US3523426A

US3523426A - Process and apparatus for driving tunnels in rock having zones differing in stability

Info

Publication number: US3523426A
Application number: US715872A
Authority: US
Inventors: Ernst Lauber
Original assignee: Atlas Copco MCT AB
Current assignee: Atlas Copco MCT AB
Priority date: 1967-04-13
Filing date: 1968-03-25
Publication date: 1970-08-11
Anticipated expiration: 1987-08-11
Also published as: SE316497B; FR1574752A; AT286345B; DE1658751A1; ES352657A1; GB1209246A; CH461562A

Description

PROCESS AND APPARATUS FOR DRIVING TUNNELS IN g. 1970 ELAUBER 3,523,426

ROCJK HAVING ZONES DIFFERING IN STABILITY Filed March 25. 1968 5 Sheets-Sheet 1 INVENTOR. E R HSTF LA LLB ER AGENT- Aug. 11, 1970 E. LAUBER 3,523,426

PROCESS AND APPARATUS FOR DRIVING TUNNELS IN ROCK HAVING ZONES DIFFERING vIN STABILITY Filed March 25. 1968 5 Sheets-Sheet 2 r V INVENTOR E R MST LA WE; e K

Aug. 11, 1970 E. LAUBER 3,523,425

PROCESS AND APPARATUS FOR DRIVING TUNNELS IN ROCK HAVING ZONES DIFFERING IN STABILITY Filed March 25. 1968 5 Sheets-Sheet 5 F/GJO 1 l 1- FIG]! I 1 10 20 901 7 14 121519 19 21 FI 5 9a 7 9 19 1a 1517 20 2 1 INVENTOR. EIQMST LWBQQ Aug. 11, 1970 E. LAUBER 3,523,426

PROCESS AND APPARATUS FOR DRIVING TUNNELS IN ROCK HAVING ZONES DIFFERING IN STABILITY Filed March 25, 1968 5 Sheets-Sheet 4 FIG. 77

F /G. 79 F/G.76

. INVENTOR. 61m Lm/csefi 3,523,426 LS IN E. LAUB PARA'IUS FOE FROG AND AP DRIVING TUNNE R0 HAVING ZONES DIFFERING IN STABILITY Filed March 25, 1968 5 Sheets-Sheet 5 INVENTOR. ERMgT: LALGBQQ BY m- M United States Patent Int. Cl. E61 3/03 US. Cl. 61-85 11 Claims ABSTRACT OF THE DISCLOSURE Zones of stable rock are excavated by a tunnel-driving machine having unshielded rotatable and radially adjustable cutting tools. The tunnel-driving machine is advanced in steps in a zone of stable rock to the transition to a zone of rock of low cohesion and is operated during such steps to excavate on a diameter which is larger than the diameter of the excavation in the preceding portion of the zone of stable rock. The tunnel driving machine is retracted after each of said advancing steps with the cutting tools in a radially inwardly adjusted position. A section of a tubular shield is installed ahead of the tunnel-driving machine after each of said retracting steps and before the next succeeding advancing step. Each section consists of a plurality of separable segments. The tunnel-driving machine is then anchored in the shield and the latter is advanced through the zone of rock of low cohesion until stable rock is reached. The tunnel behind the shield is lined with lining rings. The tunnel-driving machine is released from the shield when the stable rock is reached and is used alone for excavating the stable rock. The shield is knocked down.

In the construction of tunnels in loose rock or in strata having only a low cohesion, it is known to use tubular shields having an annular cutting edge at their leading end. Under the protection of the shield, the excavation is carried out manually. It is also known to provide an excavating device within the shield. The shield is always moved ahead by means of hydraulic jacks, which act by means of thrust-bearing ring on the tunnel lining which is arranged behind the shield. This method of excavation is not applicable to relatively hard rock or Where hard rocks occur in soft material.

Other tunnel-driving machines have been disclosed, which are provided with rotating cutting tools that may be radially adjustable so that the entire cross-section of the tunnel can be excavated even in moderately or very hard rock. Particularly satisfactory service has been given by a tunnel-driving machine which has cutting tools in the form of milling cutters which rotate on their own axes and are mounted at the end face of a drum, which is rotatable on the axis of the machine. This machine may be used without a shield if the rock is so stable that it is suflicient to install temporary supports adjacent to the machine or behind the same. The propelling unit of the machine consists in most cases of hydraulic jacking means and bears 'with static friction directly on the surface of the tunnel so that it is not required to install expensive lining rings. The final lining is formed by pumping concrete with the aid of a sliding formwork behind the tunnel-driving machine.

