A modified expansive soil, its preparation method and application

Abstract

This invention relates to a modified expansive soil, its preparation method, and its application. The preparation method of the modified expansive soil includes: mixing an EICP treatment agent, a fiber material, and expansive soil, compacting the mixture to carry out a modification reaction, thereby obtaining the modified expansive soil. By adding sisal fiber to the EICP technology, the synergistic effect of the two not only enhances the modification effect on the expansive soil but also alleviates the brittle failure problem caused by EICP technology in modifying expansive soil.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering technology, and in particular to a modified expansive soil, its preparation method, and its application. Background Technology

[0002] Expansive soil is a type of highly plastic clay primarily composed of expansive clay minerals such as montmorillonite, exhibiting swelling, shrinkage, fissure, and overconsolidation properties. Under natural conditions, expansive soil has low strength and is highly susceptible to repeated expansion and contraction due to changes in humidity and climate, leading to further strength reduction. Therefore, expansive soil is a special type of soil with extremely poor engineering properties. Its properties are highly unstable under the influence of climate change and engineering activities, which can severely damage the integrity of engineering structures, leading to engineering problems such as slope instability, structural cracking, and uneven foundation settlement, potentially causing significant economic losses.

[0003] Expansive soil is widely distributed in my country. With the rapid development of the national construction and transportation industries, the demand for land resources is increasing. Future highway and railway plans will inevitably encounter engineering problems involving expansive soil. The damage caused by expansive soil is long-term and recurring, posing a significant potential threat to construction projects and people's property safety. Therefore, improving expansive soil to address the potential dangers posed by its inherent characteristics is of paramount importance.

[0004] Currently, the main methods for improving expansive soil include physical and chemical methods. Physical methods modify expansive soil by using non-expansive materials such as fibers and weathered sand, altering its particle composition and gradation to improve its engineering properties. Physical methods are low-cost and simple to implement, but because they do not fundamentally solve the expansibility problem, their effectiveness is generally poor. Chemical methods use chemical materials to inhibit the expansibility of expansive soil, thereby improving its engineering properties. The most common chemical grouting materials include sodium silicate, acrylates, polyurethane, and resins, but these synthetic materials are mostly toxic and pose a serious pollution threat to the surrounding environment, especially groundwater. Therefore, bio-geotechnical engineering technologies, represented by MICP and EICP technologies, are increasingly being used in expansive soil improvement due to their environmental friendliness, good treatment effects, and low cost.

[0005] The principles of MICP (Microbial Induced Carbonate Precipitation) and EICP (Enzyme Induced Carbonate Precipitation) technologies are as follows: urease extracted from microorganisms and plants hydrolyzes urea to produce carbon dioxide and ammonia, respectively. Ammonia dissolves in the water within the system to form hydroxides and ammonium ions, creating an alkaline environment. Under alkaline conditions, carbon dioxide dissolves in the water to produce carbonate ions, which combine with added calcium ions to form calcium carbonate precipitate. The deposition of calcium carbonate then binds soil particles together, thereby improving the properties of the soil and rock materials.

[0006] Currently, MIP technology is most widely used in geotechnical engineering, but it is often ineffective in treating expansive soils. This is because: firstly, microorganisms typically have a diameter greater than 3μm, making it impossible for them to survive in the pores of expansive soil; secondly, the calcium carbonate crystals produced by MIP technology are usually large, which can clog the pores of the expansive soil during treatment, preventing the treatment solution from seeping in and resulting in uneven solidification; thirdly, the cultivation and storage of urease-producing bacteria are costly; and finally, MIP treatment leaves microorganisms in the soil, requiring regular checks to ensure that the microorganisms do not harm the environment surrounding the remediation site.

[0007] The EICP technology using free urease overcomes the limitations of MICP technology in expansive soil remediation. First, urease, with a size of approximately 12 nm, can exist in very small pores, and the calcium carbonate crystals produced by EICP are smaller, significantly improving the uniformity of the treated soil. Second, urease can degrade naturally, eliminating long-term impacts on the ecosystem. Finally, EICP technology avoids the need for prior storage and cultivation of microorganisms, greatly reducing costs and workload. Therefore, EICP technology is more suitable for the remediation of expansive soils.

[0008] However, current research on expansive soil improvement methods based on EICP technology is limited and yields poor results. This is because the key to EICP technology lies in adding a treatment solution composed of crude urease and a cementing agent (urea and calcium source) to the soil. The calcium carbonate generated through the reaction binds soil particles together, effectively improving the properties of the expansive soil. Typically, a circulating grouting method is used to introduce a large amount of the treatment solution into the expansive soil, generating a large amount of calcium carbonate to improve its properties. However, this method not only uses a large amount of EICP treatment solution, resulting in high costs, but more importantly, due to the poor permeability of expansive soil, the treatment solution has difficulty migrating within the pore throats. This prevents the formation of calcium carbonate-cemented soil in the fine pores and deep soil layers through repeated grouting, leading to extremely poor uniformity of the solidified soil.

