The present invention relates to a bed structure, such as a dumping ground, for deposi¬ tion of refuse and other waste materials. One of the main problems in relation to the construction of dumping grounds and other bed structures for the deposition of refuse and other waste materials is to prevent leakage of polluting substances from the waste material to the surroundings.

10 Known bed structures of this type comprise a substantially horizontally extending, liquid tight liner or membrane of plastic material and an overlying layer of gravel on the upper surface of which the refuse or waste material is deposited, and in which seepage drain pipes are embedded. Polluted seepage may then continuously or intermittently be withdrawn from the drain pipes and transferred to a seepage decomposing system of

15 some kind. Wells or borings extending below the ground water level are normally ar¬ ranged outside the periphery of the substantially dish-shaped liner or membrane. In case a leak occurs in the liner or membrane so that polluting liquid flows through the membrane and down into the underlying ground, pollution of the ground may be re¬ stricted by pumping of water from the surrounding borings or wells.

20

It has been proposed to detect a possible leakage by geoelectrical measurements be¬ low the liner or membrane, whereby moisture caused by a heavy leakage may be dis¬ tinguished if the liner is locaced well above the ground water level.

__ It has also been proposed to place an auxiliary liner or membrane below the main liner

25 and to embed seepage monitoring pipes in a layer of gravel arranged between the liners or membranes. If a leak occurs in the main liner so that a substantial amount of polluted liquid or percolate flows into the gravel layer between the membranes, perco¬ late may be pumped from the seepage monitoring pipe whereby the leak may be dis- -. covered. However, it is normally not possible to repair the main liner or membrane without completely removing the waste materials deposited and reconstructing the complete bed structure, because it is normally not possible to locate the leak of the main liner or membrane. The auxiliary liner may, of course, prevent the seepage or percolate from penetrating into the subgrade as long as the auxiliary liner remains intact. If, however, a leak occurs also in the auxiliary liner, polluting percolate may flow into the ground water which may have serious consequences.

The present invention provides a bed structure of the above type which allows detec¬ tion of even a relative small membrane leakage by geoeletrical measurements.

The bed structure according to the invention is of the type comprising a first liquid tight liner or membrane, and electrical moisture detecting means arranged below the bottom surface of said first liner for detecting possible leakage of liquid through the liner, and the bed structure according to the invention is characterized in a second liquid tight liner or membrane vertically spaced from said first liner, and in an intermediate layer of porous material arranged between said first and second liners, the electrical moisture detecting means being arranged in the intermediate layer. When the vertically spaced forst and second liners or membranes are completely liquid tight neither percolate nor any other liquid may penetrate into the intermediate layer in which the electrical mois¬ ture detecting means are arranged. Therefore, the moisture detecting means may de¬ tect a small leakage even when the liners or membranes and the intermediate layer therebetween are totally or partly located below ground water level. When a leak occurs in one of the liners or membranes and liquid flows into the intermediate layer through the leak, it is possible to judge whether polluted liquid is leaking through the upper liner or whether pure ground water is leaking upwardly through the lower liner, because seepage or percolate will improve the conductivity of the porous material to a higher extent than pure ground water.

The electrical moisture detecting means arranged adjacent to the bottom surface of the liner for detecting possible leakage of liquid through the liner. Such electrical moisture detecting means may be of any suitable type which may generate an electrical signal to . indicate that the humidity exceeds a predetermined level at the location where the respective moisture detector is positioned. As these moisture detecting means may be distributed over the area of the liner in a uniform or non-uniform manner, it will be possible to substantially identify the location of a leak on the basis of the position or positions of the moisture detector or detectors from which an alarm signal is received. The moisture detecting means may, for example, comprise any known type of hy¬ grometers or humidity sensing devices generating electrical measuring signals.

However, in the preferred embodiment the moisture detecting means comprise a plurality of mutually spaced electrical conductors and means for applying a potential difference to adjacent conductors as the electrical conductivity of the intermediate layer is dependent on the humidity of the porous material, such as sand or gravel, in which the conductors are embedded, a relatively heavy current between adjacent conductors to which a potential difference is applied will indicate leakage of liquid through one of the liners.

The moisture detecting means preferably comprises a first set of substantially parallel, mutually spaced conductors and a second set of substantially parallel, mutually spaced conductors extending transversely to the conductors in. the first set. The location of a possible leakage may then be rather closely identified because the two adjacent parallel conductors between which the leakage is located may be determined in each set of conductors. This means that the leakage will be located within the quadrangle - which may be a rectangle, square or parallelogram - defined by the crossing pairs of conduc¬ tors between which the electrical conductivity has been increased.

