China has unveiled the world’s largest multi-tool as it rolled out the world’s first Boring and Blasting Machine (BBM) that combines a tunnel-boring machine with the ability to set explosives to blast through solid rock.
In constructing deep underground tunnels, there are essentially two types of machines to do the job. Both are designed to pull themselves along a bit like an earthworm, with most of the machine’s length taken up by giant electric motors and apparatus to carry away spoil and lay down a watertight tunnel lining of concrete blocks.
The real difference is at the business end that does all the cutting.
TBM
The first is the Soft Ground Tunnel Boring Machine (TBM), which has a large rotating shield covered in cutting tools that slice into soil, clay, sand, gravel, or water-bearing strata. The purpose of the shield is both to dig through the earth and to support the face so it doesn’t collapse as the muck is removed by a screw conveyor. If the face is too unstable, a pressurized bentonite slurry is injected to shore things up.
The second machine is the Hard Rock TBM. This, as the name implies, is for intact rock strata like granite, basalt, and sandstone, and it has a harder time of things. This has no shield behind the cutterhead, and the machine grips directly to the rock wall for support while special heavy cutters grind slowly away at the solid rock.
There is a third hybrid machine for dealing with soft soil littered with boulders like nuts in an otherwise innocent bit of cake, but it’s a bit of a compromise whose only real virtue is that the digging can still go on, though not as well as one would like.
Xianhua
What the China Railway Science & Industry Group (CRSIC) in Wuhan and Tsinghua University have come up with is something to deal with digging tunnels in areas where the geology is aggravatingly diverse, with soft soils giving way to rock faces. The result is the BBM.
With a diameter of 14.76 ft (4.5 m), the BBM, officially called “Xianglong” or “Lucky Dragon,” has a special cutterhead with a hollow point in the center and the ability to pull back slightly against the machine, which is protected by a shield to prevent damage to the machinery and sensors behind.
The clever bit is that the hollow center and specialized access channels have integrated drilling equipment for boring holes many meters deep into the rock face that can then be stuffed with explosives and tamped with sand or clay. When the special low-yield, high-velocity explosives are detonated, they create micro-fractures in the rock without throwing out debris or massive concussion. What force is generated is deflected and absorbed by the protective shield. The cutter head can then move back into position and sweep up and crush the pulverized rock.
Xianhua
According to Chinese state media, Xianglong can improve excavation efficiency by 30% over conventional boring machines, and the shield can vary its diameter to accommodate different lining segments.
However, although it’s been built, how it actually functions remains to be seen.
Source: Xinhua
