Roadbed filling construction method for internal and external source carbonization reinforcement

Abstract

The roadbed filling construction method specifically comprises the following steps: preparing source soil and a site, preparing a microbial bacterium solution and a microbial bacterium solution-urea mixed solution, preparing a wet mixture, paving a roadbed layer, performing endogenous and exogenous carbonization maintenance, performing quality inspection, performing supplementary carbonization and the like. Through a synergistic reinforcement mechanism of endogenous carbonization of CO3 < 2-> generated by urea hydrolysis induced by microbial bacteria and exogenous carbonization of CO2 injection, carbonization and cementation of the MgO-containing low-carbon-based curing agent are promoted, the soil body strength is synchronously improved, CO2 is fixed, and the compressive strength reaches 2.5-4.0 MPa in 7 days. Biological risks can be avoided, cement dependence is reduced, gas diffusion and carbonization effect uniformity are optimized through the drain board grating, the drain board grating serves as a reinforced material, the integrity and bearing capacity of a roadbed are improved, and cracks are avoided; the method has the advantages of being high in construction speed, controllable in strength, low in cost, high in adaptability, remarkable in environmental and economic benefits and the like, large-scale engineering application requirements can be met, and solid waste resource utilization and roadbed engineering carbon emission reduction are achieved.

Description

Technical Field

[0001] This invention belongs to the technical field of environmental soil and rock solid waste resource utilization and roadbed filling, and particularly relates to a roadbed filling construction method with internal and external carbonization reinforcement. Background Technology

[0002] With rapid economic development and urbanization, infrastructure construction, including highways, is increasing. In this process, weak foundations or roadbeds are frequently encountered, becoming a pressing problem. Weak silt, due to its high water content and low strength, is difficult to use directly in engineering construction; direct dumping or use not only occupies arable land but may also pollute groundwater and damage the ecological environment. Simultaneously, the rapid development of transportation construction has led to a massive demand for soil and rock fillers. However, increasingly stringent environmental policies have resulted in a supply shortage and soaring prices for quarrying (slag), severely hindering infrastructure construction. Therefore, improving silt or engineering waste along roads to transform it into suitable roadbed fillers has become a highly promising solution that urgently needs further exploration. Currently, how to achieve large-scale improvement of silt or engineering waste using low-cost, efficient methods to meet strength, stability, and environmental safety requirements has become a key challenge that urgently needs to be overcome in the engineering construction field.

[0003] Currently, various technologies exist for treating soft silt soil, but mainstream treatment technologies have significant limitations. Existing technologies for silt treatment include heat treatment, sedimentation and drying, solidification with curing agents, and magnesium oxide carbonization. Among these, heat treatment is limited by low processing capacity and efficiency, high cost and energy consumption; sedimentation and drying are restricted due to space requirements and the potential for secondary environmental pollution. Physical treatment technologies such as vacuum preloading, while reinforcing the foundation to some extent, suffer from high costs and energy consumption, long cycles, and narrow applicability. Biological reinforcement is difficult to widely apply in engineering projects due to high costs, long cycles, and unstable effects. Chemical solidification, although the most widely used and promising technology, faces problems such as high curing agent consumption, high cost, difficulty in degradation, and the potential for secondary pollution. Conventional curing agents such as silicate cement, polymers, electrolytic solutions, and slag silicates have many drawbacks. Polymer-based curing agents have poor water resistance, and their strength decreases significantly upon contact with water. Electrolytic solution-based curing agents have complex compositions, high costs, and are prone to causing soil and environmental pollution. Cement and slag silicate-based curing agents are used in large quantities, but their strength improvement after curing is limited. Moreover, cement production consumes a lot of resources and energy, and is accompanied by high CO2 emissions, which does not meet the "dual carbon" target. In summary, although the above technologies can improve the properties of silt to some extent, they are difficult to simultaneously meet the comprehensive requirements of high efficiency, environmental protection, economy, and large-scale application.

[0004] In recent years, emerging low-carbon soil improvement technologies have become a research hotspot, mainly focusing on MgO carbonization and solidification technology and microbial induced carbonate precipitation (MICP) technology. For example, research papers such as "A Soil Carbonization and Solidification Method" (201210097042.2) and "A Soil Carbonization and Solidification Method and Apparatus" (201010604013.1) both use active MgO as a soil solidification agent, carbonizing under high CO2 concentration and high pressure conditions, which can rapidly increase soil strength. Compared with traditional cement-solidified soil, it has advantages such as fast solidification speed, high strength, and good environmental benefits. However, when treating fine-grained soil with high moisture content, it can easily affect CO2 infiltration and carbonization uniformity. MIP technology, on the other hand, utilizes microorganisms (such as Bacillus pasteurellii or urease) to catalyze the decomposition of urea to produce CO3. 2 - and with Ca in CaCl2 2+ The formation of calcium carbonate, which binds soil particles and improves soil mechanical properties, is mainly used in loose sandy soils. However, its treatment effect is limited in clay with poor permeability. For example, "A method for reinforcing coarse-grained soil based on microbial-induced calcium carbonate deposition" (201811304804.5) fixes coarse-grained soil by adding a viscous bacterial solution and then uses urea and calcium ion treatment solution to bind the coarse-grained soil; "An apparatus and method that can simultaneously realize MIP grouting to reinforce soil and remove its by-products" (201910325596.5).

