WO2017108785A1

WO2017108785A1 - Monitoring system for monitoring the integrity of impermeable barriers, method using the system and bottom barrier provided with the system

Info

Publication number: WO2017108785A1
Application number: PCT/EP2016/081915
Authority: WO
Inventors: Lorenzo Peruzzo
Original assignee: Tessilbrenta S.R.L.
Priority date: 2015-12-21
Filing date: 2016-12-20
Publication date: 2017-06-29
Also published as: ITUB20159693A1; EP3394587A1

Abstract

A system (1) for monitoring an impermeable barrier (12) installed with one side directed toward a wet environment and the other side directed toward a dry environment, in the absence of leaks from the structural sealing membrane (12). According to the invention, the system (1) comprises: a non- impermeable nonwoven geotextile (13), in which two opposite faces (131, 132) are distinguished and which is designed to be installed on the dry side of the sealing membrane (12) and is provided with at least one pair of electrical conductors (24, 25, 24', 25') which are coupled to the same face (132) of the nonwoven geotextile (13), each conductor (24, 25, 24', 25') being provided with a first terminal (241, 251) and a second terminal (242, 252) a system for measuring electrical resistance (4, 4') functionally connected to the first and second terminals of each conductor (241, 251, 242, 252) in order to detect at least one variation of a value of electrical resistance.

Description

MONITORING SYSTEM FOR MONITORING THE INTEGRITY OF IMPERMEABLE BARRIERS, METHOD USING THE SYSTEM AND BOTTOM BARRIER PROVIDED WITH THE SYSTEM TECHNICAL FIELD

The present invention relates to the field of methods and systems for monitoring the integrity of impermeable barriers, for example barriers that can applied to the bottom and to the walls of landfills, storage pits, embankments, dikes, dams, underground structures and more generally all built structures that require delimiting a portion of space that is subjected to the constant or occasional presence of liquids, wherein the impermeable membrane divides the total volume into two partial volumes, one located on the dry side and one wet one of the barrier itself.

BACKGROUND ART

In view of the technical field, in order to describe the most widely used solutions it is convenient first of all to reference the field of landfills, also known as "storage facilities".

A storage facility is considered essentially by:

a bottom and wall lining,

a system for collecting the percolated liquids at the bottom,

a final covering system and optionally

other auxiliary systems (system for extracting landfill gas in the presence of biodegradable waste),

a system for rainfall management and

a monitoring system.

In particular, the bottom and wall lining is designed to limit the flow of contaminants (percolate and gases) into the ground that surrounds the landfill and to constitute the percolate collection surface.

For this purpose, one of the provisions for the bottom and wall lining is that it must be impermeable; for this purpose, since the lining is provided by means of a layering of different materials, it must comprise at least one hydraulic barrier, generally constituted by an upper geomembrane made of HDPE, to which it is possible to couple a lower layer of compacted clay, in direct contact with the geomembrane.

Briefly, the HDPE membrane might be subjected to discontinuities (for example, holes or tears, both at origin and as a consequence of waste piercing in use) and the presence of the clay layer (with low permeability) directly below the membrane itself allows to limit the effects of these discontinuities, limiting the infiltration of the percolate and of the gases into the ground.

It should be noted that normally the condition of the bottom of the landfill is characterized - for a good part of its operating life - by the presence of a continuous percolate head and therefore checking the proper operation of the hydraulic barrier is fundamental.

For this purpose, some solutions have been made available over time which allow to monitor the landfill, in the sense that it is possible to check for the presence of leaks of percolate, which are an indicator of the lack of integrity of the hydraulic barrier and of the need for intervention.

Some of these solutions use optical sensors arranged in various manners in the lining layer; however, although they are interesting and functional, the presence of optical sensors entails a certain constructive complexity that one wishes to avoid.

One apparently simpler and more robust alternative solution - which is commercially available - provides for installing a grid of electrodes in the lining layer of bottom and walls.

In particular, in this solution there is a layer of a geotextile below which the grid of conducting electrodes is arranged. A leak in the membrane causes the escape of liquids below it, impregnating locally the geotextile and thus placing the affected electrode in electrical contact with the substrate.

Locating the leak obviously entails installing a rather dense electrode network, with a consequent relatively high overall cost.

Furthermore, in case of high proper humidity (not caused by leaks) of the substrate, false positives and errors in monitoring might occur.

