Low-carbon method for improving contractility of special soil

Abstract

The invention discloses a low-carbon method for improving the contractility of special soil, and belongs to the technical field of soil body improvement. The low-carbon improvement method for the contractility of the special soil comprises the following steps: performing oxygen-limited pyrolysis on bagasse to obtain bagasse biochar; the special soil is naturally aired and then dried, the bagasse biochar is added into the special soil to be mixed, and the low-carbon type improved special soil is obtained. The special soil is expansive soil or red clay. The method is good in environmental protection property, low in cost and lasting in improvement effect, the dry shrinkage rate of expansive soil and red clay can be effectively reduced, crack development is reduced, and the soil structure stability is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of soil improvement, and more particularly relates to a method for improving the shrinkage of special soil in a low-carbon mode. BACKGROUND

[0002] Expansive soil is a high-plasticity clay containing a large amount of montmorillonite and other hydrophilic minerals, which is extremely sensitive to changes in environmental temperature and humidity, and is prone to swelling when encountering water and shrinking and cracking when losing water, and is known as "engineering cancer"; red clay is a special clay formed by carbonate rocks or granite under the long-term physical and chemical weathering and laterization in a humid and hot climate, and also has the undesirable water-physical property of easy shrinkage after losing water, and belongs to a typical water-sensitive clay.

[0003] In view of the undesirable shrinkage properties of expansive soil and red clay, the commonly used improvement methods currently include physical improvement and chemical improvement. The physical improvement often adopts mechanical compaction, which can improve the compactness of the soil to some extent, but has no obvious effect on improving the shrinkage of the soil. The chemical improvement mainly uses solidifying agents such as lime and cement, which can effectively improve the strength of the soil, but changes the chemical properties of the soil, and causes problems such as high carbon emission, soil hardening and secondary pollution, which is contrary to the concept of green and sustainable development.

[0004] Based on this, the present application is proposed. SUMMARY

[0005] The purpose of the present application is to provide a method for improving the shrinkage of special soil in a low-carbon mode to solve the problems existing in the prior art. The method has good environmental protection, low cost and long-lasting improvement effect, and can effectively reduce the dry shrinkage rate of expansive soil and red clay, reduce the development of cracks and improve the structural stability of the soil.

[0006] To achieve the above-mentioned purpose, the present application provides the following solutions. One of the technical solutions of the present application provides a method for improving the shrinkage of special soil in a low-carbon mode, which comprises the following steps: subjecting bagasse to limited oxygen pyrolysis to obtain bagasse biochar; drying the special soil after natural airing, and adding the bagasse biochar to the special soil to obtain improved special soil; The special soil is expansive soil or red clay.

[0007] Preferably, the temperature of the limited oxygen pyrolysis is 250-350 DEG C, and the time is 1-3 h.

[0008] Preferably, the temperature of the limited oxygen pyrolysis is 300 DEG C, and the time is 2 h.

[0009] Preferably, the addition amount of the bagasse biochar is 2-10% of the mass of the expansive soil.

[0010] Preferably, the added amount of the bagasse biochar is 2-10% of the mass of the red clay.

[0011] The second technical solution of the present application provides a test method for improving the shrinkage of special soil in a low-carbon manner, comprising the following steps: S1, compacting the improved special soil obtained by the above low-carbon improvement method into a soil cake, and then saturating; S2, weighing and measuring the volume of the saturated soil cake, and placing it into a low-field nuclear magnetic resonance instrument to test the T 2 distribution curve of the soil cake, and obtaining the water distribution of the soil cake; S3, placing the soil cake after the S2 process on a porous shrinkage plate, and placing it into a supersaturated salt solution humidifying tank of K2SO4 to perform a shrinkage test, and weighing the soil cake every 1 day; S4, when the mass of the soil cake decreases by 0.5g, sealing and storing it for 3 days, and then weighing and measuring the volume, and placing it into a low-field nuclear magnetic resonance instrument to test the T 2 distribution curve in the shrinkage process, and further obtaining the water distribution evolution law in the shrinkage process of the soil cake; S5, when the mass of the soil cake no longer changes, placing the soil cake into supersaturated salt solution humidifying tanks of NaCl, K2CO3 and LiBr in turn to perform shrinkage tests, until the mass of the soil cake no longer changes, and the test is ended.

