Composite expansive soil containing microorganisms-sisal fibers and preparation method of composite expansive soil

Abstract

The invention provides composite expansive soil containing microorganisms-sisal fibers. The composite expansive soil comprises the sisal fibers, expansive soil, a bacillus pasteurii solution and a cementing solution. Wherein the mass part ratio of the bacillus pasteurii liquid to the cementing liquid is 3: (2-6); the mass part of the sisal fiber is 0.4-1.0; the concentration of the cementing liquid is 1.0 to 3.0 mol / L; the cementing liquid comprises the following components in parts by mass: 1 part of urea and 1-6 parts of calcium chloride. The composite expansive soil provided by the invention is improved by adopting a means of combining a microbe induced carbonate precipitation technology and fiber reinforcement, and the advantage complementation of microbe mineralization and physical reinforcement can be realized. The invention further provides a preparation method of the composite expansive soil containing the microorganisms-sisal fibers.

Description

Technical Field

[0001] This invention belongs to the field of soil improvement technology, specifically, it relates to a composite expansive soil containing microorganisms and sisal fibers and its preparation method. Background Technology

[0002] Expansive soil is a typical special type of cohesive soil, exhibiting significant deformation characteristics of both water absorption and expansion, and water loss and shrinkage. The clay particles in this soil are mainly composed of hydrophilic clay minerals such as illite and montmorillonite. Due to the swelling and shrinking characteristics of expansive soil, repeated water absorption and loss lead to the formation of numerous cracks within the soil, disrupting its original structural integrity. This increases the permeability of the expansive soil, severely reducing its cohesion and shear strength, resulting in engineering problems such as uneven foundation settlement and slope instability.

[0003] Expansive soil is widely distributed in my country, and most of its distribution areas are in monsoon climates with distinct dry and wet seasons. The cycle of wet and dry conditions causes repeated changes in the volume of expansive soil, leading to a continuous decrease in its strength. This poses significant safety hazards to engineering projects such as highways, bridges, and buildings. Therefore, to avoid the negative impact of the swelling and shrinking characteristics of expansive soil on engineering projects, it is necessary to improve the expansive soil before construction to ensure that it meets the engineering requirements.

[0004] Currently, scholars both domestically and internationally have conducted extensive research on expansive soil improvement measures, with the main findings falling into two categories: physical improvement and chemical improvement. Common physical improvement measures include incorporating weathered sand and other materials into expansive soil to reduce its swelling and shrinkage and improve its overall performance through mechanical action. Chemical improvement measures primarily involve incorporating industrial waste, solid waste, cement, lime, fly ash, and soil stabilizers into expansive soil to improve its physical and mechanical properties. These traditional improvement measures are widely used and have achieved certain improvement effects. However, these methods still have certain drawbacks, such as environmental pollution, high energy consumption, and complex construction processes. They cannot fully meet my country's current advocacy of "dual carbon" targets and green environmental protection development strategies.

[0005] Therefore, developing a new type of expansive soil improvement technology that is economical, environmentally friendly, green, and sustainable is of great practical significance. Summary of the Invention

[0006] The purpose of this invention is to provide a composite expansive soil containing microorganisms and sisal fibers and its preparation method, in order to solve the technical problems existing in the background art.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is: to provide a composite expansive soil containing microorganisms and sisal fibers, comprising: sisal fibers, expansive soil, Bacillus pasteurellium liquid, and cementing liquid;

[0008] The mass ratio of Pasteurella multocida liquid to cementing liquid is 3:2-6;

[0009] The mass fraction of sisal fiber is 0.4-1.0;

[0010] The concentration of the cementing solution is 1.0-3.0 mol / L; the cementing solution includes the following components in parts by mass: 1 part urea and 1-6 parts calcium chloride.

[0011] Preferably, it includes sisal fiber, expanded soil, Bacillus pasteurellium solution, and cementing solution;

[0012] The mass ratio of Pasteurella multocida solution to cementing solution is 3:2.

[0013] The mass fraction of sisal fiber is 0.63;

[0014] The cementing solution has a concentration of 1.04 mol / L and comprises the following components in parts by mass: 1 part urea and 1.12 parts calcium chloride.

[0015] Preferably, it includes sisal fiber, expanded soil, Bacillus pasteurellium solution, and cementing solution;

[0016] The mass ratio of Pasteurella multocida solution to cementing solution is 3:2.

[0017] The mass fraction of sisal fiber is 0.78;

[0018] The cementing solution has a concentration of 1.04 mol / L and comprises the following components in parts by mass: 1 part urea and 1.12 parts calcium chloride.

[0019] Preferably, the components of the original expansive soil include: quartz, potassium feldspar, plagioclase, calcite, clay, illite-montmorillonite mixed layer, illite, kaolinite, and chlorite.

[0020] A method for preparing composite expansive soil containing microorganisms and sisal fibers, used to prepare composite expansive soil containing microorganisms and sisal fibers as described in any of the above-mentioned methods, characterized by comprising the following steps:

[0021] Preparation of Bacillus buski bacteria culture;

[0022] Collect expansive soil samples and pre-treat the expansive soil samples.

