WO2012049269A1

WO2012049269A1 - Method and device for draining off water seeped in a soil underlying hydraulic structures

Info

Publication number: WO2012049269A1
Application number: PCT/EP2011/067930
Authority: WO
Inventors: Alberto Scuero
Original assignee: Gsi Geosyntec Investment B.V.
Priority date: 2010-10-14
Filing date: 2011-10-13
Publication date: 2012-04-19
Also published as: RO128934B1; RO128934A2; ITMI20101877A1; EG27031A; IT1402028B1; AR083440A1

Abstract

A method and a device for draining off water seeped in a soil underlying a hydraulic structure (10), such as a canal, basin, dams and the like. A protective and waterproofing covering (13, 14), is laid down on the bottom wall (11) and side walls (12) of the hydraulic structure (10), by providing the covering (13, 14) with one-way gravity drainage valves (17). The waterproof covering (14) along the side walls (12), is anchored by a concrete ballast (18A) within a longitudinal trench (15), and by one or more tiltable concrete slabs (18) overlapping the waterproof covering (14), in which the slab or slabs (18) downwards extend from the anchoring ballast (18A). The water seeped in the soil, having a level higher than that of the water in the hydraulic structure, flows off by gravity towards the one-way valves (17), along an interface between the waterproof geomembrane (14) and the soil, slightly raising the tiltable concrete slab or slabs (18) by pressure of the same water seeped into the soil, having a level greater than the level of the water into the hydraulic structure (10).

Description

METHOD AND DEVICE FOR DRAINING OFF WATER SEEPED IN A SOIL UNDERLYING HYDRAULIC STRUCTURES

BACKGROUND OF THE INVENTION

The invention relates to a method and a device for draining off by gravity water seeped into a soil beneath hydraulic structures, such as canals, basins, dams and the like.

Devices for waterproofing hydraulic structures comprising a protective covering consisting of a geomembrane in a synthetic material, which is fastened to a surface of the hydraulic structure to be in contact with the water, are well known, e.g. from US-A-4.913.513 and US-A-5.720.576.

In the above-mentioned applications, a requirement exists to drain off the water seeped into the soil beneath side and bottom walls of the hydraulic structure, in order to avoid the pressure of the seeped water to cause the soil to sink, or the hydraulic structure to be damaged.

In order to partially overcome this drawback, US-A-4.913.513 suggests to embed a waterproofing membrane into the hydraulic structure, providing a number of microperforated pipes to drain off the water seeped into the soil. On the other hand, US-A-5.720.576 suggests to anchor a waterproofing covering to those surfaces of a hydraulic structure which are facing and intended to come in contact with the water within a basin, by a number of metal sections suitably configured to allow the water seeped into the soil, to be drained off to the outside.

DE-U-20 2008 014 492, and corresponding WO-A-2010/028959, which are regarded as being the prior art closest to the present invention, discloses a method and device suitable for waterproofing canals and the like, according to the preamble of claim 1 ; nevertheless said document proposes the use of a waterproofing method and device having different features, to solve a different problem in respect to present invention.

More properly, DE-U-20 2008 014 492, discloses the waterproofing of canals by web materials, fastening said web materials into a trench at an upper edge of the side walls, by any conventional fastening means. To protect the web materials in extreme climatic conditions, a layer of concrete is sprayed on a stabilization blanket and a wire-mesh fence; pumping stations in correspondence of drawdown wells, are also provided to lower the water level in the soil.

WO-A-2007/059924 also proposes to provide the waterproof covering sheets of a hydraulic structure, with a number of one-way drainage valves, and to use conventional anchoring means for fastening the waterproof sheets as well as the drainage valves.

While these solutions have provided satisfactory results for certain applications, the construction of a waterproofing and drainage device is not always possible in existing hydraulic structures, or it is very difficult and expensive.

OBJECTS OF THE INVENTION

The problem to be solved by the present invention consists in providing a different method and device for the waterproofing and drainage of water seeped into a soil beneath a hydraulic structure, such as a canal, water basin, dams and the like, as an alternative to the known systems of the prior art, to avoid damage to the hydraulic structure caused by the same seeped water.

An object of the present invention is therefore to provide a method and device for draining water seeped into the soil, beneath side and bottom walls of a hydraulic structure to solve the problem referred to which are of simple construction, less expensive than conventional systems, and take advantage of the gravity and pressure of the same water seeped into the soil, in order to allow the seeped water to flow out from one or more one-way drainage valves, while maintaining the valves in proper operative conditions, such that any sedimentary material can be dragged and removed by the same drainage water flow.

