AU2001295755A1 - Trenching method and apparatus - Google Patents

Description

TRENCHING METHOD AND APPARATUS

The present invention relates to a method of trenching, and to a trenching apparatus, both of which are applicable in particular, but not exclusively, for cutting a trench in rock.

There are known many trenching machines for digging trenches in soil by use of a prime mover such as a crawler tractor, or a conventional tractor pulling a trailer, in which a cutting device is positioned in the trench on a boom. The main examples are a cutting rotor rotating about an axis transverse to the trench, known as a ripper cutter, or one or more cutting rotors rotating about an axis aligned along the length of the boom, generally known as a milling cutter, or a cutting device having an elongate endless support means carrying a plurality of cutting elements and trained along upper and lower runs on the boom, known as a chain cutter. Where a cutting rotor is used this is mounted on the distal end of a boom projecting forwardly and downwardly from the prime mover, relative to the direction of digging of the trench. Where a chain cutter is used, this is normally mounted on a boom extending downwardly and rearwardly from the prime mover relative to the direction cutting of the trench. In such an arrangement the elongate support member moves in a direction such that the cutting elements move downwardly around the distal end of the boom and upwardly and forwardly along the lower run of the moving elongate support member. There is normally provided in all these forms of trenching machines, a positioning device for raising and lowering the distal end of the cutting boom to vary the height of the trench. Examples of such trenching machines are found in, for example, CH-A-239498 (Entreprise de Grands Travaux SA), and WO 95/13433 (Mastenbroek & Company Limited).

All these forms of trenching machine are generally satisfactory for trench cutting in normal soil conditions, but are not suitable for cutting trenches in rock or other hard ground materials. For cutting trenches in hard rock, labour intensive methods are generally adopted using percussion tools and explosives.

In another art unconnected with trenching, tunnelling machines for tunnelling into rock are known in which a cutting rotor known as a ball cutter protrudes forwardly on a boom from a prime mover, and is movable in a vertical plane by pivoting of the boom on the prime mover. In use the cutting rotor is lowered to the floor of the tunnel and the prime mover advances with the cutting rotor to engage the end face of the tunnel at floor level, to produce an undercut. The cutting boom is then pivoted in an upward direction by hydraulic rams so that the cutting rotor or rotors are lifted upwardly to cut a slice of material from the end face of the tunnel. The cutting boom is raised by application of a force between the cutting boom and the floor of the tunnel. In a variation of such a device, there is provided a form of chain cutter trained along upper and lower runs along the cutting boom and around the distal end of the cutting boom, for use for example in mining coal or soft stone. In such a machine the endless moving support means carrying the cutters is driven in a direction such that the cutters move upwardly around the distal end of the boom and move rearwardly along the upper run of the moving support member. Examples of both forms of tunnelling equipment are found in leaflets published in 1982 by Hawker Sidley Dosco Overseas Engineering Limited and entitled DOSCO 1982 "The Twin Boom TB600" and DOSCO 1982 "Mark II Heavy Duty Dintheader".

In addition to this prior art, there is disclosed in EP-A-0080802 (Wallace) a machine for cutting a trench in rock by use of a ripper cutter. EP-A-0080802 also acknowledges as prior art a known trench cutting machine comprises a giant chain saw mounted on a crawler chassis and equipped with tungsten carbide teeth which literally saw away the rock. However, this machine suffers from the disadvantage that considerable bounce is generated along the saw, especially when cutting the harder rocks, and this reduces the cutting efficiency. Primarily the problem arises because of the long unsupported length of the cutting arm. There is also discussion in EP-A-0080802 of ball cutting machines such as have been described above in tunnelling, and it is said that other rock cutting machines are known for face mining operations and these comprise a rotatable cutting head carried at the end of a boom pivotably attached to a crawler chassis. However, it is said that these known machines cannot be used for cutting trenches and furthermore suffer from a similar vibration problem because they too have a long unsupported boom carrying the cutting head.

