US3589135A

US3589135A - Trenchless laying of underground pipes

Info

Publication number: US3589135A
Application number: US806912A
Authority: US
Inventors: Ainsley Neville Ede
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-03-15
Filing date: 1969-03-13
Publication date: 1971-06-29
Anticipated expiration: 1988-06-29
Also published as: NL6903909A; FR1600083A; GB1263464A; DE1913101A1

Abstract

A method of laying lengths of underground pipe, for example drainage pipes, by forming an underground tunnel by means of a mole plough, and drawing a length of pipe into the tunnel into its required final position, characterized in that a liquid slurry is injected into the tunnel formed by the mole before the pipe length is drawn into the tunnel, the slurry serving to lubricate the pipe during its travel along the tunnel. Preferably the principal ingredient of the slurry is Bentonite. The pipe may be towed directly behind the mole and drawn progressively into the tunnel as it is formed or the pipe may be drawn into the slurry-filled tunnel after the mole plough has completed its traverse.

Description

Unlted States Patent 1 1 3,589,135

[72] lnventor Ainsley Neville Ede 1,314,279 8/1919 McCrary 61/72.2 36 Thornton Way, Cambridge, England 1,765,185 6/1930 Umbarger 61/72.2 X

[21] Appl. No. 806,912 1,904,666 4/1933 Sack 61/72.7

[221 Filed Mar. 13,1969 3,100,381 8/1963 Case et a1. 6l/50X [45] Patented June 29,1971 3,214,920 11/1965 Jacobs 61/72.7 X

[32] Pl'lOl'lly Mar. 15, 1968 FOREIGN PATENTS [33] Great Britain [3 2783/68 976,421 1964 Great Br1ta1n 6l/72.5

[S4] TRENCHLESS LAYING OF UNDERGRGUND PIPES Primary Examiner-Jacob Shapiro Attorney-watson, Cole, Grindle & Watson ABSTRACT: A method of laying lengths of underground pipe, for example drainage pipes, by forming an underground tunnel by means of a mole plough, and drawing a length of pipe into the tunnel into its required final position, characterized in that a liquid slurry is injected into the tunnel formed by the mole before the pipe length is drawn into the tunnel, the slurry serving to lubricate the pipe during its travel along the tunnel. Preferably the principal ingredient of the slurry is Bentonite. The pipe may be towed directly behind the mole and drawn progressively into the tunnel as it is formed or the pipe may be drawn into the slurry-filled tunnel after the mole plough has completed its traverse.

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SHEET u' UF 5 vvvv Fla] INVENTOR ATTGRNEY IINVENTOR Amway NEVILLE EM ATTORNEY TRENCHLESS LAYING OF UNDERGROUND PIPES This invention relates to the laying of lengths of underground pipe, for example drainage pipes, in situ by forming an underground tunnel by means of a mole plough and drawing a length of pipe into the tunnel into its required final position. By this method, known as trenchless pipe laying, a considerable length of a pipe, perhaps a single length up to 350 feet long and up to 18 inches or more in diameter, can be laid in position underground without the need for opening up a trench to receive the pipe. Either the underground tunnel is first formed by means of a complete traverse of the mole plough and then a length of pipe is drawn into the preformed tunnel as by winching, or else the pipe length is towed behind e the advancing mole so as to be drawn progressively into the tunnel as the latter is being formed by the traverse of the mole through the ground.

A difficulty that has been encountered in the trenchless laying of pipe arises from the very considerable soil friction which has to be overcome when drawing the pipe length into the newly formed tunnel, especially where the tunnel lies at a considerable depth below ground, since the soil friction is found to vary in almost direct proportion to the soil depth on account of the soil pressure due to compaction. It is an object of the present invention to reduce this variation of soil friction with tunnel depth, and also to reduce the total soil friction encountered by the pipe when being drawn into the tunnel; and in addition, when working in loose earth or running sand or in waterlogged conditions, to stabilize the tunnel against collapse before the pipe is drawn into it, as well as to provide an improved and stabilized bed for the pipe.

According to the present invention, a liquid slurry of clay or cement or soil, or an admixture of two or more of the same, with or without any other ingredient or ingredients and similar to the slurries and muds commonly used in the drilling of soil, is injected into the tunnel formed by the mole before the pipe length is drawn into the tunnel, the slurry serving to lubricate the pipe during its travel along the tunnel. It is found that the presence of the slurry in the tunnel isolates the pipe to a certain extent from direct contact with the tunnel wall, and not only substantially reduces the frictional drag on the pipe but also makes this drag substantially independent of the depth of the tunnel below ground level.

