SOIL TREATMENT

Abstract

A method of treatment of foundation soil beneath a structure, comprising treating the foundation soil with an ionic treatment agent, and treating the foundation soil with a soil stabiliser to stabilise the foundation soil.

Description

The invention relates to a method of treatment of foundation soil beneath a structure.

Structures such as buildings and areas of flat concrete are prone to movement if built on reactive soil. Changes in moisture content cause these soils to swell or shrink. An increase in moisture causes the soil to swell (increase in volume) and a reduction in moisture causes the soil to shrink (decrease in volume).

If a structure that is built on reactive soil is not heavy enough to resist the forces of swelling, or its footings do not extend down to a moisture stable stratum of soil, it will be lifted/moved through a process known as clay heave. Similarly, shrinkage of the foundation soil will result in the structure being unsupported and potentially dropping. Causes of change in moisture content may be cyclical (seasonal temperature and precipitation), the result of an event (leaking pipework or flooding), or, a change in long term equilibrium (e.g., growing/removal of trees, new construction that affects water catchment, or variance between moisture content immediately pre-construction and the established long term equilibrium).

As the extent of shrink/swell is unlikely to be uniform across a foundation it causes differential movement that damages the structure above.

Regarding cyclical changes, because reactive soils swell with equal pressure in all directions but tend to only shrink vertically, cyclical changes commonly result in a long term reduction in foundation soil volume as the soil moves laterally with each cycle. The lateral movement occurs because the weight of the footing and vertical outer face of the footing offer resistance to the swell and provide a more stable mass off which to push. The path of least resistance and the predominant direction of swell is therefore lateral. Similarly, clays on grade/slopes tend to move laterally in the direction of fall. The movement cycle causes damage to the structure that is greater than would be expected if the structure had moved just once to a new level (e.g., through settlement or subsidence).

Long term reduction in equilibrium moisture content is typically associated with tree growth or the result of the soil's pre-construction moisture content being particularly high, for example because the foundations were excavated and the footings poured during a wet period. Over time the foundation soil will reduce in moisture content and volume, i.e. shrink. The structure above will lose support and drop. In addition to such a long term effect on the structure, cyclical changes in moisture content may still occur. If shrinkage is caused by tree growth, removal of trees in future will cause the soil to rehydrate and regain some, or all, of its original volume.

In many parts of the world, subsidence caused by foundation soil shrinkage is a major problem, requiring remedial action which can be extensive and expensive.

Various remedial systems involve the use of a stabiliser to stabilise the soil.

One such system involves the injection of a stabiliser comprising a fast reacting two component polyurethane into the soil underneath a structure. The system is designed to increase the volume of settled and subsided soil and can also be used to lift structures that have dropped as a result of the change to the soil. Once injected, the polyurethane rapidly expands with great force. The force of the expansion consolidates the soil and causes the structure above to be lifted.

The first system works well in non-cohesive soil where the polyurethane can expand uniformly through the soil and the soil above is more easily compressible (i.e., the soil can be compacted, and water expelled, at a pressure lower than that required to effect lift). In cohesive (less permeable) soil, however, the polyurethane tends to expand only through existing cracks and fissures in the soil. As a result of this confinement, the cell structure of the polyurethane may be damaged and weakened, although generally the strength of the polyurethane is sufficient to support most structures.

The soil is usually injected through tubes at a depth leaving an untreated portion of soil above the injected portion and below the base of the structure. In reactive soils, the system generally relies on vertical cracks remaining in the untreated portion which will provide pressure relief to accommodate future swelling i.e. the soil will swell to fill these cracks rather than over lift the structure. However, we have realised that although vertical cracks may frequently be evident at the surface, they may not be present below the structure and consequently continued movement can be expected with this system. Moreover, if the soil is already in a swollen state at the time of injection, future shrinkage is probable.

