EP0041516B1

EP0041516B1 - Liquid retaining structures

Info

Publication number: EP0041516B1
Application number: EP19800902360
Authority: EP
Inventors: Michael James Durack
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-12-03
Filing date: 1980-12-03
Publication date: 1984-09-19
Also published as: EP0041516A4; WO1981001585A1; EP0041516A1; DE3069242D1

Abstract

A method and apparatus for construction of swim pools comprised of a plurality of wall panels (23) aligned at their bottom edges by brackets (13) and at their top edges by preformed coping members. The coping members have apertures to locate and retain a tensioning cable used to withstand outwardly directed forces on the wall members. The pool has a concrete base (30) and a vinyl membrane lining.

Description

The present invention relates to liquid retaining structures and in particular, but not exclusively, to a prefabricated or semi-prefabricated swimming pool.
There are basically two general forms of prefabricated swimming pool available, the rigid fibreglass pool, which is supported in an excavation, and the liner-type pool which commonly takes the form of an above-ground pool having upstanding peripheral walls lined with a waterproof liner and supported by a frame. Such prefabricated pools are normally manufactured to a particular shape and this shape cannot be easily varied. In the case of prefabricated below- ground pools such as fibreglass pools, if variations are required, then different moulds have to be provided for each variation. As the production of such moulds is expensive it is difficult for a manufacturer to provide a range of different shaped pools. Accordingly if a person requires a non-standard pool, then usually it would be custom made or alternatively the person may select a concrete pool which may be shaped to his requirements, but of course both of these alternatives are more costly in comparison with standard prefabricated pools.
Further, conventional liner-type pools require a substantial amount of buttressing to support the walls from collapsing inwardly due to ground pressure in the absence of water in the pool or alternatively to support against expansion outwardly when the pool is filled with water and particularly when the pool walls are fully or partly above ground level. Also as it is often desirable to keep the number of buttresses to a minimum in view of constructional costs; the pool walls between buttresses are long in respect to their height and accordingly require strengthening and/or reinforcing to withstand the working pressures along their length. Thus the need for strong walls tends to preclude the use of lightweight and less costly materials in the construction of the pool.
United States Patent No. 2,902,157 discloses a thin walled reinforced concrete prefabricated swimming pool shell. Unitary or longitudinally split shell constructions are described in which hollow conduits are cast into the wall structure perimetrally of the pool and laterally of the pool with the conduit passing transversly within the floor and vertically within the side walls. Tensile members are slidingly located within the wall and are post-stressed by a threaded nut on the ends of the tensile members to provide a substantial multidirectional dynamic compressive force within the structure to prevent cracking during transportation and use. The tensile members are tensioned by providing a substantial torque of 90 ft. lb. to the tensioning nuts. The main disadvantages of such a system are that a very large mould is required for each pool shape thus mitigating against freedom of choice for a pool customer and further the high costs of transportation and likely damage during transportation.
United States Patent No. 2,864,098 describes a structure comprising a plurality of interconnected wall panels forming a continuous arched construction The wall panels include outwardly facing flanges on their upper lower and side edges, the side flanges permitting adjacent wall panels to be bolted together. A membrane liner is located within the perimeter of the wall with its upper edge clamped between the top flanges of the wall panels and coping members resting thereon. The coping members are located atop the wall structure by a centrally located groove and are brought into compression by a tensile member located on the outside edge of the coping members to form a substantially continuous coping. It is the assembly of wall panels presenting the continuous arched construction which resists external forces and the solid packing of backfill which resists internal forces. The coping member, not being rigidly interlocked with the wall of the pool, does not contribute to the strength of the structure to any great extent. The purpose of the cable arranged about the outside edge of the coping members is only to hold the coping members together.
Australian Patent Application No. 84889/ 75 (corresponding to French Patent No. 2,288,201 ) describes a construction similar in principle to that described in United States Patent No. 2,864,098. The constructions both employ interlocking wall panels and the membrane liner is retained in position by placing the upper edge of the liner over the top of the wall panels and clamping it between the coping members and the wall panels. Similarly, a tensile member is arranged about the coping members to lock them together. The construction of Australian Patent Application No. 84889/75 differs slightly in that when the coping members are locked together, this serves to lock the wall panels together.
All of the above constructions suffer two major disadvantages. Firstly, all require a continuous arched structure which resists externally applied forces by placing the wall structure in compression. Thus any discontinuity in the continuous arched structure i.e. a long straight wall or an inwardly directed curve (concave) cannot be placed in compression to resist both internally and externally applied forces. As used in this specification the expressions "internally" and "externally" applied forces relate respectively to an outwardly directed force applied by a body of water in the pool and an inwardly directed force applied by the surrounding earth structure. Further the expressions "concave" and "convex" relating to curve direction of a pool perimeter, coping member or wall member, relate respectively to axes of curvature located outwardly and inwardly of the pool structure.
Secondly the other main disadvantage concerns the means of securing a membrane liner within the pool structure. In both Australian Patent Application No. 84889/75 and United States Patent No. 2,864, 098, the liner is placed in position within the pool and is at least partially filled with water before the coping members are placed on top of the outer wall. During this time backfill must be added carefully to support the force applied by the water in the pool but great care must be taken to prevent inward collapse of the wall. Further, once positioned and clamped in place by the coping members it is almost impossible to make slight adjustments to compensate for stretch or wrinkling. Removal of the liner for repair or replacement requires removal of the coping members. While the pool is empty and the coping members are not in place, it is necessary, with the structure of Australian Patent Application No. 84889/75 at least, to support the walls against inward collapse.
It is an aim of the present invention to provide a method and apparatus for the construction of a liquid retaining structure adaptable to a wide variety of shapes and sizes and which permits rapid and economical yet sturdy and durable construction while retaining an aesthetically pleasing appearance.
According to one embodiment of the present invention there is provided an apparatus for constructing a liquid retaining structure with at least one concave curve in an outer perimeter, said structure comprising:-

