EP2231934B1

EP2231934B1 - Method for hydrating a cohesive soil by electro-osmosis to prevent its volumetric reduction by dehydration

Info

Publication number: EP2231934B1
Application number: EP08867805.7A
Authority: EP
Inventors: Carlo Falugi
Original assignee: Falugi Carlo
Current assignee: Falugi Carlo
Priority date: 2007-12-21
Filing date: 2008-10-22
Publication date: 2016-05-11
Anticipated expiration: 2028-10-22
Also published as: WO2009083811A1; EP2231934A1; ITPR20070098A1

Description

TECHNICAL FIELD AND BACKGROUND ART.

The present invention relates to a method for hydrating a cohesive soil by electro-osmosis to prevent its volumetric reduction by dehydration. The term "cohesive" soil means a soil comprising colloidal materials (such as clays, silts or clayey sands), so that the soil has the property of perfectly adhering to itself.
When a cohesive soil dehydrates, for natural causes (e.g., evaporation or aspiration of water by trees) or human causes, tends to undergo a cracking process that causes its weakening.
The dehydration of a cohesive soil causes a volume decrease with repercussions (even severe ones) on infrastructures having both private and industrial use and also, possibly, on buildings with historical and/or architectural value (of particular note are lesions, cracks, fissures on buildings, pipelines, infrastructures).
Note that the reduction in volume of the cohesive soil, in itself, entails an increase in the load-bearing capacity of a single portion of soil; however, the reduction in volume, as a whole causes the (total or partial) detachment of the soil from the construction (in particular from the foundation walls) constituting a load for the ground itself. Said detachments, due to the lowering of the soil underlying said construction by effect of the volume reduction due to dehydration, cause structural collapses and problems of various types, as described below.
The reason for this behaviour resides in the fact that soils of a clayey (or marl or silt) nature have a particular, complex crystalline structure which makes them unique from the viewpoint of their response to the load in relation to the presence of water.
In particular, the clayey minerals whose volume varies according to the humidity present are phyllosilicates, whose bonds in turn are based on the two-dimensional repetition of tetrahedric units having at the centre a silica atom, or multiple octahedric units having at the centre an aluminium atom or similar smectites that present an excess of negative charges that is offset by molecules of water and positive metal ions. Therefore, a dehydrated cohesive soil presents a reduced ability to withstand a load (e.g. a building or an embankment supporting a pipeline) positioned on the soil itself. This entails numerous risks and drawbacks due to the collapse of the soil subjected to the load.
In light of the above consideration, the need would emerge to hydrate cohesive soils weakened by dehydration to make the able to withstand loads; it is often necessary to hydrate dehydrated cohesive soils whereon are already present loads (e.g. construction works with their foundations, or tanks, pipeline support structures or other works).
On the contrary, it should be noted that ordinarily in order to increase the ability of a cohesive soil to withstand a load, consolidation methods are used that are based rather on a dehydration of the soil itself, in combination with a mechanical action thereon.
In this light, it should be noted that the traditional methods used to reduce and/or eliminate volume changes in cohesive soils are generally invasive and costly.
Among such methods, for example, can be mentioned the technique known as "jet grouting", providing for the use of posts driven or drilled into the soil to be consolidated, as well as injections of cement material or resins to fill the voids created in the soil underneath the foundations. Such interventions leave profound changes in the building or in the soil, which may even hamper subsequent interventions; moreover, in some cases they present a difficulty of precision, in the sense that they do not reach the set goal but compress areas of soil that are distant from those that should have been treated.
With regard to soil consolidation, also known is the use of electro-osmosis, i.e. of the application of an electrical field to the soil to move water from one area of the soil to another.
However, in the prior art electro-osmosis is used essentially to dehydrate the soils to be consolidated.
For example, the patent documents ( EP0870875 , EP1108817 and US2099328 ) describe methods for consolidating a soil using electro-osmosis to dehydrate the soil.
According to other known techniques for consolidating a soil, electro-osmosis is used to operate a crystallisation of the soil (as described in the patent document US5347070 ) or a modification of the chemical. properties of the soil (as described in the patent document US5616235 ). Therefore, said technical solutions do not confront the problem related to the humidity loss in cohesive soils due mainly to phenomena of evaporation or aspiration, of the lowering of the water-bearing strata, of water suction by the vegetation, of the self-draining of cohesive soils by their self-fissuring.
In practice, currently used techniques do not confront the problem linked to the weakening of cohesive soils due to their dehydration in the absence (or in the impossibility) of mechanical actions aimed at consolidating the soils themselves.
In this light, the patent document EP1108817 by the same applicant describes a method for consolidating soils and buildings subject to drainage and/or to the separation of the water-bearing stratum, which entails use of electro-osmosis.
According to the teachings of said document, the electro-osmotic process moves the ions towards the cathode, offsetting the charge deficit; by so doing, the aim is to cause the dehydration phenomenon to stop, using the electrical field applied to prevent or slow down the migration of water from the dehydrated areas.
However, this technical solution also fails to solve the dehydration problem effectively, because it does not enable significantly to increase the volume of the dehydrated soil, restoring its compactness. I.e., this technical solution is effective essentially as a preventive measure, i.e. to prevent a cohesive soil from dehydrating.
Instead, the goal of the present invention is to hydrate in an effective and selective manner a previously dehydrated portion of soil.
In this light, it should also be noted that, as a result of rain or of the human action of watering a surface of a soil, a superficial layer of the soil tends to become impermeable, preventing a subsequent infiltration of water in the depth of the soil.
Studies by Armillotta disclosed on the magazine I1 Geologo dell'Emilia Romagna show a method for hydrating soil by electro-osmosis for cohesive soils according to the preamble of claim 1.