[0023] According to the preset ratio, sisal fiber is evenly mixed into the pretreated expansive soil. Then, Bacillus busulosa bacterial solution is evenly mixed into the pretreated expansive soil and left to simmer for 2-4 hours to allow the bacteria to be fully distributed on the surface of the soil particles.

[0024] The cementing solution is evenly mixed into the product prepared in the previous step and thoroughly stirred. The mixture is then sealed and left to stand for 24-48 hours to obtain composite expansive soil containing microorganisms and sisal fibers.

[0025] Preferably, sisal fiber is evenly mixed into the pretreated expanded soil in a preset ratio, and then Bacillus busulosa bacterial solution is evenly mixed into the pretreated expanded soil and left to simmer for 2-4 hours to allow the bacteria to be fully distributed on the surface of the soil particles.

[0026] The cementing solution is then evenly mixed into the product prepared in the previous step and thoroughly stirred. The mixture is then sealed and allowed to stand for 24-48 hours to obtain a composite expansive soil containing microorganisms and sisal fibers.

[0027] The composite expansive soil comprises: a mass ratio of Bacillus pasteurellium solution to cementing solution of 3:2; a cementing solution concentration of 1.0-3.0 mol / L; and a cementing solution comprising the following components in parts by mass: 1 part urea, 1 part calcium chloride; and / or 0.63-0.78 parts sisal fiber.

[0028] Preferably, the preparation of Bacillus buskiform bacterial suspension includes:

[0029] Use a sterile pipette to draw sterile liquid culture medium and inject it into the lyophilized culture bottle. Gently shake the bottle to completely dissolve the lyophilized culture.

[0030] The dissolved bacterial solution was aspirated into a sterile test tube and liquid culture medium was added. The test tube was sealed with sealing film. The bacterial solution test tube was then placed in a constant temperature shaker at 37°C and 180 rpm for incubation. After activation, the bacterial solution became slightly turbid and the bacteria began to grow.

[0031] Solid culture medium was isolated and purified. The activated bacterial solution was dipped into a sterile inoculation loop and streaked onto the solid culture medium. The inoculated plates were then placed in a 37°C incubator and incubated for 24 hours.

[0032] Preferably, the process of collecting and pretreating the expansive soil includes:

[0033] The natural moisture content of the original expansive soil is 27.17%; the maximum dry density is 1.63 g / cm3; the optimum moisture content is 22%; the liquid limit of the original expansive soil is 51.8%, the plastic limit is 28.5%, the plasticity index is 23.3%; and the free swelling rate of the original expansive soil is 55%.

[0034] Preferably, the process of collecting and pretreating the expansive soil includes:

[0035] The soil sample was crushed using a rubber hammer, then passed through a 2mm soil sieve and placed in an oven to dry.

[0036] The beneficial effects of the composite expansive soil containing microorganisms and sisal fibers provided by this invention are as follows: Compared with the prior art, the composite expansive soil containing microorganisms and sisal fibers of this invention is improved by combining microbial-induced carbonate precipitation technology with fiber reinforcement, which can achieve the complementary advantages of microbial mineralization and physical reinforcement. The process for preparing the composite expansive soil of this invention is green, sustainable, and has both strength and deformation control capabilities. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 A physical image of the original expansive soil in its initial state, used in the composite expansive soil containing microorganisms and sisal fibers provided in the embodiments of the present invention.

[0039] Figure 2 The images are SEM images and corresponding binarized images of unmodified expansive soil (i.e., original expansive soil) and composite expansive soil containing microorganisms and sisal fibers, respectively, at a scale of 50 μm.

[0040] Figure 3 The XRD diffraction patterns are those of unmodified expansive soil and expansive soil modified by different methods, respectively.

[0041] Figure 4 The stress-strain curves of expansive soil solidified by different improvement methods under a confining pressure of 100 kPa are shown.

[0042] Figure 5 The peak deviatoric stress curves of solidified expansive soil with different modified materials are shown under three confining pressures;

[0043] Figure 6 The failure modes of triaxial shear test specimens of expansive soil with different modified materials are shown. Detailed Implementation

[0044] To make the technical problems to be solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0045] Please refer to the following: Figures 1 to 6The present invention will now describe a composite expansive soil containing microorganisms and sisal fibers. The composite expansive soil containing microorganisms and sisal fibers comprises sisal fibers, original expansive soil, Bacillus pasteurellium solution, and a cementing solution; wherein the mass ratio of Bacillus pasteurellium solution to cementing solution is 3:2-6; the amount of sisal fiber incorporated is 0.4-1.0g per 100g of original expansive soil (i.e., the amount of sisal fiber incorporated is 0.4-1.0%, or it can be expressed as the mass fraction of sisal fiber being 0.4-1.0); the concentration of the cementing solution is 1.0-3.0mol / L; the cementing solution comprises the following components in parts by mass: 1 part urea and 1-6 parts calcium chloride.