BRIEF DESCRIPTION OF THE INVENTION

The above has been made possible by a method according to claim 1 , as well as by a device according to claim 2.

According to the invention, a method has been provided for draining off water seeped into a soil beneath side walls and a bottom wall of a hydraulic structure, according to which a protective covering comprising a plurality of side by side arranged waterproofing sheets, having overlapped and sealingly connected side edges, are laid down over the side and bottom walls;

in which the waterproofing sheets of the side walls are fastened into a longitudinal trench which extends parallel to an upper edge of the side walls; and

in which a concrete layer is provided over said waterproofing sheets of the side walls;

characterised by the steps of:

laying down a first set of waterproofing sheets over the bottom wall of the hydraulic structure;

laying down a second set of waterproofing sheets, over the side walls of the hydraulic structure, from said longitudinal trench;

providing normally-closed one-way drainage va lves by partial ly overlapping the waterproofing sheets of the side walls, to the waterproofing sheets of the bottom wall; anchoring the waterproofing sheets of the side walls by overlapping a ballast in the longitudinal trench, and at least one concrete slab tiltably connected to, and extending from the ballast along the side walls; and

discharging the seeped water from the soil into the hydraulic structure, by allowing the seeped water to flow off along an interface between the waterproofing sheets of the side walls, and the same soil, by tiltably slightly raising the concrete slab and opening the one-way valves by back pressure of the water seeped into the soil, having a level greater than that of the water into the hydraulic structure.

According to another aspect of the invention, a drainage device has been provided suitable for draining off water seeped into a soil beneath a hydraulic structure having bottom and side walls, according to the method of claim 1 , the device comprising:

a plurality of side by side arranged waterproofing sheets laid dawn over the bottom and side walls, said waterproofing sheets having overlapped and sealingly connected side edges;

in which the waterproofing sheets of the side walls are fastened into a longitudinal trench parallely extending to an upper edge of the side walls; and

in which a concrete layer is provided over the waterproofing sheets of the side walls;

characterised in that said plurality of waterproofing sheets comprises: a first set of waterproofing sheets laid down on the bottom wall;

a second set of waterproofing sheets laid down and downwardly extending on the side walls from the longitudinal branch;

means for retaining the second set of waterproofing sheets into the trench and onto the side walls, said retaining means comprising a concrete ballast in such trench and at least one concrete slab tiltably connected to the ballast into the trench;

said concrete slab extending from the concrete ballast, over the waterproofing sheets along the side walls; said concrete slab being tiltably connected to the concrete ballast of the retaining means;

and normally closed one-way valves in the waterproofing sheets of the side walls and/or the bottom wall.

BRIEF DESCRIPTION OF THE DRAWINGS

These and further characteristics of the drainage method and device according to the invention, will be apparent from the description, with reference to the examples of the attached drawings, in which:

Fig. 1 is a top view of a canal length, comprising a waterproof covering and a drainage device according to a first embodiment of the invention;

Fig. 2 is a cross-sectional view taken along the line 2-2 in Fig. 1 ;

Fig. 3 is an enlarged cross-sectional view of a first detail, taken along the line 3-3 in Fig. 1 , showing the connection between a concrete ballast and a concrete slab;

Fig. 4 is an enlarged cross-sectional view of a second detail, taken along the line 4-4 in Fig. 1 , relating to a first embodiment of a one-way drainage valve for the side walls;

Fig. 5 is a sectional view similar to Fig. 4, relating to a second embodiment of a one-way drainage valve, in the closed condition, for a side wall;

Fig. 6 is a sectional view of the drainage valve in Fig. 5, in the opened condition;

Fig. 7 is a sectional view similar to that in Fig. 3, relating to a hinge connection between a concrete ballast and a concrete slab;

Fig. 8 is an enlarged sectional view of a second detail, taken along the line 8-8 in Fig. 1 , relating to a first embodiment of a one-way drainage valve for the bottom wall;

Fig. 9 is a top view of a canal length, sim ilar to Fig. 1 , comprising a waterproof covering and a drainage device according to a further embodiment of the invention; Fig. 10 is an enlarged cross-sectional view similar to Fig. 8, relating to a second embodiment of a one-way drainage valve for the bottom wall of the canal in Fig. 1 ;

Fig. 1 1 shows another type of drainage valve for the bottom wall of the canal in Fig. 1 and 9;

Fig. 12 is a cross-sectional view taken along the line 12-12 in Fig. 1 1 .

