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Abstract According to the influence of climatic conditions on vegetable production in the Yangtze River Basin， the existing arched tunnel was improved through investigation to provide guidance for the construction of arched steel tunnels in this area.
Key words Earth-inserted tunnel； All-steel structure； Construction technology
The common plastic tunnel is the most important cultivation facility in China， with an area of more than 560 000 hm2[1]. With the advantages of low investment， low construction cost， flexible construction， strong resistance， long service life， short production cycle， high degree of industrialized production and high economic benefits， the plastic tunnel has become one of the main production methods for the growth of agricultural efficiency and increase of farmers income， achieving remarkable economic and social benefits[2-4]. At present， the steel structure of plastic tunnel has gradually replaced the common plastic tunnels of bamboo structure and cement structure[5-7].
The most common plastic tunnel in the Yangtze River Basin is the earth-inserted tunnel， which is made of galvanized steel tube as the arching， and adopts the all-steel structure plastic tunnel with shoulder-type arch structure. Without concrete foundations， earth-inserted tunnels have the advantages of fast construction speed， easy assembly and disassembly apart from the advantages of common plastic tunnels. In this study， improvement was made to the original structure of earth-inserted tunnel to make it suitable for the production of vegetables in the tunnel， thereby promoting the development of protected vegetables in the Yangtze River Basin.
Overall Requirements for Earth-inserted Tunnel Construction
The earth-inserted tunnels should be built in the plots exposed to the sun on the lee side， which has flat terrain， convenient irrigation and drainage， fertile soil， low groundwater level， no pollution and shadings. Its safety and stability are mainly determined by the bearing capacity of the structural load-bearing components， which is expressed by the load， including permanent load （dead load）， crop load， wind load and snow load. The basic load requirements of structural design are： wind load > 0.25 kN/m2； snow load > 0.3 kN/m2； dead load > 0.55 kN/m2； crop load > 0.15 kN/m2.
According to the Technical Code for Horticultural Plastic Tunnel Engineering （GB/T 51057-2015）[8]， the service life of the main steel structure of the tunnel should be 10 years. The normal service life of plastic film， shade net， insect proof net and film line should not be less than 1 planting season， and the normal service life for card slot and circlip should be less than 5 years. The span should not be greater than 20 m， ridge height should not be less than 2.4 m， shoulder height of shouldered greenhouses should not be less than 1.6 m， and length should not be less than 30 m but not more than 100 m. The tunnel orientation （ridge direction） should be arranged from the south to the north， and the spacing of tunnels should be 1-2 m in the span direction and not less than 3 m in the length direction. Structure Parameters for Earth-inserted Tunnel
Length
In general， the length of a tunnel is determined according to the site. However， the shorter the length is， the higher the construction cost per unit area is. The longer the length is， the harder it is to adjust the environmental factors such as temperature and humidity. Thus， the length should be about 60 m （Fig. 1）.
Span
In central China， the arching is usually connected by 2 standard hot-dip galvanized steel piles of 7 m long using vault steel fasteners， so the span is usually 8 m.
Height
In order to make the tunnel be proper for growing seedlings and for the growth of hanging crops such as melons， the earth-inserted tunnel is designed into the tunnel with shoulders. The investigation on the structure of tunnels in central China found that when the height of the tunnel was about 3 m， the inside temperature could reach up to 50 ℃ during the productive phase of melon vegetables in early summer， which severely restricted the vegetable growth and agricultural operation in tunnel. Therefore， the shoulder of the earth-inserted tunnel is 2 m high with the ridge height of 4.5-5.0 mm.
Arching spacing
The designed earth-inserted tunnel had main and sub-arches. Since the bending strength of circular tube is not as good as that of square tube， the horizontal distance between main arches is 3 m. The sub-arch is the hot-dip galvanized circular steel tube， and there are 2 sub-arches between the main arches with the spacing of 1 m.
Earth-inserted Tunnel Construction Materials
Arching material
The costs for arching materials account for about 50% of the total costs of the tunnel. All the materials are Q235A galvanized materials with the adhesion amount of zinc in the steel tube of no less than 570 g/m2， coating thickness of no less than 80 μm in line with the relevant provisions in Metallic Coating-hot Dipped Aluminum Coatings on Ferrous Articles-Specification （GB/ 18592-2001）[9].
