AU2007203032B2 - Void forming and suspension system for a structural concrete slab - Google Patents

Description

AUSTRALIA Patents Act 1990 SuperSlab Tech Pty Ltd COMPLETE SPECIFICATION Invention Title: Void forming and suspension system for a structural concrete slab The invention is described in the following statement: 2 Field of the Invention The present invention relates to a void forming and suspension system for a building/construction and in particular to an apparatus and method for arranging a void 5 forming system for a structural concrete slab of a building/construction such that the building/construction is suspended above the ground and is able to withstand changes in the level of the underlying ground support. Background Art 10 Buildings, such as residential homes or commercial buildings generally rely upon a solid footing upon which to support the considerable weight of the structure. The footing generally provides a flat and level base upon which the building is supported, with the base being in communication with the underlying ground surface. 15 Many modem buildings are built upon a flat, concrete slab that provides both a base for the structure as well as the bottom floor for the building. The concrete is typically placed over a prepared ground surface and is shaped to the desired dimensions to form the footings of the building. This may be achieved through laying pre-cast components of concrete, or through pouring wet concrete into a mould and allowing the concrete to 20 set in position. Typically such concrete slabs consist of a system of structurally engineered concrete beams and platforms which together form what is collectively referred to as a slab. A common problem with concrete slabs supported on a ground surface is that 25 large areas of land available for residential development have soil profiles with high clay content. Such soil profiles are not considered stable, as the surface level of the soil can change as the moisture content of the soil changes. This is due to the volume of the soil being directly dependent upon the soil's moisture content. In particular, as the moisture content of the soil increases, the volume of the soil increases, resulting in the 30 surface level of the soil changing. Such a phenomena causes uplifting of a concrete slab that may be supported on the surface of the soil. This uplifting of soil is commonly referred to as the concrete slab being considered to have "heaved". With such soils, when their moisture content reduces, the soil reduces (shrinks or settles) in volume. 35 c:\nrportbl\syddocs\sxy\507975 1.doc 3 Heaving and settling of soil due to moisture content change therein tends to occur at varying rates across the surface plane of the soil. This variance is termed differential movement. As a concrete slab being supported on the surface of such soil will heave and settle accordingly, this differential movement can result in a concrete 5 slab supported on such a soil surface experiencing a variety of loading forces over the life of a building construction. The factors contributing to variations in soil moisture content which generates such concrete slab movements are various and complex. Some common factors 10 include: natural moisture variations due to seasonal considerations such as wet and dry seasons; soil drying due to trees growing or being introduced into a zone of influence of the soil; soil wetting due to the removal of trees and other related flora such that previously drier soils re-hydrate resulting in the phenomena of "rebound" occurring in the soil; garden watering (or lack thereof) by owners/occupiers of a subject site; or a 15 combination of any of these sources. Any or all of these factors can significantly alter the soil moisture content resulting in a concrete slab deflecting or becoming damaged. This can result in damage to the super structure supported on the slab, such as a residential dwelling or the like, requiring time consuming and expensive corrective action. 20 Typically, soil moisture content is not consistent and may vary across regions, resulting in further differential shrinking/swelling of the soil. Historically, construction areas have been chosen to avoid soils having a high clay content to reduce the occurrence of soil heave. However, as populations increase, more and more land 25 having such high clay content soil is being used for development. In this regard, areas which have previously been avoided such as beach/river frontages; steeply sloping hinterland areas beyond the coastlines; and the grazing plains (some of which are flood prone) which surround most cities and towns are increasingly becoming the site for a variety of constructions and developments that have previously been considered 30 unsuitable for such a soil base. A number of methods have been proposed to address the soil heave phenomena and to prevent highly expansive soils from damaging the structures supported on such soil. Typically, these methods are employed prior to construction and rely upon 35 chemical treatment of the soil; engineered fill and compaction of the soil; and/or the c:\nrportbl\syddocs\sxY\507975 1.doc 4 formation of a void space beneath the concrete slab to accommodate expansion of the soil into these voids without damaging the structure. Most void systems previously proposed have been arranged such that the beams 5 of the concrete slab are placed between the void forming elements, the beam regions of the concrete slab do not have a void space and hence are more susceptible to soil heave. 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 10 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. 15 Summary of the Invention 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 20 any other element, integer or step, or group of elements, integers or steps. In a first aspect, the present invention provides a system for creating a suspended structural slab, comprising: at least one pile, the or each pile having a first end and a second end, the first 25 end adapted to be embedded into a ground surface to be in contact with load bearing ground beneath said ground surface and the second end arranged to extend above said ground surface; and a plurality of void forming element, comprising first void forming elements and second void forming elements, the void forming elements arranged to substantially 30 cover a ground surface upon which the structural slab is to be placed, the void forming elements each having a first surface adapted to receive at least a portion of the concrete slab and a second surface adapted to contact the ground surface, a distance from the first surface to the second surface of the or each second void forming element being less than a distance from the first surface to the second surface of the or each first void 35 forming element, the first and second void forming elements being laid adjacent one another to form one or more recesses in which one or more beam portions of the slab 5 are formed, wherein the void forming elements are sacrificial, being formed to be capable of withstanding forces placed thereupon during formation of the structural concrete slab and being arranged to collapse under a force of ground movement occurring after the slab is formed; 5 wherein said structural slab is supported above said ground surface by said at least one pile such that said void forming elements are positioned between the structural slab and the ground surface, and wherein the second end of said at least one pile extends through at least one of the second void forming elements and aligns with a respective one of the one or more beam portions of the slab. 10 According to a second aspect, the present invention provides a method of forming a suspended structural slab, comprising: installing at least one pile, the or each pile having a first end and a second end, the first end adapted to be embedded into a ground surface to be in contact with load 15 bearing ground beneath said ground surface, and the second end arranged to extend above said ground surface; installing a plurality of void forming elements, comprising first void forming elements and second void forming elements, the void forming elements arranged to substantially cover a ground surface upon which the structural slab is to be placed, the 20 void forming elements each having a first surface adapted to receive at least a portion of the concrete slab and a second surface adapted to contact the ground surface, a distance from the first surface to the second surface of the or each second void forming element being less than a distance from the first surface to the second surface of the or each first void forming element, the first and second void forming elements being laid 25 adjacent one another to form one or more recesses in which one or more beam portions of the slab are formed, wherein the second end of said at least one pile extends through at least one of the second void forming elements and aligns with a respective one of the one or more beam portions of the slab ; wherein the void forming elements are sacrificial, being formed to be capable of 30 withstanding forces placed thereupon during formation of the structural concrete slab and being arranged to collapse under a force of ground movement occurring after the slab is formed; and forming the structural slab whereby the structural slab is supported above the ground surface by the at least one pile such that the void forming elements are 35 positioned between the structural slab and the ground surface.