In EP-A-0080802 it is said that these difficulties are overcome by providing a machine for cutting a trench in rock having a pivoted boom with a rotatable cutting head at the distal end and a telescopic control arm extending between a mobile work platform and the end of the boom adjacent the cutting head, the telescopic control arm being an hydraulically operable ram serving to move the cutting head on an arcuate path about the pivot axis of the boom. The machine operates by the hydraulic ram applying to the cutting head a force having a major vertical component. In operation the machine is positioned to straddle the line of the trench and the boom is lowered into contact with the ground. The cutting head is rotated whilst applying force by way of the hydraulic ram to move the cutting head on an arcuate path downwardly and to remove rock from the forward end of the trench. A conveyor is positioned on the floor of the trench and the spoil is carried away. The boom is then raised and the machine moved forward and the process is repeated.

It is said that the problem of vibration and bounce with the relatively long boom is avoided because the position at which the ram is coupled to the boom adjacent to the cutting head adds to the stability of the cutting head so eliminating the vibration and bounce problems which would be expected if the ram were omitted and the cutting force was applied by the long boom. It is said that because the control arm applies the necessary loading of the cutting head, and because of the relatively short distance between the cutting head and the point of support, the problem with cutter head bounce is largely avoided. However a disadvantage arises with this form of machine, as is acknowledged in EP-A-0080802, in that the force applied at the cutting head tends to lift the machine out of contact with the ground. It is suggested that additional ballast may carried, but it is nevertheless inescapable that there is a limit to the amount of force which can be applied to move the cutting head on its arcuate path, without the surface mounted components of the apparatus lifting from the ground surface.

It is one object of the present invention to provide a trench cutting apparatus in which the problems outlined above are avoided or reduced.

In accordance with the present invention there is provided a method of trenching comprising: positioning in a trench a cutting device mounted on a prime mover movable on the ground surface above the trench; engaging an end face of the trench with the cutting device; and moving the cutting device in a vertical plane while operating the cutting device, so as to cut material from the end face of the trench; in which the method includes: positioning the cutting device against the end face of the trench below the level of the ground surface, and moving the cutting device forward in the trench while operating the cutting device, so as to produce an undercut in the end face of the trench; and lifting the cutting device upwardly from the undercut by exerting a force between the cutting device and the ground surface above the trench and operating the cutting device during the upward movement so as to cut material from the end face of the trench.

Preferably in the step of producing the undercut the cutting device is positioned to engage the end face substantially at the bottom of the trench. It is to be appreciated that where features of the invention are set out herein with regard to a method according to the invention, such features may also be provided with regard to apparatus according to the invention, and vice versa.

In particular, there is provided in accordance with the invention trenching apparatus comprising a cutting device; mounting means for mounting the cutting device on a prime mover and for positioning the cutting device in a trench with the prime mover movable on the ground surface above the level of the trench, the cutting device being movable in a substantially vertical plane when engaged with an end face of the trench so as to cut material from an end face of the trench; in which the apparatus includes lifting means arranged to lift the cutting device upwardly while engaged with the end face of the trench by exerting an upward force between the cutting device and the ground surface above the trench; and control means arranged to operate the lifting means and the cutting device concurrently so as to lift the cutting device when in cutting engagement with the end face.

In connection with yet a further aspect of the present invention, there may be provided trenching apparatus comprising a cutting device; mounting means for mounting the cutting device on a prime mover and for positioning the cutting device in a trench with the prime mover movable on the ground surface above the level of the trench, the cutting device being movable in a substantially vertical plane when engaged with an end face of the trench so as to cut material from an end face of the trench; in which the apparatus includes lifting means arranged to lift the cutting device upwardly while operating the cutting device in cutting engagement with the end face of the trench, by exerting an upward force between the cutting device and the ground surface above the trench; and in which the cutting device comprises a boom having an elongate endless support means carrying a plurality of cutting elements and trained along upper and lower runs on the boom, the boom being mounted to project forwardly and downwardly relative to the intended direction of cutting the trench, and drive means arranged to drive the endless support means in a direction such as to carry the cutting elements upwardly around the distal end of the boom and rearwardly along the upper run of the endless support means.

The method of the invention brings the advantage that during the upward cutting stroke the force exerted between the cutting device and the ground surface is limited only by the force generated and applied, and is not limited by potential raising of components of trenching apparatus from the ground as in the prior art where the cutting is effected on the down stroke of the cutting device. It is not necessary to provide substantial weight in the part of the apparatus on which the lifting means is mounted, as would be the case if the lifting means were pressing downwardly on the cutting device during the cutting stroke.

