EP1778817A1

EP1778817A1 - Soil consolidating method and composition therefor

Info

Publication number: EP1778817A1
Application number: EP05857857A
Authority: EP
Inventors: Jean-Pierre Hamelin; Gérard Evers; Sabine Darson-Balleur
Original assignee: Compagnie du Sol SARL
Current assignee: Soletanche Freyssinet SA
Priority date: 2004-07-28
Filing date: 2005-07-26
Publication date: 2007-05-02
Also published as: JP2008508450A; FR2873725B1; FR2873725A1; US20080298901A1; WO2006131611A1; AU2005332811A1

Abstract

The invention concerns a method for soil consolidation, whereby a combination of calcifying bacteria and denitrifying bacteria is used. The invention also concerns a composition for implementing said method.

Description

Method of consolidation of the soil and composition for the implementation of this process

The invention relates to a method for soil consolidation, and a composition for carrying out this method. Fine or liquefiable soils have a very low permeability, of the order of

10 ^"5 m / s The consolidation or the sealing of such soils implies the injection of solutions whose maximum size of the grains it contains must be less than one micron, in this context mineral solutions have been developed with as against counterpart, short setting times which limit their penetration into the soil and the release of polluting phases such as ammonia or nitrates.

Moreover, the use of calcifying bacteria in renovation of facings is described in patent applications EP-A-388,304 and FR-A-2,734,261. The advantage of calcifying bacteria is that they can penetrate deeply into Soils through the culture medium in which they are, live there as nutrients are brought to them and grow carbonates on the surface of the soil grains maintaining an open porosity if necessary.

It should be noted that the idea of using bacteria to "cement" the soil has been studied in Australia (see wwwJnnovation.wa.qov.au/Innovation/News/2002/ 07 ^' ). However, to the knowledge of the Applicant, this study has not been the subject of concrete implementation to date

Continuing research in this field, the Applicant has found that calcifying bacteria produce nitrogen compounds which are released into the surrounding environment. This disadvantage can have adverse consequences for the environment and / or health, for example if the soil to be consolidated is near a water point.

An object of the invention is therefore to have a soil consolidation method, which is environmentally friendly and / or health.

Another object of the invention is to provide a method in which the consolidation "cement" does not exhibit a setting phenomenon.

Another object of the invention is to have a method which is less restrictive to implement than conventional methods. These objects are achieved according to the present invention using a combination of calcifying bacterial solutions and solutions bacterial denitrifying.

Thus, according to a first aspect, the invention relates to a soil consolidation method which comprises the following steps: a) feeding the soil with one or more solution (s) of calcifying bacteria; b) the circulation of this (these) solutions in the soil; c) feeding the soil with one or more nutritive solution (s) for the calcifying bacteria and the circulation of this (these) solution (s) in the soil; d) feeding the soil with one or more solution (s) of denitrifying bacteria and the circulation of this (these) solution (s) in the soil.

By "calcifying bacteria" is meant bacteria that make or grow carbonates. These bacteria are also known under the name of carbonatogenic bacteria (see in particular the patent application FR-A-2,734,261). As examples of calcifying bacteria that may be used in the method of the invention, mention may be made of bacteria belonging to the following families:

Bacillaceae (preferably of the genus Bacillus such as Bacillus cereus, Bacillus pasteur / I, etc.). Pseudomonadaceae (preferably of the genus Pseudomonas, for example Pseudomonas stutzerii, ..)

• Enterobactehaceae (preferably of the genus Proteus as for example Proteus mirabilis, ..)

• Myxococcal (preferably of the genus Myxococcus such as Myxococcus xanthus, ..).

By "denitrifying bacterium" is meant soil bacteria that convert nitrate nitrogen to molecular nitrogen and ammonia.

As examples of denitrifying bacteria that may be used in the process of the invention, mention may be made of the following bacteria: • Pseudomonas denitrificans,

• Azospirillum brasilense,

• Pseudoxanthomonas broegbernesis,

• Luteimonas mephitis,

• Stenotrophomonas nitritireducens, • Thermomonas brevis or Thermomonas fusca,

• Nitratireductor aquibiodomus. By "bacteria solution" (or bacterial solution), it is to be understood that the bacteria are in solution in a suitable culture medium, from about 10 ⁶ to about 10 ⁹ cells / ml.

As an indication, an example of a culture medium for calcifying bacteria and an example for denitrifying bacteria are given below. • Culture medium for calcifying bacteria o 0.01 M phosphate buffer pH 6.5 o 1% (w / v) Casitone o 0.1% (w / v) MgSO ₄ .7H ₂ O • Culture medium for denitrifying bacteria o 1000 ml Distilled water o 5 g Peptone o 3 g Meat extract.

Each medium is sterilized by autoclaving for 20 minutes at 121 ° C. After cooling, it is inoculated with the above bacteria.

