AU706192B2 - Method of building slab construction - Google Patents

Description

2 THIS INVENTION relates to a method of constructing floors or foundation slabs, particularly suitable for, but not limited to, domestic buildings.

The foundations for domestic buildings are usually designed on a once-off basis. While this is expensive, it is necessary to take into account the characteristics of the soil on which the foundation slab is to be laid. Some soils, eg., black soil and clay, are highly reactive and it is not unknown for the uplift force generated, when the soil expands, to exceed the total weight of the building.

One solution to the problem of reactive soils is to erect a building on a concrete slab or "raft", which is designed to support the building so that the building "floats" on the soil.

While the use of such a slab minimises the likelihood of structural damage due to uneven movement of the soil, it is only a partial solution as it does not allow the building to be supported via a stable, or neutral soil formation rock) below the reactive soil ~zone. This means that the building may move relative to adjacent structures, eg., pavements and retaining walls and to essential services, eg., plumbing, electricity, gas and sewerage.

20 It is an object of the present invention to provide a method of floor construction which takes into account the uplifting S•forces of reactive soil.

It is a preferred object of the present invention to provide o a method of construction which can accommodate the secondary settling of the soil.

It is a further preferred object to provide a method of construction which can accommodate the secondary settling of the soil.

It is a still further preferred object of the present invention to provide a method of construction where the floor or slab is supported on screw piers.

Other preferred objects of the present invention will become apparent from the following description.

In one aspect, the present invention resides in a method of construction for a building slab including the following steps: wetting soil, over which the slab is to be formed, to cause the soil to at least substantially achieve its maximum expansion; driving a plurality of screw piers into the soil in a predetermined array; and ~pouring the slab supported on the array of screw piers.

In a second aspect, the present invention resides in a o°•oo** method of constructing a floor for a building, including the following 20 steps: wetting soil, over which the floor is to be constructed, to oo cause the soil to at least substantially achieve its maximum expansion; driving a plurality of screw piers into the soil in a oeoo• predetermined array; and constructing the floor supported on the array of screw piers.

Preferably, the screw piers are driven through the soil into a stable, or neutral, soil zone underlying the soil.

Preferably, the soil is ripped up or dug to a predetermined depth, eg.,200-5OOmm, depending on the soil, to assist in the wetting of the soil.

Preferably, the soil is wet over a period, eg., 24-48 hours to enable the soil to reach its full expansion.

Preferably, the screw piers are provided at regular intervals along the side and end edge beams of the slab or along the sides and ends of the floor.

Preferably, further screw piers are provided in a regular array beneath the slab or floor.

Preferably, the steel reinforcing in the slab is welded or ~tied to the upper ends of the screw piers or to anchoring bars or rods extending from the upper ends of the screw piers.

Preferably, the slab has "drop panels" of increased *go* thickness centred on the screw piers intermediate of the slab.

To enable the invention to be fully understood, preferred 0° embodiments will now be described with reference to the accompanying drawings, in which: °°•oo FIGS 1(a) to show schematic views of the types of buildings to which the invention is applicable; FIG 2 is a perspective view of a typical screw pier suitable for the method; FIG 3 is a side view showing the installation of a screw pier; FIG 3a is a perspective view of the driving machine for the screw pier; FIG 4 is a schematic side view of a building slab constructed in accordance with the present invention; FIG 5 is a similar view of an alternative method; FIGS 6, 7 and 8 to are illustrative plan views of the location of the screw piers below the slab; FIGS 9 to show details of a typical slab constructed by the method, showing typical engineering details therefor; and *FIG 10 is a schematic end view showing bracing for the screw piers.

The invention is applicable to single storey dwellings of 20 the type with a "slab-on-ground" floor (FIG two storey dwellings of the type with a "slab-on-ground" lower floor and timber upper floor (FIG two storey dwellings of the type with a part "slab-onse ground"/part suspended slab lower floor and timber upper floor (FIG and two storey dwellings of the type with timber lower and upper floors.

Referring to FIG 2, a typical screw pier 10, suitable for the method, is assembled in sections, where a screw unit 11 has a tubular shaft 12 provided with a single octagonal helix plate 13.

An aggressive point attack bit for rock or shale 14 is engaged in a square drive 15 at the lower end of the shaft 12 and a square drive assembly 16 allows one or more extensions 17 to be connected to the screw unit 11 to achieve the desired length of the screw pier 10. Typically, the shaft 12 of the screw unit 11 is 1000mm or 1500mm long, and the shaft(s) 18 of the extension(s) 17 are 1000mm or 2000mm long.

The screw pier 10 illustrated in FIG 2 has a single-helix plate 13, but it will be readily apparent that double- or triple-helix plates 13 may be provided along the shaft 12 at spaced intervals. The helix plates are typically hexagonal or octagonal in plan view and, eg., of 250-300mm diameter and 8-10mm thickness.

As shown in FIGS 3 and 4, the screw pier 10 is driven into the soil 100 (on which the plate or floor is to be laid or supported) 20 by a driving machine 20 having a hydraulic power head 21 and adaptor 22. The torque applied to the screw pier 10 can be indicated by a hydraulic pressure gauge (not shown) which measures the hydraulic pressure in the lines connecting the power head 21 to a 7 hydraulic pump (not shown), the reading being given in x 100 Nm of applied torque following suitable calibration of the gauge.

