GB2594981A

GB2594981A - Prefabricated permanent formwork structure

Info

Publication number: GB2594981A
Application number: GB2007138.7A
Authority: GB
Inventors: Olof Eagle Timothy; Hill Austin
Original assignee: Dutypoint Ltd
Current assignee: Dutypoint Ltd
Priority date: 2020-05-14
Filing date: 2020-05-14
Publication date: 2021-11-17
Anticipated expiration: 2040-05-14
Also published as: GB202007138D0; GB2594981B

Abstract

Prefabricated permanent formwork structure 10, for forming a pit, comprising a vertical side wall 16 around the periphery of a base 12 and having an outer layer 18 extending from the base and an inner layer 20, with a space between the inner and outer layers defining a first cavity, a floorpan spaced above the base and extending between the inner layer to define a chamber of the pit, the space between the floorpan and base defining a second cavity, wherein concrete can flow from the first cavity into the second cavity and when set the concrete and the formwork form a unitary wall and base of the pit. The first cavity may have a strengthening element 34 in the first cavity about a corner in the outer layer coupled to the outer layer on either side of the corner along its height. The side wall may be formed from a plurality of modular wall elements 28 each having an outer wall portion and an inner wall portion, and a plurality of modular corner elements 30 each having an outer wall portion and an inner wall portion. The formwork may be for manholes, draw pits, or inspection chambers.

Description

PREFABRICATED PERMANENT FORMWORK STRUCTURE

The present invention relates to a prefabricated permanent formwork structure for a pit, particularly for a draw pit.

BACKGROUND TO THE INVENTION

Pits, or subterranean chambers, are formed in the ground along a subterranean ducting route to facilitate access to the ducting. These are commonly known as draw pits, manholes or inspection chambers. They allow for installation, maintenance and monitoring of any utilities, such as piping and electrical cabling, carried within the ducting and other equipment used with utility ducting, such as connectors and valves.

Draw pits are typically installed at crossover points, changes in ducting route direction, or at regular distance intervals.

Draw pits, or other subterranean chambers, must be strong to resist the weight of structures above the chamber, and also hydrostatic pressure on the walls and base of the chamber in locations where the chamber extends below the water table. Such pits or chambers are therefore often constructed on site with reinforced concrete walls and a separately poured concrete base. This construction process requires excavation of a hole, and assembly of shuttering within the hole for receiving the concrete. This requires operatives to work in the excavation for extended periods to assemble the shuttering, posing health and safety risks. Shuttering assembly also increases the time required for construction. This is particularly inconvenient in certain locations, such as in roads and pathways.

A partial solution has been to provide precast concrete draw pits with or without precast bases. Precast concrete draw pits are cast off-site using moulds. A standard set of moulds may be used to save cost and may or may not include set apertures. Once cast, they are delivered to site and placed within the excavation. Precast concrete draw pits tend to be heavy which causes issues with transportation and fitting. Moreover, if standard moulds are used, there are limited options available and significant modification may need to be made on site.

Another partial solution has been to provide prefabricated plastic draw pits which are manufactured offsite and fitted in situ. These can be crate type draw pits which include a base or merely walls which are placed on concrete bases cast in the excavation. Plastic draw pits offer weight savings at a cost of strength when compared to precast concrete draw pits. The reduction in strength and weight limits the environments in which they can be used effectively because of structures above the chamber or hydrostatic forces. Furthermore, prefabricated plastic draw pits tend to be offered in set sizes because of cost of creating specific moulds.

A further partial solution has been to provide a set of modular wall elements, either plastic or precast concrete. The modular elements are delivered to the excavation and are used to construct the walls of the draw pit on top of a precast concrete base. The modular construction provides some flexibility in on-site construction; however, they tend to suffer from the draw backs of the material used.

Some of the partial solutions, particularly those which do not include bases, still require operatives to work in the excavation for extended periods to cast and construct the walls. Moreover, further work, which can be easily forgotten, has to be carried out to seal the constructed walls to the base. This can lead to unexpected flooding.

It is an object of the present invention to reduce or substantially obviate the aforementioned problems.

