A method for the stabilisation of soils and restoration of any collapses

Description

[0001] The present invention relates to a method for the stabilisation of soils and the restoration of any collapses, with particular reference to foundation soils.

[0002] The problem of foundations on yielding soils falls within the broader context of the hydro-geological disorder which now chronically afflicts Italy.

[0003] The continual and inconsistent lowering of the water-bearing stratum, occasionally alternating with elevations of local weather origin, causes a constant series of damages both to private and industrial infrastructure and also to historically and/or architecturally valuable buildings. In particular, the lowering of the water-bearing strata and/or the draining of free water causes the start or resumption of the collapses of the soil (subsidence) which lead to fissures in buildings.

[0004] Clay soils have a particular and complex crystalline structure which makes them unique from the standpoint of their reaction to loading in relation to the presence of water.

[0005] Clay minerals are hydrated phyllosilicates with structures having superposed and linked sheets, based in turn on the two-dimensional repetition of tetrahedral units (T) having at their centre a silica atom, or multiple octahedral units (O) having at their centre an aluminium atom.

[0006] The alternating superposition of layers (O) and of layers (T) gives rise to various groups of clay minerals:

minerals with two-layer packets (P/O) such as kaolinite,

minerals with three-layer packets (P/O/T) such as smectites,

minerals with packets having three layers plus one.

[0007] The possibility of isomorphic replacements of silica atoms with aluminium atoms produces, in the packet system, an excess of negative charges which is compensated by molecules of water and positive metal ions, generally alkaline or earthy alkaline, in addition to hydrogen and ammonium ions.

[0008] The cohesive energy between the packets of the layers is not sufficient to contrast the hydration force of the ions so that, by way of compensation, the packets surround themselves with a high and variable number of layers of water and of cations, appearing as lenticular shaped structures contained in sacks of pellicular water molecules and positive ions, having an average diameter of less than 2µm.

[0009] When the free water moves away it creates spaces to the interstitial water which flows off and no longer serves as a support and contour the pellicular water, favouring drainage and the decrease in the natural humidity of the soil, with its consequent loss of volume.

[0010] The pellicular water, no longer held, tends to escape, increasing the values of the geo-technical characteristics of the soil and reducing its volume (consolidation).

[0011] Part of the soil underlying the foundations of the building loses its hydrostatic thrust, increasing its loading to about twice the previous one; i.e. a few years after the construction of the civil works, the soil shifts from the elastic phase to the plastic phase and these phases can involve the structure of the clay micelles. This behaviour in the presence of active clays (smectites) appears with the closure of the fissures during rainy periods and their re-opening during dry periods.

[0012] Traditional methods used for soil consolidation aim at curing the effect but not the causes, are highly invasive and generally very expensive as well.

[0013] Among such method one can mention for instance the "jet grouting" technique, the use of driven or drilled posts, and the injections of cement material or resins to fill the empty spaces created in the soil below the foundations.

[0014] Such interventions in any case leave profound changes in the foundation of the building or in the soil, which can even hinder subsequent interventions and in some cases also present a challenge in terms of precision, since cement injections oftentimes fail to achieve the pre-set goal but go to compress areas far away from those that were to be treated.

[0015] Less invasive techniques such as electrosmosis which allows to move the water in porous septa by means of electrical fields have so far been employed solely for drying soils that are subject to landslides or unstable or for dehumidifying soils and buildings, i.e. for the exact opposite purpose.

[0016] The aim of the present invention is to eliminate the aforesaid drawbacks by making available a method that acts on the causes and not on the effects of the collapses, retains its effectiveness unaltered over time, does not get altered and insofar as possible restores the original conditions of the soil, is not highly invasive and not at all destructive and also has reduced costs.

[0017] Said aims are fully achieved by the method of the present invention, for the stabilisation of soils, in particular soils underlying the foundations of constructions, subject to drainage and/or removal of the water from the soil (free and/or pellicular and/or zeolitic). Said method, unlike those of the prior art, takes place in the absence of a draining cathodic pit, and it is further characterised by the content of the claims set out below and in particular in that it entails the use of electrosmosis to obtain the cessation of drainage and the superficial link, and/or inside the crystal of the clay mineral of the water (present naturally and/or supplied artificially) and of the ions (present naturally and/or supplied artificially), stabilising soil behaviour by stopping collapses and possibly obtaining a partial restoration thereof.

