GB2136861A - Process for the construction of insulated sites in particular for the discharge of polluant products or the formation of impervious barricades or barriers, and works thereby constructed - Google Patents

Description

SPECIFICATION Process for the construction of insulated sites in particular for the discharge of pollutant products or the formation of impervious barricades or barriers, and works thereby constructed The present invention has as its object a process for the construction of insulated sites, in particular for the discharge of pollutant products, and it relates inter alia to the construction of barricades against liquids, placed in the ground. The conflicting and contradictory requirements of at the same time protecting natural sites poses the problem, which is becoming more and more difficult and costly to solve, of constructing sites which are insulated sufficiently securely so that the polluting products or the liquids which are to be retained or stored in the sites cannot migrate into the surroundings, in particular under the influence of natural flow or flow produced by the water in the ground. This is a very general problem in the storing up of industrial and urban waste and the construction of hydraulic works, dams, reservoirs, and catchment areas where the water is required to be held back. Unless special precautions are taken to isolate such water, it migrates into the soil and renders the site or the work unsuitable for its proper purpose. To solve such problems, it has been proposed to surround or enclose the site locally by means of an impervious casing descending sufficiently deeply into the ground. If the ground is normally stratified into substantially horizontal layers or only slightly inclined layers (as is most frequently the case), the phenomena of migration are virtually confined to the upper layers of the terrain, which are frequently more permeable than the substratum, the depth to which the permeable layers reach obviously depending upon the structure and constitution of the given terrain. Under these conditions, the construction of an insulating "tubbing" or casing would prevent or at least reduce migration of the products stored in the insulated site to the surrounding environment. The most suitable technique available at present for the construction of such a "tubbing" consists of forming a flexible casing in the terrain by pouring it into a trench, this casing normally consisting of a mixture, in suitable proportions, of a product which ultimately sets and containing fillers, water, bentonite and cement. Such a relatively flexible casing is capable of following any local deformations of the terrain without cracking, so that the construction will be of very long duration, persisting over many years. Unfortunately, if the casings are limited to thicknesses which are economically viable, this technique does not provide sufficient imperviousness, and certainly not perfect imperviousness, at least during the first phase of exploitation of the site. It is an object of the present invention to resolve this difficulty. For this purpose, there is carried out, according to this invention, a process by which flexible casings sunk into the ground are rendered impervious, characterised in that just after the mixture of which the flexible casing is constituted has been poured into position in the trench designed to receive it, and no matter what the nature of this mixture, one or more than one impervious film is lowered into this mixture substantially to the bottom of the trench, the film being weighted down by a ballast weight at its lower edge for this purpose, and this process is carried out step by step by separate widths corresponding to the width of the sheet used, these widths being arranged to overlap sufficiently to ensure impermeability between adjacent widths. Complete imperviousness of the casing is thereby obtained without significantly increasing the cost of construction while the casing preserves its flexibility which enables it to adapt to shifting movements of the terrain. The film must, of course, be chosen of a quality which is chemically and mechanically resistant to the products with which it will come into contact, in particular the type of water, which depends upon the particular environment, and the effluents from the pollutant products discharged on the site. The sealing product need not be cast into the trench immediately after excavation. It has become increasingly common to produce the trench directly by a process of "continuous" hydraulic circulation with a drilling fluid which subsequently solidifies or sets when left to stand. The process indicated above is obviously also applicable to trenches in which a sealing product which subsequently solidifies or sets, such as concrete or mortar, is cast after drilling. The invention and its characteristics will become clearer with the aid of the description given below with reference to the annexed drawings illustrating one embodiment by way of example. In these drawings, Figure 1 is a schematic vertical section showing the construction of an impervious casing according to the invention, Figure 2 is a horizontal section taken substantially at the level of the plane II-II of Fig. 1, Figure 3 is an enlarged view of the detail encircled at III in Fig. 1, showing means for positioning the impervious film, Figure 4 is a view from above, showing the insulated site on a smaller scale, Figure 5 is a schematic horizontal section showing one phase of the formation of an impervious casing according to the invention, Figures 6 and 7 are views similar to Fig. 5 of variations in the formation of casings, Figure 8 is a perspective view, with a portion torn out and showing schematically and on an enlarged scale an element of impervious film which may be used according to the embodiment illustrated in Fig. 7. In the embodiment illustrated in Figs. 1 to 4, a site 2 containing a product 3 to be isolated is insulated locally or totally from the surrounding terrain 1. Insulation of the site 2 from the surrounding terrain 1 is obtained by means of a casing of separation 4 completely or partially surrounding the site and, for example, substantially circular in form. This casing 4 must extend down to a sufficient depth H in the