and the coal produced to supply an accurate mass of backfill materials.

3 Definition of backfill-mining mass ratio

As shown in Fig. 2, when the raw coal in Region I is unexploited, the mass of the raw coal in Region I is set to mass m_c. If the coal is mined out, backfill material is needed in Region I, to a compacted mass m_b. On this basis, backfill-mining mass ratio e is the ratio of the mass of the backfill body backfilled into the goaf (m_b) to the mass of raw coal (m_c), which can be expressed by

                                     (1)

Owing to the volume of mined coal equalling the volume of the backfill body, the backfill-mining mass ratio is equivalent to the ratio of the density of the backfill (ρ_b), to the density of raw coal (ρ_c) namely:

                                     (2)

From Fig. 2, the backfill materials formed a dense backfill body once compacted. It is necessary to design the backfill-mining mass ratio by considering the condition of surface structures in the mining area to guarantee that sufficient backfill material is used so as to control overlying strata movement and surface subsidence. Thus, the backfill-mining mass ratio is regarded as a direct control indicator which could guarantee the backfill effect. Besides, changes in the density of such backfill materials with changing compaction force are able to be obtained experimentally.

4 Compaction properties test of backfill materials

4.1 Test description

The test system for the compaction properties of these backfill materials is shown in Fig. 3. A YAS-5000 electro-hydraulic servo-pressure test machine produced by Changchun Kexin Test Instrument Co., Ltd, China was used. The maximum axial force provided by the test machine was 5 MN. Besides, the compaction device was a self-designed steel cylinder with inner diameter, outer diameter, and height of 125 mm, 137 mm, and 305 mm, respectively. The loading platen installed with this compaction device was adopted to bear load from the test machine, while the loading platen had a radius of 124 mm and a height of 40 mm (see Fig. 4).

Five types of test material were involved: gangue, fly ash, loess, aeolian sand, and slag as shown in Fig. 5. These are respectively selected from coal gangue piles, a loess slope, an opencast slag discharge field, and the discharge point for fly ash from the nearby power plant. To meet the demands of solid backfill mining, gangue and slag had to be crushed to a grain size of less than 50 mm. Then the samples were prepared according to the broken grain-size grade, while the samples of fly ash, loess, and aeolian sand were prepared according to their original sizes. After sample preparation, each backfill material sample was put into the compaction device and then subjected to an imposed axial stress of 8 MPa. Each set of tests was performed for three times.

Fig. 1 Basic principle of solid backfill mining

Fig. 2 Schematic diagram of backfill-mining mass ratio:

Fig. 3 Test system

4.2 Test results and analysis

1) Stress-strain relationship

The stress-strain curves for the five backfill materials under compaction are shown in Fig. 6.

Fig. 4 Plan and elevation views of steel cylinder:

Fig. 5 Backfill material samples

Fig. 6 Stress-strain curves of backfill materials

From Fig. 6, the stress-strain relationships of the five backfill materials are similar during compaction, and all have an exponential form: the goodness of fit is high. With increasing applied stress, the strain in the samples increases, but the increase reaches an asymptote. Under the same stress, the strain, in descending order is that loess, fly ash, slag, gangue, and aeolian sand. This suggests that the deformation resistances of these backfill materials are different. The compaction process comprised three stages: fast compaction, slow compaction, and stable compaction.

2) Density-stress relationship

During compaction, the backfill mass is conserved, namely ρ₀Ah=ρ(σ)Ah(1-ε(σ)), which is simplified to a function relating density and strain:

                              (3)

where ρ₀ is the initial bulk density of the backfill; A is the cross-sectional area of the compaction device; and h is the initial loading height.

Figure 7 shows the density-stress curves of the backfill materials during compaction (obtained from Eq. (3)).

Fig. 7 Density-stress curves of backfill materials

From Fig. 7, the density-stress relationships are non-linear. With increasing applied compaction stress, the density of these backfill materials increases gradually to an asymptote. During compaction, the changes in the density-stress, and stress-strain relationships are similar (i.e. the former also experienced three stages, including fast compaction, slow compaction, and stable compaction). The higher the density of the backfill materials, the stiffer the material. Each backfill has a different stiffness. An applied stress of 1.0 to 2.0 MPa is sufficient to improve the density of these materials and increase their stiffness.

5 Design of backfill-mining mass ratio

From the above compaction tests, there is an exponential form of stress-strain relationship therein, solving this relationship and obtaining strain-stress formula as

                              (4)

where a, b and c are fitting parameters. For different kinds of backfill materials, fitting parameters can reflect unique properties of backfill materials.

Substituting Eq. (4) into Eq. (3) gives:

                        (5)

By solving Eqs. (2) and (5) simultaneously, the formula for backfill-mining mass ratio becomes:

                  (6)

According to the compaction test results for these backfill materials, the relationship between the backfill- mining mass ratio and strain in the backfill during compaction is as shown in Fig. 8.

Fig. 8 Backfill-mining mass ratio versus stress

From Fig. 8, there is a non-linear relationship between backfill-mining mass ratio and stress. Moreover, as the compaction stress increases, the backfill-mining mass ratios increase to an asymptote. During compaction, the change in backfill-mining mass ratio and stress, and their stress-strain relationships are similar and also contain three stages: fast compaction, slow compaction, and stable compaction. The backfill-mining mass ratio of these materials could be improved by applying 1.0 to 2.0 MPa compaction stress.