The use of machines in the excavation of tunnels has not been fully successful before owing to the geological structure of rock. The rock which is encountered along the length of a tunnel, particularly one of great length, varies widely in hardness and strength and the condition of the rock may also vary. Difiiculties arise when zones 3,523,426 Patented Aug. 11, 1970 of hard rock having a high stability are immediately succeeded by zones of rock which is loose and has only a low cohesion. Another difficulty is due to the fact that unexpected conditions are apt to occur even .if a thorough geological investigation as to the rock conditions to be expected has been carried out before the construction of a tunnel and has been evaluated to obtain information as to the classes of rock.

If soft and hard rocks are to be expected in alternation in the construction of a tunnel, it will be impossible to use tunnel-driving machines which are suitable only in soft rock. On the other hand, it will be impossible to use machines that are suitable both for hard and soft rocks if strata will or may be encountered which have only a low cohesion. It might appear obvious to replace the well known excavating machines for soft, homogeneous material by a known tunnel-driving machine for soft and hard rock and to install such machine permanently in a tubular shield. The adoption of this method is impossible in most cases in view of the high expense which is involved in it because the use of a shield requires the subsequent installation of lining rings and a complete lining of long tunnels with lining rings is much more expensive than, e.g., a lining consisting of concrete placed with the aid of a formwork.

It is an object of the invention to provide an economical process which enables the use of machine in the driving of tunnels at least under rock conditions which are frequently encountered.

Based on a process for the construction of tunnels or the like in rock having zones differing in stability and with the aid of a tunnel-driving machine having rotating and radially adjustable cutting tools, which preferably consist of milling cutters that are adapted to revolve in unison about the longitudinal axis of the machine, the process according to the invention is essentially characterized in that the tunnel is driven in zones of stable rock in the usual manner with the aid of the tunnel-driving machine alone whereas upon the arrival at a zone of rock having a low cohesion a tubular shield consisting of a plurality of sections which can be knocked down into segments is installed one section after the other in the zone of stable rock ahead of the tunnel-driving machine while the latter is advanced in steps and operated to excavate at a diameter which is larger than the previous tunnel diameter, the tunnel-driving machine is subsequently retracted with the cutting tools in a radially inwardly adjusted position, and is then firmly anchored in the shield, the latter is advanced, the tools of the tunnel driving machine are operated, if desired, during the advance of the shield, and the tunnel or the like is lined with lining rings behind the shield until the next zone of stable rock is reached, when the tunnel-driving machine is released from the shield and propelled along and the shield is knocked down for its next use. When inferior rock is encountered in the driving of a tunnel or the like and such rock cannot remain in a stable condition until the temporary lining can be installed, the shield will be used so that the tunnel-driving machine is disposed within the protecting shield While the zone of low cohesion is being traversed. The occurrence of inferior rock may be detected, e.g., 'by test bores driven ahead of the tunnel. In the zone of inferior rock, the excavation is carried out only with the aid of the shield. In the normal case, however, the cutting tools of the tunnel-driving machine are in operation ahead of the cutting edge of the shield. Only when the rock caves in, e.g., in loose gravel, the tunnel-driving machine is retracted into the shield to such an extent that the cutting tools are behind the cutting edge of the shield. The tunnel is lined in the usual manner with lining rings behind the shield. In the normal case, however, the cutting tools of the tunnel-driving machine is released from the shield and moves out of the tubular shield with the aid of the propelling unit associated with the machine. The expensive lining with lining rings is required only where it is actually essential, namely, in the zone of rock of low stability, whereas in zones of rock having a sufiiciently high stability the tunnel is driven in the usual manner and can -be lined with concrete placed with the aid of a framework or with other means which are much more economical. It is not necessary to modify the tunnel-driving machine, which may be used also alone. The shield which can be knocked down can easily be stored, assembled and taken apart and can be used as often as desired.

