process for treating excavated soils containing pyrite

Abstract

The present invention relates to a method for treating excavated soil spoil from underground development, comprising a step of determining the iron disulfide, FeS2, content and the carbonate content in the spoil.

Description

Title of the invention: method for treating excavated soils containing pyrite FIELD OF THE INVENTION

[0001] The invention lies in the field of treatment and management of materials generated by excavation, in particular during the digging of structures. The invention aims at the treatment of materials presenting risks of oxidation and acidification. The invention thus aims at the treatment of excavated soils containing pyrite. STATE OF THE ART

[0002] Infrastructure projects involve underground developments, such as tunnels, dams, networks, foundations, which require excavation work and generate significant volumes of excavated materials during their construction. The excavation may involve marine and / or aquatic environments such as basins, ports or canals, or non-marine environments.

[0003] In aquatic environments, sedimentation forms sedimentary layers by decantation in the bottom of these basins. The cleaning of basins, such as canals or port installations, consists of the extraction of these sedimentary layers formed by decantation. The cleaned materials, that is to say those resulting from the cleaning, are extracted and generally exported to suitable deposit sites.

[0004] In non-marine and aquatic environments, digging refers to excavating or digging into the ground. Infrastructure projects use different digging methods depending on the type of soil.

[0005] The nature and characteristics of the materials excavated during digging or cleaning methods, in particular the physicochemical composition and the granulometry, vary not only according to the geological nature of the ground being dug or cleaned, but also according to the geological, chemical and biological phenomena which have previously affected this ground. The digging methods are, for example, so-called "mechanized" digging methods, as opposed to methods using explosives. Mechanized digging methods are generally implemented using machines, such as excavators, dredgers or tunnel boring machines. They can in particular be implemented with gripper tunnel boring machines - preferably used in rocky terrain - or with earth pressure or mud pressure tunnel boring machines - preferably used in the case of soft terrain (for example composed of clay, gravel, silt and / or sand).

[0006] Methods for cleaning basins, for example canals or port installations, are for example mechanical cleaning, that is to say by bucket machines, mechanical shovels, bucket dredgers, or hydraulic cleaning, i.e. by cutter dredgers or pumps.

[0007] Thus, digging methods can create a significant flow of spoil which must be stored, in particular so that it can be reused in various construction or earthwork applications, without polluting the environment.

[0008] These methods generate large volumes of excavated materials. The spoil is then most often used as backfill. However, in order to be stored or reused in various construction applications, the spoil must be environmentally inert, in the sense that it must not contribute to a risk of environmental pollution. Thus, the management of excavated materials during excavation and / or cleaning has become an important issue for these infrastructure projects.

[0009] It is therefore the responsibility of the project owners of underground excavation or cleaning works to manage the excavated materials.

[0010] It is also particularly advantageous to be able to process large quantities of excavated materials, in particular quantities greater than or equal to 1000 tonnes, in particular quantities greater than or equal to 2000 tonnes, for example approximately 2500 tonnes, a mass corresponding to the volume of a barge, per processing cycle. Statement of the invention

[0011] An aim of the invention is to treat excavated soil spoil, originating from underground excavation sites, in particular originating from tunnel boring machines, presenting risks of oxidation and acidification.

[0012] To this end, a method is proposed for treating excavated soil spoil from underground development, comprising a step of determining the iron disulfide content, FeS2, and the carbonate content in the spoil.

[0013] Advantageously, the method comprises the following steps: a) determination of the acidification potential of the spoil, noted AP, by: al) determination of the oxidizable sulfur content, noted Sox%, in the spoil; a.2) calculation of the acidification potential AP according to the following equation (1): [Math 1] AP — 2 x Sox% x b) determination of the neutralization potential of the spoil, noted NP, by: bl) determination of the total carbonate content in the spoil, noted MC03%, M representing the counter-ion, preferably Ca or Mg; b.2) calculation of the neutralization potential NP according to the following equation (2): [Math 2] NP = 2 x MCO,% x c) calculation of the NP / AP ratio; d) if the NP / AP ratio is > 3, the spoil can be stored without treatment; e) if the NP / AP ratio is < 3, the spoil is treated by adding carbonate in such a way that the resulting mixture has an NP (mixture after treatment) / AP (mixture after treatment) ratio strictly greater than 3.