The arch bars of the main arching are galvanized square steel tubes with the specifications of 35 mm × 35 mm × 2.0 mm， which are in accordance with the relevant provisions in Cold Formed Steel Hollow Sections for General Structure -Dimensions， Shapes， Weight and Permissible Deviations （GB/T 6728-2002）[10]. There are a total of three longitudinal ties and 4 diagonal braces for the earth-inserted tunnel. There is one longitudinal tie in the ridge， and 2 on the shoulders on both sides of the tunnel. The diagonal braces are set on the top ceiling of the tunnel. The materials for the shed shape braces， sub-arch bars， door headpins of the main arching as well as the longitudinal rods and diagonal braces are all the hot-dip galvanized circular tubes with the specifications of φ32 mm × 1.8 mm. Coating materials
Tunnel film should meet the lighting requirements for planting crops， so the light transmittance should not be less than 85%. According to the relevant provisions in Ethylene-vinyl Acetate Copolymer （EVA） Blown Covering Film for Agriculture （GB/T 20202-2006）[11]， it can use the EVA film with the thickness of over 0.08 mm， or use polyethylene （PE） or polyvinyl chloride （PVC） film.
The insect proof net is installed at the top of the vents and below the top skylight on both sides of the tunnel， and the selected insect proof net is the 32-mesh nylon insect proof net of 2 m wide. The quality of the insect proof net should be in line with the relevant provisions in the current national standard of the Criterion Insect Proof Net Design and Its Installation for Greenhouse[12].
Fitting materials
The fitting materials of the tunnel mainly includes steel clamp （φ3.4 mm hot-dip galvanized carbon steel spring）， U-shaped clamp （U-shaped screw of φ6mm hot-dip galvanized rebar， plate of the 2 mm thick hot dip galvanized steel）， fixed clamp hoop （0.7 mm thick， hot dip galvanized）， card slot （0.7 mm thick with high zinc layer）， card slot connector （0.8 mm thick）， slot holder （1.5 mm thick， hot-dip galvanized）， criclips （φ2.5 mm thick， hot-dip galvanized）， clamps （plastic card with steel wire）， arching connecting bends and T-shaped connecting fastenings， tube head sheath， screws， film reeler and so on. The selection of fitting materials must meet the strength requirements， fasteners and other accessories should comply with the Code for Acceptance of Construction Quality of Steel Ttructures （GB 50205-2001）[13]， hot-dip galvanized steel sheet material quality should be consistent with the Continuously Hot-dip Zinc-coated Steel Sheet and Strip （GB/T 2518-2004）[14]， and the circlip materials should be consistent with the Measuring Method of Magnetic Properties at Alternative Current for Amorphous and Nanocrystalline Soft Magnetic Alloys （GB/T 18983-2003）[15].
The materials for a new arched all-steel earth-inserted tunnel are as shown in Table 1 for the one the length of 60 m， span of 8 m， ridge height of 4.5-5 m， shoulder height of 2 m， arching spacing of 1 m.
Construction and Installation of Earth-inserted Tunnel
Arching installation
Before the installation of the arching， the ground foundation construction and the arching processing should be carried out. The ground should be rammed and leveled， and marked according to the length and span of the tunnel. Holes are drilled using the steel chisel or electric drill with the diameter of 32 mm according to the arching spacing， and the hole should be no less than 0.5 m deep. The hole has an oblique degree of 5 °， so that there is a 85° tilt angle between the arching and the ground. The spacing for holes is 1 m to ensure that the depth of the earth-inserted tunnel goes at least 0.5 m into the soil. In order to ensure the same earth-inserted depth of the arching， the depth of the earth-inserted arching on both sides should be marked. The sub-arch is connected by 2 standard hot-dip galvanized steel tubes of 7 m long with a certain arc， but the purchased steel tubes are usually straight ones. Therefore， it usually requires on-site processing to curve the tubes into the needed arching using molds according to the shoulder height and span. When insert the curved arching into the holes， marking line is flush with the holes， and then two horizontally-symmetrical arc-shaped arches are connected through connecting bends and fixed with screws.
