WO2000070152A1

WO2000070152A1 - Method and apparatus for soil stabilisation

Info

Publication number: WO2000070152A1
Application number: PCT/GB2000/001834
Authority: WO
Inventors: Peter Hay; John Hay; John Rutt; Christopher Malyzewicz
Original assignee: Bioguard Limited
Priority date: 1999-05-18
Filing date: 2000-05-18
Publication date: 2000-11-23
Also published as: GB9911478D0; AU4596700A

Abstract

The invention provides a method of stabilising a soil substrate comprising positioning within the soil a reservoir of anions and multivalent cations, and providing means for leaching the anions and polyvalent cations out of the reservoir such that they interact with each other and/or cations and anions in the soil to form insoluble reinforcing compounds.

Description

METHOD AND APPARATUS FOR SOIL STABILISATION

This invention relates to a method and apparatus for soil stabilisation using a piling technique.

BACKGROUND OF THE INVENTION

The recent impact of global warming and its effects has manifested itself in the UK, European and other parts of the World in the form of subsidence due to the drying up of the clay sub strata. In another manifestation, neglect, poor engineering and the admixture of what were deemed to be acceptable materials has meant that slip planes have rendered a significant proportion of the embankments and cuttings in the national railway network to be dangerous and in urgent need of repair. In yet another manifestation a site containing contaminated soil will have the contaminated soil dug up and removed to another part of the site or to specialised landfill or use other highly expensive processes to render the material less likely to harm the environment.

Current techniques for repairing subsidence, damaged housing and buildings vary from simple underpinning techniques such as creating new and deeper foundations in whole or part of buildings to extensive piling and rafting. This remediation work is very expensive as it occurs in built up and densely populated areas, which may require buildings to be evacuated.

Slip planes in embankments are corrected and repaired in a variety of ways, usually with piling techniques using sheet steel piling, core piling using "lime" piles, and other filled piles using concrete and steel in various forms. All of these techniques require extensive rehabilitation of the site after work has been completed. In subsidence claims the cost of the making good the damage is many times more expensive than the underpinning, which in itself is costing insurance companies in the UK many millions of pounds in claims each year.

SUMMARY OF THE INVENTION

The present invention sets out to overcome the above problems by providing a method of changing the ionic content of a soil to bring about solidification of the soil. More particularly, the invention provides a means of artificially inducing diagenesis in the soil.

Accordingly, in one aspect, the invention provides a method of stabilising a soil substrate comprising positioning within the soil a reservoir of anions and multivalent cations, and providing means for leaching the anions and multivalent cations out of the reservoir such that they interact with each other and/or cations and anions in the soil to form insoluble reinforcing compounds.

In another first aspect, the invention provides a method of reinforcing a soil substrate, which method comprises positioning in the soil a reservoir of anions and multivalent cations and anions; providing means for applying an electrical potential to the soil so as to promote migration of the anions and multivalent cations from the reservoir into the soil; such that the anions and multivalent cations interact with each other and/or with cations and anions in the soil to form insoluble reinforcing compounds.

The means for applying an electrical potential to the soil preferably takes the form of electrodes. Power may be supplied to the electrodes to effect migration of the ions, but more preferably the electrodes are constructed and disposed so as to form a battery within the soil, the potential between a cathode and an anode providing the necessary electromotive force (emf) to bring about ionic migration. By setting up in the soil what is effectively a battery, much lower potentials can be generated and hence migration of the ions occurs without significant hydrolysis taking place at the electrodes. Polarisation of the electrodes resulting from the generation of hydrogen gas is therefore substantially avoided.

The electrodes are typically in the form of piles which serve as reservoirs of the anions and/or multivalent cations as well as having the necessary structure and properties to enable them to function as electrodes. The piles can therefore take the form of elongate containers for the anions and/or multivalent cations.

A pile (e.g. electrode) can have a single polarity; for example it can function either as a cathode or an anode. If the pile does have a single polarity, a conductor is provided for linking a pair of electrodes, one forming the anode and the other forming the cathode.

However, in one preferred embodiment, the pile is constructed so as to define both positive and negative electrodes. The pile can therefore be partitioned to provide an anode compartment and a cathode compartment separated by an insulator, each compartment containing an electrolyte, and the anode and cathode being electrically linked.

