How does a tunnel excavator work? The working principle of a tunnel excavator (mainly a shield machine) is to achieve full-process automated construction of stratum cutting, slag transportation, and tunnel support through the coordinated operation of multiple systems such as machinery, hydraulics, and sensors. Its core process can be divided into the following five modules.

  I. Working principle of the five modules of a tunnel excavator

   1. Cutter cutting system

   ① Composite cutter design: The cutter disc with a diameter of 3 to 18 meters is equipped with a roller cutter (hard rock), a cutter (soft soil), and a scraper (clay). For example, the Shanghai Airport Link Project uses a cutter disc with a diameter of 14.1 meters, 432 cutters are installed, and the single cutter can bear a pressure of 35 tons.

   ② Intelligent speed control drive: The variable frequency motor drives the cutter disc to rotate at 0.5 to 3 rpm. The torque reaches 58,000 kN·m when the Qinling Tunnel is excavated in hard rock, and the power consumption accounts for 60% of the whole machine.

   2. Propulsion and attitude control system

   ① Hydraulic jack cluster: 32~40 groups of jacks are distributed along the circumference, and the thrust of a single cylinder is 350~500 tons. The total thrust of the shield machine propulsion system of Guangzhou Metro Line 18 reaches 120,000 tons, which is equivalent to 12 times the thrust of the Long March 5 rocket.

   ② Laser guidance system: The shield attitude is measured 10 times per second, and the axis accuracy of ±10mm is achieved in the Shenzhen Chunfeng Tunnel project, which is finer than the diameter of a hair.

   3. Slag treatment system

   ① Screw conveyor: The screw shaft with a diameter of 900~1200mm discharges slag at a speed of 5~15rpm. The daily slag discharge volume of Wuhan Sanyang Road Yangtze River Tunnel reaches 3000m³.

   ② Mud and water circulation system (mud and water shield): The mud pump maintains the pressure balance of the excavation chamber at a pressure of 10MPa. The mud specific gravity is accurately controlled at 1.2~1.3g/cm³ during the construction of Nanjing Yangtze River Tunnel.

   4. Synchronous support system

   ① Segment assembly machine: The 6-DOF robotic arm achieves millimeter-level positioning of 1.5-ton segments, and the assembly speed of Chengdu Metro Line 19 reaches 8 minutes/ring.

   ② Synchronous grouting technology: The double-liquid grouting system completes the filling within 3m behind the shield tail, and the grouting pressure of Beijing Metro Line 17 is stable at 0.3~0.5MPa.

   5. Environmental control system

   ① Gas monitoring module: Equipped with methane and oxygen sensors, 0.1% concentration level monitoring was achieved during the construction of Chongqing Metro Line 6.

   ② Earth pressure balance system: By adjusting the speed of the screw machine to control the amount of soil discharge, when Shenzhen Metro Line 14 passed through the water-rich sand layer, the warehouse pressure fluctuation was <0.02MPa.

   II. The whole work process (taking the earth pressure balance shield as an example)

   1. Cutter head excavation: The cutter head rotates to cut the soil layer, and the cutting efficiency can reach 80m³/h in clay and drop to 15m³/h in granite.

   2. Slag improvement: inject foaming agent into the excavation bin (volume ratio of bentonite to foaming agent is 1:20), and the plasticity of the slag flow is improved to a slump of 180~220mm.

   3. Pressure balance: the screw conveyor gate opening adjusts the discharge volume to maintain the excavation bin pressure at 0.1~0.3MPa (equivalent to 1.5 times the hydrostatic pressure).

   4. Segment assembly: 6 prefabricated segments (C50 concrete) form a lining ring with a diameter of 6.7m, and the staggered assembly accuracy is less than 2mm.

   5. Synchronous grouting: grouting is completed within 5 rings after the shield tail, the initial setting time of the slurry is controlled at 6~8 hours, and the filling rate is ≥95%.

   III. Response to special working conditions

   1. Water-rich sand layer: use high molecular polymer to improve slag, and the water seepage of Guangzhou Metro Line 22 when crossing the Pearl River is less than 0.5L/s.

   2. Soft upper and hard lower strata: Qingdao Metro Line 1 uses a mixed layout of roller cutters and cutters, reducing tool loss by 40%.

   3. Small radius turning: Hangzhou Airport Express Line project achieves a 250m turning radius construction, and the deflection angle of the articulated system reaches 2.1°.

   The coordinated operation of these precision systems enables modern tunnel boring machines to advance 20 to 40 meters per day under complex geological conditions, which is 5 to 8 times more efficient than the traditional drilling and blasting method, and the surface settlement is controlled within 3mm. China's independently developed shield machine has achieved 100% localization of key components, and the life of core components has exceeded 30 kilometers, reaching the international leading level.