with reference first and in particular to fig1 which shows the representative tunnel excavator of the invention in the act of tunnel excavation , the excavator is provided with a generally tubular shield comprising a head section 11 , an inner section 18 , and a tail section 26 . concentrically mounted to the front end , shown directed to the left in fig1 of the head shield section 11 is a cutting disk 13 which is rotatable relative to the head shield section about the same axis therewith . as will be seen also from fig2 a front end view of the excavator , a plurality or multiplicity of roller cutters 12 are replaceably mounted to the cutting disk 13 in a pattern designed for the most efficient cutting of the tunnel face 43 . the construction of each roller cutter 12 , and the method and means for its mounting to the cutting disk 13 , will be later explained in more detail . employed for the desired rotation of the cutting disk 13 with respect to the shield are a plurality of , three in the illustrated embodiment , drive motors 15 , fig1 and 3 , which are rigidly supported within the head shield section 11 . pinions 16 on the armature shafts of the drive motors 15 mesh with an internal gear 14 which is concentrically coupled fast to rear end of the cutting disk . it is thus seen that the drive motors 15 conjointly drive the cutting disk 13 via the intermeshing gear 14 and pinions 16 . a slurry chamber 17 is water - tightly defined just behind teh cutting disk 13 . fig1 and 4 indicate that the inner shield section 18 is concentrically nested in a rear part of the head shield section 11 for axial displacement relative to the same . the inner shield section 18 has a front sealing ring 19 and front bushing 20 rigidly mounted on the outside surface of its front end portion for sliding contact with the inside surface of the head shield section 11 . a rear sealing ring 21 and rear bushing 22 are rigidly mounted on the inside surface of the rear end portion of the head shield section 11 for sliding contact with the outside surface of the inner shield section 18 . it will also be observed from fig1 and 4 that a pair of slides 23 are disposed in diametrically opposite positions within the inner shield section 18 and extend parallel to its axis . the slides 23 have their front ends secured to an annular partition 11a which is formed in one piece with the head shield section 11 and which is disposed just forwardly of the inner shield section 18 . the slides 23 are slidably engaged in respective channel - shaped guides 24 secured to the inside surface of the inner shield section 18 , so that the head shield section 11 and inner shield section 18 are slidable axially relative to each other while being restrained from relative angular and radial displacement . it is , of course , understood that the head shield section 11 and inner shield section 18 could be splined together or otherwise interengaged in any other convenient manner for such relative displacement in the axial direction only . as seen in both fig1 and 3 , the head shield section 11 is provided with a plurality of , four in this embodiment , front anchor mechanisms 25 disposed at constant circumferential spacings thereon . each front anchor mechanism can be of any known or suitable construction comprising , for example , a hydraulic jack for thrusting an anchor member radially outwardly of the head shield section 11 to establish a positive anchorage thereof against the surface of the tunnel being excavated . with reference directed further to fig1 the tail shield section 26 is disposed in end - to - end relation with the head section 11 , with a spacing therebetween , and in overlapping relation to the inner shield section 18 . a collapsible seal 27 between the lapping ends of the inner shield section 18 and tail shield section 26 permits the radial play of the inner shield section , and therefore of the head shield section 11 , relative to the tail shield section 26 . in other words , the head shield section 11 as well as the inner shield section 18 is steerable with respect to the tail shield section 26 . as depicted cross - sectionally in fig6 the tail shield section 26 is provided with a plurality of , two in this embodiment , rear anchor mechanisms 28 at constant circumferential spacings . the rear anchor mechanisms 28 perform the function of positively anchoring the tail shield section 26 to , and releasing the same from , the tunnel surface . fig1 clearly indicates that the outside diameter of the tail shield section 26 is less than that of the head shield section 11 . therefore , the gap ( tunnel overcut ) δ1 between the head shield section 11 and the tunnel surface is less than the gap δ2 between the tail shield section 26 and the tunnel surface . this difference between the overcuts is intended to expedite the advance of the tail shield section 26 through the reduction of the frictional resistance . the head shield section 11 and tail shield section 26 are interconnected by a plurality of , four in this embodiment , hydraulic push jacks 29 , fig1 and 6 , disposed circumferentially within the inner shield section 18 . the push jacks 29 propel the shield , and therefore the complete excavator , as excavation proceeds . as