[0005] Furthermore, the project "Magnesium Oxide Carbonized Foundation Solidified Soil and Its Preparation Method and Application" (202510090965.2) uses in-situ soil as raw material and active magnesium oxide as cementing material. It adds Bacillus subtilis, which produces carbonic anhydrase, to improve carbon fixation capacity and adds reinforcing agents to enhance the carbonization bonding strength and CO2 absorption capacity of the raw material interface. However, the preparation process involves mixing and granulation of various raw materials, including amine solutions, making the process complex and requiring strict control over the physical properties of the raw materials, such as particle size, which limits its large-scale application. The project "An Inorganic Solid Waste-Microbial Composite Solidifying Agent and Its Preparation Method and Application" (202310242854.X) adds a composite of carbonic anhydrase and Bacillus subtilis to materials such as active magnesium oxide, then introduces CO2 to solidify sludge through inorganic solid waste carbonization and microbial mineralization. However, the material preparation process is complex, involving porous material preparation and microbial microencapsulation, resulting in high costs and hindering widespread application. The method, materials, and applications of bio-carbonized magnesium oxide solidification of geological materials based on urea pre-hydrolysis (202310003073.5) involves mixing urea and commercial bacterial solution for pre-hydrolysis, followed by mixing with a solid mixture of geological materials and activated magnesium oxide. While this improves the utilization rate of urea and activated magnesium oxide and the performance of the solidified soil, it requires more bacterial solution to produce a fluid slurry and necessitates the addition of a hydration agent for MgO pre-hydrolysis. Furthermore, it has limitations on raw materials (such as geological materials with a pH value less than 9.5), resulting in a narrow range of applications. The method of solidifying dredged sludge using bio-carbonized activated magnesium oxide technology (202211271891.5) first prepares a pre-hydrolyzed urea solution using urease-producing bacteria, then separately mixes the pre-hydrolyzed urea solution and activated magnesium oxide into the dredged sludge, stirs it into a slurry, and fills it into a designated site for curing. However, this method requires the separate addition of pre-hydrolyzed urea solution and activated MgO, resulting in harsh pre-hydrolysis conditions and limited applicability.

[0006] In summary, the main bottlenecks faced by existing technologies include: (1) the high cost of commercially available exogenous microbial agents and the biosafety risks, making it difficult to meet the requirements for large-scale application; (2) the growth and metabolism of microbial strains (including carbonic anhydrase bacteria) are greatly affected by environmental temperature, humidity, and pH. Temperature fluctuations can lead to a decrease in microbial activity and quantity, affecting the controllability and effectiveness of the solidification process; (3) the strength of the prepared slurry increases slowly and is low, making it difficult to meet the requirements for roadbed filling; and (4) MgO carbonized foundations have defects such as uneven reinforcement effect and high economic cost. To solve the above bottlenecks, this invention innovatively develops a roadbed filling construction method with internal and external carbonization reinforcement. Through the synergistic effect of internal and external carbonization, it reduces material costs, simplifies construction steps, improves the resource utilization level of poor soil and the strength, durability, and carbon fixation effect of the filled roadbed, and effectively expands the scope of application, which is of great significance to promoting infrastructure construction. Summary of the Invention

[0007] To address the problems existing in the prior art, this invention provides a roadbed filling construction method that combines internal and external carbonation reinforcement. This method utilizes carbonate ions generated from urea hydrolysis induced by microorganisms for internal carbonation reinforcement, and injects CO2 for external carbonation. The synergistic effect of internal and external sources enhances the treatment effect. Drainage boards and grids optimize gas diffusion and the uniformity of carbonation, and also act as reinforcement materials, increasing the overall integrity and bearing capacity of the roadbed, reducing or preventing crack formation. This method meets the needs of large-scale engineering applications and has significant advantages in achieving resource recycling and carbon emission reduction.

[0008] This invention is implemented as follows, specifically including the following steps:

[0009] S1. Source Soil and Site Preparation: Source soil preparation includes testing the physical, chemical and mechanical properties of the source soil, turning it over, air-drying and crushing it, and testing the compaction characteristics of the source soil-solidifier mixture; Site preparation includes cleaning, compacting and leveling the surface of the roadbed or foundation, setting out the construction section and setting out markers, and waterproofing and seepage prevention treatment for the pre-treated site.

[0010] S2. Preparation of microbial culture: Weigh urea, nutrient broth, potassium dihydrogen phosphate, and deionized water according to the mass percentage of (6-12):(4-6):(5-10):1000. First, mix the nutrient broth, potassium dihydrogen phosphate, and deionized water evenly and sterilize them in a high-temperature and high-pressure autoclave at 120-126℃ and 100-105kPa for 15-30 minutes. After cooling to room temperature, add urea and stir evenly to form a liquid culture medium for expanding the microbial mother liquor. Then, mix the microbial mother liquor with the liquid culture medium at a mass ratio of 1%-2% and carry out the expansion culture. Incubate in a constant temperature shaker at 25-35℃ at a speed of 130-160r / min for 24-48 hours to obtain the microbial culture and seal it for storage.

