The invention relates to a subterranean drainage structure and a base unit to be incorporated therein.

Drainage structures, such as groundwater attenuation structures (or soakaways) are typically used to capture and retain surface water, for instance to prevent surface flooding. Such structures are typically constructed from a lattice or mesh type structure which is structurally robust under a distributed load, but which has a large void fraction to accommodate the water. Conveniently, a system of modules or cells may be used to construct a drainage structure.

It may be necessary to provide access to the drainage structure to inspect it, for example to ensure that it is not impaired by silt or scale. In order to accomplish this, horizontal inspection channels can be provided. Such horizontal access channels may conveniently be formed as part of the drainage structure. In a modular system, specific cells may be used in which a horizontal through channel is defined, providing access for an inspection car. Such horizontal through channels may extend through a central region of each cell.

It is sometimes difficult to access inspection channels. The present invention seeks to overcome or ameliorate at least one of the disadvantages of the prior art.

According to a first aspect of the invention, there is provided a subterranean drainage structure comprising: a plurality of similarly dimensioned drainage cells formed into a regular grid comprising at least one layer; a substantially vertical access chamber formed by omission of at least one cell from the or each layer;

a base, positioned at the lower end of the chamber; and fixing means for fixing the base to at least one of the cells adjacent the chamber.

The base may extend substantially from a first side of the chamber to an opposite side in at least one direction.

The fixing means may be provided to fix the base to at least one of the cells at the first and opposite sides of the chamber The or each layer may be substantially horizontal. Cells at the first and opposite sides of the chamber may be access cells, comprising a substantially horizontal access channel therein, and the base may comprise a platform which extends from a base of the access channels across the chamber

Access cells may be positioned in at least one horizontal line to form an access channel therethrough across the grid. The platform may be arranged to enable a wheeled access car to move freely across the inspection chamber, between the access channels of cells at the first and opposite sides of the chamber.

The platform may be arranged to provide a smooth transition from the platform to an access channel.

The platform may comprise comprises a trough shaped profiled portion that communicates with an access channel.

The bottom of the base may be in substantially the same horizontal plane as the bottom surfaces of the lowermost cells in the grid

The subterranean drainage structure may further comprise a geotextile wrapped around the external surface of the grid.

The platform may be arranged to interconnect, across the inspection chamber, an access channel opening in a central region of the first wall with an access channel opening in a central region of a second, opposite wall.

According to a second aspect of the invention, there is provided a subterranean drainage structure inspection chamber base comprising: a platform extending across the base in a first direction; a substantially flat surface opposing the platform to support a geotextile membrane placed against it; and fixing means for fixing the base to an adjacent element. Instead of geotextile, a geo-membrane can be used.

The platform may form a channel that extends between opposing edges of the base. The base may comprise two sections that cooperate, together defining the flat surface and the platform.

The two sections may be substantially identical. Each base section may have a linking means that mechanically co-operates with a corresponding feature of the other section to secure the sections together.

The linking means of each section may comprise a recess and/or a protrusion. The linking means may comprise a removeable protrusion.

The platform may comprise a substantially continuous channel arranged to extend across at least half of the base.

The base may further comprise fixing means for engaging the base with a wall of the inspection chamber.

The fixing means may comprise a protrusion and/or a recess. The fixing means may comprise a fixed protrusion from the base. The fixing means may comprise a removeable protrusion, and the base may comprise a recess within which the removeable protrusion is engageable.

The flat surface may be substantially rectangular.

According to a third aspect of the invention, there is provided a drainage structure according to a first aspect of the invention, comprising a base according to the second aspect of the invention.