A second remedial system for dealing with shrinkage of reactive soil involves two stages, using two types of soil stabiliser. Cracks in the upper levels of the clay soil are first injected at low pressure with a non-foaming polymer, which is intended to fill the cracks and prevent future swell potential by limiting the ability of water to enter the soil. Second, a slow foaming polymer is injected into the soil beneath the crack filled section at high pressure through tubes. The structure is lifted by the high injection pressures. As with the first system, if the soil is in a swollen state at the time of injection, future shrinkage is probable.

However, by filling cracks in the upper levels of the soil, the system actually confines the swelling process and increases the potential for future over lift. If the clay cannot expand into these cracks then upward pressure will increase. The suppliers of this system therefore emphasise the importance of controlling the moisture content of the soil after the system is installed.

A third system for dealing with soil shrinkage, known as slab jacking, involves the use of a stabiliser in the form of a fast expanding two component polyurethane (as used by the first system described above) which is injected directly beneath the concrete footing/slab. The expanding foam takes the path of least resistance, initially filling voids, then filling cracks and consolidating weak soil until the expansive pressure is sufficient to lift the structure.

The system is effective at consolidating weak soils directly below the slab or footing and providing lift but will not prevent future shrink/swell, particularly of the soil below the treated region, because the stabiliser has limited downward penetration.

In one aspect, the invention provides a method of treatment of foundation soil beneath a structure, comprising treating the foundation soil with an ionic treatment agent, and treating the foundation soil with a soil stabiliser to stabilise the foundation soil.

The method can advantageously be used for the treatment of the foundation soil beneath any existing (i.e. already built) structure that is founded on, or set within, reactive (typically clay) soils. Examples include:

• • buildings, whether dwellings or commercial or industrial or civil
  • culverts
  • sewers, pipes and other buried services
  • manholes
  • junction and service boxes
  • flat areas of concrete or other construction types, such as paths, roads, hard standing for cars, car parks or runways
  • silos
  • equipment & tank baseplates or plinths
  • retaining walls (below and preferably also behind them, providing excellent resistance to lateral pressures).

Structures that are set within soils, such as sewers, pipes, other buried services and silos, may have the foundation soil beneath the structure treated in accordance with the invention, and may also have the soil adjacent to the structure treated in a similar manner, i.e. with an ionic treatment agent and with a soil stabiliser. Such adjacent soil may be laterally and/or upwardly adjacent to the structure. In some methods, the treatment may take place around the structure, that is below, laterally adjacent and upwardly adjacent (i.e. above) the structure, for example around a buried pipe.

The ionic treatment agent can serve to limit, or remove, the capacity of the foundation soil to shrink or swell. Ionic treatment agents, also referred to as ionic stabilisers, are known. They use cation exchange to alter the shrink/swell property of the soil. The effect is usually permanent. Ionic stabilisation is used pre-construction on roads, flat areas of concrete, and buildings. In these applications it is sprayed and mixed into the foundation soil and then compacted. In remedial applications it has only been used to a limited extent, to alleviate swell and to relax existing heaving. Once treated the soil no longer attracts and holds onto moisture, becoming free draining as well as compressible/compactable at lower pressure. Treated soils are therefore prevented from lifting further and will relax, often dropping as the weight of a structure above compresses and compacts the soil.

Examples of ionic treatment agents are “AGSS-ICS” (Ionic Clay Stabiliser as supplied by Advanced Geotechnical Soil Stabilization), “Condor SS” as supplied by Earth Science Products, and “EcSS 3000” as supplied by ESSL Environmental Soil Stabilization.

The treatment with the ionic treatment agent and the soil stabiliser may be carried out on different portions of the foundation soil (as discussed further below), or may be carried out on the same portion of the foundation soil.

The soil stabiliser may stabilise the soil by increasing its strength. The soil stabiliser may stabilise the soil by increasing its compressive strength. The soil stabiliser may stabilise the soil by increasing its weight bearing capacity.

The soil stabiliser may stabilise the soil by increasing its shear strength.

The soil stabiliser may bind with soil particles to create a cohesive mass.

The soil stabiliser may swell during curing.

The soil stabiliser may be hydrophobic, or it may be hydrophilic.

The soil stabiliser may expand to compact and compress the foundation soil, expelling and displacing water as it does so.