Preferably the sole or a principal ingredient of the slurry is Bentonite, which is a Montmorillonite clay in dried and ground form which will take up water in large quantities, up to 1,200 times its own volume, by a physicochemical effect. When mixed with water in suitable quantities Bentonite forms a jelly having thixotropic properties, which can be pumped down into the tunnel formed by the mole by means of a positive-displacement pump such as a worm-type pump or a Mono pump, either in metered quantities or in quantities limited only by the space available. Again, the slurry may be propelled down into the underground tunnel by pneumatic pressure, for instance applied to a closed hopper connected to a delivery pipe leading down into the tunnel. In this case metered delivery by means of a regulating valve is preferable, in case the whole available quantity of slurry should otherwise be lost from the tunnel into a soil cavity.

The slurry instead of being simply a mixture of Bentonite and water may comprise a mixture of Bentonite and soil in water, for example a mixture of approximately 50:50 ratio of Bentonite and soild in water has been found satisfactory. Either the mixture may be premixed before delivery, or the Bentonite slurry may be delivered separately into the tunnel and there mixed in place with the soil of the tunnel wall by means of an underground mechanical mixer towed behind the mole.

Again, a mixture of Bentonite and cement in water may also be used as the slurry, and will'set to provide a flrm bed for the pipe after the latter has been drawn into position.

As an illustration of the effectiveness of the present invention, a 350-foot length of 6-inch diameter bare pipe drawn into a bare soil tunnel formed by a mole plough might require a drag of 2,000 pounds weight at a depth ofa few inches below ground level, rising with a substantially straight line characteristic to about 40,000 pounds weight at 10 feet depth. However if the tunnel contains a Bentonite slurry into which the pipe is drawn so that the slurry surrounds the circumferential surface of the advancing pipe and isolates it from direct contact with the soil, the drag on the pipe may be expected to remain substantially constant at about 12,000 pounds weight at all depths, at least up to 10 feet.

The pipe-laying operation may be performed in either of two ways. Either a complete length of the tunnel may first be formed by a traverse of the mole plough, the slurry being pumped down through a passage in the mole plough coulter and delivered into the tunnel behind the mole so as to fill the tunnel completely, and when the mole plough has completed its traverse a length of pipe may then be drawn into the slurryfilled tunnel by means of a cable and winch, or else the pipe length may be towed directly behind the advancing mole so that the pipe is drawn progressively into the tunnel as the latter is being formed by the mole, the slurry being pumped via the mole plough coulter into the space: in front of the leading end ofthe pipe and around the circumference of the pipe.

In the former case, an expander of generally flaring hollow form may be attached at its smaller end preferably via a pivotal connection to the pipe-drawing cable immediately in front of the pipe length to expand the newly formed passage before the main part of the pipe length enters it, and the slurry will be forced into the soil forming the circumferential surface of the tunnel by the expander, as well as passing around and possibly through apertures in the expander to surround the ad vancing pipe length with a circumferential layer of slurry perhaps one-halfinch to 1 inch thick.

Preferably the expander is of such a form that the bottom of the expander follows the path made by the bottom of the mole plough so that expansion occurs in the upwards and sideways directions which thus has the effect of completely sealing the channel above the passage fon-ned by the tool stalk of the mole plough.

The expander may be any suitable flared form, its leading end being dimensioned to fit closely into the tunnel previously formed by the mole, and it should be of symmetrical, substantially circular cross section over its whole length, and it may be made free to rotate without disturbance of its effective depth below ground level. For example the expander may take the form of a simple frustum of a right cone, or a bell, or series of cone frustra or bells of successively greater cross-sectional size, with or without intervening cylindrical sections. The expander may be of longitudinally fluted form instead of being of pure circular section.

In practice it is desirable to use the same mole plough machine for laying pipes of various sizes in which case the expander size is chosen to suit the particular application. Conveniently the expander is formed of two portions the larger trailing end portion of the expander being detachable from the leading end portion, for example by a bayonettype connection, so that trailing end portions of different sizes can be fitted according to the size of pipe to be laid.