[0014] During step e) the carbonate is advantageously provided by adding calcareous material and step e) comprises the following steps: el) determination of the total carbonate content in the limestone material, noted MCO3%(limestone), M representing the counter-ion, preferably Ca or Mg; e.2) calculation of the neutralization potential of the calcareous material, noted NPcaicaire, according to the following equation (3): [Math 3] NP cakait . e = 2 x MCO.% (limestone) x e.3) calculation of the minimum rate of limestone material to be added, noted x, according to the inequality (4) following: [Math 4] 3xAP-NP NPM.e-NP+3xAP where AP and NP are as defined previously. Preferably, the content, x, of calcareous material to be added is determined according to the following equation (5): [Math 5] _ 5xAP-NP NPcalc,]ir-NP+5xAP where NP, AP and NPcaicaire are as defined previously. The excavated soils are particularly sandy-clayey in nature.

[0015] The process is advantageously carried out on an industrial scale. It advantageously comprises the following steps: 1. Deposit of excavated soil debris from the same geographical digging area on a flat, impermeable area with means of collecting and recovering runoff water; and 2. Analysis of iron disulfide content, FeS2, and carbonate content, calculation of the NP / AP ratio and determination of the minimum rate, x, of calcareous material to be added; then 3. Spreading of limestone materials including the deposit of a layer of limestone materials superimposed on the layer of spoil to be treated; then 4. Homogenization of the mixture of rubble and limestone materials by mixing; then 5. Storage of the excavated material made inert following step 4.

[0016] During step a), 2500 T of spoil are advantageously deposited. DETAILED DESCRIPTION OF THE INVENTION

[0017] The subject of the invention is a method for treating excavated soil spoil comprising a step of determining the iron disulfide content, FeS2, and the carbonate content in the spoil.

[0018] The method of treating spoil directly from excavation thus allows the treatment of spoil from, for example, excavations carried out by tunnel boring machines.

[0019] Spoil, such as that from the Sables Supérieurs / Sables de Cuise, from the Ypresian geological horizon, the Fausses Glaises, or the gypsum marls, may contain pyrite. Pyrite is a mineral species composed of iron disulfide. In the presence of air and meteoric water, this pyrite undergoes a natural oxidation phenomenon, according to the reaction (1):

[0020] [Math.6] FeS2 + ^O2 +}H2O Fe(pH\ + 230%+4^

[0021] This oxidation results in a drop in pH, a release of sulfates, and a dissolution in an acid medium of certain metals which were initially insoluble.

[0022] The carbonate makes it possible to neutralize the acid produced by oxidation, according to an irreversible acid / base reaction (2) releasing carbon dioxide:

[0023] [Math.7] 2H* + SOi + CaCO3 + H2O CaSO42H2O + CO2

[0024] When the limestone content is sufficient, the oxidation reaction (2) no longer produces acid in the form of proton H+, and results in the formation of gypsum CaSO4.2H2O and carbon dioxide (gaseous or in the form of hydrogen carbonates).

[0025] This oxidation in the presence of limestone does not lead to a drop in pH, and in fact the solubility of the metals is not increased. The sulfur present is found in a stable form of gypsum.

[0026] It is thus possible to neutralize the risk of acidification of materials containing pyrite by adding carbonate. The quantity of carbonate to be added can be determined, at the industrial level, by determining the acidification potential of the spoil, where each mole of oxidizable sulfur generates two moles of H+ acid, and the neutralization potential, where each mole of carbonate neutralizes two moles of H+ acid.

[0027] For the purposes of the invention, the terms “oxidizable sulfur” will designate iron disulfide which is found in particular in pyrite.

[0028] Oxidizable sulfur is measured as follows by a direct method for the determination of sulfides Ss%, according to EN 1744-1§ 12 of February 2014, tests to determine the chemical properties of aggregates - part 1: chemical analysis. This standard consists of to solubilize all the sulfide present with concentrated hydrochloric acid (i.e. having a density at 20°C ranging from 1.18 to 1.19 g / cm3, as defined in the cited standard), in the presence of a reducing agent. The sulfides are transformed into dihydrogen sulfide, which is carried by a gas stream into an ammoniacal solution of zinc sulfate. The precipitated zinc sulfide is determined by iodometry. It is expressed as % of sulfide ions per kg of cuttings.

[0029] The concentration of oxidizable sulfur Sox% is directly equal to the concentration of sulfide Ss% determined previously.

[0030] Alternatively, it is possible to measure the concentration of oxidizable sulfur by a difference method:

[0031] 1) The total sulfur content of the spoil, Stot%, is determined according to the standard EN1744-1§11 of February 2014, tests to determine the chemical properties of aggregates - part 1: chemical analysis, or equivalent alternative method. This standard consists of oxidizing all the sulfur present with hydrogen peroxide, and solubilizing all the sulfur present in hydrochloric acid. The sulfur thus dissolved is determined gravimetrically with barium chloride. It is expressed as a % of total sulfur per kg of spoil.