DETAILED DESCRIPTION OF THE INVENTION Figures 1 and 2 show, by way of example, a hydraulic structure provided with a waterproofing protective covering and a one-way valve drainage device for the water seeped into the underlying soil, according to the present invention, wherein the hydraulic structure consists of a water canal 10 having an indetermined length.

As can be seen, the canal 10 in the lower side 10A of figure 1 , is provided with a waterproofing protective covering and concrete ballast, both on the side walls and bottom wall thereof, as will be explained below, while the concrete ballast in the upper side 10B of figure 1 has been removed.

The canal 10 comprises a bottom wall 1 1 and side walls 12, only one of the latter being shown in Figure 2, which extend in a longitudinal direction of the canal or hydraulic structure 10. According to Fig. 1 , the canal 10 is provided with a waterproofing protective covering, consisting in a geomembrane comprising a plurality of waterproofing sheets made from an elastically yieldable synthetic material, for example thermoplastic material, thermoplastic rubber, thermosetting material, bituminous material, or other suitable material, according to the table below. TYPE BASE MATERIAL ABBREVIATION

- High-density polyethylene

HDPE

- Low-density polyethylene

LLDPE

- Chlorinated polyethylene

CPE

- Ethylene-Vinyl acetate co¬

THERMOPLASTIC E/VAC

polymer

S MATERIAL PE

- Polyethylene

PP

- Polypropylene

PVC

- Polyvinyl chloride

THERMOPLASTIC - Chlorine-sulfonate polyethylene CSPE

RUBBERS - Ethylene-propylene co-polymer EP

- Poly isobutylene PIB

- Chloroprene Rubber CR

THERMALLY-

Ethylene-propylene-diene EPDM

STABLE

monomer HR

POLYMERS

- Butyl rubber BR

- Nitrile rubber

BITUMINOUS - Oxidised bitumen Prefabricated GM

PRODUCTS - Polymeric bitumen

The geomembrane consists of a number of waterproof sheets of desired length, having a thickness ranging between 0.2 and 50 mm, with an elastic module ranging between 10 and 5,000 MPas.

With further reference to Fig. 1 , the geomembrane providing the protective covering for the canal 10, comprises a first set of side by side arranged waterproofing sheets 13 which are longitudinally laid down on the bottom wall 1 1 , the side edges thereof being partially overlapped and sealingly connected to each other by welding or adhesive. In Fig. 1 , the covering further comprises a second set of side by side arranged waterproofing sheets 14 transversally extending over the side walls 12, wherein the side edges of the sheets 14 have been again overlapped and sealingly connected to each other similarly to the longitudinal sheets 13 of the bottom wall 1 1 .

As shown in Figs. 1 to 3 and in the detailed view in Fig. 4, each transversal waterproofing sheet 14 extends from the upper to the lower edge of the side walls 12, starting from the bottom of a trench 15, along an upper planar length 16, as well as along the slope of a side wall 12, ending with a lower edge 14A freely overlapping to a side edge of a bottom sheet 13, for example along a length of 10 to 50 cm. A one-way drainage valve 17 is thereby provided between the partially overlapped sheets 13 and 14, which one-way valve 17 longitudinally extends along a length of a few meters, tens of meters, or more. As will be explained further on, the one-way valve 17 allows the water, which has seeped into the soil beneath same hydraulic structure 10, and the side walls thereof, for example through the valves 17 when a negative differential pressure exists between the upper and the lower sides at the edges 14A of the waterproof sheets 14, or from the environment.

The sheets 14 defining the waterproofing protective covering for the side walls, are fastened into the trench 15 and hold along the sloping side walls 12 of canal 10, by a ballast 18A, for example made of a concrete casting poured into the trench 15 parallel to the upper edge of the side walls 12, and by at least one flat concrete slab 18 which extends downwards from the ballast 18A, as shown, ending with a lower edge 18B at short distance from the drainage one-way valve 17; in the example shown in Fig. 4, the valve 17 results to be outside and far from the concrete slab 18.

