Tunnel Segment Bolts (Curved & Longitudinal)
Segment bolts connect the precast concrete rings of a shield-driven tunnel. They work in a wet, chloride-bearing environment and are re-torqued after assembly, so strength consistency, ductility and coating life matter as much as the thread itself.
Two main types
- Curved (arc) bolts — bent to match the segment's circumferential duct. The bending radius follows the segment geometry, so these are always produced to drawing.
- Longitudinal bolts — straight bolts through the longitudinal ducts connecting consecutive rings.
Both are normally supplied as a set with the matching nut and washer, since the torque coefficient depends on the coating of the specific batch.
Typical specification range
| Item | Range / options |
|---|---|
| Diameter | M16 – M36 common; other sizes to drawing |
| Thread | Metric coarse or fine; to drawing |
| Grade | 8.8 / 10.9 |
| Material | Carbon steel or alloy steel to the required property class |
| Bending radius | Per drawing, matched to the segment duct geometry |
| Coating | Dacromet / zinc flake, hot-dip galvanized, phosphated, or plain for later coating |
| Surface | Threads clean and free of burrs; coating thickness controlled so the nut can still be run on |
| Marking | Grade and manufacturer mark on the head as agreed |
Choosing the coating
- Dacromet / zinc flake — thin, uniform and low-friction. It does not build up on the thread, which keeps the torque–preload relationship predictable; widely used where a defined torque coefficient is needed.
- Hot-dip galvanizing — thicker, sacrificial protection for long-term exposure. Because the layer is thicker, threads are normally cut or re-tapped after coating, and the torque value should be adjusted for the higher friction.
- Phosphating or plain — used when the bolt will be coated or protected on site.
As a rule, do not apply one fixed torque value across different coatings: the torque coefficient differs, and the same torque can produce a noticeably different preload.
What to send for a quotation
- Drawing with bending radius, overall length, thread length and diameter
- Grade and coating, with the salt-spray requirement if there is one
- Matched nut and washer, if these are part of the scope
- Quantity per ring and total quantity, plus the delivery schedule
Request a quotation or send your drawing
Tell us the specification, grade, coating and quantity — or send a PDF/DWG drawing and we will come back with price and lead time.
Tel / WhatsApp / WeChat: +86 185 3100 8199
FAQ
- Do you make curved bolts to a specific radius?
- Yes. Curved segment bolts are produced to the drawing; the bending radius is matched to the duct geometry of your segment. Please include the radius, chord length and overall length on the drawing.
- Why does the same torque value give different results on different coatings?
- Because the torque coefficient depends on the friction of the surface treatment. A zinc-flake coating is lubricious while a hot-dip galvanized surface is rougher, so the same torque produces different preload. Confirm the torque value against the coating actually supplied.
- Are the nuts and washers supplied as a set?
- They can be. Supplying bolt, nut and washer from the same production batch keeps the torque coefficient consistent, which is recommended when the contractor has to verify torque on site.
- Can bolts be reused after they have been tightened?
- Reuse is not recommended for bolts that have been loaded or over-torqued. Yielding or coating damage makes the preload inconsistent between pieces.