Segment Bolts for Precast Tunnel Linings: M24-M30, Grades 8.8/10.9 and Coating Choice

Segment bolt selection starts with ring geometry. A six-segment ring typically needs roughly 90-140 bolts across its circumferential and longitudinal joints; an eight-segment ring with a smaller key often uses 20-30% more, in shorter lengths. M24 suits thinner 250-300 mm linings, M27 covers most metro work, and M30 is common where jacking forces and mixed-face loads run high. Lengths generally fall between 200 and 500 mm.

Corrosion protection is where budgets quietly diverge. Hot-dip galvanizing builds a thick sacrificial zinc layer that tolerates site handling and performs well in wet, chloride-bearing ground. Dacromet (zinc-aluminium flake) holds tighter tolerance on threads and resists abrasion in sandy desert conditions where bolts are handled repeatedly before installation. Grade 8.8 remains the default; 10.9 earns its cost under high jacking loads and thin linings, but demands disciplined torque control.

Whatever the grade, certification matters more than price per piece. Ask for mill certificates, batch-level tensile and proof-load testing, and dimensional checks on thread pitch and coating thickness — a failed bolt at depth becomes a permanent leak path and a costly repair. OUSUN supplies segment bolts, disc cutters and TBM spares from its Deyang base on a typical 2-4 week lead time; enquiries to [email protected].