STATEMENT OF INVENTION

According to a first aspect of the present invention there is provided a prefabricated permanent formwork structure for forming a pit, the prefabricated permanent formwork structure comprising a base, a substantially vertical side wall disposed around the periphery of the base, the side wall having an outer layer extending from the base and an inner layer spaced from the outer layer, the space between the inner layer and outer layer defining a first cavity, a floorpan spaced above the base and extending between the inner layer of the side wall, the floorpan and inner layer defined a chamber of the pit, and the space between the floorpan and base defining a second cavity; and a strengthening element disposed in the first cavity about a corner formed in the outer layer, the strengthening element being coupled to the outer layer on either side of the corner along its height; wherein an opening is provided between the first cavity and second cavity enabling concrete to flow through the first cavity into the second cavity; when set, the concrete and the permanent formwork structure forming a unitary wall and base of the pit.

Advantageously, the prefabricated permanent formwork can be transported easily to site, lowered into a hole in the ground, positioned and filled with concrete. The corner strengthening allows the whole formwork to be filled in a single shot, without further support or strengthening.

According to a second aspect of the present invention there is provided a prefabricated permanent formwork structure for forming a pit, the prefabricated permanent formwork structure comprising a base, a substantially vertical side wall disposed around the periphery of the base, the side wall having an outer layer extending from the base and an inner layer spaced from the outer layer, the space between the inner layer and outer layer defining a first cavity, wherein the substantially vertical side wall is formed from a plurality of modular wall elements and a plurality of modular corner elements, each modular wall element modular element having outer wall forming a portion of the outer layer and an inner wall forming a portion of the inner layer, each modular comer element having an outer wall forming a portion of the outer layer and an inner wall forming a portion of the inner layer, and a floorpan spaced above the base and extending between the inner layer of the side wall, the floorpan being connected to the inner layer, the floorpan and inner layer defined a chamber of the pit, and the space between the floorpan and base defining a second cavity, wherein an opening is provided between the first cavity and second cavity enabling concrete to flow through the first cavity into the second cavity; when set, the concrete and the permanent formwork structure forming a unitary wall and base of the pit.

Advantageously, the prefabricated permanent formwork can be transported easily to site, lowered into a hole in the ground, positioned and filled with concrete. The modular construction of the side wall provides saves cost in manufacturing as the same modular elements may be used to create different sizes and shapes of draw pits.

The prefabricated permanent formwork is preferably made from plastics, which is both very lightweight and permanently waterproof. Fibreglass may be utilised in the base and/or the floorpan. Preferably, glass reinforced plastic may be utilised in the base and/or floorpan.

It is envisaged that the use of a unitary formwork, which can stay in the ground, will significantly speed up the construction of pits. Both the inner and outer layers are waterproof and sealed to the respective bases, preventing ground contamination from the draw pit, and into the draw pit. The draw pits are also sealed against rodents.

The use of concrete ensures that the completed draw pit is able to withstand loading from above.

Most draw pits have upper openings, the opening of the chamber, of up to around 750mm x 750mm, but may be smaller, for example, as small as 200mm x 200mm depending on the application. The depth of the draw pit chamber may also vary from 250mm to 1500mm. Other dimensions and depths are envisaged.

The height of the chamber, that is from the floorpan to an upper end of the side wall, may be approximately 0.8m to 1.8m, preferable lm. Other heights are envisaged.

The longitudinal length of the chamber may be between 200mm to 1500mm, perferably 300mm, 600mm or 1000mm. The transverse length of the chamber may be between 200mm to 1500mm, perferably 300mm, 600mm or 1000mm. Other lengths or envisaged.

The volume of the first cavity and second cavity may be in the range of approximately 0.3 rnA3 to 1 m^3, preferably at least 0.5 m^3.

The prefabricated permanent formwork structure may comprise structural support means to strengthen the side wall. The structural support means may include structural support elements disposed within the first cavity. The structural support means may include structural support elements disposed to the outer layer outside of the first cavity, i.e. disposed to a surface of the outer layer not forming the first cavity.

The structural support means may include further structural support elements disposed to the inner layer, i.e. disposed to a surface of the inner layer not forming the first cavity, in combination with structural support elements described above.