6 Preferably the at least one pile engages the structural slab. There is also disclosed herein apparatus for creating a void beneath a suspended structural concrete slab, comprising: 5 a plurality of first bodies each having a first surface adapted to receive at least a portion of the concrete slab and a second surface adapted to contact a ground surface, each first body having at least one cavity provided therein, the at least one cavity extending at least partially from said second surface toward said first surface; a plurality of second bodies each having a first surface adapted to receive at 10 least a portion of the concrete slab and a second surface adapted to contact a ground surface, each second body having at least one cavity provided therein, the at least one cavity extending at least partially from said second surface toward said first surface and a distance from the first surface to the second surface of the second body being less than a distance from the first surface to the second surface of the first body, the first 15 and second bodies being configured to be laid adjacent one another to form a recess in which a beam portion of the slab is formed; wherein the first and second bodies are sacrificial, being formed to be capable of withstanding forces placed upon them during formation of the structural concrete slab and being arranged to collapse under a force of ground movement occurring after the 20 slab is formed; and a plurality of piles, each of which, when in use, anchored beneath the ground surface and extending through one of the second bodies and into structural slab to support the structural slab above the ground surface after the slab has cured.. 25 Also disclosed herein is a structural slab formed over the apparatus of the second aspect above, wherein the structural slab is a concrete slab consisting of a plurality of platforms formed over the first bodies and beams formed over the second bodies and in which each of the piles extends through one of the second bodies and into the structural slab. 30 Engagement between the at least one pile and the slab will typically be in the form of a wider portion located in the upper part of the pile such that the concrete formed over the pile surrounds and engages the widened portion and a narrower portion beneath it. Preferably the wider portion comprises a rod or bar extending horizontally 35 through a hole in a vertical portion of the pile located above the lower extremity of the 6a structural slab. Preferably also the piles are located in beam portions of the structural slab and the rod or bar extends longitudinally of the beam portions. In another embodiment, the at least one pile is a screw pier foundation pile. The second end of the at least one pile may be raised above the ground surface to a 5 predetermined height. Where more than one pile is employed, the piles may be arranged in a grid to form a consistent and level loading point above the ground surface. The structural slab may be a concrete slab consisting of a plurality of platforms 10 and beams. The structural slab may be supported above the ground surface by the beams receiving the second end of the at least one pile. The depth to which the second ends of the piles are received by the beams is determined by the bearing capacity of a building/construction supported by the slab. In this regard, the installed height of the pile above the ground enables the second end of the piles to penetrate to a desired depth 15 into the beams of the structural slab. In this regard, the piles are subsequently structurally connected to the concrete slab within the beam. Brief Description of the Drawings 7 By way of example only, the invention is now described with reference to the accompanying drawings: Fig. I is a perspective view of the void forming and suspension arrangement of 5 an embodiment of the present invention; Fig. 2 is a cross sectional side view of the void forming and suspension arrangement of Fig. 1, supporting a concrete slab; 10 Fig. 3 is a plan view of the slab of Fig. 2; Fig. 4 is a perspective view of an embodiment of a spacer void forming element of the void forming arrangement of Fig. 1; Fig. 5 shows a plan view of a second embodiment of a spacer void forming 15 element of the void forming arrangement of Fig. 1; Fig. 6 shows an elevation view of the embodiment of a spacer void forming element of Fig. 5 with a first height; Fig. 7 shows an elevation view of the embodiment of a spacer void forming element of Fig. 5 with a second height; and 20 Fig. 8 shows (a) plan, (b) elevation and (c) end elevation views of an embodiment of a shallow void forming element of the void forming arrangement of Fig. 1. Detailed Description of an Exemplary Embodiment of the Present Invention 25 A void forming and suspension system 10 in accordance with one embodiment of the present invention is shown in Fig. 1. The void forming and suspension system 10 generally consists of a plurality of 30 void forming elements 12, 16 arranged to extend over a ground surface 22 upon which the building is to be constructed. As will become more apparent in the following description, the void forming system 10 functions to act as an interface between the ground surface 22 and a concrete slab 20 of the building to be constructed. 