In the golden autumn season, the Yangtze River flows endlessly past Yichang in Hubei Province. In the waters of the Gezhouba Three Gorges navigation channel, construction vessels have moved into position one after another, with equipment aimed directly at the water surface. Before long, two "waterborne great walls" will rise here, enclosing a stretch of dry ground within the surging river to lay the foundation for new ship locks.

The Yangtze golden waterway runs from east to west and connects rivers to the sea, serving as a key support for the high-quality development of the Yangtze River Economic Belt. With rapid economic and social development, demand for shipping has continued to climb. In 2011, cargo throughput at the Three Gorges ship locks exceeded 100 million tonnes for the first time, reaching designed capacity 19 years ahead of schedule. Annual throughput at the downstream Gezhouba ship locks also grew from several million tonnes in the early years of operation to more than 170 million tonnes. Behind the efficient operation of the ship locks lies the strong pulse of China's large-scale logistics and sustained economic improvement.

To better unlock the benefits of the Yangtze golden waterway, the state made a major decision to build a new Three Gorges water transport corridor. As the first national landmark project to begin construction during the 15th Five-Year Plan period, the new Three Gorges water transport corridor project started construction in June this year and consists of two parts: a new Three Gorges hub corridor and the Gezhouba shipping capacity expansion. The Gezhouba shipping expansion, which has now begun cofferdam construction, will "demolish the small and build big" and "demolish one and build two" for the No. 3 ship lock that has operated for more than 40 years, constructing two new single-stage ship locks on the original site. The effective chamber dimensions are 280 meters long and 40 meters wide, with a minimum water depth of 8 meters over the sill, capable of accommodating 10,000-tonne vessels.

For the new ship locks to rise from the riverbed, the first step is to build cofferdams. On October 10, cofferdam construction officially began upstream and downstream for the Gezhouba shipping expansion of the new Three Gorges water transport corridor project. The photo shows the excavation site of the upstream cofferdam project. Photographer: Tang Wei.

Enclosing a patch of dry ground in the river

What is a cofferdam? Simply put, it is a temporary "wall in the water" built to create construction conditions. As navigation structures, ship locks must take root in the riverbed and be poured with huge amounts of concrete, but the river never stops flowing, so how can construction proceed? The answer is the cofferdam. Builders first erect cofferdams around the lock construction area using earth and rock, concrete, steel pipe piles and other materials to separate the work zone from the river. Then they start pumps to drain the water inside the cofferdam, exposing the riverbed and creating a dry environment for construction operations.

"The cofferdam is both the 'advance guard' of the main project and a 'safety barrier.' Without the cofferdam, subsequent construction could not even begin, and the quality, safety and schedule of the entire project depend on it," said Sun Zhifeng, a senior professional at Three Gorges Group's Three Gorges New Corridor Company and director of the Gezhouba Capacity Expansion Project Management Department.

Although the cofferdam is a "temporary project," its standards are not low. The Gezhouba shipping expansion requires two cofferdams, one upstream and one downstream. The upstream cofferdam starts at the Huangbai River mouth dike and ends between the No. 2 ship lock and the Three Gorges sand sluicing gate, with a total axis length of 835.94 meters and a maximum dike height of 28 meters. The downstream cofferdam begins at the right side wall of the No. 2 ship lock and connects to the left slope of the Three Gorges navigation channel, with a total axis length of 960.85 meters. Both cofferdams are built to the highest level of standards, and their diversion design flood standard is consistent with that of the Gezhouba Dam.

"During the water-retaining period, the upstream cofferdam must hold water together with the Gezhouba Dam. We will build this cofferdam to high standards to ensure that when floodwaters arrive in the flood season, the cofferdam remains as steady as Mount Tai," said Yu Wenfei, chief engineer of the upstream cofferdam project department of Gezhouba Group's Gezhouba shipping expansion.

"The downstream cofferdam has a short construction window and concentrated tasks. The main water-retaining structure must be completed during next year's dry season. We will perform efficiently and organize resources reasonably to ensure the overall progress of the main ship lock construction," said Mu Xiangcheng, chief engineer of the downstream cofferdam of the Gezhouba shipping expansion under CCCC Second Harbor Engineering.