There will now be set out a number of preferred features of the invention. Preferably the step of lifting the cutting device upwardly is carried out by exerting a force between the cutting device and the ground surface at a position substantially directly above the cutting device. Such an arrangement adds to the stability of the apparatus and allows most effective use of the upward force applied to the cutting device. Preferably the step of lifting the cutting device upwardly is carried out by exerting a force in a direction lying in a range from the vertical to 30° to the vertical, preferably in the range from 10° to the vertical to 20° to the vertical, and most preferably the direction of the said lifting force is inclined to the vertical in a direction forwardly relative to the direction of cutting of the trench.

Conveniently the cutting device is mounted on a pivoted boom extending forwardly relative to the direction of cutting of the trench, and the said lifting step is carried out by moving the cutting device along an arcuate path defined by pivotal movement of the boom. Conveniently the step of lifting the cutting device upwardly is carried out by exerting a force between the cutting device and a region of the ground surface spaced from the axis of pivoting of the boom in a forward direction along the trench.

Although a number of different cutting devices can be used in embodiments of the invention, it is particularly preferred that the step of cutting material from the end face of the trench is carried out by moving a plurality of cutting elements along upper and lower runs of an elongate endless support means trained along the boom, the cutting elements being driven at the distal end of the cutting boom in a direction upwardly around the end of the cutting boom and rearwardly along the upper run relative to the direction of cutting the trench. Such an arrangement is particularly advantageous because the cutting elements engage the undercut in a direction upwardly and rearwardly at the distal end of the boom so as to cooperate with the lifting force in bringing the cutting teeth into cutting engagement with the ground material. In the case of hard rock this allows an effective cutting action in which the movement of the cutting elements cooperates with the upward movement of the lifting means and the forward movement of the prime mover, during cutting. Also, the upper run of the elongate endless support means can be effective to carry away spoil, so that there is no need to provide a separate endless conveyor, or other means, to remove the spoil from the trench.

In alternative arrangements, the cutting device may comprise a cutting rotor mounted at the end of a boom, and the step of cutting material from the end face of the trench is carried out by rotating the cutting rotor about an axis aligned transverse to the length of the trench, or by rotating the cutting rotor about an axis aligned generally along the length of the boom.

In preferred forms it may be arranged that the lifting means is constructed to provide a power stroke in the upward direction and a return stroke in the downward direction, and to provide a greater force in the power stroke than in the return stroke.

It is particularly preferred that the cutting device comprises a boom having an elongate endless support means carrying a plurality of cutting elements and trained along upper and lower runs on the boom, the boom being mounted to project forwardly and downwardly relative to the intended direction of cutting the trench, and drive means arranged to drive the endless support means in a direction such as to carry the cutting elements upwardly around the distal end of the boom and rearwardly along the upper run of the endless support means.

Although in some arrangements the lifting means may be mounted on the prime mover, conveniently it is arranged that the lifting means comprises a base spaced from the prime mover and coupled thereto for movement with the prime mover, a source of the lifting force mounted on the base, and a link extending between the cutting device and the source of the lifting force. Preferably the link is mounted so as to be inclined to the vertical in use in a direction forwardly relative to the direction of cutting of the trench. In preferred forms, the cutting device is mounted on a pivoted boom extending forwardly relative to the intended direction of cutting of the trench, and the said lifting means is arranged to move the cutting device along an arcuate path defined by pivotal movement of the boom. Preferably the base is coupled to the prime mover so as to be spaced from the axis of pivoting of the boom in a forward direction relative to the intended movement of the prime mover along the trench. The base may consist of a structure mounted on skids which slide over the ground when propelled forwardly by the prime mover. In other arrangements the base may be mounted upon wheels, or in some cases may be mounted on a second prime mover arranged to cooperate with the first prime mover in moving the cutting device along the trench. Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-

Figures 1 and 2 show perspective views of a known trench cutting apparatus described in EP-A-0080802, Figure 2 showing details of the cutting device of the apparatus;

Figure 3 is a diagrammatic side view of a trench cutting apparatus embodying the present invention and utilising a chain cutter;

Figure 3a is a side view of the distal end of a chain cutter suitable for use in the embodiment of Figure 3;

Figures 3b and 3c are views of components shown in Figure 3;

