“Heart Surgery” 20 Meters Underground

In the shield tunneling section of the Xiamen Metro Line 6 project in Fujian, beams from searchlights pierced the darkness. Wu Jichang, Safety Director of the Shield Center of The Sixth Engineering (Xiamen) Co., Ltd. of CCCC Third Harbor Engineering Co., Ltd., was meticulously inspecting the maintenance of the shield machine equipment as usual. Suddenly, the flashing red numbers on the monitor screen, like an alarm, shattered the calm in the tunnel. The screen displayed: Thrust cylinder pressure too low! Technicians urgently conducted a comprehensive inspection of the shield machine, finding the direct cause to be severe wear on the cylinders.

The cylinders, acting as the heart of the shield machine, are not only the core power source but also the central control hub for its posture. The Xiamen Metro Line 6 project, a key project under Xiamens Cross-Island Development strategy and constructed by Third Harbor Engineering Bureau, features a complex and variable composite stratum within its sections, presenting significant inherent construction challenges. This wear on the thrust cylinders directly affected the equipments power output. If left unaddressed, deviation of the tunneling trajectory would become a Sword of Damocles hanging overhead. However, the characteristic of shield equipment being only able to advance, not retreat meant that major overhauls typically had to wait until the entire machine exited the tunnel. Yet, at this point, the shield machine had only advanced 300 meters, with over four months of construction remaining until full-line breakthrough. Operating the shield machine with this illness was absolutely not an option. After comprehensive discussion, performing an in-tunnel cylinder replacement became the only feasible solution, provided safety was ensured.

Replacing cylinders within the tunnel can be described as a highly difficult underground surgery. In the confined space 20 meters underground, barely enough for two people to stand, technicians felt shackled in their movements. The intertwined hydraulic pipelines and electrical systems inside the equipment were like crisscrossing blood vessels and nerves; the slightest misstep could trigger deadly dangers such as high-pressure oil injection or electrical short circuits. Facing these challenges, the project team sprang into action. They conducted a thorough physical examination of the shield machine, carefully studied similar domestic and international engineering cases, and meticulously developed a detailed plan tailored to the sections unique hard rock geological conditions and the shield machine model. The core of the plan lay in the seamless transition between removing the old and installing the new, which required both protecting the already installed shield segments and achieving millimeter-level precision in the installation of the new cylinders.

To ensure the plan was foolproof, the team conducted countless simulated surgeries on the surface. Every drill for cylinder disassembly and installation was practiced down to the tightening torque of the smallest screws, all repeatedly tested. Everyone understood that any oversight could lead to disaster. Through countless drills and reviews, improvement measures were continuously refined until the drill results far exceeded the standards.

Finally, the cylinder replacement officially commenced. Operators carefully squeezed into the tail chamber of the shield machine, as if entering the interior of a precise steel heart, and began the tense disassembly. The most perilous moment arrived as the old cylinder was about to be detached from the machine body. The flexible joint connecting the cylinder body to the main shield machine structure—the cylinder ball head—had only a 50-centimeter safety gap from the shield segments. The slightest movement could cause severe scraping. The project team cleverly utilized the oscillating characteristic of the cylinder ball head, precisely adjusted the shield machines posture, and managed to create a precious operating space, achieving a millimeter-level safety clearance for equipment disassembly. Ultimately, the old cylinder was successfully detached with a perilous gap of only 5 millimeters.

With the old cylinder removed, installing the new one was an even greater test of precision. After the new cylinder was precisely positioned using three locators, the team immediately secured it to the shield body with flanges. At this point, controlling the gap between the cylinder and the flange surface became paramount. Technicians meticulously adjusted the erectors angle in increments of 1 degree, strictly implementing a triple-check process: preliminary measurement with a vernier caliper, precise calibration with a laser rangefinder, and final verification with a feeler gauge. As the last bolt was tightened into place, the parameters on the monitor finally returned to normal. Wu Jichang, who had been on edge throughout, said excitedly, With the new cylinder replaced, the shield machines thrust performance has greatly improved. Completing the remaining construction tasks wont be a problem!

Initiate trial advance! The command was given. With a deep rumble, the shield machine slowly started up again. The flashing green numbers on the monitor screen announced that the thrust cylinder pressure was normal. At that moment, relieved smiles spread across everyones faces. This nerve-wracking heart surgery had finally been successfully completed.

Content provided by: Liang Feiyan

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