The soil stabiliser may occupy at least some pore spaces in the soil. Such pore spaces may include interstitial spaces between soil particles, and/or cracks in the soil, and/or fissures in the soil. In embodiments where the ionic treatment agent and the soil stabiliser are applied to the same portion of the foundation soil, the soil stabiliser may occupy the volume in the soil previously occupied by water and then vacated as the soil becomes free draining as a result of treatment with the ionic treatment agent. Thus, the soil stabiliser may displace the water. Depending on the sizes and spatial distribution of the soil particles and the pore spaces, the travel of the soil stabiliser may be limited to filling cracks, fissures, and interconnected voids with a throat size which permits entry of the stabiliser, as determined by the viscosity, injection pressure, and cure time of the stabiliser.

The soil stabiliser may form a solid or a gel in the foundation soil and thereby stabilise it. The solid or gel may be injected as a low viscosity resin, aqueous solution, or be a suspension in water, and will spread through pores in the foundation soil. Other examples of solids and gels are described below. The solid or gel may at least partially occupy interstitial spaces between soil particles.

The soil stabiliser may comprise a material which expands in the foundation soil. For example the stabiliser may comprise two components which react to create foam, or may comprise a material which reacts with water and/or with minerals in the soil to create foam. The expansion may lift the structure above the foundation soil or it may be more modest and only occupy pores in the soil without causing lift.

The plural component example usually expands with great force. Adjacent soil is displaced and compacted by the expansive force and the soil above, being confined by the weight of the structure, is compacted. A single component example generally has a week expansive force and will self-inject through the surrounding soil as it expands.

The soil stabiliser may comprise a non-expanding material. Such a material may be a single or plural component material.

The soil stabiliser may comprise a soil stabilising polymer.

The soil stabiliser may comprise polyurethane.

The polyurethane may be a plural component polyurethane which foams and provides lift through chemical reaction between the components. Examples are Tam Geotek HS as supplied by Normet Singapore Pte Ltd, Cormix's “Contite” range of plural component polyurethanes, and “Purl SJ-64” and “Purl SJ-90” as supplied by Liquimix Australia. These plural component polyurethanes each form a rigid foam, i.e. a solid.

The polyurethane may be a single component polyurethane which foams by reacting with water and/or minerals in the soil. Such a product may not provide lift. An example is Tam Geotek LV as supplied by Normet Singapore Pte Ltd. This reacts when it comes into contact with water, forming a rigid foam, i.e. a solid.

The polyurethane may be a non-foaming polyurethane. This may be a plural component product. An example of a non-foaming polyurethane is TamPur 116 as supplied by Normet Singapore Pte Ltd. This is a two component product which sets to form a semi-flexible solid.

The soil stabiliser may comprise a colloidal silica. An example is Tam Geo Tek CS as supplied by Normet Singapore Pte Ltd. This is a single component material, which when set forms a gel.

The soil stabiliser may comprise an acrylic (acrylate). The acrylate stabiliser may be soluble. Examples of acrylate stabilisers are Tam Geo Tek AC as supplied by Normet Singapore Pte Ltd, and “InjectPro-PM3811-SoilStabiliser” as supplied by Aquaf in. These are acrylic grouts, which when set form a resilient gel.

The soil stabiliser may comprise a cementitious material, i.e. a solid.

The soil stabiliser may comprise a microfine cement. An example is Tam Geo Tek MFC as supplied by Normet Singapore Pte Ltd. This sets into a solid form.

The soil stabiliser may comprise silica (silicate). The silicate stabiliser may be soluble. An example of a silicate stabiliser is Tam Geo Tek SS as supplied by Normet Singapore Pte Ltd. This is a sodium silicate, which sets into a solid form. Another example is Tam Geo Tek CS, which is a colloidal silicate, as supplied by Normet Singapore Pte Ltd. This is a single component material, which when set forms a gel.

The soil stabiliser may comprise an organic monomer.

The soil stabiliser may comprise a vinyl acetate.

The soil stabiliser may comprise a urea.