35 Prior to positioning the void forming system as shown in Fig. 1, a plurality of load bearing compression and tension piles 14 are driven, drilled, jacked, tied or c:\nrportbl\syddocs\sxy\507975_1.doc 8 otherwise fixed into the deeper soil or rock of high bearing capacity to provide stability to the structure which is to be built. The piles 14 may be in a variety of forms, such as screw piles which communicate directly with the load bearing ground beneath the ground surface 22. The piles 14 perform in both tension and compression and are 5 raised above the ground surface to a predetermined height. In this regard, the piles are arranged in a grid to form a consistent and level loading region above ground upon which to found the beams of the finished concrete slab. The depth to which the piles 14 are installed is determined in relation to where 10 they will achieve the engineered and designed bearing capacity for the said building/construction. The installed height of each pile 14 above ground will allow the end of the piles 14 to penetrate to an engineered depth into the finished beams of the concrete slab above. The piles 14 are subsequently structurally connected to the concrete slab within the beam. It will be appreciated that the size and shape of the slab 15 can be readily adapted to suit the size and shape of the building to be constructed. As such, the placement location and number of piles 14 utilised is variable, though based on the relevant modular system engineered design for that project. Following the positioning of the piles 14 in accordance with the design of the 20 structure to be built, void forming elements 12 are positioned over and around the piles 14. The void forming elements 12 are in the form of elongate shallow members which are designed to effectively absorb extreme ground movement as well as allow the piles 14 to pass therethrough. As the void forming elements 12 extend along the piles 14, they are typically arranged in parallel rows in accordance with the specific design of 25 the structure to be built. It will be appreciated that the arrangement as shown in Figs. I to 3 represents a substantially rectangular footing wherein the piles are regularly spaced and positioned in rows and hence the void forming elements 12 extend in rows, however it will be appreciated that the footing structure may vary in accordance with the specific design of the building to be constructed. 30 Between each of the rows of shallow void forming elements 12, there are provided spacer void forming elements 16. The spacer void forming elements 16 are in the form of elongate void former structures, such as but not limited to, foam plastic elements as discussed above. The spacer void forming elements 16 extend between 35 each of the shallow void forming elements 12 as shown to provide an additional surface, elevated from the ground surface, offering a holistic void forming body upon c:\nrportbl\syddocs\sxy\507975 1.doc 9 which the concrete slab (platform regions and beams) can be placed. In this regard, the platform and beam regions of the concrete slab are supported upon the spacer void forming elements 16 and the shallow void forming elements 12 and are suspended from the ground surface upon placement. 5 It will be appreciated that prior to setting and positioning the void forming system 10 of Fig. I, the area upon which the void forming system is placed is prepared. Such preparation typically comprises excavating the site to form a level surface upon which the concrete is to be placed. Trenches can also be dug to position deep edge 10 beams and the like to define the perimeter of the foundation to enable the piles 14 to be positioned as discussed above. A vapour barrier/retarder may also be laid upon the soil surface, below the shallow void forming elements 12 and spacer void forming elements 16 to prevent the transmission of moisture upward through the slab from the soil below. Once this has been done the void forming elements 12, 16 can be positioned as shown 15 in Fig. I, for placement of the concrete. As the spacer void forming elements 16 and the shallow void forming elements 12 are made from a foam or polystyrene material, such as a closed cell expanded polystyrene (EPS), they can be readily shaped or cut to fit around plumbing pipes and 20 the like, through the use of a hand or power saw. Generally, this may not be necessary as the spacer void forming elements 16 and shallow void forming elements 12 can be custom made and delivered to the building site to suit the building plans provided. It will be appreciated that whilst the void forming elements 12, 16 of one embodiment of the invention are made from a foam or polystyrene material, the void forming elements 25 may be made in a variety of shapes and from a variety of materials to enable the void forming system 10 to perform its function, as will be appreciated below. Prior to placement of the concrete slab 20, reinforcing