The invention may be carried into practice in various ways, but one specific embodiment will now be described by way of example only with reference tothe accompanying drawings, in which:

FIG. 1 is a sectional side view of a pipe-laying apparatus incorporating a mole plough and an expander,

FIG. 2 is a detailed view showing the expander and pipe connected thereto, as used in FIG. 1,

FIG. 3 is a perspective diagrammatic view of the trailermounted slurry reservoir to which the mole plough of FIG. 1 is connected,

FIG. 4 is a diagram showing schematically the valve and pump circuit associated with the reservoir,

FIG. 5 is a perspective view of a modified form of expander, the trailing end portion of which is detachable,

FIG. 6 is a view similar to FIG. I of a modified form of the apparatus incorporating the two-piece expander shown in FIG. 5,

FIG. 7 is a view similar to FIG. 1 of a modified form of the apparatus incorporating a rotary mixer, and

FIG. 8 is a view similar to FIG. 1 further modified to incorporate an expander which is towed by a cable after the mole plough has made its initial tunnelling pass.

In the first embodiment, a mole plough 10 comprising a mole 11 is mounted at the foot of a coulter blade 12 having a rigid slurry feed pipe 13 attached generally vertically to its rear edge and leading at its lower end via a flexible connection to a rearwardly directed tube 14 mounted on an expander 15 which is towed directly behind the mole plough, the tube 14 communicating with a passage 16 within the expander 15 which comprises a simple, rigid, flared member of generally frustoconical external form and which is attached by means of a pivotal linkage 18 at its nose to the rear end of the mole 11, the pivotal linkage l8 permitting up and down and sideways pivotal movement of the trailing expander 15. The expander 15 has a short extension of cylindrical form 17 extending rearwardly from its rear end within which is mounted the leading end of a continuous length of plastics pipe 24, the detachable manner by which the pipe 24 is connected to the expander being shown in FIGS. 2 and 6.

As shown in FIG. 2 the leading end of the plastics pipes 24 is attached by a pin 25 which passes through transverse holes in a short cylindrical spigot 26 located within the end of the pipe the spigot 26 being loose fitting in the bore of the pipe and being closed at its forward end, and having forwardly projecting lugs 27 to which are attached corresponding lugs 28 which cooperate with a releasable catch secured to the expander IS. A plug 29 of foam rubber is compressed between the links and the end of the pipe 24 the purpose ofwhich is to prevent liquid slurry entering the pipe 24 or getting into the locking and release mechanism of the releasable catch. The details of the releasable catch are shown in FIG. 6 and will be described later.

The upper end of the feed pipe 13 attached to the coulter I2 is connected by a hose 27 to a trailer-mounted reservoir 30 shown in FIGS. 3 and 4 and provided with a positive-displacement pump 31 of the worm type, to whose delivery outlet the hose 27 is connected.

The reservoir 30, the bottom part of which is of V-shaped cross section, is filled with a slurry of Bentonite and water in the form of a thixotropic jelly 32, which can be pumped along the hose 27 and down through the slurry feed pipe 13 and then through the tube 14 and the passage 16 in the expander 15 and around the foam plug 29 and into a tunnel 33 newly formed in the soil by the traverse of the mole as shown in FIG. 2. As shown in FIGS. 3 and 4 the Bentonite slurry is pumped out from the lowest part of the V bottom of the reservoir 30 by the pump 31 and then under pressure via a gate valve 34 to the delivery pipe 35 which as shown in FIG. 3 communicates via a T connection with a transverse pipe 36 providing alternative delivery points to the left or to the'right below the reservoir frame. The hose 27 is connected to one of the delivery points and a cap 37 is fitted to the delivery end which is not in use.

A bypass return line 38 connects the outlet from the pump 31 back to the reservoir 30 via a gate valve 39 the return line 38 leading to a pipe 40 leading through the interior of the reservoir 30 which pipe 40 within the reservoir has two pairs of small diameter forwardly directed jets 41 for discharging the slurry back into the reservoir. A pressure control valve 42 is provided above the return line 38 in a second bypass line 43, the pressure control valve being controlled by means of weights 44.

Initially for mixing the slurry, the reservoir 30 is filled with water and the pump 31 started with the return line valve 39 open and the delivery valve 34 closed so that all the pump delivery circulates back to the reservoir 30 via the return jets 4I. Bentonite powder is then added until the desired consistency is reached. With the weights 44 suitably set, the return valve 39 is closed so that the circulation of slurry is forced through the pressure control valve 42 back to the reservoir 30. The slurry can then be drawn off for delivery to the hose 27 and expander 15 by opening the delivery valve 34.

