Technical Note
Three-dimensional analysis of coal barrier pillars in tailgate area adjacent to the fully mechanized top caving mining face

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Introduction

In underground coal mining, an increase in coal recovery rate can be achieved by a decrease in the size of the supporting coal pillars [1]. The design of the coal pillar is important for both the success and the effective operation of coal extraction. Pillar design studies have been carried out by many engineers and researchers, and many strength formulae and design methods have been proposed, and these are reviewed in [2], [3], [4]. Pillar formation in bord and pillar coal mining is required for (1) panel isolation, (2) protection of roadways, shafts and surface features, and (3) protection of mine workings from flooding and roof caves in the face area while depillaring [4]. Different methods are used for the design of the pillar for different situations or functions.

A goaf is an extracted coal panel during coal mining. Here, we consider the pillar along the goaf, which is a chain pillar used for the protection of the roadway and the extraction panel and exists only along one side. Sheorey [4] has suggested three methods for chain pillar analysis and design: (i) choose a pillar strength formula, determine the average load (depending on one-sided or two-sided goaf, caving or stowing) and the width of pillar with a suitable safety factor; (ii) choose the width of pillar such that the gate road beyond is not much affected by the longwall or depillaring face movement; and (iii) perform numerical stress analysis with different chain pillar sizes and various design parameters, and the pillar size is defined by applying a suitable failure criterion to the seam. When the first two methods are selected, Wilson’s formula [5] and Sheorey’s formula [6] can be used, and these are derived from analysis of the abutment pressure across the width of the pillar. When the third method is selected, it is not necessary to choose the strength or load formulae as an estimation of the pillar load and the strength is not required, but it is important to model the realistic constitutive model using suitable caved goaf properties with the adoption of a suitable failure criterion for the in situ seam. Numerical stress analysis of this problem has been done in [7], [8], [9], [10], [11], [12].

Although the use of design formulae is more popular in chain pillar design and analysis, the use of numerical modeling is necessary for some complex and special situations, particularly when the excavation sequence is important for overall stability. Moreover, the use of design formulae relies on the assumption that the abutment pressure is not affected by the roadway position (Fig. 1a). From numerical simulation, Wei and Cheng [13] have shown that the distance of the roadway excavation from the goaf has some influence on the abutment stress distribution in the roof strata (Fig. 1b), which cannot be accounted for by simple design formulae. On the basis of a two-dimensional numerical simulation, Wei and Cheng [13] pointed out that an intermediate pillar width is not good for roadway stability, and this conclusion is similar to that reached by Whittaker and Singh [14], who recommended that a pillar 10–30 m wide is optimal for a gate road, because this range will give the highest gate road closure. Wilson [5] pointed out that a roadway placed in the highly stressed area of the ribside will suffer damage.

Here, the pillar in a top-coal mining panel with an inclined thick coal seam is analyzed by numerical modeling. The 3D distribution of stress and failure zone is analyzed in detail, and the effects of the coal pillar shape are discussed. It is revealed that both the height and the width of the coal pillar are responsible for the stability of the roadway system. Finally, the results obtained from the present study have been used for a real project in Xieqiao with satisfactory outcome.

The present analysis is based on the mining conditions of the 1151(3) mining panel in the Xieqiao colliery, Huainan City, PR China. Mining panel 1151(3) is a top-coal caving mining face, and on the north of this face is a mining panel 1141(3), where the coal has been extracted (Fig. 2). The elevation of the ground surface is from +20.4 to +25.8 m and the elevation of the working face is from –588 to –662 m. The mining face is 1674 m along the strike and 231.8 m along the dip. The average thickness of the coal stratum is 5.4 m and the dip angle is 13°. The main roof is siltstone or fine sandstone with an average thickness of 6.2 m. The immediate roof is mudstone or sandy mudstone with an average thickness of 3.26 m. The immediate floor is mudstone and the thickness is about 1.5 m. The main floor is siltstone ∼2.8 m thick. The properties of the rock, which are based on laboratory tests and are consistent with the design parameters commonly adopted for design in Xieqiao, are given in Table 1. In this analysis, six different coal pillar widths are considered: 3, 5, 7, 10, 15, and 20 m.

