Experimental method for the improvement mechanism of expansive soil based on microbial-induced calcium carbonate

Abstract

This invention discloses an experimental method for the improvement mechanism of expansive soil based on microbial-induced calcium carbonate. The method involves taking four containers and adding dry expansive soil and pure water to form a water film on the surface of the expansive soil. Then, the water film thickness is obtained under different conditions: no water film, ammonium salt, calcium salt, urea, calcium salt, and microorganisms are added. The changes in thickness are analyzed to obtain the influence mechanism of microbial-induced calcium carbonate on the surface properties of expansive soil. This invention can qualitatively and quantitatively determine how microbial-induced calcium carbonate technology affects the surface properties of expansive soil, thus providing a quantitative component ratio reference for the microbial improvement of expansive soil to achieve the best improvement effect.

Description

Technical Field

[0001] This invention relates to the field of expansive soil improvement testing technology, specifically to a test method for the expansive soil improvement mechanism based on microbial-induced calcium carbonate. Background Art

[0002] Expansive soil is a type of cohesive soil that expands dramatically in volume when soaked in water and shrinks significantly in volume when dehydrated; it is also known as "expansive-shrinking soil." It often causes uneven vertical or horizontal expansion and contraction deformation in buildings, leading to displacement, cracking, tilting, and even damage. These deformations often occur in clusters, especially severe in low-rise bungalows. Cracks are characterized by vertical cracks on exterior walls, diagonal cracks at the ends, horizontal cracks under window sills, and symmetrical or asymmetrical inverted V-shaped cracks on inner and outer walls. Longitudinal strips and grid-like cracks appear on the floor. These typically appear six months to five years after building completion. Therefore, expansive soil can easily cause serious damage to buildings. Civil engineering technicians need to study methods to improve expansive soil to better suppress its expansion characteristics and avoid harm to construction projects.

[0003] The main mineral component of expansive soil is montmorillonite, which exhibits highly unstable properties, including swelling upon water absorption, shrinkage upon water loss, repeated expansion and contraction deformation, decreased bearing capacity when submerged in water, and development of shrinkage cracks. This is the primary reason why expansive soil swells upon contact with water. Montmorillonite is a layered hydrous aluminosilicate mineral containing small amounts of alkali metals and alkaline earth metals. Its structural layers are of the 2:1 type, with water molecules and exchangeable cations between the layers, forming a dioctahedral aluminosilicate. Heterovalent isomorphic substitution within the crystal lattice is the most fundamental and primary structural characteristic of montmorillonite. The crystal structure of montmorillonite consists of two layers of silicon-oxygen tetrahedra and one layer of aluminum (magnesium)-oxygen (hydroxyl) octahedral sheets sandwiched between them. Si⁴⁺ in the silicon-oxygen tetrahedra is often replaced by Al³⁺, while Al³⁺ in the aluminum-oxygen octahedron can be replaced by low-valence cations such as Mg²⁺ and Fe²⁺, resulting in excess negative charges (permanent negative charges) between the crystal layers (structural layers).

[0004] Due to the layered structure and surface electrical properties of montmorillonite, most current research generally suggests that its swelling is primarily caused by the formation of a water film between the crystal layers due to the adsorption of water and other cations, leading to an increase in interlayer spacing. The swelling ratio can range from ten to thirty times. To improve the swelling properties of montmorillonite or expansive soils, sodium or calcium salts are often added. These cations, such as calcium or sodium ions, neutralize the surface electrical properties of montmorillonite, thereby reducing its ability to adsorb water and achieving a certain degree of improvement, thus reducing its swelling.

[0005] With the development of microbial technology, significant progress has been made in improving the crack propagation of concrete through microbial-induced calcium carbonate precipitation. The basic principle involves the metabolic activity of specific types of bacteria producing urease. Urease promotes the hydrolysis of urea, producing ammonia and carbon dioxide. The dissolution of ammonia in water causes a change in the solution's pH, causing carbon dioxide to exist as carbonate ions. These carbonate ions then combine with calcium ions in the solution to form calcium carbonate precipitate, filling the cracks in the concrete structure and achieving a bioremediation effect. Therefore, some scholars have introduced this technology into the field of geotechnical engineering to improve the porosity of soil and rock, thus achieving a certain degree of improvement. However, due to the surface electrical properties and layered structure of expansive soil, it has a strong adsorption effect on water. Qualitative and quantitative research on whether the aforementioned microbial improvement technology can achieve good improvement results, and how the activity of microorganisms affects the surface properties of expansive soil and consequently its expansibility, is relatively scarce. Summary of the Invention