The structural support elements may be disposed horizontally, vertically and/or diagonally.

The structural support means may include structural support elements disposed within the second cavity.

The structural support elements may be rebar.

It is envisaged that no extra reinforcing need be provided for particular sizes of pit.

However, the provision of structural reinforcing can increase the range of pit sizes as it ensures that the side wall does not buckle due to the load applied by concrete during pouring.

The side wall may be formed from a plurality of modular elements. Each modular element may have an outer wall forming a portion of the outer layer and an inner wall forming a portion of the inner layer. The plurality of modular elements may comprise a plurality of modular wall elements and a plurality of modular corner elements.

Each modular corner element may comprise a strengthening element disposed between the inner wall and outer wall about a corner formed in the outer wall. The strengthening element may be coupled to the outer wall on either side of the corner along its height.

A further cavity may be defined between the strengthening element and the corner of the outer layer or outer wall.

The further cavity may be a non-concrete receiving cavity. That is to say the further cavity may not be fluidic communication with either the first or second cavities.

A plurality of strengthening elements may be disposed about each corner formed in the outer layer.

The strengthening element may be disposed closer to the outer layer than the inner layer.

A plurality of webs may be provided between the inner layer and outer layer. Each web of the plurality of webs may be connected to the inner layer and outer layer.

The strengthening element may extend between at least two webs.

Each modular wall element and modular corner element may include two coupling means. The coupling means may be provided by co-locating connectors.

Each modular wall element may comprise a plurality of webs extending between the inner wall and outer wall.

Each of the modular corner element may comprise two webs. The strengthening element may extend between each webs.

The floorpan may comprise a sump. The sump may be disposed to a corner formed in the inner layer.

The floorpan may comprise an upper surface having a fall to a corner formed in the inner layer.

The floorpan may be variably spaced from the base dependent upon the weight of the pit, or base of the pit, required. The variable spacing may be a predetermined distance. The predetermined distance may be based on the final weight of the pit, or base of the pit, required to resist buoyancy forces. The final weight is considered to be the weight once the poured concrete has set. The final weight may be based on the cavity volume.

For example, the greater the spacing the greater the cavity volume which in turn increases the final weight of the pit.

Advantageously, by spacing the floorpan by a predetermined distance during prefabrication of the permanent formwork structure it is possible to control the volume of concrete to be received in the cavity and the approximate final weight of the pit. This is particularly useful to address buoyancy forces arising from areas with high underground water levels. That is to say, the predetermined distance allows the prefabricated permanent formwork structure to be designed to form a pit which will resist buoyancy forces by being heavier.

A port may be formed between the outer layer and inner layer for providing a route for a utility within a conduit into the chamber of the pit. A plurality of ports may be provided. Each port of the plurality of ports may be symmetrically disposed.

By providing ports already formed in the prefabricated permanent formwork structure it is possible to decrease installation time on-site.

The port may include a port assembly. The port assembly may comprise a port liner extending between the inner structure and outer structure, the port liner sealing the concrete receiving cavity.

By providing port assemblies already formed in the prefabricated permanent formwork structure it is possible to ensure there is no subsequent leakage of concrete during pouring due to poor installation on-site.

The port assembly may include two wall abutment flanges, each wall abutment flange may be disposed an end of the port liner and have an surface which abuts either inner or outer layer in use. The port assembly may be a two-part port assembly. The first part assembly may comprise a first port liner and a first wall abutment flange and the second part assembly comprising a second port liner and a second wall abutment flange, the first port liner being received within the second port liner in use.

A two-part port assembly is particularly advantageous as it speeds up installation of the port assembly.

The port may comprise utility conduit engagement means for engaging and sealing the utility conduit to a portion of the port. There may be a utility conduit engagement means disposed at either end of the port or port assembly.

The modular corner elements may be movable, or removable, to provide access to the space between modular wall elements.

The plurality of modular corner elements may be substantially similar. The plurality of modular wall elements may be substantially similar.

The prefabricated permanent formwork structure may be a prefabricated draw pit permanent formwork structure The outer layer of the side wall and the base may be considered an outer structure of the prefabricated permanent formwork structure. The inner layer of the side wall and the floor pan may be considered an inner structure of the prefabricated permanent formwork structure. The first and second cavities may be considered together as a concrete receiving cavity.

