Microtunneling is the process of using a microtunnel boring machine (MTBM) to drill small tunnels into the Earth, usually for the purpose of placing a pipe into the hole. The pipe is usually right behind the MTBM and enters the hole at the same rate of speed. The tunnels and MTBMs are relatively small, around 2 feet to 4 feet (0.61 meter to 1.2 meters) in diameter, there is no way for someone to directly work in the machine. Instead, the MTBM is controlled remotely from another location, and the machine is laser-guided. The process of microtunneling presents several advantages, such as being more cost-effective than large-scale drilling and being safer for urban environments.
When a construction site, either above ground or below the surface, needs to bore small holes into the Earth, microtunneling is employed. The same type of technology used with large-scale tunneling is used here, but on a smaller scale. Except for its size, an MTBM is exactly like a tunnel boring machine (TBM), because it can cut through rock, sand and many other difficult materials. There is no way a human can fit into an MTBM, so it must be controlled remotely from an on-site computer that allows the operator to cut through the material, and the MTBM has cameras so operators can see what is happening.
The average size of a microtunneling machine ranges from 2 feet to 4 feet (0.61 meter to 1.2 meters) in diameter, and the pipes are just slightly thinner. When the hole is being drilled, the pipe is usually placed right behind the MTBM in a process called pipe jacking. This means the pipe will enter the Earth at the same rate of speed as the drilling machine.
To make pipe jacking easier, microtunneling operators will do a few things to reduce the friction of the entering pipe. The tunnels are usually drilled to be slightly larger than the pipe, around 0.5 inch to 4 inches wider (1.2 to 10 centimeters), to give it room to move. A lubricant is squeezed into this crevice so the pipe can easily slide in. If the friction is not reduced, the pipe may be difficult to move or could be damaged.
Two advantages of microtunneling are cost-effectiveness and safety. Running an MTBM is cheaper than operating a TBM, and the pipes used are not very wide, so large drilling is unnecessary. The parts for an MTBM also are cheaper to replace, because they are much smaller. Safety is the prime advantage, especially in urban settings. Drilling large tunnels under buildings and streets could make the ground unstable if done too often, which would compromise land integrity.