Preferably, the groundwater attenuation structure is wrapped in a geotextile. This can limit the ingress of slit and particles. Such geotextile wrapping may be stably supported against the pressure of the material surrounding it by the groundwater attenuation structure. A system of pipes may be connected to the drainage structure, for instance to direct surface water thereto. Embodiments of the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic of a drainage structure according to an embodiment of the invention;

Figure 2 is a schematic of a base for an inspection chamber according to an embodiment of the invention; Figure 3 is an exploded diagram of a base for an inspection chamber comprising two sections according to an embodiment;

Figure 4 is a schematic of an assembled two section base according to an embodiment of the invention;

Figure 5 is a schematic showing a base section adjacent to a drainage cell according to respective embodiments of the invention;

Figure 6 is a schematic showing a detail view of a drainage cell according to an embodiment of the invention; and

Figure 7 is a further schematic of a drainage structure according to an embodiment of the invention. Referring to Figure 1 , a drainage structure 50 is shown which may be used to control groundwater. The drainage structure 50 comprises a plurality of cells 30, 40 and a base 20. The drainage structure is, in use, wrapped in a geotextile (or geomembrane) layer which communicates water while inhibiting the ingress of silt and solid matter. Each of the cells 30, 40 is formed from a lattice or mesh like structure with a high void fraction. The cells 30, 40 thereby can accommodate a large internal volume of water, and are mechanically strong enough to support large compressive loads. The cells 30, 40 in this embodiment are substantially rectangular cuboidal, and comprise a 4x4x8 repeating arrangement of a substantially cube shaped structural sub-cell. Each cell 30, 40 thereby has a long axis that is twice the length of each of the two orthogonal remaining axes, and has four rectangular external faces and two substantially square external end faces. However, the invention is not limited to such dimensions.

In the present embodiment, each of the cells 30, 40 comprises two identical symmetric sections, coupled together by an intermediate section. The drainage cells 30, 40 comprise a plastics material. In alternative embodiments, other relative dimensions of the cells can be used and they may be formed from materials other than plastics.

Two forms of cell are provided. These are drainage cells 40 and drainage access cells 30, the latter of which include a substantially horizontal access channel 32 which extends through each access cell 30 between openings 31 which are disposed in a central region of the opposite end faces of the cell 30. The access channel 32 extends parallel to the long side of the cuboidal access cell 30 and has a sectional perimeter comprising two opposite arcs linked by a straight and vertical central region. The section of the channel 32 thereby has a curved lower profile, the bottom of which is offset from the lower edge of the cell 30. The access channel 32 is not bounded by a continuous sheet of material, but is instead defined by the lattice or mesh like structure of the cell 30. Drainage cells 40 do not have an access channel, although further access cells 30 could be used instead of drainage cells 40 of the present embodiment.

The drainage structure 50 has a vertical access chamber which is defined by the omission of a cell 30, 40. It will be appreciated that in other embodiments the drainage structure 50 may comprise a plurality of stacked layers of cells, and that the vertical access chamber may be defined by the omission of a substantially vertical stack of cells. One or more cell may be omitted from each layer of cells. A substantially cuboidal void is thereby defined in the drainage structure 50 which is suitable for obtaining access, for instance for inspections. The inspection chamber 50 is bounded on each of two opposite sides by an access cell 30 with an internal channel 32 with an opening 31 facing into the inspection chamber. In an alternative embodiment, the inspection chamber 50 may be bounded by a different number of access cells 30.

The horizontal channel opening 31 is provided in a central region of the access cell end wall, and the edge of the channel 32 is thereby offset from the base of the access cell 30. Omitting a cell 30, 40 to define the vertical access chamber therefore results in a step down transition from the channel 32 to the bottom of the structure where the horizontal channel exits to the chamber wall (which is formed by the end of the cell next to the chamber), and a similar step up in the other direction.

For clarity, some of the cells 30, 40 are omitted in the drawings. It will be appreciated that the inspection chamber of this embodiment is bounded on all four lateral sides by cells 30, 40, although this is not an essential feature of the invention.

The base 20 of Figure 1 comprises two identical sections 10, and extends from the face of the access cell 30 in which the opening 31 is disposed, to the opposite face of the access chamber. Each section 10 extends horizontally halfway across the access chamber 50. Where the access chamber 50 is defined by omission of more than one cell 30, 40 in each layer, a plurality of bases may extend across the chamber 50. Alternatively, a base of a larger dimension may be used in such cases to extend across the chamber 50.

