AU2007237161B2 - Building Method and Apparatus for Use Therein - Google Patents

Description

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AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant Actual Inventor Address for Service: Invention Title: Trista Technology Pty Ltd Kym Plotkin CULLEN CO Patent Trade Mark Attorneys, 239 George Street Brisbane QId 4000 Australia Building Method and Apparatus for Use Therein Details of Associated Provisional Application AU 2007900597 7 February 2007 The following statement is a full description of this invention, including the best method of performing it, known to us: Building Method and Apparatus for Use Therein Z Field of the invention The present invention relates to a method for constructing a building slab. In another aspect, the present invention relates to a building slab system. In a further aspect, the present invention relates to a pile cap for use in construction of a building slab.

r- Background to the invention Construction of buildings, such as residential housing and commercial buildings, on building sites that have reactive soils or problems soils, has posed many challenges to the construction industry. As is known to persons skilled in the art, reactive soils undergo significant swelling as their moisture content increases and significant shrinkage as their moisture content decreases. Consequently, reactive soils typically exhibit significant variations in soil height. Therefore, the construction industry faces significant issues in designing and building foundations and slabs for buildings located on reactive soil sites.

A number of building sites are now becoming available that have soils that are classified as "problem soils". Such building sites typically have large quantities of fill placed on those sites. This can cause difficulty because the long-term behaviour of the fill is often unknown. For example, if the building site has had relatively recent additions of fill, the fill may undergo significant settlement as time passes.

A number of possible solutions have been tried to combat the difficulties faced in building on building sites that have reactive soils or problems soils. These include: a) construction of a suspended slab. In this construction technique, piers are formed by boring holes into the ground until a depth greater than the "zone of influence" with reactive clay soils, or beyond the zone of potential settlement of low load bearing soils or fill (problem soils) is reached. The "zone of influence" is the region of reactive soils. Effectively, the pier depth should extend down to a depth where the ground 2

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structure is relatively stable. The pier holes then have appropriate reinforcement material placed in them and are filled with concrete. When the concrete sets, strong 0 Z concrete piers are formed in the ground, with the bottom of those piers resting in a stable soil zone. A very strong slab is then constructed. This slab rests on the piers. If the soil shrinks or settles, the slab is supported by the piers.

This solution is suitable for use in soils that have the potential to settle. However, it is not appropriate for use in reactive clay soils as swelling of the soil can cause the slab to flex or twist under the hydraulic forces exerted by the swelling clay soils, which leads to cracking in the concrete slab and lifting of the concrete slab with the concrete piers. Further, this slab construction method can be quite expensive as the concrete piers normally have to be of great depth. A larger ratio of bored piers are also used as engineers space those piers quite closely to overcome deleterious effects arising from inconsistency in quality in the piers and the accumulation of debris in the bored holes prior to concreting of the piers. It will be understood that debris underneath the concrete pier forms a compressible layer under the pier, thereby adversely affecting its load capacity.

b) as a variation of the system discussed in above, screw piles are used instead of concrete piles. Screw piles typically comprise a long steel shaft having a screw flight located towards the lower end thereof. In this construction method, the screw piles are screwed into the ground and the slab is built and suspended on the screw piles.

The piles are connected to the slab or keyed into the slab. Again, this solution is useful in settling soils but has limitations in reactive soils because the slab can be lifted off the screw piers. A traditional suspended slab is not designed to cope with lifting loads applied by a swelling reactive clay soil. These slabs are designed to handle tensile forces applied to the bottom of the slab (by virtue of placement of steel reinforcing at the bottom of the slab) but the top part of the slab has only poor resistance to tensile forces. Thus, lifting forces applied by a swelling soil tend to cause cracking in such slabs.

In order to address these issues, screw piling suppliers, contractors and their associated engineers have taken the step of ripping or scarifying the top part of the soil 3 prior to forming the slab. This loosens the top part of the soil and, in some instances, soil is removed from under the position of the slab in an attempt to create an aerated 0 Z cushion of soil to mitigate the pressure applied by swelling soils. This can have the unfortunate consequence of causing water to pool under the slab at the base of the ripped or scarified area, thereby concentrating water at a lower depth in the soil.