The first step of the process according to the invention consists in feeding the soil that it is desired to consolidate with one or more solution (s) of calcifying bacteria.

Beforehand, care should be taken to create on the site to be treated: - on the one hand, supply boreholes, preferably located around the area to be consolidated, and, on the other hand, boreholes for pumping or capture (eg filter tips), preferably arranged in the center of the area to consolidate. This network of supply and capture wells (hereinafter also called "firing plan") is designed to delineate the flow of fluids to the volume of the soil to be treated. As a general rule, the number and position of supply and capture wells is determined in such a way that the time required to replace the total volume of water (naturally occurring in the soil) in the area to be consolidated does not does not exceed half of the practical life of bacteria in their culture medium.

The calcifying bacteria are thus injected into the soil, by gravity, by means of the power holes. Then a depression is created in the area to be treated by means of the boreholes (for example, filter points).

Typically, capture systems operate under vacuum at a pressure maximum of - 0.7 bar. Depression thus established has the effect of circulating the bacterial solution (s) in the area to be treated and to remove the water present in the soil; in this way we obtain a "leaching" of the soil. Obviously, the speed of circulation of the bacterial solution is governed by the permeability of the soil, which itself governs the flow rate and the injection speed of said solution. The circulation is continued until the filling in bacterial solution (s) has reached the desired level; this level is determined by an analysis of the bacterial solution content in water, recovered by the capture systems. The calcifying bacteria are then supplied with the necessary nutrients to ensure the carbonation of the soil and thus the desired consolidation. The nutritive solution (s) is (are) injected into the soil (always by gravity) through the supply boreholes, then circulates in the zone to be consolidated according to the principle indicated above, the depression created to circulate the calcifying bacteria being maintained. Depending on the nature and volume of the soil to be treated, as well as the desired degree of consolidation, it may be necessary to repeat this step one or more times. The time required to reach a satisfactory level of carbonation (thanks to the contribution of nutrients) is of the order of 30 to 45 days. Nutrient solutions are standard solutions, well known to those skilled in the art. They provide calcifying bacteria with a source of organic carbon, nitrogen, and other elements essential for their physiological function. It is understood that if several types of calcifying bacteria are used, it may be necessary to use different nutrient solutions, meeting the needs of each type of bacteria.

As an indication, an example of a nutrient solution for the production of carbonate by calcifying bacteria is given below.

• 0.01 M phosphate buffer at pH 8

• 1% (w / v) Casitone • 1% (w / v) Ca (CH ₃ COO) ₂ .4H ₂ O

• 0.02% (w / v) K ₂ CO ₃ .1 / 2H ₂ O.

This solution is sterilized by autoclaving for 20 minutes at 121 ° C. and then cooled before use.

After stopping the injection of the nutritive solution (s), the calcifying bacteria die to leave only the original soil that was carbonate. As indicated previously, during carbonation, bacteria Calcificants produce nitrogenous compounds that are released into the surrounding environment. So, to "neutralize" these rejects, denitrifying bacteria are used.

The advantage of the process lies in the fact that the denitrifying bacteria use the degradation products of calcifying bacteria as nutrients to their own metabolism, which makes the process perfectly ecological.

The solution (s) of denitrifying bacteria is (are) injected (s) into the soil, always via the drilling of feeding, then circulates (it) in the zone to consolidate thanks to the depression created to make circulating calcifying bacteria and nutrient solutions.

According to a particular embodiment of the invention, the solution (s) of denitrifying bacteria is (are) injected (s) in step a), simultaneously or after the injection of the solution (s). ) of calcifying bacteria.

The method according to the invention can be optimized by using means of control of the evolution of the treatment, to adapt it accordingly, such as:

• means of analysis of pumped products and gas emissions,

• adapted geotechnical or geophysical tests. Thus, the evolution of the degree of carbonation is evaluated preferably by geophysical methods, for example by measuring the evolution of the shear wave velocity in the field between supply boreholes and pumping or capture wells.

The bacterial content and / or degradation products of the calcifying bacteria of the water recovered by the capture systems is also analyzed. If this analysis reveals the presence of nitrogen compounds, for example nitrates, it is possible to treat the water recovered with a solution of denitrifying bacteria as defined above. The treated water can then be reinjected into the soil, for example in the water table. Depending on the type of microorganisms used, the process according to the invention makes it possible to preserve the porosity of the ground, which is favorable in certain cases (maintenance of the underground circulation, particular case of the liquefaction of the soil in the event of an earthquake) , or on the contrary to clog the pores.

