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 Jacking rig in the Drive Shaft/Pit or to systems in which the primary counter-balance of earth pressures is supplied by pressurized drilling fluid.