a plurality of wall panels adapted for location in edge to edge engagement;
lower retaining means for retaining the lower edges of said panels in desired edge to edge alignment;
a membrane liner releasably securable within said structure by liner retention means to an inner face of said wall panels adjacent the upper edges thereof; and,
a plurality of preformed coping members having channels to recieve the upper edges of said wall panels to retain said wall panels in desired edge to edge alignment;
said coping members being adapted to support a tensile member adjacent the upper edge of said wall panels such that in use when said tensile member undergoes tension as a result of inwardly or outwardly directed forces on said wall panels, said coping members act in compression as a substantially continuous beam to withstand flexure from said inwardly or outwardly directed forces on said wall panels.

Preferably the wall panels comprise fibre reinforced concrete and most preferably comprise FIBRESTEEL (Regd. Trade Mark) reinforced concrete.
Preferably the lower retaining means comprises a channel sectioned member, most preferably of galvanized iron.
Preferably the coping members are provided with a channel to engage the upper edges of said wall panels and most preferably, the channel is tapered inwardly to permit firm frictional engagement with the upper edges of said panels.
Preferably said wall members and said coping members are formed in a plurality of shapes including straight section, concave curves and convex curves to permit construction of liquid retaining structures selected from a wide variety of perimetral shapes.
Preferably one or more of said coping members includes a hollow aperture extending lengthwise thereof to support the tensile member.
Preferably the tensile member comprises a steel cable arranged about an outer wall of said coping members adjacent the upper edges of said wall panels and most preferably is retained in a desired position by brackets extending between adjacent edges of said coping members.
According to another aspect of the invention there is provided a liquid retaining structure comprising:-

a plurality of preformed wall panels arranged in edge to edge relationship to form on a base a wall member of predetermined shape with at least one concave curve in an outer perimeter;
said wall panels being maintained in a desired abutting edge to edge relationship at their respective lower edges by lower retaining means and at their respective upper edges by a plurality of preformed coping members having channels (3) to receive the upper edges of solid wall panels;
a membrane liner releasably secured within said structure by liner retention means attached to an inner face of said wall member adjacent the upper edge thereof; and,
a tensile member associated with said coping members adjacent the upper edges of said wall member such that in use said tensile member undergoes tension as a result of inwardly or outwardly directed forces on said wall member, said coping members act in compression as a substantially continuous beam to withstand flexure from said inwardly and outwardly directed forces on said wall member.

