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.

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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.

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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. But partly due to the influence of magmatic rocks, coal metamorphism degree increased, up to poor coal, anthracite, coal seam gas content increased. The Permian longtan formation coal is mainly of medium to high metamorphism, and the coal is coking coal ~ anthracite. The coal grades of this group of coal seams are zonal distribution, and the gas content of coal seams is also zonal distribution. When the coal is ultra high metamorphic anthracite no. 1, the gas content in the coal seam is very low. The carboniferous survey water group coal is mainly of high metamorphism and belongs to high gas generation stage, so the gas content is generally very high. Vol. 24 [2019], Bund. 02 408

Figure 3: Contour map of gas content in coal seam 6 of the Permian longtan formation of baisha coal-bearing syncline Vol. 24 [2019], Bund. 02 409

Relationship between gas concentration and buried depth of

coal seam

The gas bodies in the exposed surface coal seam are easy to escape, and the air gas permeates into the coal seam, so the coal seam contains gas bodies such as CO2 and N2, and the gas concentration is low. With the increase of burial depth of coal seam, the proportion of gas increased. The data of each evaluation area show that the gas concentration, gas emission and gas pressure are proportional to the increase of depth below the gas weathering zone.

3. 4 relationship between gas concentration and magmatic rocks

The influence of igneous rocks on coal seam gas is obvious, which is mainly manifested in the increase of coal metamorphism degree due to magmatic heat in the evaluation area, and the enhancement of gas generation ability of coal seam (especially the coal seam in longtan formation). Therefore, the gas concentration in the whole meitian mining area is higher than that in other longtan formation anthracite evaluation area, which is mainly related to the qitianling granite mass in the north. In addition, as the coal-bearing strata are directly subjected to the baking and squeezing of rock masses, they are often not far away from the contact zone. The metamorphism of coal is very high until graphitization, the porosity decreases, and the structure becomes more complex. The coal seam has a low ability to absorb coal seam gas, and the gas concentration is generally low, as shown in figure 4.

Figure 4: relationship between the gas content of A coal seam and the buried depth of A coal seam in A mining area (according to hunan coal research institute, 1985)(A is the gas geological section of exploration line 44, and B is the gas geological section of exploration line 59) Vol. 24 [2019], Bund. 02 410

Composition, content and zoning of coal seam gas Composition of coal seam gas The natural composition of coal seam gas is mainly gas, usually more than 80%, the concentration of gas in most mining areas is usually more than 90%. The first well of xiangmei coal in hongshandian mining area is as high as 97.68%. Secondly, there are more nitrogen gas and carbon dioxide, but less heavy hydrocarbon components. The heavy hydrocarbon component of coal seam gas is mainly ethane, followed by propane. The hydrocarbon with more than 3 carbons is rare, as shown in figure 3. Coal seam gas zoning Weathering zone refers to the natural gas belt with different properties formed by air gas percolation and biochemical action in the near-surface hydrological activity zone of coal seam gas. According to the proportion of CH4, CO2 and N2, the coal seam gas belt can be divided into four zones (figure 5). In the weathering zone, with the increase of depth, the content of CH4 changes between 0% and 80%, which are all worthless coal seam gas. Therefore, the depth of weathering zone directly affects the economic cost of coal seam gas exploration, as shown in figure 6.

Figure 5: schematic diagram of weathering belt of coal seam containing gas

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Figure 6: characteristics of coal seam gas zonation in the evaluation area

Coal seam gas content 1) coal seam gas content

Coal seam gas content refers to the total amount of gas body contained in the unit weight of underground coal seam under natural conditions. Coal seam gas has three kinds of occurrence states: adsorption state, free state, dissolved state. Generally, adsorption state accounts for more than 80% of the total amount of coal seam gas. Coal seam gas content is the basis of calculating coal seam gas resource quantity and exploitation and utilization, and it is one of the key parameters to predict coal seam gas well productivity. Generally speaking, high gas content in coal seam leads to good gas accumulation.

2) mine gas emission

The amount of gas gushing out of the mine is the actual amount of gas gushing out in the production process of the mine. Although the amount of gas gushing out of the mine is affected by the mining intensity, mining technical conditions and other factors, it is closely related to the content of coal seam gas and generally in a positive ratio. The data can be used to illustrate the variation trend of gas content in coal seam from another perspective.

3) Distribution characteristics of coal seam gas content

The above - mentioned coal seam gas - rich area is closely related to the ancient sedimentary environment. The palaeogeographic environment shows that the sedimentary center and direction of coal measures are similar to those of the high concentration areas. It should be pointed out that there are also low enrichment zones in the high enrichment zones, such as xiangyong mining area in the high enrichment zone of southeast hunan of longtan formation, and chaoyang mining area in the high enrichment zone of central hunan of zhenshui formation. This is closely related to the influence and control of surrounding rock, geological structure and structure of coal seam.

