referring to the drawings , fig1 shows a shield constituted by three generally cylindrical shield sections 10 , 11 and 12 , arranged end - to - end in the direction in which they are to be moved during use . each section consists of a shield member 13 , which supports the roof and sides of the excavated tunnel , and also of a frame 14 rigidly connected to its shield member . each frame 14 is constituted by a number of steel arc segments which are interconnected , in the floor zone , via a floor beam 15 , to form a rigid self - contained frame . as may be seen from fig1 the shield members 13 of the two rear shield sections 11 and 12 are bent inwards at 13 &# 39 ; by a distance approximately equal to the thickness of the sheet metal from which they are made , so that the shield members overlap one another . advancing rams 16 are provided for moving the shield sections in the direction v in the socalled &# 34 ; retarded follow - up sequence &# 34 ;, that is to say the front section 10 is advanced in the direction v whilst the other two sections 11 and 12 are stationary , the second section 11 is then advanced to follow up the first section 10 and finally the third section 12 is advanced . the length of overlap between each pair of adjacent sections 10 , 11 and 12 is arranged to be greater than the strokes of the rams 16 so that the three sections are always telescoped together during this advancing movement . the rams 16 , positioned in the roof zone are flexibly interposed between the frames 14 of the successive shield sections , and rams 16 &# 39 ; are also provided adjacent to the floor and act flexibly between the floor beams 15 and a floor sill 17 via joints ( not shown ). the frames 14 are each provided , in the floor zone , with expansion elements in the form of hydraulic piston - and - cylinder devices 18 which are supported on the floor 20 by skids 19 , and which act on connecting brackets 21 attached to the frames 14 . the arc segments of each frame are detachably and adjustably interconnected via mechanical connections such as bolts or straps 22 . the structure of the floor sill 17 ( see fig2 and 3 ) is subdivided longitudinally into three sill sections 23 , 24 and 25 , of which the middle sill section 24 has a width considerably greater than that of the outer sill sections 23 and 25 . each sill section in itself consists of a number of sill parts 23 &# 39 ;, 23 &# 34 ; and 23 &# 39 ;&# 34 ;, 24 &# 39 ;, 24 &# 34 ; and 24 &# 39 ;&# 34 ;, and 25 &# 39 ;, 25 &# 34 ; and 25 &# 39 ;&# 34 ;, interconnected , at joints 26 , to form an articulated chain . the sill parts are given a sufficient clearance to ensure that , within certain limits , they are flexibly movable with respect to one another in all directions . the sill sections 23 , 24 , and 25 are guided ( see fig2 ), on their respective longitudinal side edges which face towards one another , by means of longitudinal guides 27 , which have sufficient clearance to allow of directional control of the shield . control members in the form of hydraulic piston and cylinder devices 28 are provided for bending the sill sections slightly aside from one another , in the horizontal plane , so that the direction of advance of the shield sections 10 , 11 and 12 can be influenced as desired . as can be seen in fig1 the rams 16 &# 39 ; and the floor beams 15 are situated underneath the floor sill 17 , and the sill sections are provided , in their undersides , with chambers or recesses 44 each of whose length is at least equal to the stroke of the rams 16 &# 39 ; and each of whose height is greater than that of the beams 15 , so that , on the retraction and extension of the expansion elements 18 , the beams 15 have sufficient vertical play in the recesses 44 . thus , by applying pressure to the rams 16 the shield sections 10 , 11 and 12 can be advanced independently of the floor sill 17 , in successive steps in the direction v . during this process the shield sections 10 , 11 and 12 are supported , via their frames 14 and the expansion elements 18 , against the floor 20 . they are also supported on one another by the overlapping parts of the shield members 13 . when the shield sections 10 , 11 and 12 are advanced , the expansion elements 18 can be retracted until the shield members 13 are completely relieved of their load . alternatively , the shield sections 10 , 11 and 12 can also be shifted with the shield members 13 under load , with the expansion elements 18 extended . as soon as all the shield sections 10 , 11 and 12 have been advanced , the advance of the floor sill 17 can be effected . this operation is performed by means of the rams 16 &# 39 ;, which act against the braced frames 14 of the shield sections 10 , 11 and 12 . in this process it is possible , for example , to advance the two outer sill sections 23 and 25 as a first step , the middle sill section 24 being advanced subsequently thereto . an excavating machine 30 , such as a cutter , is provided with a transporter having caterpillar tracks 31 . normally , these tracks 31 run on the middle sill section 24 of the floor sill 27 , but the machine 30 is provided with hydraulically extensible side brackets 32 by means of which it can be supported on continuous supporting rails 33 provided on the outer sill sections 23 and 25 . thus , the machine 30 , together with its caterpillar tracks 31 , can be lifted off the middle sill section 24 . when in use , the machine 30 is usually supported on the outer sill sections 23 and 25 via the brackets 32 . if the machine 30 is to be moved forwards , the brackets 32 are hydraulically retracted , so that the caterpillar tracks 31 are lowered onto the middle sill section 24 . the machine 30 may be of any known type . the machine shown has a pivotable extension arm 34 provided with a rotary cutting device 35 . it will be apparent , therefore , that the shield sections 10 , 11 and 12 can be advanced independently of the floor sill 17 without the necessity of interrupting the work performed by the machine 30 . the operation of advancing the sill sections 23 , 24 and 25 of the floor sill 27 can be carried out without subjecting them to any load . hydraulic alignment piston - and - cylinder devices 36 are interposed between the frame 14 of the middle shield section 11 and the floor sill 17 , these devices 36 providing a means of aligning the apparatus . the joints 26 connecting the sill parts , the joints between the rams 16 and the frames 14 , and the joints connecting the alignment devices 36 and the sill 17 , are all constructed as articulated joints which enables the entire shield apparatus to travel around bends . for the same purpose the connecting joints of the expansion elements 18 and the rams 16 &# 39 ; are designed as articulated joints . the overlapping shield members 13 are provided with sliding air ducts 37 ( see fig2 ) designed on spacesaving lines and serving for ventilation . the rear shield section 12 is also provided with a lining setting device 38 having a magazine 39 loaded with lining segments 40 ( see fig1 ) of the arch - type . these members 40 are to constitute the permanent lining immediately behind the shield and are pushed individually out of the magazine 39 by means of hydraulic piston - and - cylinder devices 41 and onto a lifting device 42 which can be raised towards the roof of the tunnel by means of hydraulic piston - and - cylinder devices 43 . in practice , the rear shield section 12 may be shifted by a distance corresponding to that provided between the individual lining arches 40 of the permanent lining , rather than by the full distance of its overlap with section 11 . after this shifting operation the arch 40 furthest from the devices 41 is pushed by these devices onto the lifting device 42 , which is then raised towards the roof by means of the lifting devices 43 . this lining arch is thus moved into the lining position , after which it can be bolted and tightened . this process is then repeated for the next arch 40 so as to provide a simple and rapid means of lining which is independent of the progress of the machine 30 . needless to say , the lining setting device 38 also enables other lining elements to be introduced into the tunnel . in place of a lining setting device and magazine , it is also possible for a hydraulically displaceable scaffolding for shaped brakes to be built onto the rear shield section 12 if the lining selected is of the type consisting of such shaped brakes . the shield described above can be shifted in a stable and reliable manner even under extremely difficult conditions , that is to say when there is no contact at all with the rock or when loose rubble rests on the entire working . any pressure exerted by the strata is taken up by the expansion elements 18 . in the event of any appreciable subsidence the shield can be relieved of its load mechanically .