Study on Strength Development Mechanism of Organic Soil in Dianchi Lake, China

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Introduction
Since the 21st century, due to the rapid development of urbanization and the continuous increase of social demands, urban construction has continued to develop toward the city's edge. Many projects must be located in various areas, such as expansive soil, peat soil, and frozen soil [1,2]. Peat soil is a kind of soft soil with unique properties [3], with high organic matter content, large void ratio, low bearing capacity, high water content, and low permeability [4,5]. Te unique properties of peat soil have brought many adverse efects to the actual engineering construction. How to build projects in these areas has become a widely concerned issue for many scholars and engineers.
Te organic matter content in peat soil is high, and the humic group (HG) is the main component of organic matter [3]. Te solubility diference of HG in acid and alkali solutions can be divided into three categories: humic acid (HA), which is only soluble in alkali solution but not in acid solution. Fulvic acid (FA) is soluble in acid and alkali, and humin is insoluble in any solution. HG contains a variety of active oxygen-containing functional groups, such as carboxyl, phenol hydroxyl, alcohol hydroxyl, methoxy, and ether [6], making HG's properties very complicated. It was found from research and practice that HG can interact with metal ions, oxides, and clay minerals, in peat soil, thereby changing its physicalchemical properties.
In 1987, Hobbs [7] pointed out that peat soil's organic matter content and the humifcation degree signifcantly impact the soil's structure and mechanical characteristics. In 1998, Lan et al. [8] studied the efects of HA on the decentralization of the soil. Te results show that HG has a signifcant dispersal efect on the soil. After removing the hydrogen peroxide solution, the decentralization of the mucous particles decreases signifcantly. In 2004, Chirdchanin et al. [9] studied the engineering properties of organic soil through laboratory tests. Te results show that HG can improve the compressibility of soil but reduce the water permeability and shear strength of soil. In 2007, Duraisamy et al. [10] proposed that the increase of HG in peat soil will cause the change in soil shear strength; Yan et al. [11] studied the physical and mechanical properties of peat soil through experiments. Te results show that peat soil's physical and mechanical properties are afected by many factors, and organic matter is the most crucial factor. With the increase of organic matter content in peat soil, the compressibility increases, the consolidation coefcient decreases, and the permeability decrease. In 2019, Zou et al. [12] studied the efect of HG on soil-available cadmium (Cd) through adsorption experiments, scanning electron microscopy, infrared spectroscopy, and other experimental methods. Te results show that HG has activation and passivation efects on Cd, and based on these efects, it can signifcantly change soil's physical and chemical properties. In 2021, Liu et al. [13] found that adding HG to montmorillonite would change montmorillonite's morphology. In 2021, Yue et al. [14] studied the efect of organic matter on the soil-water characteristics of peat soil by artifcially preparing peat soil with diferent organic matter content. Te results show that organic matter signifcantly afects peat soil's soil-water characteristic curve, and peat soil's water-holding capacity increases with increased organic matter content. In 2022, Cao et al. [15,16] were used to simulate the peat soil environment (PES) by mixing HA reagent into cohesive soil and soaking it in an FA solution. Te test result shows that the PES with diferent HA and FA contents can simulate by this method, but soaking samples in FA solution cannot reach the actual efect.
In summary, many scholars have conducted much research on the efect of HG on the engineering properties of peat soil. However, scholars mainly study HG as a whole, ignoring that HG is a complex mixture composed of HA, FA, and humin. Diferent components of HG may have diferent efects on the engineering properties of organic soil. On the other hand, fewer scholars have studied the efect of HG on the strength of organic soil. Terefore, this paper studies the efect of diferent components in HG on the strength of organic soil. Te results have signifcant theoretical value and guiding signifcance for practical engineering.
In natural peat soil, humin, humic acid (HA), and cohesive soil particles together form the soil skeleton in the form of solid particles. Te chemical structure and properties of humin are similar to those of HA, but humin is an inert humic group (HG) with relatively stable chemical properties [19,20] and can exist in the soil environment for a long time. Compared with HA and humin, fulvic acid (FA) exhibits stronger reducing and complexing ability due to more acidic oxygen-containing functional groups such as carboxyl. To sum up, considering the chemical characteristics of diferent components in HG, HA reagent, and FA reagent selected as the test materials in this paper. Tis paper adopts the blending and steeping methods to conduct the simulation test according to the diference in the existing form and solubility of HA and FA in peat soil. In this method, HA reagent is mixed into cohesive soil with low organic matter content to prepare samples and then soaked in FA solution.
According to the previous research results, the total content of HG in the peat soil of Dianchi Lake is between 7.15% and 50.06%. Among them, the content of HA is between 2.36% and 28.13%, and the content of FA is between 0.79% and 8.34% [16]. Te results provide the basis for incorporating HA and FA reagents in the samples.