mesh and bar 18 (as shown in Fig. 2) may be placed over the void forming elements 12, 16 to further 30 reinforce the slab 20. Such a mesh material may be particularly applicable if the concrete is to be poured over the void forming elements 12, 16 to form the slab 20. As is clearly evident in Fig. 2, the slab 20 is fully supported above the surface of the soil 22 by the void forming elements 12, 16 and piles 14, thereby completely isolating the slab 20 from the ground soil 22. In this regard, following setting/positioning of the slab 35 20 and construction of a building over the slab 20, the slab is suspended above the ground surface 22, with the load of the structure being supported by the piles 14 which c:\nrportbl\syddocs\sxy\507975_1.doc 10 are in turn supported by the deeper soil or rock of high bearing capacity. It will be appreciated that in this arrangement, the piles 14 have a direct/positive engagement with the slab 20 and the ground, thereby offering load bearing capacity for the concrete slab in both tension and compression. The engagement will typically be in the form of 5 a wider portion located in the upper part of the pile such that the concrete formed over the pile surrounds and engages the wider portion and a narrower portion beneath it. Preferably the wider portion comprises a rod or bar 15 extending horizontally through a hole in a vertical portion of the pile located above the lower extremity of the structural slab. Preferably also the piles are located in beam portions 13 of the structural slab and 10 the rod or bar 15 extends longitudinally of the beam portions. Fig. 3 shows a plan view of one such slab arrangement 20 where the shallow void forming elements 12 are arranged to form two internal parallel beams which extend the length of the slab 20. Perimeter beams 24 are also formed by placement of 15 shallow void forming elements 12 about the perimeter of the site such that the parallel beams work in conjunction with the perimeter beams 24 to form a grid arrangement upon which the slab 20 is supported, separate from the ground surface. It will be appreciated that the size and shape of the slab 20 can be readily adapted to suit the size and shape of the building to be constructed. As such, the void forming and suspension 20 system 10 is a modular arrangement that can be easily assembled to accommodate a variety of sized and shaped slabs. In this regard, a variety of sizes are provided in both the void forming elements 12, 16 to accommodate differing dimensions of the slab design and positions and lengths of the piles 14 to accommodate varying depths of soil penetrated to achieve the appropriate bearing capacities required. 25 Referring to Fig. 4, one embodiment of the structure of the spacer void forming element 16 is shown. The void forming element 16 is in the form of a box-like element having four vertical side surfaces 26 and a top surface 28. The surfaces 26, 28 of the void forming element 16 are configured to interface with the concrete slab 20 in the 30 manner as shown in Fig. 2. The underside 30 of the void forming element 16, namely the side which is supported on the ground surface 22, is open thereby providing a cavity within the void forming element 16. The cavity is defined by the inner walls of the surfaces 26, 28, and in the embodiment as shown in Fig. 4, represents a substantially rectangular space. 35 c:\nrportbl\syddocs\sxy\507975 1.doc I1 A plurality of strut members 32 are provided within the cavity and extend from the inner surface of the top surface 28 to the ground surface 22 when the void forming element 16 is positioned thereon for use. The strut members 32 support the top surface 28 of the void forming element 16 along its length such that the top surface 28 is able 5 to support the load of the reinforced concrete during placement of the slab 20, as well as the load of workers or machinery used during the placement process. In this regard, the shape of the struts 32 and their location within the cavity defines the strength of the void forming element. It will be appreciated that the arrangement and shape of the struts 32 can be altered in accordance to the specific construction. Such an 10 arrangement of struts 32 provides structural integrity and enables the void forming elements 12, 16 to carry higher loads without failing or disintegrating during the formation of the structural slab 20. However this configuration of walls and individual struts also provides an ability to cope with any soil heave that may occur over time as the struts and walls will collapse under pressure such that movement is not transmitted 15 to the slab. Whilst not shown, it will be appreciated that the shallow void forming elements 12 may also be constructed in the same manner as the spacer void forming element 16 of Fig. 4. It will be appreciated that the structure and material of the void forming 20 elements 12, 16 ensure that they compress under load during the construction process without deforming or failing, and also ensures that they act as sacrificial members in the event of soil heave. In this regard, in the event of