Effectively, the ripping process has the potential to move the zone of influence deeper into the undisturbed ground and the scarifying or ripping process can cause the zone of influence to move to a level below the screw piles. This is very undesirable.

c) on building sites where reactive soils are present, floating concrete slabs are frequently employed, particularly in residential housing and small commercial buildings. Floating concrete slabs are formed on the soil surface. The floating slab is designed to be very stiff and very strong. If the soil shrinks (for example, during a prolonged dry spell), the soil shrinks away from the concrete slab, particularly around the edges of the slab. These slabs are designed to be strong enough so that any unsupported regions underneath the slab do not break. Similarly, if the soil expands, the slab can shift upwardly with the expanding soil. Such slabs are known as heavy duty grillage rafts and are exceedingly strong. However, they require very large amounts of concrete and reinforcing steel and require significant site preparation to complete. As a result are very expensive to construct.

In order to address these issues, so-called "waffle raft" slabs were developed. Waffle raft slabs are formed by setting out formwork on the ground, positioning a plurality of void forming elements (typically polystyrene boxes or, on occasions, cardboard boxes) at desired positions, placing appropriate reinforcement material and pouring concrete.

The void forming elements reduce the amount of concrete required in the slab and result in the formation of a slab having a plurality of ribs, set out typically in a grid pattern. If the slab could be viewed from underneath, it would resemble the surface of a waffle, hence the name "waffle raft slab".

Waffle raft slabs have been widely used in building on reactive soil sites. However, to account for the maximum possible swelling and shrinkage potential of the soil, waffle raft slabs have to be engineered with great strength. This, of course, increases the cost 00 4 of the slab. Furthermore, waffle raft slabs do not perform as well as suspended slabs on sites where differential settling is possible. Waffle raft slabs also suffer from a ;problem known as hogging, which occurs if the soil shrinks away from the edge beam of the slab. If this occurs, the load carried by the edge beam of the slab is not transferred into the ground below the edge beams but rather is transferred to the interior ribs of the slab. These ribs are not designed to carry the load of the structure Iand they will flex and bow under that load, causing structural failure.

Brief description of the invention lO In one aspect, the present invention provides a building slab system comprising a plurality of screw piles positioned in the ground and a slab constructed on and formed above the screw piles, characterised in that a structural slip joint exist between the slab and the screw piles.

In a second aspect, the present invention provides a building slab system comprising a plurality of screw piles positioned in the ground and a waffle raft slab constructed on and formed above the screw piles, characterised in that one or more of the screw piles are fitted with pile caps having an upper surface that extends beyond an outer periphery of an upper part of the screw piles, the pile cap forming a structural slip joint between the slab and the screw pile.

In a third aspect, the present invention provides a pile cap for placement on an upper part of a screw pile in the construction of a building slab, the pile cap having an upper surface that extends beyond an outer periphery of the upper part of the screw pile, the pile cap further including a downwardly extending part that extends downwardly relative to the screw pile, the pile cap forming a structural slip joint between the screw pile and the slab.

In a fourth aspect, the present invention provides a method for constructing a building slab as claimed in any one of the preceding claims comprising the steps of positioning a plurality of screw piles in the ground, placing pile caps on to upper portions of the screw piles, said pile caps having an upper surface that extends beyond an outer 00 periphery of the upper part of the screw piles, and forming a waffle raft concrete slab N above the screw piles, wherein the pile caps are positioned between the upper part of ;the screw piles and the concrete of the waffle raft concrete slab.

The pile cap of the present invention allows screw piles to be used in conjunction with waffle raft concrete slabs. Previous attempts to use screw piles with waffle raft IN concrete slabs have hitherto been unsuccessful due to various building codes and regulations and due to the methods of connecting the slab to the screw piles.

In one embodiment, the pile cap has a downwardly extending sleeve having an inner diameter that is larger than the outer diameter of the screw pile. Placement of the pile cap on the screw pile in this embodiment results in the downwardly extending sleeve being positioned over and extending downwardly around the outer periphery of the screw pile.

The pile cap of this embodiment may include centring means for centring the screw pile within the downwardly extending sleeve. The centring means may comprise inwardly extending gussets that extend from the inner wall of the downwardly extending sleeve. Alternatively, the centring means may comprise inwardly extending fingers that extend from the inner wall of the downwardly extending sleeve. In either case, the gussets or fingers come into contact with the outer surface of the screw pile and this acts to ensure that the screw pile is centred relative to the downwardly extending sleeve.

In another embodiment, the downwardly extending portion includes a portion that is inserted into and extends into the screw pile. In this regard, it will be appreciated that screw piles are generally formed from a hollow steel shaft and, in this embodiment, the downwardly extending portion is inserted into the hollow steel shaft of the screw pile.