Materials.
In order to consider the infuence of natural humic acid on the test results, the undisturbed soil used in the test is alluvial-proluvial cohesive soil with low humic acid content (0.32%). Te classifcation method of alluvialproluvial cohesive soil is based on the "Standard for engineering classifcation of the soil" (GB/T 50145-2007) [17]. Te undisturbed soil is taken from Sanhe Village, Xinjie Town, Jinning County, Kunming City, which is brownishyellow and brownish-red. According to the "Standards for Geotechnical Test Methods" (GB/T50123-2019) [21], the undisturbed soil is tested indoors. Its physical and mechanical properties are shown in Table 1. Its compressive modulus is 4.65 MPa.
Tianjin Guangfu Chemical Reagent Factory produces the HA reagent. Te FA reagent is prepared by purifying young lignite in Yunnan through hydrogen peroxide degradation. Te test water is distilled water. Each test material is shown in Figure 1.

Sample Preparation.
Te test is carried out according to the "Standards for Geotechnical Test Methods" (GB/T50123-2019) [21]. First, the undisturbed soil used in the test is crushed, air-dried, and then passed through a sieve with an aperture of 2.00 mm. Ten, mix the test soil, humic acid reagent, and water evenly and put it into a three-part mold (the inner diameter of the three-part mold is d � 39.10 mm, and the height h � 80.00 mm). Finally, the mixed material in the mold is compacted to make a sample, and after the mold is removed, it is wrapped with PVC for curing. Te sample preparation process is shown in Figure 2. To ensure the accuracy of the test, the moisture content (ω) � 24%, and the void ratio (e) � 0.8.
After the sample preparation is completed, soak the sample in FA solution. Since FA is acidic, the higher the concentration of the prepared solution, the lower the pH value. Te test controls the concentration of FA by controlling the pH value of the solution. Te pH value of the FA solution is measured with an electronic pH tester, and the pH value of the FA solution is kept constant by adding a FA reagent [22].

Analysis of the Efect of Humic Acid on the Strength of
Organic Soil. To preliminarily analyze the efect of humic acid (HA) on the unconfned compressive strength (UCS) of organic soil, the samples soaked in distilled water (pH value: 7.0) for the same time are regarded as the same group of samples for comparative analysis. Figure 3 shows the relationship between the UCS of the sample soaked in distilled water simultaneously and the HA reagent's content. Te UCS continuously decreased with the increased content of HA reagent added to the samples immersed in distilled water simultaneously. In addition, this study refers to some novel techniques to predict complex engineering problems [23][24][25]. Analysis of the mechanism: the surface of clay particles is usually negatively charged. Under the action of electrostatic attraction, clay particles will adsorb cations to their surface. Te electric double layer of clay particles comprises the charged layer (adsorption layer) on the surface of clay particles and the ionic layer (difusion layer) formed by cations on the surface of clay particles. Te efect of HA on the sample's UCS is based on a series of physicalchemical interactions of clay particles, HA, cations, water, and other substances. HA has a signifcant molecular weight (compared with fulvic acid) and a complex molecular structure. A variety of oxygen-containing active functional  Advances in Materials Science and Engineering groups, such as carboxyl, alcohol hydroxyl, and methoxy, are combined on the aromatic core structure of the HA monomer [7]. Te presence of these functional groups makes HA show high surface reaction activity. When the samples containing HA are immersed in distilled water, the molecular structure and functional groups of HA are wetted by water, showing higher surface reactivity. Te above makes HA adsorb on the surface of clay particles by relying on its various oxygen-containing acidic functional groups, react with high-valent cations on the surface of clay particles, reduce the valence of cations, and weaken the electrostatic attraction between cations and negative charges on the surface of soil particles. Tis phenomenon leads to an increase in the thickness of the diffusion layer of soil particles. Te bond between soil particles is weakened [26]. On the other hand, HA particles are smaller than most clay mineral particles and have a more developed electric double layer than clay mineral particles. Tis structural feature determines the strong adsorption of HA particles. If there is a specifc content of HA in the cohesive soil, the HA adsorbed on the clay particles' surface will enhance the cohesive soil's dispersibility [8]. It hinders the coagulation between clay particles and causes the deterioration of the physical properties and structure of the soil. Finally, HA can signifcantly reduce the UCS of the sample and will continue to decrease with the increased content of the HA reagent incorporated.
To preliminarily analyze the long-term efect of HA on the UCS of organic soil, Figures 4(a) and 4(b) show the relationship between the sample UCS soaked in distilled water and the soaking time. With the increase in soaking time, the UCS of the sample without HA gradually increased, and the sample containing HA decreased slightly. Analysis of the mechanism: the samples in this test are all remodeled soil samples. With the increase in soaking time, the recovery of the connection structure between soil particles makes the UCS of the samples gradually increase slightly. However, the HA adsorbs with the clay particles [27,28], reducing the valence of high-valent cations on the surface.
Moreover, the electrostatic attraction between the cations and the negative charges on the surface of the clay   particles gradually weakened. Te thickness of the electric double layer of the clay particles increases, the coagulation between them is weakened, the dispersibility is enhanced [8], and the recovery of the interparticle connection structure is inhibited. Te soil structure gradually deteriorated with the increase in soaking time, and fnally, the UCS of the samples containing HA decreased slightly with the increase in soaking time.