soil expansion in the direction of arrows A of Fig. 2, the soil is able to expand into the cavities of the void forming elements 12, 16, where appropriate. As the void forming elements 12, 16 and the struts 25 32 are made from a foam material such as polystyrene, in the event of soil expanding into the cavities, the struts 32 can break away from the body of the void forming element 12, 16 to further accommodate the expanding soil. As discussed above, the void forming elements 12, 16 create a moisture, an 30 acoustic and a thermal barrier between the slab 20 and the ground 22, as well as protect the slab 20 from cracking and shifting due to expansion of soil. Referring to Figs. 5 and 6, a more preferred embodiment of the structure of the spacer void forming element 16 is shown in plan and elevation respectively. The void 35 forming element 16 in this embodiment is in the form of a box-like element having four vertical side members 46, 47 and a top platform member 49 forming the concrete c:\nrportbl\svddocs\sxv\507975 1.doc 12 supporting surface 48. Lower portions 46 of the side walls are corrugated and while upper portions 47 of the side walls are corrugated on an inner surface and have the corrugations filled on the outer surface. The members 46, 47 49 of the void forming element 16 are configured to interface with the concrete slab 20 in the manner as shown 5 in Fig. 2. As with the previously described embodiment, the underside 30 of the void forming element 16, namely the side which is supported on the ground surface 22, is open thereby providing a cavity within the void forming element 16. The cavity is defined by the inner surfaces walls of the members 46, 49, and in the embodiment as shown in Figs. 5 and 6, again represents a substantially rectangular space, although in 10 this instance with corrugated walls. Figs 7 show an elevation of an alternative version of the spacer void forming element 16 having differing heights to that of Fig. 6. In other respects the embodiments shown in elevation in Figs. 6 and 7 are similar to one another. 15 A plurality of strut members 42 are provided within the cavity and extend from the inner surface of the platform member 49 to the ground surface 22 when the void forming element 16 is positioned thereon for use. The strut members 42 provide support for the platform member 49 of the void forming element 16 along its length 20 such that the top surface 48 is able to support the load of the reinforced concrete during placement of the slab 20, as well as the load of workers or machinery used during the placement process. In this regard, the shape of the struts 42 and their location within the cavity defines the strength of the void forming element. In the figure 5 embodiment the struts have a substantially rectangular cross section with two opposing concave 25 sides. It will be appreciated that the arrangement and shape of the struts 42 can be altered in accordance to the specific construction. Fig. 8 illustrates an embodiment of the structure of the shallow void forming element 12. The shallow void forming element 12 in this embodiment is also in the 30 form of a box-like element having four vertical side members 56 (in this case not corrugated) and a top platform member 59 forming the concrete supporting surface 58. The height of the shallow void forming element 12 is preferably the same as the height of the corrugated lower portion 46 of the walls of the spacer void forming element 16. Therefore the overall height of this shallow void forming element 12 is shorter than that 35 of the spacer void forming elements 16 to allow the formation of concrete beams between the spacer void forming elements 16. The members 56, 59 of the shallow void c:\nrportbl\syddocs\sxy\507975_1.doc 13 forming element 12 are also configured to interface with the concrete slab 20 in the manner as shown in Fig. 2. A single internal strut 52 is provided extending from the inner surface of the platform member 59 to the ground surface 22 when the void forming element 12 is positioned thereon for use and the strut 52 divides the internal 5 space of the shallow void forming element into two cavities 60. Each cavity is defined by the inner surfaces of walls of the members 56, 59 and the strut 52, and in the embodiment as shown in Fig. 6, again represents a substantially rectangular space. It will be appreciated that the system described in the embodiment above 10 comprises a concrete slab supported on deep footings, typically screw piles, drilled piles and piles designed for both tension and compression. The slab is designed to be suspended between the deep footings and the void forming elements allow concreting to be easily accomplished during construction and serves as a compressible/crushable zone during the life of the slab as the underlying soil expands. It will be appreciated 15 that in the event of the void forming elements becoming compressed/crushed, the slab remains thermally insulated. 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 20 embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. c:\nrportbl\syddocs\sxy\507975_1.doc