[0006] To address the shortcomings of the existing technologies, the technical problem to be solved by this invention is: how to provide a test method for the mechanism of expansive soil improvement based on microbial-induced calcium carbonate that can qualitatively and quantitatively determine how microbial-induced calcium carbonate technology affects the surface properties of expansive soil, thereby providing a quantitative component ratio reference for the microbial improvement of expansive soil to achieve the best improvement effect.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A test method for the mechanism of expansive soil improvement based on microbial-induced calcium carbonate, characterized by the following steps:

[0009] a. Take four containers and add dry expansive soil to them. Then add pure water to each container and stir to saturate the expansive soil and make sure there is a certain amount of free water in the containers so that the surface of the expansive soil adsorbs pure water to form an adsorbed water film. Obtain the first thickness data of the water film in the first container.

[0010] b. Gradually add ammonium salt to the second container and measure the concentration of ammonium ions in the free water until ammonium ions are present in the free water and the concentration of ammonium ions is stable (stable concentration means reaching saturation), and obtain the thickness of the water film at this time as the second thickness data.

[0011] c. Continue to gradually add calcium salt to the second container and measure the concentration of calcium ions in the free water until calcium ions are present in the free water and the concentration of calcium ions is stable. Obtain the thickness of the water film at this time as the third thickness data and measure the concentration of ammonium ions in the free water at this time.

[0012] d. Add urea, calcium salts in the corresponding proportion and microorganisms (specifically microorganisms that can be used for microbial-induced calcium carbonate precipitation technology) to the third container. Under the action of microorganisms, calcium carbonate is induced to be produced, and the concentration of calcium ions in the free water is measured until calcium ions are present in the free water and the concentration of calcium ions is stable. The fourth thickness data of the water film is obtained.

[0013] e. Add urea, calcium salts and microorganisms in the corresponding proportion to the fourth container. Under the action of microorganisms, calcium carbonate is induced to be produced, and the concentration of calcium ions in the free water is measured until there are no calcium ions in the free water. The fifth thickness data of the water film is obtained.

[0014] f. Based on the thickness data of the first, second, third, fourth, and fifth water films obtained in step ae, obtain the mechanism of influence of microbial-induced calcium carbonate on the surface properties of expansive soil.

[0015] Therefore, this invention enables the addition of different substances to expansive soil to form a water film, thereby detecting the effect of these substances on the thickness of the water film. Comparative analysis then reveals the mechanism by which microorganisms induce calcium carbonate to affect the surface properties of expansive soil. This invention is characterized by its ease of operation, rapid implementation, and low cost.

[0016] As an optimization, the expansive soil uses montmorillonite particles with a certain purity.

[0017] Since the main component of expansive soil is montmorillonite, directly using montmorillonite particles for testing is more targeted and representative, and can better test the mechanism and methods of expansive soil improvement. A certain purity is typically greater than 95%.

[0018] As an optimization, the four containers and the amount of substance added in step a are kept consistent. This facilitates subsequent comparative analysis.

[0019] As an optimization, the thickness data of the first, second, third, fourth, and fifth layers of the water film were all obtained using proton NMR data acquired by a nuclear magnetic resonance (NMR) device.

[0020] Specifically, because water molecules are polar molecules, and the surface of expansive soil carries a permanent negative charge that forms a certain electric field, combined with factors such as surface tension, it has a strong adsorption effect on water molecules and causes the water molecules to arrange in a certain regularity. Therefore, a layer of water molecules can be adsorbed on the surface of montmorillonite to form a water film. During the experiment, the distribution of water molecules affected by the surface electric field of expansive soil can be characterized by measuring the T2 relaxation time curve (H2 spectrum) of H atoms in the solution in a nuclear magnetic resonance (NMR) instrument. Pore water, being free or gravitationally dependent, is not affected by the surface electric field of expansive soil and exhibits a relatively uniform T2 relaxation time. Based on this conclusion, the distribution of water molecules in the adsorbed state can be calculated. Combined with quantitative data on the theoretical specific surface area of ​​expansive soil, the theoretical thickness of the water film formed by the adsorbed water molecules can be obtained. In other words, for a certain amount of expansive soil particles, based on their particle size or diameter, they have known theoretical surface area data. The total amount of water molecules in the adsorbed state can be obtained through NMR. Based on the total amount of adsorbed water film and the surface area data of the expansive soil particles, a theoretical data model of the water film thickness of a single particle can be established, thus obtaining the water film thickness data. This makes the detection of water film thickness very convenient and does not affect the experimental process.

[0021] As an optimization, all four containers are transparent. This makes it easier to perform nuclear magnetic resonance analysis.