[0012] Preferably, the compaction method is a static compaction method; the diameter of the compacted soil cake is 39.1mm, and the height is 7.5mm.

[0013] Preferably, the saturation includes: first vacuum saturation for 4h, then water injection and vacuum for 2h, and finally static saturation with deionized water for 3 days.

[0014] Preferably, the surface of the porous shrinkage plate is coated with vaseline to eliminate the boundary effect in the shrinkage and deformation process of the soil cake and prevent cracking.

[0015] The sugar cane residue biochar prepared by the application has the characteristics of low specific gravity (0.99), high porosity, high adsorption and high stability, can effectively change the pore structure of the soil body, affect the stress change of the soil body in the shrinkage process, and thus inhibit the formation and development of cracks. The low specific gravity feature means low density, that is, the incorporation of biochar will not significantly increase the total mass of the improved soil body, which is an important advantage in engineering applications and helps to reduce additional load. The fiber structure characteristics of the sugar cane residue itself and the physical and chemical changes in the pyrolysis carbonization process are the core reasons for its high porosity. At the micro level, the developed pore network (including micropores, mesopores and macropores) on the surface and inside of the sugar cane residue biochar particles can directly intervene and change the pore structure system of the original soil body when it is incorporated into the swelling soil or red clay. These new pores introduced by the biochar, as well as the filling and bridging of the original pores of the soil body by the particles, optimize the pore size distribution of the soil body. In particular, it can increase the number of beneficial small and medium pores and improve the pore connectivity. This change in microstructure directly affects the soil-water interaction. During the soil shrinkage process, the path and rate of water migration change, and the dissipation of pore water pressure is more uniform, thus effectively relieving the rapid increase of tensile stress between soil particles caused by local stress concentration, and fundamentally inhibiting the initiation and expansion of macro cracks. The rich surface functional groups give the biochar strong adsorption and storage capacity, which can effectively retain calcium, magnesium, potassium and other cations in the soil solution through electrostatic adsorption, complexation and other actions. These retained cations can further promote the agglomeration of soil particles to form more stable and anti-crushing aggregate structures. This stable aggregate is not easy to break up during the shrinkage process, thereby significantly improving the structural strength of the soil body resisting shrinkage deformation. The sugar cane residue biochar converted by pyrolysis has a highly aromatic carbon skeleton structure, and its decomposition rate in the natural environment is extremely slow, and it has strong resistance to biological and chemical degradation. This means that the pore structure support and chemical improvement effect brought by the biochar is not temporary, but can be maintained for a long period of time within the service life of the project. It is particularly important for treating swelling soil and red clay subgrade, slope and other engineering problems with repeated swelling and shrinking characteristics, avoiding the disadvantages of repeated treatment of traditional organic improvement materials due to decomposition failure, and reducing the whole life cycle cost.

[0016] In addition, as an agricultural waste, sugar cane residue is widely available and low in cost. Preparing it into biochar for the improvement of special soil not only realizes the resource utilization of waste, but also effectively improves the shrinkability of special soil, and has good application prospect.

[0017] The application discloses the following technical effects: (1) The method disclosed by the application prepares biochar from sugarcane residue, realizes the reduction and high-value utilization of agricultural solid waste, and reduces the environmental pollution caused by solid waste incineration or landfill. Compared with traditional lime and cement modifiers, the raw material cost of sugarcane residue biochar is low, and the energy consumption in the preparation process is low. At the same time, the sugarcane residue biochar can slowly release nutrients in the soil, improve the ecological environment of the soil, promote the growth of vegetation, and realize the coordination and unity of engineering benefit and ecological benefit.

[0018] (2) The chemical properties of sugarcane residue biochar are stable, which can meet the long-term requirements of soil mechanics performance in slope support, roadbed filling and other engineering.

