EP3074574B1 - Method for restoring a structure having a crack by following a curve representing the separation of the edges of the crack - Google Patents

Description

Technical area

The present invention relates to a method of rehabilitation of a structure resting on a collapsed soil having at least one crack resulting from the subsidence of the soil.

More particularly, the invention relates to a method for rehabilitating a structure resting on a floor, by treating its foundation soil by injecting an expansive substance to close said crack.

Background of the invention

A crack appearing on a structure can be a sign of a break in the materials constituting the structure and occurs when the constraints generated in the work concerned, exceed the limits of rupture for which it was conceived.

Some cracks appear due to a subsidence of the structure which is then the seat of parasitic stresses, that is to say of tensile and / or compressive stresses, normally nonexistent on the intact structure but appeared following the removal of the base of the structure, and can significantly weaken the work and thus generating weaknesses and additional disorders.

• d'un défaut de portance initiale
• d'une décompression accidentelle du sol (suite à une fuite de canalisation ou la réalisation d'un terrassement incontrôlé, etc.)
• d'un tassement différentiel.

The causes of soil subsidence are multiple. This is usually:

• initial lift defect
• accidental decompression of the ground (following a leak in the pipeline or uncontrolled earthworks, etc.)
• differential settlement.

As a result of the settling phenomenon, the structure undergoes a movement which results in the appearance of cracks in the superstructure as represented on the figures 1 , 3 and 5 for example. Part A of the structure follows the settlement of the soil on which the structure of the structure rests, said ground floor and which will be defined in detail later. On the other hand, other parts of building B (often placed above the first) remain temporarily immobile in an unstable equilibrium, the two parts A, B being separated by cracks. This state of unstable equilibrium creates stresses that are often harmful for the durability of the structure.

In an ideal case, the work of reinforcing the floor and the ground deeper, must allow the book to return to its original state.

However, the diffusion of stresses in the soil is a function of the depth in the soil and the effects of the reinforcement work on the closing of the cracks are also dependent on the depth at which the soil is reinforced.

Boussinesq's formula indeed shows that the stresses are localized in a small radius around the vertical of the point of application of the load q for a depth z.

Thus, as represented on the figure 10 the diffusion is at an angle to the vertical, the stress sp being uniform within the diffusion area on a given horizontal plane.

We express the value B ₁ : $${\mathrm{B}}_{\mathrm{1}}=\mathrm{B}+\mathrm{2}*\mathrm{z}*\mathrm{tg\; \alpha }$$

From where : $${\mathrm{\sigma }}_{\mathrm{z}}=\mathrm{q}/\left(\mathrm{1}+\mathrm{2}*\mathrm{z}/\mathrm{B}*\mathrm{tg\; \alpha }\right)$$

Generally, α is a value between 30 ° and 45 °.

The figure 9 schematically shows the diffusion of σ _z with the depth from the previous equation.

We observe that from a depth of z~2B, the value of σ _z becomes less important with respect to the loads brought by the structure on the ground. It converges to zero for a depth of 8B.

With regard to soil compaction, we can establish a direct relationship between stresses and deformations. In other words, the maximum settlement value is observed at the base of the foundations and this settlement decreases with the depth to converge towards zero for a depth of 8B.

1. 1. Sol d'assise - Il comprend une couche dite d'assise Z1 qui se situe en surface c'est à dire immédiatement sous les fondations de l'ouvrage, ainsi que les vides éventuellement présents entre la couche d'assise et les fondations de l'ouvrage. Lorsqu'il présente une mauvaise portance, il est susceptible de subir sous l'effet d'une charge, un tassement et provoquer des dégâts au sein de l'ouvrage. Le sol d'assise s'étend, conformément à la figure 5, sur une profondeur comprise entre « 0 » c'est à dire le niveau inférieur des fondations, et « 2B », profondeur à laquelle la contrainte σ_z est de l'ordre de 0,2 σ₀, « B » étant la largeur de l'élément de fondation (semelle ou dallage) par l'intermédiaire duquel la charge est exercé. La valeur de « 2B » peut donc être par exemple comprise entre 1,2m à 2m..
2. 2. Sol profond - Sous la couche d'assise, une couche profonde Z2, est située à une profondeur comprise entre 2B et 8B. La contrainte σ_z y est comprise entre 0,2 σ₀ et 0,01 σ₀ et est donc négligeable. A cette profondeur, une couche de sol même présentant une mauvaise portance subit un moindre tassement du fait de sa profondeur relativement importante vis à vis de la charge appliquée « q ».