[0022] As an optimization, in step b, heavy water is added first, and then the second thickness data of the water film is obtained by nuclear magnetic resonance.

[0023] Specifically, during the experiment, in order to prevent the hydrogen atoms in the ammonium ion from affecting the proton spectrum, heavy water was added during the nuclear magnetic resonance measurement. Since the hydrogen atoms in the ammonium ion are free hydrogens, they are easily replaced by deuterium atoms. This allows the proton spectrum data of hydrogen atoms bound to the ammonium ion to be excluded, making the proton spectrum results more accurate and more reliable.

[0024] As an optimization, the calcium salt in steps c, d, and e is calcium sulfate.

[0025] This is because neither sulfur nor oxygen atoms have spin, which reduces interference during nuclear magnetic resonance (NMR) measurements and allows for the determination of the concentration of ammonium ions in free water. Once ammonium ions are present in the free water and their concentration is stable, better data on water film thickness can be obtained.

[0026] As an optimization, the ammonium salt in step b is ammonium sulfate.

[0027] This works on the same principle as calcium sulfate, which better reduces interference during nuclear magnetic resonance measurements and improves the reliability of water film detection.

[0028] As an optimization, the microorganism used in step d is Bacillus pasteurellii.

[0029] This is because *Bacillus pasteurellii* produces a highly active urease during its metabolism. This urease significantly increases the hydrolysis rate of urea in the environment. Urea hydrolyzes into ammonia and carbon dioxide. The ammonia produced continuously raises the pH of the surrounding environment, causing carbon dioxide to exist in the form of carbonate ions. These carbonate ions combine with calcium ions in the solution to form calcium carbonate crystals, which have a cementing effect. Calcium carbonate crystals precipitate and continuously grow using expanded soil particles or the *Bacillus* itself as nuclei, thus forming a coating layer on the surface of the expanded soil particles. However, how the consumption of ions in the solution and the development of the calcium carbonate crystallization process on the surface of expanded soil particles affect the surface properties of the expanded soil is currently unknown and requires further research.

[0030] As an optimization, the determination of calcium or ammonium ion concentrations in steps b, c, d, and e includes determination by chemical methods, physical methods, or ion sensors.

[0031] Specifically, in this method, the determination of ammonium ion and calcium ion concentrations can be carried out using chemical methods, such as titration and chromatography, or physical methods, generally in combination with chemical methods, through chemical reactions followed by heating and separation. In addition, there are now many related single-parameter or multi-parameter ion sensors or detectors to determine the ion concentration in solutions or water, which are relatively mature existing technologies and will not be elaborated on here. The determination of ion concentrations in the following sections will all use the methods described above.

[0032] As an optimization, step f includes:

[0033] f1. By comparing the first, second, and third thickness data of the water film, the effects of ammonium ions (the influence of ammonium ions is necessary because urea decomposition produces ammonium ions) and calcium ions on the thickness of the water film in expansive soil are obtained. In practice, this step reveals that after adsorbing cations, the surface charge of the expansive soil water film changes, and the negative charge is neutralized to some extent, weakening the surface electric field of the expansive soil particles. This leads to a decrease in adsorption capacity, resulting in a thinner water film and inhibited expansibility. Simultaneously, by comparing the second and third thickness data, it can be seen that the inhibitory effect of monovalent cations, i.e., ammonium ions, is less than that of divalent cations, i.e., calcium ions.

[0034] f2. By comparing the second and third thickness data, the degree of ion exchange between calcium ions and ammonium ions is obtained. In practice, through the analysis of this step, it can be known that when ammonium ions and calcium ions are present in the solution, calcium ions have a stronger ion exchange capacity than ammonium ions. Therefore, calcium ions will enter the water film and exchange ammonium ions. Thus, according to step f2, the degree of ion exchange between calcium ions and ammonium ions, or whether calcium ions can completely exchange ammonium ions, can be known. Furthermore, by comparing the second and third thickness data, the influence of this ion exchange process on the thickness of the water film can be known. In further practice, two containers can be taken separately. After performing step a to form a water film, ammonium salt and calcium salt can be added separately as control groups. If the third thickness data is consistent with the data of the control group with only calcium salt added, it indicates that calcium ions can completely exchange ammonium ions. If the third thickness data is between the data of the two control groups, it indicates that ammonium ions cannot be completely exchanged. In this way, the intermediate value of the third thickness data can be used to characterize the degree of exchange between calcium ions and ammonium ions.