[0009] Therefore, most current expansive soil improvement technologies based on EICP technology employ mixed compaction to improve the uniformity of the solidified soil. Mixed compaction involves mixing an EICP solution (a mixture of plant-derived urease, calcium source, and urea) with the soil sample and then compacting it. However, when treating expansive soil using mixed compaction, the EICP solution immediately flocculates after mixing, while the compaction process requires time, preventing calcium carbonate from forming a good bond between soil particles, resulting in poor solidification. Furthermore, the solidified soil treated with EICP technology fails under extremely low strain, reaching peak strength before the stress decreases sharply, leaving almost no residual strength; its failure mode exhibits extreme brittleness. Especially for expansive soil, the brittleness index increases by 100%–200% after EICP treatment. With a deeper understanding of the unique engineering characteristics of expansive soil, flexible prevention and control concepts such as "softening expansity" are gradually becoming the main trend, which clearly limits the application of EICP technology in expansive soil improvement. Summary of the Invention

[0010] To address at least one of the aforementioned technical problems, this invention provides a modified expansive soil, its preparation method, and its application. By combining EICP technology with sisal fiber reinforcement technology, the modification effect on expansive soil is enhanced, and the brittle failure problem caused by EICP technology in modifying expansive soil is alleviated.

[0011] In one aspect, the present invention provides a method for preparing modified expansive soil, the method comprising: mixing EICP treatment agent, fiber material and expansive soil, compacting and carrying out modification reaction to obtain modified expansive soil.

[0012] In some embodiments of the present invention, the method for preparing the EICP treatment agent includes: mixing an equal volume of cementing solution and a urease solution to prepare the EICP treatment agent.

[0013] In some embodiments of the present invention, the cementing solution contains calcium chloride solution and urea solution, preferably with a molar concentration ratio of calcium chloride solution to urea solution of 1:1 to 1:1.5, more preferably 1:1.5; preferably, the concentration of calcium chloride solution is 0.5 to 2.5 mol / L, more preferably 1 to 1.5 mol / L.

[0014] In some embodiments of the present invention, the extract used in the urease solution is selected from one or more of soybeans, sword beans, chickpeas, watermelon seeds, Chenopodium album plants or mulberry leaves; and / or, the urease activity of the urease solution is greater than 10 mM / min.

[0015] In some embodiments of the present invention, the amount of EICP treatment agent added is calculated using the following formula, which includes:

[0016]

[0017] In formula (1), M is the mass of expansive soil in g; y is the moisture content of expansive soil in %; m is the mass of EICP treatment agent in mixed expansive soil in g; m1 is the mass of solute in cementing solution in g; m2 is the mass of solute in urease solution in g; m3 is the mass of cementing solution in g; m4 is the mass of urease solution in g; and the optimum moisture content of expansive soil -2% ≤ y ≤ optimum moisture content of expansive soil +2%.

[0018] In some embodiments of the present invention, the mass of the fiber material is 0.3% to 0.5% of the mass of the expansive soil; and / or, the fiber material is selected from one or more of sisal fiber, polypropylene fiber, polyester fiber, glass fiber, and basalt fiber; and / or, the length of the fiber material is 5 to 10 mm.

[0019] In some embodiments of the present invention, the compaction state is determined as follows: the optimal dry density of expansive soil - 0.1 ≤ the dry density of mixed expansive soil ≤ the optimal dry density of expansive soil + 0.1.

[0020] In some embodiments of the present invention, the temperature of the modification reaction is 15–25°C; and / or the modification reaction time is 3–7 days.

[0021] In one aspect, the present invention provides a modified expansive soil obtained by the above-described preparation method.

[0022] In another aspect, the present invention provides a modified expansive soil obtained by the above preparation method or the application of the above modified expansive soil in the field of construction engineering.

[0023] The technical solution provided by the embodiments of the present invention has the following advantages:

[0024] The low-pH-EICP technology can improve the improvement effect and engineering feasibility of expansive soil. Before mixing the crude urease solution with the cementing solution, the crude urease solution is adjusted to a low pH (weakly acidic). Under low pH conditions, urease catalyzes the hydrolysis of urea, initially neutralizing the pH. The EICP cementing process only begins when the pH rises to the alkaline environment required for calcium carbonate deposition. Compared to traditional EICP technology, the low-pH-EICP technology delays calcium carbonate formation, meaning the formation of calcium carbonate clusters takes longer and occurs mostly after the solution is mixed with soil particles and compacted. The resulting calcium carbonate clusters are concentrated at the contact points with soil particles, providing a significantly larger contact area. This calcium carbonate effectively binds the soil particles together, resulting in a better improvement effect on expansive soil.