The pile cap may include reinforcing means positioned between the upper surface and the downwardly extending part of the pile cap. For example, the reinforcing means may comprise one or more gussets extending between an underneath part of the upper 00 O surface of the pile cap and the downwardly extending part of the pile cap. These C N reinforcing means act to distribute the load incident upon the upper surface of the pile 6

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cap on to the downwardly extending part of the pile cap.

0 Z In another embodiment, the reinforcing means may comprise reinforcing ribs on an underside of the upper surface of the pile cap. In a further embodiment, the reinforcing means may include one or more the downwardly extending walls extending downwardly from an underside of the upper surface. If reinforcing gussets are present, the one more downwardly extending walls may extend between the reinforcing gussets and form, together with the reinforcing gussets, an effective open box section reinforcement.

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In instances where the reinforcing means comprises one or more gussets, and the pile cap includes reinforcing means in the form of one more inwardly extending gussets, the reinforcing gussets are suitably in alignment or register with the inwardly extending gussets such that external load can be transferred from the reinforcing gussets to the internal gussets and on to the screw pile.

The pile cap may have a flexible upper edge.

The pile cap in accordance with the present invention may be made from a plastics material, such as polyvinyl chloride (PVC), polyethylene or indeed any other plastic material. Pile caps that are made from plastics material provide the advantage that the pile cap electrically isolates the screw pile from the concrete slab.

Alternatively, the pile cap may be made from a metal, such as mild steel, aluminium or any other metal. If the pile cap is made from metal, it is desirably made from a metal that does not cause galvanic corrosion when in contact with the screw piles.

The pile cap in accordance with the present invention may also be made form a large number of other materials. Examples include high strength concrete, rubber (especially high density rubber), nylon, resins, polymers, laminates, composite materials, and Kraft wood. It will be appreciated that the present invention encompasses a pile cap that may be made form any suitable material.

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In some embodiments, it is possible to place a member between the top of the screw pile and the pile cap. This member may be used, for example, to increase resistance to

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Z "cookie cutter breakthrough" in which the top of the screw pile pushes through the pile cap, to provide a separate to separate the top of the screw pile from the pile cap in instances such as where the top of the screw pile is rough cut and would otherwise be more likely to cut through or damage the pile cap. The member may comprise a ring that is positioned adjacent to the underside of the upper surface of the pile cap and comes into abutment with the top of the screw pile. Alternatively, the member may comprise a plate that is similarly positioned. Several different members may be provided for use with the pile cap. For example, different members may be provided for use in high load bearing constructions; for use with rough cut screw piles or for use in low load bearing situations.

The pile cap in accordance with the present invention assists in spreading the load between the top of the screw pile and the waffle raft concrete slab. In some embodiments, slight horizontal movement between the pile cap and the screw pile is allowed. In some embodiments, relative vertical movement between the pile cap and the screw pile is allowed. This is advantageous when the pile cap is used in reactive soils. For example, as the soil swells, the waffle raft concrete slab will lift with the swelling soil. However, as the screw pile is screwed in to the ground below the zone of influence, the screw pile does not move relative to the swelling soil. Consequently, the waffle raft concrete slab moves upwardly relative to be screw pile. The pile cap is likely to move with the waffle raft concrete slab. As the soil shrinks, the waffle raft concrete slab moves downwardly and the pile cap also moves downwardly until it fully seats on the top of the screw pile. If the soil continues to shrink, the screw piles will support the waffle raft concrete slab and further settling of the slab will not occur.

Accordingly, the cap acts to form a structural slip joint between the screw pile and the waffle raft.

In the method of the present invention, a plurality of screw piles are positioned in the ground, such as by screwing the screw piles into the desired position in accordance with an engineering design for the slab. The screw piles are either screwed in until their height above the ground surface is at the correct relative level (RL) or the screw 8 O piles are screwed into the ground until they reach the minimum required depth and the top of the screw piles may then be cut to the desired relative level. Laser leveling may Z be used to ensure that the top of the screw piles are at the correct level. Suitably, the Cc top of all the screw piles are at the desired relative level. Pile caps may then be positioned on top of the screw piles. Suitably, each screw pile is fitted with a pile cap.