[0035] f3. By comparing the third and fourth thickness data, the influence of calcium carbonate adhering to the surface of expansive soil induced by microorganisms on the thickness of the water film in expansive soil was obtained. Since calcium ions were present in the water film of expansive soil in both the third and fourth thickness data, the calcium ions in the solution were not completely consumed. The addition amount of each component in the fourth thickness data was also based on the principle that the carbonate ions produced by the decomposition of urea were consumed first, thus leaving excess calcium ions in the solution. This ensures that the carbonate ions will not preferentially combine with the calcium ions in the water film to form calcium carbonate. In this way, the influence of the presence or absence of calcium ions in the water film on the water film is eliminated, and only the influence of the calcium carbonate coating layer formed on the surface of expansive soil particles on the water film is considered. Through this comparison, it can be determined how the calcium carbonate coating layer affects the surface characteristics of expansive soil particles. If the T2 relaxation time curve of hydrogen atoms in the fourth thickness data shows a similar trend to that in the third thickness data (i.e., the peak value decreases, but the slope of the curve is basically the same), it indicates that the coating layer only occupies the space of the original water film, causing the water film to become thinner, and has no significant impact on the surface electrical properties of the expansive soil, that is, on the range of influence of the electric field caused by the negative charge. If the measured water film shows a tendency to thicken, it can be analyzed that this is because calcium carbonate precipitation forms irregular calcium carbonate crystals on the surface of the expansive soil particles, leading to an increase in the relative surface area of ​​the expansive soil particles, i.e., the specific surface area. Under the action of surface tension, it also has a large adsorption effect. Therefore, this analysis can also be used to deduce whether the surface electrical properties of the expansive soil particles or the increase in surface tension caused by the increase in specific surface area plays a major role after the coating layer is formed on the surface of the expansive soil particles under the action of microorganisms, thus revealing how calcium carbonate precipitation affects the surface properties of expansive soil particles.

[0036] f4. By comparing the third, fourth, and fifth thickness data (regarding the fifth thickness data, it can be considered that a certain amount of calcium salt is added, the amount doesn't matter, but calcium ions cannot be present in the free water, thus ensuring that the calcium ions in the solution are all present in the water film; if the microbial activity can utilize the calcium ions in the water film, then the measured result will be biased towards or close to the fourth thickness data; if it cannot utilize the calcium ions in the water film, i.e., calcium carbonate cannot be produced, then the result will be the same as the third thickness data), we can obtain the utilization of calcium ions in the water film by microorganisms during the process of inducing calcium carbonate production; this analysis result can determine whether microorganisms can utilize the calcium ions in the water film during the process of inducing calcium carbonate production. Calcium ions—specifically, whether they are more readily adsorbed by expansive soil particles or more readily bound to carbonate ions—are crucial factors. Since both are ionic bonds, if calcium ions bind more strongly to expansive soil particles, the amount of calcium salt added during the actual improvement process may not be strictly controlled. However, if calcium ions bind more strongly to carbonate ions, microbial activity will strip calcium ions from the water film, thereby increasing the adsorption capacity of the expansive soil particles and having the opposite effect. Therefore, in actual improvement processes, the amount of calcium salt added must be strictly controlled to ensure that calcium ions remain in the water film. Only in this way, combined with the coating layer, can the best improvement effect be achieved, thereby maximizing the suppression of expansibility.

[0037] f5. Based on the conclusions of f1-f4, the influence of microbial-induced calcium carbonate activity on the surface properties of expansive soil is obtained. Analysis of the above experimental results reveals how the microbial-induced calcium carbonate technology affects the surface properties of expansive soil, thus theoretically facilitating the establishment of a model for improving expansive soil using microorganisms. Furthermore, based on the parameters of the experimental process, the optimal improvement strategy under this influence can be quantitatively determined, providing theoretical and data support for subsequent research on expansive soil improvement.

[0038] As an optimization, step g is also included, which involves obtaining the optimal ratio of urea, calcium salt and microorganisms for improving expansive soil based on microbial-induced calcium carbonate based on the mechanism of influence of microbial-induced calcium carbonate on the surface properties of expansive soil obtained in step f and the amount of each component added in step ae.

[0039] Compared with the prior art, this application has the following advantages:

[0040] This invention employs grouped experiments to investigate the effects of single-ion, multi-ion, and microbial coupling on the thickness of water films in expansive soil. It aims to determine how the activity of various ions and microorganisms influences the surface properties of expansive soil during the induction of calcium carbonate activity by microorganisms. Simultaneously, it examines the utilization of adsorbed ions in the water film by microorganisms and their impact on the water film. Based on the addition amount and concentration of each ion measured during the experiments, the optimal formulation for improving expansive soil is determined, providing necessary guidance and reference for the microbial improvement of expansive soil. This invention, through qualitative and quantitative experiments, elucidates the mechanism of action of microorganisms in improving expansive soil. The experimental process is simple, and data acquisition is feasible and reliable, providing strong data support for research on the improvement of expansive soil.