Thus, depending on the soil depth, two types of soils can be identified:

1. 1. Seating ground - It includes a so-called Z1 layer which is located on the surface, ie immediately below the foundations of the structure, as well as the voids that may be present between the base layer and the foundations of the book. When it has a poor lift, it is likely to suffer under the effect of a load, a settlement and cause damage within the structure. The seating floor extends, in accordance with the figure 5 , on a depth between "0" ie the lower level of the foundations, and "2B", depth at which the stress σ _z is of the order of 0.2 σ ₀ , "B" being the width the foundation element (footing or pavement) through which the load is exerted. The value of "2B" can therefore be for example between 1.2m to 2m.
2. 2. Deep soil - Under the base layer, a deep layer Z2 is located at a depth between 2B and 8B. The constraint σ _z y is between 0.2 σ ₀ and 0.01 σ ₀ and is therefore negligible. At this depth, a soil layer even with poor lift is less compacted due to its relatively large depth with respect to the applied load "q".

However, for a process of rehabilitation of a structure to close the cracks, is effective, it is necessary to treat the part of the soil where the constraints are maximum, ie the ground seat.

But it is advisable, before attempting to repair them to return the work to its original appearance, to ensure that the deep ground has sufficient lift.

And if the deep soil requires consolidation, it is first necessary to begin the treatment of this layer Z2, by any consolidation process: "jet groutting" (for injection of high-pressure grout), injection of microcement, or by the process described in the European patent EP 0 851 064 filed by Uretek, according to which it is recommended to inject into the deep soil (layer Z2), an expansive material, while controlling the moment when, following this injection, the building and / or the soil, begins to get up.

The processes for rehabilitating a structure with a crack do not therefore include the implementation of such a deep consolidation method.

They are used on the seat floor including the Z1 foundation layer, while the Z2 layer of soil on which the Z1 layer rests, is consolidated in depth, either because it has never undergone any constraints. resulting in a loss of lift, either because it has been redensified due to such constraints, or has been consolidated by appropriate techniques.

For example, the traditional techniques of rehabilitation of a structure presenting a crack consider, after comforting the ground in depth, filling of cracks with opening, pinning, then filling with a mortar without shrinkage.

• la partie (A) qui a subi le tassement, reste affaissée. Ainsi le propriétaire du bien, ne retrouve pas son ouvrage réparé dans le même état qu'avant le sinistre.
• Le rebouchage entre la partie (A) et (B) comporte souvent un désaffleurement qui n'est pas esthétiquement satisfaisant.
• Le rebouchage réalisé depuis l'extérieur a ses limites et ne permet pas d'assurer un contact parfait entre les éléments (A) et (B)
• et lorsque la fissure est située sur une paroi de l'ouvrage revêtue de motifs répétés, par exemple une paroi carrelée, le matériau additionnel rompant la répétition des motifs et dégradant l'aspect de la paroi en question.

These techniques are however not satisfactory for the following reasons:

• the part (A) which has undergone settlement, remains collapsed. Thus the owner of the property, does not find his repaired work in the same state as before the disaster.
• Filling between part (A) and (B) often has a misalignment which is not aesthetically pleasing.
• The filling made from the outside has its limits and does not allow to ensure a perfect contact between the elements (A) and (B)
• and when the crack is located on a wall of the structure coated with repeated patterns, for example a tiled wall, the additional material breaking pattern repetition and degrading the appearance of the wall in question.

And among the known processes concerned with the injection of expansive substance into the ground, the process described in the document US 4,567,708 filed by the company Uretek in 1983, consists of bringing a slab slab, to its original flat configuration by injecting in the ground seat, an expansive material. It is planned to measure the height of the slab before additional injection. However, this document is totally tacit on how to close a crack that would appear on a wall that would be supported by the deformed slab. The object of the present invention is to improve the method and the system for the rehabilitation of works disclosed in the document EP1914350A1 . The invention proposes a method of comforting the superficial ground surface of a structure, with a quality of finish not yet achieved, the deep soil having been previously consolidated by an appropriate technique.

Object and summary of the invention

The object of the present invention is therefore to provide a method for the rehabilitation of a damaged structure, including a structure having at least one crack due to the subsidence of its ground, allowing both to improve the properties of the foundation soil of the structure and to neutralize the parasitic stresses caused by soil compaction, appeared consecutively on the structure.