terrain to prevent any infiltration of effluent from underneath the casing 4, for example as indicated by the arrows 5 in Fig. 1. The height H may be determined in practice from a knowledge of the terrain, all the less height being required the more the terrain is rich in relatively impervious, substantially horizontal layers or strata. In the layers close to the surface, on the other hand, for example, in a layer of pebbles such as 6 or sand 7, the permeability is very great and insulation of the site 2 against the terrain 1 can only be ensured if the casing 4 is completely water-tight. The particular manner of producing the casing 4 according to this invention will now be described with reference to Figs. 1 and 2. A trench 8 is first hollowed out of the terrain 1 to a predetermined height H by known processes of excavation, proceeding by separate slices of predetermined length, the width e of the trench being equal to the width of the casing which is to be placed in position. This excavation is normally carried out in a drilling fluid of a suitable nature and density to prevent collapse of the side walls. When a slice of sufficient length has been excavated, the fluid used for excavation is progressively drawn off if the final product is different from the drilling fluid, and in that case the fluid drawn off is replaced by the required mixture which after setting constitutes the semiflexible casing 4. When the corresponding slice of casing has been formed, an impervious film 9 taken from a supply roll 10 over guide rollers 11 at the edge of the trench is placed in position along one of the two walls of the trench, for example the "inner" wall 8i, as will be seen more clearly in Fig. 2. The film descends close to the wall 8i of the trench, passing, as it does so, through the as yet fluid medium of the casing 4 by means of a weight 12 previously suspended to the lower end of the film 9. A strip of film of width L is thus placed close to the wall 8i, care being taken to ensure that this strip overlaps by a sufficient amount 1 the strip of film on the preceding portion of casing. Fig. 2 thus shows that the strip of film 9k overlaps the preceding strip 9j by a width 1. A practical method of excavation for placing into position the next portion of casing and the next sealing strip of film will now be described. In Fig. 2, the strip 9k lowered into the portion of casing 4k cast to the extreme edge of excavation 13 has just been placed in position. This edge may be bounded by a pile plank, for example, if this is advisable in view of the nature of the terrain, or by any other means of covering the edge of the drilled hole. The space of trench 8 extending the portion of casing 4k already in place is then hollowed out in one or more passes, leaving a narrow portion of terrain 14 between the separation 13 and this newly excavated portion of width E. The space E should be of such a width that, taking into account the overlap 1 desired on the strip 9k, a sufficient length L + AL as indicated in the drawing will be available to leave a certain space between the righthand edge 15 of the strip 91 and the end wall 16 of the newly excavated portion of width L. Thereafter, the portion 14 of terrain separating the slice which has just been excavated from the slice of casing 4k ending behind the separation 13 is dug away and the pile plank, if used at 13 as indicated above, is removed. The fresh slice of casing is then poured into the volume of trench just excavated, in cases where this is indicated, the procedure consisting, as before, of progressive replacement of the drilling fluid by the required mixture. Immediately after or before this procedure, the impervious film 91 is placed in position, overlapping by the desired amount 1 the film 9k which has previously been placed in position in the slice 4k which has not yet set solid. This procedure is then continued step-bystep until the round of the site to be protected has been completed. The procedure may, of course, be carried out right round the site by simultaneously operating in the two opposite directions. If this method is not adopted, it is advantageous to reserve a width 1 of impervious film at the level of the first slice of casing 4k to ensure an impervious junction with the last slice of casing with which it will join when the work has been completed round the whole site. A drilling guide, for example a pile plank or any other device for guiding the drilling over the whole height of the excavation behind the band L may be placed along this length L for protection. After drilling, the strip of film in the last slice will bear against the drilling guide or against the band up to the strip of the first slice so as to seal off the construction. It will be seen that the process according to the invention allows for several successive slices to be hollowed out without any interference from the strip of film in the preceding slice already placed in position in the trench. Furthermore, the width of overlap of the strips is sufficient to prevent any leakage in the event of the strips diverging from the vertical when in position. Positioning of the strips may in certain cases, particularly if the trenches are very deep, require the use of an additional, heavy weight 40 to be lowered by a rope 41 wound over a hoist 42 when positioning the film 9. This additional weight may be held, for example, on suspension hooks 43 fixed to the weight 12 which is attached to the lower edge of the film 9 (Figs. 1 and 3). The weight 40 is raised when the film has been placed in position. It will be noted that by placing the film 9 into the hardenable fluid which is already in position, the film is effectively protected from the edges of the trench and will not tear. The imperviousness opposite the overlap of the strips may be improved by filling a tube having a diameter at least equal to the thickness of the trench (e), this tube being either a separate piece welded to the film over the whole height of the trench or formed by folding the edge of the film back over itself and sealing it down to the main width of film, as will be understood more clearly hereinafter. This tube, which is closed at the bottom, will subsequently be filled before the product used for filling the trench solidifies, so that two