[0011] S3. Preparation of microbial inoculum-urea mixed solution: Mix urea with deionized water to prepare a urea solution with a concentration of 1-5 mol / L. Mix the urea solution with the microbial inoculum obtained in step S2 at a mass percentage of (5-20):100, or mix urea with the microbial inoculum at a mass percentage of (6-12):100. Carry out a biochemical reaction at 20-30℃ for no less than 6 hours to prepare the microbial inoculum-urea mixed solution.

[0012] S4. Preparation of wet mixture: According to the preset curing agent dosage and preset moisture content, weigh the curing agent and microbial inoculum-urea mixed solution, and mix the curing agent and microbial inoculum-urea mixed solution with the air-dried source soil in sequence to make a uniform wet mixture.

[0013] S5. Paving the subgrade: According to the layout and stake setting, first pave the wet mixture to form a subgrade layer with a thickness of 20-40cm, then lay the drainage board grid, and then continue to pave the wet mixture to form the subgrade layer. A layer of drainage board grid needs to be laid every 60-80cm. Before laying the drainage board grid, it is necessary to roll and roughen the surface. The compacted subgrade layer and the subgrade layer form the subgrade compaction layer.

[0014] S6. Intrinsic carbonization curing: Connect the ends of each layer of drainage board grid in parallel to the branch pipe, cover the roadbed compaction layer with a sealing film, and fix the sealing film with a pressure plate and cable. The sealing time is 3 to 7 days to complete the intrinsic carbonization curing of the roadbed compaction layer.

[0015] S7. External carbonization curing: Connect the branch pipe to the CO2 gas tank through the gas injection pipe, open the pressure regulating valve to introduce CO2 gas into the roadbed compaction layer. If there are multiple layers of drainage board grid, start from the bottom layer and implement the gasification layer by layer. After the gasification is completed, continue the sealing curing for 12-24 hours to complete the external carbonization curing of the roadbed compaction layer.

[0016] S8. Quality Inspection and Supplementary Carbonization: Remove the pressure plate, cable and sealing film in sequence, and set up no less than 2 inspection points at each pile location for quality spot checks. If the compressive strength and resilient modulus of the roadbed compaction layer meet the requirements of the highway roadbed treatment specifications, the construction is completed; if not, repeat step S7 for supplementary carbonization.

[0017] Preferably, the source soil in step S1 is the foundation soil of the construction site, excavated waste soil, shield tunneling slag, engineering mud, or dredged silt; the physical, chemical, and mechanical properties of the source soil include moisture content, liquid limit, plastic limit, relative density, particle size distribution, pH value, etc., and the particle size of the source soil after crushing is no greater than 1cm; the flatness of the pretreatment site is controlled within 2cm; the construction section layout extends 50-100cm beyond the roadbed width, and the spacing of the marker stakes is 10-15m; the water-proof and seepage-proof layer treatment includes spraying cement-clay slurry and curing, wherein the cement admixture ratio is 10-20%, the water-solid ratio is 0.45-0.60, the curing time is 24-48h, and the clay is mainly bentonite.

[0018] As a preferred embodiment, the curing agent in step S1 is mainly active magnesium oxide powder, supplemented by quicklime, hydrated lime and industrial solid waste. The industrial solid waste includes one or more of carbide slag, steel slag powder, olivine powder and serpentine powder. The mass percentage of active magnesium oxide in the mixture is not less than 50%, and the steel slag content does not exceed 5%.

[0019] As another preferred embodiment, the nutrient broth in step S2 is composed of one or both of plant peptone and animal peptone, with a content of 60-100 g / L, a urea content of 30-60 g / L, and a potassium dihydrogen phosphate content of 5-10 g / L; the microorganisms are indigenous urease bacteria or a mixture of indigenous urease bacteria and Bacillus pasteurellii, and the volume fraction of indigenous urease bacteria is not less than 50%.

[0020] The indigenous urease-inducing bacteria suspension is prepared as follows: Urea, nutrient broth, and deionized water are weighed and mixed according to a mass percentage of (8-16):(8-12):1000 to prepare the urease-inducing solution; based on the tested microbial indicators, soil containing a relative urease-inducing bacteria content of not less than 50% is selected as the target material, and the target material is mixed into the urease-inducing solution at a mass percentage of 0.2-0.5%, and cultured in a 30℃ constant temperature shaker incubator at a speed of 100-130 r / min for 24-48 h to form an indigenous urease-inducing suspension; next, a suspension is prepared according to a mass percentage of (6-12):(4-6):(5-10). Weigh 1000g of urea, nutrient broth, potassium dihydrogen phosphate, and deionized water. Mix the nutrient broth, potassium dihydrogen phosphate, and deionized water evenly and sterilize them in a high-temperature autoclave at 120-126℃ and 100-105kPa for 15-30 minutes. After cooling to room temperature, add urea and stir evenly to form a liquid culture medium for the large-scale cultivation of indigenous urease bacteria. Mix the indigenous urease bacteria suspension with the liquid culture medium at a mass percentage of 1%-2% and incubate in a constant temperature shaker at 25-35℃ at a speed of 130-160r / min for 24-48 hours to obtain the indigenous urease bacteria solution.

[0021] As another preferred method, the microbial inoculum and the microbial inoculum-urea mixed solution in steps S2 and S3 need to be sealed and refrigerated at a temperature of 0-6°C for no more than 15 days.

