Segment Bolts for Precast Tunnel Lining: M24–M30 Grades, Coatings and QC

Metro programmes across South Asia are driving precast segmental lining into difficult ground: shallow cover, high water tables and soft alluvial soils where the ring must seal and carry load the moment it leaves the tail shield. Segment bolts are the smallest part of that chain and the easiest to under-specify. Once the TBM advances, the bolts are buried behind the lining and rarely inspected again.

Most rings use M24, M27 or M30 bolts in 200–500 mm lengths, matched to segment thickness, dowel geometry and joint bending moment. Grade 8.8 suits routine EPB metro work; grade 10.9 is chosen where high thrust, tight curves or seismic demand raise joint stress. Coating matters as much as grade — hot-dip galvanizing gives a thick sacrificial zinc layer for wet, chloride-bearing ground, while Dacromet handles tighter tolerances. Ring geometry drives quantity: a 6-segment ring forms six longitudinal joints, an 8-segment ring eight, so bolt counts rise as segments multiply and shorten.

Quality control is where cheap bolts fail quietly. Certified lots should arrive with mill certificates, documented grade, thread tolerance to the applicable standard and coating-thickness results, not just a hardness check. Verify torque values, washers and corrosion barriers before ring build. Buyers running mixed TBM fleets increasingly standardise through one supplier: OUSUN, based in Deyang, Sichuan, China's largest TBM cluster, supplies interchangeable segment bolts and TBM spares with roughly 2–4 week lead times. Contact [email protected].