Figure 4 is a diagrammatic end view of the front of the apparatus shown in Figure 3, taken in the direction A;

Figure 5 is a diagrammatic side view of details of the chain cutter shown in Figure 3, and Figure 5a is a partial plan view taken in a direction B in Figure 5 and illustrating the lower end of the chain cutter of Figure 5;

Figures 6a and 6b show respectively a block circuit diagram and a flow chart;

Figures 7a to 7g are diagrammatic illustrations showing a series of steps in the operation of an embodiment of the invention shown in Figures 3 to 6; and

Figure 8 shows a diagrammatic side view of a yet further alternative embodiment of the invention, in which the cutting device comprises a milling cutter. Figures 1 and 2 show a known trench cutting apparatus described in EP- A-0080802 for cutting a trench in rock. Two crawler chassis 3 and 5 are coupled together by tie bars 7, the rear crawler 3 having a cutting boom 2 pivotally mounted at 6. The forward end of the boom 2 has a cutting rotor 7 driven in rotation about an axis transverse to the trench to be cut. The distal end of the boom 2 is coupled to the forward crawler 5 by a telescopic control arm 8 comprising an hydraulic ram and telescopically extending sleeves coupled to the boom 2 by a coupling 9. In operation the machine is positioned to straddle the line of the trench and the boom 2 is lowered into contact with the ground. The cutting head 7 is rotated while applying force by way of the hydraulic ram 8 to move the cutting head 7 on an arcuate path downwardly and to remove rock from the forward end of the trench. An endless chain conveyor 4 is positioned behind the cutting rotor 7 and the spoil is carried away. The boom 2 is then raised and the machine moved forward and the process is repeated.

In such a machine, the disadvantage arises that the force applied to the cutting head 7 tends to lift the forward crawler 5 out of contact with the ground. To attempt to counteract this problem, first the hydraulic ram 8 is mounted on a heavy, independent, crawler chassis 5, and secondly additional ballast may be carried by either or both of the forward and rearward crawlers 5 and 3.

In Figures 3 to 5a there is shown a trench cutting apparatus embodying the present invention. In general, components of the embodiment of the invention which are known in the art, may be as set out in the previously mentioned specification EP-A-0080802, modified as necessary in the light of the features of the invention embodied in the apparatus shown. Referring firstly to Figure 3, trenching apparatus for cutting a trench in rock or the like has a single prime mover 21 consisting of a crawler chassis for movement over the ground surface 22. A cutting device indicated generally at 23 is mounted on the prime mover 21 by mounting means indicated generally at 24. A lifting means indicated generally at 25 is coupled to the cutting device 23 in the region of the distal end thereof, and is itself mounted on a mobile base indicated generally at 26 and coupled to the prime mover 21 by coupling means 27. The entire operation of the machine is controlled by control means indicated diagrammatically at 34, located in a cab 35 of the prime mover 21.

Considering now the detailed construction of the embodiment shown in Figure 3, the cutting device 23 consists of an endless chain cutter comprising an elongate endless support means 28 such as a chain, carrying cutting teeth 29, indicated in more detail in Figure 3a. The chain 28 is trained along upper and lower runs 30 and 31 on a boom 32. The cutting teeth 29 pass around the end of the boom 32 around a pulley 33, being driven in a sense such that at the distal end of the boom 32 the cutting elements move upwardly around the distal end of the boom and rearwardly along the upper run 30, relative to the intended direction of forward movement of the prime mover 21 , which is indicated by the direction X in Figure 3. Figure 3a shows the distal end of the boom 32 and details of the mounting of the teeth 29 on the endless support means 28. The chain cutter 23 is driven by drive means including an hydraulic drive motor mounted in or on the prime mover 21 , and an upper driven pulley 49 indicated in Figure 5, to be described hereinafter. In general the cutting device 23 may be a chain cutter as shown in prior published patent application WO 95/13433, although the chain cutter in that publication is driven in movement in the opposite sense to that shown in the present embodiment, so that the directional alignment of the teeth is reversed in the prior publication mentioned.