[0022] As another preferred method, the preset curing agent dosage in step S4 is determined based on the curing agent composition and indoor trial mixing test, and is 10-30% of the source soil mass; the preset moisture content in step S4 is determined based on the compaction test of the source soil-curing agent mixture, and is 2-4% higher than the optimal moisture content obtained from the compaction characteristic curve. The moisture content is controlled by the mass percentage of the microbial liquid-urea mixture to the air-dried source soil.

[0023] As another preferred method, in step S5, the transverse spacing of the drainage boards in the drainage grid is 0.8-1.6m, and the longitudinal spacing may or may not be provided. If longitudinal drainage boards are provided, the spacing is 1-3m. The compaction sequence of each layer is from both sides to the middle. The compaction degree of the lower layer of the subgrade is 75-85%, and the compaction degree of each layer of the upper layer of the subgrade is 80-90%. When the upper layer of the subgrade is multi-layered, the drainage boards of the drainage grid between adjacent layers should be staggered. The drainage board is a concave-convex hollow rib structure.

[0024] As another preferred method, the air injection pipe in step S7 can be connected to a branch pipe on one or both sides of the roadbed; the plastic film covers the top surface and both sides of the compacted roadbed layer laterally, and extends 1-2m beyond the pre-carbonization construction section longitudinally; the CO2 is an industrial gas with a CO2 concentration of not less than 40%, which can be collected from industrial exhaust gases such as those from power plants; the air injection pipe is equipped with a pressure regulating valve and a heater, and the air pressure of the pressure regulating valve is controlled at 100-1000kPa; intermittent air ventilation is adopted, and the ventilation time for each layer is determined according to the compacted layer density and permeability, with a cumulative ventilation time of 6-24h. The worse the soil permeability, the longer the ventilation time.

[0025] The advantages and technical effects of this invention are as follows:

[0026] (1) Special site waterproofing and seepage prevention treatment was carried out, and the seepage prevention material mainly composed of cement clay slurry soil was used to block the transmission of water and pollutants in the filling road base and the lower foundation.

[0027] (2) Using commercially available or indigenous microbial inoculum as the stock solution for expanded culture, microbial inoculum was prepared for hydrolyzing urea and replacing water in soil samples, which improved the carbonization degree and compaction density of alkaline materials such as magnesium oxide in the mixed soil.

[0028] (3) The drainage board grid is laid in special layers. It not only serves as a transmission channel for CO2 gas during the construction and maintenance period, but also as a reinforcing material, which improves the integrity of the roadbed foundation, avoids uneven settlement under the roadbed load, and can also serve as a transverse drainage channel for the roadbed during operation.

[0029] (4) Active MgO and solid waste such as carbide slag and olivine powder are added to the fill material, which are low-carbon and environmentally friendly materials. They have a good CO2 carbon fixation effect and the pH value of the solidified soil is low. This achieves resource recycling and carbon emission reduction, which meets the "dual carbon" goal and has the advantages of low-carbon environmental protection and sustainable development.

[0030] (5) A constant heater is specially installed on the gas injection pipe to avoid condensation and water vapor accumulation in the gas pipe caused by low temperature compressed gas. A sealing membrane is laid on the top of the paved soil layer and the gas is ventilated intermittently to avoid CO2 leakage.

[0031] (6) The carbonate generated by the urea hydrolysis induced by microbial bacteria is used for endogenous carbonization reinforcement, and CO2 is injected into the drainage board grid for exogenous carbonization. The dual carbonization of endogenous and exogenous sources is coordinated to optimize the uniformity of gas diffusion and enhance the carbonization effect.

[0032] (7) The construction method is simple and each step is carried out in sequence, which greatly improves the construction efficiency of roadbed paving and soil reinforcement. The strength of the roadbed filling is controllable, the cost is low, and the adaptability is strong, which meets the needs of large-scale engineering applications. Attached Figure Description

[0033] Figure 1 A construction flowchart for a roadbed filling construction method that uses both internal and external carbonization reinforcement;

[0034] Figure 2 A plan view of a grid mesh containing only horizontal drainage panels;

[0035] Figure 3 A plan view of a grid mesh containing transverse and longitudinal drainage boards;

[0036] Figure 4 The effect of magnesium oxide content on the strength of microbial-MgO solidified soil under conventional curing conditions;

[0037] Figure 5 The effect of magnesium oxide content on the strength of microbial-MgO solidified soil under CO2 carbonization conditions;

[0038] Figure 6 The effect of bacterial solution content on the strength of microbial-MgO solidified soil under conventional maintenance conditions;

[0039] Figure 7 The effect of bacterial solution content on the strength of microbial-MgO solidified soil under CO2 carbonization conditions;

[0040] Figure 8 The effect of urea concentration on the strength of microbial-MgO solidified soil under conventional curing conditions;

[0041] Figure 9 The effect of urea concentration on the strength of microbial-MgO solidified soil under CO2 carbonization conditions.

[0042] In the diagram: 1. Horizontal drainage board, 2. Fiberglass mesh, 3. Quick-connect connector, 4. Vertical drainage board. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0044] This invention is implemented as follows, specifically including the following steps:

[0045] S1. Source Soil and Site Preparation: Source soil preparation includes testing the physical, chemical and mechanical properties of the source soil, turning it over, air-drying and crushing it, and testing the compaction characteristics of the source soil-solidifier mixture; Site preparation includes cleaning, compacting and leveling the surface of the roadbed or foundation, setting out the construction section and setting out markers, and waterproofing and seepage prevention treatment for the pre-treated site.