[0032] 2) The content of acid-soluble sulfate ions, SO4SOi_ac%, is determined according to the EN 1744-1 §12 of February 2014, tests to determine the chemical properties of aggregates - part 1: chemical analysis. This standard consists of solubilizing all the sulfate present in dilute hydrochloric acid. The sulfides present are transformed in an acidic medium into gaseous dihydrogen sulfide, and are therefore not measured. The sulfate thus dissolved is measured gravimetrically with barium chloride. It is expressed as a % of sulfate ions soluble in acid per kg of spoil. It can be recalculated as a % of sulfur present in the form of sulfate soluble in acid, by the relationship Ssol-ac% = SO4Sol-ac% / 3.

[0033] The oxidizable sulfur content, noted Sox%, is determined by subtracting the sulfur content in the form of sulfates soluble in acid, Ssoi aC%, from the total sulfur content, Stot%: Sox% = Stot% - Ssol_ae%.

[0034] The method advantageously comprises the following steps: a) determination of the acidification potential of the spoil, noted AP, by: al) calculation of the oxidizable sulfur content of the spoil, noted Sox%, advantageously estimated by the direct method or the difference method described above; Once the oxidizable sulfur content is determined, the acidification potential of the spoil, noted AP, is calculated according to equation (1): a.2) calculation of the acidification potential AP according to the following equation (1): [Math 8] AP = 2 x Sox% x b) determination of the neutralization potential of the spoil, noted NP, by: bl) determination of the total carbonate content in the spoil, noted MCO3%, M representing the counter-ion, preferably Ca or Mg; The total carbonate content is determined according to the standard NF P 94-048 of October 1996, soils: reconnaissance and tests - determination of the carbonate content - calcimeter method, (or by an equivalent method). This method consists of attacking the sample with hydrochloric acid. The carbonates present are converted into gaseous CO2. The reaction is carried out in a set-up allowing the volume of gas produced to be measured. The total carbonate content thus measured is recalculated in the form of MC03 in % of MC03 per kg of spoil, with M representing the counter-ion, preferably Ca or Mg. Once the total carbonate content is determined, the neutralization potential of the spoil, denoted NP, is calculated according to equation (2). b.2) calculation of the neutralization potential NP according to the following equation (2): [Math 9] NP = 2 x MCO.% x Once the acidification potential AP of the spoil and the neutralization potential NP of the spoil are calculated, the NP / AP ratio is calculated. c) calculation of the NP / AP ratio; d) if the NP / AP ratio is > 3, the spoil can be stored without treatment; e) if the NP / AP ratio is < 3, the spoil is treated by adding carbonate in such a way that the resulting mixture has an NP (mixture after treatment) / AP (mixture after treatment) ratio strictly greater than 3.

[0035] This ratio makes it possible to determine whether the spoil intrinsically comprises sufficient carbonate to ensure that the pyrite does not undergo oxidation when the spoil is left in contact with air and meteoric water.

[0036] It has been determined that for this the NP / AP ratio must be strictly greater than 3.

[0037] Thus, when the NP / AP ratio is less than or equal to 3, the spoil must be treated by adding carbonate. This treatment makes it possible to obtain a mixture of cuttings and carbonate, also called a mixture after treatment, which has a NP (mixture after treatment) / AP (mixture after treatment) ratio strictly greater than 3.

[0038] The carbonate can be any carbonate, with a counterion preferentially chosen from calcium or magnesium, more advantageously calcium.

[0039] It is particularly interesting to use a mineral material as a source of carbonate. As an example of a suitable material, we can cite all calcareous materials such as chalk.

[0040] Advantageously, during step e) the carbonate is provided by adding material limestone and step e) comprises the following steps: el) determination of the total carbonate content in the limestone material, denoted MCO3%(limestone), with M representing the counterion, preferably Ca or Mg; e.2) calculation of the neutralization potential of the calcareous material, noted NPcaicaire, according to the following equation (3): [Math 10] NPcalcaire = 2 x MC03% (limestone) x e.3) calculation of the minimum rate of limestone material to be added, noted x, according to the inequality (4) following: [Math 11] and 3xAP-NP X NPcMre-NP+3xAP OR AP represents the acidification potential of the spoil, NP represents the neutralization potential of the spoil, AP and NP are determined as explained previously.