Another known solution, which uses electrodes as sensors to detect leaks, places the electrodes on opposite sides of the membrane, so that said membrane insulates the electrodes electrically.

In case of membrane tearing, the electrodes are placed in electrical contact, allowing detection of the damage.

This solution, too, is generally functional, but it entails the installation of electrodes at the face of the membrane that is directed toward the waste, with possible damage thereof, in addition to generating an increase in costs and times for installation.

Furthermore, the voltage required for correct operation is high.

SUMMARY OF THE INVENTION

The aim of the present invention is to overcome the drawbacks of the background art.

In particular, the aim of the present invention is to provide a system, a method and a bottom barrier for landfills that are relatively simple to provide and are robust and precise.

Furthermore, an object of the present invention is to provide an alternative to known methods.

This aim and these and other objects of the present invention are achieved by means of a method and a system and a bottom barrier that incorporate the characteristics of the accompanying claims, which are an integral part of the present description.

The general idea on which the present invention is based provides for detecting leaks that are a consequence of damage to the structural sealing membrane by detecting a variation in the measurement of an electrical resistance across terminals of pairs of conductors associated with a nonwoven geotextile which are arranged on a side of the geotextile which, in the active condition, is in a dry environment.

Preferably, in the exemplifying case of landfills, said side is the one arranged below the geotextile itself, which - in turn - is arranged below the sealing membrane.

This solution offers the advantage of allowing a simple, relatively inexpensive and inherently robust production. This last characteristic is maximized, furthermore, if the face of the nonwoven geotextile that is provided with the conductors is arranged in a condition in which it does not face the sealing membrane, i.e., is directed toward the ground, on the opposite side with respect to the waste: the conductors are thus protected against any damage caused by them.

The proposed solution allows to use the usual constructive method for covering the excavation, minimizing the additional procedures associated with the laying of the monitoring system.

Accordingly, the invention relates to a system for monitoring an impermeable barrier installed with one side directed toward a wet environment and the other side directed toward a dry environment, in the absence of leaks from said structural sealing membrane,

wherein the system comprises a non-impermeable nonwoven geotextile in which two opposite faces are distinguished and which is designed to be installed on the dry side of said sealing membrane and is provided with at least one pair of electrical conductors, said at least one pair of electrical conductors being coupled to the same face of the nonwoven geotextile, each conductor being provided with a first terminal and a second terminal, said electrical conductors extending substantially parallel to each other for their whole length, in particular not crossing each other, on said face of the nonwoven geotextile, a system for measuring electrical resistance functionally connected to the first and second terminals of each conductor in order to detect at least one variation of a value of electrical resistance.

In another embodiment, the invention relates to a system for monitoring an impermeable barrier installed so that one side is directed toward a wet environment and the other side is directed toward a dry environment, in the absence of leaks from said structural sealing membrane,

wherein the system comprises

a nonwoven non-impermeable geotextile, in which two opposite faces can be distinguished, designed to be installed on the dry side of said sealing membrane and provided with at least one pair of electrical conductors which are coupled to the same face of the nonwoven geotextile, each conductor being provided with a first terminal and a second terminal

a system for measuring electrical resistance which is functionally connected to the first and second terminals of each conductor in order to detect at least a variation of an electrical resistance value.

According to an advantageous characteristic that is dependent or independent with respect to the other characteristics, the face of the nonwoven geotextile to which said electrical conductors are coupled is the phase that is not in direct contact with (or the one directed toward) the impermeable membrane in the condition in which the geotextile is installed; this allows indeed to protect the conductors.

According to another advantageous characteristic that is dependent or independent with respect to the other characteristics, the electrical conductors are extended substantially along an entire dimension of said face to which they are coupled; this allows a measurement that affects substantially the entire area of the installation.

According to an advantageous characteristic that is dependent or independent with respect to the other characteristics, the geotextile is a mechanically needle-loomed nonwoven fabric made of a material chosen between polyester and polypropylene, with grammages comprised

- between 400 and 2000 g/m² for landfill applications, - between 200 and 800 g/m² for applications in underground structures,

- between 150 and 400 g/m² for applications in the building sector, preferably

- between 600 and 2000 g/m² for landfill applications,

- between 500 and 800 g/m² for applications in underground structures,

- between 200 and 400 g/m² for applications in the building sector;

this allows the percolate or the liquid that has optionally been lost by the sealing membrane to impregnate the nonwoven geotextile and generate a conducting bridge - which allows to detect the leak - before it is drained into a drain.