As shown in figures 2 and 3, the ballast 18A and the slab 18 are cast in concrete as a integral unit, longitudinally extending for a required length; depending on the shape and dimension of the hydraulic structure 10, one or more side by side arranged ballast and slab units may be provided to retain the waterproofing sheets. The connection between the ballast 18A and the slab 18 as well as the thickness of the same slab 18 should be conformed to provide a joint or a connection configured to al low the slab 1 8 to be elastically deflected and tiltably raised under the thrust generated by the pressure of the water seeped into the soil. In order to avoid that the waterproofing sheets 14 of the side walls may be damaged, between the concrete slab 18 and the waterproofing sheets 14 a layer of anti-puncturing material 19 has been disposed, such as a textile material suitable for geology applications normally defined as "geotextile", which is capable of withstanding to perforations caused by the weight or slight displacements of the concrete slab.

Lastly, in Figs. 1 and 2, with reference number 20, continuous concrete bands have been designated, which are overlapped to the longitudinal waterproofing sheets 1 3 of the bottom wall 1 1 , in order to allow service vehicles to move and carry out maintenance operations.

In the example in Fig. 4, the one-way drainage valve 17 consists of overlapped edges of the longitudinal sheets 13 and transversal sheets 14; to this purpose, as shown in Fig. 4, the edge of the sheet 1 3 has been anchored in order to be kept adhered to the soil, by means of any mechanical fastening system, schematically shown by reference number 21 , while the edge of the sheet 14, overlapped to the above-mentioned one, is free both to raise to open the valve 17, and allow the water that has seeped into the soil to flow off, and to lower to close the valve 17 preventing the water within the canal, or hydraulic structure, to flow out through the same valve 17 in the case of a positive differential pressure existing between the water in the canal 10, or the hydraulic structure, and the water into the soil. This may be further explained with reference to Fig. 2, where a first high level of the water within the canal 10 has been indicated by L1 , whereas a possible level of the water that has seeped into the soil, lower than L1 , has been indicated by L2. In these conditions, it is understood that the one-way membrane valve 17 will remain closed due to the positive differential pressure resulting from the fact that the water within the canal 10 is higher than the pressure of the water that has seeped into the soil. In contrast, when the water level within the canal decreased at a level L2 lower than the water in the soil, as indicated by L3, between the two faces of the edge 14A of the sheet, or each sheet 14, a negative pressure differential is generated, due to the fact that the level and the pressure of the water in the soil is higher than the level and the pressure of the water in the canal 10, which will cause the valve 17 to open and the water to flow out from the soil, into the canal or hydraulic structure 10.

The water can flow off from the soil by gravity along an interface existing between the same soil and the waterproof sheets 14 and through the one way valve or valves 17 in the opened condition, due to a slight bending or raising of the concrete slab 18 and the edge 14A of the waterproofing sheets 14, caused by the strong thrust exerted by the pressure of the water into the soil, as a result of the long extension of the surfaces in contact to each other at said interface.

In the example in Fig. 3 and 4, the provision of a one-way valve 17 outside and far from the concrete slab 18 has been illustrated; Figs. 5 to 7 illustrate, on the other hand, an example in which the one-way valve 17 is positioned under the lower edge 18B of the concrete slab 18 along the side walls of the canal or hydraulic structure 10; accordingly, in Figs. 5 to 7 the same reference numbers, as in the preceding figures, has been used to designate similar or equivalent parts. The difference between the solution of Figs. 3 and 4 and the solution of Figs.

5 and 6 not only resides in the different positioning of the one-way valve 17, but also in the provision of a hinge joint 22 between the slab 18 at the upper edge of the side wall 12, and an extension of the ballast 18A which partially protrudes from a side of the trench 15, as shown in Fig. 7. The hinge connection 22 acts such that the concrete slab 18 along the wall 12, under the thrust of the pressure of the water that has drenched the soil, can be tilted up and slightly raised by a few millimetres, or less, providing a gap in order to cause the water seeped into the soil to flow off along the existing interface, and out from the one-way valve 17, into the canal or hydraulic structure 10. To this purpose, as shown in the enlarged detail in Fig. 7, the hinge 22 can comprise a pad 23 in rubber or other elastically-compressible synthetic material, which extends throughout the slab 18 between facing edge surfaces of the ballast 18A and the slab 18; U-shaped reinforcement and connection iron rods 24, 25 pivotally engaged to each other, are embedded in the concrete of the ballast 18A and slab 18.

Fig. 5 schematically shows the closed condition of the valve 17, whereas Fig. 6 shows the same valve 17 in an opened condition. Fig. 8 shows a one-way valve 17 that is substantially similar to that in Fig. 5 and 6, between two longitudinal waterproofing sheets 13.1 and 13.2 covering the bottom wall 1 1 of the canal 10, in a position beneath a concrete band 20 of Fig. 1 ; accordingly, in Fig. 8 some of the reference numbers used in the preceding figures, has been also used to designate similar or equivalent parts.