Analysis of the Efect of Fulvic Acid on the Strength of
Organic Soil. To preliminarily analyze the efect of fulvic acid (FA) on the UCS of organic soil, the samples without HA soaked for the same time are regarded as the same group of samples for comparative analysis. Figure 5 shows the relationship curve between the sample UCS without HA and the pH value of the FA soaking solution with diferent pH values simultaneously after soaking in the FA solution. When the sample without HA is soaked for the same time, the sample UCS continues to decrease with the increased pH value of the FA solution. Analysis of the mechanism: FA and HA are amorphous, polydisperse polymer organic mixtures. FA is an essential part of HG, and its molecular structure contains many oxygen-containing functional groups, such as carboxyl, phenolic hydroxyl, and methoxy [7]. Compared with HA, FA is soluble in both acidic and alkaline solutions and has a lower relative molecular weight (generally less than 2000). Moreover, the content of acidic functional groups such as the carboxyl group and phenolic hydroxyl group of FA is higher than that of HA [29][30][31], showing more apparent weak acidity and higher surface reactivity than HA.
When the sample is soaked in the FA solution, the FA will invade the pores of the cohesive soil with the soaking solution depending on its small molecular structure and low molecular weight. It has various efects, such as coordination exchange, ion exchange, cation bridge, and hydrogen bond with the surface of clay particles [27,28]. Te clay particles can adsorb the free FA in the pore water through the above two or more reaction forms. FA is wrapped on the skeleton of the sample soil by adsorption, and changes from a free state to a bound state, forming a stable organic-inorganic composite together with the clay particles, and due to the colloidal properties of FA itself [32] the colloidal connection between clay particles [26].
On the other hand, the FA in the soaking solution invades the sample's interior and can remain as a fller in some pores inside the sample. Te pore size of some pores in the sample is reduced, and the structure's connection is strengthened. At the same time, as the pH value of the solution decreases, the concentration of FA in the solution increases, and the content of FA that penetrates the pores of the sample increases as the soaking solution increases   Advances in Materials Science and Engineering 5 [27,28]. Moreover, the decrease in pH value will weaken the protonation of acidic functional groups in the FA monomer [33], resulting in the enhancement of the hydrophobicity of some nonpolar components in the FA, weakening the water solubility and the electrostatic repulsion in the organic matter-mineral system will decrease accordingly. Te results favor the adsorption of FA, and fnally, FA can signifcantly enhance the sample UCS. Furthermore, it continued to increase with the increased concentration of FA (that is, the decrease of the pH value of the solution).
To preliminarily analyze FA's long-term efect on organic soil's UCS, Figure 6 shows the relationship between UCS and soaking time of samples without HA when soaked in FA solution with diferent pH values. It can be concluded from Figure 6 that (1) [27,28]. Terefore, it adsorbs on the surface of clay particles and relies on its colloidal properties to connect the clay particles [26]. Moreover, the FA remaining in the sample pores can reduce the pore size of some pores inside the sample and strengthen the connection of the sample structure.
On the other hand, the pH value of the solution is controlled to be constant in this experiment, and the FA reagent is continuously supplemented in the soaking solution. Te interaction between the FA and the clay particles inside the sample continued with the increase in immersion time. However, due to the limited adsorption capacity of clay particles to HG, and the adsorption of FA preferentially occurring on the surface of clay particles at the edge of pores in the sample, FA can continuously invade the sample in the early stage of immersion and preferentially react with clay particles that are easy to bind at the edge of pores. However, with the increase in soaking time, the adsorption reaction between FA and the surface of the clay particles at the edge of the inner pores of the sample gradually tends to be saturated, and the number of binding sites on the surface of the clay particles that can produce adsorption decreases. Moreover, the clay particles far away from the pores are difcult to contact with FA and cause adsorption, making it difcult for the clay particles far away from the pores in the sample to adsorb FA. FA gradually flls the internal pores of the sample, resulting in a decrease in the pore size of some pores inside the sample, which prevents the FA in the solution from invading the sample to a certain extent. Terefore, with the increase of soaking time, the sample UCS soaked in FA solution increases rapidly at frst and then slightly. , it is found that (1) for the samples immersed in the FA solution with the same pH value for the same time, the UCS gradually decreases with the increase of the content of HA reagent added. Moreover, the sample UCS decreased rapidly with the addition of HA reagent from 0% to 10%. Moreover, the decrease rate of UCS decreased when it increased from 10% to 25%. (2) When the content of HA reagent is increased in the range of 0%∼10%, the samples immersed in FA had a more signifcant UCS drop rate than those immersed in distilled water. When the HA reagent content increased from 10% to 25%, the samples immersed in FA solution had similar UCS decline rates to those immersed in distilled water. Analysis of the mechanism: when the sample is immersed in FA solution, the hydrophobicity of HA is enhanced in an acidic environment. Moreover, the electrostatic repulsion between HA and the clay particles in the sample is weakened. Compared with the samples soaked in distilled water, the HA in the samples soaked in FA has a higher surface reactivity. As a result, the adsorption capacity of the clay particles to HA increased. Moreover, the efect of HA on dispersibility is enhanced. Finally, the UCS of the sample decreases rapidly with the increased content of HA  Advances in Materials Science and Engineering reagent from 0% to 10% and has a more efective UCS decrease rate than that of the sample immersed in distilled water.