The soil stabiliser may comprise an epoxy.

At least in the preferred methods of the invention, the soil stabiliser comprises grouts, which may be pumped into the soil. The grouts may be cementitious grouts, or chemical grouts or polymer grouts. Examples of grouts include particulate mineral materials, such as fly ash, sand, or a mixture of fly ash and sand. In the case of cementitious grouts and particulate mineral materials, it is the hydraulic pressure of the pump that provides lift to the structure. A system known as mud jacking uses particulate mineral materials which are pumped into soil to create lift, and this is a suitable form of soil stabilising treatment which can be used to stabilise the foundation sale in the present invention. Also suitable are any other products which are pumped at sufficient pressure to cause lift.

The characteristics of the soil stabilisers of the embodiments of the invention in terms of the interaction of the soil stabilisers with the foundation soil may be summarised as follows:

• • 1. Non-expanding soil stabilisers are typically low viscosity and are generally pumped at low pressure. The stabilisers move into the available pore spaces without pushing the particles about too much.
  • 2. Single component polyurethane soil stabilisers react with water in the soil to create a foam. The expansion force of the foam is quite weak, but sufficient that the foam self-injects into the soil. There is some movement of particles within the soil.
  • 3. Plural component polyurethanes create a foam ball within the soil, creating high expansive forces which move and compact the surrounding soil. The foam remains localised and is not intended to fill in situ pore spaces.

The soil may be treated with the ionic treatment agent and the soil stabiliser at the same time. Thus the ionic treatment agent and the soil stabiliser may be simultaneously discharged into the foundation soil, as a single “shot”. The ionic treatment agent and the soil stabiliser may be mixed together before being discharged into the foundation soil. Simultaneous application of the two treatment components may be helpful in providing an efficient and quick method of treatment of the foundation soil.

However, it will often be convenient to carry out the treatment steps sequentially. This may then allow the same injection tubes to be used for each treatment step. The treatment steps may be commenced within 6 months or 4 months or 2 months or 1 month of each other. If for example the foundation soil is swollen, it may be beneficial to treat it with the ionic treatment agent and then allow time for shrinkage before carrying out the treatment with the soil stabiliser. The treatment steps may be commenced within 30 days of each other, such as within 14 days, 7 days, or 5 or 4 or 3 or 2 days. The treatments may be commenced on the same day.

The treatment with the soil stabiliser may be carried out before the treatment with the ionic treatment agent. In most cases, however, the treatment with the ionic treatment agent will be carried out before the treatment with the soil stabiliser. This is particularly effective if the same portion of the foundation soil is subjected to the two treatments. The portion of the foundation soil may effectively be conditioned by the ionic treatment agent, ready for treatment with the soil stabiliser.

The method may comprise injecting the ionic treatment agent into the foundation soil and injecting the soil stabiliser into the foundation soil.

Tubes may be used to inject the ionic treatment agent and the soil stabiliser into the foundation soil. The same tubes may be used for both injections. Such a method is non-disruptive, in that both materials are installed through the same tubes.

At least some of the tubes may be left in place after the respective treatments have been carried out. This could provide sheer strengthening of the soil, in addition to that which may be provided by the treatment with the soil stabiliser.

Foundation soil is considered to extend both beneath a structure and outwardly at an angle from the perimeter of the structure, referred to as the angle of repose. The method involves treatment of at least the foundation soil beneath the structure, and may additionally involve treatment of the foundation soil situated outwardly of the perimeter of the structure.

The foundation soil extends in a depth direction beneath the structure.

In some methods the treatment with an ionic treatment agent may be carried out on a first portion of the soil above a second portion of the soil, and the treatment with soil stabiliser may be carried out on the second portion. The first portion would then have a reduced shrink/swell capacity and it may not be necessary to treat the first portion with the soil stabiliser. If an expanding soil stabiliser is used, the first portion and the structure above would be lifted by expansion of the second portion. For example, the upward force of a strongly expanding plural component polyurethane would compress and compact the first portion that has only been treated with the ionic treatment agent.