This objective is achieved with a method of rehabilitating a collapsed structure having at least one crack resulting from subsidence, wherein a reinforcement substance is injected into the soil of the structure (Z1) at the right of said crack in a primary drilling hole, the method being characterized in that it further comprises, simultaneously with the injection, the acquisition of a revealing curve of the approaching of the edges of the crack, the injection being stopped as soon as a sudden slope decrease in absolute value is detected on the curve.

More specifically, when a soil reinforcement substance is injected into the soil or at the interface between the soil and the foundation of the structure, in a quantity and / or at a sufficient pressure, it exerts, on the basis of the cracked structure, an upward thrust that tends to close the crack of the structure, that is to say to bring one to the other, the lower and upper edges of the crack.

At first, the approximation between the edges of the crack is fast, since these edges are not yet in contact.

As soon as the edges come into contact, the approach is suddenly slowed down.

The beginning of the contact between the edges of the crack corresponds to a moment when the parasitic stresses exerted on the work because of the slump, are neutralized. Indeed, once the two edges in contact, the descent of the loads of the upper part (B) can be done satisfactorily towards the ground as it was the case initially.

According to the invention, it is therefore decided to stop the injection as soon as this moment is detected.

It has been established that this moment can be easily detected by plotting the variation curve of a revealing parameter of the approximation between the edges of the crack, for example a variation curve of displacement or pressure.

Thus, according to an exemplary implementation of the method according to the invention, a sensor of a revealing parameter of the approximation between the edges of the crack is placed between two points located on the facade of the work, on the part and other of the crack. The sensor is connected to a reading device, and, simultaneously with the injection, the curve is read on the reading device.

The sensor may for example be a displacement sensor adapted to measure the approximation between the edges of the crack.

According to an advantageous arrangement of the invention, the sensor makes it possible to detect infinitesimal displacements of the order of 10 microns.

The sensor may also be a pressure sensor adapted to measure between the edges of the crack a pressure increase revealing the approximation between said edges.

In this case also, the pressure variations are measured with a high degree of accuracy, for example of the order of 0.005 bar.

At the moment when the edges of the crack come into contact, a curve as evoked above has a sharp decrease in slope in absolute value.

By drawing the curve of a revealing parameter of the approximation of the edges of the crack, it is thus possible to determine with great precision the optimum moment for stopping the injection.

Thanks to these provisions, it is possible both to avoid the creation of excessive stresses in the structure (for example an excessive compressive stress between the edges of the crack due to an excessive injection), and to neutralize in a manner satisfactory parasitic stress due to the disaster (avoiding to stop the injection prematurely).

The method also allows the densification and the improvement of the surface soil lift in the Z1 layer by injection of the reinforcing substance, which increases the bearing capacity of the soil (in a soil of insufficient lift) and / or decreases its permeability in the case of soil sensitive to water changes. In the same way, it makes it possible to fill the empty spaces between the ground and the foundation of the structure.

According to an exemplary implementation, to detect the abrupt rupture of slope signaling the stop of the injection, the slope of the curve is evaluated during a first interval ΔT _S and then during a second time interval ΔT _D , ΔT _S and ΔT _D being between 10 seconds and 2 minutes, and preferably of the order of 1 minute, and even more preferably, 15 or 30 seconds.

For example, it is considered that a sudden decrease in slope is detected when the slope in absolute value on the second interval is more than 30%, preferably more than 50% lower than the slope in absolute value on the first interval.

On a cracked structure having a plurality of cracks, the method will generally be applied to those among the cracks that are most representative of the state of the structure in relation to a soil compaction problem.

More precisely, we will start by treating the part of the structure where the subsidence is the most important, and in this part, by the most significant fissures, that is to say the highest and widest, and the nearest angles of the structure, and among the most significant cracks, by the most extensive.

And when a crack is identified as very extensive, for example when it has a length greater than 1 meter, it will be treated by multiple injections, or by starting the injection at the level of the largest gauge at within the crack, by a first injection point I, then by successive injection points J, J ', K, K' alternately on either side of this first injection point I, either by starting injection by a first injection point K located at a first end of the crack considered, continuing the injection at a second injection point K 'located at a second end of this crack, then by successive points J , I alternately on either side of the center of the crack to close it gradually from its ends.