According to a third aspect of the present invention there is provided a method of manufacturing a prefabricate draw pit, the method comprising the steps of providing a base and a plurality of modular elements for forming a wall, wherein each modular element for forming a wall includes an inner wall and an outer wall, constructing a substantially vertical side wall from the plurality of modular wall elements around the periphery of the base, wherein the inner wall and outer wall of each modular element define a portion of an inner layer of the substantially vertical side wall and an outer layer of the substantially vertical side wall respectively, the constructed side wall having the outer layer extending from the base and the inner layer spaced from the outer layer, the space between the inner layer and outer layer defining a first cavity for receiving concrete, providing a floorpan in a position spaced above the base and extending between the inner layer of the side wall, and sealing the periphery of the base to the outer layer of the side wall and the floorpan to the inner layer of the side wall.

The prefabricated permanent formwork structure in the third aspect of the present invention may be the prefabricated permanent formwork structure according to the second aspect.

The third aspect of the invention shares the advantages associated with the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which: Figure 1 shows a perspective schematic view of a prefabricated permanent formwork structure; Figure 2 shows a cross-sectional side schematic view of the prefabricated permanent formwork structure in Figure 1; Figure 3 shows a cross-sectional overhead schematic view of the prefabricated permanent formwork structure in Figure 1; and Figure 4 shows a perspective cross-sectional schematic view of the prefabricated permanent formwork in Figure 1 with port assemblies installed.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring firstly to Figures 1, 2, 3 and 4 a prefabricated permanent formwork structure is generally indicated at 10.

The prefabricated permanent fomiwork structure includes a base 12. The base 12 has a base portion which is polygonal in shape, preferably a quadrilateral, with an upper surface and a lower surface and a circumferential flange portion 14 extending orthogonally from the edge of the base portion. The base 12 is preferably made from a strong rigid lightweight material, such as a plastic, fibreglass, glass reinforced plastic, metal or similar material.

A side wall 16 is provided extending substantially vertical from the base 12 around its periphery. The side wall 16 has an outer layer 18 and an inner layer 20 defining a first cavity therebetween. The outer layer 18 and inner layer 20 are substantially concentric. The inner layer 20 has a smaller cross section than the outer layer 18. The side wall 16 is made from plastics and is waterproof. In the current embodiment, the side wall has a substantially quadrilateral square cross section, but in other embodiments the cross section may be different.

In the current embodiment, base 12 is coupled to the side wall 16 by attachment means such as adhesive and/or mechanical fastenings. The circumferential flange portion 14 lies against a portion of the outer layer 18 and may be used to couple the base 12 to the side wall 16. The side wall 16 is substantially sealed to the base 12 and/or the circumferential flange portion 14, preferably the circumferential flange portion 14 is sealed to the outer layer 18. In other embodiments, the side wall 16 may sit on the base 12.

A plurality of openings 22 are formed in the inner layer 20 close to the base 12. The plurality of openings provide a path for the concrete to flow out of the first cavity into another cavity, specifically the second cavity described further below. In other embodiments, the openings may be formed by the inner layer 20 having areas of shorter length than the outer layer 18 A plurality of webs 24 are provided between the outer layer 18 and inner layer 20. Each web 24 is a planar elongate element, having a first long edge and a second long edge.

The first long edge of each web 24 is joined to the outer layer 18 and the second long edge of each web is joined to the inner layer 20. Each web 24 therefore bridges the cavity between the inner and outer layers 18, 20. Each web 24 includes a plurality of apertures 26 for allowing concrete to flow through the webs during pouring.

At the upper end of the side wall 16, the cavity is open for providing an entry point for concrete.

In the current embodiment, a plurality of modular elements, connected together during manufacturing of the prefabricated permanent formwork structure, form the side wall 16. The plurality of modular elements include a plurality of wall elements 28 and a plurality of corner elements 30. Each of the modular elements has an outer wall spaced from an inner wall by a number of the webs 26, the outer wall forming part of the outer layer 18 and the inner wall forming part of the inner layer 20.

