It is May, and the river city of Wuhan is full of spring.

　　Amid blooming flowers, Wuhan's urban rail transit construction is in full swing. According to local media reports in Wuhan, this year, a total of 13 metro lines are under construction in Wuhan. Among them, Phase I of Line 6 and the Airport Line will be completed and put into trial operation this year; Phase II of Line 8 and the Wuchang section of Line 11 will newly start construction; and 9 lines including Phase I of Line 7 will continue to be advanced.

　　Wuhan residents are very proud, because Wuhan already has 126 kilometers of metro lines in operation and 260 kilometers of metro lines under construction. Wuhan strives to build and operate 400 kilometers of metro lines by 2020. To this end, starting this year, Wuhan aims to open two metro lines within one year.

　　They do not know that to build these metros, several construction forces are fighting day and night underground in Wuhan, overcoming one problem after another.

　　Minimum clear distance between two tunnels: 1.315 meters

　　In March 2014, Section 13 of Wuhan Rail Transit Line 6, undertaken by the Second Company of China Railway No.3 Engineering Group Co., Ltd., officially started construction. The section includes "one station and two intervals," namely: the Jianghan Road Station–Dazhi Road Station interval, Dazhi Road Station, and the Dazhi Road Station–Miaoli Road Station interval. The total length of the double-line tunnels in the two intervals is about 3,681 meters.

　　When Xu Jianfeng saw the design document stating that "beneath the anti-uplift piles of the approach tunnel of the Wuhan Yangtze River Highway Tunnel and the underground historic building complex of the Republic of China period, the Line 6 tunnel crosses over the Line 2 tunnel at a skew angle of 68°, with a minimum clear distance of 1.315 meters," he felt the challenge had come. This is because the disturbance zone of shield machine construction is generally 0–9.3 meters, and the closer the distance, the greater the disturbance. "At 1.315 meters, shield construction is almost at the limit," Xu Jianfeng explained.

　　Since the Line 2 tunnel is already in operation, construction must be carried out during the nighttime operational "window." "We can only work strictly according to accurately calculated parameters. For example, reduce the advance speed and make every effort to maintain a uniform speed; strictly extract soil according to the calculated volume and conduct 24-hour monitoring of the existing Line 2 tunnel," said Zhou Heping, the project chief engineer.

　　Hard work pays off. On April 30, 2016, the station and interval main structures of Section 13 of Phase I of Wuhan Rail Transit Line 6 were successfully completed ahead of schedule, making it the first section in the central urban area where the entire shield tunnel was successfully holed through.

　　700 isolation steel piles obliquely inserted beneath each old building

　　Walking into the old streets of Hankou, a series of European-style buildings standing facing the river come into view: Gothic, Rococo, and Baroque styles. These are the remains of the Hankou Concession from the 1860s to the first half of the 20th century, and they are also witnesses to Wuhan's modern history. The old streets are being enclosed section by section; it turns out that the construction of Metro Line 6 is in full swing. Wuhan Metro Line 6 connects Hanyang and Hankou across the Han River, with a total length of 35.95 km and 27 stations. It is expected to open on December 28, 2016.

　　Section 13 of Metro Line 6 is considered by the owner to be one of the two most difficult sections. "It combines all kinds of difficulties: the most complex construction environment, the most complex station dewatering, the most difficult earthwork transport, the greatest construction organization difficulty, the greatest interval construction difficulty (large slope, small turning radius), the most intervals crossing historic building complexes, and so on," said project manager Xu Jianfeng, listing so many "mosts" in one breath.

　　When constructing beneath historic building complexes, cultural relic protection is the top priority. "Underground construction is least afraid of the tallest buildings; the taller the building, the deeper the pile foundation, and the most disturbance. But the foundations of old buildings are very shallow, especially heritage buildings, where no damage is allowed," Xu Jianfeng said.

　　To resolve the contradiction between construction and cultural relic protection, after full demonstration, the project department pioneered the small-diameter steel pipe isolation pile method. Each pile has a diameter of about 1,300 mm, and 700 are obliquely inserted beneath each old building in advance to reinforce outstanding historic buildings, becoming a successful case of historic building protection in Wuhan urban metro construction.

　　Cover-and-cut top-down construction underground: construction deviation of 64 permanent steel columns only ±5 mm

　　The under-construction Wangjiadun Central Station of Wuhan Metro has a main structure of a three-story underground five-span rectangular structure. Standing on the construction ladder and looking down, at a depth of 26 meters is a huge construction foundation pit, with the widest point at 58.6 meters. The most striking feature is 64 steel pipe columns standing in it.

　　"These steel pipe columns are the load-bearing structure of this metro station, and their installation positioning and construction precision are the biggest technical difficulties of this section," said project manager Wang Xiaoming.

　　In July 2014, Section 4 of Wuhan Rail Transit Line 7, undertaken by the Second Company of China Railway No.3 Engineering Group Co., Ltd., started construction.

　　How difficult is the section? First look at this string of numbers.

　　In addition to the widest point of the foundation pit being 58.6 meters, the deepest point is 26.9 meters; the co-construction section with the Huanghai Road Tunnel at the east end is 233 meters long, with a main structure of a three-story underground four-span rectangular structure, a soil cover thickness above the roof of 3 meters, a foundation pit width of 54.1 meters, and a depth of 26.9 meters.

　　These numbers depict the scale of the future Wangjiadun Central Station, which will become one of the largest central stations in domestic metros; they also show that this section has characteristics such as high groundwater level, poor geological conditions, tight schedule, and high safety risks.

　　To save 20 million yuan in temporary support investment, the station is designed for cover-and-cut top-down construction. That is, for a three-story station, first build the negative first floor, then the negative second floor, then the negative third floor.

　　Going down the construction ladder more than 10 meters and examining these tall steel pipe column piles at the broad station building site, each one is very upright, slender, and smooth. "Because these columns are permanent, we built them to the standard of artworks during construction," Wang Xiaoming said.

　　As permanent structures of the station after completion, the construction and installation precision requirements for the steel pipe columns are extremely strict: first, the allowable deviation of the column centerline and the foundation centerline is ±5 mm; second, the maximum allowable deviation for column non-verticality is no more than 15 mm.

　　Wang Xiaoming introduced that during construction, the project department actively researched and explored new technologies. On the basis of drawing on peers' construction experience and combining the project's own construction characteristics, they communicated with the design institute. Ultimately, technological innovations were made in two aspects: "quick-install integral formwork for the outer wall of the metro station and steel pipe column pile construction." The metro steel pipe column pile construction method won "provincial and enterprise-level construction method" honors and was submitted for a national invention patent.

　　These two technological innovations not only provided a safe construction environment but also improved work efficiency and construction quality, and saved costs for the project department. The successful application of the metro steel pipe pile construction method alone saved the project department more than 7 million yuan in costs and won unanimous praise from the owner and the supervision unit.