[0046] As a specific embodiment of the present invention, a composite expansive soil containing microorganisms and sisal fibers is used for slope stability reinforcement. The soil includes sisal fibers, original expansive soil, Pasteurella multocida liquid, and cementing liquid. The sisal fiber content is 0.4-1.0%, and the mass ratio between Pasteurella multocida liquid and cementing liquid includes 1:2, 2:3, 1:1, 3:2, and 2:1.

[0047] The optimal ratio can be selected as follows: the mass ratio of Bacillus pasteurellium solution to cementing solution is 3:2. The sisal fiber incorporation amount is 0.63-0.78%. The cementing solution concentration is 1.04 mol / L; the cementing solution consists of the following components in parts by mass: 1 part urea and 1.12 parts calcium chloride.

[0048] As a specific embodiment of the present invention, the expansive soil used is expansive soil from the slope of Provincial Highway 305 in Zhuozi County, Inner Mongolia. The expansive soil comprises a layered, flaky structure, a rough, irregular particle surface, and micropores distributed between the particles. Specifically, the natural moisture content of the expansive soil is 27.17%, and the maximum dry density is 1.63 g / cm³. 3 The optimum moisture content is 22%. In the original expansive soil, fine-grained components with a particle size smaller than 0.075 mm account for over 80%. Specifically, particles larger than 2 mm account for 0.47%; particles between 0.075 and 2 mm account for 16.65%; and particles smaller than 0.075 mm account for 82.88%. The liquid limit of the original expansive soil is 51.8%, the plastic limit is 28.5%, and the plasticity index is 23.3%. The free swelling rate of the original expansive soil is 55%.

[0049] The original components of expansive soil include: quartz, potassium feldspar, plagioclase, calcite, clay, illite-montmorillonite mixed layer, illite, kaolinite, and chlorite.

[0050]

[0051]

[0052] As a specific embodiment of the present invention, the *Bacillus pasteurellii* strain used in the liquid was purchased from Beijing Baocang Biotechnology Co., Ltd., with strain number ATCC 11859. *Bacillus pasteurellii* belongs to the genus *Bacillus* of the family Coccidaceae in the order Bacillusales and is an aerobic bacterium.

[0053] This invention also provides a method for preparing composite expansive soil containing microorganisms and sisal fibers, used to prepare the composite expansive soil containing microorganisms and sisal fibers as described above. Please refer to the following: Figures 1 to 6 This includes the following steps:

[0054] Step S1: Prepare Bacillus buskiform inoculum. The strain was purchased from Beijing Baocang Biotechnology Co., Ltd., strain number ATCC 11859. Urea medium was used as the culture medium. The specific preparation process of urea medium was as follows: Weigh 5g of peptone, 3g of beef extract, and 15g of agar powder, dissolve them in 900mL of distilled water, adjust the pH to 7.0 with 1mol / L NaOH, and then sterilize in a high-temperature sterilizer at 121℃ for 15min. After sterilization, when the temperature drops to about 60℃, add 100ml of 20% urea solution (sterilized by filtering with a disposable syringe filter).

[0055] This step can be implemented by:

[0056] Step S1.1: Use a sterile pipette to draw 1 mL of sterile liquid culture medium and inject it into the lyophilized culture bottle. Gently shake the bottle to completely dissolve the lyophilized culture. The sterile liquid culture medium is specifically the 0907 medium recommended by CGMCC (China General Microbiological Culture Collection Center).

[0057] Step S1.2: Aspirate the dissolved bacterial solution into a sterile test tube and add 10 mL of liquid culture medium. Seal the test tube with sealing film, and then place the bacterial solution test tube in a constant temperature shaker at 37°C and 180 rpm for incubation. After activation, the bacterial solution will become slightly turbid and the bacteria will begin to grow. The liquid culture medium is specifically the 0907 medium recommended by CGMCC (China General Microbiological Culture Collection Center).

[0058] Step S1.3: Finally, the solid culture medium is isolated and purified. Using a sterile inoculation loop, the activated bacterial solution is dipped into the solid culture medium and streaked (using the "three-segment streak method"). The inoculated plates are then placed in a 37°C incubator for 24 hours. The final result is round colonies with regular edges and smooth surfaces. The solid culture medium is obtained by adding agar to urea medium.

[0059] Step S2: Collect the original expansive soil and pre-treat it.

[0060] The expansive soil sample was selected from the slope of Provincial Highway 305 in Zhuozi County, Inner Mongolia. It has the following characteristics: the natural moisture content of the expansive soil is 27.17%, and the maximum dry density is 1.63 g / cm³. 3 The optimum moisture content is 22%. Fine-grained components with a particle size less than 0.075 mm account for over 80% of the expansive soil. Specifically, particles larger than 2 mm account for 0.47%; particles between 0.075 and 2 mm account for 16.65%; and particles smaller than 0.075 mm account for 82.88%. The liquid limit of the original expansive soil is 51.8%, the plastic limit is 28.5%, and the plasticity index is 23.3%. The free swelling rate of the original expansive soil is 55%.

