Method for treating soils prone to differential settlement

Abstract

The present invention relates to the field of treating soils that have a clay fraction in order to reduce their sensitivity to hydric variations. The term "clay" designates both a particle size class (<80 µm) and a mineralogical nature corresponding to the phyllosilicate family.

Description

Process for treating soils subject to differential settlement Field of invention

[0001] The present invention relates to the field of treating soils which have a clay fraction to reduce their sensitivity to water variations. The term clay designates both a granulometric class (< 80 µm) and a mineralogical nature corresponding to the family of phyllosilicates.

[0002] At the microscopic level, clay minerals are characterized by a layered mineralogical structure. These consist of an assembly of silicates (SiO3) and aluminates (Al2O3) between which water molecules are interposed. The majority of clay minerals belong to the 2:1 phyllosilicate family (two tetrahedral layers framing an octahedral layer). The structure of the crystalline assemblies varies depending on the type of clay. Some of them, such as montmorillonite, have weak bonds between layers, which allows the acquisition or departure of water molecules. The hydration of the cations located on the surface of the layers causes them to widen, which results in an increase in the volume of the mineral. This is the phenomenon of intracrystalline or interfoliar swelling. Swelling is linked to the phenomenon of water adsorption on the hydrophilic sites of the clay. This process is reversible.A loss of water leads to a reduction in the volume of the mineral. This is the shrinkage phenomenon. Shrinkage-swelling phenomena are expressed preferentially in clay minerals belonging to the smectite group (montmorillonite, beidellite, nontronite, saponite, hectorite, sauconite) and to a lesser extent to the interstratified group (more or less regular alternation of layers of different natures, for example illite – montmorillonite). On a macroscopic scale, these micro-aggregates of layers are organized into more or less anisotropic and coherent assemblies, depending on the shape of the elementary particles that compose them, and depending on the strength of the bonds between particles.

[0003] These are ensured by intercalated water molecules. This assembly mode, which defines the texture of the "clayey soil", depends on the mineralogical nature of the clays, the sedimentation mode and the state of consolidation of the material. In particular, a mud clay will not have the same texture – and therefore not the same cohesion – as an overconsolidated clay, for example following burial at great depth.

[0004] Clayey soils are therefore characterized by a strong influence of water content on their mechanical behavior. In geotechnics, the different types of clayey soils are identified on the basis of this criterion. To do this, the water contents (known as Atterberg limits) are determined from which the behavior of the material changes. Atterberg, and later Casagrande, conventionally defined, based on the water content, the limits of various states of consistency of a given soil: - the liquidity limit WL separates the liquid state from the solid state; it corresponds to the water content from which the clay begins to flow under its own weight; - the plasticity limit WP separates the plastic state Clayey soils are therefore characterized by their sensitivity to water variations, which has the effect of causing a modification of their volume. In geotechnics,

[0005] identifies the different types of clay soils on the basis of this criterion. To do this, the water contents (called Atterberg limits) are determined from which the behavior of the material changes. Atterberg, and later Casagrande, conventionally defined, based on the water content, the limits of various states of consistency of a given soil: - the liquid limit WL separates the liquid state from the solid state; it corresponds to the water content from which the clay begins to flow under its own weight; - the plasticity limit WP separates the plastic state from the solid state (with shrinkage); it corresponds to the water content below which the clay can no longer deform without microcracking; The extent of the plastic domain included in these two values ​​is called the plasticity index: IP = WL – WP. It represents the ability of the clay to acquire water.-the shrinkage limit WR: when the water content decreases below WP, the volume of clayey soil gradually reduces, but the material remains saturated with water up to a value called the shrinkage limit which separates the solid state with shrinkage from the solid state without shrinkage. From this stage, if the drying continues, it results in cracking of the material. In the event of rehydration of the clay, water will be able to circulate quickly in these cracks. Beyond WR, the arrival of water will be accompanied by an increase in volume, proportional to the additional volume of water incorporated in the structure. The Atterberg limits, which are specific water contents, are expressed, like the water content W, in %.

[0006] The phenomena of shrinkage (linked to a reduction in the volume of the material which results, vertically by settlement, and horizontally by cracking), and swelling (linked to an increase in volume), are therefore essentially caused by variations in water content. In reality, however, the phenomenon is also governed by variations in the state of stress, and more precisely by the appearance of negative pore pressures. In the case of a saturated soil, the total vertical stress, which prevails in the soil at a given depth, is the sum of the pore pressure due to the water and a so-called effective stress which governs the behavior of the solid phase of the soil (intergranular pressure). The total stress is constant since it is linked to the load exerted by the overlying ground (possibly increased by an overload due, for example, to the presence of a construction on the surface).The appearance of negative pore pressure, called suction, therefore results in an increase in the effective stress (i.e. consolidation of the granular skeleton) and expulsion of water. A clayey soil located above the water table, and which is saturated, is thus subjected to a suction pressure which allows it to draw water from the water table, by capillarity, and to maintain its state of saturation. This suction pressure can reach very high values ​​at the surface of the soil, especially if it is subject to intense evaporation.