[0018] This is obtained by placing in the soil a first plurality or set of negative electrodes and a second plurality or set of positive electrodes so that, by connecting the two pluralities of electrodes to a power supply, the electrical field thereby applied causes the interruption of said outflow and the inversion of the direction of motion of the water, recalling the water itself.

[0019] Preferably the power supply provides a difference in potential that is opposite to the one caused by the outflow of the water.

[0020] The negative electrodes are normally positioned in the soil immediately underlying the foundations, preferably at the centre of the pressure bulb, whilst the positive electrodes are normally positioned in the soil surrounding the walls a few meters away therefrom and at a distance of about 4-10d from the corresponding negative electrodes, where d is the thickness of the foundation wall overlying the negative electrode.

[0021] This and other features shall become more readily apparent from the following description of a preferred embodiment described purely by way of non limiting example.

[0022] The present method originally provides for the application of electrosmosis technology not to dry or dehumidify but rather to restore to its place the stratum that had moved farther away or had lowered, or the constitution of a humid environment (at least 85% saturation).

[0023] This is done by exploiting the intimate structure of clay soils, which can be compared to a network of innumerable capillaries obtained from the silicate surfaces of the clay lenticules.

[0024] Although obviously the soil as a whole is electrically neutral, in its interior the interstitial water is in the condition of having an electrical potential exceeding that of the surface of the lenticules so that the application of an external electric field induces its migration towards the negative electrodes.

[0025] On the contrary, if interstitial water has the possibility of flowing and moving away, an imbalance of charges will be created with a consequent electrical field, measurable as a difference in potential between different points of the soil, and in fact the present method acts on this.

[0026] If the soil surrounding the masonry does not contain sufficient water because of an excessive removal of the water-bearing stratum, the latter will be created artificially by means of an external sources that injects water directly in proximity to the positive electrodes (anodic wells), which will migrate towards the negative electrodes.

[0027] Electrosmosis technology has the advantage of being economical, non invasive and reversible, so that it allows to place the building in a safe condition for a nearly indefinite time interval, thereby allowing sufficient space, should it be deemed necessary, for the study and planning of restorations and possible consolidation of a different kind.

[0028] The reconstitution of a humid environment with saturation of at least 85% with the salts naturally dissolved, allows to stabilise clays and possibly partially to restore any collapses which may have taken place already.

[0029] Another advantage of the electrosmosis technique of the present invention is given by the increased resistance which may be produced in the soil through a change in the grain size of the fine claim (about 2 µm) existing near the anodes.

[0030] In any case the cost of an intervention using the electrosmosis technology is smaller by about an order of magnitude than the cost of a classic stabilisation intervention by means of micro-posts or jet-grouting.

[0031] In regard to the electrical powers to be installed and to the times required for stabilisation, reference can for instance be made to a typical construction site with a foundation perimeter of 50 m and one can state that the system for reconstituting the stratum needs to absorb a direct current in the order of 1÷3 A at a voltage of about 10÷30 V.

[0032] The water requirement through the anode wells amounts to a few tens of litres for each cubic metre of foundation.

[0033] In the course of the process, in any case, the current flow is reduced asymptotically down to zero due to the conductivity variations in the soil, whilst the demand for water in the system is reduced with an irregular trend.

[0034] The extension of the humid area is completed within a few weeks, returning the conditions of the soil substantially to the original ones.

[0035] The present method comprises the following phases:

• drilling down to a depth which is below the lower plane of the sub-foundation by about 2d (with d= thickness of the foundation wall), and possible desalting of the diffusion area in proximity to the positive electrodes, using one of the known technologies;
• insertion of contact and anti-salt material (for instance, mortar with layered laminar graphite added to 3-3.5% by weight) in proximity to the positive electrodes;
• insertion of the positive electrodes in the holes treated as described and insertion of metallic negative electrodes in the area of the pressure bulb; preferably, each positive electrode is formed by a pair of electrodes: one made of resin and a metallic one, made of a two-component eutectic alloy (for instance tin-lead) or a three-component eutectic alloy (for instance tin-lead-silver), the latter acting as a sacrificial electrode, for it is destined to be consumed;
• application, which may be alternating, of the systems for re-hydrating and dehydrating the pressure bulb until the result is obtained;
• possibly re-supply of external water, with the possible addition of Calcium Chlorides (CaCl₂), Magnesium Chlorides (MgCl₂) to lead to a stabilisation of the soil with the possible restoration of any collapses;
• periodic monitoring of the injected water, of the piezometric levels and of load-bearing capacity.

