US20130199113A1

US20130199113A1 - Floatable Constructions

Info

Publication number: US20130199113A1
Application number: US13/537,872
Authority: US
Inventors: Carl Richard Nelson
Original assignee: LIZZANO Ltd
Current assignee: LIZZANO Ltd
Priority date: 2012-02-02
Filing date: 2012-06-29
Publication date: 2013-08-08
Also published as: GB2499011B; EP2820193A2; WO2013114134A3; GB2499011A; WO2013114134A2; GB201201877D0

Abstract

A plurality of floatable constructions are provided, the constructions including a floating base for a building, the base having at least one buoyant basement unit defining a basement level, and a reinforced concrete transfer platform atop the basement unit. The basement level can provide habitable or functional space for the building, and the transfer platform has at least one access opening giving access to the basement level which is enhanced by windows for light and ventilation. Methods and means of tying modular components of the structures together, and materials suitable for manufacturing such ties, are also provided.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. 119(a), the instant application claims priority to prior United Kingdom application number GB 1201877.6, filed Feb. 2, 2012.

FIELD OF THE INVENTION

The present invention relates to constructions that are normally at rest on the ground or some form of solid support, but can float during periods of flooding. In particular, but not exclusively, the invention relates to constructions having a buoyant basement structure which support conventional buildings.

BACKGROUND OF THE INVENTION

In modern urban environments, the development and construction of large buildings for residential, commercial, leisure or industrial use can often be beset with problems.
For example, a site in UK may have been previously developed, but with a change of legislation the land may now be classified as within a flood zone, e.g. Planning Policy Statement 25 (PPS 25) & the Development and Flood Risk Practice Guide dated June 2008. PPS25 is part of the holistic approach to managing risk set out in the Government's strategy for flood and coastal erosion management, Making Space for Water (Defra, 2005).
Flooding can cause substantial damage to property and threaten human life, as the floods of 2007 in the UK showed. Such damage is a consequence of previous decisions about location and nature of settlement and land use.
Floating or floatable buildings, which are not based on vessels, are known. For example, U.S. Pat. No. 6,199,502 describes the use of connectable concrete flotation modules with polystyrene cores to create a floating pontoon on which structures can be supported. The flotation modules are designed to be transportable by land vehicles, so that a large number of modules are required to create a floating platform of modest size, and the weight that can be supported by the platform is limited.
U.S. Pat. No. 5,647,693 describes a floatable building having a watertight concrete basement of unitary construction which provides buoyancy in the event that the site of the building is flooded. As in a conventional building with a basement, the walls of the basement structure support the floor joists and walls of the building above. This limits design freedom and compromises access to the basement. The basement is constructed at the site of the building, and remains in place after construction until floodwater raises the building.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a construction defined by claim 1.
According to a second aspect of the invention, there is provided a method of constructing a structure that can float defined by claim 26.
According to a third aspect of the invention, there is provided a construction defined by claim 51.
Such a construction preferably comprises a floating base for a building, the base having at least one buoyant basement unit defining a basement level, and a reinforced concrete transfer platform atop the basement unit. The basement level can provide habitable or functional space for the building, and the transfer platform has at least one access opening giving access to the basement level which is enhanced by windows for light and ventilation.
The basement unit may be manufactured from 300 mm micro fibre reinforced concrete. On the top of the wall sections ties may be cast in to connect the walls to the transfer platform. In embodiments in which the transfer platform is made of concrete, the walls may comprise a plurality of ties, each tie extending partly within the transfer platform. In such an embodiment, the ties may be connected to the reinforcement of the transfer platform.
Preferably, the ties extend from the basement unit into the transfer platform, so as to securely connect the transfer platform to the basement unit. The ties may, for example, be cast into the basement unit during construction of said unit, or may be bolted or otherwise affixed to the basement unit.
Optionally, the ties may extend from the transfer platform into the basement unit. In this case, the ties may be inserted into holes drilled in one or more basement units, and the ties may be retained in the holes by adhesive filler, such as a resin grout or mortar.
Preferably, where a part of a tie extends within a basement unit, that part of the tie is approximately 400 mm to 750 mm in length. Preferably, the ties comprise reinforcing bars.
Preferably, additional starter bar ties may be cast into the concrete wall sections to provide a means of attachment to a walkway discussed in detail below.
The ties may, for example, be cast into the basement unit during construction of said unit, or may be bolted or otherwise affixed to the basement unit.
Preferably, the starter bar ties comprise reinforcing bars.
The transfer platform preferably comprises a lightweight reinforced concrete slab. For example, the transfer slab may include an array of voids, optionally formed by an array of void formers. Alternatively, the transfer platform may be formed of a plurality of wooden joists, which are preferably secured to the walls of the basement unit by galvanized straps. In this way, the mass of the floating basement unit can be kept to a minimum, and the centre of gravity can be low in the base so as to provide stability to the base.
The upper surface of the transfer platform may include a layer of tiles or timber floorboards to form a finished floor.
The construction comprises guide means for preventing horizontal movement of the basement unit. The guide means may comprise locating means which are fixed relative to the ground and engagement means arranged to engage with the locating means. The engagement means may, for example, comprise rollers arranged in rolling contact with the locating means, or sliders arranged in sliding contact with the locating means.
The locating means may comprise either timber or steel piles set into the ground. Advantageously, steel hollow piles could house apparatus for extracting heat from the ground for supply to the building, such as ground source heating apparatus.
The basement units are preferably micro-fibre reinforced concrete which, advantageously, is approximately 300 mm thick. However, it is conceivable that the basement units could be formed of other materials, such as steel.
Sheet steel piling (preferably corrugated) may form the walls of the excavated pit.
A depth of 500 mm of water in the pit is preferably sufficient to lift the basement unit through displacement pressure.
Advantageously, the basement unit can provide a load-bearing platform to support the weight of a superstructure thereon.
In preferred embodiments, the construction comprises a floating basement unit in accordance with the first aspect of the invention, and a superstructure upon the basement unit. A transfer platform provides a load-bearing surface to distribute the weight of the superstructure across the basement unit.
The basement unit having the transfer platform provides a mechanically uniform platform upon which a superstructure of substantially any design and construction can be built. In some embodiments, the transfer platform may provide a driveway upon which vehicles may be parked. The weight of the superstructure is distributed across the base via the transfer platform, so there is no need for correspondence between the position of the load-bearing parts of the superstructure and the position of features within the basement structure. Thus, the present invention offers a flexible and adaptable way of constructing floating buildings.
As well as providing support for the superstructure, the transfer platform can act as a fire barrier. Thus, if fire were to break out in the basement level, unlike an open-framed load bearing structure, the transfer platform would act to slow passage of fire up into the superstructure. Consequently, the basement level can be arranged to house plant for the building, such as equipment associated with electricity generation, metering or distribution, gas supply, water treatment, waste processing and so on.

DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 shows a cross-sectional view of a construction forming a preferred embodiment of the present invention;

FIG. 2 shows a plan view of the construction of FIG. 1;

FIG. 3 shows a cross-sectional view of a walkway of the construction of FIG. 1;

FIGS. 4 a and 4 b show the construction of FIG. 1 in two positions;

FIG. 5 shows a cross-sectional view of a construction forming a preferred embodiment of the invention in a first position;

FIG. 6 shows a cross-sectional view of the construction of FIG. 5 in a second position;

FIG. 7 shows a footbridge extending from ground level to a superstructure in a preferred embodiment of a construction;

FIG. 8 shows a plan view of the construction of FIG. 7;

FIG. 9 shows a plan view of a locating pile; and

FIG. 10 shows a detailed cross-section of a transfer platform.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a first embodiment of a construction comprising: a pit 100; a buoyant basement unit 22; and a superstructure 20 built upon the basement unit 22.
Preferably, the basement unit 22 comprises a floor and external walls and one or more internal walls to define rooms in the basement level. Optionally, the floor is generally rectangular in plan, so that the rooms may be generally cuboidal.
Preferably, an external floor 33 extends from the basement unit 22. Preferably, the external floor 33 is formed integrally with the basement unit 22. The external floor 33 may be polystyrene encased concrete, and can therefore act as an additional float. Optionally, the external floor 33 substantially surrounds the top of the basement. Thus, the external floor 33 can provide a walkway to ensure that nobody can fall into the excavated pit in which the basement unit 22 is located. Preferably, there is a gap of no more than 75 mm between the edge of the external floor 33 and the pit 100 when the basement unit 22 is not floating. This can allow flood water to cascade down into the pit 100.
The pit 100 is formed by digging below ground level 23 to a depth sufficient to accommodate the majority of the buoyant basement unit 22. The size of the pit 100 is greater than the size of the basement unit so that a recess 24 will surround the basement unit 22 when it is not floating. Preferably, the recess 24 will have a width (i.e. the distance between the basement unit 22 and the walls of the pit 100) of between 75 mm and 100 mm.
The pit 100 is preferably formed by excavating below ground level 23, for example, in a flood plain area 23.
The buoyant basement unit 22 comprises outer walls 26 and a floor. It can provide a floating base upon which the superstructure 20 is built or placed.
The basement unit 22 may comprise a transfer platform 25 which spans the entire basement unit 22. The transfer platform 25 may comprise a single unitary transfer slab, or a plurality of joists (e.g. timber joists).
When a plurality of joists form the transfer platform 25, these may abut each other to form a substantially continuous floor. Alternatively, the joists may be provided with an additional surface mounted thereon, such as a plurality of abutting floor boards, to form a substantially continuous floor.
The bottom face of the transfer platform 25 rests upon and is attached to the tops of the outer walls 26 of the basement unit 22. In this way, the transfer platform 25 may close the open top of the basement unit 22.
The basement unit 22 may be a habitable space comprising one or more rooms separated by internal walls. For the comfort of the user, one or more windows 27 may be provided to provide light and/or ventilation. Preferably, the depth of the pit 100 is chosen such that the lower extent of the window(s) 27 is at ground level when the basement unit 22 is not floating. In which case, the basement unit may extend above the external floor by between 0.8 m and 1 m.
The basement unit 22 may be formed of concrete in which is cast reinforcing bars 44 (as can be seen in FIG. 3). Preferably, the basement unit 22 is formed of fibre-reinforced concrete.
When a single unitary transfer platform 25 is used, this is preferably formed as a lightweight reinforced concrete slab, for example of the type marketed as BubbleDeck®. The slab 25 contains a plurality of voids, preferably defined by void formers in the form of hollow plastic spheres, arrayed within a lattice of reinforcing bars. The reinforcing bars and voids are set within a concrete matrix.
The transfer platform 25 is preferably permanently attached to the basement unit 22. The connections between the basement unit 22 and the transfer platform 25 may consist of reinforcing bars 44 which extend upwardly from the outer walls 26 of the basement unit 22 and into the transfer platform 25 (as can be seen in FIG. 3).
