Method and installation for compacting and consolidating the underwater bed

Abstract: Method for consolidating and compacting the underwater bed by applying and maintaining a reduced pressure in the underwater bed. This can be achieved, for example, with the aid of draining said underwater bed. To this end, a drainage system is produced in the underwater bed and it is ensured that the water located above the underwater bed has as little opportunity as possible to penetrate into the underwater bed. For example, to this end a liquid-tight sheeting is applied on top of the underwater bed. In every case, the drainage system is constructed well below the surface of the underwater bed, so that that part of the underwater bed located above said system in itself forms a liquid barrier.

Description: The invention relates to a Installation for compacting and/or consolidating an underwater bed, comprising a floating body with at least one trench-cutting element extending downwards from said floating body, which trench-cutting element defines a box-shaped body which has a height substantially longer than its width, and comprising a feed channel from the floating body to the box shaped body for inserting a flexible drainage pipe horizontally into the trench. The invention also relates to an installation for carrying out said method. In particular, the invention relates to compaction of the underwater bed in such a way that, during and/or after carrying out said method with the aim of lowering the underwater level of and/or of compacting the underwater bed, the surrounding environment is polluted as little as possible by impurities originating from the underwater bed. More particularly the invention relates to work to be carried out on the underwater bed where the top layer of the underwater bed contains sludge. A current method for lowering the level of the underwater bed and thus increasing the navigable depth is to remove some of said bed by dredging. Especially if there is question of contaminated sludge, there is the disadvantage that removal by dredging is environmentally unfriendly and is associated with appreciable costs. After all, measures must be taken during removal by dredging to prevent sludge from swirling up in the water and reaching other locations with the water current. Moreover, the sludge dredged out constitutes a waste problem. If the aim is to use the underwater bed as foundation and unstable layers are removed by dredging, it is frequently the case that additional sand has to be applied thereafter in order to compensate for the underwater bed removed by dredging. A device according to the preamble of claim 1 is known from JP-A-60 040410. In this Japanese patent application a horizontal drain is placed in an underwater bed of extra soft ground underneath the bed surface. One end of the drain is connected to a vacuum pump, which is operated without sucking in any surface water or air. The known method has as a disadvantage that the consolidation will be relatively ineffective. If the underwater bed is relatively fluid-permeable, the drain must be buried deep beneath the surface in order to prevent surface water and air being pumped up. Thereby the top layer of the soil will not be effectively consolidated. For relatively impermeable underwater beds, consolidation at some distance from the drain will be inadequate. The aim of the present invention is therefore to provide an effective and rapid installation and method of compacting and consolidating an underwater bed which avoids the above drawbacks. A further aim of the invention is, firstly, to lower the level of the underwater bed without it being necessary to remove sludge by dredging. Secondly, sludge consolidated by said method can be removed with extensive protection of the environment and thus as cost-effectively as possible. Thirdly, another aim of the present invention is to prepare the underwater bed for use as foundation, likewise with extensive protection of the environment and with an improvement in the efficiency. To this end, the device according to the present invention is characterised in that the box-shaped body is open at the rear to the height of the drainage trench to be formed and is connected to a source of porous fluid-permeable material such as sand. By use of the box-shaped trench cutting element of the present invention, the porous material and the drainage pipe are introduced into the trench simultaneously, in one processing step, allowing fast and efficient compacting. The effect can, if necessary, be intensified by also applying a top load, of for example sand. Consequently, a consolidation effect is rapidly produced and the underwater bed is compacted over a large area, substantially over the height of the trench. This results, on the one hand, in a fall in the level of the underwater bed. On the other hand, the load-bearing capacity of the underwater bed increases. Especially, when the top layer of the underwater bed consists of sludge, a significant degree of consolidation and compaction, accompanied by a lowering of the level, will be found. If required, following compaction in the manner envisaged by the invention, the compacted layer of the underwater bed can still be removed by dredging. The compacted layer is appreciably more concentrated and contains little/no water and/or other liquids or gases. With the method according to the invention, the sludge becomes firmer for digging and consequently easier to dump. It is also possible to construct dams, dykes or banking on the compacted underwater bed. Removal of the initial soft sludge layer by dredging is not necessary in this case, and likewise, it is not necessary to replace the sludge removed by dredging by sand or another material. Consequently, fewer building materials are needed to produce a body of this type. According to the present invention, the drainage system may be installed beneath the surface of the underwater bed, in particular about 1 meter below the sludge surface. To this end, for example, drainage trenches are made in the underwater bed and a discharge drain is laid in said trenches, which drain is connected to a suction system in order to create and maintain a reduced pressure. The trenches are stabilised using a porous material, such as sand. The sand acts both as a filter and as a discharge drain. Instead of sand, another draining material, for example a filter mat, could also be used for installing in the drainage trenches. The sludge layer or other layer of the underwater bed located above the drainage system acts as a water-sealing layer, so that the ingress of water from above is prevented. In order to increase and/or ensure the sealing effect, it is also possible to lay sheeting on the underwater bed. If sheeting is used in order to create a water-sealing layer, the drainage system can also be installed at a lesser depth below the level of the underwater bed. In that case, it would be possible for the drainage trenches even to extend up to the surface of the underwater bed, since they are closed off at the top by the sheeting. Instead of sheeting it would also be possible to use other water-sealing elements, for example plates or something similar. The invention will be explained in more detail below with the aid of a nonlimiting illustrative embodiment with reference to the appended drawings. In the drawings: Fig. 1 shows a perspective view of an installation for installing a drainage system in the underwater bed, for carrying out the method according to the invention; and Fig. 2 shows a cross-sectional view of the underwater bed provided with a drainage system installed using the installation shown in Fig. 1. Fig. 1 shows an installation 1 for making trenches in the underwater bed beneath the surface of the underwater bed, in order thus to create a reduced pressure in the underwater bed and so to consolidate the underwater bed. To this end, the installation 1 has a floating pontoon 2 which, for example, can be propelled from the shore with the aid of a tug or something similar, or can be propelled independently. Chutes 5 run vertically downwards from the top of the pontoon and extend to well below the surface of the underwater bed 4. Optionally, it is also possible to attach a single chute 5 to the pontoon 2. The chutes, or trench cutters, 5 run essentially parallel to one another and are arranged aligned to one another some distance apart. Each trench cutter 5 is essentially box-shaped and open at the rear to the height of the drainage trench to be made in the underwater bed 4. The front of the trench cutter 5 is shaped in such a way that said cutter is able to move easily through the underwater bed, the material in the underwater bed being pushed aside and optionally upwards/downwards. In this way a trench is formed in the underwater bed 4 by the trench cutter 5 as the pontoon 2 moves in the direction of the arrow A. Sand or other material is supplied at the top of the trench cutter 5, in order to fill the trench 8, made in the underwater bed by the trench cutter 5, with sand. To this end, the trench cutter 5 is provided with transport-promoting elements (not shown in more detail), such as vibrators. A hollow pipe 3 runs from the top of the pontoon 2 into the trench cutter 5. A flexible drainage pipe 7 is fed through said hollow pipe 3 in order to run in the longitudinal direction of the trench made in the underwater bed 4. The trench cutter 5 can, moreover, also be designed to introduce other materials into the drainage trench, for example a plastic drainage mat. The arrangement of the drainage trenches 8 in the underwater bed 4 is shown more particularly in Fig. 2. As is shown, the trenches 8 are located well below the top surface of the underwater bed 4. A sealing sheet 9 has also been applied on top of the underwater bed 4, which sheet prevents water from being able to penetrate into the underwater bed 4 from above the sheet 9. The trenches 8 are filled with sand or other draining material and a drainage pipe 7 is installed close to the bottom of the trenches. The drainage pipes 7 are connected, in a manner which is not shown in more detail, to a suction system in order to remove liquids and/or gases from the underwater bed 4 and thus to create a reduced pressure in the drainage system. As a result of said reduced pressure, the said liquids/gases are removed from the underwater bed, as a result of which the compaction of the underwater bed is achieved.

