The invention relates to a cutter suction dredger, comprising a pontoon, a ladder, one end of which is suspended from the pontoon, which ladder can be brought into a downwardly directed position with respect to the pontoon, a cutter head for cutting off rock or soil on the other end of the ladder, an extraction unit for extracting rock or soil which has been cut loose, at least one spud pole, interacting with the pontoon, for anchoring the pontoon with respect to a water bed, as well as a supporting device close to the cutter head for supporting the cutter head with respect to the water bed.

A cutter suction dredger of this type is known from NL-A-671 1816. At the free end of the ladder of this known cutter suction dredger is found a supporting plate, which is adjustable by means of hydraulic piston/cylinder devices. By pressing the supporting plate against the bed, the cutter head can continue to follow the bed profile, even if swell occurs. During operation, the cutter head is moved back and forth over the bed, and moved forwards in steps after each swinging motion of the pontoon and the ladder. The supporting plate must here slide over the bed.

The disadvantage of this known supporting device is that the stabilization of the cutter head with respect to the bed is limited. Only the undesirable up and down movement can thus be combated. The object of the invention is to provide a cutter suction dredger of the described type, which offers improved stabilization of the cutter head. This object is achieved according to a first embodiment by the supporting device comprising at least one projection, which can be plunged into the water bed to stabilize the cutter head and/or ladder with respect to and along the plane defined by the water bed.

In the cutter suction dredger according to the invention, the cutter head can be stabilized not only in the vertical direction, but also transversely thereto in the plane of the bed. This further stabilization is obtained by the projection penetrating into the bed. The supporting device produces a vocationally fixed positioning of the cutter head with respect to the bed and obstructs the forward movement, whilst the swinging motion remains possible. The supporting device thereby largely assumes the function of the spud pole. Normally, the spud pole positions the cutter head in a locationally fixed manner with respect to the bed during the swinging motion about the spud pole, namely according to the radius of the swinging motion. As a result, a regular cut can be made in the bed.

In the use also of the supporting device, the spud pole still plays a role, namely of setting the radius at the start of the swinging motion with the use of the sliding member. In connection with the maintenance of the radius, the spud pole is, however, no longer necessary, since the function thereof is then assumed by the supporting device.

An important advantage thereof is that the spud pole no longer needs to be rigidly connected to the pontoon during the swinging motion. This enables the spud pole to be spring mounted, whereby it will be less heavily loaded. If it is desired to set the cutter head to a specific depth, the supporting device can preferably be suspended from the ladder such that it is vertically adjustable, for example by means of a piston/cylinder device.

Furthermore, the supporting device can be suspended from the ladder rotatably according to a substantially vertical centre axis. This enables the direction of displacement to be set at an angle a with respect to the transverse hauling direction. In particular, the supporting device can possess a supporting surface which can be turned towards the water bed and which can be supported on the water bed, and the at least one projection projects from the supporting surface.

The supporting device can be realized such that this is tiltable between a stabilization position, in which the supporting surface is directed towards and supportable on the water bed and the projection can be plunged into the water bed, and a release position, in which the supporting surface and the projection are supportable at a distance from the water bed. In the release position, the supporting device is freely displaceable; in that case, it has no stabilizing action on the forward movement. In particular, the supporting device can be realized with a sliding member, which is tiltable is between a rest position at a distance from the water bed and a sliding position in which the sliding member is supportable and slidable on the water bed, which rest position of the sliding member is associated with the stabilization position of the supporting device and which sliding position of the sliding member is associated with the release position of the supporting device.

The supporting device can be variously realized. According to a first option, it can be shaped as a foot (Fig. 5-6b). In that case, both the foot and the sliding member can be accommodated on a frame which is tiltably suspended from the ladder and which is drivable, for example, by a hydraulic piston/cylinder device. By tilting the frame into the desired position, either the foot can be supported on the bed, for stabilization of the cutter head in the swinging motion, or the sliding member can be supported on the bed, for displacement of the cutter head transversely to the swinging motion in the plane of the water bed.

According to another embodiment, the supporting device can be realized as a roller having a rotationally symmetric surface, which surface is provided with the at least one projection (Figs. 7a- 10). The cutter head can here be displaced in the swinging direction by rotation of the roller. In particular, the centre axis of the roller can here have a position in a vertical plane parallel to or coincident with the longitudinal direction of the pontoon and the peripheral surface of the roller is provided with a row of mutually spaced projections and/or with a blade running in the peripheral direction (Figs. 7a-8c). When the roller is rolled, the projections of the row come successively into interaction with the bed. The alternative blade, upon the rolling of the roller, can constantly take hold in the bed, for the stabilization of the cutter head during the swinging motion.

