For growing cities having tunnels is no longer considered a luxury – they are a necessity as land is a precious commodity. Going underground frees up surface land for other development, and in fact, having metro stations nearby increases the value of land, which could be recouped (if necessary) from higher property assessments and quit rents.
For example, a city the size of Shanghai has 12 metro lines, and the city won’t be stopping underground construction any time soon. Xian, the capital of the Shaanxi province, will have up to six metro lines by 2020 (two are already in operation).
“At any given time, there are around 700 TBMs (Tunnel Boring Machines) running throughout China,” says Li in trying to give us a sense of the “underground movements” that are taking place in that country. Even at the peak of tunnelling in Kuala Lumpur for the Klang Valley mass rapid transit (MRT) project, only eight TBMs ran simultaneously.
Tunnels for speed and efficiency
If the Chinese Government approves the proposed Bohai Strait tunnel, then China will hold the record for the longest undersea tunnel at 122km – 2.5 times longer than the Channel Tunnel.
The proposed Bohai tunnel will run between the northern city of Dalian and Yantai on the east coast, slicing off nearly 1,300km off the current overland route between the two cities.
Straighter high speed rail lines enable faster speeds. Straighter lines require more tunneling.
Tunneling also goes together with bridges. Bridges to go over obstacles. Tunnels to go under or through obstacles.
China’s expertise in tunnelling is being brought to bear in Malaysia. This image is of a Chinese tunnel-boring machine excavating the second longest portion of the 9.5km-long MRT tunnels in Kuala Lumpur.
All this means that China has had – and will continue to have – lots of practice in tunnelling and TBMs.
This has served to raise the confidence levels of Chinese TBM manufacturers, who now intend to enter the export market.
In fact, two such Chinese-made TBMs – specifically, made by the China Railway Tunnelling Equipment Corp Ltd (CRTE) – are now quietly working on our MRT project, excavating the second longest portion of tunnels (2.9km) that start from the Semantan portal all the way to the Pasar Seni station in KL.
Costing around RM25mil apiece, these TBMs are the first ever to be exported outside of China by CRTE, and serve as a reference project for other cities that are also considering Chinese-made TBMs.
Market and details on TBM
Shield tunnel boring machine are a kind of specialized engineering machinery for tunneling. Currently it is widely used in the construction of subway, railway, road and hydropower projects. With the trend of three-dimensional cities and pipelines buried underground, output of China’s shield tunnel boring machines reached to 141 units in 2012, with sales revenue harvesting CNY 7.045 billion (US$1.1 billion).
A single shield machine uses pre-fitted segments to exert the blast force on the working face. Digging time represents approximately 80% of operating time, the remaining 20% corresponding to segment fitting.
Double shield TBM
Double shield machines allow for raster tunnel excavation as the segments are fitted in concurrent operation time during blasting (digging). In this case, digging time approaches 100% of operating time. The rear shield is fitted with grippers.
Machine progress is a 3-step process:
* Gripping of the rear shield onto the excavated surfaces,
* Digging by the cutter head (front shield advance), and laying of the lining,
* Rear shield advance (hence gripper advance) to the next digging position and back-up traction.
Grippers are shoes that press radially against the walls of the excavated tunnel.
Contrary to shelded hard rock TBM, this tunnel-boring machine therefore does not rest against the lining. The force exerted by the grippers must be very high to prevent the thrust applied to the head from causing shield recoil.