Gas Distribution Characteristics of Coal Seam in Central Hunan and Its Impact on Anping Tunnel and Structural Design Xiao Wengui1 Fu Helin2 Wu Wuxing1 1 Hunan expressway construction and Development Co., Ltd. 410031) 2 Central South University,410075 e-mail:[email protected] ABSTRACT Anping Tunnel is a very long tunnel on Longlang Expressway, which integrates slope accumulation, large deformation of soft rock, karst and gas in goaf. Starting from the mechanism of gas distribution in coal seam, this paper analyzes the gas distribution characteristics and spillover mechanism of coal seam in central Hunan, and then puts forward the reasonable structural design of gas tunnel and the structural measures of drainage and gas. It provides technical support for the construction and maintenance of the coal seam passing through the tunnel area. KEYWORDS: coal seam gas/ distribution characteristics/ tunnel/ influence/structure design INTRODUCTION Located between Longtang Town of Lianyuan City and Langtang Town of Xinhua County, the main line is about 74 kilometers long. It connects the completed Changshao-Lou Expressway and Xinwei Expressway. It is a key project at the provincial level. It is an important part of the fourth horizontal section of Hunan Province's "seven vertical and seven horizontal" Expressway network, and also a horizontal connection line between Erguang National Expressway and Hubei National Expressway.Anping Tunnel is the only super-long tunnel in the whole line, which passes through slope accumulation, large deformation of soft rock, karst, goaf and gas-prone areas. The tunnel is adjacent to Chetianjiang Reservoir. It is the longest tunnel with the most complicated geological and environmental conditions since the construction of Hunan Expressway. In order to ensure the safety of the construction and operation of the tunnel, this paper studies the distribution characteristics of coal seam gas in the tunnel crossing area and the influence and Countermeasures of the tunnel, in order to provide technical support for the construction and maintenance of the tunnel. - 403 - Vol. 24 [2019], Bund. 02 404 GENERAL CONDITIONS OF ANPING TUNNEL In Anping Tunnel, the pile numbers of the left tunnel are ZK13+130~ZK16+735 with a length of 3605 m, and those of the right tunnel are K13+180~K16+750 with a length of 3570 M. Longtang end of the left line of the tunnel is located in the curve section with a radius of 1160 m, Longtang end of the right line is located in the curve section with a radius of 1180 m, Longtang end of the left and right line is located in the straight section, Longtang end of the left and right line is located in the uphill section of 2.72% and Langtang end is located in the downhill section with a radius of -0.5%. The design elevation (504.495m, 505.85m) of the abutment of the tunnel's left and right Langtang end is 10.455m and 11.81m higher than the dam top elevation (494.04m) of Chetianjiang Reservoir, and the distance from the slope change point of the left and right line of the tunnel body to the tunnel mouth of Langtang end is 887.67m and 860.31m, respectively. The slope change point of the left and right tunnel is 15.27m and 16.0m higher than the dam top elevation of Chetianjiang Reservoir. The section of Chetianjiang Reservoir's dam top elevation (494.04m) is 1435m and 1450m long respectively; the design elevation of the tunnel's left and right coal crossing (goaf) position is 507.64m and 509.05m respectively, 13.60m and 15.01m higher than the Chetianjiang Reservoir's dam top elevation (494.04m), 7.676m and 9.086m higher than the main inclined shaft entrance elevation (499.964m) of the new mining area of Changchong Coal Mine; the location of the tunnel crossing coal (goaf) is in the direction. There are 880m and 860m starting points respectively. The design elevation of long tunnel is higher than that of dam top of reservoir. At the end of Langtang tunnel, there are enough water barriers to prevent Chetianjiang Reservoir from backfilling into the tunnel, which meets the risk control requirements of preliminary design risk assessment for preventing Chetianjiang Reservoir from backfilling into the tunnel. The net distance between the left and right lines at Longtang end and Langtang end is 25.3m and 20.1m respectively, which is a non-small net distance tunnel. The plane Brown section ofAnping Tunnel is shown in Fig. 1 and Fig.2. Vol. 24 [2019], Bund. 02 405 Figure 1: Plane of Anping Tunnel Figure 2: Longitudinal Section of Anping Tunnel DISTRIBUTION CHARACTERISTICS OF COAL SEAM GAS Geological control factors of coal seam gas distribution Anping tunnel is located near the new mining area of Changchong coal mine and doulishan coal mine. The variation of gas content in coal seam is affected by many factors, among