[0019] (3) The raw material treatment, sugarcane residue biochar preparation and soil modification process of the method disclosed by the application all use conventional equipment, and the operation process is simple, without complex special process. The addition ratio of sugarcane residue biochar can be flexibly adjusted according to the properties of special soil in different regions, and the adaptability is strong. In actual engineering, the sugarcane residue biochar can be directly mixed with the soil on site, which is convenient for construction, shortens the construction period, has low requirements for construction environment, and is convenient for popularization and application in large-scale engineering such as highway, railway and water conservancy. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The shrinkage characteristic curve of the Baise expansive soil improved by the sugarcane residue biochar; Figure 2 The shrinkage characteristic curve of the Guilin red clay improved by the sugarcane residue biochar; Figure 3 The shrinkage characteristic curve of the Nanning red clay improved by the sugarcane residue biochar; Figure 4 The moisture distribution characteristics of the Baise expansive soil improved by the sugarcane residue biochar; Figure 5 The moisture distribution characteristics of the Guilin red clay improved by the sugarcane residue biochar; Figure 6 The moisture distribution characteristics of the Nanning red clay improved by the sugarcane residue biochar; Figure 7 The moisture distribution evolution in the shrinkage process of the Baise expansive soil improved by the sugarcane residue biochar, wherein the left graph is the Baise expansive soil, and the right graph is the Baise expansive soil improved by the sugarcane residue biochar; Figure 8 The moisture distribution evolution in the shrinkage process of the Guilin red clay improved by the sugarcane residue biochar, wherein the left graph is the Guilin red clay, and the right graph is the Guilin red clay improved by the sugarcane residue biochar; Figure 9 The moisture distribution evolution in the shrinkage process of the Nanning red clay improved by the sugarcane residue biochar, wherein the left graph is the Nanning red clay, and the right graph is the Nanning red clay improved by the sugarcane residue biochar; Figure 10Schematic diagram of mechanism for improving special soil shrinkage by sugarcane bagasse biochar. DETAILED DESCRIPTION

[0021] Various exemplary embodiments of the present application will now be described in detail, which should be considered to be illustrative of certain aspects, features and embodiments of the present application, but not a limitation of the present application.

[0022] It is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. In addition, for a range of values of, for example, the upper limit and the lower limit of the range are not included in the range unless the context clearly indicates otherwise. Any intermediate value or any other stated or intervening value of any stated value or range not specifically mentioned in this specification is also included in the application. The upper and lower limits of these smaller ranges can be independently included or excluded in the range.

[0023] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although preferred methods and materials are described, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In case of conflict between the content of the specification and that of any document incorporated by reference, the content of the specification prevails.

[0024] Many modifications and variations of the present application described in the specification are possible without departing from the scope or spirit of the application. Other implementations of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. The specification and examples given are exemplary only.

[0025] As used herein, the terms "comprise", "comprising", "include", "including", "have", "having" and the like are open-ended terms that are intended to mean "including but not limited to".

[0026] It should be noted that the present application does not describe in detail the conventional operation means in the art, and is not the focus of the present application.

[0027] The expansive soil used in the following examples of the present application is taken from Baise City, Guangxi Zhuang Autonomous Region; the red clay is taken from Guilin City, Guangxi Zhuang Autonomous Region and Nanning City, Guangxi Zhuang Autonomous Region; other reagents used are commercially available products, and the source of the commercially available products does not affect the technical effects of the present application.

[0028] Example 1 This embodiment provides a method for improving special soil with different amounts of sugarcane bagasse biochar and a test method for shrinkage. Step one: Fresh, mildew, low impurity content of sugarcane bagasse was selected, which was uniformly crushed by a small crusher and then pyrolyzed in a muffle furnace to obtain the test sugarcane bagasse biochar. Specifically, the sugarcane bagasse was dried for 8h and then crushed, and then pyrolyzed in a muffle furnace at 300℃ for 2h, and then cooled and sealed for storage.

