What Are the Three Types of TBM?  Tunnel Boring Machines (TBMs) are specialized engineering equipment designed to excavate tunnels efficiently and safely.The following is the classification of TBMs under different circumstances。

   I. Classification based on full-section tunnel boring machines

   1. Open TBM

   ① Features: Uses a support system anchored to the tunnel wall to provide thrust and stability. It does not have a complete protective cover, so it is suitable for stable rock formations such as hard or medium-hard rock.

   ② Use: Very suitable for tunnels in self-supporting rocks (such as granite, basalt), which have a low risk of collapse. Commonly used in

   2. Single Shield TBM

   ① Features: Has a cylindrical shield to protect the machine and workers. It relies on segmented lining to provide thrust and is suitable for mixed geological conditions (soft soil to broken rock).

   ② Use: Effective in unstable or variable formations, such as soft rock or broken zones. Pre-installed segments are required for advancement.

   3. Double Shield TBM

   ① Features: Combination of open and single shield designs. It has two shields: a front shield for cutterhead support and a rear shield for segmented installation. This allows simultaneous excavation and support, maximizing efficiency in hard rock formations with localized fractures.

   ② Application: Best suited for long tunnels in hard rock with intermittent weak interlayers, which can operate continuously without frequent failures.

   II. Classification based on geological conditions and operating mechanisms

   1. Hard Rock TBMs (Open-Type TBMs)

   ① Design: Optimized for stable, non-fractured rock formations (e.g., granite or basalt). These machines lack a shield and rely on gripper systems to anchor against tunnel walls during excavation.

   ② Functionality: They use disc cutters to crush rock and conveyor systems to remove debris. No immediate structural support is needed in stable geology.

   ③ Applications: Ideal for mining, hydropower tunnels, and mountainous regions. Examples include the Robbins Main Beam TBM used in the Gotthard Base Tunnel.

   2. Soft Ground TBMs

  Subtypes:

   ①  Earth Pressure Balance (EPB) TBMs: Use pressurized soil to stabilize tunnel faces in soft, cohesive soils (e.g., clay). Screw conveyors manage excavated material while maintaining pressure balance.

   ②  Slurry Shield TBMs: Employ a slurry mixture (bentonite or water) to counteract groundwater pressure in permeable soils like sand or gravel. A separation plant filters slurry for reuse.

  Applications: Urban metro systems (e.g., London Crossrail) and subaqueous tunnels.

   3. Shielded TBMs

   Variants:

   ①  Single Shield TBMs: Equipped with a rotating shield and thrust cylinders. Precast segments are installed immediately behind the cutterhead, suitable for mixed or unstable rock.

   ②  Double Shield TBMs: Combine gripper and shield mechanisms, enabling simultaneous excavation and segment installation. Effective in alternating hard and fractured rock.

   Applications: Long-distance rail tunnels and geologically variable environments, such as the Alps.

   The choice of TBM depends on geological complexity, project scale, and safety requirements. Hard Rock TBMs excel in stable formations, Soft Ground TBMs address loose soils, and Shielded TBMs adapt to heterogeneous conditions. Innovations like hybrid designs further enhance flexibility, ensuring TBMs remain pivotal in modern tunneling projects.