seen in fig1 and more clearly in fig5 a pair of antirolling jacks 30 are mounted between the inner shild section 18 and the tail shield section 26 , and in symmetrical positions with respect to the shield axis . disposed in a plane at right angles with the shield axis , the antirolling jacks 30 extend parallel to each other in positions of symmetry about the shield axis . each antirolling jack 30 is coupled at one end to the rear end of the inner shield section 18 and at the other end to the front end of the tail shield section 26 . thus the reactive ( rolling ) force exerted on the head shield section 11 by the cutting disk 13 during excavation is transmitted to the tail shield section 26 via the interfitting slides 23 and guides 24 , the rear anchor mechanisms 28 , and the antirolling jacks 30 . both fig1 and 7 show a water conduit 31 extending through the tail shield section 26 and inner shield section 18 into the slurry chamber 13 for the delivery of water thereto . the water thus fed into the slurry chamber 13 will mingle with the spoil to form a slurry . the slurried spoil is to be withdrawn from the slurry chamber 17 . through a slurry conduit 32 extending rearwardly therefrom through the inner shield section 18 and tail shield section 26 . at 33 in fig1 and 6 are shown auxiliary push jacks designed to transmit the thrusts of the primary push jacks 29 to a series of tunnel liner segments 36 which has been installed by an erector mechanism 35 of any known or suitable construction within the tail shield section 26 . additionally , as required , the auxiliary push jacks 33 may be used for propelling the excavator by themselves . coupling bars 34 serve the purpose of interconnecting the inner shield section 18 and tail shield section 26 so as to permit the relative bending and rolling of the head and tail shield sections 11 and 26 . as drawn on an enlarged scale in fig8 and 9 , each roller cutter 12 comprises one or more , two in this embodiment , cutter rings 12a rotatably mounted on a cutter shaft 38 in a manner to be described presently . it will also be noted from fig8 in particular that each roller cutter 12 is replaceably mounted to the cutting disk 13 via mounting means 37 comprising fixed cutter supports 39 and clamps 49 . the cutter shaft 38 has a pair of extensions 38a of rectangular cross sectional shape extending collinearly from its opposite ends . fig8 illustrates how each cutter shaft extension 38a is supported by the mounting means 37 via a rotary mounting disk 40 , as set forth in detail hereafter . the cutter support 39 and clamp 49 define in combination a circular opening 41 for receiving the rotary mounting disk 40 . the axis of this circular opening 41 is aligned with , or parallel to , the axis of the cutter shaft 38 . approximately one half of the circular opening 41 is formed by a semicircular recess 44 in the cutter support 39 , and the other half by a semicircular recess 50 in the clamp 49 . the semicircular recess 44 in the cutter support 39 is open to a guide channel 42 extending radially therefrom toward the interior or rear side of the cutting disk 13 for the passage of the cutter shaft extension 38a during the mounting and dismounting of the cutter roller 12 . the cutter support 39 has further formed therein a generally rectangular recess 46 which is bounded in part by a pair of seating surfaces 45 for supporting the clamp 49 , although the provision of this recess 46 is not a necessity . as viewed in fig8 the circular opening 41 , guide channel 42 and recess 46 are of bilateral symmetry with respect to a plane passing the axis of the cutter shaft 38 and extending perpendicular to the tunnel face 43 . the rotary mounting disk 40 has itself defined therein a radial slot 47 for receiving the cutter shaft extension 38a and filler 48 with a sliding fit . the filler 48 has a surface 48a which is flush with the outer surface 40a of the mounting disk 40 and which is curved with the same radius as the mounting disk surface 40a , so that the filler surface 48a and mounting disk surface 40a provides in combination an unbroken cylindrical surface . the clamp 49 has a pair of leg portions 49a seated against the seating surfaces 45 of the cutter support 39 . machine screws 51 are inserted into and through clearance holes in the cutter support 39 and engaged in tapped holes 51a in the leg portions thereby fastening the clamp 49 to the cutter support and so clamping the rotary mounting disk 40 together with the cutter shaft extension 38a engaged therewith . the clamp 49 serves to prevent the cutter shaft 38 from falling off the cutter support 39 toward the tunnel face 43 . therefore , as had been stated , the mounting of the clamp legs 49a to the recessed seating surfaces 45 on the cutter support 39 is unessential ; instead , the clamp legs could be mounted to the extreme front face of the cutter support . so mounted to the cutting disk 13 , the roller cutters 12 are arranged thereon for the even cutting of the complete tunnel face 43 . it will also been seen from fig2 that the cutting disk 13 is additionally provided with scrapers 53 , disposed adjacent spoil intakes 52 , which coact with the roller cutters 12 for optimal excavation . fig9 clearly illustrates