Flexible Drainage Cell

Cross-Reference to related applications

This application claims priority from Australian provisional patent application No 2008902415 entitled "Flexible drainage cell" filed 16 May 2008, the entire contents of which are hereby incorporated by reference.

Field of the Invention

This invention relates to an improved drainage cell.

Background to the Invention

The control and flow of surface water, such as rain water or storm water, is important in preventing the build up of surface water adjacent to the foundations of buildings or other structures, and in other areas such as playing fields, golf courses, landscaped decks, gardens and the like. Roof gardens are becoming increasingly popular both to "green" buildings and make them more environmentally friendly, and also to provide recreational spaces. The use of drainage cells in roof gardens is critical in allowing excess water to escape quickly from the roof garden to drainage outlets. This reduces the potential for leakage of water through roofs, and also helps prevent load issues due to the weight of water, growth media and the like on top of a building's roof.

A typical drainage cell (often known as a dimpled drainage sheet) is formed from an extruded sheet of plastic in which is formed an array of protruding frusto- conical dimples . A layer of geotextile is adhered to the base of the frusto conical sections of the drainage cell. The cells are placed with the base of the frusto-conical sections and the geo-textiles facing upwards. The opposite face, or base, of the cell typically lies on a sheet of waterproof membrane or like when used in a roof garden application.

However, the problem with the existing drainage cells is that flow tends to be inhibited by the volume of the conical dimples occupying the space beneath the geotextile layer. More significantly, the geotextile tends to sag due to the weight of soil or other growth medium lying on top of the geotextile layer. This tends to block or partially block the spaces between the conical dimples, thus impeding water flow and slowing the rate of water flow through the drainage cell. The problem of sagging geotextile tends to increase as the drainage cell and geotextile age.

The present invention aims to provide an improved drainage cell which addresses some of the problems of prior art drainage cells discussed above.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Summary of the Invention In a first broad aspect, the present invention provides a drainage cell comprising a backing plate or sheet defining a plurality of holes, a series of tubular structures having side walls and a base extending from the backing layer, the tubular structures being in fluid communication with at least some of the holes in the backing layer thereby allowing fluid to pass through a hole into a tubular structure, wherein the side of the tubular structures define apertures thereby permitting fluid flow out of the tubular structures through the walls of the tubular portions.

Typically, the perforated or open area of the tubular structures is at least 20% of the area of the side walls of the cones, more preferably at least 40%, more preferably 50% or greater, and most preferably of the order of 70 to 75%. Advantageously, providing apertures in the side walls of the tubular structures significantly improves not only the rate of flow of the water through the cell but also creates a higher internal drain void volume area as the water can occupy the interior of the tubular structures and not just the volume between cones. This contrasts with existing drainage cells in which the interior of the conical structures is not occupied by water.

Typically, the backing sheet is generally planar and is covered with a layer of a fluid permeable geotextile which allows a restricted flow of fluid (usually water) through it.

Typically, half of the holes in the backing layer coincide with tubular structures with the other half of the holes being located in between the tubular structures. Thus only 50% of the apertures are typically located above tubular structures.

In use the drainage cell is used in the opposite orientation to existing drainage cells with the cell resting on and supported by the base of the tubular structures. The total area of the base of the tubular structures is typically less than 35%, most preferably less than 25% of the total area of the base of the cell. In one specific preferred embodiment the area is about 20%. The geo-textile layer is better supported by the planar backing layer and this reduces the tendency for the geo-textile to sag.

Because the geotextile lies against the backing sheet and not on the ends of the tubular members, as in prior art drainage cells, it is possible for the drainage cells to be nested with the tubular structures from one drainage cell nesting in between the gaps between the tubular structures of another drainage cell. This significantly reduces the volume occupied by the drainage cells and makes transport of the drainage cells more cost effective.

The tubular structures are typically frustro-conical. However it will be appreciated that the structures need not be circular in cross-section but may be square or have any other suitable cross-sectional shape.

