GB2355997A - The underpinning of buildings - Google Patents

Abstract

A method of underpinning of buildings 18 erected upon clay soils by way of the electrophoretic introduction of particular cations, such as calcium ions into the clay soil. Predetermined ions are introduced into the soil by alternate cathodes 11 and anodes 10 being placed in holes 12 and supplied with a chemical substance which releases the ions upon solution in water. This serves to harden the clay soil by effecting stabilisation of various mineral lattice structures within the soil, and thereby helps to reduce the effects of water-induced swelling and subsequent shrinking which can cause heave or subsidence in structures founded on the clay soil in question.

Description

2355997 TFIE UNDERPINNING OF BUILDINGS

The present invention relates to the underpinning of buildings erected upon soils containing clay minerals, especially of montmorillonite and other related types, in situ by using electrophoretic introduction of chemical substances.

Domestic houses and other buildings, constructed of, founded on or supported by clay soils can be susceptible to subsidence or ground heave. This is because clay soils tend to swell significantly upon absorption of water, and conversely tend to shrink when the soil dries. This cycle of expansion and contraction is often linked to the seasonal climatic cycle from winter to summer, and is exacerbated in areas planted with trees, since these extract a much greater volume of water from the soil during summer than winter.

Traditionally the effects of subsidence or ground heave are countered by structural underpinning of affected buildings which serves to stabilise the foundations thereof, achieved by providing concrete piles into the ground. Such a technique is expensive and often disruptive.

According to the present invention there is provided a method of underpinning of buildings erected upon clay soils, wherein at least one hole is formed in the soil, the or each hole is provided with at least one electrode, and wherein, the hole containing the electrode is further supplied with a chemical substance releasing a predetermined ion upon solution in water, and direct current is supplied to the electrode so as to cause electrophoretic penetration of the ion into the clay soil.

The return electrode may be either a further similar electrode in a similar hole or a simple earth electrode.

To provide uniformity of electrophoretic treatment the polarity of the electrodes may be reversed with the necessary chemical substance being added to the desired electrode. 5 The volume occupied by certain types of clay soils, such as those containing montmorillonite clay materials and related mineral types, can be regulated by the addition of potassium or calcium ions. This is because these clay minerals are largely made up of aluminium silicate structures comprising extended lattice layers of aluminium and silicon atoms. These lattices are bonded together by interchangeable ions to a significantly lesser degree by sodium ions than by calcium or potassium ions, which fit the spaces between the aluminium and silicon atoms in the lattice more exactly. In this way the soil beneath a building may be made less susceptible to water induced expansion and shrinkage.

However it is generally not possible to introduce these ions simply by pouring an appropriate aqueous solution over the ground surrounding an affected building, since clay soils of this type have exceedingly small pore spaces and therefore are relatively impervious. By means of the invention a source of these ions would be provided at the anodic electrode.

In order to determine whether the method of the present invention will be appropriate in any given situation, it is preferred to conduct a relatively simple site investigation so as to allow an analysis of the clay soil to be made. In particular, analysis of the clay mineralogy and soil moisture content is required to assess the suitability of the clay for electrophoretic treatment.

It is advantageously determined whether the clay soil is inter-bedded with soil types of permeability greater than that attributable to electroosmotic and/or 2 electrophoretic processes (normally around 5 x 10-7 M/S), since the method of the present invention will then not be effective.

A method of underpinning buildings built upon clay-containing soils embodying the invention will now be described, by way of example, with reference to the drawings, in which figures I and 2 are an embodiment in use, viewed in section and plan respectively.

Referring to figures I and 2, in a presently-envisaged embodiment the electrodes 10, 11 may take the form of metal (e.g. steel) rods, although it is to be appreciated that any suitable electrical ly-conductive material may be used, such material preferably being resistant to corrosion in the conditions of operation. For example, proprietary materials having flexibility, conductivity and resistance to corrosion may be used. When rod shaped electrodes are used, the holes 12 in the ground 20 into which they are to be inserted will generally be formed as bore holes 12 with a diameter slightly greater than that of the rods, thus providing a generally annular gap 25 around each rod 10,11 when this is inserted centrally into a bore-hole. Typical diameters for the bore-holes are from 50 to 500 mm preferably around 75 to 100 mm, and the depth may be from I to in excess of 10 in, typically around 6 in, although it will be appreciated that other dimensions may be useful in particular situations It is envisaged that the electrodes may be spaced from 0. 1 to 5m from one another, although again the precise electrode spacing will need to be determined in accordance with prevailing ground conditions. It has been found that a spacing of around 0.5m is effective for electrophoresis driven by a safe potential difference of around 50V. Preferably, electrodes are positioned so as to enable a relatively uniform electric field to be generated in the volume of soil to be treated. Consideration will therefore also need to be given as to which of the electrodes are to be designated anodes and which cathodes, and to their alignment according to 3 circumstances of access. Sophisticated mathematical modelling is required to determine the most favourable array of electrodes.