In one particular embodiment, the roller can comprise a cylindrical surface portion, as well as a sliding member realized as a spherical or conical head. By tilting the roller into the release position, the head comes to rest on the bed. The projections or blade present on the cylindrical surface portion then come free from the bed, such that the cutter head can be displaced, for example can be moved forward in steps.

According to yet another embodiment, the supporting device can be realized as a roller having a rotationally symmetric surface, the centre axis of which is directed horizontally and transversely with respect to the ladder, as well as a row of projections distributed evenly over the periphery of the roller, such as blades directed parallel to the centre axis of the roller (Figs. 9, 10). Such blades allow the swinging motion, yet obstruct the forward movement. A forward movement is made possible, however, by rotation of the roller, whereupon successive blades are pushed into the bed. In connection with the control and stabilization of the cutter head, the roller can interact with a braking member in order to set a fixed rotation position of the roller. In addition, the roller can interact with a drive mechanism to set the roller in rotation.

Preferably, the ladder, at one end, is rotatably suspended from the pontoon and, at a distance from the rotatable suspension mounting, is suspended from the pontoon by means of a hoisting cable and a swell compensator. By lowering the ladder on the hoisting wire, the supporting device can be placed on the bed. With the aid of the swell compensator, the supporting device can reliably be held supportively on the bed. In this state, the cutter head can gradually penetrate into the bed, under the influence of the adjustment of the position of the supporting device with respect to the cutter head.

In the above, an embodiment in which the supporting device is stabilized with respect to the bed by one or more projections is described. However, it is not always necessary to use such projections for stabilization purposes on the supporting device. In place of such projections, or in addition thereto, according to a second embodiment of the invention a connecting means of adjustable length can extend between the spud pole and the ladder.

By means of an adjusted connection of this type, the greatest movement of the supporting device and the cutter head with respect to the ground surface in the horizontal direction can be widely limited. During the swinging motion, the connection is located above the bed between the bottommost portion of the spud pole and the bottom edge of the ladder. The force in the connection gives rise to a rearwardly directed force on the ladder. As a consequence thereof, and by fixing the distance of the connection during the swinging motion, any movement of the cutter head in the horizontal direction along the longitudinal axis of the pontoon is opposed.

Preferably, a length adjusting device is provided, by means of which the length of the connection between the spud pole and the ladder can be set. According to a first option, the connection can for this purpose be fixedly connected to the spud pole. It is possible that the fastening of the connection to the spud pole is situated sufficiently low that the connection, in the tensioned state, cannot hit the top edge of the water bed during the rotary movement of the cutter head. However, it is not necessary to connect the connection fixedly to the spud pole. It is also possible for the fastening of the connection to the spud pole to be located on a guide, for example a ring or a tubular member, which is led through the spud pole and can move in and/or around the spud pole and the height of which on the spud pole can be adjusted before and/or during the rotary movement of the cutter head by, for example, a piston-cylinder device located in or on the spud pole.

According to a second, alternative option, it can be provided, for example, that the connecting means is realized as a wire which is displaceably led along a guide on the spud pole and extends from this guide rearwards towards and is connected to an anchorage which is anchored in the water bed and which is preferably located on that side of the spud pole which is facing away from the cutter head. The guide can consist of an eye-shaped member, which is fastened to the spud pole. It is also possible for this eye-shaped member to be fastened to a ring or a tubular member, which is fastened around the spud pole and can move or be moved over a certain height along the spud pole.

The invention is explained in greater detail below with reference to the illustrative embodiments represented in the figures.

Fig. 1 shows a schematic longitudinal section through a known cutter suction dredger.

Fig. 2 shows a schematic top view of a known cutter suction dredger.

Figs. 3 a and b show vertical cross sections of examples of ground profiles in a water bed which are possibly to be cut with the cutter head.

Fig. 4a shows the orbital movement of a long surface wave on shallow water and Fig. 4b shows a definition diagram of the 6 main movements of a floating cutter suction dredger.

Fig. 5 shows a schematic longitudinal section through a first embodiment of the cutter suction dredger according to the invention.