[0036] The crystalline grid of clays is constituted by sheets of phyllosilicates and ionic layers. With the water recall obtained by the present method, there is a transfer of Al, Mg, Ca ions which enter between the sheets and replace pre-existing ions (Na and K ions). Since Al, Mg, Ca ions have greater diameter than Na and K ions, they surprisingly cause the clays to expand, which allows to restore collapses.

[0037] The use of positive Al and/or Mg ions has the same effect as the addition of calcium or magnesium chlorides into the soil, because said electrodes are destined to be consumed releasing Mg⁺⁺ and Al⁺⁺⁺ ions.

[0038] If "d" is defined as the thickness of the foundation wall, the pressure bulb extends significantly below said wall for about 4d in depth and 2d in width.

[0039] The negative electrode is normally positioned as close as possible to the centre of the pressure bulb (for instance with an oblique hole into which is introduced a metal tip serving as an electrode), whilst the positive electrode is positioned at a distance of4 - 10 d from the negative electrode.

[0040] Electrosmosis consists of the movement of a liquid through a capillary (or through the innumerable capillaries of a porous diaphragm) by effect of the application of a difference in electrical potential.

[0041] The quantity of liquid transferred through the porous septum is proportional to the intensity of the current that traversed the septum and is independent from the area and thickness thereof. When a liquid flows through a porous diaphragm, at its ends a difference in potential (flow potential) is generated which is directly proportional to the pressure difference generating the flow and is independent from the area and thickness of the diaphragm.

[0042] When a glass tube is immersed in a solution containing electrolytes, the solid surface is able to adsorb selectively thereon a part of the dissolved negative ions.

[0043] This leads to an excess of negative charges being built up on the surface of the tube.

[0044] The free cations of the solution tend to crowd in proximity to the surface of the solid phase, initially forming a second, rather orderly, layer of positive charges and progressively towards the interior of the solution a third diffuse layer of cations.

[0045] The set of the first two layers is called Helmoltz compact double layer and its behaviour is similar to that of a planar capacitor whose negative plate coincides with the glass surface and whose positive plate has a diffuse shape starting from the partially orderly layer of cations.

[0046] The thickness of the compact double layer is in the order of about 10^-8 m, i.e. a length equal to the diameter of some cations whilst if the diffuse area is also included a value of 10^-7 m is obtained.

[0047] The effects of the double layer, being superficial, are more evident in very small cylindrical cavities such as capillaries or between two lenticular packets of the clays and this in relation to the fact that for them the surface / volume ratio is very large.

[0048] The cavity of interest are the innumerable small channels which are present in the most common lithoid materials, which, being made up almost exclusively of silicates, have the same adsorption selectivity as glass.

[0049] The difference in potential between the "far" solutions, assumed as zero value, and that of the solid surface, is called Electrokinetic Potential or Zeta Potential, conventionally indicated by the symbol ζ; its value can be positive, nil, or negative and reach a maximum observed absolute value of about 0.2 V (generally, it is a value ranging between 10 mV and 100 mV).

[0050] The positive ions contained in the diffuse area create a region of solution with a non-zero average charge density which, under the influence of an external electrical field, is able to move because of the viscous driving force exerted by the migrating cations.

[0051] It can be demonstrated that if
E (V/m) = intensity of the applied electrical field,
η (Pa^. s) = dynamic viscosity of the solvent,
ε (F/m) = dielectric constant of the solution, then the flow velocity v (m/s) of the electrolytic solution through the capillary is given by the relationship v=Eεζ / 4πη.

[0052] Multiplying times the section of the capillary πr² we obtain the electro-osmotic permeability, i.e. the flow rate of liquid D = Eεζr² / 4η, expressed in cubic metres.

[0053] Recalling Ohm's law, and introducing the specific conductance of the solution K (Ω^-1 . m^{- 1}) and L the length of the capillary in metres, one obtains I = V / R = EL/R = Lπr²E and therefore the flow rate of liquid D = EζI /4πηK which remains valid, in indicative terms, also for porous diaphragms.

[0054] The potential that is originated at the ends of the capillary forcing the solution to flow through the capillary by means of a pressure differential Ap (Pa), is given by V = εζΔP / 4πηK, also valid for porous diaphragms.