[0041] In summary, this invention can qualitatively and quantitatively determine how microbial-induced calcium carbonate technology affects the surface properties of expansive soil, thereby providing a quantitative component ratio reference for the microbial improvement of expansive soil to achieve the best improvement effect. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the structure of the expansive soil particles after steps d and e of the present invention. Detailed Implementation

[0043] The present invention will now be described in further detail with reference to specific embodiments.

[0044] Implementation method: An experimental method for improving expansive soil based on microbial-induced calcium carbonate, characterized by the following steps:

[0045] a. Take four containers and add dry expansive soil to them. Then add pure water to each container and stir to saturate the expansive soil and make sure there is a certain amount of free water in the containers so that the surface of the expansive soil adsorbs pure water to form an adsorbed water film. Obtain the first thickness data of the water film in the first container.

[0046] b. Gradually add ammonium salt to the second container and measure the concentration of ammonium ions in the free water until ammonium ions are present in the free water and the concentration of ammonium ions is stable (stable concentration means reaching saturation), and obtain the thickness of the water film at this time as the second thickness data.

[0047] c. Continue to gradually add calcium salt to the second container and measure the concentration of calcium ions in the free water until calcium ions are present in the free water and the concentration of calcium ions is stable. Obtain the thickness of the water film at this time as the third thickness data and measure the concentration of ammonium ions in the free water at this time.

[0048] d. Add urea, calcium salt in the corresponding proportion (referring to calcium salt in a higher proportion than in step e, with some calcium salt that has not been reacted by microorganisms and has reached saturation) and microorganisms (specifically microorganisms that can be used for microbial-induced calcium carbonate precipitation technology) to the third container. Under the action of microorganisms, calcium carbonate is induced to be produced, and the concentration of calcium ions in the free water is measured until calcium ions are present in the free water and the concentration of calcium ions is stable. The fourth thickness data of the water film is obtained.

[0049] e. Add urea, calcium salt in the corresponding proportion (meaning the proportion of calcium salt is relatively low compared to step d so that all calcium salt is used to react with microorganisms and precipitate completely or is partially located in the water film) and microorganisms to the fourth container. Under the action of microorganisms, calcium carbonate is induced to be produced, and the concentration of calcium ions in the free water is measured until there are no calcium ions in the free water, and the fifth thickness data of the water film is obtained.

[0050] f. Based on the thickness data of the first, second, third, fourth, and fifth water films obtained in step ae, obtain the mechanism of influence of microbial-induced calcium carbonate on the surface properties of expansive soil.

[0051] Therefore, this invention enables the addition of different substances to expansive soil to form a water film, thereby detecting the effect of these substances on the thickness of the water film. Comparative analysis then reveals the mechanism by which microorganisms induce calcium carbonate to affect the surface properties of expansive soil. This invention is characterized by its ease of operation, rapid implementation, and low cost.

[0052] In practice, the expansive soil is made of montmorillonite particles with a certain purity.

[0053] Since the main component of expansive soil is montmorillonite, directly using montmorillonite particles for testing is more targeted and representative, and can better test the mechanism and methods of expansive soil improvement. A certain purity is typically greater than 95%.

[0054] During implementation, the four containers and the amount of substances added in step a were kept consistent. This facilitates subsequent comparative analysis.

[0055] During implementation, the thickness data of the first, second, third, fourth, and fifth layers of the water film were obtained using hydrogen spectrum data acquired by nuclear magnetic resonance equipment.

[0056] Specifically, because water molecules are polar molecules, and the surface of expansive soil carries a permanent negative charge that forms a certain electric field, combined with factors such as surface tension, it has a strong adsorption effect on water molecules and causes the water molecules to arrange in a certain regularity. Therefore, a layer of water molecules can be adsorbed on the surface of montmorillonite to form a water film. During the experiment, the distribution of water molecules affected by the surface electric field of expansive soil can be characterized by measuring the T2 relaxation time curve (H2 spectrum) of H atoms in the solution in a nuclear magnetic resonance (NMR) instrument. Pore water, being free or gravitationally dependent, is not affected by the surface electric field of expansive soil and exhibits a relatively uniform T2 relaxation time. Based on this conclusion, the distribution of water molecules in the adsorbed state can be calculated. Combined with quantitative data on the theoretical specific surface area of ​​expansive soil, the theoretical thickness of the water film formed by the adsorbed water molecules can be obtained. In other words, for a certain amount of expansive soil particles, based on their particle size or diameter, they have known theoretical surface area data. The total amount of water molecules in the adsorbed state can be obtained through NMR. Based on the total amount of adsorbed water film and the surface area data of the expansive soil particles, a theoretical data model of the water film thickness of a single particle can be established, thus obtaining the water film thickness data. This makes the detection of water film thickness very convenient and does not affect the experimental process.

