Type of microtunneling or tunneling machine in which mechanical pressure is applied to the material at the face and controlled to provide the correct counter-balance to earth pressures in order to prevent heave or subsidence. The term is usually not applied to those machines where the pressure originates from the main Pipe JackingA system of using hydraulic jacking from a drive shaft to directly install pipes behind a shield machine so that they form a continuous string in the ground. rig in the Drive ShaftSee Jacking Shaft./Pit or to systems in which the primary counter-balance of earth pressures is supplied by pressurized drilling fluid.