Figure 2 shows a base section 10 in more detail. A single such section 10 may be used as a base 20 in its own right, and is accordingly an embodiment of the invention. For example, in other embodiments the access chamber 50 may be dimensioned to be bridged with a single section 10. A single section 10 may, for example, be used to bridge across the short dimension of a chamber formed by the omission of the modules 30, 40 of the example embodiments. In still other embodiments, the base 20 may comprise a single section 10, that is dimensioned to be the same length as the example cells 30, 40 (in the long direction). Each section 10 comprises a platform 11 , a lower portion 2, fixing means 3, 3a, 4, and a shoulder profile 5.

The platform 11 has a trough shaped profiled portion 1 that defines a channel across the section 10. The lower portion 2 is joined on two opposite sides to a substantially flat portion 6 of the platform 1 1 by upwardly extending respective shoulder profiles 5. Each shoulder profile 5 is stepped to improve the strength of the section 10. The flat portion 6 is provided with indents 7 and corresponding protrusions 7a that cooperate when two sections 10 are stacked with their platforms 1 1 adjacent. This would allow the upper side of the channel formed by the profiled portion to be closed by another corresponding section mounted on top of the section and inverted relative thereto. The lower portion 2 is joined on the two remaining opposite sides to the platform 1 1 by respective vertical end walls 12 that are substantially flat. Horizontally spaced either side of the centre of the end wall 12 are first and second holes 4, which form part of a fixing means for engaging the base sections 10 with each other and adjacent drainage cells 30.

The lower portion 2 is joined on the two remaining opposite sides to the platform 11 by respective vertical end walls 12 that are substantially flat. Horizontally spaced either side of the centre of the each end wall 12 are a first and second circular hole 4, which form part of the fixing means for engaging the base sections 10 with each other and adjacent drainage cells 30.

The fixing means of the section 10 comprises recesses 4 or protrusions 3, 3a, 9 arranged to cooperate with corresponding features of an adjacent section 10 or cell 30, so as to fix the section 10 and adjacent section 10 or cell 30 together.

In Figure 2 a cylindrical protrusion 3 is shown engaged with the first circular hole 4, and the position of a second identical protrusion 3a to be engaged with the second hole 4 is indicated. It will be appreciated that if an adjacent section 10 or cell 30 comprises only recesses, it is appropriate for the section 10 to comprise only corresponding protrusions and vice versa. Similarly, each section 10 (or assembled base 20) may be provided with a fixing means comprising both one or more protrusions and one or more recesses. Figures 3 and 4 show more clearly how the two section base 20 of Figure 1 is assembled, with the corresponding protrusions and recesses of the fixing means of each section 10 cooperating with each other to fix the base 20 together. The platform 1 1 and lower portion 2 of each section 10 combine to form a single platform 1 1 and lower portion 2 of the assembled two section base 20. Protrusions 9 for fixing each section 10 (and hence the base 20) to an adjacent cell 30 are placed in the recesses 4 of one end wall of each section 10. The protrusions 9 cooperate in use with corresponding fixing means of the adjacent cell 30 to fix the section 10 (and base 20) to the adjacent cell 30. Cooperation between fixing means may be enhanced by an interference fit between corresponding features. Referring to Figures 5 and 6, the corresponding fixing means of a cell 30 is shown, which comprises two recesses 33, that have a spacing that corresponds with that of the fixing means of the base section 10. The fixing means of the cell 30 and base section 10 are arranged to cooperate to position the platform 1 1 in communication with the channel opening 31. The protrusions 9 have an interference fit with the recesses 33, thereby securing the section 10 (and base 20) to the cell 30 in use. As is shown in Figure 6, the recesses 4 in the base section 10 have a larger diameter than the recesses 33 of the cell 30, necessitating the different design of the protrusion 9 for fixing the base 20 to the cell 30. It will be appreciated that in other embodiments matching fixing means may be used, facilitating interchangeable protrusions for either purpose.

