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Microtunneling or microtunnelling is a tunnel construction technique used to construct 
Microtunnel boring machines (MTBM) are similar to larger
utility tunnel
A utility tunnel, utility corridor, or utilidor is a passage built underground or above ground to carry utility lines such as electricity, steam, water supply pipes, and sewer pipes. Communications utilities like fiber optics, cable television ...
s from approximately in diameter. Because of their incredibly small diameter, it is not possible to have an operator driving the tunneling machine, so they have to be remotely operated.
Microtunnel boring machines (MTBM) are similar to larger tunnel boring machine
A tunnel boring machine (TBM), also known as a "mole", is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata. They may also be used for microtunneling. They can be designed to bore throu ...
s (TBMs). The MTBM and jacking frame are set up in a shaft at the required depth. The operator monitors the MTBM's location, orientation and hydraulic devices via a computer console, a CCTV camera or Gyro unit. Some systems use video cameras in the jacking shaft and at the separation plant. Gyro Control have generally replaced cameras for location and digital feedback.
Operation
In most microtunneling operations the machine is launched through an entry eye and pipes are pushed behind the machine. Thispipe jacking Pipe ramming (sometimes also called pipe jacking) is a trenchless method for installation of steel pipes and casings. Distances of 30 m (150 feet) or more and over 500 mm (20 inches) in diameter are common, although the method can be used for m ...
process is repeated until the MTBM reaches the reception shaft at the far end. The speed of the advancing machine is limited to the speed at which the pipe is inserted into the entry eye via the hydraulic rams in the jacking frame.
The friction of the ground around the pipe increases in proportion to the tunnel length. Two practices can minimize this friction. First, over-cutting is used to provide a gap between the inner edge of the tunnel and the outer edge of the liner. The gap is ½ inch (12mm) to 1.5 inches (35mm). A lubricant
A lubricant (sometimes shortened to lube) is a substance that helps to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, t ...
, often bentonite
Bentonite () is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-mon ...
slurry
A slurry is a mixture of denser solids suspended in liquid, usually water. The most common use of slurry is as a means of transporting solids or separating minerals, the liquid being a carrier that is pumped on a device such as a centrifugal pu ...
, is injected into this gap. The pressure of the lubricant prevents the gap from collapsing. Depending on the geology a 35mm overcut may create ground subsidence. For road and rail crossings, this 35mm is reduced so prevent more than 10mm subsidence.
As much as hundreds of tons of force may be required to push the machine and liner forward. The jacking frame containing hydraulic rams
A hydraulic ram, or hydram, is a cyclic water pump powered by hydropower. It takes in water at one " hydraulic head" (pressure) and flow rate, and outputs water at a higher hydraulic head and lower flow rate. The device uses the water hammer e ...
produces these forces. The entrance shaft must be strong enough to support the forces it generates.
Interjack
In addition to the jacking frame, smaller jacks, called “interjacks”, may be inserted between sections of tunnel liner. These push two liner sections apart. Friction on the liner sections between the interjack and the tunnel entrance helps to prevent the liner from sliding out backwards. So while the liner behind the interjack does not move, those sections in front of it receive additional pushing force.Thermal cutter
San Francisco startup Petra demonstrated a thermal drilling robot that can tunnel throughSioux Quartzite
The Sioux Quartzite is a Proterozoic quartzite that is found in the region around the intersection of Minnesota, South Dakota, and Iowa, and correlates with other rock units throughout the upper midwestern and southwestern United States. It was ...
, "the hardest rock on earth" that would normally require dynamite. It can create diameter tunnels. It disintegrates the rock with a hot, high-pressure spallation
Spallation is a process in which fragments of material (spall) are ejected from a body due to impact or stress. In the context of impact mechanics it describes ejection of material from a target during impact by a projectile. In planetary p ...
head. The machine combines remote control and machine vision and can reverse out of the tunnel. It is claimed to cost 30-90 percent less.
See also
*Trenchless technology
Trenchless technology is a type of subsurface construction work that requires few trenches or no continuous trenches. It is a rapidly growing sector of the construction and civil engineering industry. It can be defined as "a family of methods, mate ...
* Directional boring
Directional boring, also referred to as horizontal directional drilling (HDD), is a minimal impact trenchless method of installing underground utilities such as pipe, conduit, or cables in a relatively shallow arc or radius along a prescribed und ...
* Hobby tunneling
Hobby tunneling is tunnel construction as a diversion. Usually, hobby tunnelers dig their tunnels by hand, using little equipment, and some can spend years or even decades to achieve any degree of completion. In some cases tunnels have been dug ...
References
{{reflist Trenchless technology Tunnel construction