Underground Boring Machines: A Guide to TBMs for Tunnels

Underground boring machines, also called tunnel boring machines (TBMs), are advanced pieces of construction equipment used to excavate tunnels with precision through a variety of soil and rock formations.

This guide will provide a deep look into TBMs - how they work, types, components, operations, costs, top brands, and more.

How Do Underground Boring Machines Work?

TBMs are complex mechanical systems customized to the geological conditions they will encounter. Here is an overview of how TBMs work:

• The circular cutter head on the front of the TBM rotates and presses against the tunnel face. Cutters break up and excavate the soil or rock.
• Hydraulic propel cylinders thrust the TBM forward as the head rotates and tunnel face is excavated.
• Conveyors carry the excavated material continuously through openings in the cutter head to be transported away.
• A circular shield or housing around the TBM protects it from cave-ins and supports the tunnel as progress is made.
• Precast concrete liner segments are automatically erected along the interior of the tunnel walls as boring proceeds.
• Grout is injected to fill voids behind the liner segments for structural integrity.
• The TBM pushes forward using the previously installed tunnel lining for support, advancing the tunnel.
• Laser guidance, monitoring sensors, and operator controls direct and adjust boring functions.

Types of Tunnel Boring Machines

TBM types are classified by geological conditions, excavation method, and lining erection system:

• Hard Rock TBM – For boring through solid rock using disc cutters.
• Soft Ground TBM – Uses counter-rotating tungsten cutters and pre-support of tunnel face in soft soils.
• Earth Pressure Balance – Maintains pressure against tunnel face to prevent cave-ins and excessive groundwater seepage.
• Slurry Shield – Uses pressurized slurry pushed against the face to hold up loose ground.
• Open-Face Shield – No soil support. Used in stable rock formations.
• Double Shield – Separate shields for tunnel face and lining erection.
• Gripper TBM – Powerful grippers brace against tunnel lining for forward thrust.
• Laser Guided – Highly precise laser guidance for tunnel alignment.

Key Parts and Components

Critical TBM system components include:

• Cutter head – The rotating excavation wheel fitted with disc cutters, scrapers, or tungsten carbide teeth.
• Main bearing – Rotates cutter head under heavy loads. Needs frequent maintenance.
• Thrust cylinders – Hydraulic cylinders that push the TBM forward.
• Conveyor system – Collects and removes muck through openings in the cutter head.
• Shield – Cylindrical housing protects and supports around the TBM.
• Lining segment erector – Assembles pre-cast concrete or steel lining sections.
• Grout injection system – Fills space between tunnel walls and liner with cement.
• Laser guidance – Monitors and controls tunnel alignment.
• Automation – Modern computerized controls for TBM functions.

TBM Operations and Processes

Operating a TBM involves meticulous process control:

• Pilot tunnel and access shafts are constructed to lower parts of the TBM underground.
• The TBM is assembled and thrust cylinders brace against the initial tunnel section.
• Rotation of the cutter head begins excavating the face as propel cylinders advance the TBM.
• Conveyors remove excavated material continuously. Workers add sections to conveyors as needed.
• Laser guidance keeps the TBM steering straight on the planned path.
• Lining segments are erected and installed tightly against tunnel walls by the TBM.
• Gaps between lining and earth are filled with pressurized grout.
• The previous lining provides friction for the thrust cylinders to push off and extend the tunnel.
• This cycle repeats round-the-clock with shifts operating the TBM.

What is the Cost of Tunnel Boring Machines?

TBM costs can range from $2 million for smaller machines up to $50+ million for mega-TBMs over 40 feet in diameter. Costs depend on size, materials, complexity of technology, and geological factors.

Operational costs like labor, maintenance, electricity, supplies, and logistics account for the majority of a TBM project’s budget.

Top TBM Brands

Leading global manufacturers of tunnel boring machines include:

• Herrenknecht – Pioneer in TBM innovation and technology based in Germany. Custom designed for all geologies.
• China Railway Construction Heavy Industry – Massive Chinese state-owned company producing heavy tunnel boring systems.
• Hitachi Zosen – Major Japanese corporation manufacturing a wide range of TBMs for over 50 years.
• The Robbins Company – Original inventor and leading U.S. producer of TBMs since the 1950s.
• Kawasaki Heavy Industries – Diversified Japanese conglomerate and major supplier of specialty TBMs.
• Karam Boring – Reliable India-based manufacturer of TBMs for metro and utility tunnel projects.

FAQ

How do underground boring machines work?

They work by a rotating cutter head that breaks up soil and rock. Hydraulic cylinders thrust the TBM forward while conveyors remove debris continuously.

What is the principle of TBM?

The principle is using a shielded, rotating full-face cutting wheel to simultaneously excavate and install precast concrete segment lining to securely advance a tunnel.

What is the function of TBM?

The functions are face excavation via disc cutters or tungsten teeth, removal of muck, installing tunnel support lining, grouting, steering guidance, and propulsion forward using hydraulic thrust.

Tunnel boring machines enable safe, efficient, precision boring of underground tunnels through a vast range of geological conditions. This guide provided an overview of how they work, types, major components, operations, costs, and top manufacturers. TBMs are complex but revolutionary tunneling technology.