[0025] By combining fiber reinforcement technology with EICP technology, the improvement effect of expansive soil can be enhanced, and the brittle failure problem caused by EICP technology in improving expansive soil can be alleviated. When an appropriate amount of fiber material is added, the randomly distributed fibers will intertwine in the soil, forming interlacing points. These interlacing points combine to form a fiber stress network. When the soil is subjected to external forces, the interlacing points will move along with the nearby soil particles, and the fibers connected to them will generate tensile stress, hindering their movement. This will link the entire fiber stress network, greatly increasing the integrity of the soil and thus alleviating its brittleness. In addition, the fiber material can provide nucleation sites for calcium carbonate in EICP technology, promoting the formation of calcium carbonate on the fiber surface and within the soil pores. The calcium carbonate generated on the fiber surface increases the interfacial friction between the fiber and soil particles, and also enhances the network constraint of the fiber on the soil particles. The addition of the fiber and the generated calcium carbonate can further reduce the pore volume and improve the bonding efficiency between calcium carbonate and soil. The synergistic effect of fiber reinforcement technology and EICP technology on the improvement of expansive soil can greatly improve the strength of expansive soil and reduce its expansibility. Attached Figure Description

[0026] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 Comparison of stress-strain curves of unconfined compressive strength for various expansive soil samples;

[0029] Figure 2 A comparison diagram of the unconfined compressive strength of various expansive soil samples;

[0030] Figure 3 A comparison chart of the brittleness indices of various expansive soil samples;

[0031] Figure 4 A comparison diagram of the cohesion of various expansive soil samples;

[0032] Figure 5 A comparison diagram of the internal friction angles of various expansive soil samples;

[0033] Figure 6 A comparison chart of the unloaded swelling rates of various expansive soil samples. Detailed Implementation

[0034] To better understand the above-mentioned objectives, features, and advantages of the present invention, the solutions of the present invention will be further described below. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0035] Many specific details are set forth in the following description in order to provide a full understanding of the invention, but the invention may also be practiced in other ways different from those described herein; obviously, the embodiments in the specification are only some embodiments of the invention, and not all embodiments.

[0036] According to an embodiment of the present invention, a method for preparing modified expansive soil is provided. The method includes: mixing EICP treatment agent, fiber material and expansive soil, compacting and carrying out a modification reaction to obtain modified expansive soil.

[0037] The modified expansive soil preparation method provided in this invention employs low-pH-EICP technology to delay calcium carbonate formation, effectively increasing engineering operability and enhancing the improvement effect of EICP technology based on mixed compaction on expansive soil. By adding sisal fiber to the EICP technology, the synergistic effect of sisal fiber reinforcement and EICP technology not only enhances the improvement effect on expansive soil but also alleviates the brittle failure problem caused by EICP technology in improving expansive soil.

[0038] In some embodiments of the present invention, the method for preparing the EICP treatment agent includes: mixing an equal volume of cementing solution and a urease solution to prepare the EICP treatment agent.

[0039] In some embodiments of the present invention, the materials used in a method for preparing modified expansive soil include deionized water, soybean flour, urea, calcium chloride, dilute hydrochloric acid, and sisal fiber.

[0040] In some embodiments of the present invention, a method for preparing modified expansive soil includes the following steps:

[0041] 1. Extraction of crude urease solution

[0042] (1) Place the soybean powder in a 60℃ electric heating drying oven and dry for 24 hours;

[0043] (2) Prepare a soybean powder solution with dried soybean powder and deionized water at a concentration of 50-120 g / L (the urease activity of the crude urease solution extracted (measured by conductivity method) must be greater than 10 mM / min). Place the solution in a magnetic stirrer and stir for 2 hours to make it evenly mixed.

[0044] (3) After the mixture has stood for 1 hour, it is placed in a high-speed centrifuge and centrifuged at 4000 r / min for 15 min. The mixture is then filtered through a 1000-mesh filter to remove the soybean residue. The resulting supernatant is the extracted crude urease solution. The prepared crude urease solution is refrigerated at 4℃ for later use.

[0045] 2. Preparation of the bonding solution

[0046] (1) Prepare a cementing solution with a calcium chloride concentration of 1 to 1.5 mol / L and a calcium chloride to urea ratio (molar concentration ratio) of 1:1.5.