Fig. 6a shows a side view of the bottommost end of the ladder with supporting device.

Fig. 6b shows a top view according to VIb of Fig. 6a.

Fig. 7a shows the side view of a second variant of the supporting device in a first position.

Fig. 7b shows a top view according to Vllb of Fig. 7a.

Fig. 7c shows a second position of the supporting device.

Fig. 7d shows a side view of the roller of the supporting device.

Fig. 7e shows a front view of the roller.

Fig. 8a shows a side view of a third variant of the supporting device.

Fig. 8b shows a front view of the roller of Fig. 8a.

Fig. 8c shows a top view according to VIIIc according to Fig. 8a.

Fig. 9 shows a fourth variant of the supporting device.

Fig. 10 shows a top view according to X of Fig. 9. Fig. 1 la shows a head-on view of a variant of the roller.

Fig. 1 l b shows a side view of the roller of Fig. 1 la.

Fig. 12 shows a schematic longitudinal section through a second embodiment of the cutter suction dredger according to the invention.

Fig. 13 shows a schematic longitudinal section through a third embodiment of the cutter suction dredger according to the invention.

Figs. 1 and 2 show a typical embodiment of a cutter suction dredger.

This suction dredger has a pontoon 1 as a float, with a ladder 2 articulately suspended therefrom. The bottom end of the ladder 2 hangs from a hoisting wire 6, which can be adjusted with a ladder winch 7. The top end of the ladder 2 hangs in hinge points 20 on the pontoon 1. The firm ground surface 24 is cut into with the aid of a cutter head 3 rotating about the longitudinal axis of the ladder 2. The pontoon 1 held in its place along its longitudinal axis by means of the (working) spud pole 4. An (auxiliary)spud pole 5 is used in the event of the displacement of the spud pole 4. The spud poles 4 and 5 can be hoisted up with a piston/cylinder device 8. In the lowered working state, the spud pole 4 stands vertically in the water bed 24 with a certain penetration depth 26 and at the top stands in two guide rings 42, which are fastened to a spud carriage 9 with wheels 12 having axes 13, which spud carriage can be moved horizontally over a horizontal guide 1 1 by means of a piston/cylinder device 10.

During a dredging run (swinging motion or rotary movement) of the cutter head

3, the front side of the cutter head 3 is kept at a fixed distance R from the spud pole 4 by the securement of the piston/cylinder device 10 and the winch 7, whilst a circular swinging motion Z with radius R is made about the spud pole 4. At the end of each swing Z, a step s forward is made in the angle by rearward displacement of the spud carriage 9 with piston/cylinder device 10 by one step length s with respect to the pontoon 1. During a swinging motion, the bottom side of the ladder 2 is laterally shifted by on one side, with a side winch 19, drawing in the side wire 18, anchored to the front side anchor 17, via a snatch block 47 and, on the other side, paying out the side wire 18. The position of the cutter head is thus controlled in the lateral direction by two wires 18, whilst these wires still allow a lateral movement and rotary movement in the vertical plane through the longitudinal axis of the pontoon and also a rotation of the pontoon about the axis formed by a line through the virtual hinge point 31 and the anchor point 41 of the spud pole. The cut-off soil is sucked up by a pump 16 via a suction pipe 14 and flexible suction bag 15. After a number of swings with step size s, the maximum extension P of the piston/cylinder device 10 of the spud carriage 9 is reached. The swinging motion is now stopped in the middle, in the centre axis 48 of the cut. Next the spud pole 5 is lowered into the water bed 24, after which the spud pole 4 is hoisted and the spud carriage 9 with piston/cylinder device 10 is withdrawn again to the starting position. The spud pole 4 is then lowered into the water bed 24 and the spud pole 5 is hoisted again. After this, the swinging motion is continued and at the end of the cut is advanced in the angle again by a step s. In Figs. 3 a and 3b, a possible cut pattern is given by cuts A to H with step size s and cut height d.

Fig. 4a shows the orbital movement of a long surface wave in relatively shallow water. Cutter suction dredgers operate in relatively shallow water and this kind of long waves has a great effect on the motional behaviour of the pontoon. It can clearly be seen that, in the case of a long wave on shallow water, both a horizontal movement with amplitude u and a vertical movement with amplitude w occurs. The cutter head will hence likewise undergo movements, with the result that both horizontal loads and vertical loads are exerted on the cutter suction dredger.