The base 20 has a number of functions in use, as shown in Figures 1 and 7. Firstly, the platform 1 1 communicates with the opening 31 of the channel 32 in the cell 30, thereby enabling access for inspection to the channel 32 from the chamber. The platform 11 further enables communication of inspection means across the chamber, between access channels 32 of the two opposite access cells 30. Secondly, it has a lower portion 2 that mechanically supports the adjacent geotextile material by bridging between the opposite walls of the inspection chamber. The lower portion 2 of the base 20 is arranged to be at substantially the same height as the base of the adjacent cells 30, 40 in use.

In use, the trough shaped profiled portion of the platform 11 defines a substantially horizontal channel linking the opposite access channel openings 31 of the chamber 50. In this embodiment, the profiled portion 1 is arranged at the same height as the corresponding curved lower profile of the channel opening so as to provide a smooth transition from the platform 1 1 of the base 20 into the channel 32 of the adjacent cell 30. This smooth transition allows an inspection car to travel between the base 20 and channel 32 without needing to traverse a step, which might otherwise prevent movement of the inspection car. The platform 11 provides a convenient surface from which to inspect the drainage structure 50. Because the access channel opening 31 is relatively close to the lower edge of the cell 30, the height of the platform 11 is correspondingly relatively low in relation to its length and width. The base section 10 has a longitudinal axis parallel to the axis of the channel 32 with which the platform 1 1 communicates in use. The lower portion 2 is substantially rectangular and flat, but is provided with lateral corrugations 8 that increase the flexural stiffness thereof.

In use, the section 10 is dimensioned to extend fully halfway across the length of the access chamber in the longitudinal direction, not to extend fully across the chamber in the lateral direction. The assembled base 20 thereby abuts the opposite walls of the access chamber in the longitudinal direction, but a gap is provided between the edges of the base 20 and the walls of the chamber (again formed by sides of the cells adjacent the chamber) that are parallel to the longitudinal direction, as is shown in Figure 1. In an alternative embodiment, however, the base may extend such that no such gap is provided. Figure 7 clearly illustrates the substantially smooth and continuous transition provided by the profiled portion 1 1 of the base 20 between the respective channels 32 of cells 30 that are adjacent to opposite sides of the vertical access chamber. An inspection means, for example a wheeled inspection car, is thereby able to pass smoothly across the inspection chamber, without becoming stuck due to a sudden step, which arises at the edge of the chamber in the absence of the base 20.

In use (as shown in Figures 1 and 6) the base 20 is engaged securely by the fixing means with a first and second cell 40 on opposite sides of the chamber. The protrusions 9 and corresponding recesses 33 resist upward loads on the base 20. The relatively long protrusions 3 of the fixing means that couple each section 10 of the base 20 together provides sufficient flexural strength in the central coupling of the base 20 to adequately resist upward pressure on the base 20. The fixing means also serves to counteract any buoyancy that the base 20 may have when submerged. The external surface of each section 10 is substantially continuous, which may for instance allow each section 10 to be filled with a ballast material such as sand. Such ballast may be used to enhance the ability of the base 20 to support the adjacent geotextile under pressure and prevent bulging of the base 20. In other embodiments, the platform may comprise perforations or openings. Similarly, other portions of the base 20 may comprise perforations or openings. Alternatively, the base may be formed from an open lattice, for example of substantially vertical baffles, webs or the like.

Although a base 20 with a sectional construction has been described it will be appreciated that, in other embodiments, the base 20 may comprise a unitary body. Similarly, although an example base 20 has been described that extends from one wall of the access chamber to the opposite wall of the chamber, this is not essential. In other embodiments, the base 20 may provide access to only a single wall. For example, each section 10 is an embodiment of the invention.

Although a profiled portion 1 1 has been described, this is not essential. Where the base 20 comprises a profiled portion 11 , it is not essential that it is trough shaped.

It will be appreciated that a wide range of modifications and alterations may be made to the arrangements described hereinbefore without departing from the scope of the invention, as defined in the appended claims.