Sand or other particulate material may be used to fill between the screw piles The

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sand or other particulate material may then be screed to be level with the top of the Cc pile caps. This results in the sand or other particulate material being at the correct level by using a relatively simple process. In contrast, preparation of a site for pouring of a waffle raft concrete slab presently requires the use of laser levelling to get the sand or other particulate material level to the correct level.

Once the sand or other particulate material has been levelled, plastic sheeting is placed over the sand and the pile caps. This further ensures separation between the slab and the screw piles, in accordance with building codes. Appropriate form work and void formers are positioned on the plastic sheeting, reinforcement material laid as required and the concrete is poured to form the waffle raft concrete slab.

In order to more clearly understand the benefits and advantages arising from the present invention, embodiments of the present invention will now be described with reference to the accompanying drawings.

Brief description of the drawings Figure 1 shows an underneath perspective view of a pile cap in accordance with embodiment of the present invention; Figure 2 shows another perspective view of the pile cap shown in figure 1; Figure 3 shows a side view of the pile cap shown in figure 1; Figure 4 shows a side view, in cross section, of the pile cap shown in figures 1 to 3, as fitted to the top of a screw pile; 9 Figure 5 shows an underneath perspective view of a pile cap in accordance with Z another embodiment of the present invention; Figure 6 shows a side view of the pile cap shown in figure

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Figure 7 shows an underneath perspective view of a screw pile and waffle raft Cc concrete slab system incorporating a pile cap in accordance with the embodiment shown in figures 5 and 6; Figure 8 is an underneath perspective view of a pile cap in accordance with a further embodiment of the present invention; Figure 9 is a perspective view showing a screw pile and waffle raft concrete slab system incorporating the pile cap shown in figure 8; and Figure 10 shows an underneath perspective view and of a pile cap as shown in figure 1 and an insert ring for use with the pile cap.

Detailed description of the drawings It will be appreciated that the drawings have been provided for the purposes of illustrating embodiments of the present invention. Therefore, it will be understood that the present invention should not be considered to be limited solely to the features as shown in the accompanying drawings.

The pile cap 10 shown in figures 1 to 3 includes an upper flange 12 having an enlarged upper surface 14. Upper surface 14 it is essentially flat and planar. The pile cap has a downwardly extending sleeve 16 that extends downwardly from the underneath surface 18 of upper flange 12. The downwardly extending sleeve 16 suitably has an inner diameter that is larger than the outer diameter of a screw pile to which the pile cap 10 is to be fitted. In very reactive soils that exhibit a large amount of swelling and shrinkage, caps having a longer downwardly extending sleeve may be

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used, when compared with caps used in less reactive soils. The longer sleeve facilitates retention of the cap on the top of the screw pile during swelling of the soil.

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z The pile cap 10 includes a plurality of gussets 20 (not all of which are numbered). As best shown in figure 2, the gussets 20 extend between the underneath surface 18 of the upper flange 12 and the outer side wall of the downwardly extending sleeve 16. The gussets 20 provide reinforcement to the pile cap. The gussets 20 also assist in transferring load from the pile cap to the screw pile. This will be discussed in greater detail hereunder.

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To further strengthen the pile cap, a downwardly depending annular wall 22 extends downwardly from the underneath surface 18 of the upper flange 12. As can be seen from figures 1 and 2, annular wall 22 also extends between adjacent gussets 22 to further strengthen the gussets 20. Indeed, adjacent pairs of gussets 20, in conjunction with the relevant sections of annular wall 22 and sleeve 18, essentially form an open box section, which assists in reinforcing the pile cap The pile cap 10 also includes a plurality of inwardly extending gussets 24 (only some of which are numbered). Inwardly extending gussets 24 extend inwardly from the inner wall of sleeve 18. As can be seen from figures 1 and 2, gussets 24 are in alignment or in register with gussets 20. When the pile cap 10 is placed over a screw pile, gussets 24 come into contact with the screw pile and act to centre the screw pile in the sleeve 18. Further, as the gussets 24 are in alignment with gussets 20, forces applied from the pile cap through gussets 20 are transferred to gussets 24 and thereafter to the screw pile. Thus, gussets 24 assist in locating the pile cap on the screw pile and also in transferring loads to the screw pile.

Suitably, the gussets 24 have a degree of flex to enable the pile cap to move horizontally relative to the screw pile.