According to an advantageous characteristic that is dependent or independent with respect to the other characteristics, the electrical conductors are in electrical contact with the nonwoven geotextile, having at least one part of the conductor in direct contact with said geotextile; this allows precise and localized measurements.

According to an advantageous characteristic that is dependent or independent with respect to the other characteristics, each electrical conductor comprises a first polyester supporting filament around which a second electrically conducting filament, preferably made of stainless steel, is wound in turns; this allows to provide relatively inexpensive electrical conductors which are mechanically tough and chemically adapted for use in chemically aggressive environments.

According to an advantageous characteristic that is dependent or independent with respect to the other characteristics, each electrical conductor comprises a polymer filled with powders of conducting metals and/or graphite, in order to be able to determine the resistance thereof in a relatively simple manner.

According to an advantageous characteristic that is dependent or independent with respect to the other characteristics, for each pair of electrical conductors, one of the conductors has a linear electrical resistivity, while the other one has an electrical conductivity that can be approximated to that of an ideal conductor.

According to an advantageous characteristic that is dependent or independent with respect to the other characteristics, the resistance measurement is performed by means of the ratio between the instantaneous values of voltage and current applied to the conductors by operating with alternating current; this allows to reduce ion transport phenomena.

According to an advantageous characteristic that is dependent or independent with respect to the other characteristics, the electrical resistance measurement system comprises an alternating-current voltage/current measurement device, in which an electrical current is supplied to a circuit that is formed

- by the two conductors of the same pair of conductors

and

- by a resistive bridge formed by a liquid that has escaped from a leak of said structural sealing membrane,

in which measurement device the voltage at the two terminals of the two conductors is measured, wherein the resistance is obtained as a ratio between the instantaneous values assumed by voltage and current, wherein sampling is performed at least for a time equal to the periodicity of a signal that is used.

Another aspect of the invention is a method for detecting damaged points in a sealing membrane for landfills, comprising the steps of

- providing a structural sealing membrane installed so that one side is directed toward a wet environment and the other side is directed toward a dry environment, in the absence of leaks from said structural sealing membrane

- providing a non-impermeable nonwoven geotextile to be installed on the opposite side with respect to the one that faces the structural sealing membrane - providing at least one pair of electrical conductors coupled to the same face of the geotextile

- providing a system for measuring electrical resistance which comprises an alternating-current voltage/current measurement device

- connecting opposite terminals of each electrical conductor of said pair of electrical conductors to said electrical resistance measurement system

- detecting, by means of said electrical resistance measurement system, at least variations of the electrical resistance between one terminal of a first conductor of the pair of conductors and one terminal of a second conductor of the pair of conductors.

According to an advantageous characteristic that is independent or independent with respect to the other characteristics, the detection step is performed by means of a measurement of resistance in alternating current, with the advantages indicated above.

Another aspect of the present invention is a bottom barrier for landfills which comprises, from the top downward in the operating condition:

- a structural sealing membrane

- a nonwoven, non-impermeable geotextile provided with electrical conductors

said geotextile membrane having an extension in plan view that is substantially equal to the plan extension of the structural sealing membrane, said geotextile being part of a system according to the invention, preferably a system specifically adapted to provide said method of the invention.

According to an advantageous characteristic that is dependent or independent with respect to the other characteristics, the geotextile is in direct contact with the structural sealing membrane.

Further advantageous characteristics are the subject of the accompanying claims, which are understood to be an integral part of the present description. BRIEF DESCRIPTION OF DRAWINGS

The invention is described hereinafter with reference to nonlimiting examples, given by way of nonlimiting example in the accompanying drawings. These drawings illustrate different aspects and embodiments of the invention and, where appropriate, reference numerals that illustrate structures, components, materials and/or elements that are similar in different figures are indicated by similar reference numerals.