In one embodiment, the electrodes can be formed as discrete electrode elements linked by a conductor (e.g. an insulated conductor) passing between the compartments.

In another embodiment, the electrodes can be defined by portions of a single conductor. For example, the electrodes can be defined by portions of a conducting rod or bar (e.g. formed from a ferrous metal such as iron or steel) passing through the pile. The conductor can be coated in a specific manner. The portions of the conductor can be coated or otherwise modified by suitable treatment in order to provide the correct polarity. For example, an anode can be formed by coating a portion of a steel bar or rod with carbon or copper, whilst a cathode can be formed by coating a portion of the bar or rod with zinc. The anode and cathode portions of the conductor are delineated by means of a body or layer of an insulating material surrounding the conductor, the body or layer of insulating material forming a barrier which divides the interior of the pile into anode and cathode compartments and separates the electrolytes associated with the two electrodes.

Accordingly, in a further aspect, the invention provides a pile for use in stabilising soils, the pile comprising an elongate casing having an interior divided into an anode compartment and a cathode compartment by an insulator, each compartment containing an electrolyte, and a conductor rod or bar passing along the interior of the pile through the insulator, rod or bar, the portion of the rod or bar passing through the anode compartment serving as an anode, and the portion of the rod or bar passing through the cathode compartment serving as the cathode.

The pile of the invention can have a penetrating end and a non- penetrating end, the penetrating end being shaped (e.g. pointed) so as to enable the pile to be driven into the earth. The penetrating end can be constructed of a material such that it forms either the negative plate (cathode) of the battery set up, or the positive plate (anode). Thus, for example, the penetrating end can be formed from a metal which constitutes one of the electrodes. Where the pile has a penetrating end it can be driven into the soil by means of a suitable force, e.g. a percussive force exerted on its upper end. However, the pile need not have a penetrating end and, in such a case, can be installed in the soil by pre-boring a hole into which the pile is inserted.

The pile is provided with perforations or openings in its walls, or is otherwise permeable or semi-permeabie to allow ions to migrate into the surrounding soil from the anode and cathode compartments.

The materials selected for the anode, cathode and electrolyte can be varied as required to suit the different chemistries of different types of soils and the characteristics of the soil substrate being treated.

Soil substrates typically will contain clays. Clays can be ordered in terms of their plasticity characteristics, and the plasticity of the clays can be can be modified by substitution of metallic ions of the higher order of substitution according to the scale: Li < Na > NH₄< K < Mg < Rb < Ca <AI. Thus, the multivalent cations employed in the methods and piles of the present invention are preferably metals to the right hand side of the scale, and are typically selected from divalent and trivalent cations such as calcium, magnesium and aluminium, calcium ions being particularly preferred. The calcium ions are preferably derived from a calcium sulphate, and more particularly to a calcium ferrosulphate (calcium ferrite) , for example a calcium ferrosulphate having the formula (CaSO₄)₂.H₂O, 10% Fe(OH)₆.

The cations and anions migrating or leaching out of the pile recombine with each other, or combine with counter ions in the soil, to form insoluble complexes thereby bringing about solidification of the soil substrate in the region of the pile. It is most preferred that the combination or recombination of the cations and anions takes place in the interstices between soil particles thereby binding the soil particles together. More preferably still, the cations and anions recombine with each other or combine with counter-ions in the soil to form crystals which grow in the interstices between soil particles. Thus, by means of the invention, the soil is subjected to accelerated diagenesis which can lead to the soil assuming a rock-like structure within a relatively short period of time, for example less than two years.

Accordingly, in another aspect, the invention provides a method of stabilising a soil substrate by inducing the formation of crystalline substances in the interstices between soil particles, thereby to bind the soil particles together, wherein the induction of the formation of crystalline materials is effected by positioning within the soil a reservoir of anions and multivalent cations, and providing means for leaching the anions and multivalent cations out of the reservoir such that they interact with each other and/or with cations and anions in the soil to form the crystalline substances.