The delivery pressure will be determined by the weights 44 of the pressure control valve 42, and the volume of slurry which is delivered underground, dependent upon the speed of the mole and thesize of the tunnel 3, is in practice always less than the capacity of the pump, the excess slurry being returned via the pressure control valve 42 to the reservoir 30. The return jets 41 cause the recirculated slurry to stir the bulk of the Bentonite slurry in the reservoir and being thixotropic the slurry in the reservoir and being thixotropic the slurry thus remains liquid as long as it is circulated.

The reservoir 30 may be trailer mounted for drawing alongside the mole ploughing machine or may instead be mounted on the mole plough machine itself.

In use the mole 11 and the rigid coulter blade 12 are forced through the soil by means of a tractor or a winch to which the mole plough 10 is coupled, with the mole running at a graded depth of perhaps 2 feet, to form a tunnel 33. The expander 15 and the leading end of the pipe 24 attached thereto are towed directly behind the advancing mole 11, the expander 15 expanding the tunnel by the upward and sideways compaction of the soil structure, thereby to close the channel above formed by the coulter blade 12, the bottom wall of the tunnel remaining substantially undisplaced, by virtue of the expander pivotal connection 18 which permits angular freedom of the expander 15. Bentonite slurry is continuously pumped via the hose 27 and feed pipe 13 through the expander 15, the slurry passing around the leading end of the pipe 24 to occupy the annular space 38 between the circumference of the pipe 24 and the wall of the expanded tunnel, so that the pipe length 24 will be towed behind the expander 15 through a layer of Bentonite slurry perhaps one-half inch thick at its minimum below the pipe, and somewhat thicker above and around the sides of the pipe. This layer of slurry surrounding the pipe 24 acts as a lubricant to reduce frictional drag as already described.

In a modified form of the apparatus shown in FIGS. 5 and 6, a two-piece expander is used, the larger portion 51 of which at the trailing end is detachable so that it can be interchanged for larger or smaller end portions according to the size of the pipe 24 to be laid. The detachable trailing portion 51 of the expander can be fitted into the leading nose portion 52 by a bayonet type connection, by means of projecting lugs 53 on a tubular shank 54 which is of common size for all expanders, which lugs are inserted through slots 55 provided in the nose portion 52, the trailing portion 51 then being rotated slightly and locked by means of a suitable locking bolt (not shown).

At the completion of a pipe-laying run it is convenient to release the pipe underground at a specific point without having to dig a hole around the mole plough, and for this purpose a releasable catch is provided as shown in FIG. 6. The releasable catch comprises a pivotal hook mounted in the expander which engages a pin attached to the lugs 28 (see FIG. 2) connected to the end of the pipe 24, the hook 60 being locked in the traction position by a spring loaded withdrawable bolt 61, the hook 60 being spring biased to its released position by a spring 62. At the end of the run the hook 60 is opened by withdrawing the locking bolt 61 by pulling on a cable 63 attached thereto which cable passes around a pulley 64 in the mole and up through the coulter to above ground level. A toggle 65 is provided on the hook 60 for use when attaching the end of the pipe 24 initially.

FIG. 7 shows a further modification of the apparatus incorporating a mechanically driven rotary mixer mounted directly behind a rearwardly directed nozzle 71 through which the slurry is discharged, the mixer comprising a power-rotated drum having radically protruding teeth by which the newly discharged slurry is mixed into the surface layer of soil forming the .wall of the tunnel. In this case the slurry 32 will preferably comprise a mixture of Bentonite alone with water, no soil being present in the slurry or pumped down through the coulter. If no mechanical mixer is used, however, a proportion of soil may be included in the slurry in the reservoir for pumping down into the tunnel.

In a modified engagement arrangement and process, illustrated in FIG. 8 the mole plough first makes an initial tunnelling pass during which the expander and pipe 24 are left at the entry point of the tunnel 33 and thereafter the expander and pipe 24 are winched along the tunnel 33.

In this case the coulter blade 80 has an internal duct 81 leading to a rearwardly directed circular nozzle 82, and the expander 83 comprises a simple rigid, flared tubular sheet metal member of frustoconical form, about 8 feet long, which is formed with a series of apertures 84 in its wall and a central aperture 85 in its nose, is attached by means of a swivel joint 86 at its nose, is attached by means of a swivel joint 86 at its nose to the rear end of a towing cable 19 associated with the mole plough. The rear diameter of the mole 83 is 8 inches and the diameter of the leading end of the expander is about the same; at its rear end the expander is 24 inches in diameter.