DISCLOSURE OF THE INVENTION.

An object of the present invention is to eliminate the aforesaid drawbacks and make available a method for hydrating a cohesive soil by electro-osmosis.
Said object is fully achieved by the method of the present invention, which is characterised by the content of the appended claims.

BRIEF DESCRIPTION OF DRAWINGS.

This and other characteristics shall become more readily apparent from the following description of a preferred embodiment, illustrated purely by way of non limiting example in the accompanying drawing tables, in which:

figure 1 schematically shows a sectioned lateral view of an apparatus for implementing a method according to the present invention;
figure 2 shows a plan view of the apparatus of figure 1.

BEST MODE FOR CARRYING OUT THE INVENTION.

In figure 1, the reference number 1 indicates a dehydrated cohesive soil to be hydrated; the number 2 indicates a superficial portion of said soil, made impermeable by the infiltration of water from the surface (e.g. in the form of precipitation), or able to be made impermeable by the infiltration of water from the surface. Said superficial portion 3 of the soil 1 constitutes a layer of about 20 ÷ 30 cm of thickness. The reference number 3 indicates an area of soil underlying the dehydrated soil 1, i.e. a portion of soil not affected by the dehydration phenomenon because it is particularly deep.
The reference number 4 schematically shows a structure (e.g. a building) constituting a load for the underlying soil 1.
The reference number 5 schematically indicated a pressure bulb defined starting from a terminal portion of the structure 4 constituting the foundations of the structure 4.
The present invention makes available a method for hydrating a cohesive soil by electro-osmosis and an electro-osmotic system constituting an apparatus for implementing said method.
Said method provides for placing into the cohesive soil 1 to be hydrated a plurality of negative electrodes 6 and a plurality of positive electrodes 7, connected to the positive and negative pole of a direct voltage power supply 8, respectively.
Preferably, each electrode is made of aluminium; in particular, it is constituted by a rod-like body made of aluminium which, originally, contains within it a tubular element made of titanium. The insertion of the titanium has the advantage of preserving the electrode over time, because aluminium tends to be consumed.
It should be noted that, for the implementation of the method according to the present invention, it is sufficient to have available at least one positive electrode 7 and a negative electrode 6. However, if the soil 1 underlying the foundation of a structure 4 needs to be hydrated, the method preferably provides for placing a plurality of pairs of electrodes (a positive and a negative) along the perimeter of the structure 4.
The negative electrodes 6 are inserted into the soil 1 to be hydrated at a depth of over 30 cm, preferably within the pressure bulb 5 (e.g. at a preferred depth of 1 ÷ 2 m).
The positive electrodes 7 are inserted in the soil at a predetermined distance (e.g. 3 ÷ 5 m) from the corresponding negative electrodes. The positive electrodes 7 are inserted preferably at the same depth as the corresponding negative electrodes 6.
Originally, a step is provided of drilling the soil to obtain holes (or wells) into which are inserted conduits 9 (preferably piezometers, constituted by tubes made of plastic material) to a desired depth.
In this way, each electrode is inserted into the soil through a corresponding conduit 9, until it is driven into the soil underlying the lower end of the conduit 9.
Originally, a step is provided of supplying water to a predetermined depth of at least 20 cm into a portion of ground between the two electrodes, positive 7 and negative 6, of each pair; said supply may consist of pumping water, although said pumping is not indispensable (hereafter, the terms supply and pumping shall be used in substantially indifferent manner.
Originally, the supply of water takes place through the conduits 9.
A direct voltage of predetermined value (e.g. 10 ÷ 50 V) is applied between the positive electrodes 7 and the negative electrodes 6.
In this way, originally, the inflow of water into the soil where the applied electrical field is the greatest is assured, minimising interventions in the soil.
Therefore, connecting together the two electrodes (positive and negative, respectively) to said power supply 8, the electrical field thus applied causes the displacement of water towards the set of negative electrodes 6.