[0046] S2. Preparation of microbial culture: Weigh urea, nutrient broth, potassium dihydrogen phosphate, and deionized water according to the mass percentage of (6-12):(4-6):(5-10):1000. First, mix the nutrient broth, potassium dihydrogen phosphate, and deionized water evenly and sterilize them in a high-temperature and high-pressure autoclave at 120-126℃ and 100-105kPa for 15-30 minutes. After cooling to room temperature, add urea and stir evenly to form a liquid culture medium for expanding the microbial mother liquor. Then, mix the microbial mother liquor with the liquid culture medium at a mass ratio of 1%-2% and carry out the expansion culture. Incubate in a constant temperature shaker at 25-35℃ at a speed of 130-160r / min for 24-48 hours to obtain the microbial culture and seal it for storage.

[0047] S3. Preparation of microbial inoculum-urea mixed solution: Mix urea with deionized water to prepare a urea solution with a concentration of 1-5 mol / L. Mix the urea solution with the microbial inoculum obtained in step S2 at a mass percentage of (5-20):100, or mix urea with the microbial inoculum at a mass percentage of (6-12):100. Carry out a biochemical reaction at 20-30℃ for no less than 6 hours to prepare the microbial inoculum-urea mixed solution.

[0048] S4. Preparation of wet mixture: According to the preset curing agent dosage and preset moisture content, weigh the curing agent and microbial inoculum-urea mixed solution, and mix the curing agent and microbial inoculum-urea mixed solution with the air-dried source soil in sequence to make a uniform wet mixture.

[0049] S5. Paving the subgrade: According to the layout and stake setting, first pave the wet mixture to form a subgrade layer with a thickness of 20-40cm. Then lay the drainage board grid. Then continue to pave the wet mixture to form the subgrade layer. Every 60-80cm thick layer, a drainage board grid needs to be laid (the drainage board grid mainly includes transverse drainage board 1 and longitudinal drainage board 4, fiberglass mesh 2 and quick connector 3). Before laying the drainage board grid, it is necessary to roll and roughen the surface. The compacted subgrade layer and the subgrade layer form the subgrade compaction layer.

[0050] S6. Intrinsic carbonization curing: Connect the ends of each layer of drainage board grid in parallel to the branch pipe, cover the roadbed compaction layer with a sealing film, and fix the sealing film with a pressure plate and cable. The sealing time is 3 to 7 days to complete the intrinsic carbonization curing of the roadbed compaction layer.

[0051] S7. External carbonization curing: Connect the branch pipe to the CO2 gas tank through the gas injection pipe, open the pressure regulating valve to introduce CO2 gas into the roadbed compaction layer. If there are multiple layers of drainage board grid, start from the bottom layer and implement the gasification layer by layer. After the gasification is completed, continue the sealing curing for 12-24 hours to complete the external carbonization curing of the roadbed compaction layer.

[0052] S8. Quality Inspection and Supplementary Carbonization: Remove the pressure plate, cable and sealing film in sequence, and set up no less than 2 inspection points at each pile location for quality spot checks. If the compressive strength and resilient modulus of the roadbed compaction layer meet the requirements of the highway roadbed treatment specifications, the construction is completed; if not, repeat step S7 for supplementary carbonization.

[0053] Preferably, the source soil in step S1 is the foundation soil of the construction site, excavated waste soil, shield tunneling slag, engineering mud, or dredged silt; the physical, chemical, and mechanical properties of the source soil include moisture content, liquid limit, plastic limit, relative density, particle size distribution, pH value, etc., and the particle size of the source soil after crushing is no greater than 1cm; the flatness of the pretreatment site is controlled within 2cm; the construction section layout extends 50-100cm beyond the roadbed width, and the spacing of the marker stakes is 10-15m; the water-proof and seepage-proof layer treatment includes spraying cement-clay slurry and curing, wherein the cement admixture ratio is 10-20%, the water-solid ratio is 0.45-0.60, the curing time is 24-48h, and the clay is mainly bentonite.

[0054] As a preferred embodiment, the curing agent in step S1 is mainly active magnesium oxide powder, supplemented by quicklime, hydrated lime and industrial solid waste. The industrial solid waste includes one or more of carbide slag, steel slag powder, olivine powder and serpentine powder. The mass percentage of active magnesium oxide in the mixture is not less than 50%, and the steel slag content does not exceed 5%.

[0055] As another preferred embodiment, the nutrient broth in step S2 is composed of one or both of plant peptone and animal peptone, with a content of 60-100 g / L, a urea content of 30-60 g / L, and a potassium dihydrogen phosphate content of 5-10 g / L; the microorganisms are indigenous urease bacteria or a mixture of indigenous urease bacteria and Bacillus pasteurellii, and the volume fraction of indigenous urease bacteria is not less than 50%.