[0057] During implementation, all four containers were transparent. This made nuclear magnetic resonance analysis more convenient.

[0058] In implementation, in step b, heavy water is added first, and then the second thickness data of the water film is obtained by nuclear magnetic resonance.

[0059] Specifically, during the experiment, in order to prevent the hydrogen atoms in the ammonium ion from affecting the proton spectrum, heavy water can be added during nuclear magnetic resonance measurement. Since the hydrogen atoms in the ammonium ion are free hydrogens, they are easily replaced by deuterium atoms, thereby excluding the proton spectrum data of hydrogen atoms bound to the ammonium ion, making the proton spectrum results more accurate and more reliable.

[0060] During implementation, the calcium salt in steps c, d, and e is calcium sulfate.

[0061] This is because neither sulfur nor oxygen atoms have spin, which reduces interference during nuclear magnetic resonance (NMR) measurements and allows for the determination of the concentration of ammonium ions in free water. Once ammonium ions are present in the free water and their concentration is stable, better data on water film thickness can be obtained.

[0062] In practice, the ammonium salt in step b is ammonium sulfate.

[0063] This works on the same principle as calcium sulfate, which better reduces interference during nuclear magnetic resonance measurements and improves the reliability of water film detection.

[0064] During implementation, the microorganism used in step d is Bacillus pasteurellii.

[0065] This is because *Bacillus pasteurellii* produces a highly active urease during its metabolism. This urease significantly increases the hydrolysis rate of urea in the environment. Urea hydrolyzes into ammonia and carbon dioxide. The ammonia produced continuously raises the pH of the surrounding environment, causing carbon dioxide to exist in the form of carbonate ions. These carbonate ions combine with calcium ions in the solution to form calcium carbonate crystals, which have a cementing effect. Calcium carbonate crystals precipitate and continuously grow using expanded soil particles or the *Bacillus* itself as nuclei, thus forming a coating layer on the surface of the expanded soil particles. However, how the consumption of ions in the solution and the development of the calcium carbonate crystallization process on the surface of expanded soil particles affect the surface properties of the expanded soil is currently unknown and requires further research.

[0066] Figure 1 The diagram shows the structural composition of the expansive soil particles after steps d and e. Figure 1 In the figure, 1 represents expansive soil particles, 2 represents the coating layer formed by the precipitation of calcium carbonate crystals on the outside of the expansive soil particles, 3 represents the water film, and 4 represents the free water inside the container.

[0067] During implementation, the determination of calcium or ammonium ion concentrations in steps b, c, d, and e includes determination by chemical methods, physical methods, or ion sensors.

[0068] Specifically, in this method, the determination of ammonium ion and calcium ion concentrations can be carried out using chemical methods, such as titration and chromatography, or physical methods, generally in combination with chemical methods, through chemical reactions followed by heating and separation. In addition, there are now many related single-parameter or multi-parameter ion sensors or detectors to determine the ion concentration in solutions or water, which are relatively mature existing technologies and will not be elaborated on here. The determination of ion concentrations in the following sections will all use the methods described above.

[0069] During implementation, step f includes:

[0070] f1. By comparing the first, second, and third thickness data of the water film, the effects of ammonium ions (the influence of ammonium ions is necessary because urea decomposition produces ammonium ions) and calcium ions on the thickness of the water film in expansive soil are obtained. In practice, this step reveals that after adsorbing cations, the surface charge of the expansive soil water film changes, and the negative charge is neutralized to some extent, weakening the surface electric field of the expansive soil particles. This leads to a decrease in adsorption capacity, resulting in a thinner water film and inhibited expansibility. Simultaneously, by comparing the second and third thickness data, it can be seen that the inhibitory effect of monovalent cations, i.e., ammonium ions, is less than that of divalent cations, i.e., calcium ions.