The cutter head is supported on the ground surface by the shallow support 52 located in front of it; the support thus acts as a hinged anchor point of the pontoon and the ladder. Moreover, the support can assume the positioning function of the spud pole, as already stated. The spud pole is now still necessary in order to set the distance R at the start of the swinging motion, but the maintenance of the distance R can afterwards be left to the supporting device.

With the aim of limiting the problems associated with these movements of the cutter head, the tilting arm 27 represented in Figure 5 is provided with a supporting surface 52 between the ladder 2 and the ground surface 24. With the aid of the actuating cylinder 28, the tilting arm 27 can be rotated, and during the swinging motion, for example, adjusted up or down such that the supporting surface 52 remains in contact with the ground surface and the cutter head remains at a desired depth. In this way, the position of the cutter head 3 can be adjusted with respect to the ground surface 24 both in the vertical and in the horizontal direction. The position of the supporting surface 52, and thus of the cutter head 3, in the vertical direction can be measured with respect to the ground surface 24, with the aid of the length measurement 56 and the DGPS height measurement, with a pickup 57. With this measuring apparatus, the supporting surface with the cutter head can be set at such a depth that the depth of the cutter head remains at the desired level. At the same time, the supporting surface, during the swinging motion, opposes the undesirable large movements of the cutter head, caused by waves, in both the horizontal and the vertical direction. The forces upon the spud poles 4, 5 and the pontoon 1, which forces are generated by the wave motion, hence decrease. The risk of damage also decreases, whilst the workability increases.

With the aim of enabling the forward step movement, according to the variant of

Figure 6a, 6b a supporting surface in the form of a slide shoe 54 is provided. This slide shoe is tiltable by means of the actuating cylinders 51 , as represented with the position shown in dashed representation in Figure 6a. In this position, the slide shoe 54 enables sliding movements transversely to the swing direction. In the position represented with solid lines in Figure 6a, the supporting surface 53 rests on the bed, and the projection, such as a blade edge 59, sticks in the bed such that it is oriented in the swing direction. The movement in the longitudinal axis of the pontoon, transversely to the swinging motion, is then opposed. Moreover, the supporting surface 53 is suspended rotatably about the vertical axis 61, such that it can be rotated through the angle a according to Figure 6b. The supporting surface 53 can now be manipulated about the vertical axis 61 such that the distance R can be maintained. In this phase, the function of the spud pole can therefore be assumed by the supporting surface 53 with projection 59.

In the variant of Figure 7a, 7c, the support is realized in the form of a roller 49, which is rotatable about an axis 50. The roller bears a projection 59, for example in the form of a blade edge or a row of punctiform teeth (see figures 7d, 7e). The toothed embodiment is, in particular, suitable for a relatively hard ground surface, into which a blade can less well be plunged. In addition, the roller 49 is rotatable through an angle a about the vertical axis 61 , by means of an assembly 63 of worm and gearwheel, as represented in Figure 7b. The radius R of the swinging motion can thus be maintained. Furthermore, the roller 49 is tiltable into the position represented in Figure 7c, in which the roller 49 then rests on the spherical portion 60 thereof. This spherical portion can roll /slide over the ground surface during this tilting movement such that the blade edge 59 is freed from the ground surface. In this position of the roller 49, the step s can be realized at the end of the swinging motion. By means of the actuating cylinders 28, the roller 49 can be adjusted in height with respect to the ladder 2 and the cutter head 3.

Figures 8a-8c show an alternative in which the adjustment of the roller 49 about the vertical axis is realized by means of an assembly 63 of worm and gearwheel. The roller 49 can be rotated about the vertical axis 61 through 90°, as represented in Figure 8b. In this position, the step s can be made at the end of the swinging motion.

Figures 9 and 10 show a further alternative, in which a roller 49 is rotatable about an axis 50 directed horizontally and transversely to the longitudinal axis of the pontoon 1 , in the direction, therefore, of the swinging motion. The roller 49 bears blades 65, which extend according to the swinging motion, so that the displacement of the supporting device and the cutter head during the swinging motion is not hampered by the blades 65 which reach into the ground surface. The roller also interacts with the braking device 70, which influences the rotation of the roller about the axis 50. If the roller is thus prevented from rotating about the axis 66, the bottom edge of the ladder 2 with the cutter head 3 is not displaceable horizontally in the longitudinal direction of the pontoon. At the end of the swinging motion, the step s can be made by releasing the brake and rotating the roller until the desired step is reached, after which the brake is activated for the non-rotatable securement of the roller 49. After this, the swinging motion can be executed in the opposite direction. With a cylinder 64, the axis 50 of the roller 49 can be rotated in a slot 67 through an angle a about an axis 61.