[0056] The indigenous urease-inducing bacteria suspension is prepared as follows: Urea, nutrient broth, and deionized water are weighed and mixed according to a mass percentage of (8-16):(8-12):1000 to prepare the urease-inducing solution; based on the tested microbial indicators, soil containing a relative urease-inducing bacteria content of not less than 50% is selected as the target material, and the target material is mixed into the urease-inducing solution at a mass percentage of 0.2-0.5%, and cultured in a 30℃ constant temperature shaker incubator at a speed of 100-130 r / min for 24-48 h to form an indigenous urease-inducing suspension; next, a suspension is prepared according to a mass percentage of (6-12):(4-6):(5-10). Weigh 1000g of urea, nutrient broth, potassium dihydrogen phosphate, and deionized water. Mix the nutrient broth, potassium dihydrogen phosphate, and deionized water evenly and sterilize them in a high-temperature autoclave at 120-126℃ and 100-105kPa for 15-30 minutes. After cooling to room temperature, add urea and stir evenly to form a liquid culture medium for the large-scale cultivation of indigenous urease bacteria. Mix the indigenous urease bacteria suspension with the liquid culture medium at a mass percentage of 1%-2% and incubate in a constant temperature shaker at 25-35℃ at a speed of 130-160r / min for 24-48 hours to obtain the indigenous urease bacteria solution.

[0057] As another preferred method, the microbial inoculum and the microbial inoculum-urea mixed solution in steps S2 and S3 need to be sealed and refrigerated at a temperature of 0-6°C for no more than 15 days.

[0058] As another preferred method, the preset curing agent dosage in step S4 is determined based on the curing agent composition and indoor trial mixing test, and is 10-30% of the source soil mass; the preset moisture content in step S4 is determined based on the compaction test of the source soil-curing agent mixture, and is 2-4% higher than the optimal moisture content obtained from the compaction characteristic curve. The moisture content is controlled by the mass percentage of the microbial liquid-urea mixture to the air-dried source soil.

[0059] As another preferred method, in step S5, the transverse spacing of the drainage boards in the drainage grid is 0.8-1.6m, and the longitudinal spacing may or may not be provided. If longitudinal drainage boards are provided, the spacing is 1-3m. The compaction sequence of each layer is from both sides to the middle. The compaction degree of the lower layer of the subgrade is 75-85%, and the compaction degree of each layer of the upper layer of the subgrade is 80-90%. When the upper layer of the subgrade is multi-layered, the drainage boards of the drainage grid between adjacent layers should be staggered. The drainage board is a concave-convex hollow rib structure.

[0060] As another preferred method, the air injection pipe in step S7 can be connected to a branch pipe on one or both sides of the roadbed; the plastic film covers the top surface and both sides of the compacted roadbed layer laterally, and extends 1-2m beyond the pre-carbonization construction section longitudinally; the CO2 is an industrial gas with a CO2 concentration of not less than 40%, which can be collected from industrial exhaust gases such as those from power plants; the air injection pipe is equipped with a pressure regulating valve and a heater, and the air pressure of the pressure regulating valve is controlled at 100-1000kPa; intermittent air ventilation is adopted, and the ventilation time for each layer is determined according to the compacted layer density and permeability, with a cumulative ventilation time of 6-24h. The worse the soil permeability, the longer the ventilation time.

[0061] The mechanical properties of the solidified soil layer in roadbed filling are affected by many factors, including magnesium oxide content, initial moisture content, soil type, type and activity of microbial inoculum, urea concentration, carbonation time, and compaction degree, among which the first three have the most significant impact. To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0062] Example 1

[0063] A nutrient broth was prepared using soybean peptone (40 g / L) and tryptone (20 g / L). Urease-inducing broth, urea (100 g / L), and deionized water were mixed at a specific mass ratio to prepare a urease-inducing solution. A small amount of soil sample was collected from the site and added to the urease-inducing solution at a mass ratio of 0.2-0.5% to form a soil-urease-inducing suspension. A liquid culture medium was prepared by mixing soybean peptone (30 g / L), tryptone (30 g / L), urea (40 g / L), potassium dihydrogen phosphate (8 g / L), and deionized water, and then sterilizing the mixture. The urease-inducing suspension and the liquid culture medium were mixed at a 1.5% mass ratio and cultured for 24 hours to obtain a microbial culture containing urease-inducing bacteria.

[0064] Urea was first dissolved in water to prepare a 1.5 mol / L urea solution, which was then mixed with microbial inoculum and pre-hydrolyzed at 25-35℃ for 12 hours to obtain a microbial inoculum-urea mixed solution. Silty clay was used as the treatment object; naturally air-dried silty clay was crushed and sieved through a 2 mm sieve. The microbial inoculum-urea mixed solution was used as the water in the soil mixed using the traditional method, prepared at 30% of the dry soil mass; the magnesium oxide content (as a percentage of dry soil mass) was used as an influencing variable, set at 10%, 15%, and 20%. The microbial inoculum-urea mixed solution was added sequentially to the dry soil, and after uniform mixing, cylindrical samples were prepared. The samples were cured using conventional methods and conventional curing plus carbonization curing. After 7 days and 28 days of conventional curing alone, the strength was measured and found to increase with increasing magnesium oxide content and curing age, with the 28-day strength being approximately 1 MPa (e.g., ...). Figure 4After conventional curing for 7 and 28 days, followed by continued carbonization curing, the unconfined compressive strength of the 7-day-cured and carbonized specimens increased significantly with increasing magnesium oxide content, from approximately 0.5 MPa to approximately 3 MPa; the unconfined compressive strength of the 28-day-cured and carbonized specimens increased from approximately 2 MPa to approximately 7 MPa. Figure 5 ).