Claims (19)

1. A system for creating a suspended structural slab, comprising: at least one pile, the or each pile having a first end and a second end, the first end adapted to be embedded into a ground surface to be in contact with load bearing ground beneath said ground surface and the second end arranged to extend above said 5 ground surface; and a plurality of void forming element, comprising first void forming elements and second void forming elements, the void forming elements arranged to substantially cover a ground surface upon which the structural slab is to be placed, the void forming elements each having a first surface adapted to receive at least a portion of the concrete 10 slab and a second surface adapted to contact the ground surface, a distance from the first surface to the second surface of the or each second void forming element being less than a distance from the first surface to the second surface of the or each first void forming element, the first and second void forming elements being laid adjacent one another to form one or more recesses in which one or more beam portions of the slab 15 are formed, wherein the void forming elements are sacrificial, being formed to be capable of withstanding forces placed thereupon during formation of the structural concrete slab and being arranged to collapse under a force of ground movement occurring after the slab is formed; wherein said structural slab is supported above said ground surface by said at 20 least one pile such that said void forming elements are positioned between the structural slab and the ground surface, and wherein the second end of said at least one pile extends through at least one of the second void forming elements and aligns with a respective one of the one or more beam portions of the slab.

2. The system of claim 1 wherein each of the void forming elements comprises a 25 body on which is defined the first surface and the second surface, said body having at least one cavity provided therein, the at least one cavity extending at least partially from said second surface toward said first surface.