Preferably, the first borehole is located in a plane substantially aligned with the sensor and perpendicular to the facade.

According to an advantageous arrangement, the injected reinforcement substance is an expansive substance, especially polyurethane foam.

Brief description of the drawings

• La figure 1 est une illustration schématique d'un ouvrage sinistré pouvant être réhabilité grâce au procédé de l'invention ;
• La figure 2 montre un détail de mise en oeuvre du procédé selon l'invention ;
• La figure 3 montre l'ouvrage de la figure 1 soumis au procédé de l'invention ;
• Les figures 4A et 4B illustrent une fissure de l'ouvrage respectivement avant et après la mise en oeuvre du procédé ;
• La figure 5 illustre schématiquement par une vue en perspective les points de forage pouvant être réalisés notamment le long des fissures étendues selon un mode de mise en oeuvre de la présente invention ;
• La figure 6 est une courbe établie grâce à un capteur de pression fixé de part et d'autre de la fissure ;
• La figure 7 est une courbe établie avec un capteur d'élongation fixé de part et d'autre de la fissure
• La figure 8 illustre schématiquement par une vue en coupe du sol d'assise d'un ouvrage, comprenant un vide entre le dallage et une couche d'assise Z1, ainsi qu'une lance d'injection à orifices multiples répartis le long de son axe à son extrémité, et permettant une distribution de la substance expansive pour combler le vide et consolider la couche Z1,
• La figure 9 représente par une vue schématique en coupe, le sol à l'aplomb d'un ouvrage supporté par une semelle, définissant une surface d'assise de profondeur z=0, une couche de surface ou d'assise Z1 à une profondeur z comprise entre 0 et 2B et une couche profonde à une profondeur comprise entre 2B et 8B,
• La figure 10 représente par une vue schématique la répartition des contraintes σ_z dans le sol, sous l'effet de l'application d'une charge « q ».

The invention will be better understood on reading the following description of an embodiment of the invention given by way of non-limiting example, with reference to the appended drawings, in which:

• The figure 1 is a schematic illustration of a damaged work that can be rehabilitated by the method of the invention;
• The figure 2 shows a detail of implementation of the method according to the invention;
• The figure 3 shows the work of the figure 1 subjected to the process of the invention;
• The Figures 4A and 4B illustrate a crack of the structure respectively before and after the implementation of the method;
• The figure 5 illustrates schematically by a perspective view the drilling points that can be made in particular along extended cracks according to an embodiment of the present invention;
• The figure 6 is a curve established by a pressure sensor fixed on both sides of the crack;
• The figure 7 is a curve established with an elongation sensor fixed on both sides of the crack
• The figure 8 schematically illustrates a sectional view of the base of a structure, comprising a gap between the pavement and a seat layer Z1, and a multi-port injection lance distributed along its axis to its end, and allowing a distribution of the expansive substance to fill the void and consolidate the Z1 layer,
• The figure 9 is a diagrammatic sectional view of the ground perpendicular to a structure supported by a soleplate, defining a seat surface of depth z = 0, a surface or seat layer Z1 at a depth z between 0 and 2B and a deep layer at a depth between 2B and 8B,
• The figure 10 is a schematic view of the distribution of stresses σ _z in the soil, under the effect of the application of a load "q".

Detailed description of several modes of implementation

In the first years of the life of a structure, its base is always a little under the effect of loads (weight of the structure, permanent loads, operating costs). We speak of settlement under loads, distributed at any point of the work.

In some cases, however, the settlement is not the same at any point in the structure. We are talking about differential settlement.

The causes of this phenomenon can be multiple, either that the soil has been more compressible in some places than others, or that the loads applied to the structure have been unequally distributed, or still due to soil moisture variations. such as orientation (the south and west facing flanks are more desiccated than the north and east flanks), the presence of tree roots absorbing water from the soil, insufficient drainage, network leaks, etc ...

On the figure 1 , there is shown a flag 10 whose seat has undergone such a differential settlement. There is a significant settlement of the ground and, in fact, a subsidence of a lower portion of the flag 10, on its right side while an upper portion B remains at the same level as before the settlement.

On this collapsed side, the transmission of loads of the structure on the ground is modified, the interface between the foundation plate and the ground being uncertain or even non-existent in the event of formation of an intermediate gap between these two elements .

The response of the construction to the modification of the transmission of the loads of the structure on the ground, and the parasitic stresses which result, results in a rupture with the cracking of the building, illustrated by the network of cracks 20 on the figure 1 .

