The invention relates to a method for dredging a layer of material from an area of water. Such a method is generally known. In this known method the whole area of the rele- vant water is dredged from the bottom by means of a suction mouth of a dredging device, wherein the collected sludge is collected in floating vessels or carried away by means of a pressure conduit. The danger exists here of the part of the area of water already dredged being contaminated by the material originating from the part of the pool not yet dredged, so that after completion of the whole dredging process the bottom is not yet free of the material to be removed.

With normal dredging processes this is often not so important because the object of the dredging is usually to obtain a required navigable depth of the water.

There are situations, however, in which it is important to remove a specific type of material which extends over a considerable part of the bottom of the area of water. Here there is indeed the danger of the material from the non-dredged part moving to the other part of the area of water - lower lying due to the dredging - which has already been dredged.

This problem arises in particular when for in- stance a determined layer of sludge is removed.

The object of the present invention is to provide a method with which above-mentioned drawbacks are avoided.

This object is achieved in that the area of water is divided into sections and that, before a section is dredged, the section to be dredged is separated from the rest of the area of water with screens extending upward from the bottom of the area of water.

As a result of this step a separation is obtained

between the dredged and the non-dredged parts of the area of water, so that there is no longer the danger of the sludge or other material for removing by dredging contami¬ nating the part of the water not yet dredged. The sub-claims relate to favourable preferred embodiments.

The invention further relates to a screen that can be used with the above-mentioned method and to a suction dredger adapted for the above-mentioned dredging method. The present invention will now be elucidated with reference to the annexed drawings, in which: fig. 1 shows a schematic perspective view of a dredging device and of the associated screens during per¬ forming of a dredging method according to the invention; fig. 2 shows a schematic partially broken away perspective view of a sampling bore and the associated holders which can be used when performing the method ac¬ cording to the present invention; and fig. 3 shows a partially broken away perspective view of a suction head for use with the present invention. Shown in fig. 1 is a pool 1 on the bottom 2 of which there is a layer of sludge 3 which has to be removed by means of dredging. The sludge has the tendency to dis¬ place over the bottom of the pool so that there is the danger that the sludge coming from the part of the pool not yet dredged will displace to the part of the pool which has been dredged.

To combat this effect the pool 1 is divided by means of screens 4 into sections, or at least into a part already dredged and a part not yet dredged. It is recommended however that the section not yet to be dredged be separated from the non-dredged part and from the part that has already been dredged because at the start of dredging of the relevant section the sludge layer is present in said section and can contaminate the part not yet dredged, and because after dredging of the section the danger exists that the sludge from the part not yet

dredged finds its way into said section.

The above-mentioned drawbacks are avoided by arranging the screens 4. For this purpose the screens 4 are arranged such that over a predetermined length they are anchored folded over on the bottom by means of weights 5. Instead of separate weights 5 as shown in the figure, it is also possible to employ a chain.

On their top edge 6 the screens 4 are provided at regular intervals with float bodies 7 which exert an upward force such that the screen is held in the vertical position. Floating markers 9 which indicate the position of the screens 4 are further arranged at the location of the float bodies 7 by means of a rope 8. It is noted here that the screens preferably do not extend up to the water surface in order to prevent damage and displacement by wave movement.

For the dredging use is made of a classical dred¬ ging vessel 10 which will not be discussed further since it corresponds with the usual dredging vessels. Use is further made during dredging of a suction head 11 which is connected to the dredging vessel 10 by means of a rod system 12.

The rod system 12 herein forms a so-called paral¬ lel construction; herein, when the dredging vessel 10 is ballasted such that it is as horizontal as possible, the suction head 11 also remains horizontal so that the bottom edge of both suction mouths extends horizontally, which is important with respect to sucking up the sludge 3 for removal as accurately as possible. The rod system 12 is therefore formed by a bottom ladder 13 and a top ladder 14 which are connected to the dredging vessel 10 by means of appropriate pivot points 15 and are connected to the suc¬ tion head 11 by means of pivot points not shown in the drawing. A suction conduit 16 further extends parallel to the ladder 12 from the suction head to the dredging vessel 10 and a pressure conduit 17 extends from the dredging vessel 10 for discharging the sucked-up sludge.

Using the above dredging device the whole section between both screens is dredged wherein the screens prevent the dredged part becoming contaminated with displacing sludge. Dredging further takes place at an accurate height, which height can easily be regulated by means of the lifting device 18 present on the dredging vessel 10. The method by which the height of the sludge layer present is determined will be described hereinafter. When the section shown in the drawing between both screens 4 is thus wholly freed of sludge, the screen 4 situated on the right-hand side of the drawing is super¬ fluous and this is placed on the left-hand side of the left screen 4. A following section is thus obtained which is then dredged empty. It is however noted herein that, as shown in fig.