[0047] 3. Improvement of expansive soil

[0048] (1) Air dry the soil sample naturally, crush it, and pass it through a 2mm standard sieve;

[0049] (2) Use dilute hydrochloric acid to adjust the pH of the extracted crude urease solution to 5-5.5, and then mix the cementing solution and the crude urease solution in equal volumes to prepare a low pH-EICP treatment solution.

[0050] (3) Weigh sisal fiber at 0.3% to 0.5% of the dry mass of the expansive soil to be treated, with a fiber length of 5 to 10 mm; weigh low pH-EICP treatment solution at ±2% of the optimum moisture content of the expansive soil to be treated. Quickly add the weighed EICP solution and sisal fiber to the soil sample and mix thoroughly for 10 to 15 minutes using an electric mixer to ensure uniform mixing.

[0051] (4) Compact the mixed soil sample to the optimum dry density of the expansive soil to be treated ±0.1, seal it with plastic film, and cure it for 3 to 7 days at 15 to 25°C.

[0052] In some embodiments of the present invention, the cementing solution contains calcium chloride solution and urea solution. Preferably, the molar ratio of calcium chloride solution to urea solution is 1:1 to 1:1.5, for example, selected from any one of the molar ratios of 1:1.1, 1:1.2, 1:1.3, 1:1.35, 1:1.4, and 1:1.45, and more preferably 1:1.5. Preferably, the concentration of calcium chloride solution is 0.5 to 2.5 mol / L, for example, selected from any one of the concentration values ​​of 0.68 mol / L, 0.76 mol / L, 0.93 mol / L, 1.69 mol / L, 1.98 mol / L, 2.16 mol / L, and 2.36 mol / L; more preferably, it is 1 to 1.5 mol / L, for example, the concentration of calcium chloride solution is selected from any one of the concentration values ​​of 1.18 mol / L, 1.26 mol / L, 1.33 mol / L, 1.41 mol / L, and 1.49 mol / L.

[0053] In some embodiments of the present invention, the extract used in the urease solution is selected from one or more of soybeans, sword beans, chickpeas, watermelon seeds, Chenopodium album plants, or mulberry leaves; and / or, the urease activity of the urease solution is greater than 10 mM / min. For example, the urease activity of the urease solution includes any one selected from 12 mM / min, 15 mM / min, 18 mM / min, 20 mM / min, 25 mM / min, and 30 mM / min.

[0054] In some embodiments of the present invention, soybeans have extremely high urease content and are inexpensive and readily available; therefore, soybeans are the preferred extract for this invention. In specific engineering projects, alternative extracts such as sword beans, chickpeas, watermelon seeds, Chenopodium album, and mulberry leaves can be selected based on the material costs at the project location, but it is necessary to ensure that the urease activity (measured by conductivity method) of the extracted crude urease solution is greater than 10 mM / min.

[0055] In some embodiments of the present invention, urease activity is measured using the conductivity method.

[0056] In some embodiments of the present invention, the steps for measuring urease activity by conductivity method are as follows: (1) Prepare a 1.11 mol / L urea solution, add 27 mL of urea solution to a beaker, and add 3 mL of crude urease solution to the beaker using a pipette to ensure that the urea concentration in the mixed solution is 1 mol / L; (2) Under 25°C conditions, use a conductivity meter to measure the change in conductivity of the mixed solution within 15 min (record the conductivity value every 5 min), calculate the average value of the change in conductivity per minute, and take the average value after three measurements; (3) Characterize urease activity by the amount of urea hydrolyzed by urease per minute. The amount of urea hydrolyzed is proportional to the rate of change in solution conductivity. 1 mS / min conductivity change is converted to 11.1 mM / min of urea hydrolysis. The formula for calculating urease activity is as shown in formula (2):

[0057] U=11.11Δσn (2)

[0058] In the formula, U (mM / min) is the urease activity characterized by the amount of urea hydrolyzed per minute; Δσ (mS / min) is the change in conductivity per minute; and n is the dilution factor, which is 10.

[0059] In some embodiments of the present invention, the pH value of the urease solution is between 5 and 5.5. For example, the pH value is selected from any one of the values ​​including 5.06, 5.16, 5.23, 5.31, 5.39, 5.46, and 5.49.

[0060] In some embodiments of the present invention, the amount of EICP treatment agent added is calculated using the following formula, which includes:

[0061]

[0062] In formula (1), M is the mass of expansive soil in g; y is the moisture content of expansive soil in %; m is the mass of EICP treatment agent in mixed expansive soil in g; m1 is the mass of solute in cementing solution in g; m2 is the mass of solute in urease solution in g; m3 is the mass of cementing solution in g; m4 is the mass of urease solution in g; and the optimum moisture content of expansive soil -2% ≤ y ≤ optimum moisture content of expansive soil +2%.