An engineer specialized in construction can easily differentiate two types of cracks: stabilized cracks, which no longer affect the structure, and so-called "active" cracks, which, on the contrary, are the seat of parasitic stresses.

Generally, active cracks are located above stabilized cracks.

Once the structure and the underlying soil stabilized after comfort after a disaster, for example, we will treat the part of the structure where the slump is the most important, and in this part, by the most significant cracks ie the highest and the widest, and the closest to the lateral extremities of the structure (corners) , and among the most significant cracks, by the most extensive.

When a fissure is identified as very large, for example when it is more than 1 meter in length, it will be treated with multiple injections, starting with the injection at the level of the largest of the crack, by a first injection point I, then by successive injection points J, J ', K, K' alternately on either side of this first injection point I, or by starting injection by a first injection point K located at a first end of the crack in question, by continuing the injection at a second injection point K 'situated at a second end of this crack, then by successive points J, I alternately on either side of the center of the crack to close it gradually from its ends.

On the figure 1 for example, it will be considered that the crack 22 is the most representative of the state of the structure.

To rehabilitate the damaged structure, three conditions must be met.

A prerequisite is that the soil in depth, that is to say located at a depth greater than 2B, B being the width of the sole supporting the structure, the closest to the crack, has never undergone any constraints having caused a loss of lift, because it has been redensified or consolidated by appropriate techniques.

This preliminary condition must be fulfilled before carrying out the process according to the invention.

A second condition is the improvement and homogenization of the properties of the soil S vis-à-vis the applied loads and / or water variations, in order to avoid the subsequent occurrence of new problems of differential settlements.

This second condition can be fulfilled by the injection into the soil S of a reinforcing substance, in particular an expansive substance, making it possible to densify the soil and thus to improve both its lift and its permeability.

A third condition is the neutralization of parasitic stresses generated in the structure due to differential settlements and breaks in the structure. Indeed, a part of the superstructure could for example remain in overhang.

The process according to the invention, the successive stages of which are described below, makes it possible to rehabilitate the damaged work illustrated on the figure 1 .

In a first step of the method illustrated on the figures 2 and 3 , a stress sensor 30 is placed on the front of the structure, on either side of the crack 12, preferably perpendicular to the mean direction of the crack.

The stress sensor 30 is for example a pressure detector, connected respectively to two fixed points Q3, Q4 located on either side of the crack 22.

An increase in pressure measured by the sensor 30 means that the two edges 24, 26 of the crack 22 tend to get closer.

The pressure sensor 30 is connected to a reading device 40, here a computer, on which a first operator can read the curve illustrating the pressure values measured over time using the sensor 30.

On the figure 5 , which illustrates a particularly advantageous embodiment of the invention, a borehole 50 as wide as an injection lance is drilled in line with the crack 22, in a slightly oblique trajectory directed towards the inside of the building 10 towards the soil layer Z1.

This first borehole 50 is generally located in a plane P perpendicular to the facade 12 of the structure 10 and comprising the pressure sensor 30 (see FIG. figure 1 ) and is generally within the maximum gap of the crack (often coinciding with the center of the crack).

An injection lance 51 is inserted in the borehole 50: the lance 51 is positioned so that its lower end is placed in the layer Z1 under the foundation of the flag 10. This lance 51 has at its penetrating end the seat floor, successive orifices in order to be able to spread the expansive substance both in the Z1 bed layer and in the possible voids under foundation existing between the sole and the Z1 layer.

Thus, as illustrated by the upper bubble of the figure 3 by means of the upper openings of the injection lance 51, the reinforcing substance is injected at the interface between the foundation plate 14 and the floor S to fill the voids and to ensure that the charges are well transmitted between these two elements. In the following, this step is called "keying injection".

And as illustrated by the lower bubble of the figure 3 by means of the lower orifices of the injection lance 51, the expansive substance is injected into the base layer itself Z1, called consolidation injection.

The consolidation injection is parameterized (volume of substance injected, injection pressure, coefficient of expansion of the reinforcing substance if necessary, phasing of injections, etc.) so that the ground around the injection point is reinforced, and an upward thrust is exerted on the collapsed base A of the work to the right of the crack to be treated.