In other methods the treatment with an ionic treatment agent may be carried out on a first portion of the soil below a second portion of the soil, and the treatment with soil stabiliser may be carried out on the second portion. These methods are at least applicable to reactive foundation soil which has reduced in volume and shrunk. The ionic treatment agent may be installed before the soil stabiliser, with the soil stabiliser being used to fill any voids in the foundation soil beneath the structure, fill cracks, and consolidate loose soil near the surface, and provide lift if needed. The soil stabiliser may be one which can add significant volume and provide lift, so would likely be a plural component polyurethane, providing lift through a chemical foaming reaction, or a grout, providing lift through hydraulic pressure of the injection pump.

It is expected however that in most cases, in particular where lift is not required, both the ionic treatment agent and the soil stabiliser would be carried out on the same portion of the foundation soil. This would allow the use of the same treatment equipment to be used for both operations, for example the same injection tubes and the connectors to those tubes.

By combining treatment of the foundation soil with an ionic treatment agent and with a soil stabiliser, it is possible effectively to remediate shrinkage of the foundation soil, whether existing or potential. There is a benefit not just for structures which have subsided, but the treated foundation soil will also no longer be subject to swelling in the case of increasing moisture content. Therefore if surrounding conditions are changed in a way which would increase moisture content, such as with tree removal, heaving of the structure can be mitigated.

The treatment may be used to bring a structure back to a level condition.

The treatment may be used to lock in the foundation soil, i.e. to avoid the long-term reduction in foundation soil volume as the soil moves laterally with each wetting and drying cycle.

The major direction of shrinkage under a structure is downwards rather than lateral, hence there can be a problem with soil swell beneath an adjacent structure such as a path. The footing of a structure such as a building may prevent swell beneath and in the direction of the footing and consequently increase its impact on a path. These paths are commonly caused to lift at the edge furthest from the building and as a result water starts to run off towards the building, where it is channelled through a newly developed gap between the path and the building. The extent of swell outwardly adjacent to the footing, owing to moisture changes, therefore increases with each cycle and the extent of shrink diminishes. Thus by using the treatment to reduce or avoid long-term reduction in foundation soil volume beneath a first structure, such as a building, the problem of swelling beneath an adjacent second structure, such as a path, may be mitigated. In this example, application of the ionic treatment agent beneath the path and outward of its perimeter would also be beneficial.

The use of the ionic treatment agent can create a homogeneous and porous, i.e. free draining, soil, which can then be stabilised by the soil stabiliser.

When the method is carried out along the length of a footing, changes in the moisture level of the soil below the structure inwardly of the footing can be minimised. The treated soil below the footing can effectively create a curtain which protects the soil inwardly thereof against moisture ingress or egress.

The treatment with the ionic treatment agent may be carried out directly under a central region of a slab of the structure. This can be of assistance where the foundation soil has heaved. Differential moisture content between the soil beneath the slab and the surrounding area promotes the suction of water towards equilibrium, causing the slab itself to “dome” or “dish”. If this problem is not addressed in its early stages and soil beneath the slab heaves, then usually the only repair option is to remove and replace the entire floor slab. Installation of the ionic treatment agent directly under the central region of the slab can alleviate an existing heave problem.

The method may further comprise treating soil around a perimeter of the structure with the ionic treatment agent. This would be done in addition to treating the soil beneath the structure with the ionic treatment agent. The soil may be treated around the perimeter of the structure with the ionic treatment agent over a region extending up to 1 m or 2 m or 3 m from the structure. By treating soil around the perimeter of the structure, suction by the soil in this region of moisture away from the soil under the structure, and conversely, suction by the soil under the structure of moisture away from this region, is avoided, helping to ensure that the soil beneath the structure is further protected from the effect of moisture changes. Since the moisture content of the soil beneath a structure usually changes in response to moisture changes in the soil around its perimeter (because the structure itself prevents rainfall onto the soil beneath it), treatment of the soil around the perimeter of the structure with the ionic treatment agent can provide a major benefit.