Preferably said liquid retaining structure is formed within a cavity in the ground and most preferably concrete is cast between the outer surface of the wall member and the cavity wall.
Preferably the liquid retaining structure includes a concrete floor cast within the wall member.
Preferably the floor is formed with a concave depression.
Preferably the liquid retaining structure includes a liquid impervious membrane releasably retained by retaining means adjacent the upper part of the wall member.
Preferably the structure includes means for evacuating the space between the membrane and the wall and floor members.
Preferably the structure includes means for removal of liquid accumulated between the membrane and the wall member.
In yet a further aspect of the invention, there is provided a method for the construction of a liquid retaining structure comprising:-

arranging a plurality of preformed wall panels to form a substantially continuous wall member of predetermined perimetral shape with at least one concave curve therein;
retaining the lower edges of said wall panels in desired edge to edge relationship with lower retaining means;
retaining the upper edges of said wall panels in desired edge to edge relationship with preformed coping members (1 a to 1α) having channels (3) to receive the upper edges of said wall panels;
releasably securing a membrane liner within said structure by liner retantion means attached to an inner face of said wall member adjacent the upper edge thereof;
retainably positioning a tensile member in association with said coping members adjacent the upper edge of said wall panels whereby in use said tensile member undergoes tension as a result of inwardly or outwardly directed forces on said wall member, said coping members act in compression as a substantially continuous beam to withstand flexure from said inwardly and outwardly directed forces on said wall member.

Preferably said structure is formed within an excavation within a ground surface.
Preferably concrete is cast around the external perimeter of said structure.
Preferably a concrete floor is cast within the perimeter of said structure.
Preferably said floor is formed with a concave depression.
Preferably the space between the membrane and the inner wall and floor of the structure is evacuated prior to filling with liquid.
One particular embodiment of the invention will now be described with reference to the construction of in-ground swimming pools.
In the accompanying drawings, FIG. 1 illustrates plan and vertical cross-sectional views of a series of coping members the shading patterns on the plan views being incorporated merely to distinguish the coping sections in FIGS. 12-19 as hereinafter described.

FIG. 2 illustrates plan and vertical cross-sectional views of wall panels;
FIG. 3 illustrates a lower retaining member;
FIG. 4 illustrates a cable support bracket;
FIG. 5 illustrates a membrane retaining member;
Fig. 6 illustrates the wall structure of a swimming pool under construction;
FIG. 7 illustrates a partially completed pool structure;
FIG. 8 illustrates the structure of a step out;
FIG. 9 illustrates a completed, tiled step out;
FIG. 10 illustrates an auxiliary membrane retaining member;
FIG. 11 illustrates the construction of a spa unit;
FIGS. 12-19 illustrate a number of pool shapes which may be constructed in accordance with the invention;
FIGS. 20 and 21 illustrate alternative methods of finishing the upper surface of the coping sections.