Furthermore, the transfer platform 25 may be formed of concrete cast directly onto the top faces of the basement units 22 to form the connections. In this way, the transfer platform 25 and the basement units 22 can be considered as a continuous reinforced concrete basement structure.
The basement structure 22, and hence the building, may be constrained from lateral movement by a number of locating piles 28. Preferably, each locating pile 28 consists of a 300 mm steel column pile, which is driven into the ground adjacent the basement structure 22. At least one locating pile 28 is provided adjacent at least two of the outer sides 26 of the basement structure 22.
Pile guides are attached to the outer surface of the basement structure 22, just above the water line. Each pile guide 28 may comprise one or more rubberised rollers (not shown) mounted on a galvanized steel frame. The frame of each pile guide extends around one of the locating piles 28, and the rollers bear upon the outer surface of the associated pile 28. In this way, the basement structure 22, and the superstructure can rise or fall to accommodate changes in the water level. However, lateral or side-to-side motion of the basement structure 22 is prevented so that the building remains in the desired position above its normal resting place.
Within the basement structure 22, windows 27 can be position at the top of the walls above ground level 23.
The basement unit 22 may be located on one or more (preferably two) concrete spreader bars 31 which preferably have a rectangular cross-section (preferably 500 mm deep×300 mm wide) and a length sufficient to extend across the majority of the basement unit 22.
These may be located on top of vertically oriented 300 mm diameter piles 32, which are driven into the ground in the conventional manner. The depth of these piles 32 may vary depending on the weight of the structure.
Alternatively, a blinding layer of concrete 41 may be provided on the ground.
In which case, the spreader bars 31 could be replaced small blocks cast into the binding layer 41.
The spreader bars 31 or blocks (preferably formed of concrete) can allow flood water to trickle underneath the basement unit 22 to prevent a vacuum forming between the floor of the basement unit 22 and the surface on which it rests.
The side walls 102 of the excavation can be kept in the vertical position by the use of steel sheet piling 21 or by the use of other materials such as pre-cast concrete planks or engineering brickwork.
FIG. 3 shows a section of the external floor 33 around the basement unit 22. Preferably, the external floor 33 comprises fabric reinforcement 35 encased in concrete. The external floor 33 is preferably joined to the basement unit 22. This may be achieved by the addition of splice reinforcing bars 34, which are attached to the reinforcing bars 44 of the outer walls 26.
Most preferably, the external floor 33 encapsulates buoyant float material 36, such as polystyrene, to thereby increase the buoyancy of the basement unit 22.
In cross-section, the external floor 33 may be tapered from its upper surface, which can provide a walkway.
The external floor 33 is preferably attached to the basement unit 22 so that its upper surface is at ground level 23 to provide a cover over the pit 100. The cover preferably does not necessarily entirely close the pit 100 and there may be a gap around its periphery to allow water to fill the pit 100. Additionally a grill such as a metal grating can be placed over the remaining gaps between the pit 100 and the external floor 33 for safety. Preferably, the gap is not more than 75 mm.
The external floor 33 may continuously surround the basement unit 22 or may be formed of one or more discrete sections separated by gaps. In either case, the external floor covers a portion of the recess 24 when the basement unit 22 is not floating.
Preferably, the upper surface of the external floor 33 is flush with ground level 23.
The external floor 33 may be an integral part of the basement unit 22.
The superstructure 20 can be pre-fabricated or manufactured on the basement unit 22.
Renewable energy sources can be positioned on the roof of the superstructure 20, such as solar photovoltaic panels and wind turbines.
Whilst the description above has been directed to the use of a single buoyant basement unit 22, the inventors have envisaged the use of multiple buoyant basement units 22, connected together to form a single floating structure. Preferably, the multiple basement units 22 would have a single transfer platform 25 affixed thereon and may together be substantially surrounded by an external floor 33.
It will be appreciated that the access ramps and other connections between the ground and the basement unit 22, external floor 33 and/or superstructure 20 are arranged to accommodate the rising and falling motion of the building.
The superstructure is preferably a building having a plurality of rooms (for example a house).
