Segment Bolts for TBM Tunnel Lining: M24–M30 Grades, Coatings and Ring Counts

Dense metro and intercity rail programmes across Southeast Asia are driving demand for precast concrete tunnel lining, frequently bored through soft soils and water-bearing karst where groundwater is chemically aggressive. In that setting the segment bolt is not a minor consumable — it is the element that lets a ring of discrete segments behave as one continuous structure. Getting diameter, length, grade and coating right at specification stage costs far less than remedial work after ring closure.

Most linings use M24, M27 or M30 bolts in grades 8.8 and 10.9, with lengths from 200 mm to 500 mm matched to segment thickness plus socket, washer and curvature allowance. A 6-segment ring has fewer joints but heavier segments and typically 12–18 circumferential bolts; an 8-segment ring carries more joints and often 16–24. Curved inter-ring bolts generally add one or two per segment edge, so total per-ring counts climb quickly.

Corrosion protection usually decides coating choice. Hot-dip galvanizing provides a thick, abrasion-tolerant zinc layer for aggressive ground, while Dacromet gives a thin, dimensionally stable film with strong chloride resistance and lower hydrogen-embrittlement risk for 10.9