[0007] The volume variations of the soil or of outcropping to sub-outcropping lithological formations are due, on the one hand, to the water-solid interaction, at microscopic and macroscopic scales, and, on the other hand, to the modification of the stress state in the presence of water.

[0008] These variations can be expressed either by swelling (increase in volume) or by shrinkage (reduction in volume).

[0009] In a temperate climate, clays located at shallow depths are often deconsolidated, moistened and have exhausted their natural swelling potential. But they are in a state far from their shrinkage limit (water content below which deformations due to the shrinkage-swelling phenomenon become insignificant) and can shrink if their water content decreases significantly. These shrinkage-swelling phenomena of certain clay soils cause differential settlements which manifest themselves in disorders mainly affecting individual buildings. In this context, damage occurs on buildings constructed on such soils, especially when a period of intense or prolonged drought causes the appearance of negative pore pressures in the surface layer of the soil, subject to evapotranspiration.

[0010] Exceptional weather events are the main triggering factor for the shrink-swell phenomenon. Variations in soil water content are due to seasonal climatic variations. The depth of soil affected by seasonal variations in water content hardly exceeds 1 to 2 m in our temperate climates, but can reach 3 to 5 m during an exceptional drought, or in an unfavorable environment (nearby vegetation). The two important parameters are precipitation and evapotranspiration. In the absence of a water table, these two parameters control variations in water content in the surface layer of soils. Evapotranspiration is the sum of evaporation (linked to temperature, wind, and sunlight conditions) and transpiration (water absorbed by vegetation).This parameter is measured at some weather stations, but its spatial distribution is difficult to understand because its value depends closely on local vegetation conditions. We generally reason on effective rainfall heights, which correspond to precipitation less evapotranspiration. State of the art

[0011] Known in the state of the art is patent FR2637925A1 describing a process for consolidating loose soils and more particularly clayey soils, characterized in that, in a first step, the clays are flocculated with an acidic aqueous solution in order to make them permeable, and in that, in a second step, the flocculated clayey soil is consolidated by adding sodium carbonate to the surface of the soil to be treated, so as to limit the water input.

[0012] This solution is suitable for road works, in the construction of paths, tracks in hardened ground, industrial floors, by surface treatments. However, it is not suitable for deep treatments for the treatment of constructed or constructible soils.

[0013] Patent AU732848B2 describes another example of treating clay soils to make them substantially impermeable to water, by applying to the soil a water-soluble calcium salt which comprises an aqueous solution of calcium acetate, allowing the calcium ions present in the salt to penetrate into the soil, and then applying an aqueous solution of silicate.

[0014] Also known is the article Fleury, Marc & Sissmann, Olivier & Brosse, Etienne & Chardin, M.. (2017). A Silicate Based Process for Plugging the Near Well Bore Formation. Energy Procedia. 114. 4172-4187. 10.1016 / j.egypro.2017.03.1558 which discloses a method for treating the surroundings of a well using a reactive suspension is studied, using silicate-based solutions and exploiting the precipitation of low-cost potassium silicate solutions, using a weak acid to lower the pH. The experiments were carried out between 20 °C and 60 °C, adding an acidic compound to the silica-based solution. Bulk gelation times were estimated between a few minutes and 4 days. A core injection experiment was then carried out on an analog sandstone sample, representative of CO2 formation, using an optimal mixture with a viscosity close to that of water.This solution is suitable for plugging a well, but not for reducing the water sensitivity of a deep soil. Disadvantages of the prior art

[0015] The solutions of the prior art are not satisfactory because they concern the treatment of surface soils, which is not possible for built-up land and poorly suited to deep treatment of land intended to receive constructions. Solution provided by the invention

[0016] In order to overcome these drawbacks, the present invention relates to a method whose characteristics are set out below.

[0017] The invention relates to a method for treating soils subject to differential settlements to reduce their sensitivity to water variations having the characteristics set out in claim 1. The method consists of injecting a gelling aqueous solution deep into the soil, characterized in that a step of injecting a gas under a pressure greater than 0.5 MPa is first carried out.

[0018] Advantageously, said aqueous gelling solution is composed of silica.

[0019] According to one variant, said aqueous solution comprises microorganisms capable of causing a soil calcification reaction.