1, the dredging is not performed in the vicinity of the screen because there is otherwise the danger of the screen being sucked in or otherwise impeding the dredging process. This means that after completing the dredging of a section an edge remains in the vicinity of a screen. For this purpose, when the following section is dredged, the relevant screen is moved over a distance onto the first dredged section, wherein the screen comes to hang at a distance from the remaining layer of sludge such that this layer of sludge can be removed when the next section is dredged.

It is also possible after dredging the section to move the right-hand screen in front of the left, which prevents the remaining edge of sludge contaminating the "clean" part of the pool, and to then use a third screen to separate the new section still to be dredged and to subsequently remove the left-hand screen, but it is also possible to directly move the left-hand screen wholly to the left to delimit the new section. With reference to fig. 3 the construction of the suction head will now be described. The suction head 11 is, as already described, connected to the top ladder 14

and to the bottom ladder 13 (not shown in fig. 3) in order to hold the suction head 11 as horizontal as possible. For movement in the vertical direction the suction head can be- raised by means of the lifting device 18. The suction head 11 is itself formed by a box-shaped structure 19.

Arranged in the box-shaped structure 19 are two funnel-shaped suction mouths 20, 21 which are separated from the interior of the suction head 11 by obli-quely extending and tapering side walls 23 and by an obliquely extending and tapering upper wall 24.

Two transverse walls 25 are arranged in the vici¬ nity of the centre for the purpose of reinforcement. Each of the suction mouths 21, 20 debouches into a vertically extending pipe 26, 27 respectively which debouches at its top into a collection chamber 27 which is connected to the suction conduit 16. Between the upper wall of the box- shaped structure 19 and the collection chamber at the point of the vertical pipes 26 is arranged a slidable plate 28 in which an opening 29 is arranged which is movable by means of a linear drive means 30 to above one of both vertically extending pipes 26. Using the linear drive means the hole 29 can thus be placed above one of the pipes 26 so that alternating use can be made of one of the suction mouths 20, 21. For accurate localized sucking away of sludge, being able to apply suction alternately on both sides is important, as it is for sucking away the sludge without sludge eddying and contaminating the water, which is particularly important in the case of contaminated sludge. Before actual dredging is started, a check is made by means of depth measurements at which height the layer of sludge for removal is situated. Use can be made for this purpose of a sampling device as shown in fig. 2.

This sampling device is formed by a steel tube 31 that is provided on its bottom part with a chamfering 32 on the outside. Arranged inside the steel tube 31 is a transparent pipe 33 which is provided at its top with two

protrusions 34 which engage in correspondingly formed channels 35 arranged on the inner wall of the steel tube 31. A sort of bayonet fitting is thus obtained. After taking depth measurements, whereby the height of the bottom 2 of the pool can be established, it is determined using the sampling device shown in fig. 2 how thick is the layer of sludge above the permanent bottom and what is the composition thereof. To this end the sampling device shown in fig. 2 is driven through the water from above into the layer of sludge, wherein, after contact has been made with the bottom, it is pushed further down over a short distance so that a closing plug is formed in the bottom of the transparent pipe 33, whereafter a turn is made through a quarter stroke for the purpose of locking and the device is brought to the top.

After being released from the steel tube the sample obtained can be pressed out into a transparent pot 38 by means of a cover 37 provided with a plunger 36 which can be placed to fit closely over the transparent pipe 33. The cover 37 is herein fixed, again by means of the protrusions 34. The contents of the transparent pot 38 thus give a clear picture of the sample obtained.

It is of course also possible to make use of other sampling devices; it is for example possible to connect the top of a pipe made of transparent material to a gas pipe and to drive the transparent pipe into the sludge, to cause it to rotate until a closing plug is formed and to raise the transparent pipe above water again and to release the gas pipe. Thereafter the plunger 36 and the cover 37 can be applied as in the above described method for placing the sample into a pot.

According to the above method the depth measure¬ ments are taken initially in the whole pool or section b- section and the samples therefrom are then brought to the surface, wherein the samples are placed in the relevant transparent pots, whereafter it is determined possibly visually by means of interpolation at which height the

dredging operations have to be performed. An accurate performance of the dredging operations is thus obtained, wherein only the desired layer of material is removed.

It is also possible after completing the dredging operations to take samples again so that it can be established on the basis thereof whether the required effect has indeed been achieved. Use can be made herein of statistical methods, for example the Student's T-test.