It is everybody’s first time in the mad dash to complete an undersea tunnel off the southern coast of China. A vital water link is needed to regenerate the largely vacant island and boost the region’s economy. East Sea Island will eventually host a massive steelworks and accommodate the 20,000 workers expected to flock to the island. The island lies in Zhanjiang Bay, which is home to China’s first modern port, the enormous 39 wharf dock.

It is the first time the tunnelling crew crossing Zhanjiang Bay has used a slurry TBM. And the first time the machine’s manufacturer has ever built one. Tunnelling crews are racing to meet a tight construction schedule with breakthrough penned for this month.

The tunnel will carry water south from Wuchuan, located north of Zhanjiang, to feed the steel works being constructed on East Sea Island. The entire water transfer scheme is 26.4km long. The tunnel is needed to cross Zhanjiang bay and is being driven from East Sea Island, northwards 2.7km.

The designer specified a minimum overburden of 22m. This means the ideal alignment sees the tunnel launching from a shaft that ends a little above the rock layer. Then drops on a steep gradient of around four per cent under the bay to a total depth of some 60m. It then rises almost equally as sharply and ends in a reception shaft excavated on Nanshan Island.

The geology is made up of stayed clays, middle coarse sand, gravelly sand and fine silty sand. The sand is quartz.

Shaft excavation
The launch shaft on East Sea Island is cut 26.5m deep and the reception shaft on Nanshan Island reaches 28.5m. The bottom of each shaft is at the same depth below sea level. There were two limiting factors on the shaft depths, the height of the rock layer, which would have been costly to excavate through and the water pressure.

David Han is project manager for US-based TBM manufacturer Robbins. He explains that the shaft depth was limited to ensure the safety of the TBM launch. As the ground has a high water pressure the TBM must be launch through a seal in the shaft wall. The deeper the shaft, the higher the pressure, and the greater the stress on that seal.

Excavation on the launch shaft started in December 2010. It was completed in July 2011. The retrieval shaft started construction in August 2011 and was due for completion in August this year. Shi Quinghua, chief engineer for contractor Guangdong Hydropower Engineering explains that the reception shaft is founded in a much less stable geology, which slowed construction. The launched shaft is excavated in stayed clay while the reception shaft, to the north, is excavated in an unstable sand.

Piles were driven to beyond the full depth of the shaft. Excavation then began and as the shaft was excavated a secondary lining was cast for ¬ nal support. The initial piles on the launch shaft were driven 37m deep for a finished shaft depth of 26.5m. In the poorer geology on the north side the piles had to be driven 52m deep for a finished shaft depth of 28.5m. The secondary lining was installed as the shaft was excavated within the piled ring. The piles and the secondary lining are made of reinforced concrete.

The machine
As this is the first slurry machine Robbins has manufactured, the company partnered with Mitsubishi Heavy Industries Mechatronics Systems (MHI-MS) of Japan, which supplied the slurry system.

The contractor chose a slurry TBM for the project because, project manager Liang Mindong says, the system is safer in the sandy ground conditions, where an EBPM could cause a collapse of the ground above by removing soil faster than the machine advances. A sinkhole would be a major problem for the project as it could lead to the flooding of the machine.

The slurry also offers the cutterhead some protection against the abrasive quartz sands. Muck removal is simplified using a slurry system in the tight launch shaft as all of the earth can be shifted through the slurry pipes.

The machine has an outside diameter of 6.26m and a shield thickness of 45mm. Three rows of wire brushes are fitted to the tale of the machine to prevent the six bar of pressure escaping down the side. The annulus space is filled with a single component liquid grout. The machine has 20 sets of thrust cylinders giving a thrust of 46,000kN. The high power is needed to overcome the high pressures at the face. It has a working torque of 5,442kNm and a maximum torque of 5,896kNm. The cutter head is a spoke type with plates and a 30 per cent opening ratio.

The cutter tools include discs, knife edge bits and cutter bits.

The machine is fitted with a two chamber man lock in case crews need to access the cutterhead. So far the project has not needed any interventions.

The launch
The TBM was delivered on 1 August 2011. The machine was originally intended to be delivered in February but at that point there was no road to site that could take the large trucks needed to haul the machine components. The machine was assembled, tested and on 17 November last year was launched.

The machine was lowered into the shaft and seated on a steel cradle for launch. A launch frame was constructed that moved forward with the TBM as it pushed into the ground. This allowed the contractor to keep the congested launch shaft free of segments until the machine was through the shaft lining. According to Liang, this is the first time the method has been used in China.

Chief engineer Shi explains that the high surrounding water pressure meant that is was impossible to totally seal the shaft. He explained that: "Some water is entering the shaft but it is at an acceptable level." The shaft structure was successful in preventing water ingress but when the TBM launched through the side of the shaft the seal was unable to completely hold back the water. A small, 3kW pump is in place to remove water from the bottom of the shaft, while crews work on a mesh platform a few meters above the floor.

To control the water during the launch of the TBM, a 25m wide, 12m deep grout injected block was cast from the surface. The grout block was the initial 12m of ground that the TBM cut through on breaching the shaft.

The lining
The tunnel is segmentally lined using 1.5m long, 450mm thick segments in a five plus key arrangement. The thicker-than-usual segments were needed to cope with the high pressures. The lining has an outer diameter of 6,000mm and an inner diameter of 5,100mm. Each segment is cast on site and then allowed to cure in a cooling pool before being stacked, coated in waterproofing, lowered down the shaft, drawn through tunnel, erected and bolted into place.

The hydraulic design institute that prepared the designs for the tunnel prepared a very robust speci-fication, building in many redundancies according to the contractor. For example, the rubber gasket between the segments that is placed to hold back the high water pressures is supported by a second sealant that is cast in the joint once the segments are in place.

This rubber paste sets in the joint in case the primary sealant fails. A pump system is being installed in the tunnel upon completion to cope with any water ingress that occurs during the operation of the tunnel.

The drive
After launch the contractor took a slow and steady approach to excavation so the crew could get accustomed to the machine and its behaviour. An average of four rings a day, or 6m per day, was achieved initially. But this ramped up to four to six rings per 12-hour shift, or 12 to 18m per day, once through the launch process.

The contractor is on a tight schedule so has to press ahead 24-hours a days and avoid as much downtime as possible. Maintenance is limited as much as possible with the aid of wear detection on the cutter discs and some cutterhead components. The crew typically needs to shut down the machine once a week.

The machine can progress four rings before excavation work stops to allow slurry lines and other support lines to be extended. It takes the operators about eight hours to extend the lines. Gate valves positioned along the slurry line allow a section to be parted to insert the extension piece.

A bentonite slurry was used during the launching of the machine as it cut through the shaft wall and grout block. Beyond launch no additives are typically used, water is sent to the face and the mud slurry is pumped back, loaded into trucks and the truck dump the grey coloured slurry on the land surrounding the site.

In the high summer temperatures the slurry quickly dries and the land surrounding the steel works is changed from a vibrant red and orange to a cement like grey.

Along the route the ground has occasionally become sticky and risks clogging the system so the contractor adds sodium carbonate to help reduce the viscosity.

With no one above other than boats, subsidence is not a big concern for the project. Nevertheless, the material being excavated is measured against the advance of the machine to ensure that no over excavation is occurring. However, a sinkhole would be a major problem for the project as it could lead to the flooding of the machine.

There is no requirement for probing or pre grouting. The machine is fitted with a boulder-collecting gate to deal with any boulders that are encountered on the drive.

The TBM is due to breakthrough this month and the tunnel would become operational by the end of the year