On July 22, with the successful detonation of the last digital electronic detonator, the primary support of the mined tunnel on the Huangtian Station–Airport East Station section (hereinafter referred to as the Huangji section) of Shenzhen Metro Line 12 was successfully holed through, creating favorable conditions for the completion of the major milestone of full-line breakthrough.

The Huangji section runs roughly south–north, with a total double-track length of about 2,590 meters and a tunnel depth of 14 to 28 meters. Constructed by the mined method, it is one of the first green and eco-friendly construction sites on Shenzhen Metro Line 12 and the longest and deepest mined tunnel on the entire line. In addition, the tunnel mostly passes through slightly weathered and moderately weathered mixed granite and other hard rock strata of grades II to VI, with hard rock strength reaching 120 MPa at one point, making it the most difficult and largest-volume mined tunnel blasting excavation on Metro Line 12.

There were four major challenges in the mined tunnel construction of the Huangji section:

First, the tunnel passes beneath one of the busiest routes in Bao'an District, known as the "main axis of Bao'an's development"—National Highway 107, the Bao'an Avenue sunken tunnel, and the elevated bridge of Metro Line 1 (all north–south urban expressways with an average daily traffic volume of more than 201,000 vehicles)—imposing strict construction control requirements. Under the guidance and assistance of the Municipal Rail Transit Office, the Traffic Police Brigade, the Metro Group and others, Southern Company and the project department actively organized surveys of the surrounding environment, repeatedly simulated and rehearsed potential difficulties, and continuously studied and optimized the plans. They also established a joint control mechanism with road maintenance departments and road emergency units, forming a closed-loop process control for the occurrence, development and handling of problems, so that any issue found was dealt with immediately, ultimately completing the breakthrough of the Huangji section smoothly.

Second, the tunnel is adjacent to multiple important structures. The inevitable blasting vibration during tunnel construction imposed numerous constraints on blasting operations. In response to the vibration impact of mined tunnel blasting, the project adopted multiple measures, formulated targeted special measures, continuously optimized the construction plan, and repeatedly organized experts to conduct on-site inspections. Closely integrating design, construction and production with production needs, it continuously optimized construction techniques and refined production parameters to ensure safe production. The interval length between the upper and lower benches was increased to reduce stress concentration in cross-section blasting and control blasting vibration. On the basis of using the latest digital electronic detonators for precise blasting control, low-vibration machinery such as hydraulic breakers and splitters was used for excavation in sections with strict vibration requirements, reducing single-cycle blasting advance, reducing explosive charge per blast, controlling blasting vibration, and increasing the frequency of surface settlement monitoring, thereby keeping vibration impact and uneven settlement to a minimum. This effectively ensured that the Huangji section passed smoothly through the hard rock strata and created favorable conditions for the section breakthrough.

Third, the geological conditions of the Huangji tunnel are complex, with great variation in the surrounding rock within the tunnel profile and uneven distribution of hard rock. While extensive hard rock blasting was carried out, there were also "soft upper and hard lower" surrounding rock conditions, with alternating soft and hard rock restricting the mined tunnel excavation of the Huangji section, and blasting exacerbated pore water flow in the tunnel strata, affecting the quality of the finished tunnel. In response to the "soft upper and hard lower" characteristics of the surrounding rock, the project department refined construction parameters, adopting curtain grouting pre-reinforcement on the upper bench for soft rock to form a hard shell at the tunnel crown, and using "small blocks, short advance, quick closure" measures to strengthen backfill grouting behind the primary support. At the same time, in light of the actual conditions of silty fine sand strata and fissure water between rocks in the mined tunnel, retreating grouting with "cement + water glass + phosphoric acid" was adopted to improve the quality of tunnel curtain grouting reinforcement and ensure construction safety.

Fourth, the tunnel is the deepest mined tunnel on the entire Line 12. With a relatively enclosed environment, construction encountered unfavorable factors such as thin air, high temperature and humidity, seriously affecting underground workers during long shifts. To address this, in addition to strengthening ventilation management and adding conventional ventilation facilities, the project department actively added underground cooling circulation systems in response to Shenzhen's long summer high temperatures, and regularly purchased heatstroke prevention and cooling medicines and ice blocks and transported them underground for cooling and heat relief.