As can be seen from FIGS. 4 a and 4 b, when constructed as set out above, the buoyant basement unit 22 may be movable between a position in which it rests upon the floor of the pit (either directly, or via supports 31) to a position in which it floats upon a sufficient volume of water. The minimum volume of water suitable for initial displacement of the mass of the basement unit 22 and the superstructure 20 preferably corresponds to a depth in the range of 0.5 m to 1 m within the recess 24.
As shown in FIG. 5, a second embodiment of a construction may include access to the pit 100 via a manhole 40 (preferably, a 600 mm×600 mm manhole with a replaceable cover). A metal ladder 41 between the floor of the pit 100 and the manhole 40 may be provided. This allow for maintenance and the clearance of any silt or debris which may accumulate in the pit 100.
Optionally, in either embodiment, a fence or handrail 60 is attached around the walkway.
Preferably, a barrier 61 extends down into the pit 100 from the basement unit 22 or, more preferably, from the outer edge of the external floor 33. This may be secured in place by one or more brackets 62 attached to the sheet piles 21.
Optionally, the barrier 61 may extend past the external floor 33 to form the fence or hand rail 60.
FIG. 6 depicts the construction in a raised position when the water level 50 has risen above ground level 23. In this position, the barrier 61 can prevent debris from entering the pit 100.
The barrier 61 may be arranged to prevent the passage of debris therethrough but allow the passage of water. The barrier 61 is preferably formed of a mesh or an apertured sheet. Preferably, reinforcement is provided to maintain the shape of the barrier 61.
FIG. 7 shows an optional footbridge 70 extending from ground level 23 around the pit 100 to the superstructure 20. Alternatively, the footbridge 70 may span from ground level 23 to the basement structure 22, or to the external floor 33.
Small metal ramps 73 may be provided at either end of the footbridge 73 ensure ease of access for wheelchairs.
Optionally, a drive way may be provided to allow access for vehicles to the basement unit 22 when it is not floating.
The footbridge 70 is free to pivot at either end to compensate for movement of the construction. Preferably, at least one end of the footbridge 70 is free to move laterally relative to the ground 23 and/or the construction, to compensate for large displacements of the construction. In preferred embodiments the end of the footbridge 70 at the construction is free to pivot while the other end of the footbridge 70 rests on rollers.
Preferably, the pivot at the construction end of the footbridge 70 is mounted on the external floor 33, the transfer platform 25, or the basement unit 22.
As can be seen from the plan view of FIG. 8, a driveway 74 for a vehicle may also be provided to span the pit 100. This would have the same general construction as that set out above for the footbridge 70.
A garage or porch 76 can be constructed adjacent to the superstructure 20 for sheltering a vehicle and/or providing an area for bin storage.
FIG. 9 shows a plan view of a preferable arrangement of locating pile 28. As shown in the figure, the locating pile 28 stands outside the basement unit 22 and passes through an opening in the external floor 33. At least one roller 80 is provided to maintain the position of the locating pile 28 relative to the opening. Preferably, a pair of rollers 80 are provided to retain the locating pile 28 therebetween.
As shown in FIG. 9, the locating pile 28 may be an I-beam, which provides channels in which rollers 80 may be provided.
Preferably, the rollers 80 comprise Teflon.
Optionally, the rollers 80 are attached to a metal bracket 83, which is attached to the external floor 33 using bolts 81.
FIG. 10 shows a detailed cross-section of the transfer platform 25 to the wall of a preferred superstructure 20. In this figure, the transfer platform 25 comprises a plurality of timber joists 87, upon which a surface 86 (preferably formed of medium density fibreboard) is provided. Surface 86 provides a continuous surface on which a floor finish 85, such as floor boards or carpeting, may be laid.
The joists 87 may abut or be spaced apart. When the joists 87 are spaced apart, an insulating material is preferably provided therebetween.
Preferably a layer is provided upon the transfer platform 25 of a breather membrane (such as Tyvek DuPont® Airguard® Control). Thus, an air and vapour tight base may be provided.
In preferred embodiments, the breathable membrane may extend up a at least a portion of the walls of the superstructure 20.
The wall of the superstructure 20 may be formed of timber 83 on which a surface of acrylic render 86 is provided. A layer of vapour check barrier and a layer of fibre board may be provided on the inner surface.