Typically the holes in the sheet have a diameter of about 15 to 25mm, most preferably about 22.5 mm, to maximise the open area of the cell without compromising its strength. The diameter at the base of the conical structures is in the order of 18.5 mm for a 22.5 mm diameter hole. The total area of the holes in the sheet is typically around 60 to 65% of the area of the sheet. This allows for rapid drainage.

The conical structures and apertures are preferably arranged in parallel arrays with one line of conical structures separated from the next line of conical structures by a line of simple, unsupported, holes. The centres of the holes and the conical structures is offset so that each hole is located in the centre of four conical structures and vice versa.

It is preferred that the interior of the tubular structures is reinforced. Typically this is achieved by means of an internal rib or ribs which extends down the internal side walls of the structure. Four such ribs may be provided for each conical structure and these may meet to define a cruciform structure at the base of the conical structure.

It is preferred that reinforcing ribs are also defined on the under side of the backing sheet extending between each pair of adjacent conical support structures.

Brief Description of the Drawings A specific embodiment of the present invention will now be described by way of example only with reference to the accompany drawings which:

Figure 1 is an isometric view from above of a drainage cell embodying the present invention;

Figure 2 is an isometric view of the drainage cell of Figure 1 seen from below; Figure 3 is an enlarged isometric view of one corner of the drainage cell of Figure 1;

Figure 4 is an enlarged side view of part of the drainage cell shown in Figure 1; Figure 5 is an enlarged top plan view of one corner of the drainage cell of Figure i;

Figure 6 is an isometric view showing two drainage cells stacked one within the other;

Figure 7 is a side view of the stacked arrangement shown in Figure 6; Figure 8 is an isometric view showing two drainage cells embodying the present invention nested facing each other for transport or storage; and

Figure 9 shows a side view of a drainage cell covered with a layer of geotextile.

Detailed Description of the Preferred Embodiment

Referring to the drawings, Figures 1 and 2, show a drainage cell 10 embodying the present invention. The drainage cell is typically injection molded from a plastics material such as polypropylene, HDPE or LDPE. Recycled polypropylene may be used. However, any other suitable materials which can be molded or cast may be used. In the specific embodiment, the cell is about 30mm high but it may range from 15mm to 50mm or more.

The drainage cell 10 comprises a generally square sheet of plastics material defining a perforated backing plate 12 which defines a plurality of parallel rows 14 of first and second circular holes 16a and 16b. A tubular support member 18 which is generally frusto-conical, extends from each second hole 16b.

More specifically, with reference to Figures 3 and 5, it can be seen that in each row 14, running parallel to the side of the cell, a support member 18 depends from every second hole 16b in the row, with unsupported holes 16a being located between the holes 16b associated with supports. A similar arrangement is defined in the adjacent row M ₁ except that the supported and unsupported holes are offset by one. Thus as can be seen in the Figures, there is an unsupported hole 16a in the centre of four supported holes 16b and a supported hole 16b is located at the centre of four unsupported holes 16a. The unsupported holes 16a are the same size as the supported holes 16b, in the described embodiment but in a variant (not shown) may be larger.

This arrangement allows two cells to be nested facing each other as shown in Figure 8 to reduce the volume occupied by the cells for storage and/or transport by about 50%.

Alternatively, one can view the drainage cell as having alternating rows of supported and unsupported holes extending at 45° to the edges of the cell. With reference to Figure 3, one side edge 30 of the backing plate defines a series of alternating generally triangular cut out portions 32 and generally triangular projections 34. Depending from each triangular projection 34 is a male connecting member in the form of depending prong 36.

The adjacent side edge 40 of the cell also defines a series of triangular cut out portions 42 alternating with triangular protrusions 44. A slot 46 or female engaging means is defined below each of the triangular cut out portions 42. The slot is shaped and configured to receive the male connecting member so that the cells can be interconnected using the prongs and female slots. The prongs are ideally much longer than the slots as this helps keep the cells together when they are interlocked and prevents accidental disengagement.