Attached to the end of a towing cable 88 (which may conveniently comprise the same cable as that used for releasing the catch 60 shown in FIG. 6 if the same mole plough is used for this embodiment) is the leading end of a pipe towing rod 89. At its rear end the towing rod 89 is secured by a quickrelease coupling 90 to the leading end of a continuous length of pipe 24 some 350 feet in length and 18 inches in diameter. The towing rod 89 extends through the interior of the conical expander 83 from its leading end, and the quick-release coupling 90 is located just inside the rear end of the expander 83. The towing cable 88 leads forwardly from the expander 83 to a pulley 91 in the base of the hollow coulter blade 80 of the mole plough, whence it is passed upwardly through the coulter and around a second pulley 92 above ground level to a winch (not shown), the winch having a free spooling drum on which the cable is stored.

In use the mole makes an initially tunnelling pass with the mole running at a graded depth of perhaps 2 feet, to form a tunnel 33 initially 8 inches in diameter, which is filled with Bentonite slurry 32 pumped down through the coulter passage 81 as the tunnel is being formed by the traverse of the mole. The expander 83 and pipe 24 will be left at the entry point of the tunnel 33, and as the mole plough advances the free-spooling drum of the cable winch pays out cable 92 which passes down the coulter blade 80. When the mole plough reaches the forward end of the required pipe run it is stopped and the full power of the tractor or winch motor is applied to the cable winch to wind in the pipe-towing cable 92 and draw the expander 83 and the pipe length 24 through the slurry-filled underground tunnel 33 into the final required position of the pipe length. This pipe-drawing operation is performed very quickly, and as the expander 83 is drawn along the slurry-filled tunnel 33 it will expand the tunnel to about 24 inches diameter, forcing some slurry 32 into the tunnel wall. The expansion of the tunnel will again be effected by the upward and sideways compaction of the slurry-impregnated soil structure, and the pipe 24 when drawn into the slurry-filled tunnel 33 behind the mole 11 will rest directly on this undisturbed slurryimpregnated bottom of the tunnel in its predetermined grad ing. The slurry 32 previously pumped into the interior of the tunnel 33 in front of the advancing towed expander 83 will pass around and through the nose aperture 85 of the expander and will occupy the annular space between the circumference of the pipe 24 and the wall of the expanded tunnel.

In this process, either the correct amount of Bentonite slurry 32 required to fill the annular space around the pipe 24 is metered and delivered into the tunnel 33 before the pipe is towed into it, and the return of slurry through the coulter passage 81 during pipe towing is prevented by the closure if the stop valve 34; or else the tunnel 33 is first completely filled with the slurry, and the pipe 24 is then towed into the filled tunnel and excess slurry is forced back up through the conduit 81 and the hose 27 back into the reservoir 30 by the back pressure due to the advancing pipe and expander, the stop valve 34 being open and the pressure'loaded relief valve 42 operating to permit and control this back flow.

The use of the expander 83 facilitates the maintenance of the grade of the pipe 24 by utilizing the undisturbed bottom surface of the tunnel 33 as the reference line for the grade, despite the considerable distance between the entry point for pipe drawing and the foot of the mole plough which previously determined this reference line.

Whilst, as described above and illustrated, the attachment of the pipe tow rod 89 is to the main towing cable 92, and the expander 83 is free to pivot in any direction independently of the pipe 24, it is also possible, if required, to utilize the pipe draught tension to impose a small biassing force on the expander 33 towards its centralized position. This may be done by attaching the leading end of the pipe tow rod 89 rigidly to the leading end of the expander 83 itself, for example by means of a spider or other suitable fitting, and to provide a swivel joint in the pipe tow rod 89 a short distance, say 2 inches, behind the connection to the expander.

What I claim as my invention and desire to secure by Letters Patent is:

1. A method of laying a length of underground pipe in situ, which comprises forming a generally horizontal underground tunnel by forcing the mole of a mole plough through the ground, towing an expander behind the advancing mole to ex pand the cross section of the underground tunnel, and towing a length of pipe behind the expander along the expanded tunnel into its required final position, the pipe being of smaller cross section than the expander so that an annular-section clearance is defined between the exterior of the pipe length and the interior surface of the expander tunnel, and simultaneously extruding a thixotropic slurry of Bentonite and water under positive pressure through an extrusion orifice in the advancing expander and into the said annular clearance around the advancing pipe length to fill the said clearance with an annular layer of said thixotropic slurry through which the pipe length advances behind the expander as said tunnel is progressively formed and expanded, the slurry being pumped to the extrusion orifice by means of a positive-displacement pump from a reservoir above ground through a duct extending down to the expander.