Each of the gussets 24 may have an upper inward extension 26 that acts to space the underneath surface 18 of the upper flange 12 from the top of the screw pile. In this regard, the inward extensions 26 rest on the upper end of the screw pile. Inward 11 Sextensions 26 also strengthen the underneath surface of the top of the pile cap at the position where the top of the screw pile bears on the pile cap. They also compensate Z for slight irregularities in the top of the screw pile's steel ring. They also add to the Cc strength of the compressive nature of the pile cap and add to resistance to punch through by the top of the screw pile under excessive load.

The pile cap 10 shown in figures 1 to 3 is suitably made from a plastic material, such Cc as polyvinyl chloride (PVC), although it will be appreciated that the present invention encompasses pile caps made from any material. The pile cap may be dimensioned such that it has a height of, for example, 80 mm, with the upper flange 12 having a diameter of, say, 200 mm. These dimensions may, of course, be varied. Further, although figures 1 to 3 show the pile cap 10 as having a circular upper surface 14, it will be appreciated that the upper surface is not limited to that shape and it may have any shape.

Figure 4 shows the pile cap 10 fitted to a screw pile 30. As can be seen in Figure 4, the end of the screw pile 30 rests on inward extensions 26. The inward gussets 24 contact the side walls of the screw pile 30. As can also be seen from figure 4, the downwardly extending sleeve 16 has an inner diameter that is larger than the outer diameter of the screw pile 30. This allows for easy fitment of the pile cap 10 to the screw pile 30. Further, if the soil swells to a degree that is sufficient to cause the waffle raft concrete slab (not shown) to lift to an extent that the lower end of sleeve 16 comes off the top of the screw pile 30, any slight misalignment of the waffle raft concrete slab during soil swelling is unlikely to stop the pile cap 10 from again becoming positioned over the top of the screw pile 30 as the soil shrinks, due to the oversized diameter of the sleeve 16. In this regard, it will be appreciated that the top face of the pile cap becomes interfaced with the concrete slab and can move upwardly with the concrete slab if the soil swells. Effectively, the pile cap is a working pile cap that works geotechnically with the soil.

Figures 5 and 6 show another embodiment of the present invention. The embodiments shown in figures 5 and 6 has a number of features in common with the embodiment shown in figures 1 to 4. For brevity and convenience of description, like 12 O features are denoted by like reference numerals but with the addition of These features need not be described further. The embodiment shown in figures 5 and 6 0 Z differs from the embodiment shown in figures 1 to 4 in that annular wall 22 of the Cc embodiment shown in figures 1 to 4 has been replaced by an annular reinforcing rib 32. The pile cap 10' shown in figures 5, 6 and 7 has a slightly lesser degree of stiffness than the pile cap shown in the embodiment of figures 1 to 4 as a result.

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Cc Figures 7 shows a pile cap 10' as shown in figures 5 and 6 used in conjunction with a screw pile 30' and a waffle raft concrete slab 40. In figure 7, the screw pile 30' is positioned in the ground. The pile cap 10' is placed on the upper end of the screw pile and the waffle raft concrete slab is then formed above the pile cap As can also be seen from figure 7, the waffle raft concrete slab 40 includes thick peripheral beams 42, internal beams 44 and internal voids 46.

The pile cap in accordance with the present invention and as shown in the embodiments of figures 1 to 7 provides the means to interface the waffle raft concrete slab with the screw piles. The pile cap isolates the screw pile from the waffle raft concrete slab so that the waffle raft concrete slab can still move vertically in reactive soils, such as swelling clays. The pile cap allows the waffle raft to rest on top of the slender screw pile during soil shrinkage, without concentrating load onto a confined area that could lead to stress failure of the slab in the area of screw pile-to-slab contact. The enlarged area provided by the upper surface of the pile cap increases the interface area between the screw pile and the slab to thereby more evenly distribute the load between to slab and the screw pile. The enlarged area of the upper surface of the pile cap also acts to minimise or avoid the formation of a "socket" effect during slab pour that could catch the edge of the pile during movement of the waffle raft concrete slab in reactive soils.

The pile cap suitably requires a push fit action to fit onto the top of the screw pile.

The internal gussets assist in centring the pile cap on the screw pile and in retaining the pile cap on the screw pile during slab construction. This minimises the likelihood of the pile cap being accidentally removed from the screw pile during the construction 13 process. The combination of the external gussets and the internal gussets also assist in transferring loads from the pile cap to the screw pile.