[0063] In some embodiments of the present invention, the mass of the fiber material is 0.3% to 0.5% of the mass of the expansive soil, for example, selected from any value including 0.31%, 0.34%, 0.36%, 0.39%, 0.41%, 0.43%, 0.46%, and 0.49%; and / or, the fiber material is selected from one or more of sisal fiber, polypropylene fiber, polyester fiber, glass fiber, and basalt fiber; and / or, the length of the fiber material is 5 to 10 mm, for example, the length of the fiber material is selected from any length value of 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, and 10 mm.

[0064] In some embodiments of the present invention, sisal fiber, a fiber material extracted from sisal plants, has advantages such as being eco-friendly, easy to process, and inexpensive. Therefore, sisal fiber is the preferred fiber material in this invention. However, its strength is relatively low compared to synthetic polymer fibers and other fiber materials. In specific engineering projects where higher strength is required for the improved expansive soil, polypropylene fiber, polyester fiber, glass fiber, and basalt fiber can be selected as alternative materials. However, it is necessary to ensure that the selected fiber material has a certain degree of acid and alkali resistance and good dispersibility in the soil.

[0065] In some embodiments of the present invention, the compaction state is determined as follows: the optimal dry density of expansive soil - 0.1 ≤ the dry density of mixed expansive soil ≤ the optimal dry density of expansive soil + 0.1.

[0066] In some embodiments of the present invention, the temperature of the modification reaction is 15–25°C; and / or, the modification reaction time is 3–7 days. For example, the temperature is selected from any one of the following temperature values: 16.8°C, 18.3°C, 19.8°C, 21.4°C, 23.6°C, and 24.9°C, and the time is selected from any one of the following time periods: 3.5 days, 4 days, 4.5 days, 5 days, 5.5 days, 6 days, and 6.5 days.

[0067] In some embodiments of the present invention, the modification reaction is carried out by a curing reaction. The curing reaction temperature is 15–25°C, and the curing reaction time is 3–7 days. For example, the curing reaction temperature is selected from any one of the following temperature values: 16.8°C, 18.3°C, 19.8°C, 21.4°C, 23.6°C, and 24.9°C, and the curing reaction time is selected from any one of the following times: 3.5 days, 4 days, 4.5 days, 5 days, 5.5 days, 6 days, and 6.5 days.

[0068] According to an embodiment of the present invention, a modified expansive soil obtained by the above preparation method is provided.

[0069] According to embodiments of the present invention, a modified expansive soil obtained by the above preparation method or the application of the above modified expansive soil in the field of construction engineering is also provided.

[0070] The present invention will be further explained and illustrated below through specific embodiments.

[0071] Example 1

[0072] The expansive soil used was taken from Baise City, Guangxi Zhuang Autonomous Region, and was yellowish-white in color. According to the "Standard for Geotechnical Testing Methods" (GB / T50123-2019), indoor geotechnical tests showed that the liquid limit of the test soil was 47.6%, the plastic limit was 21.4%, the plasticity index was 26.2, and the free swelling rate was 62%, classifying it as moderately expansive soil. The specific experimental procedures are as follows:

[0073] 1. Extraction with crude urease solution:

[0074] (1) Place commercially available soybean powder in a 60℃ electric heating drying oven and dry for 24 hours;

[0075] (2) Prepare a soybean flour solution with a concentration of 100 g / L using dried soybean flour and deionized water. Place the solution in a magnetic stirrer and stir for 2 hours to make it evenly mixed.

[0076] (3) After the mixture has been left to stand for 1 hour, it is placed in a high-speed centrifuge and centrifuged at 4000 r / min for 15 min. The mixture is then filtered through a 1000-mesh filter to remove the soybean residue. The resulting supernatant is the extracted crude urease solution. The prepared crude urease solution is refrigerated at 4℃ for later use.

[0077] The activity of the crude urease solution extracted from 100 g / L soybean flour solution was measured to be 12.13 mM / min.

[0078] The steps for measuring urease activity using the conductivity method are as follows:

[0079] (1) Prepare a 1.11 mol / L urea solution. Add 27 mL of urea solution to a beaker and use a pipette to add 3 mL of crude urease solution to the beaker to ensure that the urea concentration in the mixed solution is 1 mol / L.

[0080] (2) At 25℃, the conductivity change of the mixed solution was measured in 15 min using a conductivity meter (the conductivity value was recorded once every 5 min), and the average value of the conductivity change per minute was calculated. The average value was taken after three measurements.

[0081] (3) Urease activity is characterized by the amount of urea hydrolyzed by urease per minute. The amount of urea hydrolyzed is directly proportional to the rate of change in solution conductivity. 1 mS / min conductivity change is converted to 11.1 mM / min of urea hydrolysis. The formula for calculating urease activity is shown in equation (2):

[0082] U=11.11Δσn (2)

[0083] In the formula, U (mM / min) is the urease activity characterized by the amount of urea hydrolyzed per minute; Δσ (mS / min) is the change in conductivity per minute; and n is the dilution factor, which is 10.