In FIG. 1 (enlarged), coping section la a is formed as a straight section from reinforced concrete. In cross section it is formed in essentially a "T" shape with face 1 formed essentially at a right angle to upper surface 2.
The channel 3 is formed with an outwardly sloping face 4 to permit close frictional engagement with a wall panel (not shown).
Coping section 1 is essentially identical to 1 a except that the rear or outwardly facing portion is formed as a solid section without the recess shown in 1 a. A hollow aperture 5 extends lengthwise of coping section 1 b to support a tensioning cable (not shown). The position of the hollow aperture 5 corresponds with the boundary of the two faces forming the recess in the rear of section 1 a.
Coping section 1 c is shaped to permit access to a conventional skimmer box and includes an access aperture and a cut-away portion defining the upper part of a skimmer box opening through a wall member.
Coping sections 1d, 1 e, 1 and 1g illustrate a "fast" curve, a "slow" curve, a "spa" curve and an inverted "step out" curve, the purpose of which will be described later.
FIG. 2 illustrates plan and vertical cross-sectional views of various wall members from which it may be seen that the wall members are available as a standard height or as fractional heights.
FIG. 2a illustrates an enlarged view of the upper edge of a wall panel showing an inwardly facing rebate 11 for attachment of a membrane liner retaining member (not shown). Adjacent the upper vertical edges of the wall panel are positioned holes 12- which permit the adjacent edges of wall panels to be temporarily held together with a wire tie.
FIGS. 2c-2g illustrate respectively a "fast" curve, a "slow" curve, an inverted "fast" curve, a "spa" curve and an inverted "step out" curve.
FIG. 2h illustrates a wall section with skimmer box fitting attached.
FIG. 3 illustrates a lower retaining member 13 for retaining the lower edges of the wall panels together in parallel face to face relationship.
FIG. 4 illustrates a tensioning cable retaining bracket 14 comprising an upright let 15 and a right angled flange 16 extending therefrom. A tensioning cable aperture 17 is provided in leg 15.
FIG. 5 illustrates a membrane retaining member comprising extruded sections 18 and 19 of plastics material. Section 18 is adapted to be affixed to a wall panel and a portion of membrane folded over upstanding leg 20 is releasably secured thereto by section 19.
The construction of a swimming pool according to the invention will now be described with reference to FIG. 6.
Having selected a suitable site, an excavation 21 of slightly larger dimensions than the required pool size is made in the ground. The floor of the pool is shaped as a smooth concave depression 22 within the perimeter of the excavation. The wall panels 23 are set vertically in the excavation, side by side, and are temporarily held together by wire twitches 24 through preformed holes 12 in the side edge portions of the panels. At each junction of succeeding panels, the two panels are supported on concrete blocks 25 and located in a galvanized iron base bracket 13 fitting loosely under the joint.
One of the wall panels is formed, near to its top, with a rectangular aperture in which is fixed a skimmer extension throat, to enable a skimmer box and return fittings to be installed.
With the wall panels installed and levelled as required with some pieces of, for example, fibro- cement between their bottoms and their supports, coping sections of corresponding curvature are set in place.
The coping sections are arranged to overlap the edge joins of the wall panels and the inwardly tapered groove therein ensures accurate face to face location of the wall panels. Prior to positioning the coping sections, the upper and outer portions of the wall panels are coated with adhesive grout. The adhesive mixture is used also in vertical joints between coping pieces, and in each joint there is cemented a cable bracket 14 which may be of galvanized sheet steel with a top flange 16 bearing on one coping piece, the lower part of the bracket having a hole 15 located at the junction of the coping piece top and the outside face of the downward projection. A cable is passed about the coping pieces and through the apertures of the cable brackets and is tensioned by the use of a turnbuckle or the like. Certain coping pieces, including any that may be made with a reverse curve, convex when viewed from within the pool, are made with holes 5 through them from end to end to receive the cable, the cable brackets being omitted.
FIG. 7 illustrates a partially completed pool in which a vacuum pipe is installed which may suitably include an elbow 24 with one end opening into the pool through a hole at or near the bottom of one wall panel, its other end connected to an upright pipe 25 to the outside of the panel, the top of this pipe being connected, through a further elbow 26, to one end of a discharge pipe 27 below the coping and inclining down towards its other end. The purpose of the vacuum pipe is to facilitate the positioning of the pool liner membrane and to allow for discharge of any water which may find its way between the liner and the pool interior. Water seeping inwardly via hydrostatic valve 28 from surrounding ground is forced up to the perimetral join between the pool wall and floor and is discharged by a siphon action via pipe 27.
The hydrostatic valve simply comprises a tube 28 extending from a drainage pit 29 beneath the pool floor.
A concrete bottom 30 is laid for the pool and is trowelled smooth, and concrete is also poured between the wall panels and the excavation.
A liner retaining section 18 as illustrated in FIG. 5 of any suitable material such as plastics is riveted to the upper inside parts of the wall panels, within the rebates thereof, the section consisting of a flat upright section which is riveted adjacent its lower edge flat against the rebated section 11 of wall panels the upper edge being retained in the tapered aperture 3 of the coping section.
A membrane liner, preferably vinyl, for the pool has its upper edge portion folded over a liner retaining strip 19 which is of inverted- channel form, the excess liner portion hanging down outside the main part of the liner wall. The retaining strip 19, with part of the liner wrapped about it, is hooked into engagement with the channel of the liner retaining extrusion, the overhang of the excess liner being adjusted to suit the pool and to ensure that the seam where the wall of the liner joins the liner bottom is correctly located. A particular advantage associated with this liner retention means is that not only does it firmly retain the liner edge in position by a self-locking action, but the retention means is readily releasable to allow adjustment of tension on the liner material e.g. to take up stretch or to reduce tension.
A vacuum pump connected to the pipe 27 is used to withdraw air from between the liner and the pool bottom and sides before the pool is filled with water.
A pool may be made with a step out section as illustrated in FIGS. 8 and 9 by using reduced height wall panels for one section of the lower part of the pool wall, and shallow wall panels including panels reversed for inverted curves, above the level of this lower wall section, the result in a shallow outward extension of the pool structure, this outward extension being provided with a concrete floor and a coping. The vinyl liner has a part cut away at the opening to the step out section, the edge of the opening formed in the liner being secured to the pool wall panels with the auxiliary retaining member 30 illustrated in FIG. 10. The function of the auxiliary retaining member is similar to that of the main liner retaining member of FIG. 5 in that it is riveted to the edge of the step out opening through the lower channel 31 and the edge of the liner is retained in the upper channel 32 by forcing insert 33 and the liner thereinto. Excess liner material is then trimmed away and tiles and any appropriate sealing means are provided along the bottom and sides and the entry of the step out section to the main part of the pool as illustrated in FIG. 9.
If required a spa may also be formed at the perimeter of the pool as shown in FIG. 11. A step out construction is formed as described above but an inner wall 34 is formed to isolate the spa from the pool. Naturally, the vinyl liner is not cut away at this region but continues around the inner wall of the pool. Instead of positioning a coping section on the upper part of wall 34, it may be tiled. A false floor 35 is provided with a plurality of apertures to permit air to enter the spa from a source of pressurized air 36. Inlet and outlet orifices 37 are provided for circulation of heated water.
FIGS. 12-19 illustrate some of the many pool shapes which may be constructed with the apparatus and according to the method of the invention. As illustrated, the optional features of step out and/or spa may be readily included in the construction.
A particularly advantageous feature of the present invention resides in the retaining means for the tensioning cable. Should the cable slip downwards during the tensioning step, the coping sections may become misaligned and thus become difficult if not impossible to realign before the grouting adhesive sets. The cable retaining brackets obviate the possibility of such a misalignment. A further advantageous feature is the provision of the apertured coping sections for use in those regions where inverse curvatures would otherwise preclude a perimetral tensioning cable. The cable, being held captive within the inversely curved coping section exerts an outwardly directed force under tension on the coping section which counteracts an inwardly directed force as the section undergoes compression from neighbouring coping sections.
It can be readily seen therefore that the features of the present invention may be adapted to a wide variety of pool shapes while at the same time retaining structural integrity.
The coping sections of the present invention are adapted to permit a wide variety of surface finishes.
FIGS. 20 and 21 illustrate two different types of finishes. In FIG. 20, the inwardly projecting lip 38 may be tiled and a conventional nosing tile 39 is situated adjacent the front upper edge of the coping section. An aggregate finish 40 may be applied to the remaining upper surface of the coping section and a rebate 41 in the rear edge permits the aggregate finish to be taken over the rear portion of the coping section. Not only does this feature give the illusion of a thicker aggregate finish but it prevents chipping of the finish at the rear edge.
FIG. 21 illustrates a conventional brick finish comprising bricks 42 set in motor 43.
Although the invention has been described with reference to a swimming pool, it will be readily apparent to a skilled addressee that similar structures, with appropriate modification will be suitable for other liquid retaining purposes e.g. water or chemical storage, chemical treatment vats, settling tanks, sewerage treatment plants etc.
Accordingly it will be readily apparent to a skilled addressee that many modifications may be made to the invention without departing from the scope of the claims.

