On the 2.1-kilometer-long section tunnel, the shield machine overcame a series of challenges in succession: crossing rock harder than steel plate, passing beneath a subway station, passing beneath Moshui Lake, and passing alongside the Second Ring Road elevated highway.

On May 26, a Changjiang Daily reporter visited the under-construction Line 12, the longest under-construction subway loop line in the country. The left-line section between Dingjiazui Station and Shilipu Station had just been completed, and the shield machine's cutterhead had broken through. This shield machine set out from Gangkou Village Station and successively completed the tunneling tasks for the Gang-Ding and Ding-Shi sections.

The tunnel from Gangkou Village Station to Dingjiazui Station to Shilipu Station is 2.1 kilometers long. Ni Zhengmao, a first-level project manager of Wuhan Metro, introduced that multiple technologies explored and summarized during the construction of this tunnel section were the first of their kind in the country, representing substantial technical value and providing a good reference for the subsequent construction of Wuhan Metro.

Shield Machine Armed with "Teeth"

Biting Through Hard Rock "Like Cutting Iron"

The segment assembly in the hard-rock tunnel section on the right line between Dingjiazui Station and Shilipu Station of Metro Line 12 is smooth and of high quality.

On the 26th, the reporter entered the 20-meter-deep underground tunnel and saw that some of the originally sharp "teeth" — the disc cutters on the shield machine's cutterhead that had broken through — were already one-third worn, because during tunneling it encountered rock as hard as steel plate.

"When crossing the Gang-Ding and Ding-Shi sections, we encountered two stretches of hard rock with strength seven to eight times that of commonly used concrete. This can be said to be the hardest stratum encountered since the construction of Wuhan Metro began." According to the introduction, the conventional approach when encountering such strata was previously to use explosive blasting followed by manual tunnel excavation. However, the hard rock is located by the shore of Moshui Lake, beneath the Fangcao Road Bridge, and next to a school. The vibration and noise generated by blasting would have a relatively large impact on the lake ecology, school education, and residents' lives, so this approach was abandoned.

To this end, Wuhan Metro Group, Wuhan Rail Transit Line 12 Construction and Operation Co., Ltd., the Engineering Command of China Railway Investment Group, and China Railway Seventh Group dispatched elite personnel to form an expert group and decided to adopt the full-face hard-rock shield tunneling method — that is, letting the shield machine directly bite through the hard rock.

"We armed the shield machine to the teeth and carried out reinforced modifications, equipping it with full-face disc cutters whose blade compressive strength reaches 2,800 MPa, achieving 'cutting iron like mud.'" Wen Bin, project manager of the construction unit China Railway Seventh Group, introduced that in the face of solid hard rock, motors were added to match the shield machine with more powerful driving force.

"During tunneling, the hardest 'bone' appeared after the shield machine had advanced 9 meters into the hard rock of the Gang-Ding section; at that time, it took nearly 60 hours of continuous tunneling to advance 1 meter." According to the introduction, 13 heavy-duty disc cutters were replaced at that time, and with each rotation of the cutterhead the shield machine advanced less than 1 millimeter. The owner, project manager, and chief project engineer took turns on duty, closely monitoring the shield tunneling parameters, organizing personnel to enter the chamber at scheduled times to measure cutter wear, and ensuring that the shield machine's cutterhead and cutter structure remained intact. Ultimately, the hard-rock section was safely and smoothly passed 30 days ahead of schedule, creating a new domestic case of successful earth-pressure-balance shield tunneling through extremely hard strata.

According to the introduction, in conjunction with the construction of this section, the project department has established a research project titled "Research on Construction Technology for Earth-Pressure-Balance Shield Tunneling Through Moderately Weathered Extremely Hard Rock Beneath a Lake," and multiple papers and construction methods have been adopted.

Successfully Passing Beneath the Station

Intelligent Robots Providing Support

Construction personnel inspecting segment assembly quality inside the tunnel.

Shilipu Station is an interchange station between Line 12 and Line 4, with the two lines crossing in a cross shape. The Line 12 section passes 2.1 meters below the base slab of the Line 4 station.

The original design adopted manual mined tunnel construction. However, considering the relatively high risk of long-distance mined soft-soil tunnels, Wuhan Metro Group took the lead in organizing well-known domestic experts in geology, shield equipment, construction, and tunnel emergencies. After repeated consultation and demonstration, it was decided to adopt the scheme of using a shield machine to directly pass beneath the existing Line 4 station.

According to the introduction, before passing beneath, the two key parameters of thrust and earth chamber pressure were calculated based on stratum parameters and were basically consistent with the opinions of consulting experts. On the basis of manual monitoring, intelligent robots were used in the track running area of Shilipu Station on Line 4 for intelligent monitoring.

To avoid disturbance to Line 4 operations from the vibration of shield tunneling beneath, the shield machine tunneled during the maintenance window from 0:00 to 4:30 a.m. During tunneling, the intelligent robots scanned throughout the entire process and submitted a monitoring result every half hour. The shield machine adjusted tunneling parameters according to the monitoring results and smoothly passed through Line 4 in 5 days, with no abnormalities in the station structure. This created a precedent for a Wuhan Metro shield machine completely passing beneath a single station and provided a safer and more efficient reference method for subsequent line construction.

350-Ton Shield Machine

Successively Crossing Lake and Bridge

On May 26, the left-line tunnel between Dingjiazui Station and Shilipu Station of Metro Line 12 was smoothly completed, and the shield machine emerged.

This shield machine weighs more than 350 tons. When crossing Moshui Lake, because the strata were mostly relatively soft clay layers, according to the introduction, the shield tunneling industry has a vivid metaphor for this: "boring a hole in soft tofu."

To reduce risk, before passing beneath Moshui Lake, China Railway Seventh Group lowered the section alignment by 1.5 meters according to geological conditions to increase the thickness of the impermeable layer above the tunnel. The shield machine was stopped before entering the lake bottom, and special inspections were carried out on the screw conveyor gate and tail brushes to ensure complete shield sealing and ensure continuous and uniform tunneling through the lake. The entire process was monitored, and the lake was crossed safely.

When the shield machine passed alongside the Second Ring Road elevated bridge, the minimum horizontal distance from the bridge piles was only 1.8 meters. According to the introduction, during tunneling, manual repeated measurements of the tunnel centerline were organized to recheck the shield tunneling alignment and attitude. Various types of monitoring points were installed on the bridge abutments, piers, ground surface, and soil mass. Engineers worked alongside the machine and corrected the shield attitude and tunneling parameters according to monitoring conditions. Through 30 days of continuous tunneling, the side-crossing bridge construction was completed with nearly zero settlement.