[0061] Pretreatment includes: breaking up the soil sample with a rubber hammer to avoid excessive disturbance, then passing it through a 2mm soil sieve according to the test requirements and drying it in an oven;

[0062] Step S3: Prepare a cementing solution by mixing urea and calcium chloride. The mass ratio of urea to calcium chloride is 1:1-6, specifically 1:2, 2:3, 1:1.12, 3:2, and 2:1; more specifically, the preferred mass ratio is 1:1.12. The cementing solution provides the matrix and ion source required for urea decomposition and calcium carbonate formation in Bacillus buski.

[0063] Step S4: Prepare the dried expansive soil using Bacillus buski bacteria solution and cementing solution. Specifically, first, evenly mix sisal fibers into the pretreated expansive soil, then evenly mix Bacillus buski bacteria solution into the pretreated expansive soil and let it sit for 2-4 hours (preferably 2 hours) to allow the bacteria to be fully distributed on the surface of the soil particles.

[0064] Then, the cementing solution is uniformly mixed into the product prepared in the previous step and thoroughly stirred. The mixture is then sealed and allowed to stand for 24-48 hours (preferably 24 hours) to obtain the improved product. The cementing solution concentration is 1.0-3.0 mol / L; the mass ratio between Bacillus pasteurellosis solution and cementing solution is 3:2-6; and the fiber content is 0.4-1.0, specifically, preferably 0.63% or 0.78%.

[0065] This invention provides a composite expansive soil containing microorganisms and sisal fibers, and its preparation method. Compared with existing technologies, this invention improves upon existing technologies by combining microbial-induced carbonate precipitation technology with fiber reinforcement, achieving a complementary advantage of microbial mineralization and physical reinforcement. Furthermore, the preparation process is green, sustainable, and possesses both strength and deformation control capabilities.

[0066] Example

[0067] A method for preparing improved expansive soil includes the following steps:

[0068] Step S1: Prepare Bacillus buskiform inoculum; the strain was purchased from Beijing Baocang Biotechnology Co., Ltd., strain number ATCC 11859. Urea medium was used as the culture medium.

[0069] Step S2: Collect the original expansive soil and pre-treat it.

[0070] Step S3: Prepare a cementing solution by mixing urea and calcium chloride. The cementing solution provides Bacillus buskieri with the matrix and ion source required for urea decomposition and calcium carbonate formation. The mass ratio of urea to calcium chloride is 1:1.12.

[0071] Step S4: Prepare the dried expansive soil using Bacillus buskieri bacterial solution and cementing solution at a moisture content of 22%. Specifically, first, evenly mix sisal fibers into the pretreated expansive soil, then evenly mix Bacillus buskieri bacterial solution into the pretreated expansive soil and let it sit for 2 hours to allow the bacteria to be fully distributed on the surface of the soil particles.

[0072] Then, the cementing solution is evenly mixed into the product prepared in the previous step and thoroughly stirred. The mixture is then sealed and left to stand for 24 hours to obtain the improved product. The fiber content is 0.4%-1.0%, the cementing solution concentration is 1.04 mol / L, and the mass ratio between the Bacillus pasteurellium solution and the cementing solution is 3:2.

[0073] MIP-stabilized expansive soil was reinforced with sisal and corn silk fiber (ordinary fiber), and experimental schemes were designed. The experimental schemes in the table below were prepared in accordance with the preparation methods provided above. The specific experimental schemes are shown in the table below.

[0074]

[0075] Performance testing:

[0076] The improved product was dried and subjected to conductive treatment (gold sputtering) to improve imaging quality. Then, SEM was used to scan the sample surface to obtain detailed surface morphology images. EDS analysis was performed in selected areas, collecting and analyzing characteristic X-ray signals to determine the elemental composition and content of the sample. Finally, a comprehensive analysis of the sample's microstructure and chemical composition was conducted by combining the image and energy dispersive spectroscopy data.

[0077] Unmodified expansive soil (i.e., original expansive soil) is mainly composed of flaky and flocculent soil particles, which are loose and irregularly arranged overall. There are large pores between local particles, and there is a lack of obvious cementing material. The pores are relatively well-developed, and their morphology is mostly irregular or large cracks, indicating that the soil particles are loosely bonded. During water absorption or loss, this pore structure is prone to significant volume changes, leading to macroscopic expansion or contraction cracks. After combining sisal fiber with microbial induced carbonate precipitation (MICP) technology, the fiber surface is coated with calcium carbonate, and the pores are filled with a large number of calcium carbonate crystals. The cementing material attached to the fiber can further enhance the bond between the fiber and soil particles, further reducing porosity. In particular, the gaps between the fiber and soil particles are also partially "filled and cemented" by calcium carbonate, effectively suppressing both large pores and microcracks. Combining sisal fiber with MIP technology leverages the advantages of both, not only filling pores and enhancing cementation, but also improving the bond between the fiber and the soil, forming a denser and more stable microstructure, thereby significantly enhancing the shear strength and water resistance of expansive soil.