[0029] Step two: The swelling soil taken from Baise (Baise swelling soil), the red clay taken from Guilin (Guilin red clay) and the red clay taken from Nanning (Nanning red clay) were dried after being naturally aired, and then crushed by rubber wood hammer, and then sieved through a 2mm sieve, and then dried in an oven, and then cooled and sealed for storage.

[0030] Step three: Different proportions of sugarcane bagasse biochar were added to Baise swelling soil, Guilin red clay and Nanning red clay respectively, and then mixed to obtain improved soil, and then sealed for storage. The ratio of sugarcane bagasse biochar to swelling soil or red clay was calculated based on the mass of dry soil, and the mass fraction of sugarcane bagasse biochar was 0% (control group), 2%, 5% and 10% of the mass of dry soil. The improved soil was mixed and sealed for storage.

[0031] Step four: The improved mixed soil was compacted into a soil cake by static compaction method, the diameter of the soil cake was 39.1mm, and the height was 7.5mm, and then the soil cake was loaded into a saturator, vacuum saturated for 4h, then water was injected and vacuumed for 2h, and finally the soil cake was saturated with deionized water for 3 days.

[0032] Step five: The saturated soil cake was taken out, the height of the soil cake was measured by a dial gauge, and the area of the sample was measured by digital image processing technology, and then the volume of the soil cake in the saturated state was calculated, and then the soil cake was placed in a low-field nuclear magnetic resonance instrument, and the T 2 distribution curve was tested by CPMG analysis sequence to obtain the water distribution of different improved soil.

[0033] Step six: A layer of vaseline was coated on the surface of the porous shrinkage plate, and then the saturated soil cake was placed above the porous plate, and then moved into a supersaturated salt solution containing K2SO4 to carry out the shrinkage experiment.

[0034] Step seven: The mass of the soil cake was measured every 1 day, and when the mass of the soil cake decreased by 0.5g, the soil cake was sealed and stored in a lock box for 3 days, and then the mass and volume of the soil cake were measured again, and then the T 2 distribution curve was tested by a low-field nuclear magnetic resonance instrument to obtain the water distribution evolution rule of the soil cake during the shrinkage process.

[0035] Step eight: When the mass of the soil cake no longer changes, the soil cake is placed in the supersaturated salt solution of NaCl, K2CO3 and LiBr in turn for shrinkage test until the mass of the soil cake no longer changes, and then the test is considered to be completed.

[0036] Figure 1 Shrinkage characteristic curve of Baise expansive soil modified by bagasse biochar; Figure 2 Shrinkage characteristic curve of Guilin red clay modified by bagasse biochar; Figure 3 Shrinkage characteristic curve of Nanning red clay modified by bagasse biochar; Figure 4 Moisture distribution characteristics of Baise expansive soil modified by bagasse biochar; Figure 5 Moisture distribution characteristics of Guilin red clay modified by bagasse biochar; Figure 6 Moisture distribution characteristics of Nanning red clay modified by bagasse biochar; Figure 7 Moisture distribution evolution of Baise expansive soil during shrinkage process modified by bagasse biochar, wherein the left graph is Baise expansive soil, and the right graph is Baise expansive soil modified by bagasse biochar; Figure 8 Moisture distribution evolution of Guilin red clay during shrinkage process modified by bagasse biochar, wherein the left graph is Guilin red clay, and the right graph is Guilin red clay modified by bagasse biochar; Figure 9 Moisture distribution evolution of Nanning red clay during shrinkage process modified by bagasse biochar, wherein the left graph is Nanning red clay, and the right graph is Nanning red clay modified by bagasse biochar; Figure 10 Mechanism diagram of shrinkage of special soil modified by bagasse biochar.

[0037] Figures 1 to 9 In the formula, BC represents the biochar content, e represents the pore ratio of the soil body, w represents the water content of the soil body, represents the water content ratio of the soil body.

[0038] In the formula, the calculation formula of the pore ratio is: , in the formula, e : pore ratio, V v : the volume of the pores of the soil cake in a certain shrinkage state (unit: cm 3 ), V s : the volume of the dry soil (unit: cm 3 ),V t : total volume of soil cake in a certain shrinkage state (unit: cm 3 ), m s : dry mass of soil cake (unit: g), : specific gravity of soil particles (see Table 1 below).