The variant of Figures 1 l a, 1 l b possesses a roller 49, rotatable about an axis 50, horizontally and transversely to the longitudinal axis of the pontoon 1 in the swinging direction. The roller 49 bears rows of cutting discs 66, which rows are parallel to the axis 50 and are distributed evenly over the periphery of the roller 49. The axis 71 of each disc 66 is directed perpendicularly to the axis 50 of the roller, substantially tangentially to the surface of the roller 49 or parallel to a straight line on that surface. Such discs result in a lower load upon the side winch during the lateral shifting of the cutter head, whilst the grip on the ground surface in the longitudinal direction of the pontoon 1 continues to be maintained by penetration of the discs 66 into the ground surface/rock.

In the embodiment of the cutter suction dredger according to Figure 12, the stabilization of the supporting device is obtained by a wire 101 between the connection point 23 on the spud pole 4 and the pulley 30 on the bottom end of the ladder 2; via this pulley 30, the said wire 101 runs to the winch 22 on the ladder 2. The distance R between the spud pole 4 and the front side of the cutter head 3 is adjusted with the aid of the winch 22 on the ladder 2. At the end of the swing, the wire 101 is paid out with a step length s. During the swing, the length of the connecting means 21 is kept constant, whereby the length R between point 41 in the spud pole 4 and the front side of the cutter head is constant. At the same time, during the swing, the spud carriage 9 is forced rearwards at a constant force with the piston/cy Under device 10, whereby the connecting means 21 remains taut. The tensioning force can also be obtained by means of one or more guy ropes on anchors placed far in front of the dredger. As a result of the taut wire 101 of connecting means 21 , the horizontal position of the supporting device and of the cutter head 3 with respect to the firm ground surface remains constant.

In Fig. 13, yet another embodiment of the device according to the invention is shown, with which it can remain in use also during pile displacement. The wire 101 of the connecting means 21 between the ladder 2 and the spud pole 4 runs via a guide structure 39 through or closely along the spud pole 4 to an anchorage 40 to the water bed, which anchorage is positioned behind the spud pole 4 on the centre axis of the cut. Here too, the tensioning force Fs is set with the piston/cylinder device 10 and is absorbed, via the wire 101 and anchorage 40, by the firm ground surface. Here too, the tensioning force can alternatively be obtained by one or more guy ropes on anchors placed far in front of the dredger. During pile displacement, the axis of the pontoon lies on the centre axis 48 of the cut in the middle of the swing. As a result, the pretensioned wire 101 runs in a straight line from the pulley 30 on the ladder to the anchor point 40 and produces virtually no transverse force upon the guide structure 39 and therefrom resulting torsional force upon the spud pole 4. Hence the spud pole 4 can be lifted free from the bed during pile displacement and can be displaced along the wire 101 , in a horizontally free sliding manner, with the aid of the guide 39. This has the important and large advantage that, also during pile displacement, the anchorage of the wire 101 to the firm ground surface endures and thus a horizontal movement of the supporting device and of the cutter head is prevented.

Figs.6a and 7a are, as stated, side views of a possible embodiment of the supporting device, wherein the extreme position 55 represented with broken lines reveals that the supporting surface can be brought well below the cutter head. That this is of importance when the cutter head is lowered to the ground surface or raised from the ground surface is explained below.

The method involved in using the stabilization device according to the invention is explained below with reference to Figures 5 and 13. At the start of the work with the cutter suction dredger, when the hoisting wire 6 is moved down so as to be paid out, the swell compensator 25 is fixed in its middle position. In addition, prior to the cutter head 3 being placed on the ground, a resilient support is realized with the tilting arm 27 by producing with the cylinder 28 a cylinder force with suitable spring characteristic. The cylinder 28 is here in its most extended state and can be pushed in by an increasing force. As soon as the supporting surface 52 has fallen to sufficiently closely above the firm ground surface, it will touch the firm ground surface, as the result of a vertical wave motion, and thereupon partially push in the cylinder 28. Upon further falling, the cylinder 28 will be pushed in further and with a greater force. As soon as the cylinder 28 is largely pushed in as the result of a vertical wave motion, it must, in the next following extreme position, upon the first succeeding upward wave motion, thus within a half wave period, be fixed in its extreme position. As a result, the tilting arm 27 immediately proceeds to partially assume the operation of the hoisting wire 6. At that moment, the fixation of the swell compensator 25 must be cancelled, so that this proceeds to function as a swell compensator and holds the hoisting wire 6 taut.