Section snippets

Numerical simulation model

Six different 3D models were developed by FLAC3D for the project at Huainan City, and the pillar widths are 3, 5, 7, 10, 15 and 20 m. All the numerical models use a length of 500 m along the strike, a width of 600 m along the dip and a height of 214.17 m (Fig. 3). In these models, there are 87,898–91,036 zones and 103,531–107,134 grid-points. At the top of the models, a vertical load (p=γH) is applied to simulate the overburden weight. The elasto-plastic Mohr–Coulomb model with the non-associated

Distribution of vertical stress along the strike with respect to pillar width

The vertical stress for coal pillars of different widths along the strike is shown in Fig. 5. When the pillar width is 3 m, the vertical stress in the pillar is relatively low and the stress in front of the working face is even lower than that at the back of the working face. For pillars of other widths, the peak stress is in front of the working face. When the pillar width is 5–20 m wide, the stress is relatively low near the working face and increases quickly in front of the working face. The

Distribution of vertical stress along the dip with respect to pillar width

The vertical stress in different cross-sections along the dip with respect to pillar width is shown in Fig. 7. There is less constraint to the 1151(3) coal seam, since the coal seam at the back of the working face has been extracted. For a section less than 11.25 m from the working face, the vertical stress is very low in the coal seam, while it is relatively high in the pillar, since there are more constraints at the back of the coal pillar.

At sections further than 22.25 m away from the working

Vertical stress distribution in the plane of the coal seam

As shown in Fig. 8, there is a peak stress region in the coal seam at a distance of 15–20 m in front of the mining face along the strike, and at a distance of 7–11 m from the edge of the coal rib along the dip. If the width of the coal pillar increases, the distance between the mining face and the center of the peak stress region decreases slowly, while the peak value decreases gradually. With the increase in pillar width, a peak stress region appears gradually in the pillar similar to that in

Distribution of failure zone with respect to pillar width

The distribution of the failure zone in the coal stratum is shown in Fig. 10. When the width of the pillar is less than 15 m, the failure zone runs through the whole pillar. When the width is 20 m, the failure zone does not run through the pillar, and there is an elastic core 5–8 m wide in the center of the pillar. In coal seam 1151(3), when the pillar width is 20 m, there is little failure zone along the roadway. When the width of the pillar is 5–15 m, there is always a failure zone ∼5 m wide in the

Distribution of displacement with respect to pillar width

The contour of the vertical displacement and the displacement vector around the roadway are shown in Fig. 11. When the pillar is 5–20 m wide, the largest vertical displacement around the roadway is located at its roof. When the pillar is 3 m wide, the largest vertical displacement is located at the side of the pillar, and the vertical displacement value is much larger than that for the other pillar widths. From the displacement vector, it can be seen that the horizontal displacement for a pillar 3

Stability analysis of roadway system with respect to pillar width

The stability of the roadway system can be assessed from the distribution of the stress and failure zone as follows. When the width of the pillar is very large, such as 20 m, the vertical stress has a double peak distribution in the pillar. There are plastic zones in both sides of the pillar and there is an elastic zone in the center of the pillar. In this situation, the pillar has sufficient load-bearing capacity to maintain pillar stability. When the width of the pillar is very small, such as 3

Effect of the shape of coal pillar

As early as in 1907, Daniels and Moore [15] discovered that the larger the coal specimen, the less will be its strength. Furthermore, the strength of the sample is found to decrease with the height of the specimen. Bunting [16] called these two phenomena as size effect and shape effect. It has been determined by numerous tests that a coal pillar has a critical size. When the size of the coal specimen exceeds the critical size, the strength does not decrease with further increase in size.

Discussion and conclusions

From the present study, it was found that in front of the working face, there is a range that is seriously affected by top coal mining. In this range, stress increases rapidly to a peak value. In the coal pillar, the peak stress was located at about 5–20 m in front of the working face and in the coal seam the peak stress is at about 16–20 m in front of the working face.

Beyond the seriously affected range, with increase in pillar width, the stress in the coal seam is always decreasing, while the

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