In the example, the reinforcing substance is a polyurethane foam. Such a polyurethane foam is, for example, the result of a mixture of polyol and MDI Isocyanate. On site, these two products are stored in a truck, in separate tanks. Both components are routed through pipes to the spray gun of the spray lance. The combination of the two products mixed under pressure with air injected by a two-component pump forms by chemical reactions an expansive foam which solidifies and acquires high mechanical characteristics.

During the entire injection operation, a first operator continuously reads the curve displayed by the computer 40.

The curve of the figure 6 gives the measured pressure values (on the ordinates) as a function of time (on the abscissa).

It can be noted that the curve can be recorded indifferently by a sampling system over time (measurements at regular intervals) or continuously.

The moment T _A = 0 (point A on the curve) corresponds to the injection of the mixture of polyol and MDI Isocyanate into the depth of the seat ground with the aid of the injection lance 51. this moment, a certain time is necessary for the reaction of the two components.

The preliminary phase, during which the foam has not begun to expand and the ground has not yet been moved, is noted PH1 on the curve. The injection does not involve any modification of the stresses exerted on the structure 10, which results in a first plateau PH1 on the raised curve.

The moment T _B (point B on the curve) corresponds to the beginning of the movements of the collapsed part A of the structure due to the treatment of the seat floor Z1. As the lateral resistance of the ground is lower than its vertical resistance (important because of the weight of the building), the foam propagates essentially laterally from the end of the injection lance 51. The grains of the ground reorganize with each other. The soil becomes denser under the effect of the lateral thrust of the foam, but the level of the ground seat remains for the moment unchanged. Again, no influence on the structure is measured. The pressure curve remains flat PH2.

The seat floor, once compacted (point T _C of the curve), offers increased resistance to the propagation of the foam in the horizontal direction. The soil resistance in the vertical direction eventually becomes lower than its lateral resistance. The foam then tends to propagate upward, generating, on the base of the work 10, a thrust force directed upwards. This thrust force naturally tends to raise the collapsed portion of the structure 10, gradually bringing the lower edge 26 of the crack 22 closer to its upper edge 24. The pressure measured by the sensor then increases very rapidly, as illustrated on the curve by the phase noted PH3. The point T _C therefore corresponds to the beginning of the lifting of part A.

After a while, the increase in pressure is slower. Indeed, at a time T _D (point D on the curve), the curve shows a slope break, which corresponds to the beginning of contact between the lower and upper edges of the crack. The injection is stopped as soon as the slope break is detected. The injector gun is cut.

The slope variations of the curve are evaluated continuously throughout the injection.

In practice, an experienced operator can stop the injection as soon as he detects a sharp break in the slope of the curve with the naked eye, the slope decreasing in absolute value. To avoid any misinterpretation, the operator generally waits for this modification of the curve to be confirmed over a predetermined time interval, of the order of ten seconds, for example between 15 and 30 seconds, after the breaking point. slope D, generally less than 1 minute.

In general, the operator will stop the injection when at a time T, he has detected a decrease in the slope of at least 30%, preferably 50%, over a time interval of less than 1 minute. This detection can also be performed automatically, using appropriate software.

In the illustrated example, the injection is stopped at a time Ts (corresponding to a point Gs of the curve) where the slope on a first interval ΔT _S , is more than 30% lower than the slope of the curve on a previous time interval ΔT _D , ΔT _S and ΔT _D being less than or equal to 1 minute, for example 15 or 30 seconds.

Obviously, the stabilization of the curve is not immediate, the foam continuing to expand for a few moments after stopping the gun. This is illustrated by a slow and slow pressure variation noted PH4 on the figure 6 .

Finally, once the crack 22 is fully closed, there is a total stabilization of the measured pressure value, identifiable by the step noted PH5 on the curve.

The Figure 4B illustrates the crack 22 once the consolidation injection is complete. The low point Q1 of the work has returned to its original level Q1 '.

If the crack in question extends over a considerable length, for example 1 meter or more, and despite a first injection, according to the method above, a gap between the edges of the crack remains at a certain distance. additional injection are then continued at other injection points J and J 'and then K, K' located on either side of the first borehole I and in the areas which are in line with the crack considered in accordance with figure 5 ..

The other injection points J, J ', K, K', adjacent to the first injection point I are according to a first embodiment, made in drill holes made alternately to the left and to the right of the primary borehole. 50, substantially following the facade 12 of the building 10. The drill holes are spaced from the first borehole 50 and between them a predetermined constant distance d, for example equal to one meter.