[0039] The formula for calculating the water content ratio is: , wherein : water content ratio, V w : volume of water in a certain shrinkage state of soil cake (unit: cm 3 ), V s : dry soil volume (unit: cm 3 ), : specific gravity of soil particles (see Table 1 below), m t : total mass of soil cake in a certain shrinkage state (unit: cm 3 ), m s : dry mass of soil cake (unit: g).

[0040] Table 1 Initial conditions of soil cake As can be seen from Figure 1 , with the increase of the addition amount of sugarcane residue biochar, the shrinkage characteristic curve of the sugarcane residue biochar modified expansive soil becomes more and more flat, until the addition amount of sugarcane residue biochar reaches 5%, that is, the addition of sugarcane residue biochar can effectively inhibit the shrinkage deformation of special soil, but when the addition amount of sugarcane residue biochar exceeds 5%, its influence on the shrinkability of soil remains unchanged.

[0041] As can be seen from Figures 2 to 3 , when the addition amount of sugarcane residue biochar is less than 5%, the influence of sugarcane residue biochar on the shrinkability of Guilin red clay and Nanning red clay is relatively small, and when the addition amount increases to 10%, the improvement effect of sugarcane residue biochar on the shrinkage characteristics of Guilin red clay and Nanning red clay is significant.

[0042] As can be seen from Figures 4 to 6 , with the increase of the addition amount of sugarcane residue biochar, the peak area of the three special soils increases, which confirms from the microscopic point of view that the sugarcane residue biochar increases the porosity of the special soil.

[0043] As can be seen from Figure 4As can be seen, with the increase of sugarcane bagasse biochar content, the peak area of ​​Baise expansive soil increased by 11%, while the number of small pores and pore throats decreased slightly; however, the number of large pores continuously increased and the pore size decreased. When sugarcane bagasse biochar is added to Baise expansive soil, it replaces the space of the original soil mass, resulting in a reduction of small pores in the expansive soil. The sugarcane bagasse biochar itself has more pores, and the pore size is smaller than the large pores in the Baise expansive soil, thus reducing the pore size. Furthermore, sugarcane bagasse biochar has no significant shrinkage; after being added to the soil, it fills the spaces between aggregates within the soil during water loss and shrinkage, acting as a framework. Although it increases the internal porosity of the soil, it still effectively inhibits soil deformation.

[0044] from Figure 5 As can be seen, with the incorporation of sugarcane bagasse biochar, the peak area of ​​Guilin red clay only increased by 5%. Meanwhile, no significant changes were observed in the micropores, indicating that sugarcane bagasse biochar had no effect on the micropores within the aggregates. However, both the number and size of macropores increased.

[0045] from Figure 6 As can be seen, with the addition of sugarcane bagasse biochar, the peak area of ​​Nanning red clay increased by 13%. Sugarcane bagasse biochar has a significant impact on the shrinkage of Nanning red clay. Since the pore size of sugarcane bagasse biochar is smaller than that of Baise expansive soil, the addition of sugarcane bagasse biochar will cause peak 2 to shift to the left.

[0046] Figures 7 to 9 A comparative analysis was conducted on the shrinkage process of special soil and special soil modified with bagasse biochar during water loss. T 2. Distribution Curves. It can be observed that the addition of bagasse biochar did not alter the drainage pattern of pore water in the samples. Specifically, during the initial shrinkage stage, water was first drained from macropores; as shrinkage continued, water in macropores gradually dried out, followed by water in micropores until the shrinkage process ended. However, the addition of bagasse biochar affected the drainage time of both macropores and micropores. Specifically, the addition of bagasse biochar increased the macropores of the special soil, thus prolonging the drainage process of macropores during the drying shrinkage process. Compared to the special soil, the peak area of ​​the bagasse biochar-modified special soil only began to decrease at lower moisture contents. Furthermore, at the end of the experiment, the peak area of ​​the biochar-modified special soil was also smaller than that of the special soil, and this phenomenon was more pronounced in Baise soil and Nanning red clay. Figure 7 The right image and Figure 9 (See the right figure). This result confirms that bagasse biochar has a significant impact on the micropores of special soils, and the effect is more pronounced on expansive soils with high shrinkage.