6 Engineering practice

6.1 Mining geological conditions

The solid backfill mining under the large-scale dam at Ji′ning No. 3 coal mine is taken as a case study for the analysis of the effects of backfill-mining mass ratio. The test backfill area in the mine area is the protective coal pillar in the Nanyang Lake embankment of the No. 6 mining area. The first panel of 6304-1 is 80 m in length and 518 m in advance length. Besides, the mined coal seam is from the No. 3 coal seam of the Shanxi formation with an average thickness of 3.5 m and a mean inclination angle of 5°. The recoverable reserve is up to 182000 t and the average burial depth is 680 m. The layout of the 6304-1 panel is shown in Fig. 9.

Fig. 9 Layout of 6304-1 backfill mining panel

A large amount of gangues is produced, when the roadways are driven in Ji′ning No. 3 coal Mine. In order to dispose of gangues and reduce transportation pressure, the tunnelled gangues are used as backfill materials of the goaf in the 6304-1 panel. Through the compaction property test of the backfill materials, the strain-stress formula, obtained by inverse solution, is as follows:

                   (7)

The compaction result of tunnelled gangues is shown in Fig. 10.

Fig. 10 Compaction result of tunnelled gangues

Due to the compaction stress, a large amount of particles begin to break. These particles fill into the pores of the materials, reducing porosity causing anti-deformation capacity to increase gradually. Also, the density of the tunnelled gangues increases greatly, making the backfill materials dense.

The initial bulk density of the tunnelled gangues is 1480 kg/m³ and the density of the raw coal is 1400 kg/m³. Then the data are substituted into Eq. (6) to obtain the relationship between backfill-mining mass ratio and stress as follows:

             (8)

The backfill-mining mass ratio-stress plot, based on Eq. (8) is shown in Fig. 11.

Fig. 11 Backfill-mining mass ratio-stress of tunnelled gangues

A ZZC10000/20/40 hydraulic support with six columns is adopted for the 6304-1 panel and the compaction stress is designed to be 2.0 MPa. Then the corresponding backfill-mining mass ratio is 1.22, which meants that 1.22 t of gangues are needed for each tonne of mined coal.

6.2 In-situ monitoring

During the advancing process of the 13120 backfill panel, monitoring points are laid in the goaf to dynamically monitor the backfill-mining mass ratio and roof subsidence. The in-situ monitoring method is shown in Fig. 12.

The mass of mined coal in panel and the mass of the backfill materials are monitored, as shown in Fig. 13.

The minimum monitored backfill-mining mass ratio is 1.179, while the maximum is 1.282. Besides, the mean value is 1.245, which is greater than the design value of 1.22. In the early stages of backfill mining, the backfill-mining mass ratio is lower than the design value because the preparation of the panel is inadequate, while it was continuously higher than the design value at a later stage, which suggests that the backfill-mining mass ratio is better controlled by then. Meanwhile, the maximum measured roof subsidence is 340 mm which is deemed satisfactory, as shown in Fig. 14.

Roof dynamic subsidence with panel advancing has three stages: slight subsidence stage, main subsidence stage and stabilization subsidence stage. In slight subsidence stage, within 0-33 m away from the panel, coal body supports most stress on overlying strata. Thus,the roof shows slight subsidence. In main subsidence stage, within 33-80 m away from the panel, coal body in front of the panel fails to support most stress on overlying strata, resulting in entire subsidence of the roof, and the maximum subsidence is up to 340 m. In stabilization stage, from 80 m away from the panel, the backfill materials are compacted gradually and become the main support stress on overlying strata, so that the roof subsidence tends to be stable.

Fig. 12 In-situ monitoring method: (a) Monitoring scheme;

Fig. 13 Backfill-mining mass ratio monitoring

Fig. 14 Roof dynamic subsidence

7 Conclusions

1) According to the basic principle of solid backfill mining, the definition of a backfill-mining mass ratio is proposed. The designed backfill-mining mass ratio is equivalent to the density ratio of the designed backfill materials to that of the raw coal by using the principle that the volume of the extracted coal equals that of the backfill materials.

2) The compaction tests on the five backfill materials (gangue, fly ash, loess, wind-blown sand, and slag) provide stress-strain, and density-stress, curves during compaction.

3) On the basis of the compaction properties of the five types of backfill materials, the design formula for backfill-mining mass ratio is deduced as well as the plot showing backfill-mining mass ratio versus stress during the compaction thereof.

4) The backfill-mining mass ratio of the 6304-1 backfill mining panel under the large-scale dam of Jining No. 3 coal mine is designed. The designed backfill- mining mass ratio is 1.22, while its mean measured value is 1.245. This proves that the backfill effect in the panel could be effectively guaranteed by controlling the backfill-mining mass ratio.

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(Edited by FANG Jing-hua)

Foundation item: Project(51421003) supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China; Project(2014ZDPY02) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2015-07-26; Accepted date: 2015-11-09

Corresponding author: ZHANG Ji-xiong, Professor, PhD; Tel: +86-13912005505; E-mail: zjxiong@163.com