2. A method as claimed in claim 1, in which an expander of generally flared form is attached at its smaller end to the mole immediately in front of the pipe length and is towed through thetunnel with the pipe length so as to expand the tunnel before the main part of the pipe length enters it.

3. A method as claimed in claim 2 wherein the leading end of the pipe is connected to the expander by means ofa releasable catch, which can be released by releasing means located above ground level, whereby at the end of the pipe-laying run the pipe can be detached from the mole plough coulter whilst underground.

4. A method of laying lengths of underground pipe by forming an underground tunnel by means of a mole plough, and drawing a length of pipe into the tunnel into its required final position, characterized in that a liquid slurry is injected into the tunnel formed by the mole before the pipe length is drawn into the tunnel, the slurry serving to lubricate the pipe during its travel along the tunnel, the principal ingredient of the slurry being Bentonite, the slurry comprising a mixture of Bentonite and soil in water, forming a gel having thixotropic properties, and in which a mixture of Bentonite and water is delivered separately into the tunnel by a positive displacement pump and is there mixed in place with the soil of the tunnel wall by means of an underground mechanical mixer towed behind the mole.

5. A method of laying a length of underground pipe in situ which comprises forming a generally horizontal underground tunnel of appreciably greater cross-sectional area than the pipe to be laid therein by forcing the mole of a mole plough through the ground, whilst simultaneously extruding a thixotropic slurry of Bentonite and water under positive pressure into the tunnel through an extrusion orifice at the rear of the advancing mole to fill the tunnel completely with pressurized slurry as the tunnel is progressively formed behind the advancing mole, and, subsequently, drawing a length of pipe by means of a cable and winch along the length of the preformed tunnel and through the filling of slurry therein into the required final position of the pipe length, the pipe length being drawn lengthwise through the filling of slurry which forms an annular layer of slurry surrounding the pipe length and a smaller end to the pipe-drawing cable immediately in front of the pipe length and is towed through the tunnel with the pipe length so as to expand the newly formed and slurry-filled tunnel before the main part of the pipe length enters it.

Claims

1. A method of laying a length of underground pipe in situ, which comprises forming a generally horizontal underground tunnel by forcing the mole of a mole plough through the ground, towing an expander behind the advancing mole to expand the cross section of the underground tunnel, and towing a length of pipe behind the expander along the expanded tunnel into its required final position, the pipe being of smaller cross section than the expander so that an annular-section clearance is defined between the exterior of the pipe length and the interior surface of the expander tunnel, and simultaneously extruding a thixotropic slurry of Bentonite and water under positive pressure through an extrusion orifice in the advancing expander and into the said annular clearance around the advancing pipe length to fill the said clearance with an annular layer of said thixotropic slurry through which the pipe length advances behind the expander as said tunnel is progressively formed and expanded, the slurry being pumped to the extrusion orifice by means of a positivedisplacement pump from a reservoir above ground through a duct extending down to the expander.

2. A method as claimed in claim 1, in which an expander of generally flared form is attached at its smaller end to the mole immediately in front of the pipe length and is towed through the tunnel with the pipe length so as to expand the tunnel before the main part of the pipe length enters it.

3. A method as claimed in claim 2 wherein the leading end of the pipe is connected to the expander by means of a releasable catch, which can be released by releasing means located above ground level, whereby at the end of the pipe-laying run the pipe can be detached from the mole plough coulter whilst underground.

5. A method of laying a length of underground pipe in situ which comprises forming a generally horizontal underground tunnel of appreciably greater cross-sectional area than the pipe to be laid therein by forcing the mole of a mole plough through the ground, whilst simultaneously extruding a thixotropic slurry of Bentonite and water under positive pressure into the tunnel through an extrusion orifice at the rear of the advancing mole to fill the tunnel completely with pressurized slurry as the tunnel is progressively formed behind the advancing mole, and, subsequently, drawing a length of pipe by means of a cable and winch along the length of the preformed tunnel and through the filling of slurry therein into the required final position of the pipe length, the pipe length being drawn lengthwise through the filling of slurry which forms an annular layer of slurry surrounding the pipe length and separating it from the wall of the tunnel as the pipe length advances.

6. A method as claimed in claim 5, in which an expander of generally flared form and provided with at least one passage for the flow of slurry therethrough is pivotally attached at its smaller end to the pipe-drawing cable immediately in front of the pipe length and is towed through the tunnel with the pipe length so as to expand the newly formed and slurry-filled tunnel befoRe the main part of the pipe length enters it.