In figures 1 and 2, the cathodes I I and anodes 10 are placed in an alternating row running alongside a structure 18 whose footing 22 is embedded in clay soil 20.

Each borehole 12 extends beneath the footing, as do the electrodes. The cathodes and anodes I I are then connected to a common negative rail 16 and a common positive rail 17 respectively. Other arrangements of the electrodes are possible. For example, several cathodes could be placed alongside one side of the structure as shown, and one or more anodes, perhaps bar ely protruding into the ground, or even resting as a plate upon the ground, positioned along the opposite side of the wall.

Alternatively, similar cathodes may be placed along the wall, whilst the opposite terminal of the current source is connected to an earth remote from the electrodes.

In a preferred embodiment, the annular space 25 surrounding each electrode which is designated to be an anode is filled with quicklime, slaked lime or any other suitable source of calcium ions (or potassium or magnesium ions where this is appropriate).

In yet a further embodiment, a cations and anions may be provided in separate, successive stages, and possibly by way of a separate set of bore-holes, or by reversing the polarity of the electrodes or any combination of these.

It is to be noted that the electrodes can be operated without chemicals being added, so as to achieve transport of water out of the clay soil by electro-o smosis (water being a polar molecule), and so directly regulate ground water saturation in the clay soil, and which may subsequently be used to prepare the clay soil before electrophoresis so as to achieve predictable results.

The electrodes to be designated as anodes and those to be designated as cathodes 4 are respectively connected in parallel, generally at ground level, to a source of direct current. A potential difference of suitable magnitude is then applied across the anodes and cathodes so as to cause current to flow from the former to the latter by way of moisture in the soil, thereby causing electrophoretic migration of cations 5 and, optionally, anions into the volume of soil to be treated.

The voltage will normally need to be applied for a period of days or weeks. In a preferred embodiment, the current is applied for a period of from two to eight weeks, although shorter or longer time periods may be appropriate in certain applications. The current may be applied continuously or discontinuously. It may be necessary in some ground conditions to remove surplus ground water which has collected at the cathodes as a result of the electrophoretic process. This may be achieved, for example, by way of intermittent vacuum suction applied within cathodes having a hollow tubular form and surface perforations.

The present invention is particularly convenient for use in regulating clay soils under the foundations of domestic and commercial buildings having easy access to a pre-existing mains electricity supply.

The direct current could be supplied from a battery.

The conversion of, for example, montmorillonitic clay minerals to strongly calcium form should provide sufficient change in their expansive properties to satisfy most requirements. However, it is theoretically possible for the conversion to be reversed if the ground were to be subject to a consistent and strong flow of ground water containing free sodium ions, thereby converting the montmorillonitic minerals to the weaker sodium form (although this form is still generally more stable than montmorillonitic minerals without the presence of any lattice-stabilising, ions). Nevertheless, in clay soils which are reasonably impervious, and in the circumstances of most structures which suffer shrink and swell problems, the hydraulic gradients to drive such a flow are likely to be insignificant even if ground water containing free sodium ions is available. The regulating effect achieved by the electrophoretic introduction of appropriate cations should therefore be sufficient in most applications. If not, it may then necessary to provide an additional cementing stabiliser to induce chemical reactions between the anions and cations so as to achieve an even higher degree of regulation.

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Claims (12)

1. A method of underpinning of buildings erected upon clay soils, wherein at least one hole is formed in the soil, the or each hole is provided with at least one electrode, and wherein, the hole containing the electrode is further supplied with a chemical substance releasing a predetermined ion upon solution in water, and direct current is supplied to the electrode so as to cause electrophoretic penetration of the ion into the clay soil.

2. A method according to claim 1, wherein the predetermined ion comprises calcium ions.

3. A method according to claim I or 2, wherein the ion-releasing chemical is lime.

4. A method according to claim 3, wherein the chemical substance is slaked lime.

5. A method according to any preceding claim, wherein the electrodes are spaced from 0. 1 to 5m from one another.

6. A method according to any preceding claim, wherein the hole(s) are formed to a depth of greater than I meter.

7. A method according to any preceding claim, wherein the holes are circular boreholes 50 to 500 mm in diameter, each provided with an anode or a cathode.

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8. A method according to any of the preceding claims I to 6, wherein the at least one hole is an elongate trench.

9. A method according to any preceding claim, wherein the electric current is applied across a potential difference of 10 to I I OV.

10. A method according to any preceding claim, wherein the electric current is supplied by way of a suitable rectifier connected to an alternating current supply.

11. A method according to any preceding claim, wherein the electric current is supplied from a battery.

12. A method of underpinning of buildings erected upon clay soils substantially as described herein.

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