Abstract:
Leak prevention system for a liquid retention pond ( 10 ), the system comprising an upper membrane ( 3 ) to be covered by the liquid ( 1 ), a lower membrane ( 4 ) disposed on the bottom of the pond and joined in a sealed manner to the upper membrane on a peripheral portion to thereby form a normally sealed envelope ( 20 ) delimited by the two membranes, said envelope being filled with draining material ( 2 ), a plurality of passageways ( 6 ) disposed substantially horizontally in the draining material, a pumping device ( 5 ) adapted to generate an air vacuum in the drains, to suck leakage liquid that may have passed through the upper membrane, such that pollution of the surrounding soil is avoided in the event of a liquid leak through the upper membrane.

Description:
[0001]    This application claims priority to French Application No. 13 56000, filed Jun. 24, 2013 in France, which is incorporated by reference for all purposes as if fully set forth herein. 
       BACKGROUND 
       [0002]    The present invention relates to the leak prevention systems for retention ponds, in particular for ponds containing materials and products that can be polluting or contaminating products. 
         [0003]    Such retention ponds are used to store various products provisionally or for long periods be they industrial residues, discharge products or trash, said products being capable of containing both solid and liquid elements. 
         [0004]    More particularly, the invention may be applied to retention ponds for contaminating, notably radioactive liquids. 
         [0005]    The environmental constraints require limiting, if not completely avoiding a potential contamination of the ground on which the pond is arranged. 
         [0006]    Such pond is conceived either by excavation or by erection of a peripheral delimitation berm, the pond comprising a concave bottom in both cases. Such pond most often presents a rectangular general shape with horizontal dimensions of several tens or hundreds of meters and a depth of a few meters. 
         [0007]    In the prior art, one or more membranes are arranged on the bottom of the pond before depositing products to be stored, such that the liquids are well retained and do not penetrate the ground under the membrane. 
         [0008]    It turns out that the membranes may be prone to leakage, notably owing to the fact that such membranes consist of several plastic material strips connected to each other by welding. 
         [0009]    It is also known, from document GB2289493, systems based on a network of pockets forming a partitioned protection with individual management of the compartments; however, at the location of the junction between the compartments, the leakage risk is not really eliminated. Even if a layer of masonry is formed, prior to the membrane installation, using mortar or bentonite, this does not really solve the problem as cracks may sooner or later appear in this layer, due to the ageing or the movements or quakes of the terrain. 
         [0010]    Installing several membranes does not really solve the problem as each of them is subject to the leakage risk. 
         [0011]    There has thus arisen a need to propose a solution which allows the reduction of soil contamination risks given the potential defects of the usual membranes. 
       SUMMARY OF THE DISCLOSURE 
       [0012]    To that end, the invention provides a leak prevention system for a liquid retention pond, in particular for polluting or contaminating liquids, the system comprising:
       a sealing arrangement comprising an upper membrane to be covered by the liquid, a lower membrane placed continuously on the entire foundation surface of the pond and joined in a sealed manner to the upper membrane on a peripheral portion to thereby form a normally sealed envelope delimited by the two membranes, said envelope being filled with a continuous layer of draining material,   at least one pumping device adapted to generate at least partial air vacuum, (i.e. a pressure lower than the atmospheric pressure),   a plurality of drains arranged substantially horizontally in the draining material, in fluid communication with the pumping device, in such a way as to be able to suck leakage liquid that may have passed through the upper membrane, such that pollution of the surrounding soil is avoided in the event of a liquid leak through the upper membrane.       
 
         [0016]    Thanks to these arrangements, in the case of leakage of the upper membrane, the liquid leaking through the upper membrane is collected by the pumping and will not penetrate the soil through the lower membrane. This turns out to be effective regardless of the position of the potential leakage. 
         [0017]    In other words the risk for the presence of a leakage in the membrane is accepted, but the effects are compensated by the pumping device and the risks of soil pollution are thus eliminated in the short, medium and long term. It is particularly important if solid objects that may have acute edges are deposited in the retention pond. 
         [0018]    In embodiments of the system according to the invention, one and/or the other of the following arrangements may possibly be further used:
       the system may be configured such that the pumping device releases the leakage liquid in the retention pond; thus the leaking liquid is re-injected in the basin and the contamination remains contained in the pond itself;   the system may comprise a pressure sensor, the system being configured to generate a cyclical activation of the pumping, preferably with a duty cycle dependent on the pressure evolution in the envelope; such that the energy spending is optimized to permanently maintain an sufficient air vacuum to achieve a pumping;   the system may further comprise a collector piping between the drains and the pump, and a check valve between the collector and the pump, designed to close itself once the vacuum generation is interrupted, which maintains a vacuum pressure; such that an air vacuum is maintained in the sealed envelope as long as the leakage is minimal, insignificant or inexistent;   the draining material may be a macroporous prefabricated draining complex layer; such that the sealing device is very fast to install, and furthermore it is not necessary to bring material such as sand on the site; further, drains are advantageously integrated in such prefabricated complex layer:   the draining material may be a granular material such as sand; which is a well-controlled conventional solution, sand being available in the proximity of the majority of the worksites;   the passageways are drains in the form of pierced or porous pipes; which is a reliable conventional drainage solution,   the drains are arranged parallel to each other, at a distance between 30 cm and 3 m, and are connected to a collector in communication with the intake of the pumping device; so as to ensure an efficient pumping of a potential leak regardless of its localization in the upper membrane, using the capillarity properties of sand or the draining material;   the pumping device may comprise a separation chamber, an air vacuum pump and a liquid backflow pump, with a liquid level sensor to steer the activation of the liquid backflow pump; such that the pumping device is adapted to pump air and/or liquid and to separate the liquid so as to release the latter in an adequate location.       
 