[0065] Example 2

[0066] Urea was first dissolved in water to prepare a 1.5 mol / L urea solution, which was then mixed with microbial inoculum and pre-hydrolyzed at 25-35℃ for 12 hours to obtain a microbial inoculum-urea mixed solution. Silty clay was used as the treatment object; naturally air-dried silty clay was crushed and sieved through a 2 mm sieve. The magnesium oxide content (as a percentage of dry soil mass) was 15%. Using the microbial inoculum-urea mixed solution as an influencing variable, samples were prepared at 25%, 30%, and 35% of the dry soil mass. Magnesium oxide powder and the microbial inoculum-urea mixed solution were added sequentially to the dry soil, and after uniform stirring, cylindrical samples were prepared. These samples were cured using conventional methods and conventional curing plus carbonization curing. After only 7 and 28 days of conventional curing, the strength decreased with increasing inoculum content (e.g., ...). Figure 6 After conventional curing for 7 and 28 days, followed by carbonization curing, the unconfined compressive strength of the carbonized specimens after 7 days of curing decreased slowly with increasing microbial inoculum-urea mixed solution; the unconfined compressive strength of the carbonized specimens after 28 days of curing first decreased and then increased. Figure 7 ).

[0067] Example 3

[0068] The study investigated the effects of urea concentration as a variable. Urea was first dissolved in water to prepare urea solutions of 1.0, 1.5, and 2.0 mol / L, which were then mixed with microbial inoculum and pre-hydrolyzed at 25-35℃ for 12 hours to obtain a microbial inoculum-urea mixed solution. Silty clay was used as the treatment object. Naturally air-dried silty clay was crushed and sieved through a 2 mm sieve. The magnesium oxide content (as a percentage of dry soil mass) was 15%. The microbial inoculum-urea mixed solution was used as a variable, prepared at 30% of the dry soil mass. Magnesium oxide powder and pre-hydrolyzed urea were added sequentially to the dry soil, and after uniform mixing, cylindrical samples were prepared. These samples were cured under conventional conditions and under conventional curing plus carbonization curing. After 7 and 28 days of conventional curing alone, the unconfined compressive strength showed a trend of first increasing and then decreasing with increasing urea concentration (e.g., ...). Figure 8 After conventional curing for 7 and 28 days, followed by carbonization curing, the unconfined compressive strength of the carbonized specimens after 7 days of curing increased with increasing urea concentration, from approximately 1 MPa to approximately 2 MPa; the unconfined compressive strength of the carbonized specimens after 28 days of curing showed a trend of first decreasing and then increasing with increasing urea concentration. Figure 9 ).

[0069] The foregoing description and illustration of the basic principles, main features, and advantages of this invention are merely preferred embodiments and are not intended to limit the invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention, such as replacing materials, improving forms, or changing application scenarios, should be included within the scope of protection of this invention.

Claims

1. A method for constructing roadbed filling with internal and external carbonization reinforcement, characterized in that, The method includes the following steps: S1. Source Soil and Site Preparation: Source soil preparation includes testing the physical, chemical and mechanical properties of the source soil, turning it over, air-drying and crushing it, and testing the compaction characteristics of the source soil-solidifier mixture; Site preparation includes cleaning, compacting and leveling the surface of the roadbed or foundation, setting out the construction section and setting out markers, and waterproofing and seepage prevention treatment for the pre-treated site. S2. Preparation of microbial culture: Weigh urea, nutrient broth, potassium dihydrogen phosphate, and deionized water according to the mass percentage of (6-12):(4-6):(5-10):1000. First, mix the nutrient broth, potassium dihydrogen phosphate, and deionized water evenly and sterilize them in a high-temperature and high-pressure autoclave at 120-126℃ and 100-105kPa for 15-30 minutes. After cooling to room temperature, add urea and stir evenly to form a liquid culture medium for expanding the microbial mother liquor. Then, mix the microbial mother liquor with the liquid culture medium at a mass ratio of 1%-2% and carry out the expansion culture. Incubate in a constant temperature shaker at 25-35℃ at a speed of 130-160r / min for 24-48 hours to obtain the microbial culture and seal it for storage. S3. Preparation of microbial inoculum-urea mixed solution: Mix urea with deionized water to prepare a urea solution with a concentration of 1-5 mol / L. Mix the urea solution with the microbial inoculum obtained in step S2 at a mass percentage of (5-20):100, or mix urea with the microbial inoculum at a mass percentage of (6-12):