In the embodiment shown in Figure 3, the mounting means 24 for mounting the boom 32 on the prime mover 21 comprise a pivot shaft 33 mounted between two mounting members 34A mounted on the main frame of the prime mover 21. The base 26 comprises a support skid having skids 36 for sliding movement over the ground surface 22, driven in movement by the prime mover 21 through the coupling means 27, which may comprise skid tie beams pivotally mounted on the base 26 at pivots 37 and pivotally mounted on the prime mover 21 at pivots coincident with a forward axle 38 of the crawler track

39 of the prime mover 21. The lifting means 25 comprises an hydraulic cylinder

40 pivotally mounted at a pivot 41 to upstanding support members 42 on the mobile base 26, as shown particularly in Figure 4. A drive piston 43 extends downwardly from the ram 40 and is coupled at pivots 44 to the cutting boom 32, by way of a stirrup shaped coupling member 45. In Figure 3, the cutting device 23 is shown in full lines in a lower position at the base of the trench, and in broken lines in a raised position.

As shown in Figure 4, conveniently the cutting device has in addition to the cutting chain 28, extension drums 46 and 47 extending sideways from the pulley 33 at the distal end of the cutting boom, so as to widen the channel cut by the cutting means, (particularly as shown in Figure 4). Figure 5a shows a detailed view of the cutting device 23. There is positioned behind the distal end of the boom 32 a deflector plate assembly 48 for collecting debris cut by the cutting chain 28 and the extension drums 46, 47. The deflector plate assembly 48 guides the debris inwardly towards a central area where the debris is carried upwardly and rearwardly by the chain cutter 23. As shown in Figures 5 and 5a, at the top of the cutting boom 32, the chain cutter 23 passes around an upper pulley 49 and deposits the debris onto a side discharge conveyor 50, by way of a boom discharge hopper 51.

In Figure 6 there is shown an alternative form of mobile base 26, which consists of a movable platform 57 mounted on wheels 58, and coupled to the prime mover 21 by a curved arm 59. The arm 59 is pivotally mounted at a pivot 60 on the base platform 57, and is pivotally mounted at a pivot 61 onto upper extensions 62 of the mounting members 34 on which the boom 32 is pivotally mounted at the pivot 33. The remainder of the embodiment of Figure 6 corresponds to the embodiment of Figures 3 to 5a, and corresponding components are indicated by iike reference numerals. The operation of the embodiment will now be described particularly with reference to Figures 7a to 7g, but also with reference to Figures 3 to 5b. Figures 7a to 7g show diagrammatic representations of the different stages in the cycles of operation. Figures 7a and 7b show an initial stage of starting the trench. This may be done as shown, or alternatively may be cut by hand, explosives, percussion tools, or any other means. However referring to Figures 7a and 7b, initially the cutting device 23 is lowered to the ground level 22, and the cutting device is operated while being forced downwardly. This may conveniently be done by operating in reverse the lifting device 25 which has been described with reference to Figures 3 to 5a. As shown in Figure 7b, the result is the cutting of the beginning of a trench with an arcuate end face 54. During the step shown in Figures 7a and 7b, the cutting device 23 is operated in the manner set out in the known machine of EP-A-0080802, i.e. cutting on the down stroke.

As shown in Figure 7c, the next step is that the cutting device 23 is operated whilst the prime mover 21 is driven forwardly, so as to produce an undercut 55 in the end face 54 of the trench. The next step is that the lifting means 25 as shown for example in Figure 3 is operated to pivot the cutting device 23 upwardly from the undercut 55, while the cutting device 23 is operating, so as to cut material from the end face of the trench 54. This cutting operation is shown in detail in Figure 5, where there is shown the material 56 which is cut from the end face 54 during upward movement of the cutting device 23 by the lifting means 25. This produces the new end face 54 of the trench as shown in Figure 7d. When this is completed the cutting device 23 is lowered to the bottom 19 of the trench 18 as shown in Figure 7e. The process is then repeated by operating the cutting device and moving the prime mover forwardly as shown in Figure 7f to produce a new undercut 55. Finally the cutting device 23 is again raised upwardly from the undercut 55 to cut a new end face 54 as shown in Figure 7g. The main advantage of the embodiment of the invention which has been described is that during the upward cutting stroke of the cutting device 23 the force exerted between the cutting device 23 and the ground surface 22, by way of the mobile base 26, is limited only by the force generated by the lifting means 25 and is not limited by the potential floating of the base 25 up from the ground, as in the device shown in Figures 1 and 2 (where cutting is effected on the down stroke of the cutting device). It is not necessary to provide substantial weight in the part of the apparatus on which the lifting means is mounted, as would be the case if the lifting means were pressing downwardly on the cutting device during the cutting stroke.