The plurality of modular elements are slidably engaged during manufacturing. Each modular element is joined to another modular element through co-locating connectors 32 disposed on both the inner walls and outer walls.

In some embodiments, the corner elements 30 may be slidably disengaged from the wall elements 28 to allow rebar, or other such strengthening components, to be added horizontally through the side wall 16. For example, a length of rebar could be added to two adjacent sides of the prefabricated permanent formwork structure by slidably moving one modular corner element and inserting said rebar through the apertures 26 in the webs 24.

A corner strengthening element 34 is provided in each modular corner element 30. The corner strengthening element 34 forms a corner cavity along the height of the corner element. In the current embodiment, the corner cavity is not in fluidic communication with the other cavities of the prefabricated permanent formwork structure 10, that is the say that the corner cavity will not receive concrete poured into the first cavity.

The corner strengthening element 34 may be unitarily formed with the with side wall 16, preferably the corner module 30 In the current embodiment, the corner strengthening element 34 is disposed closer to the outer wall forming the outer layer 18 than the inner wall forming the inner layer 20. The corner strengthening element 34 extends from a first web 24a in the modular corner element 30 to a second web 24b in the modular corner element 30. The corner strengthening element 34 generally follows the outer layer 18 in a parallel manner but includes a curved section instead of a sharp corner.

The corner strengthening element is additionally connected to the outer wall by means of ribs extending therebetween.

In other embodiments not using the modular elements, the same or similar corner strengthening element may be provided in a corner formed by the outer layer of the prefabricated permanent formwork structure.

The prefabricated permanent formwork structure 10 includes a floorpan 36. The floorpan 36 is defined by an edge which follows the profile of the inner layer 20 of the side wall 16. The inner layer 20 and floorpan 36 define the inner surface and form a chamber. The floorpan 36 is disposed within the side wall 16.

The floorpan 36 includes a floor portion 38, a circumferential flange portion 40 and a sump portion 42. The floor portion 38 is spaced above the base 12 and defines a second cavity for receiving concrete from the first cavity. A fall is provided in the floor portion 38 to allow fluids to run into the sump portion 42.

The sump portion 42 includes a sump base and a sump wall extending from the sump base to the floor portion 38. The sump portion 42 is formed in close proximity to the inner layer 20, preferably in a corner formed by the inner layer 20. In the current embodiment, the sump portion extends towards the base 12 such that the sump base is in close proximity to the base 12, but in other embodiments the sump base may be spaced above the base 12.

The floorpan 36, i.e. the floor portion 38 and/or the sump base, may be spaced from the base 12 by a predetermined distance. The predetermined distance may be determined based on the required weight of the pit to resist the expected buoyancy forces within the site area.

The circumferential flange portion 40 extends around the edge of the floorpan 36 and is disposed vertically, in use, thereto. The circumferential flange portion 40 lies against and is sealed to the inner layer 20. In some embodiments, part of the circumferential flange portion 40 may be part of a portion of the sump wall.

The floorpan is fluid-fight. The floorpan is substantially sealed to the inner layer 20, allowing no fluid flow path from the second cavity to the chamber defined by the floorpan 36 and inner layer 20.

The floorpan 36 is preferably made from plastics, more preferably glass reinforced plastic, but may be made from other materials such as metals.

A plurality of ports 44 are formed in the side wall 16 during manufacture. In the current embodiment, a first set of four ports 44 is disposed in a first side of the side wall 16 and a second set of four ports 44 is disposed in a second side of the side wall 16 opposite the first side of the side wall 16. Each port 44 is substantially inline with another port disposed on the opposite side of the side wall 16, that is to say the centre of the ports sit on the same axis. Each port 44 is substantially the same size with the same diameters.

In other embodiments, the number, location and size of ports 44 may vary, for example, a single port may be provided in one side of the side wall 16, or two ports may be provided in adjacent sides of the side wall 16. In yet other embodiments, the port(s) may be formed on-site instead of during manufacture.

