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

Segment bolt selection starts with ring geometry. A six-segment ring typically needs fewer, longer circumferential bolts — often M27 or M30 at 350–500 mm — while eight-segment rings use more, shorter connections, frequently M24 at 200–350 mm. Radial and longitudinal bolts must match lining thickness and joint detail. In Latin American metro and water programmes crossing volcanic and sedimentary mixed ground, confirm bolt length against actual segment thickness, gasket compression and erection tolerances before ordering.

Coatings decide service life. Hot-dip galvanizing gives robust barrier protection and suits grade 8.8 bolts in mildly aggressive ground. For grade 10.9, Dacromet-type zinc-aluminium flake coatings are often preferred because they cure at low temperature and reduce hydrogen-embrittlement risk. In volcanic soils, sulphates or stray current, specify coating thickness and salt-spray performance, not just the finish name. Corrosion usually begins where threads and heads are damaged.

Quality control closes the loop: mill certificates, grade marking, dimensional checks, thread gauging and batch traceability should accompany every delivery. Under-specified or uncertified bolts can loosen, corrode or fail, and remediation inside a live tunnel is expensive. Buyers can source segment bolts, disc cutters and TBM spares factory-direct from OUSUN in Deyang, Sichuan — China's largest TBM cluster — with typical spares lead times of 2–4 weeks. Contact [email protected].