4) Variation law of coal seam gas content Vol. 24 [2019], Bund. 02 412

Based on the analysis of gas emission and outburst in the mine, the coal seam gas in the evaluation area can be divided into three highly enriched areas, namely, the south east area of longtan formation, the central area of longtan formation and the central area of water survey formation.Design of lining structure of gas section in 4 holes.

LINING STRUCTURE DESIGN OF GAS SECTION OF THE

TUNNEL BODY

Lining structure design of gas section of the tunnel body

According to the specifications, the lining support parameters of different grades of gas sections are summarized in table 1.

Table 1: lining support parameters of gas section

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Considering the gas pressure, the lining structure of the tunnel should be strengthened. In the initial stage, C25 wamei shotcrete is used for supporting shotcrete. Wamei agent is added into the shotcrete according to the cement dosage of 7%, and the gas permeability coefficient of the shotcrete is less than or equal to 10-10cm/s. C40 wasimi concrete was used for the second lining. Wasimi agent was added according to the cement content of 7%, and the gas permeability coefficient of concrete was less than or equal to 10-11cm/s. Non-gas work area according to the general section lining support; Low gas area, high gas area and gas outburst area should be supported by lining of gas area, and extend lining of gas area to non-gas area for 15m according to geological conditions. The lining support type, location and length of gas section should be designed dynamically according to the test results.

Design of waterproof and gas prevention in tunnel 1) Design standards (1) the structure of self-waterproof and gas tunnel USES compound lining, grouting water shutoff and small lead pipe as the first line of defense to reinforce the stratum, and shotcrete and anchor support as the second line of waterproof. The waterproof concrete with waterproof grade P6 is adopted for the secondary lining concrete in the areas with insufficient groundwater, and waterproof concrete with waterproof grade P8 is adopted as the last line of defense in the areas with abundant groundwater and high groundwater pressure.

Considering the gas pressure in the gas section, the lining structure of the tunnel should be strengthened. In order to isolate the gas and prevent the gas from escaping into the tunnel in the process of operation, the lining of the gas section should be completely closed. In the initial stage, C25 wamei shotcrete is used for supporting shotcrete. Wamei agent is added into the shotcrete according to the cement dosage of 7%, and the gas permeability coefficient of the shotcrete is less than or equal to 10-10cm/s. C40 wasimi concrete was used for the second lining. Wasimi agent was added according to the cement content of 7%, and the gas permeability coefficient of concrete was less than or equal to 10-11cm/s. (2) Add waterproof layer and gas externally 350g/m2+1.2mm EVA waterproof plate is laid between the secondary lining and the initial support in the area where groundwater is not abundant. In the high gas area and underground water rich area of Anping tunnel, 1.5mm thick double-sided self-adhesive film waterproof coils are laid in the whole section between the secondary lining and the initial support. Vol. 24 [2019], Bund. 02 414

(3) Settlement joints, construction joints waterproof, gas At the construction joints, the waterproof measures are as follows: the outer side of the secondary lining is provided with back stick type water stop belt, and the middle part is provided with slow-expansion rubber water stop strip. At the settlement joint, a back-mounted water-stop belt is set on the outer side of the secondary lining, and a buried rubber water-stop belt is set in the middle. The middle part is filled with asphalt wood wire board, and the inner side of the secondary lining is filled with polysulfide two-component sealant as a waterproof measure.

For the high water pressure section of Anping tunnel, the flexible polymer modified mortar coating 4841, which can resist 80m water pressure, 80cm width, 2mm thickness and tiny deformation, is set along the construction joints and settlement joints on the inner surface of the secondary lining construction joints and settlement joints as the last waterproof measure. 2) Design of drainage and gas in tunnel

(1) Drainage system behind lining

Immediately after the tunnel excavation section and comply with the design requirements, jet concrete closed rock fissure water thickness is 4 cm, along with the deformation of surrounding rock, the first layer of sprayed concrete surface cracks will leak, in set with strong leakage Ω type spring drain. Will Ω type spring drain outlet openings on crack, fissure water directly into the drainage tube; Fixed a drain immediately after injection of 2 ~ 3 cm coagulation pulp package is fixed, and then sprayed concrete are closed on the second floor, if there is leakage phenomenon, drilling water diversion in the leakage place, add Ω type spring drain.