[0041] The total carbonate content in the limestone material is determined in the same way as the total carbonate content in the spoil, according to standard NF P 94-048 of October 1996. It is expressed in mg of MC03 per kg of limestone material, with M representing the counter-ion, preferably Ca or Mg.

[0042] As soon as the ratio of neutralizing potential to acidifying potential is greater than 3, the spoil can be classified as “non-acidifying”. This safety factor of 3 guarantees that there is no excess acid production compared to the neutralizing capacity of the treated spoil.

[0043] Advantageously, the content, x, of limestone material to be added is equal to the following equation (5): [Math 12] _ 5xAP-NP ~ NPcatcaire.-NP+5xAP OR AP, NP and NPcaicaire are determined as explained previously

[0044] The excavated soils are most often sandy-clayey in nature. They are advantageously moist soils. However, they are preferably not sludge.

[0045] The method according to the invention can be implemented for very large quantities of spoil, of the order of one to several thousand tonnes, in particular 2500 tonnes, a mass corresponding to the volume of a barge.

[0046] The method then advantageously comprises the following steps: 1. Deposit of excavated soil debris from the same geographical digging area on a flat, impermeable area with means of collection and recovery. operation of runoff water; and 2. Analysis of the iron disulfide content, FeS2, i.e. the oxidizable sulfur content, and the carbonate content, calculation of the NP / AP ratio and determination of the minimum rate, x, of calcareous material to be added, according to the protocols previously described; then 3. Spreading of limestone materials including the deposit of a layer of limestone materials superimposed on the layer of spoil to be treated; then 4. Homogenization of the mixture of rubble and limestone materials by mixing; then 5. Storage of the excavated material made inert following step 4.

[0047] Steps 1 and 2 can be carried out in any order. In particular, step 2 can be carried out before step 1. When several batches of the same compositions are treated, step 2 is carried out for one of these batches and then the results obtained during this step 2 are used for each of the batches of the same composition, without reproducing step 2.

[0048]

[0049]

[0050] When setting up the first batches, a random sampling step can be planned after step 4 to determine the carbonate content after treatment and thus verify that the target NP (mixture after treatment) / AP (mixture after treatment) ratio is obtained. When homogeneity is deemed sufficient, statistical control can be carried out by external chemical analyses: - 5 unit samples from a section of the mixed strip to constitute an average sample; - Creation of 20 average samples distributed randomly over the mixed area; - Analysis of the carbonate rate on the 20 average samples to validate the homogeneity of the mixture; Calculation of the ratio NP (mixture after treatment) / AP (mixture after treatment), SUT UUC Random selection of 8 average samples. In a particular embodiment, the spoil comes from tunnel boring machines. It is most often wet without however being in the form of sludge. The spoil to be treated is made up of unit batches of one barge, or 2500 tonnes. The homogeneity of composition within a batch is checked, at each change of geological horizon or every 50 kt. The spoil is implemented in batches of 2500 tonnes, on leveled areas of approximately 7500 m2 in order to be able to recover runoff water. The leveled area can in particular be located on a chalky roof that the surface alteration has made impermeable; the platform is sloped and the water recovery is done via a drained ditch and a water collection basin sealed with clay. The leveled area can also be an area with clayey materials on the surface which will allow the runoff water to be channeled towards a collection basin sealed with clay. clay located on the periphery. The pH of the collection basin is continuously monitored; the water is managed by natural evaporation.

[0051] The complete treatment cycle takes 4 days: - recovery and implementation of pyritic materials: 1 day; - recovery and implementation of limestone materials: 1 day; - mixing: 1 day; - destocking and final backfilling: 1 day. The oxidizable sulfur and carbonate content of the spoil to be treated is determined using the method described above. Then the carbonate input required to obtain a NP (mixture after treatment) / AP (mixture after treatment) ratio strictly greater than 3, in particular equal to 5, and the nature of the products to be incorporated and the dosage to be used is determined using the method described above.

[0052] The spreading of limestone materials will consist of a layer deposit superimposed on the layer of spoil to be treated. The proportion of added material is ensured by GPS control of the layer thicknesses and by monitoring the tonnages of the added and treated materials.

[0053] In the case of incorporating calcareous sand, it is also possible to use a gravel truck equipped with adjustable volumetric dosers controlled by the speed of movement of the machine.

[0054] The actual mixing thickness will be done by visual monitoring.

[0055] Mixing can be carried out using a horizontal shaft mixer. There will be a match between the thickness of the layers and the effective depth of mixing.

