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Metro Bolt for Segment Tunnel

What is Metro Bolt

The metro bolt used to repair the cement structure of the subway is called a subway bolt. It is widely used in mining and subway engineering. There are two types of metro bolts: longitudinal straight bolts and circumferential arc bolts. In some projects, the specifications of 2 types of bolts are the same, while in others, they have different specifications and strength grades. Metro bolts are also known as segment tunnel bolts. There are various designs and types of segment bolts based on the diameter of the shield.

Arc-shaped segment tunnel bolt

Arc-shaped segment tunnel bolts are used to connect and fasten segments. These segments are assembled into a circular cylindrical pipeline, where the segments form the cylindrical walls. During excavation, arc-shaped segment bolts effectively control ground settlement, reduce the impact on above-ground structures, and ensure that underwater excavation does not affect surface traffic. Each segment is combined to form a pipeline, designed as straight or arc-shaped fastening connectors, which are called arc-shaped segment tunnel bolts.

Classification of Metro Segment Bolts

Metro segment connection bolts have a wide range of applications, including building segment connections, shallow-buried longitudinal segment bolts, subway shield construction, subway segment connections, tunnel segment connections, and other projects requiring connection and fastening. Generally, segment connection bolts can be classified into 6 types: hexagonal head arc-shaped segment bolts, non-standard segment bolts, double-ended straight segment bolts, double-ended arc-shaped segment bolts, hexagonal flange face arc thread segment bolts, and hexagonal head arc thread segment bolts. Additionally, segment connection bolts can be classified based on the applied forces, typically categorized into three levels: 5.8 grade (material using Q235), 6.8 grade (material using 45#), and 8.8 grade (material using 40Cr). Zinc plating is commonly used for surface treatment.

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Segment Tunnel Arc Bolt Specification:

Dimension Arc Length mm Tensile Strength Surface Treatment
M25~M39 480~880 4.6、4.8、5.6、5.8、8.8 Dacromet, Hot dip Galvanized

Segment Tunnel Straight Bolt Specification:

Dimension Arc Length mm Tensile Strength Surface Treatment
M25~M48 480~880 4.6、4.8、5.6、5.8、8.8 Dacromet, Hot dip Galvanized

Tunnel Segment Bolt Mechanical Property:

Tunnel Segment Bolt
Mechanical and Physical Properties Performance Level
3.6 4.6 4.8 5.6 5.8 6.8 8.8 9.8 10.9 12.9
d≤163) d>163)
Nominal Tensile Strength  N/mm2 300 400 500 600 800 800 900 1000 1200
Minimum tensile strength N/mm2 330 400 420 500 520 600 800 830 900 1040 1220
Vickers hardness HV F≥98N min 95 120 130 155 160 190 250 255 290 320 385
max 2206) 250 320 335 360 380 435
Brinell hardness HB F=30 D2 min 90 114 124 147 152 181 238 242 276 304 366
max 2096) 238 304 318 342 361 414
Rockwell hardness HR min HRB 52 67 71 79 82 89 / / / / /
HRC / / / / / / 22 23 28 32 39
max HRB 95.06) 99.5 / / / / /
HRC /   32 34 37 39 44
Surface hardness HV 0.3 max / 7)
Yield point N/mm2 Nominal 180 240 320 300 400 480 / / / / /
min 190 240 340 300 420 180 / / / / /
Specified non-proportional elongation stress N/mm2 Nominal / / 640 640 720 900 1080
min / / 640 660 720 940 1100
Guaranteed stress N/mm2 0.94 0.94 0.91 0.93 0.9 0.92 0.91 0.91 0.90 0.88 0.88
180 225 310 280 380 440 580 600 650 830 970
Breaking torque MB,N.m min / according to GB/T3098.13 regulations
Elongation after breaking % min 25 22 / 20 / / 12 12 10 9 8
rate of reduction in area % min / 52 48 48 44
Shock absorption work Aku,min   25   30 30 25 20 15
Head firmness No break
Minimum height of undecarburized thread E / 1/2H1 2/3H1 3/4H1
Maximum depth of fully decarburized layer G,mm / 0.015
Hardness after tempering / The difference between the average hardness before and after tempering is not more than 20HV
Surface defects According to GB/T 5779.1 or GB/T 5779.3 regulations

Metro Bolt Anti-corrosion Treatment:

Metro segment bolts are mainly used in subway construction projects to connect segments and form cylindrical pipelines. Due to their prolonged underground usage in a humid environment, metro segment bolts are prone to corrosion and rust.