As shown in Figure 4, each port 44 may include a port assembly 46. This may be fitted during manufacture, i.e. before delivery to site, or may be fitted on-site, i.e. after manufacturing before the concrete has been poured. The port assembly 46 includes a liner 48 formed from a cylindrical portion, flange portions 50 disposed at either end of the liner 48 and conduit engaging means 52 disposed at either end of the liner 48.

The port assemblies 46 may be considered conduit coupling assemblies and provide a secure and sealed connection between the conduit and pit.

The liner 48 extends through the first cavity formed by the outer layer 18 and inner layer 20. The flange portions 50 extend orthogonally from the edge of each end of the liner 48 and sit against the surfaces of the inner and outer layers 18, 20. The port assembly 46 seals the port 44 in which it is located such that when concrete is poured it flows around the liner 48 and not out through the port 44.

The conduit engaging means 52 are provided at each end of the liner. In the current embodiment, the conduit engaging means also provides a sealing surface for substantial abutment, in use, to a conduit.

In some embodiments, the port assembly 46 is a two-piece assembly. Each piece of the two-piece conduit coupling assembly comprises a liner, a flange portion extending orthogonally from an end of the liner and a conduit engaging means disposed to the end of the cylindrical portion, the liners each having different diameters such that one fits inside the other during fitment.

In some embodiments, particularly those of larger dimensions, A structural support means (not shown in the figures) may be included in the prefabricated permanent formwork structure 10. These will tend to be included when required, for example, when the desired pit size would significantly increase the loading from concrete on the side wall 16. The structural support means reinforces the side wall 16 to help prevent buckling.

The structural support means include support elements (not shown) to reinforce the side wall 16.

The support elements (not shown) may be provided in the first cavity. The elements may extend through the first cavity. The elements may be horizontal and/or vertical.

The support elements (not shown) may be provided in the second cavity. The elements may extend through the second cavity. The elements may be a lattice formed by a plurality of elements or a plurality of parallel elements.

The support elements (not shown) may be provided outside the first cavity on the extemal surface, i.e. the outer layer 18, of the prefabricated permanent formwork structure 10.

Support elements may be provided in both the first cavity and outside the first cavity on the external surface.

The structural support elements may be horizontally disposed and/or vertically disposed with respect to the prefabricated permanent formwork structure 10. For example, horizontal structural support elements may encircle the side wall 16, or if disposed in the first cavity encircle the inner layer. Vertical structural support may be disposed along the outer layer 18, either in the first cavity or outside it, from the base 12 to the upper region of the side wall 16.

The structural support elements may be diagonally disposed.

Further support elements (not shown) may be provided in combination with those described above. The further support elements may be disposed in the chamber against the inner layer 20. The further support elements disposed in the chamber may be vertically, horizontally, or diagonally disposed. The further support elements are coupled to either or both of the support elements disposed in the first cavity and/or the support elements disposed outside the first cavity on the external surface.

A method of manufacturing a prefabricated permanent formwork structure 10, such as those shown in figures 1 to 4, will now be described.

A side wall 16 having an outer layer 18 and an inner layer 20 is constructed from a plurality of modular elements. The size and shape of the pit, or underground structure, dictates the number and type of modular elements required. In the current embodiment, four modular corner elements 30 and sixteen modular wall elements 28 form the side wall 16. The four modular corner elements 30 and sixteen modular wall elements 28 are all slidably engaged using the co-locating connectors 32 to form a side wall 16 with a substantially square cross section.

Prior to construction of the wall, an opening 22 is formed in one wall to one end of each modular wall element, preferably the opening is formed in a wall which will be the inner wall and the end will be the lower end once the side wall is constructed. In other embodiments, the openings are formed once the side wall 16 has been constructed.

The base 12 is placed on the constructed side wall 16, with the openings 22 being disposed in proximity to the base. The base either being formed once the constructed side wall 16 has been placed or pre-formed to the desired dimensions. The flange portion 14 is sealed to the outer layer 18 of the side wall 16.

The floorpan 36 is preformed to the desired dimensions of the chamber. Once formed, it is positioned within the side wall 16 so the circumferential flange portion 40 abuts against the inner layer 20 and the underside of the floor portion is above the openings 22. The floorpan 36 is spaced by a predetermined distance from the base, or the end of the side wall 16 where the base will be located. The flange portion 40 is substantially sealed against the inner layer 20, for example by using a sealant.