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The downwardly extending sleeve may have a length that is greater than the average swelling potential of reactive soils. In this instance, the sleeve always has at least a portion extending over the top of the screw pile. Alternatively, if the soil swells to such an extent that the sleeve is lifted off the pile, when shrinkage of the soil occurs, the sleeve will return onto the pile, even if there has been a small amount of lateral movement of the waffle raft concrete slab, due to the oversized dimension of the sleeve when compared to the outer diameter of the screw pile. If lateral movement of the waffle raft concrete slab does occur in such circumstances, the flexibility of the internal gussets and the oversized diameter of the sleeve will allow the sleeve to return onto the top of the screw pile.

Another advantage arising from the pile cap in accordance with the present invention is that waffle raft slab design can be changed for a given soil reactivity classification.

At present, waffle raft concrete slabs have to be engineered for a worst-case scenario for soil movement. This worst-case scenario may be determined, for example, by calculating how much swell is available from the soil having 50% moisture and how much shrinkage is likely to occur, again based upon the soil having an initial moisture content of 50%. This then provides a total amount of movement that may be experienced with the soil. For example, if the soil is determined to have 50 mm swell and 50 mm shrinkage from the 50% moisture point, the total movement that the waffle raft concrete slab must be designed to meet is 100 numm. With the system and method of the present invention, the screw piles support the slab when contraction below the midpoint occurs. Therefore, for this soil, the slab only has to be engineered to meet 50 mm of movement, not 100 mm of movement. This will allow for more efficient waffle raft slab design to occur. Effectively, use of the slab system in accordance with the present invention allows for a smaller waffle raft slab for a 3 0 particular reactive soil clay classification to be used. For example, for an type reactive soil, the waffle raft slab used in the present invention may be similar to a waffle raft slab used on an class soil using a conventional waffle raft slab design, and a classification slab used for a classification soil, and so forth. This 14 O represents a great saving in cost and provides attendant environmental benefits due to the ability to use and transport lesser quantities of materials.

0 z Cc Figure 8 shows a pile cap in accordance with another embodiment of the present invention. The pile cap 50 shown in figure 8 has an upper flange 52 having an upper plane surface (not shown). The downwardly depending member 54 extends downwardly from the underneath surface 56 of the upper flange 52. A plurality of Cc flexible barbs 58 are provided on the outer surface of member 54. Member 54 is sized such that it can fit into the upper part of the screw pile shaft. Use of the pile cap 50 in conjunction with a screw pile 60 and a waffle raft concrete slab 62 is shown in figure 9.

The pile cap 50 shown in figure 9 has a push fit insert into the top of the screw pile.

Suitably, the length of the member 54 is greater than the maximum swelling potential of reactive soils. Alternatively, the member 54 has a diameter that is undersized in comparison to the inner diameter of the screw pile such that if the member 54 is lifted out of the screw pile by swelling of the soil, the member 54 will be able to re-insert itself into the screw pile as the soil shrinks. The flexible barbs 58 ensure a good fit into the screw pile and minimise the chance of accidental removal after installation.

The enlarged flange of the pile cap provides a region of separation between the slab and the pile. The enlarged size of the flange ensures no slender pile "socket" effect during slab pour that could catch the edge of the pile during movement of the waffle raft concrete slab from clay soils.

Figure 10 shows an underneath perspective view of the pile cap 10 as shown in figure 1 used in conjunction with an insert ring 80. Insert ring 80 is designed to be inserted into sleeve 16 of pile cap 10. Insert ring 80 has a periphery that includes cutouts or depressions, some of which are shown at 82. These cutouts or depressions 82 are sized and spaced such that they come into register with the inner gussets inside the sleeve 16. This allows easy insertion of the insert ring 80 into the sleeve 16. The insert ring 80 is positioned within the sleeve 16 such that it sits between the top of the screw pile and the pile cap. The insert ring 80 may be designed to withstand a greater load whilst avoiding cookie cutter breakthrough by the screw pile. Alternatively, the

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insert ring 80 may be designed so that it acts to space or pad a rough cut screw pile top from the pile cap.

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z It will be appreciated that the insert ring may be replaced by other members that space the top of the screw pile from the pile cap. For example, that member may comprise a plate that is inserted into sleeve 16. Alternatively, a washer or ring may be used for placement over downwardly extending projection 54 of the pile cap 50 as shown in figure 8.

The dimensions and properties of the member may vary in accordance with the use of the member. For example, members designed to increase the resistance to cookie cutter breakthrough may be made from a quite strong material and act to spread the load from the top of the screw pile over a greater area. Members designed to pad the pile cap from a rough cut screw pile top may be made of a thicker, softer material.

