Segment Bolts for Precast Tunnel Linings: M24-M30 Grades, Coatings and QC

Most precast lining joints are bolted with M24, M27 or M30 hex or T-head bolts in 200-500 mm lengths, chosen by segment thickness and joint pocket geometry. Grade 8.8 covers ordinary EPB and slurry rings; grade 10.9 is worth the premium on hard-rock mountain drives where jacking thrust and shock loading run high. A six-segment ring typically consumes roughly 14-20 bolts, while eight-segment rings use more, shorter connections.

Coating choice matters more in cold-climate CIS tunnels than buyers expect. Hot-dip galvanizing gives thick, sacrificial zinc and suits grade 8.8, but acid pickling carries hydrogen-embrittlement risk for 10.9 — zinc-flake Dacromet-type coatings are the safer pairing there, and they resist freeze-thaw cycling and de-icing chlorides without the bulk of a zinc bath that can interfere with thread fit.

Quality control is where cheap bolts fail: missing mill certificates, unverified grade marking, inconsistent thread tolerance and no batch traceability. Specify tensile and proof-load testing, coating thickness checks and torque verification per batch. OUSUN supplies certified segment bolts and TBM spares direct from the Deyang cluster, with interchangeable parts and roughly 2-4 week lead times — [email protected].