When test bores are driven ahead of the tunnel so that the zone of low stability is detected early, the excavation of increased diameter can be initiated at a point which is spaced from the zone where the poor rock is to be expected by a distance which is at least as large as the length of the shield. Alternatively, the tunnel may be driven with the aid of the tunnel-driving machine alone as far as to the zone of rock of low cohesion, i.e., until the cutting tools begin to enter that zone, whereafter the tunnel-driving machine is retracted by a distance which is at least as large as the length of the shield and subsequently enlarges the previously driven tunnel or the like by twice the thickness of the shield segments. The latter mode of operation has the advantage that the removal of the excavated material is much easier during the cutting on the enlarged tunnel diameter than during the excavation of the full cross-section and that segments from which the shield tube will be assembled can be stored in the free space which is created ahead of the tunnel-driving machine.

According to the invention, an annular groove is excavated in the tunnel or the like ahead of the tunnel-driving machine which has been retracted and the cutting tools of the tunnel-driving machine are adjusted to the increased excavating diameter in said groove so that this adjustment can be carried out without difficulty.

According to the invention the rearmost two sections of the shield are installed first and are knocked down behind the tunnel-driving machine when the shield has been completely assembled, Whereafter said two sections are replaced by thin sheet metal elements welded together in installed position, a thrust-bearing ring is installed in known manner into the resulting shield tail, hydraulic jacks for advancing the shield are caused to bear against said thrust-bearing ring, and the first lining ring is installed behind the thrust-bearing ring.

The apparatus for carrying out the process according to the invention comprises a tunnel-driving machine having a hydraulic propelling means, and a shield which is adapted to be knocked down into segments and to be advanced with the aid of hydraulic jacks bearing on a thrustbearing ring. The pump or like means for propelling the tunnel-driving machine is adapted to be switched over to supply the hydraulic jacks associated with the shield so that there is no need to provide a separate source of pressure oil for supplying the hydraulic jacks associated with the shield.

If the cutting tools of the tunnel-driving machine are carried by a drum which is rotatable on the axis of the machine, it is a further feature of the invention to provide on the end face of the drum a bracketlike device for installing the shield segments, which device is provided With screws or the like extending radially with respect to the drum axis. In that case, the rotation of the drum is utilized to distribute the shield segments in assembling the length sections of the shield, and the screws serve to engage the segments with the tunnel surface.

The end face of the drum may be provided between the cutting tools with compartments which are forwardly open so as to avoid a caving-in of substantial quantities of loose material during shield tunneling.

To enable an adaptation of the shield to different tunnel diameters, bars which are wedge-shaped in cross-section are provided and are insertable between those edge faces of the arcuate shield segments which are parallel to the shield axis. The shield segments themselves are always the same. The wedge-section bars are added or bars having a difierent wedge-shaped cross-section are used in dependence on the desired tunnel diameter. When the tunnel diameter increases beyond a certain size, an additional segment may be used instead of the spacing bar or bars, and the resulting section or ring may then be enlarged with the aid of spacing bars. To enable a guided movement of the heavy tunnel-driving machine in the tubular shield, the arcuate shield segments have stiffening rings which are parallel to the axis of the shield.

The process according to the invention and the apparatus for carrying out the process will be explained more fully hereinafter with reference to the accompanying drawings in which FIGS. 1 to 1-8 are diagrammatic views illustrating the various process steps. In these figures, the parts which are essential for the respective operation are indicated by thicker lines.

FIG. 19 is a diagrammatic end elevation showing the tunnel-driving machine.

FIG. 20 is a vertical sectional view taken on line XX XX of FIG. 21 and shows a portion of alength section of the shield.

FIG. 21 is an interior View showing the shield portion.

FIG. 22 is a view similar to FIG. 20 and shows a different spacer bar and a different installing device.

According to FIG. 1, test bores 3 are driven ahead of a tunnel 2, which has been constructed with the aid of a tunnel-driving machine 1. With the aid of these test bores a zone 4 of rock of low cohesion is detected. The tunneldriving machine 1 provided with radially adjustable milling cutters 5 is now retracted about two meters from the face of the tunnel. Thereafter, a worker excavates an annular groove 6 in the tunnel 2 with the aid of a pneumatic drill. The tunnel-driving machine 1 then advances as far as to the annular groove 6, and the milling cutters 5 are adjusted radially outwardly for driving a tunnel having an enlarged diameter, which exceeds the original tunnel diameter by twice the thickness of the shield segments which are to be installed.

If test bores 3 are not drilled during the driving of the tunnel, the milling cutters 5 of the tunnel-driving machine 1 will cut directly into the zone 4 of rock of low cohesion, as is indicated in dotted lines in FIGS. 1 to 7. When this occurs, the tunnel-driving machine 1 must move back to the position shown in FIG. 1 before the excavation on the increased diameter can begin so that the tubular shield can subsequently be installed in stable rock.