Three hurdles to clear when building a cofferdam in the Yangtze

Building a cofferdam in the main stream of the Yangtze beside the "throat" of an operating hub is a task of imaginable difficulty. Builders must clear at least three hurdles: holding back water, keeping navigation open and safeguarding the green environment. On October 10, cofferdam construction officially began upstream and downstream for the Gezhouba shipping expansion of the new Three Gorges water transport corridor project. The photo shows the excavation site of the downstream cofferdam project. Photographer: Chen Chen.

The first hurdle, holding back water, tests the quality of the cofferdam itself. The riverbed is not hard, intact rock but more like a "sandwich": the top layer is newly deposited soft mud, below that are silty fine sand and soft soil, and further down are sand, gravel, pebbles and bedrock. The bedrock also contains weak, slippery thin layers that dip toward the foundation pit. Building a cofferdam on such ground makes it very easy for water to seep in through gaps in the soil and rock and through interlayers in the rock formations. How is this solved? Builders first "clean the bottom," carefully removing more than 500,000 cubic meters of mud and soft soil beneath the cofferdam foundation. Then they "build the body," using more than 1 million cubic meters of earth and rock for layered filling and compaction. Most critically, they "seal the seams," pouring a plastic concrete cut-off wall that extends deep into the bedrock and covers a total area of more than 20,000 square meters, like an invisible "water-stop wall" built underground. At the bottom of the wall, cement grout is injected into rock fissures, and high-pressure jet grouting is used to treat joints, cutting off seepage channels one by one, much like putting a tight "raincoat" on the foundation pit. Day and night, water is pumped and drained to keep the pit dry.

The second hurdle, keeping navigation open, tests the coordination between construction and shipping. During construction, the Gezhouba No. 2 ship lock will remain in operation. The construction site is adjacent to the navigation channel, and construction vessels and passing ships operate in the same waters, so how can both construction and navigation be ensured? The Measures for the Administration of Navigation at the Three Gorges-Gezhouba Water Control Project During the Construction Period of the New Three Gorges Water Transport Corridor, issued by the Yangtze River Navigation Administration of the Ministry of Transport, specify that the lower approach channel of the Three Gorges will adopt "half-width excavation and half-width navigation" - half of the water surface is reserved for engineering work and the other half for ship passage. The navigable half will be subject to one-way control, with vessels entering and leaving in order and not overtaking one another.

Once ships gather at the port, the Yangtze River Navigation Administration coordinates and guides them through a mechanism of "source management, total volume control, remote dispatch, orderly release, information disclosure and supervision according to law," and reasonably controls the number of ships put through the locks according to navigation conditions at different construction stages, smoothing peaks and ordering queues to improve lock passage efficiency. Dispatch is further supported by a "smart brain" - the Three Gorges navigation large model integrates dynamic data on locks, ships, hydrology and meteorology to accurately calculate lock passage times for each vessel, produce an "arrival timetable," and set up "virtual traffic lights" on the river to guide passing ships in scientifically adjusting speed and avoiding one another in an orderly way, minimizing the impact of construction on navigation.

The third hurdle, safeguarding the green environment, tests the project's responsibility to ecology and people's livelihoods. The construction site is close to the main stream of the Yangtze and the urban area of Yichang, and the construction team insists on protecting the environment while building. All mud cleared from the cofferdam is loaded onto vessels and taken ashore to designated spoil grounds rather than discharged directly into the river. Ship oily wastewater and construction waste slurry are collected and treated, production wastewater is recycled, and domestic sewage is discharged after meeting standards to ensure no pollution of the river. On-site management is equally meticulous: enclosure, hardening, covering, washing, watering and cleaning are all carried out properly, fog cannons spray as excavation proceeds, and spoil trucks are washed before hitting the road. No high-decibel work is arranged at noon or at night, and sound barriers are set up near streets and alleys. Spoil and raw materials are transported by water as much as possible, replacing city-crossing road transport to reduce dust and noise disturbance to residents.

On the Yangtze, machines rumble. Once the cofferdams close in the river, a stretch of dry ground will support the foundation of the new ship locks. After the project is completed, the Gezhouba hub will form a four-line ship lock layout, with annual passage capacity raised to 360 million tonnes, allowing the golden waterway to carry more vessels and better connect rivers to the sea.