Claims: Installation for compacting and/or consolidating an underwater bed (4), comprising a floating body (2) with at least one trench-cutting element (5) extending downwards from said floating body (2), which trench-cutting element defines a box-shaped body which has a height substantially longer than its width, and comprising a feed channel from the floating body to the box shaped body for inserting a flexible drainage pipe (7) horizontally into the trench, characterised in that the box-shaped body is open at the rear to the height of the drainage trench (8) to be formed and is connected to a source of porous fluid-permeable material such as sand. Installation according to Claim 1, wherin the height of the box shaped body is adjustable. Method for compacting and/or consolidating an underwater bed using an installation according to either claim 1 or claim 2, comprising the steps of: digging a substantially horizontal trench (8) in the underwater bed (4) with the trench cutting element (5), inserting a substantially horizontal drainage pipe (7) in the trench (4), and applying a suction on the drainage pipe (7) to extract water and/or air from the underwater bed, characterised in that at least two substantially parallel trenches are dug in the underwater bed, the depth of the trenches being substantially larger than the width thereof, wherein the threnches are first filled with a fluid permeable drainage material and are thereafter covered by a water-sealing layer (9) to form a relatively water-impermeable top layer. Method according to Claim 3, wherein the trench walls are covered with a filter fabric, within which the porous material is enclosed.

Inventors: GROEN PHILIPPE, PLIGT LEENDERT-JAN, GROEN, PHILIPPE, PLIGT, LEENDERT-JAN
Applicants: KONINK WEGENBOUW STEVIN B V, OORD ACZ B V VAN, VERMEER GROND EN WEGEN BV, KONINKLIJKE WEGENBOUW STEVIN B.V., VAN OORD ACZ B.V., VERMEER GROND EN WEGEN B.V.
Application year: 1995
Main classification: E02D3
IPC codes: E02F5, E02D3, E02D15