In the accompanying figures:

Figure 1 is a simplified sectional view of a waste storage site, with a system according to the invention installed;

Figure 2 is a sectional view of a bottom lining which incorporates part of the system according to the invention;

Figure 3 is a perspective view of part of a geotextile that is part of the system of the invention;

Figure 4 is a sectional view of the geotextile of Figure 3;

Figures 5 and 6 are views of the geotextile in the view of Figure 4, in two active conditions, respectively in the dry condition and in the wet condition;

Figure 7 is a perspective view of a constructive embodiment of an electrical conductor that can be applied to the geotextile of Figures 2-6;

Figure 8 is a sectional view of the electrical conductor of Figure 7 applied to a geotextile according to the invention;

Figure 9 is a sectional view of a system according to the invention comprising a geotextile, electrical conductors and an electrical resistance detector;

Figure 10 is a diagram of the operation of the system according to the invention;

Figures 11 and 12 are electrical diagrams related to a first example of operation of the system according to the invention in two operating conditions (dry and in case of a leak); Figure 13 is a view of an advanced example of operation of the system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the invention is susceptible of various modifications and alternative constructions, some preferred embodiments are shown in the drawings and are described hereinafter in detail.

In any case, it should be understood that there is no intent to limit the invention to the specific embodiment shown, but on the contrary it intends to cover all the modifications, alternative constructions, and equivalents that fall within the scope of the invention as defined in the claims.

The use of "for example", "etc.", "or" indicates nonexclusive alternatives without limitation unless otherwise specified.

The use of "include" means ""includes, but is not limited to" unless otherwise specified.

Indications such as "vertical" and "horizontal", "upper" and "lower"

(in the absence of other indications) must be read with reference to the assembly (or operating) conditions and with reference to the normal terminology used in current parlance, where "vertical" indicates a direction that is substantially parallel to the direction of the gravity vector "g" and "horizontal" is a direction that is perpendicular thereto.

With reference to Figure 1, it is a sectional view of a typical waste storage site S, for example a final storage site.

It is noted that this application, described herein, must be understood merely as a nonlimiting example of application of the present invention, which might be used more generally also in different technical fields.

The final storage site consists essentially of an excavation R provided in the ground T, in which the waste is accommodated.

The waste is covered in an upward region by a covering C, which is provided in various manners depending on the requirements and which will not be dwelt upon further. The ground of the bottom wall of the excavation R is covered by the bottom barrier 10, which in some solutions can be extended to also cover, fully or partially, the ground of the side walls of the excavation.

The main function of the bottom barrier 10 is to limit the flow of contaminants (percolate and gases) in the surrounding ground T and to constitute the percolate collection plane.

With reference also to Figure 2 and in accordance with the invention, said figure shows a preferred solution of embodiment of this bottom barrier 10, which comprises, from the top downward:

- a first simple geotextile 11 (optional)

- a structural sealing membrane 12

- a nonwoven geotextile 13 with a draining function (therefore not impermeable) provided with electrical conductors 24, 25

- a layer of gravel 14

- a layer of clay 15.

It is noted that the layering proposed above is exemplifying; there are in fact numerous possible variations, which in any case retain the key elements sealing membrane 12 and geotextile 13.

When installed, the geotextile is arranged on the dry side of the membrane (in conditions of normal operation).

The first simple geotextile 11 is optional: when installed, it is preferably made of mechanically needle-loomed nonwoven geotextile, constituted by fibers of virgin polyester or polypropylene with a grammage comprised between

- 400 and 2000 g/m² for landfill applications,

- 200 and 800 g/m² for applications in underground structures,

-150 and 400 g/m² for applications in the building sector,

preferably

600 and 2000 g/m² for landfill applications, 500 and 800 g/m² for applications in underground structures, 200 and 400 g/m² for applications in the building sector.

As regards the structural sealing membrane 12, it is preferably made of HDPE (high-density polyethylene): HDPE is a practically impermeable material, which can be crossed only by chemical migration at the molecular level. Its useful life for the specific use is estimated currently at decades.

This sealing membrane 12 is preferably formed by a plurality of individual HDPE sheets which are heat-sealed in place to each other. The thickness of the sealing membrane 12 preferably varies between 0.5 and 2 mm.

As regards instead the non-impermeable geotextile 13, it will be discussed in detail shortly.

The gravel 14, usually in a layer with a thickness comprised between 30 cm and 1 m, allows to have an effect of drainage of any percolate that has passed through the sealing membrane 12, in order to convey it into adapted collection pits.

Finally, the clay layer 15 cooperates with the upper layers by virtue of the inherent swelling properties of clay: in the presence of humidity, the effect of any small defects (holes, tears) localized within the HDPE is progressively attenuated.