When the milling cutters 5 have been adjusted radially outwardly, the tunnel-driving machine 1 advances by about 1 meter. During this advance, the machine cuts only to increase the tunnel diameter or throughout the crosssection. In this phase, the tunnel-driving machine is guided in the tunnel 2 by means of a lower skid 7 and a top protecting roof 8. When the desired length of tunnel has been driven (FIG. 2), the milling cutters 5 are adjusted to the initial diameter or to a retracting diameter and the tunneldriving machine 1 is retracted about 2 meters. A worker shovels the residual loose material to the conveyor of the tunnel-driving machine (FIG. 3).

The conveyor of the tunnel-driving machine 1 is then reversed and may be used to move shield segments 9 ahead of the tunnel-driving machine 1. To distribute the shield segments 9, a bracketlike installing device 11 is secured to the end face of the drum 10 of the tunnel-driving machine, which drum carries the milling cutters 5. With the aid of the drum drive, the shield segments 9 are subsequently swung into the correct position, moved with the aid of radially extending spindles to the tunnel surface and bolted together (FIG. 4). When all shield segments have been bolted together to form a section of the shield,

the installing device 11 is removed and the tunnel-driving machine 1 advances to the face of the tunnel or to the position assumed at the end of the preceding cycle of operations. Thereafter the milling cutters 5 are adjusted radially outwardly and the tunnel is driven for an additional length of about 1 meter (FIG. 5). When this length of tunnel has been driven, the milling cutters 5 are moved back to the retracting diameter, the machine is retracted and a worker cleans the milled tunnel section for the installation of the next section of the shield. This is shown in FIG. 6. The installing device 11 is then secured once more to the drum 10. The conveyor of the tunnel-driving machine is operated in the forward direction to supply new shield segments, which are assembled to form the next section of the shield with the aid of the installing device. FIG. 7 shows the installation of the final segment at the roof of the tunnel.

The operations shown in FIGS. 4 to 7 are repeated several times until the position shown in FIG. 8 has been reached. Thereafter the tunnel-driving machine advances for the installation of the last section of the shield. During this operation, the propelling unit 12 of the tunneldriving machine 1 bears on the inside of the assembled tubular shield under the action of the pressure applied to strutting jacks 13 whereas propelling jacks 14 force the machine against the face of the tunnel.

FIG. 9 shows the insertion of the last section of the shield when the tunnel-driving machine 1 has been retracted a suitable distance. The segments 9a form a wedge-shaped annular cutting edge at the free end of the shield in order to facilitate the subsequent penetration of the shield into the rock or loose material.

When the segments 9a have been bolted together, the propelling

unit

12, 13, 14 advances the machine 1 to the face of the tunnel. The milling cutters 5 are then adjusted to the increased diameter and the entire propelling unit is fixed in the tubular shield with the aid of wedges 15. The wedges 15 must subsequently transmit the propelling pressure from the shield to the tunnel-driving machine 1. The trailing sections of the shield, which were installed first, are then knocked down and removed. This operation and the following assembling work may be carried outwith the aid of the machine for placing the lining rings. The trailing sections of the shield are replaced by thinwalled sheet metal elements, which are assembled to the correct diameter in the tunnel and welded together. A thrust-bearing ring 17 is installed in the resulting shield tail 16. Hydraulic jacks 18 bear on the thrust-bearing ring 17 as well as on abutments 19 which are secured to the shield (FIG.

FIG. 11 shows the beginning of the lining of the tunnel with the usual lining ring segments 20. The jacks 18 are retracted to pull the thrust-bearing ring 17 to the shield. The lining ring segments are inserted into the space which has thus been cleared in the shield tail 16 and are bolted together. The lining ring segments shown in the drawing consist of pressed steel plates. Alternatively, concrete lining ring segments 20a shown in FIG. may be employed. Concrete lining rings are heavier, however, and their larger thickness inhibits the retraction of the tunnel-driving machine 1. Back-filling concrete 21 is placed for transmitting the reaction forces due to the propulsion of the shield from the lining ring segments to the tunnel wall 2. To advance the shield, the jacks 18 are extended by the width of a lining ring (FIG. l2). During this operation, the jacks 18 bear through the thrust-bearing ring 17, the lining ring segments 20 and the baclofilling concrete 21 on the tunnel wall 2. By this operation, the tubular shield and the tunnel-driving machine 1 secured in it are advanced. The pump or the like of the propelling

unit

12, 13, 14 is switched over to supply pressure oil to the jacks 18. The latter are then retracted so that the thrust-bearing ring 17 follows and the segments 20 for the second lining ring can be installed (FIG. 13). FIG. 14 shows another step of advancing the shield when the second lining ring has been assembled.