[0047] Figure 10The microstructure changes of special soil and special soil amended with biochar during shrinkage deformation were compared. The sugarcane bagasse biochar used in this experiment was derived from sugarcane bagasse, which is rich in cellulose and hemicellulose. After carbonization, the surface of the biochar showed rough and angular structure characteristics. This morphological feature enables biochar to form a close combination with soil particles. The fiber components of biochar tightly link soil aggregates through the lapping effect, enhancing the interlocking effect between biochar and soil, thereby improving the anti-deformation ability of soil during shrinkage. In addition, sugarcane bagasse biochar has negative charge and abundant oxygen-containing functional groups, which makes it adsorbed when added to soil. The interaction between sugarcane bagasse biochar and clay aggregates improves the binding force between aggregates, effectively inhibiting the large deformation of special soil during shrinkage. This improvement enhances the stability of special soil after water loss and reduces the occurrence of cracks and other undesirable deformations. At the same time, the particles of sugarcane bagasse biochar fill the pore space between the internal aggregates of soil, effectively inhibiting the reduction of pore space between aggregates during shrinkage, and significantly improving the shrinkage performance of special soil.

[0048] The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other.

[0049] The above description of disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A low-carbon method for improving the shrinkage properties of special soils, characterized in that, Includes the following steps: Sugarcane bagasse was subjected to oxygen-limited pyrolysis to obtain sugarcane bagasse biochar; After the special soil is naturally sun-dried, it is then dried and mixed with the sugarcane bagasse biochar to obtain the low-carbon improved special soil. The special soil is either expansive soil or red clay.

2. The improved method according to claim 1, characterized in that, The oxygen-limited pyrolysis is performed at a temperature of 250-350℃ for 1-3 hours.

3. The improved method according to claim 2, characterized in that, The oxygen-limited pyrolysis was performed at a temperature of 300°C for 2 hours.

4. The improved method according to claim 1, characterized in that, The amount of sugarcane bagasse biochar added is 2-10% of the mass of the expansive soil.

5. The improved method according to claim 1, characterized in that, The amount of sugarcane bagasse biochar added is 2-10% of the mass of red clay.

6. A method for testing the shrinkage of low-carbon modified special soil, characterized in that, Includes the following steps: S1. The improved special soil obtained by the low-carbon improvement method according to any one of claims 1 to 5 is compacted into soil cakes and then saturated. S2. Weigh and measure the volume of the saturated soil cake, and then test the soil cake in a low-field nuclear magnetic resonance spectrometer. T 2. Distribution curves are used to determine the moisture distribution of the soil cake; S3. Place the soil cake that has completed the S2 process on a porous shrinkage plate and put it into a moisture-retaining tank of supersaturated salt solution of K2SO4 for shrinkage test, and weigh the soil cake every day. S4. When the mass of the clay cake decreases by 0.5g, seal it and store it for 3 days. Then weigh and measure its volume, and place it in a low-field nuclear magnetic resonance spectrometer to test the shrinkage process. T 2. Distribution curve, and then to determine the evolution law of water distribution during the shrinkage process of the soil cake; S5. When the mass of the clay cake no longer changes, place the clay cake in a humidified tank containing supersaturated salt solutions of NaCl, K2CO3, and LiBr in sequence to conduct shrinkage tests until the mass of the clay cake no longer changes, at which point the test ends.

7. The test method according to claim 6, characterized in that, The compaction method is static compaction; the compacted soil cake has a diameter of 39.1 mm and a height of 7.5 mm.

8. The test method according to claim 6, characterized in that, The saturation process includes: first, vacuum saturation for 4 hours, then water injection and vacuum saturation for 2 hours, and finally static saturation with deionized water for 3 days.

9. The test method according to claim 6, characterized in that, The porous shrink plate is coated with petroleum jelly.

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