successive strips of film will be pressed together, an excess pressure in this tube being sufficient to produce this effect. As impervious film one would chose a plastics film or a film of any other product which is impervious to liquids and sufficiently flexible. Referring now to Fig. 5, it will be seen that inside a trench 8 formed in the terrain 1, into which has been poured the fluid mixture forming the semi-flexible casing 4 properly speaking, various strips 19i, 19j, 19k of an impervious film have been placed in position as described above, two adjacent strips overlapping over a width 1. As described previously, adjacent strips are most efficiently pressed together in their zone of overlap by providing, close to the zone of overlap, a vertical tube 20i, 20j, 20k which is inflated after the strips have been placed into the trench. The tube 20 preferably forms part of the film 19 and is attached thereto along one of its edges (at the right in Fig. 5), the tubes being formed either by folding back a loop of film over itself along this edge or by welding or otherwise attaching a tube to the film. When the strip is placed into position, the tube is flattened, being not yet inflated, as indicated in broken lines at 20'j, 20'k, the tube being inflated only after the two strips which partially overlap opposite the corresponding tube have been placed in position. The circular diameter of the tube is advantageously greater than the width D of the trench so that when the tube is inflated it will firmly press the two partially overlapping strips of film against each other and against the adjacent wall 8i of the trench even if the trench is slightly wider than D. As mentioned above, the tubes are for this purpose closed at their base before the corresponding strip of film is lowered. They are advantageously inflated by the introduction into the tubes of a hardenable fluid having a density close to that of the semi-flexible casing 4. The tube reaches sufficiently high above the level of liquid so that when it is filled an excess pressure is obtained over the whole height of the tube immersed in the liquid. The object of this operation is to produce a force pressing the sheets of film sufficiently against the wall to cause displacement of the sheets and produce the desired imperviousness. At the same time, the strips of film are advantageously designed to extend above the top of the trench by a sufficient length to enable them to be folded over the top of the trench, thereby protecting the mixture constituting the semi-flexible casing against a drying out which would be liable to damage its subsequent properties. In Fig. 5, the elements of impervious film 19, 20 provided to perfect the imperviousness of the casing consist of flat bands of film having an inflatable tube along one edge. Numerous variations are conceivable for forming equivalent elements. Thus, Fig. 6 shows a combination of adjacent strips 21 i, 21 j, 21 k placed edge to edge without overlapping but covered by strips 22i, 22j, 22k which overlap them and are provided at each of their ends with an inflatable tube 23, 24. A third, intermediate tube 25 may also be provided if desired. Although such a sealing structure may take slightly longer to put into use inside the trench 8, it is just as easy to place into position inside the trench as the structure illustrated in Fig. 5 or that of Fig. 1. It will be noted that in this case, the overlapping structure of the films is improved, that is to say, the preferential path of escape for infiltration between two overlapping strips is longer than in the structure illustrated in Figs. 2 and 5. Such a structure may be further improved by also making the strips 21 overlap, for example as illustrated in Fig. 2. Fig. 6 also shows schematically at 26 a backing for the film 21 which is pressed against the wall 8i of the terrain. Such a backing, which may be made of any suitable material, serves to prevent perforation or puncturing of the sealing film by elements of the wall of the trench, such as pieces of flint or other hard, sharp objects. Such a backing may generally be provided at least on that side of the film which will bear against the wall of the trench. The inflatable tube could also be protected in the same manner but this backing may in most cases be omitted since the film remains protected in the relatively impenetrable mixture which offers great protection in the coarse of hardening. According to one preferred variation illustrated in Fig. 7, the elements of film 27, the structure of which is shown more clearly in Fig. 8, have the general form of a flattened X in cross-section X.As will be seen from Fig. 8, each element advantageously has two or four sheets 28, 29, 30, 31, joined together, for example by glueing or welding 32, to form the axis of intersection of the X. One of the sheets 28 carries an inflatable tube 33 at its edge remote from the centre of X, this tube being closed at the base as indicated at 34. To ensure easy descent of these elements, the sheets are weighted down at the bottom by ballasts as indicated at 35, 36, 37, 38. Additional ballasts such as those shown in Figs. 1 and 3 may also be used if necessary. The elements are placed in position in the trench before the tubes are inflated and they are interleaved so that the strips partially overlap by a length 1. After positioning of the overlapping strips, the tubes may be inflated to ensure firm application of the overlapping strips.It may be seen from Fig. 7 that the resulting path of escape of leaking fluid has a length of at least 3 1 and that at every point of the trench at least two impervious films must be traversed to pass from one side of the trench to the other. Such a structure is particularly easy to place into position and has proved to be highly effective. It is, of course, possible without departing from the scope of the invention to conceive many other structures of an equivalent type providing for overlapping of successive strips of film over a sufficient length to ensure complete imperviousness and firm pressing of these successive strips against each other to reduce infiltration between them to a minimum. Thus, for example, the elements 27 illustrated in Figs. 7 and 8 may be formed by joining together several overlapping X's.