Claims

1. An apparatus for constructing a liquid retaining structure with at least one concave curve in an outer perimeter, said structure comprising:-

a plurality of wall panels (23) for location in edge to edge engagement;

lower retaining means (13) for retaining the lower edges of said panels in desired edge to edge alignment;

a membrane liner releasably securable within said structure by liner retention means (18, 19, 20) to an inner face of said wall panels adjacent the upper edges thereof; and,

a plurality of preformed coping members (1a a to 1g) having channels (3) to receive the upper edges of said wall panels and to retain said wall panels (23) in desired edge to edge alignment;

said coping members (1a to 1g) being adapted to support a tensile member adjacent the upper edge of said wall panels such that in use when said tensile member undergoes tension as a result of inwardly or outwardly directed forces on said wall panels, said coping members act in compression as a substantially continuous beam to withstand flexure from said inwardly or outwardly directed forces on said wall panels.

2. An apparatus as claimed in claim 1 wherein one or more of said coping members (1b_' 1c, 1 g) includes a hollow aperture extending lengthwise thereof to support the tensile member.

3. An apparatus as claimed in claim 1 or claim 2 wherein the tensile member comprises a steel cable arranged about an outer wall of one or more of said coping members adjacent the upper edges of said wall panels and is retained in a desired position by brackets (14) extending between adjacent edges of said coping members.