how the two cutter rings 12a of each roller cutter 12 are mounted on the cutter shaft 38 . the cutter rings 12a are mounted fast on a rotary sleeve 58 , with an annular spacer 59 between the cutter rings . the sleeve 58 in turn is rotatably mounted on the cutter shaft 38 via cages 54 , radial bearing rollers 55 , and thrust bearing rollers 56 . a pair of sealing rings 57 are engaged between cutter shaft 38 and rotary sleeve 58 . an annular threaded member 59 locks the rotary sleeve 58 against axial displacement relative to the cutter shaft 38 . fig9 also reveals a tool grip 38b formed in one piece with the cutter shaft 38 and left exposed from the rotary sleeve 58 . as seen in an end view as in fig1 , the tool grip 38b is of hexagonal shape , although it could be of other noncircular shape , all that is required being that it be firmly gripped with a wrench or other tool for manually revolving the cutter shaft 38 together with the rotary mounting disk 40 in mounting or dismounting the roller cutter 12 . it may also be mentioned that the cutter shaft extensions 38a need not be of rectangular shape , either . the cutter shaft extensions could be , for example , circular in cross section , even though such circular shaft extensions might slip with respect to the mounting disks 40 during excavation . it is to be noted that the axis 38c of the cutter shaft extensions 38a need not be in precise alignment with the axis 40b of the rotary mounting disks 40 as shown in fig8 . alternatively , the axis 38c may be offset from the axis 40b either toward the tunnel face , as shown in fig1 , or away from the same . either way , the roller cutter 12 will be dismountable from the cutting disk 13 by revolving the roller cutter about the axis 38c or 40b , as will be more fully explained subsequently . in cases where the cutter shaft is eccentric with respect to the rotary mounting disk 40 , the clamp 49 may be provided as required with a stop 60 , fig1 , which is to be abutted upon by the periphery 38d of the tool grip 38b , thereby positively locking the cutter shaft against undesired rotation . as additional possible modifications of the invention , each rotary mounting disk 40 may be either slitted as at 61 in fig1 a or divided into segments as shown in fig1 b , with the disk segments interconnected by springs 62 . these slitted and segmented mounting disks are both intended to facilitate the insertion of the cutter shaft extension 38a in their radial slots 47 , as these slots will resiliently spread wider during such insertion . as an added advantage , the radial slots 47 will become narrower as the clamp 49 is screwed to the cutter support 39 , thereby making it possible for the mounting disk 40 to hold the cutter shaft extension 38a more firmly . the following is a discussion of a method of dismounting each roller cutter 12 from the cutting disk 13 . let us assume that the roller cutter 12 with its mounting means 37 is in the state of fig8 with the radial slot 47 in the rotary mounting disk 40 opening toward the tunnel face 43 . the operator , lying behind the cutting disk 13 , first loosen the screws 51 to an extent necessary to slightly move the clamp 49 away from the mounting disk 40 as indicated by the arrow a in fig8 . then with an appropriate hand tool inserted in the space 63 , fig8 and 11 , between the cutter supports 39 , the tool grip 38b is turned 180 degrees as indicated by the arrow b in fig8 . thereupon the rotary mounting disk 40 will revolve from its fig8 position to that of fig1 , in which latter position the radial slot 47 in the mounting disk 40 is open to the guide channel 42 in the cutter support 39 . now the roller cutter 12 together with the cutter shaft 38 and the associated bearing means and fillers 48 is withdrawn from the cutter supports 39 . in the case where the axis 38c of the cutter shaft extensions 38a is offset from the axis 40b of the rotary mounting disks 40 as in fig1 , the roller cutter 12 will orbit about the axis 40b as indicated by the phantom outlines in fig1 , with the periphery 38d of the tool grip 38b moving away from the stop 60 . it will therefore be seen that the stop 60 on the clamp 49 does not interfere with such manual revolution of the roller cutter 12 . it will now be apparent that each roller cutter 12 can be installed on the cutting disk 13 by the reversal of the foregoing dismounting procedure . with the rotary mounting disks 40 held in the angular position of fig1 , the cutter shaft extensions 38a and fillers 48 is slid successively into their radial slots 47 via the guide channels 42 in the cutter supports 49 . such insertion of the cutter shaft extensions 38a will be easier if the mounting disks 40 are constructed as shown in fig1 a or 12b , because then the radial slots 47 will widen under pressure from the shaft extensions . then the mounting disks 40 with the cutter shaft 38 engaged therewith is revolved 180 degrees from their fig1 position back to that of fig8 by manually turning the tool grip 38b . then , with the periphery 38d of the tool grip 38b held against the stop 60 , if any , on the clamp 49 , the screws 51 are tightened for drawing the clamps 49 into forced contact with the rotary mounting disks 40 and fillers 48 . now the