The circular drainage behind the lining of the tunnel adopts rectangular plastic blind ditch without middle hole. Except for the open tunnel section, the rest sections of the tunnel are set at the longitudinal spacing of 5~15m. According to the specific situation of the construction of the appropriate adjustment, after the completion of the initial branch by the seepage section appropriate encryption. In the bottom of the side wall, along both sides of the tunnel set a corrugated pipe with a diameter of 110 longitudinal pvg-u perforated double wall, the whole tunnel through, forming the tunnel lining behind the longitudinal drainage system;

Ω type spring drain pipe and rectangular holes in the plastic blind ditch to drain the bottom ring and dirt on the longitudinal drain through the fine hemp rope fixed position. According to the distribution of gas segment, water-rich goaf segment and water-rich karst segment inAnping tunnel, the tunnel is divided into three independent drainage (gas) systems.

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(2) Concrete pile number drainage and gas design

(1) tunnel entrance to K15+980 section

A rectangular side ditch of 50 35cm is set below the curb belts on both sides of the road to serve as a clear drainage ditch for the whole tunnel, and to guide and discharge the pollution-free underground water behind the lining.

At the foot of the side wall, a 110pv-u double wall corrugated pipe is set every 10m as a horizontal drain pipe, and a 110-way three-way pipe is connected with a 110-way vertical pvc-u perforated double wall corrugated pipe, to divert the retained groundwater behind the tunnel lining to the rectangular side ditch below the road surface to form a tunnel lateral drainage system.

A rectangular center drain is set below the middle line of the carriageway. The corrugated pipe with 200pvc-u double-wall is connected with the side ditch sand sinking well every 20m, and the precipitated water in the side ditch is diverted to the center ditch.

(2) The K15 + 980 to K16 + 285 section

Affected by Chetianjiang syncline and the mudstone water isolation performance in this section, there are three layers of groundwater in this section.

The water-rich section on the contact surface between the top mudstone and the upper limestone: this section is the water-rich karst section formed mainly in K16+240~K16+285 sections after karst water in the upper limestone infiltrates through cracks and vertical karst, and deposits on the surface of the karst undeveloped mudstone. This section is the goaf fracture zone, which is perpendicular to the rock face. The gas may escape along the fracture and enter the tunnel through the transverse drainage pipe.

The middle water-rich goaf: at present, 1.8-5.4 Wells gush out of the main inclined shaft (elevation: 499.96m) of changchong coal mine every day

The elevation of the tunnel passing through the coal (goaf) section is 507.88m, although it is 7.92m higher than the main inclined shaft mouth.

However, considering the influence of yong water in the mine roadway and the water source in the roadway, the underground water may flow out of the tunnel after passing through the coal. At the same time, this section is the coal passing section and the goaf collapse section, and there is gas and gas body escaping into the tunnel through the tunnel drainage pipe.

Water-rich karst section under mudstone and breeze shale: due to the existence of impermeable mudstone and shale, and under the influence of chetianjiang syncline, groundwater is discharged Vol. 24 [2019], Bund. 02 416 along the plane and karst to the interface between limestone, mudstone and shale below, and then deposited at the interface, forming water-rich karst section. According to the geophysical data, there is an underground pipe flow (dark river) intersecting with the tunnel in this section.

In this section, the vertical slope rate is 0.5%, and there is a vertical curve effect. Through analysis, set in this section of the double-hole center ditch, and at the edge of the wall every 10 meters to set a phi 110pvc-u double wall corrugated pipe as a horizontal drainage pipe, through the phi 110 tee pipe and the phi 110 longitudinal pvc-u perforated double wall corrugated pipe connected, the tunnel lining behind the retention of groundwater drainage into the double center ditch, forming a tunnel lateral drainage system.

As the gas and gas body may enter the central ditch through the drainage system, so the double central ditch as this section of the special drainage system, directly discharged to the langtang end hole, in the mouth of the water and gas separation device, underground water discharge to the roadbed side ditch, gas through the gas discharge pipe in the hole to the high air. (3) K16+285 to langtang end hole section After the groundwater in this section is discharged into the side ditch through the lateral drainage pipe, it is directly discharged out of the hole. The central ditch serves as a special discharge channel from K15+980 to K16+285, and does not participate in the drainage in this section.

When the tunnel intersects with the changshui subsurface river, the proper inverted siphon pipe diameter should be selected according to the actual situation of the site to ensure unobstructed drainage. Avoid altering groundwater discharge channels.

ACKNOWLEDGEMENT

This paper are sponsored by Project supported by Hunan transportation science and technology project(210715) here thank them.

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Editor’s note. This paper may be referred to, in other articles, as: Xiao Wengui, Fu Helin, and Wu Wuxing: “Gas Distribution Characteristics of Coal Seam in Central Hunan and Its Impact on Anping Tunnel and Structural Design” Electronic Journal of Geotechnical Engineering, 2019 (24.02), pp 403-418. Available at ejge.com.