Fig. 9 shows a further embodiment of the waterproofing protective covering and of the one-way valves 17 for the same canal 10 shown in Fig. 1 ; in the example in Fig. 9, the waterproofing sheets 13 covering the bottom wall of the canal 10, have been laid down transversally instead of being laid down longitudinally, similarly to the waterproof sheets 14 for the side walls of the canal 10. As for all the rest, particularly as to the one-way drainage valves 17, the two solutions are equivalent; accordingly, the same reference numbers as in Fig. 1 , have been used in Fig. 9 to designate similar or equivalent parts.

Lastly, in figure 9, the reference number 26 relates to an additional one-way valve for the waterproofing sheets of the bottom wall of the canal, for example of the type shown in figure 1 1 .

Fig. 10 shows the provision of an additional drainage valve 17 at a side of the concrete band 20, in a similar manner as the valve 17 in Fig. 4; the solution in Fig. 10 differs from the solution in Fig. 4 in that the upper free edge of the sheet 1 3. 1 has been welded to the lower sheet 1 3.2 by a breakable light welding 31 . By "light welding" is meant that the welding is not capable of withstanding the force caused by pressure of the water in the soil, whereby the welding may be broken when the pressure of the water in the soil exceeds a preset value, depending on the variable breakage resistance level of the welding 31 .

Optionally, at the bottom wall of the canal or hydraulic structure 10, in replacement of, or in combination with the valves in Figs. 8 and/or 1 0, additional one-way drainage pocket-type valves 26 can be provided, one of which is shown in Figs. 1 1 and 12.

As shown in Figs. 9 and 1 1 , in preset positions of the bottom wall, in several waterproofing sheets 1 3, one or more sl its or elongated cuts 27 are provided, and one lip of the cut is fixed to the soil underneath, or to a bottom wall of the canal, by a mechanical fastening stud 28, leaving the other lip free to move up and down under the pressure of the water in the soil.

When the cut 27 has been made, after one lip thereof has been fixed at 28, a waterproofing covering membrane 29 is overlapped thereto, for example having a rectangular shape and a size greater than that of the cut 27; the membrane 29 is than sealingly fastened along part of the peripheral edge, for example along three sides, by means of a strong welding 30. By "strong welding" is meant a welding that is capable of withstanding the pressure exerted by the water in the underlying soil; the remaining part of the peripheral edge, that is the fourth side parallel to the cut 27 in the example in Fig. 1 1 and 12, is welded to the waterproof sheet 13.2 by means of a weak welding 31 , as defined above for Fig. 10.

From what has been said and shown throughout the figures, it should be understood that a method and a device for draining off, by gravity, water seeped into the soil beneath a waterproofing covering of any hydraulic structure, has been provided, wherein use of normally closed one-way drainage valves has been suggested, and wherein the waterproofing sheets of a protective covering are simply hold by a concrete ballast in a trench, and by concrete slabs which extend downwards from the trench to retain the waterproofing sheets on the side walls, and in which the concrete slabs are tiltably connected to the ballast to allow the rising of the same slab and the waterproofing sheets by water pressure to allow the discharge of the water seeped into the soil by opening said one-way valves.

It should be understood, however, that what has been said and shown has been given only by way of example of the general characteristics of the invention, of the one-way drainage valves, and of the laying down of the waterproof sheets 13 and 14; in fact, alternatively to the transversal laying down of the sheets 14, the latter may be laid down in a longitudinal direction parallel to the side walls 12. Accordingly, other modifications or variations may be carried out to the method and device without however departing from the claims below.

Claims

1 . A method for draining off water seeped into a soil beneath side walls (12) and a bottom wall (1 1 ) of a hydraulic structure (10), according to which a protective covering com prising a plurality of side by side arranged waterproofing sheets (13, 14), having overlapped and sealingly connected side edges, are laid down over the side and bottom walls (1 1 , 13); and

in which the waterproofing sheets (14) of the side walls (12) are fastened into a longitudinal trench (15) which extends parallel to an upper edge of the side walls (12); and

in which a concrete layer is provided over said waterproofing sheets (14) of the side walls (12);

characterised by the steps of:

laying down a first set of waterproofing sheets (13) over the bottom wall (1 1 ) of the hydraulic structure (10);

laying down a second set of waterproofing sheets (14), over the side walls (12) of the hydraulic structure, from said longitudinal trench (15);