The required number of ports 44 are formed in the side wall 16. A port assembly 46 may be installed once the ports 44 have been formed. Any ports made, but not required, may be filled with a bung, for example, of rubber or plastics.

Once manufactured the prefabricated permanent formwork structure 10 may be delivered to a construction site.

A method of constructing a pit using the prefabricated permanent formwork structure 10 will now be described.

The prefabricated permanent formwork structure 10, such as those in Figures 1 to 4, is manufactured off-site and transported to the installation site, typically by lorry.

At the installation site, a hole is excavated in the ground, typically to a depth equal to or greater than the height of the prefabricated permanent formwork structure 10. The hole may be finished with gravel or sand bulk. The prefabricated permanent formwork structure 10 is placed in the hole. Concrete is introduced into an opening in the side wall 16 of the prefabricated permanent formwork structure 10. The concrete flows into the first cavity and, as it flows, it will move into the second cavity. While the concrete flows, the prefabricated permanent formwork structure 10 may be mechanically agitated to further liquefy the concrete and ease flow. The volume of required concrete for a single prefabricated permanent formwork structure 10 will be less than the standard volumes offered by concrete mixer trucks. This allows for a single pour to form a single prefabricated permanent formwork structure. In some embodiments, a single standard concrete mixer truck may provide enough concrete to form multiple pits using multiple prefabricated permanent formwork structures.

Prior to the excavation being re-filled, conduits are inserted through ports 44, preferably partially through port assembly 46. Conduits may be inserted, at any stage, for example it may be before or after pouring concrete.

The excavation is re-filled around the prefabricated permanent formwork structure 10 and conduits, either with excavated material or gravel or sand bulk. Re-filling may take place during, before or after pouring of the concrete.

The use of the prefabricated permanent formwork structure provides reduced site preparation time, the use of less concrete and a highly predictable amount of concrete for a given size of chamber, a quicker installation time, a stronger integral structure and reduced risk of buoyancy problems arising, compared with current methods of construction of subterranean pumping chambers.

The embodiments described above are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims

CLAIMS1 A prefabricated permanent formwork structure for forming a pit, the prefabricated permanent formwork structure comprising: a base; a substantially vertical side wall disposed around the periphery of the base, the side wall having an outer layer extending from the base and an inner layer spaced from the outer layer, the space between the inner layer and outer layer defining a first cavity; a floorpan spaced above the base and extending between the inner layer of the side wall, the floorpan and inner layer defined a chamber of the pit, and the space between the floorpan and base defining a second cavity; and a strengthening element disposed in the first cavity about a corner formed in the outer layer, the strengthening element being coupled to the outer layer on either side of the corner along its height; wherein an opening is provided between the first cavity and second cavity enabling concrete to flow through the first cavity into the second cavity; when set, the concrete and the permanent formwork structure forming a unitary wall and base of the pit.
2. A prefabricated permanent formwork as claimed in claim 1, in which a further cavity is defined between the strengthening element and the corner of the outer layer.
3. A prefabricated permanent formwork structure as claimed in claim 2, in which the further cavity is a non-concrete receiving cavity.
4. A prefabricated permanent formwork structure as claimed in any of the preceding claims, in which a plurality of strengthening elements are disposed about each corner formed in the outer layer.
5. A prefabricated permanent formwork structure as claimed in any of the preceding claims, in which the strengthening element is disposed closer to the outer layer than the inner layer.
6. A prefabricated permanent formwork structure as claimed in any of the preceding claims, in which a plurality of webs are provided between the inner layer and outer layer, each web being connected to the inner layer and outer layer.
7. A prefabricated permanent formwork structure as claimed in claim 6, in which the strengthening element extends between at least two webs.
8. A prefabricated permanent formwork structure as claimed in any of the preceding claims, in which the substantially vertical side wall is formed from a plurality of modular elements, each modular element having an outer wall forming a portion of the outer layer and an inner wall forming a portion of the inner layer
9. A prefabricated permanent formwork structure as claimed in claim 8, in which the plurality of modular elements comprises a plurality of modular wall elements and a plurality of modular corner elements.
10. A prefabricated permanent formwork structure for forming a pit, the prefabricated permanent formwork structure comprising: a base; a substantially vertical side wall disposed around the periphery of the base, the side wall having an outer layer extending from the base and an inner layer spaced from the outer layer, the space between the inner layer and outer layer defining a first cavity, wherein the substantially vertical side wall is formed from a plurality of modular wall elements and a plurality of modular corner elements, each modular wall element modular element having outer wall forming a portion of the outer layer and an inner wall forming a portion of the inner layer, each modular corner element having an outer wall forming a portion of the outer layer and an inner wall forming a portion of the inner layer; and a floorpan spaced above the base and extending between the inner layer of the side wall, the floorpan being connected to the inner layer, the floorpan and inner layer defined a chamber of the pit, and the space between the floorpan and base defining a second cavity; wherein an opening is provided between the first cavity and second cavity enabling concrete to flow through the first cavity into the second cavity; when set, the concrete and the permanent formwork structure forming a unitary wall and base of the pit.
11. A prefabricated permanent formwork structure as claimed in claim 10, in which each of the modular wall elements and the modular corner element includes two coupling means disposed at either end of the modular element.
12. A prefabricated permanent formwork structure as claimed in either claim 10 or claim 11, in which each modular wall elements comprises a plurality of webs extending between the inner wall and outer wall.
13. A prefabricated permanent formwork structure as claimed in any of claims 10 to 12, in which each modular corner element comprises a strengthening element disposed between the inner wall and outer wall about a corner formed in the outer wall, the strengthening element being coupled to the outer wall on either side of the corner along its height.
14. A prefabricated permanent formwork structure as claimed in claim 13, in which each of the modular corner elements comprises two webs and the strengthening element extends between said webs.
15. A prefabricated permanent formwork structure as claimed in any of the preceding claims, in which the floorpan comprises a sump.
16. A prefabricated permanent formwork structure claimed in any of the preceding claims, in which a port is formed between the outer layer and inner layer for providing a route for a utility within a conduit into the chamber of the pit.
17. A prefabricated permanent formwork structure as claimed in claim 16, in which the port includes a port assembly comprising a port liner extending between the inner structure and outer structure, the port liner sealing the concrete receiving cavity.
18. A prefabricated permanent formwork structure as claimed in claim 17, in which the port assembly includes two wall abutment flanges, each wall abutment flange is disposed an end of the port liner, each wall abutment flange having an surface which abuts either the substantially vertical outer wall or the substantially vertical inner wall in use.
19. A prefabricated permanent formwork structure as claimed in either claim 17 or claim 18, in which the port assembly is a two-part port assembly, the first part assembly comprising a first port liner and a first wall abutment flange and the second part assembly comprising a second port liner and a second wall abutment flange, the first port liner being received within the second port liner in use.
20. A prefabricated permanent formwork structure as claimed in any of claims 16 to 19, in which the port comprises utility conduit engagement means for engaging and sealing the utility conduit to a portion of the port.
21. A prefabricated permanent formwork structure as claimed in claim 20, in which the port comprises two utility conduit engagement means disposed at either end.
22. A prefabricated permanent formwork structure as claimed in any of claims 16 to 21, in which a plurality of ports are provided.
23. A prefabricated permanent formwork structure as claimed in claim 22, in which the plurality of ports are symmetrically disposed.
24. A method of manufacturing a prefabricate draw pit, the method comprising: providing a base and a plurality of modular elements for forming a wall, wherein each modular element for forming a wall includes an inner wall and an outer wall; constructing a substantially vertical side wall from the plurality of modular wall elements around the periphery of the base, wherein the inner wall and outer wall of each modular element define a portion of an inner layer of the substantially vertical side wall and an outer layer of the substantially vertical side wall respectively, the constructed side wall having the outer layer extending from the base and the inner layer spaced from the outer layer, the space between the inner layer and outer layer defining a first cavity for receiving concrete; providing a floorpan in a position spaced above the base and extending between the inner layer of the side wall; and sealing the periphery of the base to the outer layer of the side wall and the floorpan to the inner layer of the side wall.