No injection shall be made more than 1 m away from the two points which are respectively at the base of the two ends of a crack.

According to another embodiment of the method according to the invention, the sensor used may be a displacement detector (elongation), in particular an optical fiber detector allowing detection of displacements to within 10 microns.

The stress curve obtained with such a displacement sensor is illustrated on the figure 7 . She is the mirror of the figure 6 previously described. The elongation measured by a sensor respectively fixed at a point of the lower part of the crack and a point of the upper part of the crack actually changes inversely with respect to the pressure measured between the edges of the crack: When the pressure increases, elongation decreases and vice versa.

Phases PH1 to PH5 are represented on the curve of the figure 7 . All of the comments made previously regarding the figure 6 apply mutatis mutandis to it.

It can be seen that after a first plateau corresponding to the expansion of the moss and at the beginning of the consolidation of the soil, the curve has a significant slope, testifying to the rapid approximation of the edges of the crack. When the edges come in contact, the approximation becomes slower and the slope of the curve (in absolute value) decreases sharply.

According to the invention, the injection into the first borehole is stopped as soon as the abrupt decrease in slope in absolute value (point D) is identified.

Claims (13)

1. A method for restoring a structure resting on a collapsing ground (10), with the subgrade being reinforced, with the structure having at least one crack (22) resulting from the collapsing of the ground, wherein a reinforcing substance is injected into the subgrade of the structure at said crack (22) into a main bore hole (50), with the method being characterized in that it further comprises, simultaneously with the injection, the acquisition of a curve indicative of the moving closer together of the edges (24, 26) of the crack (22), with the injection being stopped as soon as a sudden decrease in the absolute value of the slope is detected on the curve.
2. A method according to claim 1, wherein the reinforcing substance is an expansive substance, in particular polyurethane foam.
3. A method according to claim 1 or 2, wherein at each time T, the slope of the curve is evaluated during a first time interval ΔT_S, then during a second time interval ΔT_D ranging from 10 seconds to 1 minute and preferably of the order of 15 or 30 seconds.
4. A method according to any one of claims 1 to 3, wherein a sensor (30) of a parameter indicative of the moving closer together of the edges (24, 26) of the crack (22) is placed between two points (Q3, Q4) located on the facade (12) of the structure (10), on either side of the crack (22), with said sensor being connected to a reading device (40), and, simultaneously with the injection, said curve is read on the reading device (40).
5. A method according to claim 4, wherein the sensor (30) is a pressure sensor adapted to measure a pressure increase indicative of the moving closer together of the edges (24, 26), between the edges (24, 26) of the crack (22).
6. A method according to claim 4, wherein the sensor (30) is a displacement sensor adapted to measure the moving closer together of the edges (24, 26) of the crack (22).
7. A method according to any one of claims 4 to 6, wherein a first bore hole (50) is located in a plane (P) substantially aligned with the sensor (30) and perpendicular to the facade (12) of the structure.
8. A method according to any claim 7, wherein additional injections are carried out into bore holes provided alternately on one side and the other side of the first bore hole (50), away from said first bore hole (50).
9. A method according to claim 8, wherein, at each additional injection point, the injection is stopped as soon as no further injection is possible.
10. A method according to claim 8 or 9, wherein once the injections into the bore holes are completed, the ground bearing capacity is measured.
11. A method according to one of the preceding claims, characterized in that the structure is supported by structural elements such as a slab or a sole resting on the subgrade, with the injection of the reinforcing substance being performed between the structural element and the subgrade and/or into the ground layer supporting the structure, also called the subgrade.
12. A system for restoring a structure (10) resting on a collapsing ground (10), with the subgrade being reinforced, with the structure having at least one crack (22) resulting from the collapsing of the ground, comprising:

    - a sensor (30) for measuring the moving closer together of the longitudinal edges of the crack,

    - a device for injecting a reinforcing substance into the subgrade of the structure at the crack, or between the subgrade and a structural element supporting the structure, such as a slab or a sole, with the substance having, in its expanded state, a density almost similar to that of the ground prior to the collapsing thereof,

    - a device for the acquisition for data from the sensor as a curve indicative of the moving closer together of the edges of the crack,

    - an injection stop control device for the real-time collection of the curve from the acquisition device and controlling the injection stop as soon as a sudden decrease in the absolute value of the slope is detected on the curve.
13. A system according to claim 12, wherein the density of the reinforcing substance is above 60kg.m³.

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