In another aspect, the invention provides a method of reinforcing a soil substrate by introducing insoluble salt-forming or crystal forming cations and anions into the soil and forming an electrical circuit in the soil to effect movement of the cations and anions, which method comprises placing within the soil a cathode and an electrolyte associated with the cathode, the cathode electrolyte comprising anions and counter-ions thereof; and an anode and an electrolyte associated with the anode, the anode electrolyte comprising multivalent cations and counter-ions thereof; whereby an electrical potential is established in the soil between the cathode and the anode such that the cations migrate through the soil towards an adjacent cathode and the anions migrate through the soil to an adjacent anode, the migrating cations and anions combining with each other and/or with anions and cations in the soil to form insoluble and preferably crystalline substances in the interstices between soil particles, thereby to bring about solidification of the soil.

In a still further aspect, the invention provides a method of reinforcing a soil substrate by inducing diagenesis in the soil using a pile as hereinbefore defined.

The movement of ions in the soil substrate can be controlled by, for example, controlling the positioning and orientation of the piles and/or varying the construction of the piles to give appropriate cell potentials. In this way, the movement of ions into or through selected strata or adjacent strata types can be carefully controlled.

For example, anodic and cathodic piles can be presented separately and introduced into the ground in close proximity such that there is a flow of ions between them. In such circumstances, the anodic and cathodic piles would need to be electrically linked by means of a conductor which can extend through the soil or above the soil.

In an alternative, and preferred form, each pile has both anodic and cathodic compartments. Such "bipolar" piles can be positioned such that adjacent piles are in a "head-to-head" configuration or in a "top-to-tail" configuration. The relative orientation of the piles, together with the distance between adjacent piles will determine the primary direction of migration of the ions and hence the pattern of soil solidification produced. For example, in the case of "head-to-head" configuration in which the distances between adjacent electrodes are greater than the distance between the two electrodes in a single pile, the flow of ions will tend to be primarily between the anodic compartments and cathodic compartments of the same electrodes, such that the reinforcing effect of the process extends outwardly from the pile. Conversely, if adjacent piles are arranged in a "top- to-tail" configuration and the distance between the piles is less than the flow path between the two electrodes in one pile, the ions will tend to flow between adjacent electrodes and hence the area between the electrodes will tend to become reinforced.

In a further embodiment of the invention, a battery can be formed in the soil simply by boring or otherwise forming a hole in the soil, placing an electrode (e.g. a steel bar suitably coated or surface treated as necessary to provide the desired polarity) in the hole, and surrounding the electrode with an electrolyte. In this embodiment, the container structure can be omitted, the mass of electrolyte surrounding the electrode and the walls of the hole or any backfill material therein serving as the reservoir. This embodiment is particularly suited to forming single polarity electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be illustrated by way of example only with reference to the drawings, of which:

Figure 1 which is a schematic sectional elevation through a pile according to one embodiment of the invention; and

Figure 2 is a schematic view illustrating a pair of piles according to a second embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in Figure 1 , in one embodiment, a pile comprises a tubular outer casing 2 formed from a suitably tough plastics material. Casing 2 is perforated at two regions 4,6 along its length with a plurality of small holes

8.

At the lower end of the pile is provided a penetrating tip 1 0 comprising a plastics cone 1 2 covered by a shaped steel cover 1 4. Running through the centre of the pile is a steel rod 1 6 which, at its forward end 1 6a extends into the shaped penetrating tip 10 and at its rear end is secured to a steel "push plate" 1 1 .

The interior of the pile is divided into two compartments by a dividing wall 18 formed from an insulating material such as a plastics material. The "upper" compartment 20 functions as a cathode compartment and the portion 24 of steel rod 1 6 passing through this compartment is coated with zinc so as to form a cathode. The "lower" compartment 22 functions as an anode compartment, and the portion 26 of steel rod passing through the anode compartment is coated with carbon, or copper, or another metal lower down the electrochemical series than zinc, so as to form an anode. In addition to the anode and cathode portions of the steel rod, each of the compartments contains an electrolyte. Thus, the anode compartment contains a solid mixture of calcium ferrosulphate (which is a calcium sulphate salt having the formula (CaSO₄)₂.H₂0 plus 1 0% Fe(OH)₆) containing 0.01 to 30% (w/w) of a phosphate such as sodium dihydrogen phosphate (NaH₂PO₄), ammonium dihydrogen phosphate (NH₄(H₂PO₄) and calcium hydrogen phosphate (CaHPO₄) . The cathode compartment contains calcium ferrosulphate containing 0.01 to 50% ammonium chloride.