[0056] Mixing can be carried out in successive strips with an overlap of 20 cm of the contiguous strip already mixed.

[0057] At the end of the treatment, a random sample can again be carried out in order to measure the carbonate rate and to calculate the NP ratio (mixture after mixing / A^ treatment) •

[0058] The invention therefore makes it possible to effectively treat spoil of a pyretic nature which otherwise presents a risk of pollution for the environment by acidification of the spoil and solubilization and release of the metals present in the spoil. The method is particularly suitable for the industrial treatment of large volumes of spoil.

[0059] In the following example, spoil was subjected at laboratory level to forced oxidation and then leaching test. Examples

[0060] Representative samples of the various geological horizons Sand, False Clay, Limestone were taken. They were treated with limestone, to achieve a target NP / AP ratio, as described below.

[0061] The determination of the rate of limestone to be incorporated into the oxidizable material to avoid its acidification is obtained at the end of the following steps: - Measurement of oxidizable sulfur according to EN1744-1, February 2014, as explained previously Oxidizable sulfur = Total sulfur - Acid soluble sulfates This oxidizable sulfur corresponds to sulfur in the form of pyrite, as demonstrated by mineralogical studies in X-ray diffraction. - Calculation of the acidification potential AP according to equation (1) defined previously; - Measurement of total carbonate, according to standard NF EN196.2 or standard NF P 94-048 as explained previously; - Calculation of the neutralization potential NP according to equation (2) defined previously; - Calculation of the NP / AP ratio.

[0062] If NP / AP is naturally > 3: no treatment necessary.

[0063] If NP / AP <3, a mixture with a carbonate supply is necessary.

[0064] The carbonate is provided by adding chalk. We will therefore speak of limestone in what follows.

[0065] The total carbonate of the chalk is measured as previously.

[0066] Then, the NPCaicaire Neutralization potential is calculated, as described previously.

[0067] The NP(mixture after treatment) / AP(mixture after treatment) Or mixed Obtained SC Calculated As Follows.

[0068] [Math. 13] A Papres ïniltement ( 1-.Y ) XNP+X^^PaUeaire .APnieléinge after treatment with NPcaicaire: neutralization potential of the limestone used and x: rate of limestone to be incorporated.

[0069] Demonstration of the effectiveness of the treatment

[0070] Untreated materials, when subjected to oxidation upon exposure to weather conditions, become acidic and release undesirable elements into the environment.

[0071] The following methodology makes it possible to reproduce in accelerated mode in the laboratory, a high oxidation of the sample.

[0072] Before treatment, the samples are ground to a particle size of less than 1 mm to ensure homogeneity and allow better reactivity of the material with the oxidant in order to oxidize all the sulfides present. The samples were subjected to forced, aggressive oxidation with H2O2.

[0073] For 100 g of dry material of excavated material, 100 ml of H2O2 at 10% v / v are added and 24 hours are waited, then the addition of 100 ml of H2O2 at 10% v / v is repeated with a waiting phase. 24 hours and finally 100ml of H2O2 at 10%v / v is added with a waiting phase of 72 hours.

[0074] A leaching test according to standard EN 12457-2, of December 2002, characterization of waste: Leaching: conformity test for leaching of fragmented waste and sludge - Part 2: single batch test with a liquid-solid ratio of 100L / kg and a particle size of less than 4 mm, is carried out at the end of the test on the oxidized material.

[0075] This protocol makes it possible to establish the maximum possible release, when the material will be completely oxidized by natural conditions. This protocol therefore maximizes the potential release of contaminants by pyritic materials during the natural oxidation phenomenon.

[0076] The tests and their results are summarized in the following table.

[0077] [Tables 1] Soil type Soil mass Chalk mass NP / AP PH Sulfates Ni Pb Zn S and G NT 100 0 0 2.1 20900 4.30 1.95 32.70 S and GT 100 15 3 6.9 14500 0.90 <0.005 1.80 S and GT 100 24 5 6.9 15800 0.83 <0.005 1.70 SI NT 100 0 0.31 4.1 15800 2.47 <0.005 15.00 SI T 100 5.3 1.5 6.9 14000 0.72 <0.005 1.60 SI T 100 21 5 7 13900 0.79 <0.005 2.40 S2 NT 100 0 0.5 3.1 15200 2.60 0.42 12.40 S2T 100 14 5 6.9 13000 0.74 <0.005 1.30

[0078] S, SI, S2 = sand. The sands S, SI and S2 do not have the same origin. Conversely, all the sands S, or SI, or S2, have the same origin.