[0084] 2. Preparation of the bonding solution

[0085] Prepare a cementing solution with a calcium chloride concentration of 1 mol / L, a urea concentration of 1.5 mol / L, and a calcium chloride to urea ratio (molar concentration ratio) of 1:1.5.

[0086] 3. Improvement of expansive soil

[0087] (1) Air dry the soil sample naturally, crush it, and pass it through a 2mm standard sieve;

[0088] (2) Use dilute hydrochloric acid to adjust the pH of the extracted crude urease solution to 5, and then mix the cementing solution and the crude urease solution in equal volumes to prepare a low pH-EICP treatment solution.

[0089] (3) Weigh sisal fiber at 0.3% of the dry mass of the expansive soil to be treated. The length of the sisal fiber is 10 mm. The optimum moisture content of the expansive soil to be treated is determined to be 19%. Weigh low pH-EICP treatment solution at 20% moisture content of the expansive soil to be treated. Quickly add the weighed EICP solution and sisal fiber to the soil sample and mix thoroughly for 10 minutes using an electric mixer to ensure uniform mixing.

[0090] (4) Compact the mixed soil sample to the optimum dry density of the expansive soil to be treated, 1.5 g / cm³. 3 Seal with plastic film and cure at 20°C for 3 days.

[0091] Meanwhile, following the above experimental steps, reshaped samples were prepared using deionized water, EICP modified samples without fiber were prepared using unadjusted pH EICP treatment solution, and low-pH-EICP modified samples without fiber were prepared using low-pH-EICP treatment solution. Using the above three samples as references, the improvement effect of the present invention on expansive soil was tested.

[0092] The analysis of the experimental results is as follows:

[0093] (1) Unconfined compressive strength test of expansive soil specimens

[0094] First, unconfined compressive strength tests were conducted on four types of expansive soil samples. Their unconfined compressive strength stress-strain curves are shown below. Figure 1 As shown.

[0095] from Figure 1 As can be seen, the stress-strain curves of all samples are softening curves; with the increase of axial strain, the axial stress first increases and then decreases. However, compared to the remolded samples, the EICP-modified samples and the low-pH-EICP-modified samples show a faster rate of stress increase. After reaching the peak strength, the stress of these two modified samples suddenly and rapidly drops, and the stress-strain curves become sharp, indicating that these two modified samples exhibit brittle failure. In contrast, the modified sample of this invention with added fiber shows a slower rate of stress decrease after the peak, a flatter curve, and a larger residual strength, indicating that the brittleness problem of the modified sample of this invention with added sisal fiber is significantly improved.

[0096] Unconfined compressive strength of each expansive soil sample as follows Figure 2 As shown.

[0097] from Figure 2 As can be seen, the unconfined compressive strength of the remolded sample is 213.25 kPa, and the unconfined compressive strength of the low pH-EICP modified sample is significantly greater than that of the EICP modified sample. The unconfined compressive strength of the low pH-EICP modified sample after curing for 3 days is 330.15 kPa, which is 54.8% higher than that of the remolded sample. The unconfined compressive strength of the EICP modified sample after curing for 3 days is 302.68 kPa, which is 41.9% higher than that of the remolded sample.

[0098] Both low-pH-EICP technology and EICP technology have a certain effect on improving the strength of expansive soil. The principle is...Both technologies can deposit calcium carbonate crystals to cement soil particles, and the calcium ions in the cementing solution can undergo high-valence cation exchange with minerals such as montmorillonite in expansive soil, resulting in a greater improvement in soil strength. However, because the EICP technology produces calcium carbonate in a shorter time, it often deposits before compaction, leading to a poorer cementing effect on expansive soil particles. In contrast, the low-pH-EICP technology has a "window period" after mixing, allowing the generated calcium carbonate to fully cement the soil after mixing and compaction. Therefore, the low-pH-EICP technology has a significantly higher solidification efficiency for expansive soil than the EICP technology, and its unconfined compressive strength is also greater than that of the EICP-modified sample.

[0099] The fiber-added improved technology of this invention has a more significant effect on improving the strength of expansive soil. The unconfined compressive strength after 3 days of curing is 362.45 kPa, an increase of 70.0% compared to the remolded sample. The specific reasons are explained as follows: because the fibers form a three-dimensional network structure in the soil, they simultaneously exert one-dimensional and three-dimensional reinforcement effects, as well as the synergistic effect of the fibers and EICP technology, effectively restricting the relative displacement of soil particles, increasing the integrity of the sample, and improving its unconfined compressive strength. After sample failure, the fiber network can also effectively constrain the overall deformation of the sample, bear a certain amount of tensile stress, and delay the failure process. This allows the sample to still bear a certain load after failure, exhibiting significant residual strength.