Furthermore, the clay layer 15 allows the adsorption of some pollutants during the crossing of said clay layer 15.

In theory, it might also be possible to provide different embodiments of the bottom barrier 10, although the one that has just been described allows an optimum provision from many viewpoints, in addition to a valid alternative.

Moving on now to describing the system 1 for detecting damaged points in the structural sealing membrane 12 for landfills, according to the invention, reference should be made to the accompanying Figures 2-11.

The system 1 comprises the draining (not impermeable) nonwoven geotextile 13, in which two opposite larger faces 131, 132 are distinguished.

It should be noted that the expression "not impermeable" refers to the passage of liquids.

The face 131 is directed upward, i.e., toward the waste, while the opposite face, when the membrane 13 is installed, is directed downward, i.e., toward the ground.

When installed, as described for Figure 2, the nonwoven geotextile 13 is designed to be arranged on the side of the structural sealing membrane 12 that is dry, in order to contribute to the detection of any leaks thereof (caused by holes, tears or discontinuities that cause the passage of the liquid percolate).

In the nonlimiting example provided here, therefore, the nonwoven geotextile 13 is arranged below the sealing membrane 12.

For this purpose, the nonwoven geotextile 13 comprises at least two electrical conductors 24, 25 which are coupled to the same face 132 of said geotextile 13.

The nonwoven geotextile 13 is "structural", in that it is designed to extend in all directions until it is substantially superimposable on the sealing membrane 12, so as to detect leaks of the latter. The nonwoven geotextile 13 is "not impermeable", in the sense that it does not prevent the passage of liquids and therefore has a draining function.

For this purpose, the nonwoven geotextile 13 is preferably made of polyester or polypropylene nonwoven fabric with grammages comprised between 400 and 1500 g/m²: this particular embodiment allows the geotextile 13 to be at once sufficiently flexible to be rested within the excavation or adapted to the application otherwise, following its walls and impregnating with liquid, in case of leaks.

The system 1 furthermore comprises one or more pairs of electrical conductors 24, 25 and 24', 25', which are each provided with a first terminal 241, 251 and 241', 251' and a second terminal 242, 252 and 242', 252', which are connected electrically to an electrical resistance detector 4 and 4'; each pair of conductors 24, 25 or 24', 25' is therefore connected to a dedicated electrical resistance detector 4 or 4', as shown in Figure 9.

It is noted that the accompanying figures show only some of the conductors that are present, in a nonlimiting manner.

Each electrical conductor 24, 25 of the pair is preferably arranged at a distance of approximately 1 meter from the other conductor of the same pair.

The pairs of conductors 24, 25 and 24', 25' are instead arranged at a distance that can vary between 0.5 and 3 meter with respect to each other, depending on the spatial resolution that one intends to give to the measurement system, which can vary depending on the application, on the geometry of the site and on the danger level of the liquid to be detected.

This distribution allows to obtain an optimum but not excessively onerous distribution.

Preferably, the electrical conductors are all extended substantially parallel to each other, so as to affect an entire face 132 of the geotextile 13.

In other embodiments, the pairs of electrical conductors are crossed so as to form a pattern of squares, diamonds, lozenges.

Each electrical conductor, regardless of the arrangement of the pairs, is separated and electrically insulated from the other.

The electrical conductors 24, 25, 24', 25' are not covered with insulating material (except at crossings with other conductors, in order to maintain their mutual insulation) and are coupled to the geotextile 13.

This coupling can be obtained in various manners: for example, in the preferred solution, shown in Figures 4, 5 and 6, the conductors are inserted in a pocket provided by a tape 240 which is fixed in various manners (e.g., sewn, bonded with adhesive, heat-sealed with ultrasound or heating) to the geotextile 13, so as to remain in direct contact with it.

The tape 240 is preferably made of the same material as the nonwoven geotextile 13, so as to allow simplified coupling, for example by ultrasound (or heat) sealing, when they are made of thermoplastic material. The tape is preferably permeable so as to allow the passage of the liquid; in case an impermeable tape is used, it should not be so large as to constitute a barrier to the effective impregnation of the geotextile from the tape's sides.

In an alternative embodiment, the coupling is obtained by adhesive bonding, as shown in Figure 8; in this case also, preferably, the conductors 24, 25 have a portion in direct contact with the nonwoven geotextile 13: this can be achieved, for example, by using two beads of glue 240' which run parallel to the conductor 24, between the latter and the nonwoven geotextile 13, so that the portion of conductor 24 arranged between the beads of glue 240' is directly in contact with the nonwoven geotextile 13.

