J. Cent. South Univ. Technol. (2008) 15: 350-355

DOI: 10.1007/s11771-008-0066-z

Settlement behavior of coal mine waste in different surrounding rock conditions

MA Chun-de(马春德)¹, LI Xi-bing(李夕兵)¹, HU Bing-nan(胡炳南)²,

CHEN Feng(陈 枫)¹, XU Ji-cheng(徐纪成)¹, LI Di-yuan(李地元)¹

(1. School of Resources and Safety Engineering, Central South University, Changsha 410083, China;

2. Centre Coal Research Institute, Beijing 100013, China)

Abstract: In order to investigate the influence of complex conditions of in-situ surrounding rocks on the settlement behavior of nubbly coal mine waste subjected to high gravity pressure, four kinds of loading chambers made of different similar materials with different elastic moduli in experiments were used to simulate the deformation features of in-site rocks, including soft, moderate hardness, hard and extra-hard rocks. The results show that all the settlement-axial load (or axial strain-stress) curves obtained under four different surrounding rock conditions present power-exponential function feature. The final settlement of coal mine waste under the same axial load is closely related to the lumpiness gradations and the deformation behavior of chamber materials used to simulate behaviors of different in-situ surrounding rocks. In the same surrounding rock condition, the final settlement under the same maximum axial load decreases with the decrease of the proportion of larger gradation of coal mine waste. While for the same lumpiness gradation case, the settlement increases with the decrease of elastic modulus of simulated surrounding rocks and the lateral pressure induced by axial load increases with the increase of elastic modulus of loading chambers that are used to simulate different surrounding rocks. The test results also reveal that both the compaction curve and lateral pressure curve show a three-stage behavior, and the duration of each stage, which is closely related to gradations and the deformation feature of loading chamber materials, decreases with the increase of the proportion of the small size of coal mine waste and elastic modulus of the simulated rock materials.

Key words: coal mine waste; settlement behavior; surrounding rock conditions; final settlement; lateral pressure

1 Introduction

Coal, as one of the important energy sources, has been explored several hundreds of years, which left a great amount of underground mined-out areas with different scales, depths and complexity. Every year, several thousand million tons of waste materials are generated during the process of clearing raw coal, which will not only occupy a great amount of agricultural land and deteriorate environment due to its deposition in the open air，but also bring a huge potential danger to ground architectures, highways, dams and railways. In some cases, coal mine waste hill collapse is prone to take place if it is not properly piled up^[1-5]. To deal with these problems, a new green-filling technique is developed^[6-7], by which coal mine waste, which is used to be lifted out of underground mine, can be filled directly into underground mining-out areas or abandon laneways to support underground structures, control ground settlement and collapse.

Using coal mine waste as a backfill material possesses a lot of advantages, such as simplicity, facility, low cost and easily obtaining on the spot. Some successful experiences have been achieved in this field through lots of years’ studies^[8-10]. However, no deep study has been conducted on the settlement behavior of coal mine waste in complex surrounding rock conditions^[11-15], such as the influence of size proportion of lane section to granular coal mine waste fragmentation and the deformation feature of loading chamber on the settlement. In their studies only rigid loading chambers and small caliber loading chambers were used, which is not consistent well with the surrounding environment using coal mine waste as filling material. In fact, the elastic modulus of surrounding rock has significant influence on the settlement. In order to treat these problems and obtain more precise results of settlement behavior of coal mine waste, large caliber loading chambers with four different elastic moduli, were used to simulate different surrounding rocks in the present work. The obtained results may hopefully provide a more precise theoretical prediction to the settlement of ground surface settlement above the underground mined-out area and provide a practical settlement control method as well.

2 Experimental method and procedures

2.1 Sample preparation

All coal mine wastes used in the test were chosen from the Gaokeng Coal Mine in Jiangxi Province, China, which were hoisted currently from underground coal mine. They was divided first into five granular size levels from large to small using sieving method: P1 (50-40 mm), P2 (40-30 mm), P3 (30-20 mm), P4 (20- 10 mm) and P5 (＜10 mm), and then different samples were built up according to the lumpiness gradations cases listed in Table 1.

