In most microtunneling operations the pipe is inserted from the entry and pushed behind the machine. This is a process often called Pipe Jacking. As the machine advances, more tunnel liner is pushed from the entrance. Thus, the speed of the advancing machine is controlled by the speed at which the pipe is inserted into the entrance.
As the length of tunnel increases, the friction of the ground around the pipe increases as well. Usually, two practices are used to minimize this friction. First, over-cutting is used to give a slight gap between the inner edge of the tunnel and the outer edge of the liner. Usually this is achieved by using a cutter wheel with a diameter ½ inch (12mm) to 4 inches (100mm) larger than the outside diameter of the liner. Secondly, an economical and ecologically friendly lubricant, often bentonite slurry, is injected into this gap. In addition to lubrication, the pressure of the lubricant prevents the gap from collapsing.
While friction can be reduced, it can never be eliminated, and so hundreds of tons of force are required to push the machine and liner into the ground. A large “jacking frame” is required to produce these forces. In most cases the entrance must be modified to support this frame and the forces it generates.
In addition to the jacking frame, smaller jacks, called “interjacks”, may be inserted between sections of tunnel liner. These push the two sections of liner 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.