Originally, the method also comprises a subsequent step, of applying a greater voltage than said predetermined value to the electrodes, e.g. a voltage of 200 ÷ 400 V (and in any case of at least 100 V).
The step of applying voltage continues for a predetermined time, necessary to carry out said hydration (e.g. 15 days ÷ 2 months), whilst the subsequent step of applying a greater voltage has a duration of about four months.
This step is applied when the dehydrated portion of cohesive soil has already been at least partially dehydrated; the application of a greater electrical field in the rehydrated area entails a substantial crystallisation of the soil, i.e. a transformation of the clay of the cohesive soil from colloidal to corpuscular. This has the advantage of considerably increasing the load-bearing capacity of the ground 1. It should noted that said transformation requires the presence of ions; in this light, the fact that the electrodes (and in particular the positive electrode) are made of aluminium is particularly advantageous.
In the example shown in figure 2, the electro-osmotic system according to the present invention comprises a duct 10 positioned along a perimeter (or a portion of a perimeter) that surrounds the building 4 (or any other work constituting a load for the cohesive soil 1 to be hydrated).
Within the duct 10 are housed the conductors connecting the electrodes 6 and 7 to the power supply 8 and a pipeline for distributing water to the conduits 9, said pipeline being operatively connected to a pump, to supply the pressurised water into the conduit 9.
Said configuration minimises the size of the system and allows for a cost reduction.
In the example shown in figure 2, there is also a tree 11 in the vicinity of the building 4.
Such trees often contribute to the dehydration of the soil 1, because they aspirate water through the roots extended in the direction of the building 4.
The present invention provides a solution to this problem as well, because it provides for introducing an additional negative electrode 12 into the soil from the opposite part of the tree 11 relative to the building, in such a way that the tree is between two electrodes, a positive one (positioned on the of the building 4) and a negative one positioned in line with the tree 11 and the other electrode at the opposite side.
In this way, originally, the region of soil in which the roots of the tree 11 are located is affected by an electrical field that tends to displace the water present in the soil away from the building, this entailing a progressive displacement of the roots of the tree in the direction opposite to the building (therefore, the tree will draw progressively less water from the soil underlying the building, going to seek it elsewhere).
In addition, a wall 13 is positioned to block the roots of the roots of the tree 11, inserted into the soil between the trees 11 and the duct 12, further to reduce, in a particularly rapid manner, the water suction effect by the tree 11.
According to another aspect of the present invention, the electro-osmotic system for hydrating a dehydrated cohesive soil 1 originally comprises (for each negative electrode 6 and each positive electrode 7) a supplementary negative electrode 6A and a supplementary positive electrode 7A connected to a supplementary direct voltage power supply 8A. Said supplementary electrodes are inserted in the soil 1, in respective portions of soil underlying the other electrodes. Preferably, the supplementary electrodes are inserted at a depth about 2 ÷ 3 m lower than the other electrodes 6 and 7 (e.g. 4 ÷ 6 m), within respective conduit 9, similarly to what is described above.
Note that the present invention also provides for using the supplementary negative electrode 6A in addition to the negative electrode 6 but without the supplementary positive electrode 7A and with no need to supply water at the supplementary negative electrode 7A.
In this way, a supplementary, independent electrical circuit, positioned at a greater depth, is defined.
Preferably, the supplementary power supply 8 outputs a lower value of voltage than the power supply 8, e.g. equal to about half. This advantageously enables to offset the effect of gravity in the water displacement action.
The supplementary electrodes, defining a supplementary pair of electrodes, are also preferably positioned at the same depth (to exploit the fact that the mobility of water in the soil, pushed by the electrical field, is greater in the horizontal direction than in the vertical direction).
Operatively, the method according to the present invention further comprises the following steps:

inserting a supplementary negative electrode 6A and a supplementary positive electrode 7A into portions of the soil 1 underlying the negative electrode 6 and the positive electrode 7, respectively, at a predetermined depth level (e.g. about 2÷3 meters than the other electrodes 6 and 7, i.e. at a depth of about 4÷6 m), greater than said electrodes;
further pumping water into a portion of soil between said supplementary electrodes, at the level of depth of the supplementary electrodes;
applying a predetermined voltage between the supplementary positive electrode 6A and the supplementary negative electrode 7A.

Preferably, said supplementary electrodes are inserted through respective conduits 9 housed in corresponding holes, said additional pumping of water being carried out through said conduits 9.
Therefore, according to the method according to the present invention originally the negative electrode 6 is inserted into the soil 1 in the pressure bulb 5 and the supplementary negative electrode 6A is inserted in an underlying area, at a predetermined greater depth.
To determine said predetermined depth at which the supplementary electrodes are to be inserted, the method originally comprises a preliminary step of inspecting the soil 1 to be hydrated to determine the maximum depth of the dehydrated portion of soil. Therefore, the supplementary negative electrode is subsequently positioned at a slightly lower depth than the depth of the hydrated area 3 of soil.
With regard to the electrical powers to be installed and the time necessary for stabilisation, reference can be made for example to a typical construction site with 50 m of foundation perimeter and it can be stated that the electro-osmotic system has the need to absorb a direct current of the order of magnitude of some A at a voltage of at least 10-15 V.
The requirement for capillary water through the anodic wells (i.e. the water to be pumped within the conduits 9 in which are inserted the positive electrodes 6 and the supplementary positive electrodes 6A, if any) is of some tens of litres for each cubic meter of foundation.
During the process, anyway, the passage of current is greatly reduced with asymptotic law because of the variations of conductivity that occur in the soil 1, while demand for water in the system is reduced even more greatly.
The present method operatively comprises the following steps:

drilling down to a depth, below the lower foundation wall, that is about double the thickness of the foundation wall (preferably within the pressure bulb);
inserting into the hole a tube of the "piezometer" type, constituting said conduit 9 that will house the electrode driven into the soil 1;
filling the aforesaid tube with permeable material;
inserting the electrode;
connecting the two series of electrodes to one or more sources of direct voltage;
constructing a system for supplying water to the conduits 9;
pumping water into the conduits 9, preferably water with added salts such as Sodium Chlorides (NaCl), Calcium Chlorides (CaCl₂), magnesium chlorides (MgCl₂).