From the indicated torque reading, the strength of the soil through which the screw pier 10 is being driven can be calculated.

Generally, the torque reading will increase when the screw pier is being driven into the stable (or neutral) soil zone below the reactive soil zone. For example, the indicated torque will usually rise marked by when the screw pier 10 engages a rock strata zone below the reactive soil.

As described in our co-pending patent application being filed today, and also associated with Patent Application Nos. PN7149 and PN7150, the safe working load (SWL) and other parameters of the screw pier 10 can be calculated from the applied driving torque on the screw pier.

Referring now to FIG 4, a building is to be erected where a layer of (seasonally affected) reactive soil 100 overlies a neutral (stable) soil zone 101. Typically, the reactive soil 100 will comprise, or include, clay soils or fill; while the neutral soil 101 may be rock or

S.

shale.

r 20 At least the top 200-500mm of the reactive soil 100 is ripped or dug and the soil is wet over a period, eg., of 24-48 hours so that the soil will achieve its full expansion.

Screw piers 10 are driven through the reactive soil 100 Ooji

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into the neutral soil 101 therebelow until the screw piers achieve their desired safe working load (SWL) as calculated from the driving torque readings.

Loose fill 102 is laid over the (expanded) reactive soil 100 and the concrete slab 200 is constructed, eg., in accordance with the design details shown in FIGS 9 to When the reactive soil 100 dries out under the slab 200, it will contract below the slab 100 but the latter will remain suspended on the piers 10 and no damage will occur to the slab due to contraction/expansion of the reactive soil 100.

FIG 6 shows a slab 200 where the screw piers 10 have been driven through loose fill 12 and reactive soil fill 100 to a rock strata 103, the latter providing the neutral (or stable) zone.

Where the slab 200 is to be suspended above the soil laid eg., on an inclined slope, steel diagonal bearing straps 50 can provide bracing against wind flows (see FIG FIGS 6, 7 and 8 to show typical locations for the screw piers along the side and end beams 201 of the slab 200, and the array of the screw piers beneath the slab. At these locations, the 20 slab has "drop panels" 202 of increased thickness centred on the S Se screw pier 10. The reinforcing steel in the slab 200 is tied or welded to the upper ends of the screw pier 10, or to anchoring bars or rods *o So extending therefrom, to tie the slab 200 to the screw piers 10. With a #fees 0.

*Co.o timber floor, eg., of FIG the bearers and joists will be laid over, and connected to, the upper ends of the screw piers By digging and wetting the soil, and then constructing the slab or floor on the screw pier 10, the potential damage to the slab or floor, and any building erected thereon, due to contraction or expansion of the soil is minimised as the slab or floor is "suspended" by the screw piers above the soil movement.

Various changes and modifications may be made to the embodiments described and illustrated without departing from the scope of the invention.

.*oe o a *a The claims defining the invention are as follows: 1. A method of construction for a building slab including the following steps: wetting soil, over which the slab is to be formed, to cause the soil to at least substantially achieve its maximum expansion; driving a plurality of screw piers into the soil in a predetermined array; and pouring the slab supported on the array of screw piers.

2. A method of constructing a floor for a building, including the steps of: wetting soil, over which the floor is to be constructed, to cause the soil to at least substantially achieve its maximum expansion; driving a plurality of screw piers into the soil in a predetermined array; and constructing the floor supported on the array of screw piers.

3. A method as claimed in Claim 1 or Claim 2 wherein: the screw piers are driven through the soil into a stable, or neutral, soil zone underlying the soil.

4. A method as claimed in any one of Claims 1 to 3 wherein: the soil is ripped up or dug to a predetermined length, optionally 200-500mm, depending on the soil, to assist in the wetting of the soil.

5. A method as claimed in any one of Claims 1 to 4 a. a a.

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a a a

Claims (5)

1. 6. A method as claimed in any one of Claims 1 to wherein: the screw piers are provided at regular intervals along the side and end edge beams of the slab or along the sides and ends of the floor.
2. 7. A method as claimed in Claim 6 wherein: further screw piers are provided in a regular array beneath the slab or floor.
3. 8. A method as claimed in Claim 1, or any one of Claims 3 to 7 when appended to Claim 1 wherein: steel reinforcing in the slab is welded or tied to upper ends of the screw piers, or to anchoring bars or rods extending from upper ends of the screw piers. .ol°
4. 9. A method as claimed in Claim 7 wherein: the slab has "drop panels" of increased thickness centred on the screw piers intermediate of the slab. O.V. 20 10. A method of construction for a building slab substantially as hereinbefore described with reference to the accompanying drawings.
5. 11. A building incorporating a slab or floor constructed by the 12 method of any one of Claims 1 to DATED this sixteenth day of December 1996. STEEL FOUNDATIONS LIMITED By its Patent Attorneys FISHER ADAMS KELLY S* *i 13 ABSTRACT To protect a slab or floor 200 against the movement of a reactive, eg., clay) soil 100, at least the top 200-500mm of the soil 100 is dug and wet until maximum expansion is achieved. Screw piers 10 are driven through the soil (100) into a neutral (stable) zone (101) and the slab or floor (200) is constructed (over light fill (102)). As the reactive soil (100) expands, contracts, the slab or floor (200) is suspended above the soil (100) on the screw piers (10) and damage to the slab or floor (200) is minimised.