[0020] According to other variants: said gas and said aqueous solution are successively injected through pipes introduced into boreholes. The introduction of the injection tube of the gelling aqueous injection solution into the ground is carried out by a pressure of the injected fluid greater than 0.2 MPa.after the operations of injecting aqueous gelling fluid, the method comprises a step of injecting expansive polyurethane resin in order to increase the bearing capacity of the soilthe method comprises a step of visualization with a tomographic analysis device of the treated area and of comparison of the state prior to the injections with the state after the end of the injection operations and of reiteration of an additional injection campaignthe aqueous gelling solution comprises an alkaline solution of potassium silicate and an acidsaid alkaline solution further comprises an aluminatesaid preparation gas of pressurized carbon dioxidesaid gas is dried airsaid preparation fluid contains an isotopic tracersaid preparation fluid contains a tomographic marker.

[0021] The invention also relates to a tube for implementing the above-mentioned method, characterized in that it is constituted by a pipe formed by at least two perforated tubular segments joined by a connecting ring comprising a non-return valve.

[0022] Detailed description of a non-limiting example of embodiment

[0023] The present invention will be better understood on reading the following description, concerning a non-limiting example of embodiment illustrated by the appended drawings where:

[0024] represents a sectional view of an assembly of two tube segments according to the invention

[0025] represents a schematic view of soil preparation

[0026] represents a schematic view of a variant implementation of the hardware Principle of the invention

[0027] The operation of stabilizing the soils to be consolidated as a replacement for a demolition and reconstruction or repair solution by traditional underpinning solutions (piles, foundation beams, etc.) which are by nature highly carbon-intensive, contributes to sustainable development and environmental protection by improving the properties of the soil in order to make them suitable for supporting the loads of weakened constructions or to make clay soils buildable.

[0028] The main cause of this instability is linked to the presence of a significant clay fraction in the soil which becomes a source of differential compaction of the soil following periods of drought and rehydration.

[0029] The invention consists of preparing clay soils prior to treatment by injecting a gelling aqueous solution comprising, for example, a solution comprising silicates, in order to improve the effectiveness of this injection and the sustainability of soil consolidation. This first step consists of injecting a gas under a pressure greater than 5 bars into the soil to be treated. The gas is, for example, air or carbon dioxide, preferably at a temperature greater than 20°C.

[0030] This gas injection step has the effect of modifying the structure of the subsoil to then promote the penetration of the gelling solution and increase the interface surfaces where the reaction between the injected aqueous silicate solution and the clay sheets occurs. This reaction creates chemical bonds between the silicates of the clays by means of the silica gel and forms a binder between the clay sheets, thus ensuring consolidation in the porous medium.

[0031] Injection tube for implementing the process:

[0032] To carry out the succession of steps implemented for the consolidation of the soil, equipment is used, some of which is known to those skilled in the art, and an injection tube, the schematic sectional view of which is shown.

[0033] The injection pipe (1) is formed by an assembly of several rigid tubes (10, 20) having perforations (11) for the passage of the different fluids used by the process. Two tube segments (10, 20) are joined by a ring (15) ensuring a sealed connection. Each ring (15) incorporates a non-return valve (16). Preparing the injection

[0034] The first step of the method consists of drilling the foundation (100) to allow the injection tube to be inserted into this drilling and then driven into the ground under the foundations (100) to be stabilized. The passage of the tube (1) in the drilling (110) could be sealed if necessary to prevent fluid leakage during the injection steps by epoxy keying. Injection sequences

[0035] A first step – consists of injecting a gas, for example hot air, into the soil under the foundation (100) via the tube (1) to micro-crack and dry the soil to be treated. Micro-cracking the soil consists of injecting compressed air using a compressor (200) at a pressure of between 0.5 MPa and 2 MPa for 1 to 10 minutes. This micro-cracking makes it possible, on the one hand, to increase the exchange and penetration surfaces of the liquids injected during the following steps and, on the other hand, facilitates the exposure of the clay sheets to frost by driving out some of the existing interstitial water molecules in order to replace them with those charged with the injection solution, in particular consisting of silicates.

[0036] The gas can be loaded with a tracer, for example hydrogenated nitrogen or a radioactive tracer, to map the subsoil and estimate the diffusion parameters in the subsoil in order to optimize the solution injection step. The minimum injection pressure is 5 bars. If pneumatic cracking was decided in the project, an effective air injection pressure called Pair in the ground would be defined by the operator.