Claims (15)

1. Process for rendering impervious the flexible casings sunk into the ground for the construction of sites insulated from the adjacent terrain, in particular for the discharge of pollutant products or for barriers interred in the ground, characterised in that, just after the mixture constituting the flexible casing (4) has been cast into position in the trench (8) receiving it, at least one impervious film (9) is lowered into the said mixture substantially to the bottom of the trench, said film being weighted down for this purpose at its lower edge (12), and this procedure is continued stepwise with successive strips having a width corresponding to that of the sheet used, the said strips being arranged to overlap by a sufficient width 1 to ensure impermeability between two adjacent strips.

2. Process according to claim 1, characterised in that the trench is excavated by successive slices of a width (L) at least substantially equivalent to that of the aforesaid strips, and each freshly made strip is then successively filled with the mixture constituting the casing, and the corresponding strip of impervious sheet is then placed in position.

3. Process according to claim 1 or claim 2, characterised in that when placing into position each aforesaid strip which is lowered down a wall of the trench (8), the descent of this strip is assisted by means of a recoverable weight (40) which is lowered at the same time as the impervious film (9) and which is separably hooked to the lower edge of the said film.

4. Process according to any of claims 1 to 3, characterised in that the various successive strips (19, 22, 27) having at least one inflatable tube (20, 23, 34) forming a deformable envelope are placed in position while the said tubes are not inflated, and the tubes are then inflated after the corresponding strips and the one or more than strip at least partly overlapping opposite the said tube have been placed in position.

5. Process according to claim 4, characterised in that the diameter of said inflatable tubes (20, 23, 34) is chosen to be greater than the width D of the trench (8).

6. Process according to one of the preceding claims, characterised in that the surface of the strip which may come into contact with a wall (8i) of the trench (8) is protected by a backing (26).

7. Process according to one of the preceding claims, characterised in that the said tubes (20, 23, 34) are inflated by the injection of a material similar in density to that which constitutes the impervious wall (4) cast into the trench (8).

8. Process according to one of the preceding claims, characterised in that the impervious, flexible film or films (19, 22, 27) is or are folded over the top of the trench (8).

9. Process according to one of the preceding claims, characterised in that more than two strips (30, 29, 31, 28) are superimposed at the level where the strips overlap.

10. Elements of impervious film for carrying out the process according to one of the preceding claims, characterised in that they comprise at least one inflatable tube (20, 23, 34) on one of their lateral edges.

1 1 . Elements of impervious film according to claim 10, characterised in that the said inflatable tube (20, 23, 34) has a diameter greater than the width D of the trench (8).

12. Elements of impervious film for carrying out the process according to one of the claims 1 to 9, characterised in that the said element (27) has the general form of a flattened X in cross-section.

13. Sitings of terrains insulated from the external surroundings, constructed according to the process of any one of claims 1 to 9, characterised in that they comprise an insulating side wall (4) lowered into the terrain on the perimeter thereof, said wall being formed essentially from a mixture which stiffens or sets and in which is embedded a layer of thin film which is impervious to liquids (9), the said layer consisting of substantially vertical strips overlapping each other along a substantially vertical band.

14. Sitings according to claim 12, characterised in that the said strips (19, 22, 27) are pressed against each other where they overlap by means of inflatable tubes (20, 23, 34) bearing substantially against both walls of the trench.

1 5. Sitings according to claim 1 2 or claim 13, characterised in that the said strips have the general form of a flattened X in crosssection and successive X's partly overlap one another by a length 1 .