Tie rod and bracing installation
The tie rod is installed under the arching. Before installation， holes are punched at 5-8 cm at the end of the tie rods and then the ending parts are hammered flat. The tie rods under the ceilings are installed first， and then the rods on both sides of the shoulders. When installing the tie rods， steel clamps are used to lock the rods and arching to make the rods go perpendicular to the arching. When connecting the tie rods， the flattened part is first inserted to the other end of another rod， and then tightened by using screw at the hole. Both ends of the rod is packaged using tube head sheath to prevent puncturing the film. After the completion of each set of tie rod connection， it should be verified， amended and fixed to ensure that the formed surface by the rods is perpendicular to the horizontal plane and arch direction in the same direction with the length of the tunnel.
Bracing is used to connect the arches at both ends of the tunnel to prevent the arches from being tilted to one side by external forces. The diagonal bracings are installed inside the arching and are connected with the seven arch frames close to the tunnel heads through the U-shaped clamps. The lower end of the diagonal bracing is inserted into soil at the same straight line as the arch frame and the upper end is connected with the arch frame upright through fixed clips.
Tunnel heads installation
There are 4 uprights installed at each end of the earth-inserted tunnel with the depth of no less than 0.5 m into soil. The upper end is connected to the arching of the tunnel head through fixed clips.
Both ends of the tunnel head are provided with push-pull sliding doors with double doorways with the door opening of 2 m high and 2 m wide. The doorway and door opening are overlapped for 0.1 m. The door is provided with a clamping slot and the film is fixed on the door by a circlip. Above the door， the bearing rail is installed with bearings， and anti-swing device is below.
Tunnel film installation
Before coating the tunnel， card slots are needed to install outside the arch frames， and 2 card slots are installed at each side of the tunnel at 0.3 m and 1.8 m to the ground. Skylights are needed for ventilation on the top， and 2 card slots are installed at 0.5 m and 1.2 m to the tunnel ridge. Card slots are also installed to the arches at both ends of the tunnel heads. Each card slot is 4 m long， which needs to be connected through card slot connectors. The end of each card slot is fixed to the arching using screws， and in the middle part of the tunnel， the card slot is fixed on the arch frame by means of card slot holders. In order to ensure that the card slot is perpendicular to the arch bar and longitudinally aligned with the direction of the tunnel， the height of each arch frame can be marked before installation. The wind should be no stronger than level 3 when coating the tunnel. Before coating， all the tension ropes at the same side of the tunnel are fastened to the ground anchor， and it should well distinguish the outside film from the inside film. Before coating， the film is spread on one side of the tunnel to ensure that the film is flat and parallel， and then clamp springs are used to fix the film to the card slots in turn. Afterwards， the film is straightened stretched on the arches. Using tension rope to fix first is to prevent the film from being blown away by wind， and then clamp springs are used to fix the tunnel film to the card slots in turn.
The upper part of the tunnel film is stuck in the card slot， while the lower part is buried in the soil， and the depth of embedding in the soil should not be lower than the depth of the drainage channel outside the tunnel. The installation of plastic film， card slots， clamp springs and tension ropes shall comply with the relevant provisions of Code for Acceptance of Constructional Quality of Greenhouse Glazing-Plastic Film （NY / T 1966-2010）[16].
Vents and insect net installation
Vents are set at 0.5 m to the tunnel top at both sides of the arching， and all are controlled by manual film reelers. The film rolling rod is made of hot-dip galvanized circular tubes of φ25 × 1.8 mm， and the lower end of the tunnel film is fixed to the rolling rod using clamps. Swinging upward can roll up the tunnel film for ventilation. The film reelers for the vents on both sides are installed to the film rolling uprights outside the film， which are the hot-dip galvanized circular tubes of φ20 mm × 1.8 mm. The uprights for the film reelers of the vents at the top are fixed to the uprights at tunnel heads. The overlapping distance for the film at the vents should be no less than 0.2 m.
Insect proof net is installed to the vents and inside the skylights on both sides of the earth-inserted tunnel. The upper and lower ends of the insect proof net are fixed to the card slots of the arches， and the two ends of the east and west are fixed in the card slots of the arches at both ends of the tunnel.
References
[1] GUO AD. The application of Chinese Architectural Structural Load Standards in multi-span greenhouse engineering[J]. Modern Agricultural Sciences， 2009， 16（4）：218-220.