100. Carry out a biochemical reaction at 20-30℃ for no less than 6 hours to prepare the microbial inoculum-urea mixed solution. S4. Preparation of wet mixture: According to the preset curing agent dosage and preset moisture content, weigh the curing agent and microbial inoculum-urea mixed solution, and mix the curing agent and microbial inoculum-urea mixed solution with the air-dried source soil in sequence to make a uniform wet mixture. S5. Paving the subgrade: According to the layout and stake setting, first pave the wet mixture to form a subgrade layer with a thickness of 20-40cm, then lay the drainage board grid, and then continue to pave the wet mixture to form the subgrade layer. A layer of drainage board grid needs to be laid every 60-80cm. Before laying the drainage board grid, it is necessary to roll and roughen the surface. The compacted subgrade layer and the subgrade layer form the subgrade compaction layer. S6. Intrinsic carbonization curing: Connect the ends of each layer of drainage board grid in parallel to the branch pipe, cover the roadbed compaction layer with a sealing film, and fix the sealing film with a pressure plate and cable. The sealing time is 3 to 7 days to complete the intrinsic carbonization curing of the roadbed compaction layer. S7. External carbonization curing: Connect the branch pipe to the CO2 gas tank through the gas injection pipe, open the pressure regulating valve to introduce CO2 gas into the roadbed compaction layer. If there are multiple layers of drainage board grid, start from the bottom layer and implement the gasification layer by layer. After the gasification is completed, continue the sealing curing for 12-24 hours to complete the external carbonization curing of the roadbed compaction layer. S8. Quality Inspection and Supplementary Carbonization: Remove the pressure plate, cable and sealing film in sequence, and set up no less than 2 inspection points at each pile location for quality spot checks. If the compressive strength and resilient modulus of the roadbed compaction layer meet the requirements of the highway roadbed treatment specifications, the construction is completed; if not, repeat step S7 for supplementary carbonization.

2. The roadbed filling construction method for internal and external carbonization reinforcement according to claim 1, characterized in that: The source soil in step S1 is the foundation soil of the construction site, excavated waste soil, shield tunneling slag, engineering mud, or dredged silt. The physical, chemical, and mechanical properties of the source soil include moisture content, liquid limit, plastic limit, relative density, particle size distribution, and pH value. After crushing, the particle size of the source soil is no larger than 1 cm. The flatness of the pretreatment site is controlled within 2 cm. The construction section layout extends 50-100 cm beyond the roadbed width, and the spacing of the marker stakes is 10-15 m. The treatment of the waterproof and seepage-proof layer includes spraying cement-clay slurry and curing, wherein the cement admixture ratio is 10-20%, the water-solid ratio is 0.45-0.60, the curing time is 24-48 hours, and the clay is mainly bentonite.

3. The roadbed filling construction method for internal and external carbonization reinforcement according to claim 1, characterized in that: The curing agent in step S1 is mainly active magnesium oxide powder, supplemented by quicklime, hydrated lime and industrial solid waste. The industrial solid waste includes one or more of carbide slag, steel slag powder, olivine powder and serpentine powder. The mass percentage of active magnesium oxide in the mixture is not less than 50%, and the steel slag content does not exceed 5%.

4. The roadbed filling construction method for internal and external carbonization reinforcement according to claim 1, characterized in that: The nutrient broth in step S2 consists of one or both of plant peptone and animal peptone, with a content of 60-100 g / L, a urea content of 30-60 g / L, and a potassium dihydrogen phosphate content of 5-10 g / L; the microorganisms are indigenous urease bacteria or a mixture of indigenous urease bacteria and Bacillus pasteurellii, and the volume fraction of indigenous urease bacteria is not less than 50%.

5. The roadbed filling construction method for internal and external carbonization reinforcement according to claim 1, characterized in that: The microbial inoculum and the microbial inoculum-urea mixed solution in steps S2 and S3 must be sealed and refrigerated at 0-6°C for no more than 15 days.

6. The roadbed filling construction method for internal and external carbonization reinforcement according to claim 1, characterized in that: The preset curing agent dosage in step S4 is determined based on the curing agent composition and indoor trial mixing test, and is 10-30% of the source soil mass. The preset moisture content in step S4 is determined based on the compaction test of the source soil-curing agent mixture, and is 2-4% higher than the optimal moisture content obtained from the compaction characteristic curve. The moisture content is controlled by the mass percentage of the microbial liquid-urea mixture to the air-dried source soil.

7. The roadbed filling construction method for internal and external carbonization reinforcement according to claim 1, characterized in that: In step S5, the transverse spacing of the drainage boards in the drainage grid is 0.8-1.6m, and the longitudinal spacing may or may not be provided. If longitudinal drainage boards are provided, the spacing is 1-3m. The compaction sequence of each layer is from both sides to the middle. The compaction degree of the lower layer of the subgrade is 75-85%, and the compaction degree of each layer of the upper layer of the subgrade is 80-90%. When the upper layer of the subgrade is multi-layered, the drainage boards of the drainage grid between adjacent layers should be staggered. The drainage board is a concave-convex hollow rib structure.

8. The roadbed filling construction method for internal and external carbonization reinforcement according to claim 1, characterized in that: In step S7, the air injection pipe can be connected to a branch pipe on one or both sides of the roadbed; the plastic film covers the top surface and both sides of the compacted roadbed layer laterally, and extends 1-2m beyond the pre-carbonization construction section longitudinally; the CO2 is an industrial gas with a CO2 concentration of not less than 40%; the air injection pipe is equipped with a pressure regulating valve and a heater, and the air pressure of the pressure regulating valve is controlled at 100-1000kPa; intermittent air ventilation is adopted, and the air ventilation time for each layer is determined according to the compacted layer density and permeability, with a cumulative air ventilation time of 6-24h. The worse the soil permeability, the longer the air ventilation time.

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