The person skilled in the art will readily appreciate that a number of different members may be used in association with the pile caps of the present invention. The person skilled in the art will also appreciate that the members may be made from a wide variety of materials.

Use of pile caps in accordance with the present invention also has the potential to simplify the construction process when constructing waffle raft concrete slabs. The following typical steps will be taken when constructing a waffle raft concrete slab in accordance with the method of the present invention: survey the site and established the correct RL screw the screw piles into position in accordance with the engineering drawings of the slab. The screw piles should be screwed in to a sufficient depth so that the screw piles extend below the zone of influence or load bearing soils or fill that could settle.

The screw piles should be screwed in such that the tops of the pile caps fitted to the screw piles is at the correct RL. If a screw pile is unable to be screwed sufficiently deeply into the ground (for example, if the screw pile strikes bedrock), the screw pile should be cut to the correct length to establish the correct RL.

fit pile caps to each of the screw piles. The top of the pile caps enables the preset of RL for the underside of the waffle raft concrete slab. Under slab sand (or other 16 O particulate material) is built up around the screw piles and screed so that the top of the sand is level with the top of the pile caps. This is a simple screeding process that uses 0 Z the tops of the pile caps as a convenient guide to the correct RL. This enables a flat, Cc level surface to be formed at the correct RL without necessarily requiring laser levelling, as is often required when forming waffle raft concrete slabs in accordance with presently used techniques.

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lay under slab plastic over the sand and pile caps.

Cc waffle raft concrete slab is then prepped, poured and finished.

A number of further advantages accrue from the present invention. These include: a) the number of screw piles that need to be used in the engineered design of the slab system can be reduced by about 15 to 30%, when compared to using conventional concrete poured piers or conventional screw piles without pile caps. This has apparent implications for the cost of the building slab system and the time required to construct the slab; b) placing of screw piles into the ground on a building site represents a significant occupational health and safety risk. In particular, screw piles can be particularly dangerous if a workmen trips or falls on to the top part of the screw pile. The pile caps make the top of the screw piles easy to see and also covers the edges of the screw pile. This provides a degree of protection sure to a workmen trip or fall onto a screw pile that has been fitted with a pile cap in accordance with the present invention.

c) the pile cap also provides a visual indication that shows machinery operators where the tops of the screw piles are located, thereby assisting those operators to avoid contact with the screw piles. For example, on existing building sites where screw piles are used, bobcats are frequently utilised to move sand or soil or other material around the building site. However, severe damage to the tyres of the bobcat can occur should the bobcat tyres come into contact with the top of a screw pile. The pile caps in accordance with the present invention, when fitted to the top of the screw piles, are very easy to see. This makes avoidance of the top of the screw piles by the bobcat operator easier. The screw piles also protect people on the building site from injury Q)that could be caused by contact with the top of the pile.

Z When the pile caps of the present invention are made from an electrically insulating material, the pile caps provide a further benefit in that they act as an electrolysis isolators to electrically isolate the screw piles from the slab. This reduces galvanic corrosion of the screw piles due to electrolytic interactions with the slab.

Mc, Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. For example, the pile caps shown in the accompanying drawings all have a close fit with either the outer wall of the screw pile or the inner wall of the screw pile. It may not be necessary to have such a close fit. Thus, the inner gussets or the outer barbs may be omitted.

Other designs besides inner gussets or outer barbs may also be used. Other equivalent structures that offer a similar centring action may be used. The shape of the upper flange may vary from that shown. Alternative reinforcements designs may be used.

Those skilled in the art will appreciate that the present invention encompasses all variations and modifications that fall within its spirit and scope.

Claims (15)