Table 1 Lumpiness gradations cases of contrastive samples (mass fraction, %)

It can be seen from the lumpiness gradations of three contrastive samples that the proportion of larger size coal mine waste increases, while that of the smaller size decreases from sample A to sample C.

2.2 Experimental device

In order to investigate the effect of surrounding rock environment on the settlement of filled coal mine waste, four loading chambers with different elastic moduli were used to simulate different deformation features of surrounding rocks around the coal mine waste. The elastic moduli of loading chambers made of reinforced epoxy resin and concrete were about 3, 9, 24 and more than 50 GPa, respectively, corresponding to the simula- tion of the soft, moderate hardness, hard and extra-hard coal bed. These chambers are displayed in Fig.1. The chambers in Figs.1(a), (b) and (c) were all made into thick wall cylinders with 320 mm in inside diameter, 40 mm in wall thickness, and 300 mm in height, and the chamber in Fig.1(d) used to simulate the extra-hard surrounding rock was made of seamless steel pipe, with 320 mm in inside diameter, 10 mm in wall thickness, and 300 mm in height. The elastic modulus of the similar material was measured by the standard samples artificially made using the same material when the homologous loading chambers were moulded.

2.3 Experimental design and procedures

In this work, four surrounding rock conditions, corresponding to soft, moderate hardness, hard and extra- hard coal bed, were simulated to investigate the settlement behavior of coal mine waste located at about 800 m in depth underground mined-out areas where the gravity is about 20 MPa. All tests were conducted on INSTRON 1346 testing machine with the maximum load of 2 MN, under stress control mode with the loading rate of 0.062 5 MPa/s. In order to measure lateral pressure, circumferential strains were also measured during the tests using the strain gauges sticked on the mid-below part of the loading chamber’s opposite surface. The circumferential strains can be isochronously measured using DH3817 static-dynamic strain apparatus in the loading process.

The experimental procedures are as follows: firstly weighing the coal mine waste according to the proportion of the designed lumpiness gradations case and mixing them uniformly, then putting them into loading chamber and pugging with a crabstick to eliminate some large caves among them. An important thing that should be paid attention to is that the height of testing sample must be ensured to be the same (275 mm) in each test for the compaction feature of the testing results. Secondly, loading via a steel disk with 50 mm in thickness on the surface of the sample, a small load was added first to eliminate the interspace between the disk and sample and make them contacted well. In the end, the control software in the computer gave the instruction of slowly

Fig.1 Loading chambers to simulate surrounding rocks with different moduli: (a) E=3 GPa; (b) E=9 GPa; (c) E=24 GPa; (d) E＞50 GPa

loading with a rate of 0.062 5 MPa/s, and the test formally began. At the same time, the lateral strain measurement system started to logging the test data synchronously.

3 Experimental results and analysis

3.1 Data processing method

3.1.1 Compaction curves and analysis of axial settlement of coal mine waste

The original load and displacement data recorded by the corresponding transducers can be used to obtain the axial pressure-settlement relationship. The axial stress-strain curve (i.e. compaction curve) of each sample can be drawn easily through a simple calculation, and the axial settlement property of coal mine waste with different gradations and surrounding rock conditions can then be analyzed.

3.1.2 Lateral pressure curves and analysis of lateral pressure effect of coal mine waste

A lateral pressure curve of bulk material, namely a relationship curve of axial stress and lateral pressure, can be obtained as follows: calculating the average circumferential strain value first from the four strain gauges sticked on the opposite sides of loading chamber’s surface, and then substituting these values into formulas (1) and (2), which correspond to the calculation of thin and thick wall cylinder lateral pressure.

σ_thin=                                 (1)

where  σ_thin is the lateral pressure of the thin wall cylinder; E is the elastic modulus of loading chambers; ε_θ is the average circumferential strain; T is the thickness of a thin wall cylinder; r is the radius of thin wall cylinder.