[0078] like Figure 2 The image shows SEM images and corresponding binarized images of unmodified soil (i.e., original expansive soil) and sisal fiber-microbial modified soil (i.e., composite expansive soil containing microorganisms and sisal fibers). White areas represent pores, and black areas represent soil mass. The unmodified soil binarized image shows more white areas, indicating a larger number and wider distribution of pores and fissures, suggesting a looser structure, higher porosity, and susceptibility to water-sensitive deformation. The sisal fiber-MICP modified soil shows the fewest white areas, indicating that the synergistic effect of MICP precipitation and fibers significantly reduces pores and fissures, improving soil compactness and stability. The composite modification method used in this invention effectively improves the soil's microstructure. Quantitative analysis of the porosity of each image using PCAS software revealed that the porosity of the unmodified soil was 16.29%, while that of the sisal fiber-MICP modified soil was 12.18%. In summary, the binarization analysis quantitatively demonstrated the optimization effect of MICP and fiber modification on expansive soil structure, and verified the significant advantages of MICP-sisal fiber composite modification in enhancing soil compaction and reducing fissure connectivity.

[0079] Element content of unimproved soil

[0080]

[0081]

[0082] MICP-modified soil element content

[0083]

[0084] Sisal fiber - MICP soil amendment element content

[0085]

[0086] EDS (Energy Dispersive Spectroscopy) data reflect the effects of different soil improvement methods on the content of major elements in the soil. The results are shown in the table above. The main elements in unimproved soil are oxygen, silicon, and aluminum. The carbon content is low, only 2.5%, indicating a low organic matter content. The calcium content is also low, only 1.4%, indicating a low content of calcium carbonate minerals in the natural soil. The iron content of 5.5% suggests the possible presence of iron oxides in the soil. Compared to unimproved soil, MICP-improved soil showed a significant increase in calcium content to 5.7%, indicating that MIP-induced calcium carbonate precipitation formed in the soil, increasing the cementation degree. The carbon content increased slightly, further verifying the microbial-induced calcium carbonate precipitation. The oxygen content increased, presumably related to calcium carbonate formation. Silicon, aluminum, and iron decreased slightly, indicating that the MIP process affected the exposure of silicate and aluminum minerals to some extent. Sisal fiber-MICP-improved soil showed a further increase in calcium content to 9% compared to MIP-improved soil, indicating that sisal fiber promoted calcium carbonate deposition and improved the cementation strength of the soil. Overall, the EDS energy dispersive spectroscopy data support the conclusion that MICP and sisal fiber synergistically improve soil modification, providing direct evidence for microscopic mechanism analysis.

[0087] XRD was used to perform qualitative and quantitative analysis of the improved products. The specific experimental steps were as follows: First, the soil sample was dried, crushed, ground, and passed through a 400-mesh sieve. Then, the sample was evenly spread on the sample stage for testing, and the scanning range was set to 5-70°.

[0088] Reference Figure 3It can be seen that the unimproved and improved soils mainly contain minerals such as quartz, potassium feldspar, plagioclase, calcite, and illite. Among them, quartz has the highest peak intensity, indicating a high quartz content in the soil. Its peak is also the narrowest, indicating a high degree of crystallinity. From the comparison of the quartz peak intensity of the three curves, the overall difference is not significant, indicating that the improvement treatment has a relatively limited impact on the quartz content. Potassium feldspar and plagioclase often co-occur with quartz in soil. Feldspar peaks of all three minerals are present, and the peak shape does not show obvious disappearance or significant enhancement, indicating that the feldspar content does not change significantly before and after improvement. As for illite, a clay mineral, the peak value decreased after improvement, indicating that MICP treatment changed the relative content or crystal structure integrity of illite in the soil to some extent. As can be seen from the figure, compared with the calcite peak of unmodified expansive soil, the calcite peak of MIP-modified soil and MIP-fiber-modified soil is significantly enhanced. This change indicates that the MIP (microbial-induced calcium carbonate precipitation) technology forms new calcium carbonate deposits in the soil, which greatly increases the calcite content. The calcite content increased from 6.76% in unmodified expansive soil to 9.13% and 9.10% in modified soil.

[0089] Reference Figure 4 The stress-strain curves of expansive soil solidified by different improvement methods under a confining pressure of 100 kPa are shown. The fiber content of the two fiber-modified soils and the two fiber-reinforced MIP-modified soils was selected as 0.78% for analysis.

[0090] Depend on Figure 5It can be seen that after MIP modification, the curve shifts upward overall, with rapid stress growth in the initial stage, indicating that MIP modification improves soil strength. However, under low confining pressure, the curve begins to decline after reaching a certain strain, exhibiting obvious strain softening characteristics. This suggests that although MIP modification improves shear strength, it can lead to some brittle failure. The curve of fiber-modified soil is smoother than that of MIP-modified soil, with no obvious peak, indicating that the addition of fibers enhances the plastic deformation capacity of the soil, enabling it to maintain a high bearing capacity over a larger strain range and avoiding the brittle failure that may occur with MIP modification. However, the improvement in shear strength is limited. Sisal fiber-modified soil... The curves for soil improved with sisal fiber are steeper than those improved with corn silk fiber, indicating a more significant reinforcing effect of sisal fiber. The curves for soil improved with the combined use of MIP and fiber are steeper and tend to be more stable. The cementing effect provided by MIP and the reinforcing effect of fiber synergistically enhance the bearing capacity of the soil. The curve for sisal fiber-MICP improved soil is particularly stable, exhibiting both strong initial stiffness and good deformation compatibility under large strain. The confining pressure has a significant impact on the curves; as the confining pressure increases, the overall steepness of the curves increases, indicating improved shear resistance of the soil, especially with a more significant reinforcing effect in the improved soil, while the unimproved soil is less affected by the confining pressure. These curve characteristics indicate that the combined improvement of MIP and fiber can effectively improve the bearing capacity and deformation compatibility of expansive soil, with the sisal fiber-MICP improvement method exhibiting the best mechanical properties.