[2] YU YH， LU RG. The analysis for the characteristic of the bearing capacity in plastic tunnel[J]. Journal of Agricultural Mechanization Research， 2011， 1： 165-168. [3] QIE LJ， HAN JH， XUE JX. Design of the improved plastic shed and preliminary study on the temperature[J]. Chinese Agricultural Science Bulletin， 2010（1）： 284-287.
[4] WU DL， TANG Q， ZHU SD， et al. Structural design for outer shelf of a new-type double-layer plastic tunnel and its stability analysis based on ANSYS[J]. Acta Agriculturae Shanghai， 2012（4）： 100-105.
[5] ZHANG J， FENG QC， FU ZK， et al. Design and mounting technique of arched steel frame shed[J]. Journal of Changjiang Vegetables， 2014（10）： 13-16.
[6] PEN XR. Design and building techniques of concrete upright steel frame shed[J]. Science and Technology of Sichuan Agriculture， 2010（2）： 38.
[7] HAN AM. Techniques of plastic greenhouses design， construction and installation in the north[J]. Agricultural Sciences & Technology and Equipment， 2014（10）： 57-58.
[8] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 51057-2015 Technical code for horticultural plastic tunnel engineering[S]. Beijing： Standards Press of China， 2016.
[9] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 18592-2001 Metallic coating-hot dipped aluminum coatings on ferrous articles-specification[S]. Beijing： Standards Press of China， 2002.
[10] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 6728-2002 Cold formed steel hollow sections for general structure-Dimensions， shapes， weight and permissible deviations[S]. Beijing： Standards Press of China， 2002.
[11] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 20202-2006 Ethylene-vinyl acetate copolymer （EVA） blown covering film for agriculture[S]. Beijing： Standards Press of China， 2006.
[12] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 19791-2005 The criterion insect proof net design and its installation for greenhouse[S]. Beijing： Standards Press of China， 2005.
[13] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB 50205-2001 Code for acceptance of construction quality of steel structures[S]. Beijing： Standards Press of China， 2002.
[14] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 2518-2004 Continuously hot-dip zinc-coated steel sheet and strip[S]. Beijing： Standards Press of China， 2004.
[15] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 18983-2003 Measuring method of magnetic properties at alternative current for amorphous and nanocrystalline soft magnetic alloys[S]. Beijing： Standards Press of China， 2003.
[16] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. NY/T 1966-2010 Code for acceptance of constructional quality of greenhouse glazing-plastic film[S]. Beijing： Standards Press of China， 2011.
Key words Earth-inserted tunnel； All-steel structure； Construction technology
The common plastic tunnel is the most important cultivation facility in China， with an area of more than 560 000 hm2[1]. With the advantages of low investment， low construction cost， flexible construction， strong resistance， long service life， short production cycle， high degree of industrialized production and high economic benefits， the plastic tunnel has become one of the main production methods for the growth of agricultural efficiency and increase of farmers income， achieving remarkable economic and social benefits[2-4]. At present， the steel structure of plastic tunnel has gradually replaced the common plastic tunnels of bamboo structure and cement structure[5-7].
The most common plastic tunnel in the Yangtze River Basin is the earth-inserted tunnel， which is made of galvanized steel tube as the arching， and adopts the all-steel structure plastic tunnel with shoulder-type arch structure. Without concrete foundations， earth-inserted tunnels have the advantages of fast construction speed， easy assembly and disassembly apart from the advantages of common plastic tunnels. In this study， improvement was made to the original structure of earth-inserted tunnel to make it suitable for the production of vegetables in the tunnel， thereby promoting the development of protected vegetables in the Yangtze River Basin.
Overall Requirements for Earth-inserted Tunnel Construction
The earth-inserted tunnels should be built in the plots exposed to the sun on the lee side， which has flat terrain， convenient irrigation and drainage， fertile soil， low groundwater level， no pollution and shadings. Its safety and stability are mainly determined by the bearing capacity of the structural load-bearing components， which is expressed by the load， including permanent load （dead load）， crop load， wind load and snow load. The basic load requirements of structural design are： wind load > 0.25 kN/m2； snow load > 0.3 kN/m2； dead load > 0.55 kN/m2； crop load > 0.15 kN/m2.
