Research on Special Support Technology of Gob-side Entry Retaining with Filling the Non-roadway Side in the Thin Coal Seam

By taking a certain mining area of No.2 coal seam in a coal mine as the research object, special support technology of gob-side entry retaining with filling the non-roadway side was researched. Based on the geologic features of this mining area, support method of gob-side entry retaining was proposed. The method replaced the dense wood prop with single prop, and cooperated intersection hinge roof. The support parameters were calculated, and time-space coordination between each support process was researched. The field measured data showed that this support method had a remarkable effect and strictly controlled the deformation of gob-side entry. This research formed a support system and mining system based on this condition. It has important reference meaning to safe and efficient mining coal resource with similar geologic condition.


Introduction
The retention of coal columns will cause a large loss of coal resources, and the technology along the empty lanes has eliminated the protection of coal columns along the section, to a large extent, improve the recovery rate of resources [1][2][3][4]. Since the roadway along the goaf has to withstand the influence of secondary mining, the key to its success is the secondary support method and means [5][6][7][8]. China's technology along the empty lane developed in the 1950s, the main means of roadside support are stone stacking, wood-intensive pillars, and so on, and limited to the application of thin coal seams. By the 1970s, concrete blocks and dense pillars were used and applied to medium-thick coal seams. However, these lane side support has the disadvantages of insufficient support resistance, non-matchable performance, low degree of mechanization, labor intensity and poor air-sealing performance, which limit the use of the lane along the empty lane. The lane less filling technology is more used in thin coal seam mining because of its simple operation and low labor intensity.
In this paper, the application of the technology of filling along empty lanes without lanes is studied in a mining area of a mine 2and coal seams.

2．Mine overview
The main coal seams of the mine are 2coal and 2 lower coal. The 2 coal roof is a composite roof, the direct roof is gray massive siltstone with a thickness of 1.5m, the main roof is fine sandstone with a thickness of 10.2m, and the direct bottom is gray black siltstone with carbonaceous matter； the 2 lower 2 coal pseudo roof is black soft carbonaceous mudstone with a thickness of 0.1M and unstable distribution. The direct roof is gray black siltstone with a thickness of 2.4m, the main roof is gray black medium fine sandstone with a thickness of 5.8m, and the direct bottom is gray black siltstone with a thickness of 4.6m.
The mining area is a single-winged mining area, divided into four sections. 2 coal and 2 lower Coal seam Top floor slate conditions are better, the top plate is a composite top plate, the direct top is gray block powder sandstone, the direct bottom is gray-black powder sandstone, containing carbon; the top floor is stable and stable, and the work surface is easy to maintain. In addition, hydrogeological aspects, the main water-filled factors, although 2 coal seam top sandstone aquifer, adjacent mining area west wing empty area, the surface has river currents, but because of the middle of the mud rock, sandstone and other water layers, therefore, has no impact on mining. The gas content of coal seam is low, there is no tendency of spontaneous combustion, and coal dust is not explosive.
As can be seen from the above analysis, the mining area section is small, such as according to the normal situation of the jumping arrangement will exist to the following problems [9-10]： (1) Affected by adjacent mining, excavation time is limited, the working surface is difficult to achieve normal replacement.
(2) Small coal columns need to be retained and the excavation workload is large. In order to facilitate the maintenance of the back-mining laneway, the normal recovery needs to leave small coal columns to protect the roadway, which is also the traditional method of retaining coal columns. But this will greatly increase the amount of digging, but also reduce the recovery rate of coal.
(3) Affected by water accumulation in the upper mining area, the construction conditions are complex.
In view of the above characteristics, the mining area mining by way of no lane side filling along the empty lane.

One support in the alley (1) Support form
One support of the laneway is supported by an anchor net. Select the appropriate anchor support parameters according to the actual geological conditions. According to the "Coal Lane Anchor Support Design Code" of Yantai Mining Bureau, the top anchor rod is made of high-strength left-hand threaded steel body with a high strength of 22, and the full-length anchoring of the resin dosage is chosen; The length of the anchor rod is taken 2000mmand the spacing is700mm×700m.
(2) Numerical calculation analysis In order to further study the deformation law along the empty roadway and provide the basis for the design of its support scheme, a numerical simulation analysis of the support is carried out, and the results are shown in Figures 1to 2.
From the numerical simulation results can be seen, after the roof support and the support of the alley gang, the amount of the top plate sink decreases, the amount of deformation of the alley gang is also reduced, fully explains that the anchor bar support, is to meet the requirements of a support. (a) X-direction displacement before support (b) Displacement in X direction after support Figure 1 Comparison of X-direction displacement before and after support (a) Displacement in Z direction before support (b) Z direction displacement after support Figure 2 Comparison of Z-direction displacement before and after support

Secondary support in the alley
The secondary support method depends on the specific geological characteristics of the coal seam. Since the coal seam in the mining area is thin coal seam, and the top floor slate strength of the coal seam is large, easy to support, therefore, can be used without lane side filling along the empty lane way, specifically: the front work surface to make up the anchor rope to strengthen the support, while the monolith pillar with the cross articulated top beam, and hanging metal mesh way to control the flow of zircon into the roadway and promote its natural formation " roadside zircon pallets " support.