In use, the pile is driven (or more preferably placed) into a soil substrate, such as an embankment or other area of soil requiring reinforcement and stabilization, the push plate 1 1 providing a reinforced point of contact for a pile driver, or other percussive driving means. Thereafter, the pile functions as a battery with the zinc coated region of the rod or any other sacrificial element capable of producing an E.M.F. and the carbon, copper coated or transitional metal coated portion of the conductive rod serving as the negative and positive electrodes respectively, and the calcium ferrosulphate functioning as an electrolyte. The steel rod provides the direct conducting connection between the electrodes, whereas the electrolytes in the anode and cathode compartments, and the electrolytes in the soil surrounding the pile complete the electrical circuit.

An electrical field is set up within the soil surrounding the pile, the various ionic species in the soil thus becoming part of the battery system. Thus, zinc from the zinc coated portion of the steel rod passes into solution as Zn^{2 +} ions and calcium ions migrate out of the cathode compartment 20 through perforations 8 into the surrounding soil under the influence of the electrical potential between the two electrodes. At the same time, sulphate and phosphate ions migrate out through perforations 8 and into the surrounding soil under the influence of the electrical field. As the calcium ions migrate through the soil under the influence of the electrical field established between the two electrodes, they combine with counter ions, for example silicates or aluminosiiicates derived from clay, or phosphates, or sulphates or phosphates from the anode compartment to form insoluble salts or compounds. The formation of insoluble salts or compounds tends to take place within the interstices between soil particles and, over a period, constant precipitation of the insoluble materials leads to the growth of crystals in the soil interstices. This has the effect of binding the soil particles together and, over a period, the build up of deposits of insoluble crystalline materials in the soil leads to the soil gradually acquiring a much more solid and even rock-like consistency.

It will be appreciated that by positioning an appropriately large number of piles in a soil substrate (such as an embankment) requiring stabilization, what was previously an unstable soil substrate can be transformed into a stable, rock-like structure.

Figure 1 illustrates a single pile but it will be appreciated that a plurality of piles will typically be used in order to reinforce a given area of ground. The piles may be aligned in the same direction ("head-to-head") or in opposite directions ("top-to-tail"), the relative orientation of the piles determining the pattern of the depositions of calcium sulphate in the intervening soil. Where the piles are aligned in a "head-to-head" configuration, the deposits will tend to build up in the immediate vicinity of the pile, spreading outwards from the pile. On the other hand, if the piles

100, 200 are arranged "top-to-tail"as shown in Figure 2, the movement of ions will tend to be from one pile to the other, with the result that maximum deposition of the calcium sulphate will tend to take place in the intermediate zone IZ between the piles.

In Figure 1 , the piles are shown as having penetrating tips 1 0 and push plates 1 1 to enable them to be driven into the ground by pressurising or percussive pile driving equipment. In this case, compression of the soil takes place and this provides a reinforcing effect to the soil in addition to that provided by the subsequent diagenesis process. However, as an alternative, the penetrating tip and push plate can be omitted and the pile simply inserted into the pre-bored hole. This has the advantage that potential damage to the piles is avoided and the pile can be made of a less rigid material.

In a further variation of the invention, the electrodes can be formed simply by boring or otherwise creating a hole in the ground, inserting a suitable electrode material such as a steel rod, e.g. coated with a metal or carbon to provide the desired electrode polarity, and then surrounding the electrode with a suitable electrolyte. In this embodiment, the container or pile structure can be omitted, the walls of the hole confining the electrolyte and serving as the reservoir.

It will be appreciated that numerous modifications and alterations could be made to the piles shown in the drawings without departing from the principle's underlying invention. For example, alternative electrode materials could be used, or alternative electrolytes could be contained within the casing. All such modifications and alterations are intended to be embraced by this application.

Claims

1 . A method of stabilising a soil substrate comprising positioning within the soil a reservoir of anions and multivalent cations, and providing means for leaching the anions and polyvalent cations out of the reservoir such that they interact with each other and/or cations and anions in the soil to form insoluble reinforcing compounds.