Liquid leaking through the upper liner or membrane will flow downwardly through the intermediate layer of porous material to the lower liner or membrane, while ground water leaking through the lower membrane will remain close to the upper side of the lower membrane. Therefore, the moisture detecting means or conductors are preferably totally or partly arranged adjacent to the upper side surface of the lower membrane where leaking liquid will be concentrated. Consequently, even a small leakage may be detected.

The electrical moisture detecting means or conductors may be embedded in the inter¬ mediate layer of porous material and arranged in spaced relationship to the upper and lower liners. In the preferred embodiment, however, the conductors are fastened to an adjacent side surface of the liners. In that case, the conductors form part of the liners, and the arrangement of the conductors does not require separate working steps during construction of the bed structure. The conductors may be formed as normal non- insulated wires. Alternatively, at least some of the conductors may be strips of a metal ^" foil laminated with the liner or liners. When the conductors are in the form of metal foil laminated with the liners, they may form pairs of oppositely arranged capacitor plates.

The conductors of the moisture detecting system may be electrically connected to a central control and surveying system for successively and at desired time intervals applying a potential difference to adjacent conductors in the sets of conductors and for simultaneously estimating the conductivity of the porous material between these adja¬ cent conductors. The system may be adapted to generate an alarm signal when a leak of any of the liners has been detected.

The top surface of the upper liner may be covered by an upper layer of a porous mate- rial, such as sand or gravel. Furthermore, this upper layer is preferably covered by a mechanically protective layer in order to prevent that the upper liner or membrane is accidentally punctured by pointed or elongated hard objects contained in the waste

material which is dumped on the bed structure. This protective layer may, for example, be made from concrete, asphalt, tiles, flagstones, slabs, or the like.

A first drainage system may be embedded in the upper layer of porous material so that seepage or percolate collecting at the upper surface of the upper liner may be removed for further treatment in another plant or system, or the seepage or percolate or part thereof may be distributed over the upper surface of the deposited waste material for recirculation. Such recirculation may promote decomposition of the waste material as well as of the polluting components of the percolate or seepage. A second drainage system may be embedded in the intermediate layer of porous material so that polluting liquid or percolate which might have leaked through the upper liner may be removed from the intermediate layer.

The invention will now be further described with reference to the drawing, wherein Fig. 1 is a diagrammatic sectional view of a bed structure or dumping ground in accor¬ dance with the invention,

Fig. 2 is a diagrammatic sectional view in an enlarged scale showing the various layers, Fig. 3 diagrammatically illustrates a leakage detecting system incorporated in the bed structure shown in Figs. 1 and 2, and Fig. 4 is a sectional view along the line IV-IV in Fig. 3.

Fig. 1 illustrates a dumping ground or a bed structure for deposition of refuse 10 or other waste materials. The bed structure comprises upper and lower liners or mem¬ branes 11 and 12 which may, for example, be a plastic sheet of the type marketed by Schlegel Lining Technology GmbH. The liner or membranes 11 and 12 are vertically spaced and have a dished or upwardly concave shape for receiving the refuse 10. An intermediate layer 13 of filter gravel is arranged between the membranes 11 and 12, and a well 14 communicates with a drainage system embedded in the intermediate layer 13. A well 15 communicates with a drainage system embedded in an upper layer 16 (Fig. 2) of filter gravel covering the upper membrane 11. In Fig. 1, the main part of the dish-shaped liners or membranes 11 and 12 are positioned below ground water level 17 and the bed structure may comprise two or more adjacent dish-shaped hollows

18 for receiving refuse or waste material 10. It should be understood, however, that the membranes may alternatively be arranged totally above the ground water level. A well

19 with a boring 20 serves the purpose of checking the ground water quality and, possibly, also of removing polluted ground water.

As shown in Fig. 2, the lower membrane 12 may be placed on a levelling layer 21 of sand or gravel which is in turn arranged on the upper surface of a compressed, levelled

substratum 22. The top surface of the upper gravel layer 16 is covered by a mechani¬ cally protective layer 23, which may, for example, be made from concrete, for example in the form of slabs or flagstones of the interlocking type. The refuse or waste material 10, which may be divided into a lower layer 10b of finely divided refuse and an upper layer 10a of coarser refuse or waste material, is placed directly on the upper, substan¬ tially plane surface 24 of the slabs 23.

Percolate drain tubes 25 communicating with the well 15 are embedded in the upper gravel layer 16 and arranged closely adjacent to the upper surface of the upper mem¬ brane 11. Each of the tubes 25 is surrounded by pen gravel 26 or a similar material. Drain tubes 27 which are embedded in the intermediate gravel layer 13 and surrounded by pen gravel 28 communicate with the well 14 shown in fig. 1. A set of parallel extend¬ ing, mutually spaced electrical conductors 29 are fastened to the upper surface of the lower membrane 12, and a set of transversely extending parallel conductors 30 are fastened to the bottom surface of the upper membrane 11. These conductors form part of a leakage detecting system as will be further described in the following with reference to Figs. 3 and 4.

