On July 21, after 210 days, the 33-meter-deep Shuangdun Station foundation pit was successfully sealed at the bottom, marking another major construction milestone for Wuhan Metro's first loop line, Line 12.

How to carry out construction safely in a "water bag"

On that day, the reporter went deep into the bottom of the station foundation pit, equivalent to an 11-story building, where construction workers were pouring the last batch of concrete. After a month of continuous construction, this massive structure had "swallowed" nearly 7,000 cubic meters of concrete and used 1,500 tons of rebar; next, it will fully enter the main structure construction phase.

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Construction site of the Shuangdun Station foundation pit on Line 12

Sealing the bottom of a station is generally an ordinary construction step, so why is the bottom sealing of Shuangdun Station so important?

Wang Bin, a Level-2 project manager of Wuhan Metro Group, explained that Shuangdun Station is designed as a three-level underground station, 182 meters long and 23.9 meters wide. At Shuangdun Station, Line 12 and Line 3 will be able to interchange, and Line 12 must pass beneath the Line 3 station, so the station is designed to be relatively deep, with a maximum depth of 33 meters, making it the deepest station currently under construction in the Wuhan Metro.

"The station is about 1.5 kilometers from the Han River, and the foundation pit is located on the first terrace of the Yangtze River, in a typical water-rich silty fine sand layer," he said. Affected by Wuhan's flood season this year, the Yangtze River water level has continued to rise, groundwater is abundant, and the water level fluctuates greatly. The area outside the foundation pit is connected to the water of the Han River, and the water levels remain consistent, which is equivalent to carrying out construction in a "water bag."

"If we imagine the station as a rectangular box, under these geological conditions, the outside of the 'box' is all water," Wang Bin said simply and clearly. To build this "box," the first step is to build the four sides, namely the diaphragm walls. The diaphragm walls of the Shuangdun Station foundation pit are 1.2 meters thick and form an enclosure support system 56 meters deep. According to calculations, this is equivalent to construction under 27 meters of water pressure, which not only increases the difficulty of controlling foundation pit deformation but also easily leads to risks of seepage, water inrush, and sand inrush.

To control risks, the project department made controlling the water level difference inside and outside the pit a key focus of construction. "Shuangdun Station has 24 relief wells, with 17 water level monitoring points inside the pit and 24 water level monitoring points outside the pit, with measurements taken at least three times a day," introduced Zhang Duo, project manager of the Shuangdun Station project on Wuhan Metro Line 12 for the contractor China Railway No. 4 Engineering Group. After each measurement, the owner's representative, supervisor, and contractor jointly review the measurement results to identify and handle problems in a timely manner and ensure the foundation pit remains safe and controllable.

"We adhere to the principles of 'support first, then excavate; excavate in layers and sections; and strictly prohibit over-excavation,' strictly control the depth of layered excavation, and install supports within a limited time to reduce the risk of foundation pit deformation," Zhang Duo introduced. At the same time, professional teams are arranged to probe and excavate the diaphragm wall joints, seal them with steel plates, and grout them to ensure the safety of the diaphragm wall joints.

Successful bottom sealing means that the bottom of the station, this "box in the water," is closed off, reducing the safety risks to the station foundation pit during the flood season.

Emergency team on 24-hour standby

At the Shuangdun Station excavation site, there is also an emergency team on duty in 24-hour shifts to ensure the safety of foundation pit excavation.

In response to the flood season, two professional emergency teams for Wuhan Metro Line 12 follow the principle of "going wherever the construction site has the greatest safety risk to stand by." On July 21, at Shuangdun Station on Line 12, a Changjiang Daily reporter saw that the site was equipped with facilities and materials such as an integrated hole-guiding and grouting machine, mixing drum, grouting pump, cables, water pumps, and flood-control sandbags.

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At the bottom of the Shuangdun Station foundation pit on Line 12, the last batch of concrete is being poured.

Qian Pengjia, squad leader of the emergency unit, introduced that once an emergency occurs on site, all operations—from connecting cables, driving the drilling rig to the designated position, connecting water pipes, and starting the grouting machine—must be completed in one continuous flow. "The deepest excavation at Shuangdun Station is 33 meters, and the drilling rig can quickly drill down to 40 meters, fully meeting the needs of grouting reinforcement."

Ding Song, an engineer from the Quality and Safety Supervision Department of Wuhan Metro Group, introduced that in the face of the flood season, once excavation of the foundation pit begins, the exposure time of the foundation pit must be minimized, and compact and continuous construction must be organized—without stopping work or rushing work—so as to effectively avoid the occurrence of various risk events. Accordingly, at the project department of Shuangdun Station on Line 12, two teams are arranged, and during peak periods three vertical soil-grabbing machines, six excavators, and other large excavation machinery operate simultaneously, ensuring the safety and progress of foundation pit construction.