The position of the positive electrodes 7 and negative electrodes 6 depends on the specific conditions of the construction site (i.e. of the work 4 defining the load bearing on the soil 1 to be hydrated).
It should be noted that the present invention has multiple advantageous uses.
A first use consists of hydrating the cohesive soils in proximity to and/or in the presence of man-made works (distribution pipelines) that are affected by the volume variations of the cohesive soils near them. Another use consists of hydrating cohesive soils influenced by the suction of water by the vegetation.
An additional use consists of hydrating cohesive soils susceptible to volume variation because of the phenomenon of evapotranspiration. Another use consists of hydrating cohesive soils weakened by consolidation, fissuring and/or subsidence because of the lowering of the water-bearing stratum.
In all these cases, the electro-osmotic method and system according to the present invention enable to improve the mechanical strength of the soil 1 subjected to the load by its hydration.
This result is obtained exploiting the intimate structure of cohesive soils, i.e. clayey soils, which can be considered similar to a network of numberless capillaries obtained from the silicatic surfaces of the clay lenticles. In this case, while as a whole the soil is obviously electrically neutral, intimately the interstitial water is in the conditions of having a greater electrical potential than that of the surface of the lenticles, so that the application of an external electrical field induce its migration towards the negative electrode.
On the contrary, if the interstitial water had the possibility of flowing and to move away, a charge unbalance would be created, and consequently an electrical field measurable as potential difference between different points of the soil.
It should be noted that the present invention is effective even if in the soil surrounding the area of soil 1 to be hydrated there is not enough water and even if said soil 1 comprises a superficial layer made impermeable, thanks to the supply of water by pumping into a soil region at predetermined depth in which the applied electrical field is greatest; i.e. the water is artificially supplied by means of an external source that injects water directly in proximity to the positive electrodes (anodic wells), so that the water migrates underwater towards the negative electrodes in the pressure bulb, in the fastest, most efficient possible manner.
Moreover, the method according to the present invention allows for a radical solution of the problem of the weakening of the soil 1 by dehydration, thanks to the use of two electrical circuits positioned at different depth.
The electrodes 6 and 7 are substantially positioned at the depth of the pressure bulb, to restore the cohesion of the soil 1 rapidly in proximity to the foundations of the work 4 positioned on the soil; while the supplementary electrodes 6A and 7A are positioned at the maximum depth of the dehydrated area of the soil, to prevent the occurrence of subsequent collapses in deeper areas.
Other advantages of the present invention are given by the fact that electro-osmotic technology has the advantage of being economic, non invasive, ecological, reversible and adjustable.
Moreover, the method according to the present invention intervenes on the causes and not on the effects of dehydration, so it retains its effectiveness unchanged over time, it does not alter and as far as possible it restores the original conditions of the soil.
It should be noted that the present invention, relating to a method for hydrating a cohesive soil, finds application also in agriculture, because it originally enables to wash the roots of plants to clean them of the salt deposited thereon as a result of an excessive use of fertilisers.
With regard to the application of the present invention in the construction industry, it should be noted that the described method is inserted in a broader context of a process of erection and stabilisation of buildings comprising the following steps:

construction of a structure (e.g. a building or any other work) on a cohesive soil;
settling;
dehydration (simultaneous or subsequent to the settling step);
partial collapse of the structure due to the detachment of part of the cohesive soil from the structure by effect of dehydration;
hydration with electro-osmotic method / apparatus according to the present invention;
stabilisation of the structure by effect of the restoration or at least of the containment of the volume reduction of the cohesive soil.

