Trenchless technologies, method 2: Microtunneling
April 13, 2017
April 13, 2017
Understanding when and how to use microtunneling (Part 3 in a series)
This is the third blog in a series about Trenchless Technology, a rapidly growing sector of the construction and civil engineering that requires few trenches or no continuous trenches at all. If you have not been following the last few blogs, you can give yourself some background by following links below before going through this blog.
Microtunneling is a technology that describes itself clearly with its name—It is made to drill smaller tunnels and it’s mostly used for pipelines of 0.6m to 1.5m diameter. Microtunnel boring machines (MTBM) are very similar to normal tunnel boring machines (TBM) but on a smaller scale. These small diameter tunnels make it impossible to have an operator in the machine itself. Instead, the microtunnel boring machine (MTBM) must be operated remotely.
In Microtunneling, the process is supervised by the machine operator in the mobile cockpit which is located by the construction launch pit. Through computer console and precise control equipment the operator receives continuous data about the location and orientation of the boring tool as well as data about the other parts of equipment. This way, the operator can accurately guide and control the boring process directly from his cockpit.
In most microtunneling projects, MTBM is launched by hydraulic jacks through an entry portal into the ground and pipes are pushed behind the machine. This process is repeated until the Microtunneling machine reaches the reception shaft. As the machine advances, more tunnel liner or pipe is pushed from the starting shaft, through the entry portal.
As the length of tunnel increases, the friction of the ground around the pipe increases as well. Generally, two methods are used to minimize this friction. Firstly, MTBM cutting head, cuts the ground a little bit larger than diameter of pipes, therefore the slight gap between the inside perimeter of the tunnel and the outer perimeter of the liner/pipe reduces the friction considerably. Secondly, some type of lubricant, often bentonite slurry, is injected into this gap.
While these methods help a lot with friction, they can never eliminate it completely so sometimes when length of tunnel and jacked pipes increases and large amount of force is required to push the machine and liner into the ground, an “Inter-jack” containing hydraulic jacks is added between pipes to produce the required forces and push the pipe chain forward in a wormlike movement.