Claims

1. A construction for supporting a building, comprising:

a pit excavated below ground level;

a buoyant basement unit within the pit, comprising an upper load-bearing surface for supporting a building; and

guide means for substantially preventing lateral movement of the basement unit and allowing vertical movement of the basement unit,

wherein:

the pit is larger in plan view than the basement unit so that when the basement unit is not floating a recess surrounds the basement unit within the pit; and

the construction comprises an external floor extending substantially across the recess.

2. The construction of claim 1, wherein the external floor is formed integrally with the basement unit.

3. The construction of claim 1, wherein when the basement unit is not floating the external floor is flush with ground level.

4. The construction of claim 1, wherein when the basement unit is not floating the gap between the external floor and the edge of the pit is not more than 75 mm.

5. The construction of claim 4, wherein a grill extends across the gap.

6. The construction of claim 1, wherein the external floor comprises a buoyant material encased in concrete.

7. The construction of claim 6, wherein the buoyant material is polystyrene.

8. The construction of claim 1, wherein the external floor forms a walkway surrounding a majority of the basement unit.

9. The construction of claim 1, wherein the buoyant basement unit provides at least one habitable room.

10. The construction of claim 1, wherein the basement unit extends above the external floor.

11. The construction of claim 1, wherein the basement unit has windows above the external floor for light and/or ventilation.

12. The construction of claim 1, wherein the basement unit is formed from fibre reinforced concrete.

13. The construction of claim 1, wherein the basement unit comprises outer walls and a floor, which together with the upper load-bearing surface form a continuous structure.

14. The construction of claim 1, wherein the basement unit further comprises internal walls forming therein a plurality of habitable rooms.

15. The construction of claim 1, wherein:

the basement unit comprises outer walls and a floor; and

the upper load-bearing surface comprises a transfer platform supported by the outer walls.