Additional advantages arise in connection with the cutting of the undercut 55. Because the cutting area at the distal end of the cutting device 23 is relatively limited, and because the prime mover 21 is moving forwardly during the cutting of the undercut, the problem of penetration effort into hard rock is substantially reduced, compared with the difficulty of penetrating from above during a downward cutting stroke.

There will now be described with reference to Figure 8 an alternative embodiment of the present invention, in which the chain cutter of Figures 1 to 6 is replaced by a milling cutter comprising a cutting rotor mounted on the distal end of the cutting boom, and driven in rotation about an axis aligned generally along the direction of the length of the boom. Components which correspond to components previously described are indicated by like reference numerals. In the embodiment of Figure 7, the cutting device 23 comprises a cutting boom 32 having a milling cutter 71 mounted at the distal end thereof. Mounted behind the milling cutter 71 is an endless conveyor belt 73 for removing the debris cut by the milling cutter 71. In the embodiment shown, the mobile base 26 of the apparatus runs on tracks 72. The general construction and operation of the milling cutter 71 and debris removing conveyor 73 are as set out in the prior specification EP-A-0080802. The general construction and operation of the lifting means 25 of Figure 8, and of the overall operation of the trenching apparatus, is as has been described with reference to Figures 3 to 7 of the present application.

Claims (26)

1. A method of trenching comprising: positioning in a trench a cutting device mounted on a prime mover movable on the ground surface above the trench; engaging an end face of the trench with the cutting device; and moving the cutting device in a vertical plane while operating the cutting device, so as to cut material from the end face of the trench; in which the method includes: positioning the cutting device against the end face of the trench below the level of the ground surface, and moving the cutting device forward in the trench while operating the cutting device, so as to produce an undercut in the end face of the trench; and lifting the cutting device upwardly from the undercut by exerting a force between the cutting device and the ground surface above the trench and operating the cutting device during the upward movement so as to cut material from the end face of the trench.

2. Apparatus according to Claim 1 in which in the step of producing the undercut the cutting device is positioned to engage the end face substantially at the bottom of the trench.

3. A method according to Claim 1 or 2 in which the step of lifting the cutting device upwardly is carried out by exerting a force between the cutting device and the ground surface at a position substantially directly above the cutting device.

4. A method according to any preceding claim in which the step of lifting the cutting device upwardly is carried out by exerting a force in a direction lying in a range from the vertical to 30° to the vertical.

5. A method according to Claim 4 in which the direction of the said lifting force is inclined to the vertical in a direction forwardly relative to the direction of cutting of the trench.

6. A method according to any preceding claim in which the cutting device is mounted on a pivoted boom extending forwardly relative to the direction of cutting of the trench, and the said lifting step is carried out by moving the cutting device along an arcuate path defined by pivotal movement of the boom.

7. A method according to Claim 6 in which the step of lifting the cutting device upwardly is carried out by exerting a force between the cutting device and a region of the ground surface spaced from the axis of pivoting of the boom in a forward direction along the trench.

8. A method according to any preceding claim in which the step of cutting material from the end face of the trench is carried out by moving a plurality of cutting elements along upper and lower runs of an elongate endless support means trained along the boom, the cutting elements being driven at the distal end of the cutting boom in a direction upwardly around the end of the cutting boom and rearwardly along the upper run relative to the direction of cutting the trench.

9. A method according to any of Claims 1 to 7 in which the cutting device comprises a cutting rotor mounted at the end of a boom, and the step of cutting material from the end face of the trench is carried out by rotating the cutting rotor about an axis aligned transverse to the length of the trench.

10. A method according to any of Claims 1 to 7 in which the cutting device comprises a cutting rotor mounted at the end of a boom, and the step of cutting material from the end face of the trench is carried out by rotating the cutting rotor about an axis aligned generally along the length of the trench.