According to the Technical Code for Horticultural Plastic Tunnel Engineering （GB/T 51057-2015）[8]， the service life of the main steel structure of the tunnel should be 10 years. The normal service life of plastic film， shade net， insect proof net and film line should not be less than 1 planting season， and the normal service life for card slot and circlip should be less than 5 years. The span should not be greater than 20 m， ridge height should not be less than 2.4 m， shoulder height of shouldered greenhouses should not be less than 1.6 m， and length should not be less than 30 m but not more than 100 m. The tunnel orientation （ridge direction） should be arranged from the south to the north， and the spacing of tunnels should be 1-2 m in the span direction and not less than 3 m in the length direction. Structure Parameters for Earth-inserted Tunnel
Length
In general， the length of a tunnel is determined according to the site. However， the shorter the length is， the higher the construction cost per unit area is. The longer the length is， the harder it is to adjust the environmental factors such as temperature and humidity. Thus， the length should be about 60 m （Fig. 1）.
Span
In central China， the arching is usually connected by 2 standard hot-dip galvanized steel piles of 7 m long using vault steel fasteners， so the span is usually 8 m.
Height
In order to make the tunnel be proper for growing seedlings and for the growth of hanging crops such as melons， the earth-inserted tunnel is designed into the tunnel with shoulders. The investigation on the structure of tunnels in central China found that when the height of the tunnel was about 3 m， the inside temperature could reach up to 50 ℃ during the productive phase of melon vegetables in early summer， which severely restricted the vegetable growth and agricultural operation in tunnel. Therefore， the shoulder of the earth-inserted tunnel is 2 m high with the ridge height of 4.5-5.0 mm.
Arching spacing
The designed earth-inserted tunnel had main and sub-arches. Since the bending strength of circular tube is not as good as that of square tube， the horizontal distance between main arches is 3 m. The sub-arch is the hot-dip galvanized circular steel tube， and there are 2 sub-arches between the main arches with the spacing of 1 m.
Earth-inserted Tunnel Construction Materials
Arching material
The costs for arching materials account for about 50% of the total costs of the tunnel. All the materials are Q235A galvanized materials with the adhesion amount of zinc in the steel tube of no less than 570 g/m2， coating thickness of no less than 80 μm in line with the relevant provisions in Metallic Coating-hot Dipped Aluminum Coatings on Ferrous Articles-Specification （GB/ 18592-2001）[9].
The arch bars of the main arching are galvanized square steel tubes with the specifications of 35 mm × 35 mm × 2.0 mm， which are in accordance with the relevant provisions in Cold Formed Steel Hollow Sections for General Structure -Dimensions， Shapes， Weight and Permissible Deviations （GB/T 6728-2002）[10]. There are a total of three longitudinal ties and 4 diagonal braces for the earth-inserted tunnel. There is one longitudinal tie in the ridge， and 2 on the shoulders on both sides of the tunnel. The diagonal braces are set on the top ceiling of the tunnel. The materials for the shed shape braces， sub-arch bars， door headpins of the main arching as well as the longitudinal rods and diagonal braces are all the hot-dip galvanized circular tubes with the specifications of φ32 mm × 1.8 mm. Coating materials
Tunnel film should meet the lighting requirements for planting crops， so the light transmittance should not be less than 85%. According to the relevant provisions in Ethylene-vinyl Acetate Copolymer （EVA） Blown Covering Film for Agriculture （GB/T 20202-2006）[11]， it can use the EVA film with the thickness of over 0.08 mm， or use polyethylene （PE） or polyvinyl chloride （PVC） film.
The insect proof net is installed at the top of the vents and below the top skylight on both sides of the tunnel， and the selected insect proof net is the 32-mesh nylon insect proof net of 2 m wide. The quality of the insect proof net should be in line with the relevant provisions in the current national standard of the Criterion Insect Proof Net Design and Its Installation for Greenhouse[12].