4. An apparatus as claimed in any one of claims 1-3 wherein the channels (3) of the coping members (1 a to 1 g) engage the upper edges of said wall panels (23), said channel being tapered (4) inwardly to permit frictional engagement with the upper edges of said panels.

5. An apparatus as claimed in any one of claims 1-4 wherein the wall panels (23) comprise steel fibre reinforced concrete.

6. An apparatus as claimed in any one of claims 1-5 wherein the lower retaining means comprises a channel sectioned member (13).

7. An apparatus as calimed in any preceding claim wherein said wall members and said coping members are formed in a plurality of shapes including straight section, concave curves and convex curves to permit construction of liquid retaining structures selected from a wide variety of perimetral shapes.

8. A liquid retaining structure comprising:

a plurality of preformed wall panels (23) arranged in edge to edge relationship to form on a base a wall member of predetermined shape with at least one concave curve in an outer perimeter;

said wall panels being maintained in a desired abutting edge to edge relationship at their respective lower edges by lower retaining means (13) and at their respective upper edges by a plurality of preformed coping members (1a a to 1 g) having channels (3) to receive the upper edges of said wall panels;

a membrane liner releasably secured within said structure by liner retention means (18, 19, 20) attached to an inner face of said wall member adjacent the upper edge thereof; and,

a tensile member associated with said coping members adjacent the upper edges of said wall member such that in use said tensile member undergoes tension as a result of inwardly or outwardly directed forces on said wall member, said coping members act in compression as a substantially continuous beam to withstand flexure from said inwardly and outwardly directed forces on said wall member.

9. A liquid retaining structure as claimed in claim 8 wherein said liquid retaining structure is formed within a cavity in the ground and concrete is cast between the outer surface of the wall member and the cavity wall.

10. A liquid retaining structure as claimed in claims 8 or 9 wherein the liquid retaining structure includes a concrete floor (30) cast within the wall members (23).

11. A liquid retaining structure as claimed in any one of claims 8-10 wherein the floor (30) is formed with a concave depression.

12. A liquid retaining structure as claimed in any one of claims 8-11 wherein said structure includes an outwardly protuberant portion (Fig. 9) of reduced depth in the perimeter of the structure.

13. A liquid retaining structure as claimed in claim 12 wherein the outwardly protuberant portion of reduced depth having an opening thereto, the opening comprising one or more short wall panels contiguous with adjacent taller wall panels to form a downwardly extending aperture in said wall member, said membrane liner being releasably secured to the inner face of the wall member adjacent said downwardly extending aperture.

14. A liquid retaining structure as claimed in any one of claims 8-14 wherein the structure includes means (24 to 27) for removal of liquid accumulated between the membrane and the wall member.

15. A method for the construction of a liquid retaining structure comprising:

arranging a plurality of preformed wall panels (23) to form a substantially continuous wall member of predetermined perimetral shape with at least one concave curve therein;

retaining the lower edges of said wall panels in desired edge to edge relationship with lower retaining means (13);

retaining the upper edges of said wall panels in desired edge to edge relationship with preformed coping members (1 a to 1 g) having channels (3) to receive the upper edges of said wall panels;

releasably securing a membrane liner within said structure by liner retention means (18, 19, 20) attached to an inner face of said wall member adjacent the upper edge thereof;

retainably positioning a tensile member in association with said coping members adjacent the upper edge of said wall panels thereby in use said tensile member undergoes tension as a result of inwardly or outwardly directed forces on said wall member, said coping members act in compression as a substantially continuous beam to withstand flexure from said inwardly and outwardly directed forces on said wall member.

16. A method as claimed in claim 16 wherein said structure is formed within an excavation within a ground surface.

17. A method as claimed in claim 16 or 17 wherein concrete is cast around the external perimeter of said structure.

18. A method as claimed in any one of claims 16-18 wherein a concrete floor is cast within the perimeter of said structure.

19. A method as claimed in any one of claims 16-19 wherein said floor is formed with a concave depression.

20. A method as claimed in any one of claims 16-19 wherein an outwardly protuberant portion of reduced depth is formed in the perimeter of the structure, said outwardly protuberant portion having an opening thereto, the opening comprising one or more short wall panels contiguous with adjacent taller wall panels to form a downwardly extending aperture in said wall member, said membrane liner being releasably secured to the inner face of the wall member adjacent said downwardly extending aperture.

21. A method as claimed in any one of claims 16-21 wherein the space between the membrane and the inner wall and floor of the structure is evacuated prior to filling with liquid.