mounting of the roller cutter 12 has been completed . the mounting disks 40 will more firmly engage the cutter shaft extensions 38c if they are slitted as in fig1 a or sprung as in fig1 b . it should be appreciated that each roller cutter 12 is readily mountable to , and dismountable from , the cutting disk 13 merely by loosening and tightening the screws 51 and bidirectionally revolving the cutter shaft 38 together with the rotary mounting disks 40 . all such manual handling can be performed from within the cutting disk 13 . there is accordingly no need for backing the excavator away from the tunnel face 43 to enable an operator to enter the space so created for the replacement of the roller cutters 12 . mounted to the cutting disk 13 as in the foregoing , the roller cutters 12 will cut the tunnel face 43 as they rotate with the cutting disk , with the cutter rings 12a also revolving about the cutter shafts 38 . because of the unidirectional rotation of the cutting disk 13 , each roller cutter 12 will receive the reactive force of excavation in one direction only . it will therefore be seen that only one stop 60 may be provided on each clamp 49 for bearing against the reactive force of the associated cutter shaft 38 . for tunnel construction by the excavator shown in fig1 the pair of rear anchor mechanisms 28 is first be actuated , hydraulically or otherwise , for thrusting the anchor members radially outwardly of the tail shield section 26 , thereby anchoring the same against displacement relative to the tunnel surface . then the drive motors 15 is set into rotation for revolving the cutting disk 13 . the roller cutters 12 on the revolving cutting disk 13 will cut the tunnel face 43 as the push jacks 29 are extended for pushing the head shield section 11 away from the anchored tail shield section 26 . during such excavation , water is pumped into the slurry chamber 17 by way of the water conduit 31 . the water will mingle with the muck or spoil that has fallen into the slurry chamber 17 through the intake openings 52 in the cutting disk 13 . the slurried spoil is to be pumped away from the slurry chamber 17 by way of the slurry conduit 32 . as the cutting disk 13 rotates with the roller cutters 12 in cutting engagement with the tunnel face 43 , the head shield section 11 will be constantly subject to torsional forces produced by reaction from the tunnel face . such torsional forces will be transmitted to the inner shield section 18 via the longitudinal slides 23 and guides 24 , the slides and guides being substantially rigidly interengaged in the circumferential direction of the shield sections 11 and 18 , and thence to the tail shield section 26 via the pair of independent antirolling jacks 30 . finally , the torsional forces will be taken up by the rear anchor mechanisms 28 being held against the tunnel surface . upon extension of the push jacks 29 to a full or otherwise prescribed length , that is , upon excavation of a unit tunnel length , the cutting disk 13 is set out of rotation , and the front anchor mechanisms 25 is actuated for anchoring the head shield section 11 to the tunnel surface . simultaneously , the rear anchor mechanism 28 is retracted for releasing the tail shield section 26 from the tunnel surface . then , upon contraction of the push jacks 29 , the tail shield section 26 will be dragged close to the head shield section 11 . the inner shield section 18 will also travel forwardly with the tail shield section 26 as the slides 23 slide over the guides 24 . subsequently , the tunnel surface portion that has been bared by the advance of the tail shield section 26 over the unit distance is lined by the erector mechanism 35 with the liner segments 36 , as has been known heretofore . one cycle of tunnel excavation has now been completed . the same cycle may be repeated for excavating further into the ground . it is to be noted , however , that the anchoring of the tail shield section 26 can be effectuated before , rather than after , the completion of the lining of the tunnel surface portion that has been bared at the end of each cycle . thus , during the erection of the liner segments 36 , the front anchor mechanisms 25 on the head shield section 11 may be deactivated , and the cutting of the tunnel face may be recommended . if the tail shield section 26 is insufficiently anchored against backing by the rear anchor mechanisms 28 , the auxiliary jacks 33 may be used for transmitting the thrust loads of the primary jacks 29 to the erected liner segments 36 . the circumferntial arrangement of the push jacks 29 , as best seen in fig6 is well calculated for the steering of the excavator by these jacks . the strokes of the push jacks 29 may be selectively varied for steered advancement of the head shield section 11 . since the inner shield section 18 and tail shield section 26 are interconnected by the collapsible seal 27 , the head shield section 11 is steerable within limits both horizontally and vertically with respect to the tail shield section 26 . as the head shield section 11 is so steered and advances , the tail shield section 26 will follow the head shield section by virtue of the steerable connection between inner shield section 18 and tail shield section 26 .