[0037] The total injection time at each point does not exceed 1 minute. The on-site injection pressure is that from which the structure to be consolidated begins to move. The movement of the building is detected by laser monitoring (30). The pneumatic pressure that will be used for the injection of all the points is equal to 90% of that for which movement of the structure was detected. Pair = Pmax *0.9

[0038] Pmax = Pressure at which the building begins to move, according to observations by laser monitoring (30)

[0039] Pmax is in all cases less than 10 bars.

[0040] The execution procedure is as follows: a first injection lance is installed according to the design plan and lowered to the indicated depth. Then, a laser monitoring device (30) is installed for the building.

[0041] This monitoring device injects compressed air via a compressor (200) and determines Pair = Pmax *0.9. This Pair value will be used for injection through all injection lances of parts of the building that have the same load or a greater weight. If there is a lighter part, a second test must be carried out to determine a new Pair for this part.

[0042] The second step consists of carrying out a tomographic survey of the area to be treated before injection in order to compare its condition after injection and to ensure that the injection work has indeed treated the area defined in the project.

[0043] The third step is to inject an alkaline liquid to produce a reaction with the clay of the alkali-activation / geopolymerization type

[0044] Alkaline activation is a reaction between aluminosilicate precursors and alkaline solutions such as sodium (Na) or potassium (K) hydroxide.

[0045] This third step consists of preparing an aqueous gelling solution comprising an alkaline solution of potassium silicate, an acetic acid, and an aluminate.

[0046] Another example of preparing a gelling solution

[0047] After this preparation step, the alkaline liquid is injected in the usual manner. The preparation according to the invention may consist of injecting only an aqueous fluid or alkaline gel, or of successively injecting a gas and then an aqueous alkaline gelling fluid.

[0048] One option is to supplement this system with an injection complementary to those described above. This complementary injection is characterized by an injection of expanding polyurethane resin in case of need to increase the bearing capacity of the ground.

[0049] This optional variant is intended to treat soils with the aforementioned characteristics but also with a bearing defect making it unsuitable for supporting the loads induced by the building based on it.

[0050] The control device: at any time, laser tracking (30) of the building's movements allows injection operations to be stopped. Quality control

[0051] Before the injection works, an electrical tomography of the soil is carried out along the injection points. After the injection operations, a control tomography will be carried out to ensure that the soil impregnation by the injection of the product has taken place in the zone of influence of the foundations planned in the project study. The difference between the two tomographic curves makes it possible to locate the product injection zone.

Claims

Method for treating soils subject to differential settlement to reduce their sensitivity to water variations, consisting of injecting a gelling aqueous solution deep into the soil, characterized in that a step of injecting a gas under a pressure greater than 0.5 MPa is first carried out. Method for treating soils subject to differential settlement according to claim 1, characterized in that said gelling aqueous solution is composed of silica. Method for treating soils subject to differential settlement according to claim 1, characterized in that said aqueous solution comprises microorganisms capable of causing a calcification reaction in the soils. Method for treating soils subject to differential settlements according to claim 1 characterized in that said gas and said aqueous solution are successively injected through pipes introduced into boreholes. Method for treating soils subject to differential settlements according to claim 1 characterized in that the introduction of the injection tube of the gelling aqueous injection solution into the soil is carried out by a pressure of the injected fluid greater than 0.2 MPa. Method for treating soils subject to differential settlements according to claim 1, characterized in that after the operations of injecting gelling aqueous fluid, it comprises a step of injecting expanding polyurethane resin in order to increase the bearing capacity of the soil. Method for treating soils subject to differential settlements according to claim 1, characterized in that it comprises a step of visualization with a tomographic analysis device of the treated area and comparison of the state prior to injections with the state after the end of the injection operations and reiteration of an additional injection campaign. Method for consolidating soils and stabilizing building foundations according to claim 1, characterized in that the gelling aqueous solution comprises an alkaline solution of potassium silicate and an acid. Method for consolidating soils and stabilizing building foundations according to the preceding claim, characterized in that said alkaline solution further comprises an aluminate. Method for consolidating soils and stabilizing building foundations according to claim 1, characterized in that said gas under a pressure greater than 0.5 MPa is pressurized carbon dioxide. Method for consolidating soils and stabilizing building foundations according to claim 1, characterized in that said gas under a pressure greater than 0.5 MPa is dried air. Method for consolidating soils and stabilizing building foundations according to claim 1, characterized in that said gas under a pressure greater than 0.5 MPa contains an isotopic tracer. Method for consolidating soils and stabilizing building foundations according to claim 1, characterized in that said gas under a pressure greater than 0.5 MPa contains a tomographic marker. Tube for implementing the method according to claim 1, characterized in that it is constituted by a pipe formed by at least two perforated tubular segments (10, 20) joined by a connecting ring (15) comprising a non-return valve.

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