A mobile tunnel boring unit, wherein the unit comprises: a support body driven by first drive means, the first drive means for driving the mobile tunnel boring unit including a pair of spaced apart crawler tracks in contact with the tunnel floor and related track driving means to move the tracks, and thus the support body; a cutter head drive means located at an operatively front end of the support body, to which is fitted a rotatable cutter head that has been pre-installed in a starting chamber (200, 302), proximate a tunnel face (120, 306) to be bored, so as to rotatably drive the rotatable cutter head, in use; a muck hopper and a first conveyor arrangement, the cutter head comprising a full face cutter head fitted with cutters to bore a tunnel face (120, 306), the cutter head being arranged to allow cuttings to pass through the cutter head for discharge into the muck hopper and onto the first conveyor arrangement, the cutter head drive means and a rear portion of the cutter head defining aligned central apertures (430, 414) through which a front part of the first conveyor arrangement can extend; a telescopic shield arrangement to shield the boring unit, the shield arrangement comprising a front shield proximate the front of the mobile tunnel boring unit, from which the cutter head protrudes and for accommodating the cutter head drive means, the front shield also having been pre-installed in the starting chamber (200, 302) together with the cutter head, and a rear shield that surrounds at least an upper portion of the mobile tunnel boring unit, the front and rear shields (130, 132) operating telescopically relative to each other, to accommodate changes in the boring direction; and an actuating arrangement comprising a plurality of hydraulic thrust cylinders extending between the cutter head drive means and a pair of opposite first gripper assemblies. a support body driven by first drive means, the first drive means for driving the mobile tunnel boring unit including a pair of spaced apart crawler tracks in contact with the tunnel floor and related track driving means to move the tracks, and thus the support body; a cutter head drive means located at an operatively front end of the support body, to which is fitted a rotatable cutter head that has been pre-installed in a starting chamber (200, 302), proximate a tunnel face (120, 306) to be bored, so as to rotatably drive the rotatable cutter head, in use; a muck hopper and a first conveyor arrangement, the cutter head comprising a full face cutter head fitted with cutters to bore a tunnel face (120, 306), the cutter head being arranged to allow cuttings to pass through the cutter head for discharge into the muck hopper and onto the first conveyor arrangement, the cutter head drive means and a rear portion of the cutter head defining aligned central apertures (430, 414) through which a front part of the first conveyor arrangement can extend; a telescopic shield arrangement to shield the boring unit, the shield arrangement comprising a front shield proximate the front of the mobile tunnel boring unit, from which the cutter head protrudes and for accommodating the cutter head drive means, the front shield also having been pre-installed in the starting chamber (200, 302) together with the cutter head, and a rear shield that surrounds at least an upper portion of the mobile tunnel boring unit, the front and rear shields (130, 132) operating telescopically relative to each other, to accommodate changes in the boring direction; and an actuating arrangement comprising a plurality of hydraulic thrust cylinders extending between the cutter head drive means and a pair of opposite first gripper assemblies.