3. The system of claim 2 wherein the at least one cavity of each of the first and second void forming elements is open to the ground surface to accommodate a rise that 30 may occur in the ground surface following formation of the slab.

4. The system of claim 2 or claim 3 wherein one or more supporting struts are provided within the cavity of one or more of the void forming elements.

5. The system of claim 4 wherein the one or more struts extend from the first surface to the second surface of the respective void forming element, such that the 15 distal end of the struts is adapted to contact the ground surface to provide support to the first surface.

6. The system of claim 4 or claim 5 wherein the one or more struts are regularly spaced within the cavity. 5

7. The system of any one of claims 4 to 6 wherein the struts are deformable such that a rise of the ground surface into the cavity will cause deflection of the struts.

8. The system of any one of the preceding claims wherein at least one of the at least one pile is a screw pier foundation pile.

9. The system of any one of the preceding claims, wherein the second end of each 10 said at least one pile is arranged to be received into one of the one or more beam portions of the slab.

10. The system of claim 9, comprising a plurality of the piles, wherein the installed height of each said pile above the ground enables the second end of the piles to penetrate to a desired depth into one of the one or more beam portions of the structural 15 slab.

11. A structural slab formed over the system of any one of the preceding claims wherein the structural slab is a concrete slab consisting of a plurality of platforms formed over the first void forming elements and beams formed over the second void forming elements and in which each of the piles extends through one of the second void 20 forming elements and into the structural slab.

12. The structural slab of claim 11 wherein the engagement of the at least one pile in the structural slab takes the form of a wider portion located in the upper part of the pile, such that the concrete formed over the pile surrounds and engages the wider portion and a narrower portion beneath it. 25

13. The structural slab of claim 12 wherein the wider portion comprises a rod or bar extending horizontally through a hole in a vertical portion of the pile located above a lower extremity of the structural slab.

14. The structural slab of claim 13 wherein the piles engage beam portions of the structural slab and the rod or bar extends longitudinally of the beam portions. 30

15. A method of forming a suspended structural slab, comprising: installing at least one pile, the or each pile having a first end and a second end, the first end adapted to be embedded into a ground surface to be in contact with load bearing ground beneath said ground surface, and the second end arranged to extend above said ground surface; 35 installing a plurality of void forming elements, comprising first void forming elements and second void forming elements, the void forming elements arranged to 16 substantially cover a ground surface upon which the structural slab is to be placed, the void forming elements each having a first surface adapted to receive at least a portion of the concrete slab and a second surface adapted to contact the ground surface, a distance from the first surface to the second surface of the or each second void forming 5 element being less than a distance from the first surface to the second surface of the or each first void forming element, the first and second void forming elements being laid adjacent one another to form one or more recesses in which one or more beam portions of the slab are formed, wherein the second end of said at least one pile extends through at least one of the second void forming elements and aligns with a respective one of the 10 one or more beam portions of the slab ; wherein the void forming elements are sacrificial, being formed to be capable of withstanding forces placed thereupon during formation of the structural concrete slab and being arranged to collapse under a force of ground movement occurring after the slab is formed; and 15 forming the structural slab whereby the structural slab is supported above the ground surface by the at least one pile such that the void forming elements are positioned between the structural slab and the ground surface.

16. The method of claim 15 wherein the at least one pile is engaged in the structural slab during its formation. 20

17. The method of claim 15 or claim 16 wherein a wider engagement portion is provided at an upper part of each said pile and concrete of the structural slab is formed over the wider engagement portion and a narrower portion beneath it to engage the pile.

18. A system for creating a suspended structural slab, substantially as hereinbefore described with reference to the accompanying drawings. 25

19. A method of forming a suspended structural slab, substantially as hereinbefore described with reference to the accompanying drawings.