Maintaining direct contact between the electrical conductor 24, 25 and the nonwoven geotextile 13 is preferable for the correct operation of the system, although it may not be entirely necessary.

As regards the electrical conductors 24, 25, 24', 25', they can be made of different electrically conducting materials; a preferred solution is shown in Figure 7, in which the conductor 24 comprises a first supporting filament 248 made of polyester around which a second electrically conducting filament 249, for example made of stainless steel, particularly suitable for the aggressive environment to which it will be exposed once installed, is wound in turns.

As mentioned above, the pairs of conductors 24, 25 are coupled to the geotextile at the same face and, in greater detail, at the face 132 that is directed toward the ground.

In other embodiments they are instead arranged on the face 131 , which is directed toward the sealing membrane 12.

Moving on now to the description of the electrical resistance measurement device 4 or 4' (and of the electrical connections), it has at least four inputs, each connected electrically to a respective free terminal 241, 242, 251, 252 or 241', 242', 251', 252' of each conductor 24, 25 or 24', 25' of the same pair. Preferably, the electrical resistance measurement device 4 or 4' is a voltage/current measurement device.

The resistance measurement device 4, 4' preferably works with alternating current in order to avoid charge transport phenomena (in case of dissolved ions) and/or electrolysis, and preferably comprises an electrical diagram that can be traced back to the principle of the Kohlrausch Bridge, which is per se known to the person skilled in the art.

The operation of the system 1 , with reference to Figures 5, 6, 10, 11, 12), once installed (with the nonwoven geotextile 13 on the dry side (when installed) of a sealing membrane 12), is based on the detection of a conducting "bridge" between two conductors of the same pair 24, 25.

In a condition of absence of leaks of the sealing membrane 12 (see Figures 5, 11), the conductors 24, 25 of the same pair are arranged in a dry environment and are therefore mutually insulated, since the geotextile is indeed made of insulating (not electrically conducting) material.

The electric circuit between the two conductors is therefore open and the electrical resistance detector (connected to the two terminals of each wire of the pair) detects a given value for each conductor 24, 25.

In greater detail, the detector 4 detects a certain resistance R (in series) for the conductor 24 and a resistance R* for the conductor 25: in theory, if the conductors 24 and 25 are identical and of the same length, then R = R*.

In case of a leak from the sealing membrane 12 (see Figures 6, 10, 12), the liquid L (usually percolate) that flows out instead impregnates the nonwoven geotextile 13.

The rupture in the sealing membrane 12 in fact causes an outflow of liquid L, which by flowing over both of the wires that belong to a pair places them in electrical contact: this occurs both if the liquid L is the one impregnated in the nonwoven geotextile 13 (as in Figures 6) and if it is a possible puddle that accumulates below it (despite the gravel 14).

The presence of the liquid entails a variation of the resistance of the conductors 24, 25 (along its length).

In a basic embodiment, a simple check for the presence of a leak that affects a pair of wires is performed: in this case, the parameter of interest is the simple detection of the closure of the circuit between the two conductors 24 and 25 of the same pair, which indicates the presence of liquid L and therefore a leak from the sealing membrane 12.

In an evolved embodiment, it is possible to also locate the distance of the leak (therefore of the liquid L) from the detector 4: with reference to Figure 12, the liquid L not only short-circuits the conductors 24 and 25, closing the electric circuit, but has a certain resistance Rp of its own; furthermore, each conductor 24, 25 is ideally divided by the liquid L into two portions, which have respectively resistances Rl and R2 (assuming that in the absence of liquid, i.e., in the dry condition, R = R*).

By measuring the resistance Rl and if the linear resistance of the conductors 24, 25 is known, it is possible to determine the distance of the conducting "bridge" determined by the liquid L from the terminals 241 , 251 of the conductors 24, 25 with respect to which the measurement is made.

In greater detail the (unknown) term R_p can be eliminated by taking two resistance measurements at the terminals 241-251 and 242-252 of the two conductors and subtracting the two values (with the assumption that the volume and conductivity of the puddle of liquid do not change, which is always true for instantaneous measurements, i.e., where the time over which the conductivity varies appreciably is longer than the sampling time required to determine an experimental point, constituted in the example that follows by a pair of resistances to be subtracted).