Conversely, when the working with the cutter suction dredger is ended, prior to an upwardly directed hoisting movement with a hoisting wire 6 in connection with the hauling up of the cutter head 3, the supporting surface 52 must first be lowered with the cylinder 28, so that the cutter head 3 is lifted up to the point where the cylinder 28 is in its extreme position. As soon as the cylinder 25, as the result of a rising vertical wave motion of the front side of the pontoon, is largely pushed in, it must, at the extreme position of the next following falling movement of the front side of the pontoon, thus within half a wave period, be fixed in its extreme position. As a result, the hoisting wire 6, in the immediately following rising wave motion of the front side of the pontoon, proceeds to directly assume the operation of the tilting arm 27 and the cutter head is raised further above the firm ground surface. At that moment, the fixation of the cylinder 28 must be cancelled, whereby the tilting arm 27 again proceeds to spring and the hoisting wire 6 must be hauled in as quickly as possible. The resilient tilting arm 27 is in its downwardly extreme position and, in the event of an extreme downward wave motion of the cutter head, can serve as a resilient buffer to prevent a vertical collision between cutter head and firm ground surface.

The motional behaviour of cutter head, ladder, pontoon and spud pole in the use of the present invention and method is explained below.

During the swinging motion, the tilting arm 27 with support 52, wire 21 and/or projection 59 ensure a fixed position of the cutter head in the horizontal and vertical direction in the vertical plane through the longitudinal axis of the pontoon. In Figures 5, 13, the bottom edge of the ladder rests with the supporting surface 52 on the ground surface. The supporting surface 52 produces with the hoisting wire 6 the forces which are necessary for the torque equilibrium about the ladder hinge 20 with the forces resulting from the weight of the ladder and the forces of the cutting process and the tensioning force in the wire 21 and/or the projection 59. As a result of the fixed length of the wire 21, the horizontal position of the supporting surface 52 is also fixed with respect to the ground surface 24. Upon a vertical movement of the pontoon, the supporting surface 52 hence acts as a hinge about which the ladder can rotate. In order to enable the rotation of the ladder, the location of the hinge point 20 will have to be able to change not only in the vertical direction, which is necessary in order to be able to follow the vertical movement of the pontoon, but also in the horizontal direction.

This latter is possible because, according to the method of the invention, the pontoon, with the aid of the spud carriage 9 and cylinder 10, can move freely in the horizontal direction with respect to the working pole 5 with a constant pretensioning force Fc. It is also possible to attain this degree of freedom of movement with the aid of a spring mounted spud pole. The rotation of the ladder about the hinge point 52 will also bring about a change in the length of the hoisting wire 6, which is compensated by the swell compensator 25. As long as the support 52 is not resting on the ground surface, the full weight of the ladder will hang in the hoisting wire 6 and the length of the cylinder 25 of the swell damper is fixed. As soon as the support 52, following height adjustment with the cylinder 28, is in contact with the ground surface, the length of the cylinder 28 is fixed and, with the cylinder 25, a constant pretensioning force is applied in the hoisting wire 6. With the aid of the winch 7, the hoisting wire 6, where necessary, is paid out or drawn in, so that the cylinder 25 is on average in the middle position and can optimally compensate the variation in length of the hoisting wire 6 as a result of the movements of the pontoon 1 by waves. The vertical movements of the pontoon by waves lead to a rotation of the ladder about the support point 52. Because the cutter head lies very much closer to the support point 52 than does the ladder hinge 20, the movement of the cutter head 3 with respect to the ground surface will be considerably less than the movement of the hinge 20. Also the horizontal movement of the cutter head as a result of the rotation of the ladder, if a fixed length of wire 21 is used, is limited, because the line of action between the pulley 30 and the support point 52 is directed virtually horizontally and hence the distance between the anchorage 40 and the front edge of the cutter head 3 is virtually unchanged.