In the above method, a soil stabiliser which is impervious to water may be used beneath the structure, inwardly of its periphery, such as beneath a footing. There is then a double barrier to water ingress to, and egress from, the region beneath the structure, comprising the soil in the perimeter region which provides good drainage due to its treatment with the ionic treatment agent, and the soil beneath the structure treated so as to become a curtain impervious to water.

Because the ionic stabiliser changes the characteristics of the soil, it can also allow for the soil stabiliser to move more freely through the soil, enabling the creation of a homogenous, stabilised and strengthened section of foundation soil.

After treatment, the foundation soil may have the following advantages:

• • resistance to volume change as a result of moisture variation, being resilient to cyclical changes as well as long term equilibrium moisture content changes (both wetting and drying);
  • reduced reliance on surface water control and soil moisture content management (e.g., removing trees, hard standing around buildings etc.);
  • the creation of a homogenous foundation soil in which differential movement is prevented and even footing support is provided;
  • increased strength;
  • permanent results; and
  • measurable outcomes, in that penetrometer and reactivity testing can be carried out.

The method can be carried out as a remedial or a preventative method. The method is carried out as a remedial method if a problem already exists in the foundation soil of a structure. The method is carried out as a preventative method to avert or mitigate a problem in future.

The method may be used as a preventative method to prevent movement in known problem areas or to mitigate against future moisture changes (e.g., trees being planted or cut down).

When a new structure is to be built adjacent to an existing structure, it is common that excavation causes the foundation soil of the existing structure to dry out quickly, resulting in shrinkage, cracking, and crumbling. Therefore the method may be used on an existing structure when a new structure is to be built nearby.

In one possible preventative method, the soil may first be injected with the ionic treatment agent, before being injected with either a silicate or acrylic (acrylate) based stabiliser (no lift required). The best stabiliser to use will partly depend on the particle size and void factor of the existing soil and its moisture content.

In situations where there has been a long-term reduction in equilibrium moisture content, with resulting shrinkage of the foundation soil, the method may comprise treating the foundation soil by adding water. This may at least partially lift the structure using the swell potential of the foundation soil, e.g. reactive clay. The water may be injected. Injection may be carried out using the same tubes as are used for the other treatments.

In situations where there has been a long-term increase in equilibrium moisture content, after treatment of the foundation soil with the ionic treatment agent, the treated region may be allowed to relax, and therefore shrink, before treatment with the soil stabiliser is carried out.

In some methods, the foundation soil may be artificially dried before the treatment with the ionic treatment agent is carried out.

Thus, the method may involve managing the moisture content of the foundation soil. This can be done by the addition of water or by drying out. In the former case, the expansive force of the reactive soil is used to increase volume and provide uplift. Water can be added beneath a footing for a slab using a soaker hose or by deep injection through tubes.

In an exemplary method, a plurality of tubes may be spaced apart at approximately 500 mm to 1.5 m centres and be positioned in a row. More than one row with tubes spaced apart along the row may be employed, with the rows spaced apart by between 500 mm and 1.5 m. The row closest to the structure may pass (be drilled) through the footing of the structure and into the soil below. An area up to e.g. approximately 2 m outside the perimeter of the structure may be treated in its entirety. The depth of treatment may be up to between 1 m and 3 m, but may be deeper, for example up to 5 m, depending on the structure whose foundation soil is being treated and site conditions. By the depth of treatment, it is meant the deepest point at which the treatment material is discharged into the foundation soil. Some tubes used for injection have the directional valve outlets distributed over a length of the tube and can be sleeved or contain expandable packers, which enable control over the pressure and volume of treatment over a range of depths. The most distal outlet will be located at the deepest point of discharge.

The tube locations discussed above are applicable to treatment steps with both the ionic treatment agent and the soil stabiliser. Unless an unusually substantial water barrier is required, application of the soil stabiliser will in most cases be limited to beneath a footing of the structure and may extend to include the angle of repose. The same or different tubes may be used for the respective treatment steps.