Surface treatment of the bolts is crucial. Effective anti-corrosion measures can extend the service life of the bolts and pipelines. Improper anti-corrosion treatment can lead to rusting, which affects the mechanical properties and tensile strength of the bolts when corrosion reaches the interior. Therefore, proper anti-corrosion treatment is essential when using tunnel segment bolts. Common surface treatments for segment bolts include hot-dip galvanizing, zinc infiltration, and dacromet coating.

Metro Bolt Applications:

Metro bolts are widely used in subway construction projects, primarily responsible for connecting each segment to form cylindrical pipelines that support tunnels. However, since metro bolts work underground for extended periods and are exposed to a humid environment, they are prone to corrosion and rust. Therefore, surface treatment of metro bolts is crucial. Effective anti-corrosion treatment can prolong the service life of the pipelines and bolts, providing significant help for subsequent work. If anti-corrosion measures are not properly implemented and corrosion reaches the interior of the metro bolts, it can affect their mechanical properties and normal usage.

Metro Tunnel bolts are used in various fields, including the South-to-North Water Diversion Tunnel project, municipal engineering, West-East Gas Transmission project, gas pipeline engineering with shield tunneling, power cable shield tunneling, over-river shield tunneling for highways, railway shield tunneling, gas pipeline engineering with shield tunneling, open cut engineering, and power plant water intake shield tunneling, among others. These projects require the use of subway bolts.

Installation of Tunnel Segment Bolts

The selection of tunnel segments in shield tunneling is based on fulfilling the requirements of the tunnel alignment. It is essential to consider that the gap between the shield tail and the tunnel segment after installation should meet the requirements for the next excavation cycle to ensure an adequate shield tail gap and prevent direct contact between the segment and the shield tail. Generally, the selection and installation positions of the segments should be determined in advance based on the pushing instructions, aiming to minimize the differential stroke of the jacks after segment installation.

The installation of the tunnel segments should start from the bottom of the tunnel, sequentially installing adjacent segments and then the top segment. Before installing the top segment, the sealing gasket should be lubricated with a water-based lubricant. The sealing gasket should be inserted radially and pushed vertically to the top while adjusting its position. Once the segment blocks are installed, the thrust of the corresponding jacks should be extended promptly, tightening the segments. The jacking force should be greater than the force required for the stability section before removing the segment installation tool. After separating the segment ring from the shield tail, the tightness of the subway segment bolts should be doubly secured.

Metro Segment Tunnel Bolt Project:

  1. Indonesia’s Jakarta-Bandung high-speed railway: M36 tunnel segment bolts(50,300 sets)
  2. Shijiazhuang Metro Line 2 Phase I: M36 tunnel segment bolts( 221,713 sets)
  3. Chengdu Metro Line 10 and Phase 13: M30 tunnel segment bolts( 552,854 sets)
  4. Dongguan Rail Transit Line 1 Phase I: M30 tunnel segment bolts( 159,976 sets)
  5. Guangshan Railway: M30 tunnel segment bolts( 32,000 sets)
  6. Fuzhou Rail Transit Line 5: M30 tunnel segment bolts(123,984 sets) 
  7. Xi’an Metro Line 16: M30 tunnel segment bolts( 186,700 sets)
  8. Xiamen Metro Line 4: M30 tunnel segment bolts( 72,260 sets)
  9. Guangzhou Metro Line 11: M24 segment bolts (33,800 sets)
  10. Guangzhou urban rail transit: M24 tunnel segment bolts(70,456 sets) 
  11. Foshan Urban Rail Transit Line 3 : M24 tunnel segment bolts(145,200 sets) 
  12. Xi’an Metro Line 6: M27 tunnel segment bolts( 26,400 sets)
  13. Underground comprehensive pipe gallery project in Guangzhou: M24 pipe segment bolts( 258,006 sets)