         [0027]    The object of the invention is also a method used for a leak prevention system as described above, said method comprising the following steps:
       a—arranging a first membrane at the bottom of the pond, forming a lower membrane,   b—arranging a continuous layer of draining material over it, with draining passageways,   c—arranging a second membrane over it forming an upper membrane, coupling it in a sealed manner at the periphery of the first membrane to form a normally sealed envelope,   d—disposing a pumping device and performing a pumping from the draining passageways.       
 
         [0032]    Other characteristics and advantages of the invention will appear in the following description of two of its embodiments, given as non-limiting examples with reference to the accompanying drawings. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0033]    In the drawings: 
           [0034]      FIG. 1  is a general sectional view of a first embodiment of the leak prevention system for a retention pond according to the invention. 
           [0035]      FIG. 2  is a partial plan view of the system in  FIG. 1 . 
           [0036]      FIG. 3  shows a schematic view of the pumping device. 
           [0037]      FIG. 4  shows chronograms illustrating the system operation. 
           [0038]      FIG. 5  shows a second embodiment according to the invention. 
       
    
    
       [0039]    On the various figures, the same references indicate identical or similar elements. For the sake of clarity, the proportions of the various elements represented are not necessarily to scale. 
       DETAILED DESCRIPTION 
       [0040]      FIG. 1  represents a retention pond  10  containing polluting or contaminating liquid  1 . The liquid may be an industrial residue comprising chemical or radioactive products that is to be confined in such a retention pond. It should be noted that the invention equally applies when the retention pond is for storing solid products from which contaminating or polluting liquids may escape. 
         [0041]    In the case illustrated, the pond is formed by excavation in the soil, the concave shape thereby obtained presents a foundation (labeled  90 ). Nonetheless, such a shape may also be obtained by erecting a berm all around the pond to form. 
         [0042]    Before depositing liquid or waste in the pond, a sealing arrangement is installed on the bottom of the pond. First of all, a first membrane is arranged on the foundation  90  of the pond, here named lower membrane  4 . This membrane is made in synthetic material such as PVC, HDPE or other equivalent plastic material. This membrane forms a flexible wall, still called ‘tarpaulin’, forming a barrier that is normally liquid-sealed. However, taking account of the large usual dimensions of this type of pond, the membrane is formed by the juxtaposition of several material strips of the type PVC which are connected to each other by hot welding, by a thermofusing method known per se. Despite the care devoted to these welds, which besides in practice may often be doubled, imperfections may exist which lead to a non-desired liquid passageway, i.e. a leak. 
         [0043]    Once the lower membrane  4  is placed in a continuous manner on the entire surface of the bottom of the pond, a first layer of draining material, here a granular material such as sand, is installed. 
         [0044]    Then, a plurality of drains  6  is installed, each arranged substantially horizontally along the bottom of the pond and prolonged on either side by inclined portions along the border ramps. The drains are arranged parallel alongside each other as illustrated in  FIG. 2  in particular. Each drain  6  has an end coupled to a secondary collector  63  disposed transversally to the drains  6  on the sand  2 . 
         [0045]    After the installation of the plurality of drains  6 , draining material  2  is added over it to entirely cover the plurality of drains  6 . 
         [0046]    A continuous layer of draining material  2  is hence formed, wherein the drains  6  are inserted. 
         [0047]    Then, a second membrane is arranged, here called upper membrane  3 , to cover the draining material on the entire surface of the pond. The peripheral border of the upper membrane  3  is then joined to the peripheral border of the lower membrane  4  by welds  23 , which allows the formation of a normally sealed envelope  20  wherein an air vacuum may be created as seen in detail further. 
         [0048]    One can notice that, as the layer of draining material is continuous, the whole layer formed by the draining material is in fluid communication by capillarity, and precise positioning of the drains  6  is not essential. Each secondary collector  63  is coupled to a primary collector  60  disposed outside the sealed envelope  20 , as explained below. Each secondary collector  63  is coupled to a passage pipe  62  which passes through a sealing sleeve  64  arranged at the frontier between the interior and the exterior of the envelope, as illustrated in  FIGS. 2 and 3 . 
         [0049]    It should be noted that the passage pipe  62  may be produced in two parts, an internal part up to the sleeve  64  which may or not be porous and a hermetic external part which connects the sleeve  64  to the primary collector  60 . 
         [0050]    The system further comprises a pumping device  5  configured to be placed in communication which the plurality of drains  6  to create an air vacuum in the aforementioned sealed envelope  20 . 
         [0051]    In the illustrated example the pumping device  5  comprises a closed area called separation chamber  7  which includes an inlet  71  for the products to be pumped (air and/or liquid), an outlet for the backflow liquid products, and an air outlet for air expulsion to the outside of the separation chamber. 