To enable a driving of the tunnel in a prescribed direction or with a desired curvature, provision must be made for lateral and height corrections during the driving of the tunnel. For such corrections, the axis of the tunneldriving machine 1 is displaced or pivotally moved relative to the axis of the shield. Thereafter, only the milling cutters are cutting in the new direction and the shield follows owing to the reduced resistance in that area. The machine may also be advanced and retracted in the shield for making corrections.

In the position shown in FIG. 17, the tunnel has been driven in the loose zone to such an extent that the rear part of the tunnel-driving machine slides on the lining ring segments 20. That rear part consists of a platform 22 resting on skids. The body of the tunnel-driving machine is provided at its rear end with a pin 23, which is held in a steering device 24 for vertical and horizontal adjustment. This steering device may be used to effect the desired correcting adjustment of the axis of the tunneldriving machine relative to the axis of the shield.

When the zone of low cohesion has been traversed and stable rock has been reached, the jacks 18, the thrustbearing ring 17 and the abutments 19 are removed. To compensate for the difference in height, two filling rings 25 are installed in the shield tail 16 and the milling cutters are adjusted back to the normal tunnel diameter. The wedges 15 are removed to release the tunnel-driving machine and the source of pressure oil is connected to the propelling unit of the tunnel-driving machine. When the tunnel-driving machine 1 moves out of the shield (FIG. 17), the propelling unit bears temporarily on the shield segments 9, and the skids of the platform 22 slide over the filling rings 25 into the shield. The latter remains in position in the tunnel. When the tunnel-driving machine has left the shield, the latter can be knocked down and be re-used in the next zone of low stability.

The milling cutters 5 normally operate ahead of the cutting edge of the shield. When the rock conditions deteriorate to such a degree that a proper excavation by the milling cutters disposed ahead of the cutting edge of the shield can no longer be carried out, the hydraulic supply can be changed for a short time from the jacks 18 to the propelling jacks 14 of the tunnel-driving machine so that the latter is retracted into the shield. This is indicated with solid lines in FIG. 18. During this operation, the tunneldriving machine and particularly its propelling unit remains secured in the shield. Before the machine is retracted, the milling cutters must be adjusted radially inwardly so that they will not collide with the annular cutting edge of the shield. When the tunnel-driving machine has been retracted, the hydraulic source is connected to the propelling jacks 18 and the shield tunneling with installation of the lining ring segments 20 is continued as described. When underground water is encountered or the tunnel is driven under rivers, straits or the like, compressed air may be used in the shield. The limiting working conditions will be reached when a proper excavation is not possible in spite of the increased air pressure in the tunnel and with the tunnel-driving machine fully retracted.

To prevent a caving-in of substantial amounts of loose material, pockets 26 which are open in front are secured to the drum 10 of the tunnel-driving machine 1 between the milling cutters 5. This is indicated in FIG. 19.

FIGS. 20 and 21 show two shield segments 9 and a bar 27 which is wedge-shaped in cross-section and inserted between those edge faces of the segments 9 which are parallel to the axis of the shield. Such bars 27 enable an adaptation of the shield to different tunnel diameters without change of the segments 9. It is apparent from FIG. 22 that the use of a relatively thick bar 27a results in a considerable increase of the diameter of the section formed by the segments 9. A stiffening rib 28 is parallel to the axis of the shield and enables the segments 9 to carry and guide the heavy tunnel-driving machine.

The stiffening ribs 28 enable also an assembling and disassembling of the shield segments by means of a device which differs from the installing device 11 and is shown in FIG. 22 to comprise an arm 29, which is pivoted to a suitable part or frame and carries a gripper 30, which can be radially adjusted by a crank handle 31 and opened and closed by a crank handle 32.