σ_thick=                           (2)

where  σ_thick is the lateral pressure of thick wall cylinder; r₁ and r₂ are the internal and external diameters of the thick wall cylinder, respectively.

The σ_thin and σ_thick values calculated from formulas (1) and (2) can be used to draw the lateral pressure—time curve, compared with the axial stress—time curve, they possess the same time axis, and the axial stress and the lateral pressure curves can then be drawn together in the same figure. Therefore, the property of lateral pressure of coal mine waste with different gradations under complex surrounding rock conditions can be analyzed.

In this way, the axial stress, strain and lateral pressure of samples A, B and C under different surrounding rock conditions are calculated, and the obtained compaction and lateral pressure curves are given in Fig.2.

3.2 Analysis for test curves

It can be found from Figs.2((a)-(c)) that there exists an power-exponential function relationship between the axial stress and strain, and the compaction curves obtained under four different surrounding rock conditions are almost the same, which can be approximately divided into three stages.

The first stage is the initial linear stage corresponding to the filling course of the space among the coal waste particles. In this stage the axial deformation (or strain) increases quickly as the axial load (or stress) increases slowly, which reveals the deformation characteristic of loose media under lower axial load. But, due to the influence of distinct elastic modulus of the similar materials on the lateral deformation, the duration of this stage under four surrounding rock conditions is quite different, which satisfies the following formula:

＞＞＞               (3)

where   denotes the length of the stage, subscript i denotes the hardness of the surrounding rock, and the superscript j denotes the serial number of loading stage, for the first stage, j=1.

It can be seen from formula (3) that with the increase of the elastic modulus of similar material, the length of the first stage becomes short, denoting that the clearance among the coal mine waste particles will be filled up in a short time.

At the end of the first stage, most part of the clearance among the coal mine waste particles will be filled up. With further increasing the axial stress, a non- linear relationship between axial stress and strain can clearly be observed, which follows the power- exponential function law. These curves show that with the increase of the axial stress, the increasing rate of the axial strain becomes slower than that in the first stage. In this stage, the phenomena of clearance filling and block crushing are concurrent, while the latter is dominant. During this period, a crepitant noise with a feature of weak-strong-weak-disappearing can be clearly heard. In this stage, the loose coal mine waste material is almost completely compacted. The length of this stage is similar to that in the first stage:

＞＞＞               (4)

This result indicates that the hardness of the surrounding rock plays an important role in the settle- ment of coal mine waste during the compaction course,

Fig.2 Compaction and lateral pressure curves of three kinds of samples under different surrounding rock conditions: (a) Compaction curves of sample A; (b) Compaction curves of sample B; (c) Compaction curves of sample C; (d) Lateral pressure curves of sample A; (e) Lateral pressure curves of sample B; (f) Lateral pressure curves of sample C

and under a harder surrounding rock condition the time to reach the stable settlement deformation is shorter than that under the soft one.

In the third stage the axial stress increases rapidly with the slow increase of axial stain, and both the axial deformation and the increscent rate of the axial strain are smaller than those of the anterior stages. When the curves reach this stage, the granular characteristic of coal mine waste will almost vanish, and coal mine waste will present the constitutive behavior of the intact elastomer. There is a similar relation in this stage as follows:

＞＞＞               (5)

The above results show that the deformation feature of the surrounding rocks is of a significant influence on the final settlement of the coal mine waste. The filled coal mine waste quickly reaches the final settlement under the harder surrounding rock condition, which will be beneficial to the filling process. The influence of different surrounding rock conditions on the axial deformation of coal mine waste can be clearly observed from Figs.2((a)-(c)). The compaction curves obtained from the three samples with different lumpiness gradations also show some interesting results, the spacing of the compaction curves of sample A with a higher proportion of small granule obtained under the four surrounding rock conditions, is obvious sparse, which is larger than that of samples B and C. It can be reasonably deduced that the surrounding rock condition can produce a greater effect on samples with a higher proportion of small granule.