[0100] (2) Brittleness index test of expansive soil samples

[0101] The brittleness index is used to measure the brittleness of a material; the higher the brittleness index, the more brittle the material. The calculation formula is:

[0102]

[0103] In equation (3), q u q represents the peak intensity. c The residual strength is taken as the strength corresponding to a 5% increase in strain from the peak strain. The residual strength of the reshaped specimen is 163.81 kPa.

[0104] The brittleness index of each sample was calculated according to equation (3), such as Figure 3 As shown.

[0105] from Figure 3The results show that the brittleness index of the remolded sample is 0.232, while the brittleness indices of the EICP-modified sample and the low-pH-EICP sample are much higher than those of the remolded sample, at 0.830 and 0.812 respectively, representing increases of 257% and 250% compared to the remolded sample. This indicates that while these two modification techniques improve the strength of expansive soil, they also significantly increase its brittleness, which is obviously detrimental to the stability of expansive soil engineering. In contrast, the fiber-added modification technique of this invention not only has a good improvement effect on the strength of expansive soil, but the brittleness index of the modified solidified body is also not much different from that of the remolded soil, at 0.260, indicating a significant improvement in the brittleness of the solidified body.

[0106] The reason for this lies in the fact that EICP and low-pH-EICP technologies primarily improve the mechanical properties of expansive soil by depositing calcium carbonate crystals between soil particles. However, this cementation is calcareous, which significantly increases the brittleness of the soil, clearly detrimental to the improvement of expansive soil. Therefore, specific technical methods are needed to mitigate the brittleness of the solidified body. In contrast, the fiber-enriched expansive soil of this invention, when subjected to external loads, forms a network of constraints on the soil particles, hindering their relative displacement and maintaining the integrity of the sample by bearing and transferring external loads. After soil failure, due to the presence of the three-dimensional fiber network structure, fibers in other locations can still provide reinforcement, resulting in a slow decrease in stress after the sample reaches peak strength, greatly enhancing the sample's ductility. The soil retains good integrity after failure, giving it a certain residual strength.

[0107] (3) Direct shear test of expansive soil samples

[0108] Test method for direct shear test:

[0109] Subsequently, direct shear tests were conducted on each specimen. The vertical pressure was set to 50 kPa, 100 kPa, 150 kPa and 200 kPa, and the shear rate was 0.8 mm / min. The test method was carried out in accordance with the relevant provisions of the "Standard for Geotechnical Testing Methods" (GB / T50123-2019).

[0110] Analysis of direct shear test results:

[0111] Shear strength parameters of each specimen: internal friction angle And cohesion c, such as Figure 4 and Figure 5 As shown.

[0112] from Figure 4 and Figure 5As can be seen, the internal friction angle of the remolded sample was 11.2°, and the cohesion was 56.0 kPa. The internal friction angle and cohesion of the EICP-modified and low-pH-EICP-modified samples were both greater than those of the remolded sample. Specifically, the internal friction angles of the EICP and low-pH-EICP-modified samples were 13.3° and 13.8°, respectively, representing increases of 18.8% and 22.9% compared to the remolded sample; the cohesion was 72.4 kPa and 89.6 kPa, respectively, representing increases of 29.3% and 59.9% compared to the remolded sample. Both EICP and low-pH-EICP technologies can improve the shear strength of expansive soil, but the improvement effect of low-pH-EICP technology is significantly better than that of EICP technology. The fiber-added modification technology of this invention has an even better improvement effect on expansive soil. The internal friction angle and cohesion of the modified sample were 17.0° and 113.6 kPa, respectively, representing increases of 51.4% and 102.9% compared to the remolded sample.

[0113] (4) Expansive soil sample swelling rate test without load

[0114] Finally, no-load swelling rate tests were conducted on each sample to investigate the improvement effects of different modification methods on the expansibility of expansive soil. The test results are as follows: Figure 6 As shown.

[0115] from Figure 6 As can be seen, the no-load swelling rate of the remolded sample was 19.5%. The no-load swelling rates of the EICP-modified sample and the low-pH-EICP-modified sample were both lower than those of the remolded sample, at 13.3% and 12.7%, respectively, representing reductions of 31.8% and 34.9% compared to the remolded sample. This indicates that both EICP and low-pH-EICP technologies can reduce the expansibility of expansive soils, but the improvement effect of the low-pH-EICP technology is significantly better than that of the EICP technology. The fiber-infused sample of this invention...

