Main Beam Tunnel Boring Machine

A main beam tunnel boring machine, also called a gripper or open TBM, bores tunnels in rock that can stand unsupported for a short time. Instead of holding up the ground with a shield, it clamps itself to the tunnel walls and installs rock support directly behind the cutterhead. This page lists a 7 m class machine and its backup system in nine groups.

How it bores

Two gripper shoes are pushed out against the tunnel walls to anchor the machine. Thrust cylinders then push the cutterhead forward along the main beam for one stroke while the main drive turns it. Each disc cutter rolls in its own circular track on the face under a load of up to 311 kN, crushing the rock under its tip, and cracks running between neighboring tracks break off chips. Buckets at the rim of the cutterhead pick up the chips and drop them onto the machine conveyor, which runs through the main beam to the backup.

At the end of the stroke the rear support is lowered to the floor, the grippers are released and the gripper carrier moves forward along the beam. The grippers clamp again, the rear support lifts, and the next stroke begins. The machine steers during the stroke by moving the main beam against the clamped gripper unit, following the line shown by the laser guidance system.

Rock support

Support goes in close to the face, in the L1 zone under the roof shield: rock bolts from the roof drills, wire mesh, and steel ring beams in weak rock. Shotcrete is sprayed farther back, in the L2 zone, where precast invert segments can also be laid. A probe drill explores the rock ahead and can drill holes to grout it before the machine arrives. The grippers bear directly on the tunnel walls, so where the rock is weak the walls under them are reinforced first.

Backup and services

The backup is a train of decks towed behind the machine. It carries the transformers, frequency converters, hydraulic power unit and conveyors, and keeps the heading supplied with fresh air through a ventilation duct, water, compressed air and drainage. Its refuge chamber shelters the crew in an emergency. Rock leaves the tunnel on a continuous conveyor or in rail muck cars.

Other tunnel boring machines

Where rock is too broken to grip or to stand while support is installed, shielded hard-rock machines are used instead: a single shield pushes off the concrete lining it builds, and a double shield can do either. Soft ground needs a closed face, as on the earth pressure balance TBM and the slurry TBM.

How the list is organized

Quantities are for one machine of about 7.2 m bore with its backup, based on two published machines of that size listed in the sources. Numbers of cutters, drives and backup decks grow with diameter and with the services a project needs. Lines marked optional are fitted on many machines but not all. Existing BOMwiki pages, such as the transformers, switchgear and total station, expand to their own parts inside this list.

Sources

• Robbins: Main Beam TBM
• Herrenknecht: Gripper TBM
• Robbins: TBM for Queens Water Tunnel No. 3 (RETC 1997)
• Robbins: TBM operation at the Røssåga hydropower project
• Robbins: Disc cutters specification sheet
• Robbins: Long-distance TBMs (WTC 2015)
• Robbins: US patent 7,832,960, All-conditions tunnel boring machine