which the most important are coal grade, coal seam depth, tectonic movement, surrounding rock and hydrogeological conditions. For example, the gas-rich areas are Lian-shao coal-bearing area and Lin-lei coal area, which are located in Changshao fault depression and Yulan fault depression in caledonian fold belt of south China respectively. The stress concentration in the subsidence area is beneficial to the preservation of gas and is one of the reasons for the outburst. The control of gas concentration by structure can be divided into open structure, closed structure and semi-open structure. The control of gas concentration is different with different structures and combinations. On the other hand, the Vol. 24 [2019], Bund. 02 406 control of gas concentration by structure is caused by the change of coal thickness and its physical characteristics caused by structure, thus affecting the gas content. The factors controlling gas concentration by several tectonic types are as follows: 1) generally speaking, gas concentration is low in areas with complex structures, while high in areas with simple structures. The structure of the east wing of Baisha syncline is more complex than that of the west wing, and the structure of the north segment is more complex than that of the south segment. In particular, the gas concentration is higher in the Tanjiachong and Wangmiao Wells in the south section of the west wing (figure 3). located in the main anticline axis of tight fold, the gas concentration is small. In the main anticline axis of the tight fold, gas tends to escape from these tensile faults and fractures due to the development of tensile fractures, so the gas concentration is generally small. Such as coal mine Huanggang well in the zone is located near the dip end of the Huangtuling anticline, where longitudinal and tensile fractures are developed, and radial tensile faults are developed at the dip end. The subsidence zone in the coal seam is a low gas well, while the adjacent Huping and Xianghuatai Wells are both high gas Wells. 3) located near the anticline axis of gentle fold or the secondary fold anticline axis, gas outburst is easy to occur due to the increase of coal seam thickness and gas concentration. For example, Meitian mining area is located near the anticline axis, including the inverted fold crankshaft. Due to the structure, the coal becomes soft, the pore surface area increases, the permeability decreases, the gas is easy to gather, and gas outburst occurs. 4) The gas concentration at the steeply inclined part of the fold is lower than that at the gently inclined part. The west wing of the Lengshuijiang syncline has a steep stratum dip Angle, partial vertical inversion, and relatively small gas content, such as the Basang and the great leap well field; In addition, the strata overlying the coal seam have poor gas permeability, so the gas content is relatively high. For example, most of the gas concentration in the area south of the line 50 of Zhonglian mine field is low. Local abrupt steepening of steep strata makes gas outburst easy to occur. For example, the Liyutang formation in Hongshandian mining area has a relatively gentle occurrence, and the four prominent points all occur at the place where the occurrence suddenly becomes steeper. Gas outburst is also likely to occur in some areas with steep occurrence, such as aihe mountain well field, where occurrence is suddenly slowed down. 5) Tensile fracture development reduces gas concentration. The Baohetang mining area is located in the east wing of syncline with several large tensile faults, so the gas concentration in the east wing is lower than that in the west wing. Vol. 24 [2019], Bund. 02 407 6) compression-torsional fracture can make the gas relatively concentrated, which is located near the compression-torsion fracture and prone to gas outburst. 7) tectonic coal provides a good place for the occurrence of gas. The coal body is subject to the action of tectonic stress, and the original structure is damaged to varying degrees. Relationship between gas concentration and coal grade The results of thermal simulation show that the production of gas in coal seam is related to coal grade, from long flame coal to anthracite coal, with the increase of coal metamorphism, the production of gas increases. Isothermal adsorption experiments on coal samples show that the adsorption capacity of coal increases from long-flame coal to anthracite no. 2. Therefore, under normal circumstances, the gas content of in-situ coal seam increases with the increase of coal metamorphism. The metamorphism degree of coal seam in Jurassic and Triassic is generally low, with gas coal, fat coal and coking coal as the main ones. The gas content of coal seam is generally low, generally less than 5m3/t.