providing normally-closed one-way drainage valves (17) by partially overlapping the waterproofing sheets (14) of the side walls (12), to the waterproofing sheets (13) of the bottom wall (1 1 );

anchoring the waterproofing sheets (14) of the side walls (12) by overlapping a ballast (18A) in the longitudinal trench (15), and at least one concrete slab (18) tiltably connected to, and extending from the ballast (18A) along the side walls (12);

discharging the seeped water from the soil into the hydraulic structure

(10) by allowing the seeped water to flow off along an interface between the waterproofing sheets (14) of the side walls (12) and the same soil, by tiltably slightly raising the concrete slab (18) and opening the one-way valves (17) by a back pressure of the water seeped into the soil, having a level greater than that of the water in the hydraulic structure (10).

2. A drainage device suitable for draining off water seeped into a soil beneath a hydraulic structure (10) having bottom and side walls (1 1 , 12), according to the method of claim 1 , the device comprising:

a plurality of side by side arranged waterproofing sheets (13, 14) laid dawn over the bottom and side walls (1 1 , 12), said waterproofing sheets (13, 14) having overlapped and sealingly connected side edges;

in which the waterproofing sheets (14) of the side walls (12) are fastened into a longitudinal trench (15) which extends parallely to an upper edge of the side walls (12); and

in which a concrete layer is provided over the waterproofing sheets (14) of the side walls (12);

characterised in that said plurality of waterproofing sheets (13, 14) comprises:

a first set of waterproofing sheets (13) laid down on the bottom wall (1 1 ); a second set of waterproofing sheets (14) laid down and downwardly extending on the side walls (12) from the longitudinal branch (15);

means for retaining the second set of waterproofing sheets (14) into the trench (15) and onto the side walls (14), said retaining means comprising a concrete ballast (18A) in the trench (15) and at least one concrete slab (18), said concrete slab (18) extending from the concrete ballast (15) over the waterproofing sheets (14) along the side wall (12),

said concrete slab (18) being tiltably connected to the concrete ballast (15) of the retaining means;

and normally closed one-way valves (17) in the waterproofing sheets (14) of the side walls (12) and/or bottom wall (1 1 ).

3. The drainage device soil according to claim 2, characterised in that a hinge connection (22) is provided between the concrete ballast (18A) and the concrete slab (18).

4. The drainage device according to claim 3, characterised in that the hinge connection (22) comprises an elastically yielding pad member (23).

5. The drainage device according to claim 2, characterised in that said second set of waterproofing sheets (14) are laid down transversally to the side walls (12) of the hydraulic structure (10).

6. The drainage device according to claim 2, characterised in that said first set of waterproofing sheets (13) are laid down on the bottom wall (1 1 ) in a parallel direction to the side walls (12).

7. The drainage device according to claim 2, characterised in that said first set of waterproofing sheets (13) are laid down on the bottom wall (1 1 ) in a transversal direction to the side walls (12).

8. The drainage device according to claim 2, characterised in that a main drainage valve (17) is provided in spaced apart position to an edge of the concrete slab (18).

9. The drainage device according to claim 2, characterised in that a main drainage valve (17) is provided beneath an edge of the concrete slab (18).

10. The drainage device according to claim 2, characterised by comprising additional one-way drainage valves (23) in the waterproof sheets (13) of the bottom wall (1 1 ).

1 1 . The drainage device according to claim 10, characterised in that the additional drainage valves (23) are extending longitudinally to at least one side edge of a concrete band (20) overlapping the waterproofing sheets (13) of the bottom wall (1 1 ).

12. The drainage device according to claim 1 1 , characterised in that said additional drainage valves (23) comprise overlapping edges of waterproof sheets (13) of the bottom wall (1 1 ), sealingly fastened by a light welding (22) to be broken at a preset value pressure of the water seeped into the soil.

13. The drainage device according to claim 10, characterised in that said additional drainage valves (23) comprise pocket-like valves (26), each consisting of a slit (27) in a waterproof sheet (13) of the bottom wall (1 1 ), a waterproof membrane (29) being laid down on the slit (27); a portion of a peripheral edge of the waterproofing membrane (29) being sealingly fastened to the underlying waterproof sheet (13) by a strong welding (30) to withstand the pressure of the water seeped into the soil; the remaining portion of the peripheral edge of the waterproofing membrane (29) being sealingly fastened to the underlying waterproofing sheet (13) by a light welding (31 ) to yield under a preset pressure value of the water seeped into the soil.