In this case

R(241/251)= 2*R1+ R_P

R(242/252)= 2* R2 + R_p where R(241/251) and R(242/252) are the resistances measured at the same terminal of the two conductors of the same pair on the same side, i.e., the electrical resistances of the half-circuit constituted by

for 241/251 : a first portion of conductor 24 which goes from the detector 4 to the bridge formed by the leak of liquid, the bridge itself, a first portion of conductor 25 that goes from the leak of liquid to the detector 4, the first portions of the conductors 24 and 25 being the ones connected to the same ports of the detector 4;

for 242/252: a second portion of conductor 24 which goes from the detector 4 to the bridge determined by the leak of liquid, the bridge itself, a second portion of conductor 25 which goes from the leak of liquid to the detector 4, the second portions of the conductors 24 and 25 being the ones connected to the same ports of the detector 4.

From the above one can deduce that the difference between the resistances R(241/251)-R(242/252) of the two half-circuits is equal to

R(241/251)-R(242/252)= 2*R1-2*R2

and therefore, since Ri+R₂=R, one obtains

R(241/251)-R(242/252)= 4R 2R

and therefore ultimately

Rl=(R(241/251)-R(242/252)+2R)/4

The distance L of the leak from the terminal 251 can therefore be calculated as

L= Rl/p

where p is the specific electrical resistivity of the conductor 25.

The measurement is performed in alternating current in order to avoid ion transport phenomena.

The resistance measurement is performed by supplying an alternating current and reading a voltage.

The detector 4 can comprise an oscilloscope or an ADC acquisition system or similar instruments known to the person skilled in the art.

The subtraction between the two resistance measurements can be performed analytically, if the resistances associated with the portions of conductor 24, 25 involved and with the "bridge" of liquid are comparable. As an alternative, it is possible to subtract directly the two electrical signals by making them converge on an adder after passing one of the two through an adapted delay circuit in order to place it in phase opposition with respect to the other one.

In an evolved alternative embodiment of the system and of the method according to the invention, with reference to Figure 13, the two conductors of the same pair 24, 25 are not identical.

In particular, one of the two conductors of the pair (the conductor 25 in the example of Figure 13) has an appreciable resistance R, while the other one has a resistance «R (the conductor 24 in the example of Figure 13).

This last conductor 24, according to the step of the method, is likened to a perfect conductor and is resistance is ignored.

Figure 13 shows this situation.

This configuration allows to work without further assumptions and therefore works also in case of a puddle with macroscopic dimensions (as long as they do not change instantaneously).

The equation for obtaining Rl in this case becomes:

Rl=(R(241/251)-R(242/252)+2R)/2.

The distance L of the leak from the terminal 251 can therefore be calculated as

L= Rl/p

where p is the specific electrical resistivity of the conductor 25.

It is noted that although the description given above is based on a landfill, it should not be understood in a limiting manner, i.e., the system, the barrier and the method according to the invention are applicable in a fully general manner to the monitoring of the integrity of impermeable barriers, for example the ones used (as well as in landfills) at storage pits, embankments, dikes, dams, underground structures and more generally all built structures that require delimiting a portion of space that is subjected to the constant or occasional presence of liquids.

Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

1. A system (1) for monitoring an impermeable barrier (12) installed with one side directed toward a wet environment and the other side directed toward a dry environment, in the absence of leaks from said structural sealing membrane (12),

wherein the system (1) comprises

a non- impermeable nonwoven geotextile (13) in which two opposite faces (131 , 132) are distinguished and which is designed to be installed on the dry side of said sealing membrane (12) and is provided with at least one pair of electrical conductors (24, 25, 24', 25')

characterized in that

said at least one pair of electrical conductors (24,25,24', 25') is coupled to the same face (132) of the nonwoven geotextile (13), each conductor (24, 25, 24', 25') being provided with a first terminal (241, 251) and a second terminal (242, 252)

said electrical conductors (24,25,24',25') extending substantially parallel to each other for their whole length, in particular not crossing each other, on said face (132) of the nonwoven geotextile (13)

a system for measuring electrical resistance (4, 4') functionally connected to the first and second terminals of each conductor (241 , 251, 242, 252) in order to detect at least one variation of a value of electrical resistance.