Figs.2((d)-(f)) show that the lateral pressure curve under the four different surrounding rock conditions can also be divided into three stages, which are  corresponding to the three stages of compaction curves. In the first stage, a short-time fluctuation nearby 0 of the lateral pressure can be observed with the increase of axial stress. This is may be that the axial stress is mainly used to fill the clearance among coal mine waste particles in this stage. In the second stage, the coal mine waste enters into crushed course with the increase of the axial stress, which will lead to the catastrophe and asymmetry of the lateral pressure. Most of lateral pressure curves show the serrate fluctuation in this stage. The figures also show that with the increase of the axial stress there is a rapid rise for the lateral pressure in this period, and the harder the surrounding rock is, the larger the amplitude of lateral pressure is, especially under the hard or extra-hard chamber conditions. This trait also appears in the third stage, but the shape of lateral pressure becomes linear. This is because the large axial stress has already eliminated the characteristics of coal mine waste particles and made them become somewhat a intact elastomer.

3.3 Analysis for final settlement and maximum lateral pressure

The final settlement results and the maximum lateral pressure are listed in Table 2.

The test results show that the final settlement of coal mine waste under the same axial load is closely related to the lumpiness gradations and the deformation behavior of chamber materials that are used to simulate behaviors of different in-situ surrounding rocks. The final settlements (or the axial deformation) of sample A, which are tested under four simulated soft, moderate, hard and extra-hard surrounding rock conditions, are 92.51, 88.00, 86.50 and 77.98 mm, respectively; those of sample B are 94.03, 90.80, 89.60 and 85.24 mm, respectively; and for sample C, they are 94.62, 91.10, 90.75 and 89.73 mm, respectively. A visible rule can be reached that under the same surrounding rock condition, the final settlement due to the same maximum axial load decreases with the decrease of the proportion of larger gradation of coal mine waste, while for the same lumpiness gradation case, the final settlement increases with the decrease of elastic modulus of the simulated surrounding rocks.

In addition, when appropriate similar materials are used to simulate soft, moderate hardness, hard and extra-

Table 2 Final settlements and maximum lateral pressures under different surrounding rock conditions

Note: Elastic modulus of similar material was measured by standard samples artificially made using the same material when moulding the homologous loading chambers; the final settlement and the maximum lateral pressure all corresponded to the peak axail stress of about 20 MPa.hard surrounding rocks, the maximum lateral pressures are 2.70, 3.00, 4.11 and 5.72 MPa respectively for sample A; 5.21, 6.10, 6.91 and 7.52 MPa respectively for sample B; and 2.31, 4.33, 5.57 and 5.97 MPa respectively for sample C. For the same lumpiness gradation case, the maximum lateral pressure induced by the same maximum axial load increases with the increase of elastic modulus of loading chambers used to simulate different surrounding rocks.

4 Conclusions

1) All axial load—settlement (or axial stress—strain) curves obtained under simulating four different surrounding rocks present power-exponential function feature.

2) The final settlement of coal mine waste under the same maximum axial load and the same surrounding rock condition decreases with the decrease of the proportion of larger gradation of coal mine waste, while in the same lumpiness gradation case, it increases with decrease of elastic modulus of the simulated surrounding rock materials for the same final axial load.

3) The maximum lateral pressure induced by axial load increases with the increase of elastic modulus of loading chamber material used to simulate different surrounding rocks.

4) Both the compaction curve and lateral pressure curve show a three-stage behavior, and the duration of each stage decreases with the increase of the proportion of smaller size of coal mine waste and elastic modulus of simulated rock materials.

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(Edited by CHEN Wei-ping)

Foundation item: Project(50490274) supported by the National Natural Science Foundation of China; Project(06JJ4062) supported by the Hunan Provincial Natural Science Foundation, China

Received date: 2007-10-21; Accepted date: 2007-12-12

Corresponding author: MA Chun-de, Doctor candidate; Tel: +86-731-8876593; E-mail: cd.ma@163.com