[0079] G=clay, same origin

[0080] T=treated and NT=untreated

[0081] The masses of soil and chalk are expressed in g.

[0082] The contents of chemical elements are expressed in mg / kg.

[0083] The NP / AP ratio is the ratio of the mixture after treatment, NP(mix after treatment) / AP(mix after treatment), when chalk is added.

[0084] For information, the regulatory thresholds, in 2021, are indicated in the following table:

[0085] [Tables2] Sulfates Ni Pb Zn Regulatory threshold (mg / kg) 19000 1.2 1.5 12

[0086] It is observed on acidifying soils without the addition of chalk that the pH reached at the end of oxidation becomes acidic or even very acidic, that this drop in pH leads to a dissolution of metals initially present in an insoluble form, that the sulfates exceed the regulatory thresholds for soils initially containing little or no carbonate.

[0087] It is observed that when carbonate is added, the pH reached at the end of oxidation remains neutral, and that this neutral pH does not lead to a release of metals. In fact, the release of Ni, Pb, Zn remains below the regulatory threshold. In accordance with the theoretical solubility of gypsum, the sulfate content remains between 13,000 and 16,000 mg / kg.

[0088] Conclusions

[0089] Limestone treatment is an irreversible reaction which makes it possible to guarantee, with a safety factor of 3 in relation to stoichiometry, that, under the operating conditions of an inert waste storage facility, the material obtained will remain neutral and that there can be no change over time leading to acidification of the material or of the environment in which it is located.

[0090] The accelerated oxidation study demonstrates experimentally that the material will remain neutral even at the end of oxidation, which avoids any increase in the release of metals, and that the released sulfates will remain strictly in accordance with the regulatory threshold.

[0091] The main soluble species at the end of the treatment are therefore calcium sulfates and hydrogen carbonates whose release levels are limited by their solubility in water.

Claims

Claims

1. Method for treating excavated soil spoil from underground development, comprising a step of determining the iron disulfide content, FeS2, and the carbonate content in the spoil, characterized in that it comprises the following steps: a) determination of the acidification potential of the spoil, noted AP, by: al) determination of the oxidizable sulfur content, noted Sox%, in the spoil; a.2) calculation of the acidification potential AP according to the following equation (1): AP=2xS„x%x^6 b) determination of the neutralization potential of the spoil, noted NP, by: bl) determination of the total carbonate content in the cuttings, noted MCO3%, M representing the counter-ion, preferably Ca or Mg; b.2) calculation of the neutralization potential NP according to the following equation (2): NP = 2 x MCO.% x c) calculation of the NP / AP ratio; d) if the NP / AP ratio is > 3, the spoil can be stored without treatment; e) if the NP / AP ratio is < 3, the spoil is treated by adding carbonate so that the resulting mixture has a NP (mixture after treatment) / AP (mixture after treatment) ratio strictly greater than 3.

2. Method according to claim 1, in which during step e) the carbonate is provided by adding calcareous material and step e) comprises the following steps: el) determination of the total carbonate content in the calcareous material, denoted MCO3%(limestone), M representing the counter-ion, preferably Ca or Mg; e.2) calculation of the neutralization potential of the limestone material, noted NP limestone, according to the following equation (3): N p limestone = 2 x MCO3% (limestone) x e.3) calculation of the minimum rate of calcareous material to be added, noted x, according to the following inequality (4): rs 3xAP-NP where AP and NP are as defined in claim 1.

3. A method according to claim 2, wherein the content, x, of calcareous material to be added is determined according to the following equation (5): _ 5xAP-NP NP(:alca,rirNP+SxAP where NP, AP and NPcaicaire are as defined in claim 1 or 2.

4. A method according to any preceding claim, wherein the excavated soils are of a sandy-clayey nature.

5. Method according to any one of the preceding claims, comprising the following steps:

1. Depositing excavated soil spoil from the same geographical digging area on an impermeable flat area with means for collecting and recovering runoff water; and 2. Analyzing the iron disulfide content, FeS2, and the carbonate content, calculating the NP / AP ratio and determining the minimum rate, x, of limestone material to be added; then 3. Spreading the limestone materials comprising depositing a layer of limestone materials superimposed on the layer of spoil to be treated; then 4. Homogenizing the mixture of spoil and limestone materials by mixing; then 5. Storing the spoil made inert following step 4.

6. Method according to claim 5, in which during step a) 2500 T of spoil are deposited.