Fitting materials
The fitting materials of the tunnel mainly includes steel clamp （φ3.4 mm hot-dip galvanized carbon steel spring）， U-shaped clamp （U-shaped screw of φ6mm hot-dip galvanized rebar， plate of the 2 mm thick hot dip galvanized steel）， fixed clamp hoop （0.7 mm thick， hot dip galvanized）， card slot （0.7 mm thick with high zinc layer）， card slot connector （0.8 mm thick）， slot holder （1.5 mm thick， hot-dip galvanized）， criclips （φ2.5 mm thick， hot-dip galvanized）， clamps （plastic card with steel wire）， arching connecting bends and T-shaped connecting fastenings， tube head sheath， screws， film reeler and so on. The selection of fitting materials must meet the strength requirements， fasteners and other accessories should comply with the Code for Acceptance of Construction Quality of Steel Ttructures （GB 50205-2001）[13]， hot-dip galvanized steel sheet material quality should be consistent with the Continuously Hot-dip Zinc-coated Steel Sheet and Strip （GB/T 2518-2004）[14]， and the circlip materials should be consistent with the Measuring Method of Magnetic Properties at Alternative Current for Amorphous and Nanocrystalline Soft Magnetic Alloys （GB/T 18983-2003）[15].
The materials for a new arched all-steel earth-inserted tunnel are as shown in Table 1 for the one the length of 60 m， span of 8 m， ridge height of 4.5-5 m， shoulder height of 2 m， arching spacing of 1 m.
Construction and Installation of Earth-inserted Tunnel
Arching installation
Before the installation of the arching， the ground foundation construction and the arching processing should be carried out. The ground should be rammed and leveled， and marked according to the length and span of the tunnel. Holes are drilled using the steel chisel or electric drill with the diameter of 32 mm according to the arching spacing， and the hole should be no less than 0.5 m deep. The hole has an oblique degree of 5 °， so that there is a 85° tilt angle between the arching and the ground. The spacing for holes is 1 m to ensure that the depth of the earth-inserted tunnel goes at least 0.5 m into the soil. In order to ensure the same earth-inserted depth of the arching， the depth of the earth-inserted arching on both sides should be marked. The sub-arch is connected by 2 standard hot-dip galvanized steel tubes of 7 m long with a certain arc， but the purchased steel tubes are usually straight ones. Therefore， it usually requires on-site processing to curve the tubes into the needed arching using molds according to the shoulder height and span. When insert the curved arching into the holes， marking line is flush with the holes， and then two horizontally-symmetrical arc-shaped arches are connected through connecting bends and fixed with screws.
Tie rod and bracing installation
The tie rod is installed under the arching. Before installation， holes are punched at 5-8 cm at the end of the tie rods and then the ending parts are hammered flat. The tie rods under the ceilings are installed first， and then the rods on both sides of the shoulders. When installing the tie rods， steel clamps are used to lock the rods and arching to make the rods go perpendicular to the arching. When connecting the tie rods， the flattened part is first inserted to the other end of another rod， and then tightened by using screw at the hole. Both ends of the rod is packaged using tube head sheath to prevent puncturing the film. After the completion of each set of tie rod connection， it should be verified， amended and fixed to ensure that the formed surface by the rods is perpendicular to the horizontal plane and arch direction in the same direction with the length of the tunnel.
Bracing is used to connect the arches at both ends of the tunnel to prevent the arches from being tilted to one side by external forces. The diagonal bracings are installed inside the arching and are connected with the seven arch frames close to the tunnel heads through the U-shaped clamps. The lower end of the diagonal bracing is inserted into soil at the same straight line as the arch frame and the upper end is connected with the arch frame upright through fixed clips.
Tunnel heads installation
There are 4 uprights installed at each end of the earth-inserted tunnel with the depth of no less than 0.5 m into soil. The upper end is connected to the arching of the tunnel head through fixed clips.
Both ends of the tunnel head are provided with push-pull sliding doors with double doorways with the door opening of 2 m high and 2 m wide. The doorway and door opening are overlapped for 0.1 m. The door is provided with a clamping slot and the film is fixed on the door by a circlip. Above the door， the bearing rail is installed with bearings， and anti-swing device is below.
Tunnel film installation
Before coating the tunnel， card slots are needed to install outside the arch frames， and 2 card slots are installed at each side of the tunnel at 0.3 m and 1.8 m to the ground. Skylights are needed for ventilation on the top， and 2 card slots are installed at 0.5 m and 1.2 m to the tunnel ridge. Card slots are also installed to the arches at both ends of the tunnel heads. Each card slot is 4 m long， which needs to be connected through card slot connectors. The end of each card slot is fixed to the arching using screws， and in the middle part of the tunnel， the card slot is fixed on the arch frame by means of card slot holders. In order to ensure that the card slot is perpendicular to the arch bar and longitudinally aligned with the direction of the tunnel， the height of each arch frame can be marked before installation. The wind should be no stronger than level 3 when coating the tunnel. Before coating， all the tension ropes at the same side of the tunnel are fastened to the ground anchor， and it should well distinguish the outside film from the inside film. Before coating， the film is spread on one side of the tunnel to ensure that the film is flat and parallel， and then clamp springs are used to fix the film to the card slots in turn. Afterwards， the film is straightened stretched on the arches. Using tension rope to fix first is to prevent the film from being blown away by wind， and then clamp springs are used to fix the tunnel film to the card slots in turn.