16. The construction of claim 1, wherein the upper load-bearing surface comprises reinforced concrete encapsulating an array of voids.

17. The construction of claim 1, wherein the upper load-bearing surface is fire resistant.

18. The construction of claim 1, wherein the pit is supported by a plurality of sheet piles.

19. The construction of claim 1, wherein the pit is supported by walls formed of a plurality of bricks.

20. The construction of claim 1, wherein the floor of the pit is formed of concrete.

21. The construction of claim 1, wherein the guide means comprise piles extending into the floor of the pit and sliders or rollers fixed to the basement unit.

22. The construction of claim 21, wherein the piles are hollow and act as a conduit for heat extracted from the ground.

23. The construction of claim 1, further comprising on the upper load-bearing surface a building having a plurality of rooms.

24. The construction of claim 1, further comprising a bridge arranged to movably extend from the ground to the building or the external floor over a range of heights of the basement unit.

25. The construction of claim 1, further comprising a barrier extending downwardly from the distal edge of the external floor, the barrier arranged to allow the passage of water therethrough.

26. A method of providing a construction that can float, comprising the steps of:

excavating a pit below ground level;

locating a buoyant basement unit within the pit;

providing guide means to substantially prevent lateral movement of the basement unit; and

providing an external floor extending substantially across the recess,

wherein the pit is larger in plan view than the basement unit so that when the basement unit is not floating a recess surrounds the basement unit within the pit.

27. The method of claim 26, wherein the external floor is formed integrally with the basement unit.

28. The method of claim 26, wherein when the basement unit is not floating the external floor is flush with ground level.

29. The method of claim 26, wherein when the basement unit is not floating the gap between the external floor and the edge of the pit is not more than 75 mm.

30. The method of claim 26, further comprising providing a grill extending across the gap.

31. The method of claim 26, further comprising encasing a buoyant material in concrete to form the external floor.

32. The method of claim 31, wherein the buoyant material is polystyrene.

33. The method of claim 26, wherein the external floor forms a walkway surrounding a majority of the basement unit.

34. The method of claim 26, wherein the buoyant basement unit provides at least one habitable room.

35. The method of claim 26, wherein the basement unit extends above the external floor.

36. The method of claim 26, wherein the basement unit has windows above the external floor for light and/or ventilation.

37. The method of claim 26, further comprising forming the basement unit from fibre reinforced concrete.

38. The method of claim 26, wherein the basement unit comprises outer walls and a floor, which together with the upper load-bearing surface form a continuous structure.

39. The method of claim 26, wherein the basement unit further comprises internal walls forming therein a plurality of habitable rooms.

40. The method of claim 26, wherein:

the basement unit comprises outer walls and a floor and the method further comprises supporting a transfer platform upon the outer walls.

41. The method of claim 26, wherein the upper load-bearing surface comprises reinforced concrete encapsulating an array of voids.

42. The method of claim 26, wherein the upper load-bearing surface is fire resistant.

43. The method of claim 26, further comprising providing a plurality of sheet piles to support the walls of the pit.

44. The method of claim 26, further comprising providing a plurality of bricks to support the walls of the pit.

45. The method of claim 26, further comprising forming a layer of concrete to provide the floor of the pit.

46. The method of claim 26, wherein the guide means comprise piles extending into the floor of the pit and sliders or rollers fixed to the basement unit.

47. The method of claim 26, wherein the piles are hollow and act as a conduit for heat extracted from the ground.

48. The method of claim 26, wherein the basement unit comprises an upper load-bearing surface on which a building having a plurality of rooms is provided.

49. The method of claim 26, further comprising providing a bridge arranged to movably extend from the ground to the building or the external floor over a range of heights of the basement unit.

50. The method of claim 26, further comprising providing a barrier extending downwardly from the distal edge of the external floor, the barrier arranged to allow the passage of water therethrough.

51. A construction for supporting a structure, comprising:

a pit excavated below ground level;

a buoyant basement unit within the pit, comprising an upper load-bearing surface for supporting a structure,

wherein:

the basement unit is arranged to move between a non-floating position in which the basement unit is supported by a floor of the pit, and a floating position in which the basement unit is supported by a threshold amount of water within the pit.

52. The construction of claim 51, wherein the threshold amount of water is less than an amount of water required to fill the pit.