11. Trenching apparatus comprising: a cutting device; mounting means for mounting the cutting device on a prime mover and for positioning the cutting device in a trench with the prime mover movable on the ground surface above the level of the trench, the cutting device being movable in a substantially vertical plane when engaged with an end face of the trench so as to cut material from an end face of the trench; in which the apparatus includes: lifting means arranged to lift the cutting device upwardly while engaged with the end face of the trench by exerting an upward force between the cutting device and the ground surface above the trench; and control means arranged to operate the lifting means and the cutting device concurrently so as to lift the cutting device when in cutting engagement with the end face.

12. Apparatus according to Claim 11 in which the control means is arranged to carry out an operating cycle in which :-

(i) the mounting means positions the cutting device against the end face of the trench below the level of the ground surface;

(ii) the prime mover moves the cutting device forwardly in the trench while operating the cutting device, so as to produce an undercut in the end face of the trench; and

(iii) the lifting means lifts the cutting device upwardly from the undercut through the material of the end face.

13. Apparatus according to Claim 11 or 12 in which the cutting device comprises a boom having an elongate endless support means carrying a plurality of cutting elements and trained along upper and lower runs on the boom, the boom being mounted to project forwardly and downwardly relative to the intended direction of cutting the trench, and drive means arranged to drive the endless support means in a direction such as to carry the cutting elements upwardly around the distal end of the boom and rearwardly along the upper run of the endless support means.

14. Apparatus according to Claim 11 or 12 in which the cutting device comprises a cutting rotor mounted at the distal end of a boom, for rotation about an axis aligned transverse to the length of the boom.

15. Apparatus according to Claim 1 1 or 12 in which the cutting device comprises a cutting rotor mounted at the distal end of a boom for rotation about an axis aligned generally in the direction of the length of the boom.

16. Trenching apparatus comprising: a cutting device; mounting means for mounting the cutting device on a prime mover and for positioning the cutting device in a trench with the prime mover movable on the ground surface above the level of the trench, the cutting device being movable in a substantially vertical plane when engaged with an end face of the trench so as to cut material from an end face of the trench; in which the apparatus includes lifting means arranged to lift the cutting device upwardly while operating the cutting device in cutting engagement with the end face of the trench, by exerting an upward force between the cutting device and the ground surface above the trench; and in which the cutting device comprises a boom having an elongate endless support means carrying a plurality of cutting elements and trained along upper and lower runs on the boom, the boom being mounted to project forwardly and downwardly relative to the intended direction of cutting the trench, and drive means arranged to drive the endless support means in a direction such as to carry the cutting elements upwardly around the distal end of the boom and rearwardly along the upper run of the endless support means.

17. Apparatus according to any of Claims 11 to 16 in which the lifting means is constructed to provide a power stroke in the upward direction and a return stroke in the downward direction, and to provide a greater force in the power stroke than in the return stroke.

18. Apparatus according to any of Claims 11 to 17 including a prime mover for movement on the ground surface above the level of the trench, the cutting device being mounted on the prime mover for movement along the trench.

19. Apparatus according to Claim 18 in which the lifting means comprises a base spaced from the prime mover and coupled thereto for movement with the prime mover, a source of the lifting force mounted on the base, and a link extending between the cutting device and the source of the lifting force.

20. Apparatus according to Claim 19 in which the said link is mounted on the base in a position such as to extend in a direction lying in a range from the vertical to 30° to the vertical, when the base is positioned on level ground.

21. Apparatus according to Claim 20 in which the link is mounted so as to be inclined to the vertical in use in a direction forwardly relative to the direction of cutting of the trench.

22. Apparatus according to any of Claims 18 to 21 in which the cutting device is mounted on a pivoted boom extending forwardly relative to the intended direction of cutting of the trench, and the said lifting means is arranged to move the cutting device along an arcuate path defined by pivotal movement of the boom.

23. Apparatus according to Claim 22 in which the base is coupled to the prime mover so as to be spaced from the axis of pivoting of the boom in a forward direction relative to the intended movement of the prime mover along the trench.

24. Apparatus according to any of Claims 11 to 23 in which the lifting means includes an hydraulic ram.

25. A method of trenching substantially as hereinbefore described with reference to any one or any combination of Figures 3 to 8 of the accompanying drawings.

26. Trenching apparatus substantially as hereinbefore described with reference to any one or any combination of Figures 3 to 8 of the accompanying drawings.