Anchor support parameter design (1) Anchor length
The anchor rope should be anchored in a stable rock formation deep in the top plate. According to the comprehensive column chart of the work surface formation, the direct top is thick sandstone and fine 1.2m sandstone, which are more developed in layer theology, and the upper layer is a 0.2m thick coal line. The old top is sandstone, black and gray-black coal-clamping line, the layer is more developed.8.6m. According to the comprehensive layer level assignment condition and the ability to drill holes, determine the depth of the anchor rope 8.0mhole, anchor length: L. 8.3m.
(2) The spacing between the anchor ropes Since L hole /S≥2, S≤L hole /2 = 4.0m, then according to the roadway section size, the spacing between S = 2.1m; According to the bolt support, take row spacing S = 4.0m.
(3) Anchoring agent selection The anchoring agent selects the resin coil, according to the mine bureau "coal Roadway Bolt Support technical Code" provisions: the resin cement anchoring length is not less than 1.5m, according to the mine commonly used resin coil selection, S2370, Z2360 and Z2335 each one, the anchoring length is：

Single pillar support parameters
Single with metal articulated top beam (shun lane arrangement) to strengthen the roof support, down the alley to play three rows, one beam two columns, column distance. One of the first row, the second row playing in the upper gang, respectively, from the middle line of the laneway, the third row hit in the middle line of the laneway below the side. Single pillar wear shoes, to ensure that the initial support of the single body is not less than 90600mm1.7m1.2m0.2mKN, and to meet the mountain strong. A cushioned shed between the top plate and the top beam, not less than two pieces per beam.

Steel frame shed support
In the local surrounding rock broken section using beams, the upper and lower helpers are, column legs, shed distance of 2.8m2.6m2.2m1.0m, and according to the timely adjustment of the situation, the column legs and horizontal surface of 75 degrees inclination to the alley tilt. If the top plate inclination changes significantly, the length of the legs can be adjusted between 2.2, 2.4 and 2.6m specifications as needed, but the net height in the lane should not be less than 2.0m.

Point Pillar Support
In the roadway section with complete roof, point pillar support is adopted. The wooden point pillar is supported in the first row of mono on the upper side. The selection of φ≥180mm high quality Korean pine (or I-beam steel), the spacing is medium to medium 500mm.

Metal mesh
In order to prevent the mining area zircon into the alley, in the first row of single-body empty area side, hanging 1.2 x long metal long latitude network guard, mesh between the joints, and with 142.2m0.1m lead wire every double wire twisting solid. 0.2m.The concrete support form of the roadway along the goaf is shown in Figure 3.  It can be seen from the figure that the closer the roadway is to the coal wall, the greater the deformation, and the deformation will gradually decrease to a certain fixed value with the gradual advancement of the working face. The deformation of the two sides of the roadway gradually increases when the distance from the coal wall is about 0~145m, and becomes stable when it exceeds 145m, and the deformation speed of the two sides gradually decreases to zero after this stage; the maximum amount of the bottom heave of the roadway is 600mm, The distance from the coal wall exceeds about 120m and stabilizes at a normal value. The deformation of the roof and floor of the roadway shows a gradual increase before the distance of 120m from the coal wall, and basically remains unchanged after this distance, and the amount of roof subsidence tends to be stable after the distance from the coal wall exceeds 130m. The support effect is good.
In addition, by testing the air leakage and gas content of the tunnel, the mine is a low gas mine, with an absolute gas emission of 3.34m³/min and a relative gas emission of 2.33m³/t. According to the appraisal in 2010, the 2# coal mined is Class III and is not easy to ignite spontaneously, and the coal dust is not explosive. Through the detection of the air leakage rate of the reserved lane at the working face every ten days, the actual measurement of the air leakage rate and the length of the reserved lane shows that the air leakage rate per square meter area in the reserved lane is less than 0.02m³/min. Therefore, spraying is not required to prevent air leakage. measure.

Conclusion
This dissertation studies the specific application of the technology of retaining lanes along the goaf without filling, and the following conclusions can be drawn: (1) Under the condition of high strength of the roof and floor of the thin coal seam, a good support effect can be achieved by retaining the roadway along the goaf through special support methods such as single props with hinged roof beams and supplementary anchor cables; (2) It can be promoted and used under similar geological conditions.