Wuhan is the most populous city in Central China and a transportation hub for the wider area. But with a population just shy of 10 million people, it falls short of true megacity status, and a long way behind the country’s top-tier metropolitan areas of Shanghai and Beijing. With infrastructure investment over five years totalling approximately USD 300bn however, it is looking to reshape both the political landscape, and its own environment.

The city’s centre is being ‘reimagined’ – in typically bold Chinese style – with a colossal programme of demolition and reconstruction. All this is being financed through the state taking on a massive debt burden, and the explosion of the Chinese credit sector from a total value of USD 10tn in 2008 to USD 25tn in 2014 (a size increase equal to the entire US commercial banking sector). It has been an attempt to mitigate the effects of the global financial crisis that has seen investment soar to 50 per cent of GDP.

But economic rationale and impacts aside, the result is a windfall to engineers. A high tech commercial city centre is being swiftly built along with the associated skyscrapers, a second airport, rail routes, road networks, and an entire metro system.

River Obstacle
The Yangtze River, the longest in Asia, has been a hazardous natural obstacle for locals for thousands of years.

Splitting Wuhan in half, its span is approximately 1km at its narrowest point inside the city. While traditionally crossed by ferries, several permanent crossings, bridges and tunnels, now exist.

And now a new crossing is to be added. A multipurpose tunnel combining road and rail transport will be built as part of Wuhan’s Metro Line Seven, and Sanyang Road, located in the city centre.

In August 2013 the National Development and Reform Commission approved the feasibility study for the first phase of Line Seven. This phase includes the Sangyang Road tunnel, which will run as a twin tunnel with tubes 2.5km long and an excavation diameter of 15.76m. The large diameter allows for an upper deck with a three-lane road, and a lower deck for the metro service, as well as safety and other electrical and technical networks.

It is the fourth rail line, after Lines Two, Four, and Eight, to run across the Yangtze River. The route starts at the Oriental Lucky City in Hankou, passes Fazhan Avenue, Jianshe Avenue, Aomen Road, Sanyang Road, the Yangtze River, Qinyuan Road, Youyi Avenue, Zhongshan Avenue, and Lizhi Road, and terminates at Yezhi Lake in Wuchang. The entire length of Line Seven will be 30.9km, with 19 stations.

Analysis has predicted that of 640,000 journeys per day, some 250,000 will cross the river through the tunnel.

Crossing the river
The contract was awarded to Shanghai Tunnel and Engineering Company (STEC), a soft soil tunnelling firm, which began construction in 2014 and expects to be finished on schedule in 2017. The tunnel alignment passes through complex, varying geology consisting of sand and clay in places, and elsewhere a mixture of mudstone, conglomerates and sand. The tunnel will pass 39.5m beneath the river, and water pressure up to 5.3 bar is expected.

Around seven years ago, nearby road tunnel bores by 11.38m-diameter NFM slurry TBMs experienced pressures up to 7 bar through silt and sand with clay and large boulders. Daily progress of 15-20m was achieved.

Excavation this time will be carried out by two 15.76m Herrenknecht Mixshield TBMs, STEC having previously constructed large-diameter road tunnels in Hangzhou and Shanghai with the German manufacturer’s equipment. At Hangzhou a single 15.43m-diameter Mixshield made two passes under the Qiantang River for a 3.25km total distance at a depth of 27.6m. And at Shanghai a 14.9m Mixshield bored 3.4km at 59m below the Huangpu River.

The machines this time will have cutterhead power of 5,600kW, and nominal torque of 27,720kNm. The cutterhead will have 80 single cutting discs that are exchangeable under atmospheric pressure, and 52 scraper that are also exchangeable under atmospheric pressure. This is to deliver time and cost-saving access to the excavation chamber for maintenance work under pressurised conditions.

Lining the Link
The tunnel will be lined with precast segmental ring, forming an internal diameter of 13.9m, and an external diameter of 15.2m. The thickness of the segments is 650mm and they have a width of 2m. Each ring will be composed of 10 segments, comprising seven standard, two counter-key, and one key segment. The 30 static moulds were provided by CBE, which designed them in France and manufactured them in Yangling, China to reduce transportation costs.

CBE general director, Pascal Clerc added, "The complete mould design is based on the technical specifications and tolerances defined by our customer – a local precast company, as is often the case in China – and are confidential. But regarding the design and fabrication process, there really is no difference between this project and other large diameter tunnels, such as the Alaskan Way project, which we also supplied. In addition, the multi-function nature has no influence on segment or mould design. The ring is as it would be in any other ‘classic’ tunnel.

"However, every project is unique and we do individual pre-project studies regarding the specific design, taking into consideration dimensions, accessories and equipment such as sockets, shear cones, guiding rods etc. We pay special attention to the quantity of vibrators and their distribution at every project."

The local precast company uses a vacuum system to demould the 28,000 segments required by the project, which are then placed in a water basin during the later curing process. Gaskets are traditionally glued, as cast-in-place technology is not yet applied in China.