Beneath the city, tunnels stretch quietly like the "blood vessels" of the urban landscape. The core force shaping these pathways is the automated shield machine, hailed as the "underground aircraft carrier." It is not a traditional excavation device but a "mobile intelligent factory" that integrates tunneling, support, muck removal, segment assembly, and backfill grouting. With efficiency, precision, and quiet operation, it reshapes the urban spatial framework.

Subway TBM Construction: Full-process Technical Section View

Let’s take metro tunnel construction as an example and break down how this national flagship machine accomplishes "one-step tunneling" from ground formation to finished tunnel through five meticulous steps in just three minutes:

Step 1: Intelligent Tunneling – Adaptive Cutting of the Cutterhead for All Ground Types​

The rotating cutterhead at the front of the shield machine is its key tool for tackling complex geology. Based on the geological conditions along the tunnel route (such as soft soil, sand layers, or rock), engineers configure specialized cutter combinations on the cutterhead: disc cutters crush rock, scrapers cut soil, and buckets remove muck. Driven by ultra-high torque, the cutterhead rotates to efficiently break through the excavation face, paving the way for tunnel advancement.

Sectional Illustration of TBM Tunneling Principle and Components

Step 2: Dynamic Balance – Precise Pressure Control of the Excavation Face​

The excavated muck enters the sealed pressure chamber at the front of the shield. Here, one of the shield machine’s core technologies comes into play: dynamic pressure balance. In the commonly used Earth Pressure Balance (EPB) mode, the system precisely regulates the muck pressure inside the chamber to maintain real-time balance with the soil and water pressure ahead of the excavation face. This acts as an "invisible support" for the excavation face, effectively controlling ground settlement and ensuring the safety of overlying structures and pipelines. The muck is then systematically removed via a screw conveyor and transported to the surface.

Working Principle of EPB Shield: Excavation and Discharge Section View

Step 3: Self-Propelled Advancement – The Self-Sufficiency of the Hydraulic System​

The shield machine requires no external thrust. It relies on dozens to hundreds of high-thrust hydraulic jacks arranged within the shield body. Using the installed tunnel segment rings as anchor points, these jacks drive the machine forward through hydraulic power. This self-reaction propulsion method is key to achieving long-distance, continuous, and highly precise tunneling, showcasing the equipment’s high degree of autonomy.

Working Principle of TBM Thrust System and Reaction Force Mechanism

Step 4: High-Precision Assembly – Millimeter-Level Construction of Segments​

Each time the shield machine advances by the width of one segment ring (typically 1.2 or 1.5 meters), an annular space forms behind the shield tail. The segment erector then goes to work. This high-precision robotic arm handles hundreds of tons of precast concrete segments like assembling "giant Lego blocks," performing tasks such as gripping, transferring, positioning, and joining within the confined space. The result is a sturdy lining ring that provides both structural support and waterproof sealing, with assembly accuracy reaching millimeter levels to ensure tunnel smoothness and stability.

High-Precision Segment Erection & Ring Closure for TBM

Step 5: Immediate Stabilization – Synchronous Grouting and Void Filling​

The outer diameter of the shield machine is slightly larger than that of the segments, creating a gap between the segments and the surrounding ground. The synchronous grouting system activates immediately, injecting specially formulated grout under high pressure into this gap through ports in the shield tail. This process instantly fills voids, stabilizes segments, controls ground deformation, and forms the tunnel’s first waterproof barrier—a core procedure for ensuring long-term structural safety and waterproof performance.

Perspective View: Tail Void Grouting Process in TBM Tunneling

Conclusion: Integration of Wisdom, One-Step Tunneling​

Modern shield tunneling in China has evolved into a highly integrated, automated, and intelligent engineering system. It consolidates traditionally separate, high-risk processes into a continuous, synchronized operation within a single machine, achieving the technological leap of "tunnel completion upon tunneling finish."
This is not only a vivid reflection of the transition from "Made in China" to "Intelligently Made in China" but also a model of how Chinese engineering wisdom is integrated into urban underground space development. Through the perfect combination of surface prefabrication and underground excavation, it shapes the subterranean networks for sustainable urban development in a safer, more efficient, and environmentally friendly manner, building century-quality lifelines for rail transit.