[0071] f2. By comparing the second and third thickness data, the degree of ion exchange between calcium ions and ammonium ions is obtained. In practice, through the analysis of this step, it can be known that when ammonium ions and calcium ions are present in the solution, calcium ions have a stronger ion exchange capacity than ammonium ions. Therefore, calcium ions will enter the water film and exchange ammonium ions. Thus, according to step f2, the degree of ion exchange between calcium ions and ammonium ions, or whether calcium ions can completely exchange ammonium ions, can be known. Furthermore, by comparing the second and third thickness data, the influence of this ion exchange process on the thickness of the water film can be known. In further practice, two containers can be taken separately. After performing step a to form a water film, ammonium salt and calcium salt can be added separately as control groups. If the third thickness data is consistent with the data of the control group with only calcium salt added, it indicates that calcium ions can completely exchange ammonium ions. If the third thickness data is between the data of the two control groups, it indicates that ammonium ions cannot be completely exchanged. In this way, the intermediate value of the third thickness data can be used to characterize the degree of exchange between calcium ions and ammonium ions.

[0072] f3. By comparing the third and fourth thickness data, the influence of calcium carbonate adhering to the surface of expansive soil induced by microorganisms on the thickness of the water film in expansive soil was obtained. Since calcium ions were present in the water film of expansive soil in both the third and fourth thickness data, the calcium ions in the solution were not completely consumed. The addition amount of each component in the fourth thickness data was also based on the principle that the carbonate ions produced by the decomposition of urea were consumed first, thus leaving excess calcium ions in the solution. This ensures that the carbonate ions will not preferentially combine with the calcium ions in the water film to form calcium carbonate. In this way, the influence of the presence or absence of calcium ions in the water film on the water film is eliminated, and only the influence of the calcium carbonate coating layer formed on the surface of expansive soil particles on the water film is considered. Through this comparison, it can be determined how the calcium carbonate coating layer affects the surface characteristics of expansive soil particles. If the T2 relaxation time curve of hydrogen atoms in the fourth thickness data shows a similar trend to that in the third thickness data (i.e., the peak value decreases, but the slope of the curve is basically the same), it indicates that the coating layer only occupies the space of the original water film, causing the water film to become thinner, and has no significant impact on the surface electrical properties of the expansive soil, that is, on the range of influence of the electric field caused by the negative charge. If the measured water film shows a tendency to thicken, it can be analyzed that this is because calcium carbonate precipitation forms irregular calcium carbonate crystals on the surface of the expansive soil particles, leading to an increase in the relative surface area of ​​the expansive soil particles, i.e., the specific surface area. Under the action of surface tension, it also has a large adsorption effect. Therefore, this analysis can also be used to deduce whether the surface electrical properties of the expansive soil particles or the increase in surface tension caused by the increase in specific surface area plays a major role after the coating layer is formed on the surface of the expansive soil particles under the action of microorganisms, thus revealing how calcium carbonate precipitation affects the surface properties of expansive soil particles.

[0073] f4. By comparing the third, fourth, and fifth thickness data (regarding the fifth thickness data, it can be considered that a certain amount of calcium salt is added, the amount doesn't matter, but calcium ions cannot be present in the free water, thus ensuring that the calcium ions in the solution are all present in the water film; if the microbial activity can utilize the calcium ions in the water film, then the measured result will be biased towards or close to the fourth thickness data; if it cannot utilize the calcium ions in the water film, i.e., calcium carbonate cannot be produced, then the result will be the same as the third thickness data), we can obtain the utilization of calcium ions in the water film by microorganisms during the process of inducing calcium carbonate production; this analysis result can determine whether microorganisms can utilize the calcium ions in the water film during the process of inducing calcium carbonate production. Calcium ions—specifically, whether they are more readily adsorbed by expansive soil particles or more readily bound to carbonate ions—are crucial factors. Since both are ionic bonds, if calcium ions bind more strongly to expansive soil particles, the amount of calcium salt added during the actual improvement process may not be strictly controlled. However, if calcium ions bind more strongly to carbonate ions, microbial activity will strip calcium ions from the water film, thereby increasing the adsorption capacity of the expansive soil particles and having the opposite effect. Therefore, in actual improvement processes, the amount of calcium salt added must be strictly controlled to ensure that calcium ions remain in the water film. Only in this way, combined with the coating layer, can the best improvement effect be achieved, thereby maximizing the suppression of expansibility.

[0074] f5. Based on the conclusions of f1-f4, the influence of microbial-induced calcium carbonate activity on the surface properties of expansive soil is obtained. Analysis of the above experimental results reveals how the microbial-induced calcium carbonate technology affects the surface properties of expansive soil, thus theoretically facilitating the establishment of a model for improving expansive soil using microorganisms. Furthermore, based on the parameters of the experimental process, the optimal improvement strategy under this influence can be quantitatively determined, providing theoretical and data support for subsequent research on expansive soil improvement.