[0091] Reference Figure 5 The peak deviatoric stress curves of solidified expansive soil with different modified materials are shown below under three confining pressures. The fiber content of the two fiber-modified soils and the two fiber-reinforced MIP-modified soils is 0.78%. Figure 5It can be seen that the peak deviatoric stress of all samples increased with the increase of confining pressure, indicating that the shear strength of the soil increases with the increase of confining pressure, which is consistent with the Mohr-Coulomb strength theory. Compared with the unimproved soil, the addition of corn silk fiber and sisal fiber both increased the peak deviatoric stress of the expansive soil, indicating that fiber reinforcement enhances the shear strength of the soil. The fibers can form a three-dimensional network structure inside the soil, restricting the displacement of soil particles and improving the overall toughness of the soil. Due to the superior mechanical properties of sisal fiber, the peak deviatoric stress of sisal fiber-improved soil is significantly higher than that of corn silk fiber-improved soil, indicating that the reinforcing effect of sisal fiber is more significant. The peak deviatoric stress of MICP-improved soil is significantly higher than that of unimproved soil and fiber-improved soil, indicating that the calcium carbonate precipitation induced by MICP significantly improves the cementation strength between soil particles, thereby enhancing the shear strength of the soil. Compared with simple fiber improvement, MICP improvement shows a higher peak deviatoric stress, indicating that microbial mineralization has a more significant improvement effect on expansive soil. When MIPs and fibers work together, the peak deviatoric stress further increases, indicating a synergistic effect between the cementing reinforcement provided by MIPs and the reinforcing effect of fibers. Among these, the sisal fiber-MICP improved soil exhibits the highest peak deviatoric stress, demonstrating that this improvement method can maximize soil strength. This is because sisal fibers themselves have good reinforcing effects, while the cementing effect of MICP further enhances the bonding force between fibers and soil particles, forming a more stable soil structure. With increasing confining pressure, the peak deviatoric stress of all samples increases significantly, and the increase varies under different improvement methods. The MIP-fiber combined improved soil shows the most pronounced increase trend, indicating that under high confining pressure, this improvement method can more significantly improve the shear strength of the soil.

[0092] The table below shows the test data of cohesion and internal friction angle of solidified expansive soil with different modified materials. The fiber content of the two fiber-modified soils and the two fiber-reinforced MIP modified soils is 0.78%.

[0093]

[0094]

[0095] The table above shows that different amendments have varying degrees of influence on the cohesion and internal friction angle of solidified expansive soil, generally showing an upward trend, but the improvement effects of each amendment scheme differ. When only corn silk fiber and sisal fiber are added, both cohesion and internal friction angle are improved to some extent compared to unamended soil, but the improvement is small. Sisal fiber has a better effect on improving cohesion than corn silk fiber, but its effect on internal friction angle is worse. When MIP is treated alone (without fiber), both cohesion and internal friction angle show a significant increase. This indicates that MIP induces calcium carbonate precipitation through microorganisms, forming a cementing effect between soil particles, making the soil denser and enhancing the bonding force between particles. Furthermore, under the action of MIP, the increase in internal friction angle is less than that of cohesion, indicating that MIP mainly affects the cementing strength between particles, while having a smaller impact on the particle arrangement. Compared to the single MIP treatment, the addition of both fibers further enhances the improvement effect. The sisal-MICP scheme shows a larger increase in internal friction angle compared to the corn silk-MICP scheme, but the cohesion decreases slightly.

[0096] The test data of cohesion and internal friction angle of expansive soil under different modification materials are shown in the following table.

[0097]

[0098] Experimental data on the effect of different fiber content on the cohesion of solidified expansive soil

[0099]

[0100]

[0101] Experimental data on the effect of different fiber content on the internal friction angle of solidified expansive soil

[0102] Experimental data show that different fiber dosages have significantly different effects on the cohesion of solidified expansive soil: when reinforced alone, the cohesion of both fibers first increases and then decreases. Sisal fiber reaches a peak cohesion of 131.07 kPa at a dosage of 0.6%, which is 9.5% higher than that of corn silk fiber (119.71 kPa), and maintains high strength even at higher dosages (1%). Corn silk fiber, however, shows a decline in performance after the dosage exceeds 0.6%. When combined with MIP, the overall cohesion is significantly improved. Sisal fiber shows the strongest synergistic effect at a dosage of 0.6%, reaching 190.10 kPa, but the cohesion drops sharply after the dosage exceeds 0.78%. Corn silk-MICP, on the other hand, shows a stable trend at dosages of 0.78%-1%, exhibiting better dosage adaptability. The rigid skeleton of sisal fiber and the synergistic reinforcing effect of MIP are significantly advantageous at low dosages.