2. The system (1) according to the preceding claim, wherein the face (132) of the nonwoven geotextile (13) to which said electrical conductors (24, 25, 24', 25') are coupled is the face that lies opposite the face directed toward said sealing membrane (12) in the condition in which the nonwoven geotextile (13) is installed.

3. The system (1) according to claim 1 or 2, wherein said electrical conductors are extended substantially along an entire dimension of said face (132) to which they are coupled.

4. The system (1) according to one or more of the preceding claims, wherein the geotextile (13) comprises a needle-loomed nonwoven fabric which is made of a material chosen among polyester or polypropylene, preferably with grammages comprises between 200 and 2000 g/m², even more preferably between

- 400 and 2000 g/m² for landfill applications,

- 200 and 800 g/m² for applications in underground structures,

-150 and 400 g/m² for applications in the building sector.

5. The system (1) according to one or more of the preceding claims, wherein the electrical conductors (24, 25, 24', 25') are in electrical contact with the nonwoven geotextile (13), having at least one part of the conductor in direct contact with said geotextile (13).

6. The system (1) according to one or more of the preceding claims, wherein each electrical conductor (24, 25, 24', 25') comprises a first supporting filament (248) made of polyester, around which a second electrically conducting filament (249), preferably made of stainless steel, is wound in turns.

7. The system (1) according to one or more of claims 1 to 5, wherein each electrical conductor (24, 25, 24', 25') comprises a polymer filled with powders of conducting metals and/or graphite.

8. The system (1) according to one or more of the preceding claims, wherein, for each pair of electrical conductors, one of the conductors (24, 25, 24', 25') has a linear electrical resistivity, while the other one has an electrical conductivity that can be approximated to that of an ideal conductor.

9. The system (1) according to one or more of the preceding claims, wherein the electrical resistance measurement system (4, 4') comprises an alternating current voltage/current measurement device, in which an electrical current is supplied to a circuit formed

- by the two conductors of the same pair of conductors

and - by a resistive bridge formed by a liquid that has escape through a leak of said structural sealing membrane (12),

in which measurement device the voltage across the two terminals of the two conductors (24, 25) is measured, wherein the resistance is obtained as a ratio between the instantaneous values assumed by the voltage and current, wherein the sampling is performed at least for a time equal to the periodicity of a signal that is used.

10. A method for detecting damaged points in a sealing membrane (12), comprising the steps of

- providing a structural sealing membrane (12) which is installed so that one side is directed toward a wet environment and the other side is directed toward a dry environment, in the absence of leaks from said structural sealing membrane (12)

- providing a non- impermeable nonwoven geotextile (13) to be installed on the opposite side with respect to the one that faces the structural sealing membrane (12)

- providing at least one pair of electrical conductors (24, 25) coupled to the same face (132) of the geotextile (13), said electrical conductors (24,25,24',25') extending substantially parallel to each other for their entire length, not crossing each other, on said face (132) of the nonwoven geotextile (13)

- providing a system for measuring measuring electrical resistance (4, 4') which comprises an alternating current voltage/current measurement device

- connecting opposite terminals (241, 251, 242, 252) of each electrical conductor (24, 25) of said pair of electrical conductors to said electrical resistance measurement system (4, 4')

- detecting, by virtue of said electrical resistance measurement system (4, 4'), at least variations of electrical resistance between one terminal (241) of a first conductor (24) of the pair of conductors and a terminal (251) of a second conductor (25) of the pair of conductors, said method further comprising the step of

detecting by said electrical resistance measurement system (4,4') at least changes in the electrical resistance between both terminals (241 ,242) of a first conductor (24) of the pair of conductors and both ends (251,252) of a second conductor (25) of the pair of conductors , and calculating a distance (L) from one end (251) of said wires of a leakage on the base of said measured electrical resistances.

11. The method according to the preceding claim, wherein said detection step is performed by means of an alternating-current resistance measurement.

12. A bottom barrier (10), comprising, from the top downward in the operating condition:

- a structural sealing membrane (12)

- a non- impermeable nonwoven geotextile (13) provided with electrical conductors (24, 25)

said geotextile (13) having an extension in plan view which is substantially equal to the extension in plan view of the structural sealing membrane (12), said geotextile (13) being part of a system according to one or more of claims 1 to 7.

13. The bottom barrier (10) according to the preceding claim, wherein the nonwoven geotextile (13) is in direct contact with the structural sealing membrane (12).