For example, the bacterium Myxococcus xanthus has the following advantages:

• it allows to deposit a layer of CaCO ₃ on the pore wall without blocking them,

• calcite deposition is strongly related to pre-existing soil grains,

• The new calcite crystals thus formed are more resistant than the original material. • It is possible to modulate the productivity of bacteria and the nature of their deposits by modifying their culture medium. The use of bacterial solutions for soil consolidation offers the following advantages over traditional grouts:

• they have rheological properties close to those of water, thus an excellent penetrability in the ground,

• they do not show a catch phenomenon, which gives them a large radius of action; drilling can therefore be much more distant than those required for a conventional injection treatment. These two properties allow great flexibility in the design of the "firing plan", which can be further adapted during processing.

Another advantage related to the properties above is that the implementation of the process does not require specific equipment and has less constraints than conventional injection (storage of products, preparation-mixing material, implementation under high pressure , cleaning stress, etc.).

According to a second aspect, the invention relates to a composition for implementing the method described above. This composition contains separately (i) calcifying bacteria, (ii) denitrifying bacteria and (iii) optionally one or more nutritive solution (s) for calcifying bacteria. The bacteria, whether calcifying or denitrifying, are cultured on the intervention site from freeze-dried strains or in solution in a suitable culture medium. The composition can be in the form of a "kit" in two or three parts each containing the appropriate amounts of calcifying and denitrifying bacteria and, where appropriate, nutrient solutions. The invention will now be described in more detail with the aid of the following example, given purely by way of illustration.

Example: consolidation of the basement of a building Figures IA and IB show schematically the implementation of the method of the invention for the treatment of the soil under an existing building. Figure 1A is a plan view of the "firing plan" (network of supply and capture wells) used in the method of the invention. More specifically, the firing plan comprises 18 supply boreholes 1 disposed around the perimeter of the zone to be treated, and 5 filtering tips 2 disposed in the center of the zone to be treated.

Figure 1B is a sectional view of the treatment implemented under a building. The shaded area 3 represents the extent to be consolidated.

Firstly, feeding holes are fed with a calcifying bacterial solution, in this case a solution of Myxococcus xanthus bacteria in a culture medium comprising:

• 0.01 M phosphate buffer pH 6.5 • 1% (w / v) Casitone

• 0.1% (w / v) MgSO ₄ .7H ₂ O.

The filter tips, located in the center of the area to be treated, are depressed, which allows the bacterial solution to circulate within the area to be treated. By appropriate analysis of the bacterial solution content in the water, recovered by the filter tips, the time when the filling has reached the desired level is defined.

The operation is then continued with the introduction of a nutrient solution to promote carbonation by the bacteria, of composition:

• 0.01 M phosphate buffer pH 8 • 1% (w / v) Casitone

• 1% (w / v) Ca (CH ₃ COO) ₂ .4H ₂ O

• 0.02% (w / v) K ₂ CO ₃ .1 / 2H ₂ O.

During this period, the water recovered by the filter tips is analyzed and treated by denitrifying bacteria, as soon as the analysis reveals the presence of releases produced by the calcifying bacteria.

After the carbonation phase, the water recovered by the filter tips is recirculated in closed circuit and treated with denitrifying bacteria until a level compatible with the standards in force.

Claims

A method of soil consolidation, which comprises the following steps: a) feeding the soil with one or more solution (s) of calcifying bacteria; b) the circulation of this (these) solutions in the soil; c) feeding the soil with one or more nutritive solution (s) for the calcifying bacteria and the circulation of this (these) solution (s) in the soil; d) feeding the soil with one or more solution (s) of denitrifying bacteria and the circulation of this (these) solution (s) in the soil.

2. Method according to claim 1, wherein the supply of bacterial and nutritive solutions (s) is carried out by means of a network of supply boreholes.

3. The method of claim 2, wherein the supply boreholes are disposed on the periphery of the area to be consolidated.

4. Method according to one of claims 1 to 3, wherein the circulation in the soil of the various solutions is provided by means of a network of boreholes for pumping or capture.

5. Method according to claim 4, wherein the drilling of pumping or capture are arranged in the center of the area to be consolidated.

6. Method according to one of claims 1 to 5, which further comprises the control of the degree of progress of the consolidation of the soil and / or the analysis of the water recovered by the drilling of pumping or capture.

7. Method according to one of claims 1 to 6, wherein the calcifying bacteria are selected from the families Bacillaceae, Pseudomonadaceae,

Enterobacteriaceae and Myxococcales.

8. A soil consolidation composition which comprises separately (i) calcifying bacteria, (ii) denitrifying bacteria and (iii) optionally one or more nutritive solution (s) for calcifying bacteria.

The composition of claim 8, wherein the calcifying bacteria are selected from the families Bacillaceae, Pseudomonadaceae, Enterobacteriaceae and Myxococcales.

10. Composition according to claim 8 or 9, wherein the bacteria are in the form of freeze-dried strains or in solution in their culture medium.