[0103] The effects of different fiber dosages on the internal friction angle of solidified expansive soil also showed significant differences: when reinforced alone, the increase in internal friction angle was relatively small. Corn silk fiber reached peak internal friction angles of 12.97° and 14.15° at dosages of 0.4% and 1%, respectively, with its flexible properties enhancing soil particle interlocking through entanglement. In contrast, the internal friction angle of sisal fiber continuously decreased with increasing dosage, from 11.01° at 0.4% to 9.69° at 1%, because excessive rigid fibers would damage the original soil structure. Under the synergistic effect of MICP, the internal friction angle was significantly improved. Corn silk fiber reached a maximum internal friction angle of 17.24° at a dosage of 0.6%, but the improvement effect continuously weakened after the dosage exceeded 0.6%. Sisal-MICP achieved the optimal internal friction angle of 17.07° at a dosage of 0.78%, with the improvement effect decreasing until the dosage reached 1%. Comparing the cohesion data, the cohesion and internal friction angle of sisal-MICP increased simultaneously at a dosage of 0.63%, reflecting the synergistic effect of fiber skeleton and cementation. However, at a dosage of 0.78%, the internal friction angle increased further but the cohesion decreased.

[0104] Reference Figure 6 The failure modes of triaxial shear test specimens of expansive soil with different modified materials are shown, where (a) is unmodified soil, (b) is MICP-modified soil, (c) is fiber-modified soil, and (d) is MICP-sisal fiber-modified soil. Figure 6As shown, the unmodified expansive soil sample exhibited obvious layered failure, with a stepped shear surface that did not form a complete shear failure surface. After failure, the sample retained a certain degree of integrity, with localized shear deformation, but overall strong plasticity. The reason for this is that due to the weak internal cohesion of the expansive soil, the sample failed along natural weak surfaces or structural planes during shearing, rather than along a single shear surface. The MICP-modified expansive soil sample showed obvious shear failure, with a relatively clear shear surface and a prominent "bulging" phenomenon. The sample experienced more pronounced brittle failure during shearing, indicating that the calcium carbonate deposition of MICP enhanced the cementation strength between soil particles, making it easier for the sample to form a continuous shear failure surface. After shearing, the sample's integrity remained good, without severe disintegration, indicating that MICP modification improved the shear strength of the expansive soil. The fiber-modified expansive soil sample showed a large distribution of fibers on its surface, with a more uniform shear failure morphology and no obvious single shear surface; the overall failure tended towards plastic deformation. This indicates that the fibers play a reinforcing role in the soil, effectively inhibiting the concentrated development of shear cracks, dispersing stress, and improving the ductility of expansive soil, enabling it to withstand greater deformation before failure. The MICP-sisal fiber-modified expansive soil samples exhibited stronger integrity and, compared to MICP-modified soil alone, demonstrated better ductility and toughness in their failure modes, without forming a distinct shear plane. This suggests that the synergistic effect of MICP and fibers enhances soil cohesion while maintaining a certain degree of flexibility, enabling the soil to possess better deformation capacity and energy dissipation capacity while increasing shear strength, thus improving the overall stability of the soil.

[0105] In general, unmodified expansive soil is prone to plastic failure. MIP modification enhances the soil's stiffness, making it more susceptible to brittle failure, while sisal fiber modification improves the soil's ductility by enhancing tensile strength and inhibiting crack development. The MIP-sisal fiber composite modification combines the advantages of both, improving shear strength while enhancing the soil's ductility and toughness, resulting in superior engineering adaptability.

[0106] The experimental data of linear shrinkage, volume shrinkage, shrinkage limit and shrinkage coefficient of MIP-sisal fiber-modified expansive soil with different fiber contents are shown in the table below.

[0107] Experimental data on the effect of different fiber content on the shrinkage of solidified expansive soil

[0108]

[0109]

[0110] Shrinkage tests on expansive soil with different fiber contents showed that the soil shrinkage initially decreased and then increased with increasing fiber content. Specifically, as the fiber content increased from 0% to 0.78%, the linear shrinkage, volumetric shrinkage, and shrinkage coefficient all gradually decreased, indicating that the fiber effectively improved the soil structure and reduced shrinkage deformation during water evaporation. However, when the fiber content increased to 1%, the linear shrinkage and volumetric shrinkage rebounded, and the shrinkage slightly increased. This is because excessive fiber affected the uniformity of the soil and the distribution of moisture. Overall, the fiber content in the range of 0.63% to 0.78% was the most effective, significantly improving the shrinkage resistance of expansive soil and exhibiting the lowest shrinkage coefficient, indicating that the fiber improvement effect was optimal within this range.