EXAMPLE

The foundation soil is treated using “AGSS-ICS” as an ionic treatment agent. Through cation exchange the ionic treatment agent removes the ability of clay particles to attract and hold on to water. The soil will now become less cohesive and this makes the soil free draining and allows the soil stabiliser to pass more freely through the soil into pores between the particles.

The foundation soil is injected with the soil stabiliser. Depending on the strength requirement of the foundation and characteristics of the soil, the stabiliser may be a single component moisture activated foaming polyurethane, a plural component foaming or non-foaming polyurethane, a colloidal silica, acrylic, etc. The stabiliser will create a homogenous, strong and water resistant foundation/underpin.

If additional lift is required, this can be achieved through:

• • 1. Slab jacking—injecting plural component polyurethane foam through holes drilled directly into the footing. The foam is injected into the region above the newly stabilised foundation soil and uses it as a base on which to push. The slab jacking is used as a final step to fine tune the lift and ensure continuous support of the structure. Slab jacking can alternatively or additionally be carried out at any stage in the process (e.g., to fill voids beneath a raft footing) prior to the treatment with the ionic treatment agent or after that treatment and prior to the treatment with the soil stabiliser.
  • 2. Deep injection of a plural component polyurethane, in accordance with the first known remedial system discussed above. This would be installed to raise the structure.

In some embodiments, the deep injection of a plural component polyurethane may negate the need to stabilise the portion of the soil which has been treated with the ionic treatment agent. In other words, a first portion of the soil below the structure may be treated with ionic treatment agent, and then lower down, at the base level of that first portion, a second portion of the soil below the structure may be treated with the soil stabiliser.

Claims

1. A method of treatment of foundation soil beneath a structure, comprising treating the foundation soil with an ionic treatment agent, and treating the foundation soil with a soil stabiliser to stabilise the foundation soil.

2. A method as claimed in claim 1, wherein the soil stabiliser forms a solid or a gel in the foundation soil and thereby stabilises it.

3. A method as claimed in claim 1, wherein the respective treatments are carried out sequentially.

4. A method as claimed in claim 3, wherein the treatment with the ionic treatment agent is carried out before the treatment with the soil stabiliser.

5. A method as claimed in claim 3, wherein the treatment with the soil stabiliser is carried out before the treatment with the ionic treatment agent.

6. A method as claimed in claim 1, wherein the respective treatments are carried out simultaneously.

7. A method as claimed in claim 1, wherein the respective treatments are carried out on the same portion of the foundation soil.

8. A method as claimed in claim 1, wherein the treatment with the ionic treatment agent is carried out on a first portion of the foundation soil above a second portion of the foundation soil, and the treatment with the soil stabiliser is carried out on the second portion.

9. A method as claimed in claim 8, wherein the soil stabiliser in the second portion expands to cause compression of the first portion.

10. A method as claimed in claim 8, wherein the soil stabiliser is discharged into the second portion at a pressure which causes compression of the first portion.

11. A method as claimed in claim 1, wherein the treatment with the ionic treatment agent is carried out on a first portion of the foundation soil below a second portion of the foundation soil, and the treatment with the soil stabiliser is carried out on the second portion.

12. A method as claimed in claim 1, comprising injecting the ionic treatment agent into the foundation soil and injecting the soil stabiliser into the foundation soil.

13. A method as claimed in claim 12, wherein tubes are used to inject the ionic treatment agent and the soil stabiliser into the foundation soil.

14. A method as claimed in claim 1, wherein the treatment with the ionic treatment agent is carried out directly under a central region of a slab of the structure.

15. A method as claimed in claim 1, further comprising treating soil around a perimeter of the structure with the ionic treatment agent.

16. A method as claimed in claim 1, wherein the soil stabiliser comprises polyurethane.

17. A method as claimed in claim 1, wherein the soil stabiliser comprises acrylic.

18. A method as claimed in claim 1, wherein the soil stabiliser comprises silicate.

19. A method as claimed in claim 1, wherein the soil stabiliser comprises a material which expands in the foundation soil.

20. A method as claimed in claim 1, wherein the soil stabiliser fills at least some pore spaces in the foundation s