Claims

Method for hydrating a cohesive soil (1) by electro-osmosis, comprising the following steps:
- providing a supply (8) of direct electrical voltage connected to a positive electrode (7) and to a negative electrode (6);

- inserting the negative electrode (6) into the soil (1) to be hydrated;

- inserting the positive electrode (7) into the soil (1) at a pre-determined distance from the negative electrode (6);

- supplying water to a pre-determined depth of at least 20 cm into a portion of soil (1) between the two electrodes;- applying a direct voltage of predetermined value between the positive electrode (7) and the negative electrode (6);

- drilling the soil (1) to obtain at least one hole of predetermined depth, characterized in that the method comprises the further step of

- housing a conduit (9) within the hole, the step of inserting the positive electrode (7) into the soil and the step of supplying water being carried out through said conduit (9).
Method as claimed in claim 1, wherein the positive electrode (7) and the negative electrode (6) are inserted at a depth of at least 30 cm.
A method as claimed in any of the previous claims, further comprising the following steps:
- drilling the soil (1) to obtain an additional hole of predetermined depth;

- housing a conduit (9) within said additional hole, the step of inserting the negative electrode (6) into the soil being carried out through said additional conduit (9);

- supplying water within said additional conduit (9), into which is inserted the negative electrode (6).
Method as claimed in any of the previous claims, wherein said voltage is between at least 10 V and 50 V.
Method as claimed in any of the previous claims, wherein the step of inserting the negative electrode (6) entails positioning it in a pressure bulb (5) of a load (4) bearing on the soil (1) to be hydrated.
Method as claimed in claim 6, comprising the insertion into the soil (1) of pairs of electrodes positioned peripherally to a structure (4) bearing on the soil (1) to be hydrated, each pair being constituted by a positive electrode (7) and by a negative electrode (6).
Method as claimed in claim 5 or 6, wherein the supply of water takes place at the depth in which the electrodes (6, 7) are inserted.
Method as claimed in any of the previous claims, further comprising a step of inserting a supplementary negative electrode (6A) in a portion of soil (1) underlying the negative electrode (6), at a greater predetermined level of depth than said electrode (6).
A method as claimed in any of the previous claims, further comprising the following steps:
- inserting a supplementary negative electrode (6A) and a supplementary positive electrode (7A) into portions of soil (1) underlying the negative electrode (6) and the positive electrode (7), respectively, at a predetermined depth level, greater than said electrodes (6, 7);

- further pumping water into a portion of soil (1) between said supplementary electrodes (6A, 7A), at the level of depth of the supplementary electrodes;

- applying a predetermined voltage between the supplementary positive electrode (7A) and the supplementary negative electrode (6A).
A method as claimed in claim 7, wherein said supplementary electrodes (6A, 7A) are inserted through respective conduits (9) housed in corresponding holes, said additional pumping of water being carried out through said conduits (9).
Method as claimed in claim 7 or 8, wherein the negative electrode (6) is positioned in a pressure bulb (5) of a load (4) bearing on the soil to be hydrated and the supplementary negative electrode (6A) is positioned in an underlying portion of the soil (1) at a predetermined depth.
Method as claimed in claim 9, comprising a preliminary step of inspecting the soil (1) to be hydrated to determine the maximum depth of the dehydrated portion of soil, the supplementary negative electrode (6A) being subsequently positioned at said depth.
Method as claimed in any of the claims 7 to 10, wherein the negative electrode (6) is positioned substantially at the same depth as the positive electrode (7), and the supplementary negative electrode (6A) is positioned substantially at the same depth as the supplementary positive electrode (7A).
Method as claimed in any of the claims 7 to 10, wherein the voltage applied between the supplementary positive electrode (7A) and the supplementary negative electrode (6A) is lower than the voltage applied between the positive electrode (7) and the negative electrode (6).
Method as claimed in any of the previous claims, wherein the electrodes comprise rod-like elements made of aluminium and containing internally corresponding titanium elements.
Method as claimed in any of the previous claims, comprising an additional step of applying an increased voltage, subsequent to the step of applying a direct voltage of predetermined value between the positive electrode (7) and the negative electrode (6), having a predetermined duration.
Method as claimed in claim 16, in which said increased voltage is at least 100 V.
An apparatus to implement a method as claimed in any of the previous claims, consisting of an electro-osmotic system.