[0111] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite expansive soil containing microorganisms-sisal fibers, characterized in that, Comprise: Sisal fiber, swelling soil raw soil, bacillus pasteurii liquid and cementing liquid; Wherein, the mass fraction ratio between bacillus pasteurii liquid and cementing liquid is 3:2-6; The mass fraction of sisal fiber is 0.4-1.0; The concentration of the cementing liquid is 1.0-3.0mol / L;The cementing liquid comprises the following components by mass fraction: urea 1 part, calcium chloride 1-6 parts.

2. The microbial-sisal fiber containing expansive soil according to claim 1, wherein, Comprise: Sisal fiber, swelling soil raw soil, bacillus pasteurii liquid and cementing liquid; Wherein, the mass fraction ratio between bacillus pasteurii liquid and cementing liquid is 3:2; The mass fraction of sisal fiber is 0.63; 3. The microbial-sisal fiber containing expansive soil according to claim 1, wherein The concentration of the cementing liquid is 1.04mol / L;The cementing liquid comprises the following components by mass fraction: urea 1 part, calcium chloride 1.12 parts. Comprise: Sisal fiber, swelling soil raw soil, bacillus pasteurii liquid and cementing liquid; Wherein, the mass fraction ratio between bacillus pasteurii liquid and cementing liquid is 3:2; 4. The microbial-sisal fiber containing expansive soil according to any one of claims 1 to 3, wherein, The mass fraction of sisal fiber is 0.78; 5. A process for the preparation of a microbial-sisal fiber composite expansive soil for the preparation of a microbial-sisal fiber composite expansive soil according to any one of claims 1 to 4, characterized in that, The concentration of the cementing liquid is 1.04mol / L;The cementing liquid comprises the following components by mass fraction: urea 1 part, calcium chloride 1.12 parts. The components of the swelling soil raw soil include: quartz, potassium feldspar, plagioclase, calcite, clay, illite-smectite mixed layer, illite, kaolinite and chlorite. Comprise the following steps: Prepare bacillus pasteurii liquid; Collect swelling soil raw soil, and pretreat the swelling soil raw soil; 6. The method for preparing the composite expansive soil containing microorganisms-fibers of sisal hemp as claimed in claim 5, wherein, Mix sisal fiber into the pretreated swelling soil raw soil according to the preset ratio, then mix bacillus pasteurii liquid into the pretreated swelling soil raw soil, and stew the mixture for 2-4 hours to make the bacteria fully distributed on the surface of soil particles; Mix the cementing liquid into the product prepared in the previous step, fully mix and seal for 24-48 hours to obtain composite swelling soil containing microorganisms-sisal fiber. Mix sisal fiber into the pretreated swelling soil raw soil according to the preset ratio, then mix bacillus pasteurii liquid into the pretreated swelling soil raw soil, and stew the mixture for 2-4 hours to make the bacteria fully distributed on the surface of soil particles; 7. The method for preparing composite expansive soil containing microorganisms and sisal fibers as described in claim 5, characterized in that, Mix the cementing liquid into the product prepared in the previous step, fully mix and seal for 24-48 hours to obtain composite swelling soil containing microorganisms-sisal fiber; The mass fraction ratio between bacillus pasteurii liquid and cementing liquid is 3:2;The concentration of the cementing liquid is 1.0-3.0mol / L;The cementing liquid comprises the following components by mass fraction: urea 1 part, calcium chloride 1 part;And / or the mass fraction of sisal fiber is 0.63-0.

78. The preparation of bacillus pasteurii liquid comprises: Use a sterile pipette to suck sterile liquid medium into a freeze-dried bacterial strain bottle, and shake the bottle to completely dissolve the freeze-dried bacterial strain; Suck the dissolved bacterial liquid into a sterile test tube, add liquid medium, seal the test tube with a sealing film, then place the bacterial liquid test tube in a constant temperature shaker with a temperature of 37℃ and a rotation speed of 180rpm for culture, and after activation, the bacterial liquid appears slightly turbid and the bacterial body begins to grow; Solid medium separation and purification, using a sterile inoculation ring dipped after activation of the bacteria, on solid medium, line inoculation, the inoculated plate into 37℃ incubator for 24 hours.

8. The method for preparing the composite expansive soil containing microorganisms-fibers of sisal hemp as claimed in claim 7, wherein, The method comprises the following steps: collecting original swelling soil, pretreating the original swelling soil, and collecting the pretreated swelling soil. The natural moisture content of the expansive soil is 27.17%; the maximum dry density is 1.63 g / cm 3 ; the optimum moisture content is 22%; the liquid limit of the expansive soil is 51.8%, the plastic limit is 28.5%, and the plasticity index is 23.3%; and the free expansion rate of the expansive soil is 55%.

9. The method for preparing the composite expansive soil containing microorganisms-fibers of sisal hemp as claimed in claim 8, wherein, The method comprises the following steps: collecting original swelling soil, pretreating the original swelling soil, and collecting the pretreated swelling soil. The soil sample is knocked with a rubber hammer, then passed through a 2mm soil sieve and placed in an oven for drying.

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