The upper part of the tunnel film is stuck in the card slot， while the lower part is buried in the soil， and the depth of embedding in the soil should not be lower than the depth of the drainage channel outside the tunnel. The installation of plastic film， card slots， clamp springs and tension ropes shall comply with the relevant provisions of Code for Acceptance of Constructional Quality of Greenhouse Glazing-Plastic Film （NY / T 1966-2010）[16].
Vents and insect net installation
Vents are set at 0.5 m to the tunnel top at both sides of the arching， and all are controlled by manual film reelers. The film rolling rod is made of hot-dip galvanized circular tubes of φ25 × 1.8 mm， and the lower end of the tunnel film is fixed to the rolling rod using clamps. Swinging upward can roll up the tunnel film for ventilation. The film reelers for the vents on both sides are installed to the film rolling uprights outside the film， which are the hot-dip galvanized circular tubes of φ20 mm × 1.8 mm. The uprights for the film reelers of the vents at the top are fixed to the uprights at tunnel heads. The overlapping distance for the film at the vents should be no less than 0.2 m.
Insect proof net is installed to the vents and inside the skylights on both sides of the earth-inserted tunnel. The upper and lower ends of the insect proof net are fixed to the card slots of the arches， and the two ends of the east and west are fixed in the card slots of the arches at both ends of the tunnel.
References
[1] GUO AD. The application of Chinese Architectural Structural Load Standards in multi-span greenhouse engineering[J]. Modern Agricultural Sciences， 2009， 16（4）：218-220.
[2] YU YH， LU RG. The analysis for the characteristic of the bearing capacity in plastic tunnel[J]. Journal of Agricultural Mechanization Research， 2011， 1： 165-168. [3] QIE LJ， HAN JH， XUE JX. Design of the improved plastic shed and preliminary study on the temperature[J]. Chinese Agricultural Science Bulletin， 2010（1）： 284-287.
[4] WU DL， TANG Q， ZHU SD， et al. Structural design for outer shelf of a new-type double-layer plastic tunnel and its stability analysis based on ANSYS[J]. Acta Agriculturae Shanghai， 2012（4）： 100-105.
[5] ZHANG J， FENG QC， FU ZK， et al. Design and mounting technique of arched steel frame shed[J]. Journal of Changjiang Vegetables， 2014（10）： 13-16.
[6] PEN XR. Design and building techniques of concrete upright steel frame shed[J]. Science and Technology of Sichuan Agriculture， 2010（2）： 38.
[7] HAN AM. Techniques of plastic greenhouses design， construction and installation in the north[J]. Agricultural Sciences & Technology and Equipment， 2014（10）： 57-58.
[8] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 51057-2015 Technical code for horticultural plastic tunnel engineering[S]. Beijing： Standards Press of China， 2016.
[9] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 18592-2001 Metallic coating-hot dipped aluminum coatings on ferrous articles-specification[S]. Beijing： Standards Press of China， 2002.
[10] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 6728-2002 Cold formed steel hollow sections for general structure-Dimensions， shapes， weight and permissible deviations[S]. Beijing： Standards Press of China， 2002.
[11] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 20202-2006 Ethylene-vinyl acetate copolymer （EVA） blown covering film for agriculture[S]. Beijing： Standards Press of China， 2006.
[12] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 19791-2005 The criterion insect proof net design and its installation for greenhouse[S]. Beijing： Standards Press of China， 2005.
[13] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB 50205-2001 Code for acceptance of construction quality of steel structures[S]. Beijing： Standards Press of China， 2002.
[14] Ministry of Housing and Urban-Rural Development of the Peoples Republic of China. GB/T 2518-2004 Continuously hot-dip zinc-coated steel sheet and strip[S]. Beijing： Standards Press of China， 2004.
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