[0116] The improved technology has a better effect on improving the expansibility of expansive soil. The unloaded expansion rate of the improved sample is 3.7%, which is 81.0% lower than that of the remolded sample.

[0117] Expansive soil is a highly plastic clay rich in expansive clay minerals such as montmorillonite. The swelling of hydrophilic minerals upon water absorption leads to an increase in the volume of expansive soil samples, manifested as an increase in the no-load swelling rate. EICP and low-pH-EICP techniques can improve the expansibility of expansive soil. The improvement mechanism mainly involves two aspects: First, calcium ions in the cementing solution can undergo high-valence cation exchange with minerals such as montmorillonite in the expansive soil, inhibiting its expansibility by reducing the thickness of the soil particle diffusion layer. Second, this technique can deposit calcium carbonate between soil particles. This cementing effect constrains the soil's expansion deformation, and the generated calcium carbonate can also encapsulate soil particles, reducing their contact with water. Calcium carbonate itself is not sensitive to water, thus reducing the expansibility of the expansive soil. The second mechanism plays a major role.

[0118] The modified expansive soil preparation method provided in this invention further reduces the expansibility of expansive soil by adding sisal fibers based on low-pH-EICP technology. The fibers are evenly distributed and interwoven in the soil to form a three-dimensional spatial network structure. Through three-dimensional reinforcement, one-dimensional reinforcement, and synergistic effects with low-pH-EICP technology, the fibers jointly constrain soil deformation, resulting in a significant reduction in the expansibility of the sample, with its no-load swelling rate reaching a minimum. This solves the problem that "when treating expansive soil using the mixed compaction method, flocculation occurs immediately after mixing with the EICP solution, while the mixed compaction process requires a certain amount of time, leading to poor solidification due to the inability of calcium carbonate to form good cementation between soil particles," thus improving the modification effect and engineering operability of expansive soil.

[0119] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0120] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments described herein, but is to be accorded the widest scope consistent with the principles and novel features of the invention herein.

Claims

1. A method for preparing modified expansive soil, characterized in that, The preparation method includes: mixing EICP treatment agent, fiber material and expansive soil, compacting and carrying out modification reaction to obtain the modified expansive soil; The method for preparing the EICP treatment agent includes: mixing an equal volume of cementing solution and a urease solution to prepare the EICP treatment agent; The pH value of the urease solution is 5~5.5; The amount of EICP treatment agent added is calculated using the following formula, which includes: In equation (1), M - Mass of expansive soil, in grams; y - Moisture content of expansive soil, in % m - Mass of EICP treatment agent in mixed expansive soil, in grams; m1 - The mass of the solute in the cementitious solution, in grams; The mass of the solute in the m2-urease solution, in grams; m3 - Mass of the cementitious liquid, in grams; The mass of the m4-urease solution is expressed in grams. Among them, the optimum moisture content of expansive soil is -2% ≤ y ≤ optimum moisture content of expansive soil +2%; The cementing solution contains calcium chloride solution and urea solution; The fiber material is selected from one or more of sisal fiber, polypropylene fiber, polyester fiber, glass fiber, and basalt fiber.

2. The preparation method according to claim 1, characterized in that, The molar ratio of the calcium chloride solution to the urea solution is 1:1 to 1:1.

5.

3. The preparation method according to claim 2, characterized in that, The molar ratio of the calcium chloride solution to the urea solution is 1:1.

5.

4. The preparation method according to claim 1, characterized in that, The concentration of the calcium chloride solution is 0.5~2.5 mol / L.

5. The preparation method according to claim 4, characterized in that, The concentration of the calcium chloride solution is 1~1.5 mol / L.

6. The preparation method according to claim 1, characterized in that, The extract used in the urease solution is selected from one or more of the following: soybean, sword bean, chickpea, watermelon seed, quinoa plant or mulberry leaf; And / or, the urease activity of the urease solution is greater than 10 mM / min.

7. The preparation method according to claim 1, characterized in that, The mass of the fiber material is 0.3% to 0.5% of the mass of the expansive soil; And / or, the length of the fiber material is 5~10mm.

8. The preparation method according to any one of claims 1 to 7, characterized in that, The method for determining the compaction state is as follows: The optimum dry density of expansive soil -0.1 ≤ the dry density of mixed expansive soil ≤ the optimum dry density of expansive soil +0.

1.

9. The preparation method according to claim 1, characterized in that, The temperature of the modification reaction is 15~25℃; And / or, the modification reaction takes 3 to 7 days.

10. A modified expansive soil obtained by the preparation method according to any one of claims 1 to 9.

11. The application of the modified expansive soil obtained by the preparation method according to any one of claims 1 to 9 or the modified expansive soil according to claim 10 in the field of construction engineering.

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