[0055] Since flow potential can be estimated also with a simple tester, it is readily understood that through the latter relationship it is possible rapidly to attribute to the soil mass a characteristic drainage pressure which generally applies area by area and whose value is experimentally linked to the presence of areas with constant piezometric lowering.

[0056] Through thermodynamic considerations, one can also reach a relationship that links electro-kinetic potential to the concentration of salts in the solution but experimental evidence shows that the electro-kinetic potential is scarcely influenced by small saline concentrations whilst it can even switch signs at high concentrations, causing the oft-observed phenomenon of field inversion. The problem, however, can be easily solved by controlling electrode polarity with a power supply station.

[0057] If the main goal to be reached is the interruption of natural drainage, applied voltages are not such as to engender worries because of the presence of salts which in any case are generally not as influential in soils as in the masonry to be dehumidified.

[0058] Masonry is subject to the phenomenon of successive concentration whilst, on the contrary, the more superficial water-bearing strata are subject to periodic dilution due to water filtering as a consequence of atmospheric precipitation.

Claims

1. A method for the stabilisation of soils and the restoration of any collapses, in particular soils underlying the foundations of construction works, subject to drainage and/or removal of the water from the soil (free and/or pellicular and/or zeolitic water), characterised in that it provides for the use of electrosmosis to obtain the cessation of the drainage and the superficial link, and/or inside the crystal of the clay material of the water (present naturally and/or supplied artificially) and of the ions (present naturally and/or supplied artificially), stabilising the behaviour of the soil by stopping collapses and possibly obtaining a partial restoration thereof.

2. A method as claimed in claim 1, wherein a first plurality of negative electrodes and a second plurality of positive electrodes are positioned in the soil in such a way that, connecting the two pluralities of electrodes to a power supply the electrical field thus applied causes the interruption of the water outflow and the inversion of the direction of motion of the water with the recall of the water itself to the cathode.

3. A method as claimed in claim 2, wherein the power supply provides a difference in potential that is contrary to that produced by the outflow of the water.

4. A method as claimed in claim 2, wherein the negative electrodes are positioned in the soil immediately underlying the foundations preferably at the centre of the pressure bulb, whilst the positive electrodes are positioned in the soil surrounding the masonry at a few meters therefrom and at a distance of about 4 - 10 d from the corresponding negative electrodes, where d is the thickness of the foundation wall overlying the negative electrode.

5. A method as claimed in claim 2, wherein the following phases are provided:

- drilling to a depth that is below the lower plane of the sub-foundation by about 2d (with d = thickness of the foundation wall), and possibly desalting the diffusion area in proximity to the positive electrodes;

- possible injection of contact and anti-salt material (for instance, mortar enhanced with layered laminar graphite to 3-3.5% by weight) in proximity to the positive electrodes;

- insertion of the positive electrodes into the treated holes as described and insertion of metallic negative electrodes into the area of the pressure bulb;

- application, possibly alternating, of the pressure bulb re-hydration and dehydration systems until the result is obtained;

- possible replenishment of external water;

- periodic monitoring of the injected water, of the piezometric levels, and of load-bearing capacity.

6. A method as claimed in claim 5, wherein each negative electrode is formed by a pair of electrodes: one made of resin and one metallic made of iron or two-component or three-component eutectic alloy, the latter serving as sacrificial electrode destined to be consumed.

7. A method as claimed in claim 5, wherein during the replenishment of external water Calcium Chlorides (CaCl₂), Magnesium Chlorides (MgCl₂) are added to cause a slight decrease in the values of the geo-technical characteristics (non drained cohesion and internal friction angle) in favour of the stabilisation of the behaviour of the soils.

8. A method as claimed in claim 2, wherein the negative electrodes are made of iron.

9. A method as claimed in claim 2, wherein the positive electrodes are made of Al and/or Mg.

10. A method as claimed in claimed in claim 1, characterised in that it occurs in the absence of draining cathodic well.

11. A method as claimed in claim 1, wherein ions of a series of elements are transferred and inserted between the sheets of the crystalline grid of the clays replacing pre-existing ions and causing the expansion of the clays with the consequent restoration of the collapses, since the ions of the series of elements have greater diameter relative to the pre-existing ions of the crystalline grid.

12. A method as claimed in claim 11, wherein the ions of the series of elements are ions of Al, Mg, Ca, whilst the pre-existing ions of the crystalline grid are ions of Na and K.