The leakage detecting system comprises a first set of mutually spaced, substantially parallel extending electrical conductors 29 and a second set of mutually spaced, sub¬ stantially parallel electrical conductors 30, and the conductors of the first set extend at substantially right angles to the conductors of the second set. The conductors 29 and 30 may be uninsolated wires or strips of metal foil connected to the adjacent liner. Thus, in plan view, the two sets of conductors which are vertically spaced form a kind of square grid dividing the membranes 11 and 12 and the gravel layer 13 positioned therebetween into a great number of squares 31. One end of each of the conductors 29 and 30 is electrically connected to a central control and surveying device 32 by means of electrical connections 33 and 34, respectively, which may be in the form of multi- conductor cables.

When refuse or waste material 10 has been deposited in one or more of the hollows 18 defined by the bed structure, percolate or seepage will collect in the bottom of the hollow at the upper surface of the upper membrane 11. This percolate will flow into the percolate draining tubes 25 and into the well 15 from which it may be removed and transferred to another suitable system for decomposition of polluting matter contained therein. Alternatively, the percolate or part thereof may be distributed over the upper surface of the refuse 10 in the hollow 18 from which the percolate is removed, or of the refuse in any other of the hollows. The distribution of percolate may be controlled so as to maximize the rate of decomposition of the polluting components in the refuse or

waste material. The decomposition of the refuse 10 may be surveyed by means of probes or measuring devices (not shown) positioned at selected locations of the refuse 10. Furthermore, decomposition of refuse may be promoted by withdrawing gas which is generated by the decomposition processes, through gas drains (not shown). When the membranes 11 and 12 are intact and without any leaks, the intermediate gravel layer 13 is relatively dry, because percolate as well as ground water is prevented from penetrating into this intermediate layer. If a leak occurs in one of the membranes so that liquid penetrates into the intermediate layer 13, such liquid will flow into the well 14 from which it may be pumped. For the sake of further safety, ground water samples may from time to time be taken out from the boring 20 and be examined for possible pollution. In case pollution of the ground water is detected, further spreading of the pollution may be counteracted by pumping large amounts of ground water from the boring 20 and similar borings positioned in or around the bed structure.

However, the leakage detecting system illustrated in Figs. 3 and 4 makes it possible to detect a leak in one of the membranes 11 and 12 at a rather early stage and to identify the location of the leak. This means that the leak may rather easily be found so that the membrane may be repaired and further precautions for preventing or limiting pollution of the ground water may immediately be taken, if necessary.

If a leak 35 occurs in the upper membrane 11 as indicated in fig. 4, part of the seepage or percolate flowing downwardly through the refuse 10 and along the upper surface of the inclining membrane 11 will flow through the leak 35, across the gravel layer 13, and along the upper surface of the inclining lower membrane 12 as indicated by arrows in Fig. 4. The presence of seepage in the intermediate gravel layer 13 increases the elec¬ trical conductivity of that layer. From time to time a DC-potential difference is succes¬ sively applied to adjacent pairs of the conductors 29 and of the conductors 30, respec¬ tively. When a potential difference ΔV is applied to the adjacent conductors 29a and 29b positioned on either side of the leak 35, a substantially increased conductivity of the layer 13 will be detected by the central control device 32 which indicates that a leak is present in the hatched zone 36 (Fig. 3) between the conductors 29a and 29b. Similarly, an increased conductivity will be detected in the layer 13 between the conductors 30a and 30b indicating that the leak is probably positioned in the hatched zone 37 between these conductors. Consequently, it is likely that the leak is to be found within the double-hatched square 38 indicated in Fig. 3. In order to repair the membrane it is necessary only to remove such an amount of the refuse 10 that it is possible to expose that specific area of the membrane 11.

Example

In a bed structure as that shown in Fig. 2 the liners or membranes, which are of the type marketed by Schlegel Lining Technology GmbH, each have a thickness of 2 mm. The levelling layer 21 may have a thickness of about 10 cm while the thickness of the gravel layers 13 and 16 is about 20 cm and about 30 cm, respectively. The thickness of the concrete slabs 23 may be about 10 cm, and the thickness of the finely divided lower layer 10b of refuse is about 50 cm.

It should be understood that the intermediate layer 13 between the membranes 11 and 12 may be made from any other suitable material than gravel, such as fibrous material. In the latter case, the two spaced membranes and the intermediate fibrous layer con¬ taining the conductors 29 and 30 could form a single carpet-like unit.