Recently, as the cutterhead of the "Yihang No. 12" shield machine rotated out of the tunnel, the left-line shield machine for the Dajian Station to ventilation shaft section (referred to as the "Dafeng Section") of the interval from Dajian Station to Daguhe Museum Station on the Qingdao Metro Line 8 Branch, constructed by the company, was successfully received, marking the smooth achievement of **double-line breakthrough for the long-distance hard-rock shield tunnel interval** of the Line 8 Branch.

The Qingdao Metro Line 8 Branch is a rapid transit corridor connecting the Jiaozhou cluster with the Hongdao Economic Zone in the north-bank urban area, the Airport Economic Zone, and the east-bank urban area. The project starts in the east at Dajian Station, the junction station of the Line 8 main and branch lines, and ends at Jiaozhou Railway Station, with a total length of 20.4 kilometers and 12 stations in total, including 4 transfer stations, all of which are underground stations. The company undertook the construction of 5 metro stations (including the renovation of Dajian Station) and 6 metro intervals from the starting point to Haier Avenue Station (exclusive).

The Dafeng Section has a double-line length of 4,111.5 meters. The tunnel crosses over long distances through slightly weathered rhyolitic tuff, andesite, sandstone, and other rocks, with rock strength reaching up to 133 MPa. The complex geological conditions make it the shield tunnel with the **earliest starting excavation** on the Line 8 Branch. The tunnel needs to pass beneath multiple major environmental risk sources, including the Jinan-Qingdao High-Speed Railway and the operating Line 8. With numerous surrounding structures, construction is difficult and high-risk.

To ensure the smooth long-distance tunneling of the shield machine in the hard-rock interval, the project team organized various parties to hold shield machine selection seminars. By adjusting the blade edge width of the shield disc cutter, selecting a reducer with an extended screw groove cooling system, and increasing the pipe diameter and cooling water pump power, the heat dissipation coefficient was increased by 3.37 times, avoiding cutterhead stoppage caused by excessively high oil temperature during continuous hard-rock tunneling. By increasing the size of the gauge cutters, the problem of shield shell jamming was effectively solved. In addition, the project team conducted detailed survey and analysis of key hydrogeological conditions along the interval, applied advanced geological prediction and other technical means, and continuously optimized tunneling parameters, successfully overcoming difficulties such as long-distance hard-rock construction.

To ensure the shield machine safely and smoothly passes through risk sources, the project team organized repeated investigations of the structural forms of surrounding buildings and structures and the positional relationship between the existing lines and the shield interval, comprehensively and meticulously carried out hazard identification, held special risk analysis meetings, implemented standardized and grid-based checklist management, and adopted targeted control measures such as automated monitoring, 24-hour rotating station inspections by on-site personnel, on-site protection, and surface patrols. Ultimately, the cumulative surface settlement was only 0.22 millimeters, achieving precise underpassing of the Jinan-Qingdao High-Speed Railway and smoothly passing beneath a series of major environmental risk sources, including the Line 8 construction shaft, the depot access line, the operating main line, and the S62 New Airport Expressway.

In terms of tunnel quality control, the project team adhered to the core principle of building a premium project, conducting daily technical analysis of segment elevation, centerline re-measurement, convergence deformation, advancement parameters, and segment assembly quality, promptly reviewing and adjusting in real time. Strict control was exercised over segment entry, waterproofing inspection, assembly acceptance, and simultaneous grouting. The formed tunnel quality is good, with no leakage or deformation in the structure. As a quality benchmark for the Line 8 Branch, this interval also hosted the on-site field observation activity of China's First Shield Engineering Technology Youth Innovation Forum and the Shield Construction Series Group Standards Promulgation Meeting, and was rated as a "Engineering Quality Entity Model" by Qingdao Metro for 2023.

Finally, 18 days after the smooth breakthrough of the right line of the Dafeng Section, the shield tunnel with the **earliest starting excavation** on the Line 8 Branch successfully achieved double-line breakthrough.