[0075] The implementation also includes step g, which, based on the mechanism of influence of microbial-induced calcium carbonate on the surface properties of expansive soil obtained in step f and the amount of each component added in step ae, obtains the optimal ratio of urea, calcium salt and microorganisms for improving expansive soil based on microbial-induced calcium carbonate.

Claims

1. A test method for the mechanism of expansive soil improvement based on microbial-induced calcium carbonate, characterized in that, Includes the following steps: a. Take four containers and add dry expansive soil to them. Then add pure water to each container and stir to saturate the expansive soil and make sure there is a certain amount of free water in the containers so that the surface of the expansive soil adsorbs pure water to form an adsorbed water film. Obtain the first thickness data of the water film in the first container. b. Gradually add ammonium salt to the second container and measure the concentration of ammonium ions in the free water until ammonium ions are present in the free water and the concentration of ammonium ions is stable. Obtain the thickness of the water film at this time as the second thickness data. c. Continue to gradually add calcium salt to the second container and measure the concentration of calcium ions in the free water until calcium ions are present in the free water and the concentration of calcium ions is stable. Obtain the thickness of the water film at this time as the third thickness data and measure the concentration of ammonium ions in the free water at this time. d. Add urea, calcium salts and microorganisms in the corresponding proportion to the third container. Under the action of microorganisms, calcium carbonate is induced to be produced. The concentration of calcium ions in the free water is measured until calcium ions are present in the free water and the concentration of calcium ions is stable. The fourth thickness data of the water film is obtained. e. Add urea, calcium salts and microorganisms in the corresponding proportion to the fourth container. Under the action of microorganisms, calcium carbonate is induced to be produced, and the concentration of calcium ions in the free water is measured until there are no calcium ions in the free water. The fifth thickness data of the water film is obtained. f. Based on the thickness data of the first, second, third, fourth, and fifth water films obtained in step ae, obtain the mechanism of influence of microbial-induced calcium carbonate on the surface properties of expansive soil; Step f includes: f1. By comparing the first, second, and third thickness data of the water film, the influence of ammonium ions and calcium ions on the thickness of the water film in expansive soil is obtained. f2. By comparing the second and third thickness data, the degree of ion exchange between calcium ions and ammonium ions is obtained; f3. By comparing the third and fourth thickness data, we can obtain the effect of calcium carbonate induced by microorganisms adhering to the surface of expansive soil on the thickness of the water film in expansive soil. f4. By comparing the third, fourth, and fifth thickness data, we can obtain information on the utilization of calcium ions in the water film by microorganisms during the process of inducing calcium carbonate production. f5. Based on the conclusions of f1-f4, obtain the influence of microbial-induced calcium carbonate activity on the surface properties of expansive soil.

2. The experimental method for improving expansive soil based on microbial-induced calcium carbonate as described in claim 1, characterized in that, The expansive soil is made of montmorillonite particles with a certain purity.

3. The experimental method for improving expansive soil based on microbial-induced calcium carbonate as described in claim 1, characterized in that, In step a, the four containers and the amount of substance added remain consistent.

4. The experimental method for improving expansive soil based on microbial-induced calcium carbonate as described in claim 1, characterized in that, The thickness data of the first, second, third, fourth, and fifth layers of the water film were all obtained using proton NMR data acquired by a nuclear magnetic resonance (NMR) instrument. All four containers are transparent.

5. The experimental method for improving expansive soil based on microbial-induced calcium carbonate as described in claim 1, characterized in that, In step b, heavy water is added first, and then the second thickness data of the water film is obtained by nuclear magnetic resonance.

6. The experimental method for improving expansive soil based on microbial-induced calcium carbonate as described in claim 1, characterized in that, The calcium salt in steps c, d, and e is calcium sulfate; The ammonium salt in step b is ammonium sulfate.

7. The experimental method for improving expansive soil based on microbial-induced calcium carbonate as described in claim 1, characterized in that, The microorganism used in step d is Bacillus pasteurellii.

8. The experimental method for improving expansive soil based on microbial-induced calcium carbonate as described in claim 1, characterized in that, The determination of calcium or ammonium ion concentrations in steps b, c, d, and e includes determination by chemical methods, physical methods, or ion sensors.

9. The experimental method for improving expansive soil based on microbial-induced calcium carbonate as described in claim 1, characterized in that, It also includes step g, which, based on the mechanism of influence of microbial-induced calcium carbonate on the surface properties of expansive soil obtained in step f and the amount of each component added in step ae, obtains the optimal ratio of urea, calcium salt and microorganisms for improving expansive soil based on microbial-induced calcium carbonate.

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