What is claimed is:

1. A process of driving tunnels in rock having zones which differ in stability, which comprises the steps of excavating zones of stable rock only by a tunnel-driving machine having rotatable and radially adjustable cutting tools, advancing the tunnel-driving machine in steps in a zone of stable rock to the transition to a zone of rock of low cohesion and operating the tunnel-driving machine during such steps to excavate on a diameter which is larger than the diameter of the excavation in the preceding portion of the zone of stable rock,

retracting the tunnel-driving machine after each of said advancing steps with the cutting tools is a radially inwardly adjusted position,

installing a section of tubular shield ahead of the tunnel-driving machine after each of said retracting steps and before the next succeeding advancing step, said section consisting of a plurality of separable segments,

anchoring the tunnel-driving machine in the shield,

advancing the shield through the zone of rock of low cohesion until stable rock is reached,

lining the tunnel behind the shield with lining rings,

releasing the tunnel-driving machine from the shield when the stable rock is reached,

using the tunnel-driving machine alone for excavating the stable rock, and

knocking down the shield.

2. A process as set forth in claim 1, in which a tunneldriving machine is used in which said cutting tools consist of milling cutters adapted to revolve in unison about the longitudinal axis of the machine.

3. A process as set forth in claim 1, in which the cutting tools of the tunnel-driving machine are operated to excavate during the advance of the shield.

4. A process as set forth in claim 1, in which said tunnel-driving machine is used for excavation alone in the zone of stable rock until the zone of rock of low cohesion is reached,

the tunnel-driving machine is subsequently retracted for a distance which is at least as large as the length of the tubular shield, and

the tunnel-driving machine is operated during said advancing steps to increase the diameter of the previously excavated tunnel by twice the thickness of the shield segments.

5. A process as set forth in claim 4, in which an annular groove is excavated in the tunnel wall ahead of the retracted tunnel-driving machine, and

the cutting tools of the tunnel-driving machine are adjusted in said groove to the increased diameter of excavation.

6. A process as set forth in claim 1, which comprises knocking down the two rearmost sections of the shield when the tunnel-driving machine has advanced ahead of said two rearmost sections,

replacing said two rearmost sections by sheet metal elements which are installed and welded together in situ to form a shield tail,

installing a thrust-bearing ring in said shield tail,

causing hydraulic jacks for advancing the shield to bear on said thrust-bearing ring, and

installing the first lining ring behind the thrust-bearing ring.

7. Apparatus for driving tunnels in rock having zones which differ in stability, said apparatus comprising a tunnel-driving machine having a hydraulically operable propelling unit,

a shield consisting of separable arcuate segments mounted about said tunnel-driving machine and releasably connected to it,

a thrust-bearing ring,

hydraulic jacking means bearing on said thrust-bearing ring and operable to advance said shield, and

means for supplying hydraulic pressure fluid alternatively to said propelling unit and to said jacking means.

8. Apparatus as set forth in claim 7, said last-named means being a pump for supplying hydraulic pressure fluid alternatively to said propelling unit and to said jacking means.

9. Apparatus for driving tunnels in rock having zones which differ in stability, said apparatus comprising a tunnel-driving machine comprising a drum which is rotatable on the axis of said machine,

rotatable and radially adjustable cutting tools mounted on the leading end face of said drum,

said apparatus further comprising a shield consisting of separable arcuate segments mounted about said tunnel-driving machine and releasably connected to it,

means for alternatively advancing said tunnel-driving machine alone and said shield with said tunnel-driving machine and a bracketlike installing device which is mounted at the leading end of the drum and comprises handling means for handling said arcuate segments,

said handling means extending radially with respect to the axis of the drum.

10. Apparatus as set forth in claim 9, in which said handling means comprise spindles extending radially with respect to the axis of the drum.

11. Apparatus for driving tunnels in rock having zones which differ in stability, said apparatus comprising a tunnel-driving machine comprising a drum which is rotatable on the axis of said machine,

rotatable and radially adjustable cutting tools mounted on the leading end face of said drum, and

forwardly open pockets provided on said leading end face of said drum between said cutting tools,

said apparatus further comprising a tubular shield consisting of separable arcuate segments mounted about said tunnel-driving machine and releasably connected to it, and

means for alternatively advancing said tunnel-driving machine along and said shield with said tunnel-driving machine.

References (Cited UNITED STATES PATENTS 2,425,169 8/ 1947 Wilson 6l85 2,466,709 4/ 1949 Karr 61-85 3,061,287 10/1962 Robbins 29931 3,411,826 11/1968 Wallers et al. 29931 FOREIGN PATENTS 114,413 1918 Great Britain.

ERNEST R. PURSER, Primary Examiner US. Cl. X.R. 299ll, 31, 33