1. 2. A building slab system comprising a plurality of screw piles positioned in the ground and a waffle raft slab constructed on and formed above the screw piles, characterised in that one or more of the screw piles are fitted with pile caps having an Cc upper surface that extends beyond an outer periphery of an upper part of the screw piles, the pile cap forming a structural slip joint between the slab and the screw pile.
2. 3. A building slab system as claimed in claim 2 wherein the pile cap has a downwardly extending part that extends downwardly relative to the screw pile.
3. 4. A building slab system as claimed in claim 3 wherein the pile cap has a downwardly extending sleeve having an inner diameter that is larger than the outer diameter of the screw pile, the downwardly extending sleeve being positioned over and extending downwardly around the outer periphery of the screw pile.
4. 5. A building slab system as claimed in claim 4 further comprising centring means for centering the screw pile within the downwardly extending sleeve.
5. 6. A building slab system as claimed in claim 5 wherein the pile cap includes reinforcing means positioned between the upper surface and the downwardly extending part of the pile cap.
6. 7. A building slab system as claimed in claim 6 wherein the centering means are in alignment or register with the reinforcing means such that external load is transferred from the reinforcing means to the centering means and on to the screw pile.
7. 8. A method for constructing a building slab as claimed in any one of the preceding claims comprising the steps of positioning a plurality of screw piles in the ground, placing pile caps on to upper portions of the screw piles, said pile caps having 00 an upper surface that extends beyond an outer periphery of the upper part of the screw O Opiles, and forming a waffle raft concrete slab above the screw piles, wherein the pile t caps are positioned between the upper part of the screw piles and the concrete of the Z waffle raft concrete slab. ri-
8. 9. A method as claimed in claim 8 wherein a member is placed between the top of the screw pile and the pile cap. A method as claimed in claim 8 or claim 9 wherein the method comprises placing a plurality of screw piles in the ground and screwing the screw piles into the desired position in accordance with an engineering design for the slab, the screw piles being either screwed in until their height above the ground surface is at the correct relative level (RL) or the screw piles being screwed into the ground until they reach the minimum required depth and the top of the screw piles is then cut to the desired relative level, positioning pile caps on top of the screw piles whereby each screw pile is fitted with a pile cap, placing sand or other particulate material to fill between the screw piles, screeding the sand or other particulate material to be level with the top of the pile caps, and forming a reinforced concrete slab above the screw piles.
9. 11. A method as claimed in claim 10 wherein the step of forming the concrete slab comprises the steps of placing substantially impervious sheeting over the sand and the pile caps, positioning appropriate form work and void formers on or around the substantially impervious sheeting, laying reinforcement material as required and pouring the concrete to form the waffle raft concrete slab.
10. 12. A pile cap for placement on an upper part of a screw pile in the construction of a building slab, the pile cap having an upper surface that extends beyond an outer periphery of the upper part of the screw pile, the pile cap further including a downwardly extending part that extends downwardly relative to the screw pile, the pile cap forming a structural slip joint between the screw pile and the slab.
11. 13. A pile cap as claimed in claim 12 wherein the pile cap has a downwardly extending sleeve having an inner diameter that is larger than the outer diameter of the screw pile, the downwardly extending sleeve being adapted to be positioned over and 00 extending downwardly around the outer periphery of the screw pile. O t 14. A pile cap as claimed in claim 12 or claim 13 further comprising centring Z means for centering the screw pile within the downwardly extending sleeve. r- A pile cap as claimed in claim 14 wherein the centering means comprises inwardly extending gussets that extend from the inner wall of the downwardly extending sleeve or the centering means comprises inwardly extending fingers that n extend from the inner wall of the downwardly extending sleeve S16. A pile cap as claimed in claim 12 wherein the downwardly extending portion includes a portion that is inserted into and extends into the screw pile.
12. 17. A pile cap as claimed in claim 12 wherein the pile cap includes reinforcing means positioned between the upper surface and the downwardly extending part of the pile cap.
13. 18. A pile cap as claimed in claim 17 wherein the reinforcing means comprises one or more gussets extending between an underneath part of the upper surface of the pile cap and the downwardly extending part of the pile cap or reinforcing ribs on an underside of the upper surface of the pile cap or one or more the downwardly extending walls extending downwardly from an underside of the upper surface, or reinforcing gussets having one more downwardly extending walls extending therebetween such that the reinforcing gussets form, together with the downwardly extending walls, an effective open box section reinforcement.
14. 19. A pile cap as claimed in claim 18 wherein the cap includes reinforcing means comprising one or more gussets, and the pile cap includes centering means in the form of one more inwardly extending gussets, and the reinforcing gussets are in alignment or register with the inwardly extending gussets such that external load is transferred from the reinforcing gussets to the internal gussets and on to the screw pile. A pile cap as claimed in claim 12 wherein the pile cap has a flexible upper edge. 21 00 O O 21. A building slab system substantially as hereinbefore described with reference to the accompanying drawings. r 5 22. A method for constructing a building slab substantially as hereinbefore described with reference to the accompanying drawings.
15. 23. A pile cap for a screw pile substantially as hereinbefore described with Cc reference to the accompanying drawings. r-