         [0052]    More precisely, a liquid backflow pump  9  removes liquid from the lower part of the separation chamber and expels it towards a liquid backflow line  19 . An air pump  8  sucks air inside the separation chamber and expels said inside air to the outside. When this air pump is in operation, the pressure prevailing inside the separation chamber is lower than the atmospheric pressure, typically of the order of a few tenths of atmosphere, preferentially between and 3 tenths of atmosphere, or even advantageously less than a tenth of atmosphere. The air pump  8  thus creates a more or less deep air vacuum in the separation chamber  7 . 
         [0053]    As the separation chamber is in fluid communication with the plurality of drains via the inlet line  61  and the collector piping  60 , the same air vacuum pressure substantially prevails inside the pipes forming the drains, the pressure losses along the drains are not significant, or not problematic. If no liquid passes through the upper membrane  3  or the lower membrane  4 , then the pumping device  5  only sucks a very limited quantity of air from the drains. 
         [0054]    But, in the event of a leak forming in the upper membrane  3 , as illustrated in  FIG. 4  (ref  18 ), then the leakage liquid is sucked along the drain (arrow F) and is caused to flow up to the separation chamber  7  via the collector piping. 
         [0055]    The liquid backflow pump  9  then sends back the leakage liquids towards the retention pond. It is preferably activated only when a liquid level sensor  13  indicates the presence of the liquid in the separation chamber. The liquid level sensor  13  may for example provide a more elaborated information than a simple binary information, there may for example be several contactors on the reservoir height so as to be able to control the liquid backflow pump  9  as suitably as possible. 
         [0056]    As long as the leaks remain minimal or insignificant, it is not necessary to permanently activate the air pump, this can be achieved in a cyclical way with an activation sequence Ton and then a rest sequence Toff ( FIG. 4 , left side). 
         [0057]    Advantageously, a check valve  11  is arranged on the intake line  61  of the pumping device, this valve being open when the pump operates, and this valve being configured to close when the air pump stops so as to maintain the air vacuum in the drain network. A pressure sensor  12  is arranged in the proximity of the collector  60  or at the intake line  61  to monitor the pressure evolution, notably when the pump is deactivated. 
         [0058]    Advantageously, the pumping device is disposed just above the upper level of the liquid  1 , the hydraulic losses are consequently very limited. 
         [0059]    The system comprises a control unit  17  in charge of the activation of the liquid backflow pump and of the air pump activation cycling when such a cycling is required. 
         [0060]    When a significant leak appears, then the pressure in the drain network rises rapidly as soon as the air pump stops operating. The embodiment of the system then switches to an almost permanent or permanent pumping mode ( FIG. 4 , right side). 
         [0061]    In a second embodiment, illustrated in  FIG. 5 , a draining synthetic complex layer  26  (hybrid material) is used. This material is prefabricated, it either has the form of thin plates consisting of one or more macroporous synthetic materials forming a self-supporting structure, or the form of rolls of strips extending on several meters to be unrolled and laid down on the lower membrane  4 . This synthetic complex layer  26  replaces the sand and the drains from the above-described embodiment, it may be composed of a geotextile material, not necessarily homogeneous, there may be structure reinforcements combined with a relatively porous flexible material. 
         [0062]    The synthetic complex layer  26  is thus arranged in a continuous way on the entire surface of the pond between the upper membrane  3  and the lower membrane  4  placed continuously on the bottom of the pond. 
         [0063]    At one of the longitudinal ends, a funnel is placed leading into a conventional collect pipe, for example a secondary collector  63  equivalent to the one illustrated above. All the conventional pipes, secondary collectors to reach the inlet of the pumping device are identical or similar to those above-described. 
         [0064]    The pumping device  5  and the other elements, in particular the lower and upper membranes will not be described again as they are identical or similar to those above-described. 
         [0065]    Preferably, prefabricated draining material plates are installed alongside each other and/or one after the other to cover the whole bottom surface of the pond. The inclined borders of the pond may also be equipped with such plates. After the installation of all the plates, the upper membrane is arranged and welded on its periphery to the lower membrane to form the normally sealed envelope  20  as previously described for the first embodiment. 
         [0066]    Advantageously, the prefabricated draining synthetic complex layer  26  has a limited thickness between for example 2 cm and 10 cm. It does not decrease significantly the volume of the basin. 
         [0067]    It must be noted that, for both the case of the sand  2  and the case of the prefabricated draining material  26 , the installation of the sealing device provided is particularly simple and may be implemented rapidly by unskilled staff, regardless of the size of the pond to protect.