However, for storage and transport, the cut-out portions 32 allow the cells also to be simply abutted without interconnecting them. In this case, the projecting triangular portions 34 locate in the cut out portions 32 of an adjacent cell. With reference to Figure 2, it can be seen that two adjacent side edges have male connecting members and that those side walls have their slots 32 and projections 34 offset to allow that.

Figures 3 and 4 also show the tubular support structures 18 in more detail. Each support structure is generally frusto-conical in shape tapering from the backing plate towards the base 100 of the support structure, The diameter of the hole 16b at the top of the cone is approximately 22.5 mm and the diameter of the base 101 of the cone is approximately 18.5 mm. The areas occupied by the base of the cones is small, typically about 20% of the total surface area of the base of the cell. This assists when stacking and nesting the cells.

The side walls of the cone are made up of four depending legs 102 spaced at 90° apart around the circumference of the base 100. Each leg is arcuate in horizontal cross- section so that envelope created by the rotation of the legs about a vertical axis passing through the centre of the base is frusto-conical. An annular ring 104 extends around the support 18 about midway between its base at its top connecting the four legs 102. Extending down the interior of each leg from the ring 104 to the base is a strengthening rib 106. The four ribs meet at the centre of the base in a cross 108 - best seen in Figure 5. With reference to figure 4 in particular, eight apertures 109a, 109b are defined in the

sides of the support structure four (109b) between the legs 102, base 101 and ring 104 and four (109a) between the legs 102, top and ring 104. Typically, the open area of the tubular structures is of the order of 70 to 75% of the total surface area of the sides of the support structures/cones including the apertures. In other words the apertures constitute 70 to 75% of the unperforated area of the sides of the cone if the cone did not define apertures/perforations.

With reference to Figure 3 a further strengthening rib 110 extends between each pair of diagonally opposite supports 18. As can be seen the ribs extend down the exterior of the support as far as the ring 104 as well as extending under the top surface to the next support 18. Four ribs extend from each support 18.

It can be seen that the space between adjacent cones is relatively large - bigger than the diameter of the cones. The void area of the sheet (defined by the holes 16a and 16b is about 65% of the total area of the sheet (that is the solid part plus the holes). As the cones are open, water can flow through the interior of the cones and the total internal void are of the cell is about 95%. The cell allows water flow in there dimensions compared to existing dimple drainage sheets where the flow is essentially only 2-dimensional. Typically water flow rates may be around 20 litres/min which is two to three times the flow rate for typical dimpled drainage cells.

The cells have a compressive strength of about 70 tonnes/m . This is nearly twice the compressive strength of dimpled drainage sheets and allows for the cells to be used in other applications.

For most uses, use the backing sheet 10 is covered with a layer of geo-textile material 200 - refer to Figure 9. In that arrangement it is possible to nest the cells for transport as shown in Figure 8. For this purpose the top layer must be rotated through 90° so that the base of the cones of the top layer oppose holes 16a in the lower layer. Clearly, to enable nesting the cells should be square.

Figures 6 and 7 shows the cells without geo-textile stacked and nested with each support 18 of an upper cell inserted about halfway into an unsupported hole 16a of a lower cell, as best seen in Figure 7. In the stacked arrangement of Figure 6, it is anticipated that a plurality of cells may be stacked and nested one on top of another and enclosed in a permeable geo-textile or impermeable envelope thereby creating a tank for temporary storage or more permanent storage and control of water such as storm water, drainage water or the like. The construction of the cells allows the tank to support a considerably greater weight, typically doubling the strength of the cells. The cells may be used in the nested configuration for other purposes other than enclosed in an envelope as a tank, for example where increased strength is required. The stacked

cells also occupy less volume and when a plurality of cells are nested one within the other the cells occupy about 50% less volume than if they were not nested.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.