2. A method of reinforcing a soil substrate, which method comprises positioning in the soil a reservoir of anions and multivalent cations and anions; providing means for applying an electrical potential to the soil so as to promote migration of the anions and multivalent cations from the reservoir into the soil; such that the anions and multivalent cations interact with each other and/or with cations and anions in the soil to form insoluble reinforcing compounds.

3. A method according to claim 2 wherein the means for applying an electrical potential to the soil preferably takes the form of electrodes.

4. A method according to claim 3 wherein the electrodes are constructed and disposed so as to form a battery within the soil, the potential between a cathode and an anode providing the necessary electromotive force to bring about ionic migration.

5. A method according to claim 4 wherein the electrodes are in the form of piles which serve as reservoirs of the anions and/or multivalent cations.

6. A method according to claim 5 wherein the piles take the form of elongate containers for the anions and/or multivalent cations.

7. A method according to claim 6 wherein the pile is constructed so as to define both positive and negative electrodes.

8. A method according to claim 7 wherein the pile is partitioned to provide an anode compartment and a cathode compartment separated by an insulator, each compartment containing an electrolyte, and the anode and cathode being electrically linked.

9. A method according to claim 8 wherein the electrodes are defined by portions of a single conductor.

1 0. A method according to claim 9 wherein the electrodes are defined by anode and cathode portions of a conducting rod or bar passing through the pile.

1 1 . A method according to claim 10 wherein the anode and cathode portions of the conductor are coated or otherwise modified by suitable treatment in order to provide the correct polarity.

12. A method according to claim 1 1 wherein the anode and cathode portions of the conductor are delineated by means of a body or layer of an insulating material surrounding the conductor, the body or layer of insulating material forming a barrier which divides the interior of the pile into anode and cathode compartments and separates the electrolytes associated with the two electrodes.

1 3. A pile for use in stabilising soils according to the method of any one of the preceding claims, the pile comprising an elongate casing having an interior divided into an anode compartment and a cathode compartment by an insulator, each compartment containing an electrolyte, and a conductor rod or bar passing along the interior of the pile through the insulator, rod or bar, the portion of the rod or bar passing through the anode compartment serving as an anode, and the portion of the rod or bar passing through the cathode compartment serving as the cathode.

14. A pile according to claim 1 3 which is provided with perforations or openings in its walls, or is otherwise permeable or semi-permeable to allow ions to migrate into the surrounding soil from the anode and cathode compartments.

1 5. A method of stabilising a soil substrate by inducing the formation of crystalline substances in the interstices between soil particles, thereby to bind the soil particles together, wherein the induction of the formation of crystalline materials is effected by positioning within the soil a reservoir of anions and multivalent cations, and providing means for leaching the anions and multivalent cations out of the reservoir such that they interact with each other and/or with cations and anions in the soil to form the crystalline substances.

1 6. A method of reinforcing a soil substrate by introducing insoluble salt- forming or crystal forming cations and anions into the soil and forming a electric circuit in the soil to effect movement of the cations and anions, which method comprises placing within the soil a cathode and an electrolyte associated with the cathode, the cathode electrolyte comprising anions and counter-ions thereof; and an anode and an electrolyte associated with the anode, the anode electrolyte comprising multivalent cations and counter-ions thereof; whereby an electrical potential is established in the soil between the cathode and the anode such that the cations migrate through the soil towards an adjacent cathode and the anions migrate through the soil to an adjacent anode, the migrating cations and anions combining with each other and/or with anions and cations in the soil to form insoluble and preferably crystalline substances in the interstices between soil particles, thereby to bring about solidification of the soil.

17. A method of reinforcing a soil substrate by inducing diagenesis in the soil using a pile as defined in claim 1 3 or claim 14.

1 8. A method according to claim 3 wherein an electrode is formed by introducing into the soil an element such as a bar or rod formed from an electrode material, and surrounding the element with an electrolyte.

1 9. A method according to claim 1 8 wherein an elongate hole is bored or otherwise created in the soil, a bar or rod constituting the electrode element is inserted into the hole and electrolyte is filled into the hole surrounding the electrode element.

20. A pile substantially as described herein with reference to the accompanying drawings.

21 . A method substantially as described herein with reference to the accompanying drawings.