Analysis of the Efect of Humic
However, the adsorption capacity of clay particles for HA is limited. When the HA reagent content increases from 10% to 25%, the bound HA gradually increases and tends to be saturated. In addition, the FA in the soaking solution will continue to invade the sample's interior and cause adsorption to the clay particles. Te adsorption phenomenon between clay particles and HA gradually tends to be stable, and some HA that is difcult to combine with clay particles remains free and even precipitates from the sample. Te adsorption phenomenon between clay particles and HA gradually tends to be stable, and some HA that is difcult to combine with clay particles remains free and even precipitates from the sample. As a result, the UCS decline rate decreased with the increase of the HA reagent from 10% to 25%. Te UCS decline rate is similar to the samples immersed in distilled water.
To accurately analyze the long-term efect of HG on the UCS of organic soil, Figures 8-10 show the relationship curve between the sample UCS and the soaking time when the sample is immersed in the FA solution. Moreover, mark the soaking time when the sample UCS reaches the peak value in the fgure. It can be concluded from Figures 8-10 that (1) with the increase in soaking time, the sample UCS with HA reagent content less than 5% continued to increase and gradually became stable. (2) When the pH value of the FA soaking solution is fxed, with the increased content of HA reagent added, the sample UCS begins to decrease at an earlier soaking time. When the content of the HA reagent is the same as the increased concentration of FA, the UCS curve of the sample showed a decreasing trend, and there was an apparent lag.

Analysis of the Mechanism
(1) Since the efects of HA and FA are signifcantly diferent, the UCS change with the increase of soaking time results from the joint action of HA and FA. HA is the skeleton of the sample soil, can be adsorbed on the surface of the clay particles, and inhibit the recovery of the connection structure between the alluvial clay particles through a series of physical-chemical actions. Terefore, HA can make the sample UCS slightly decrease with the increase of soaking time. Soluble FA penetrates some of the pores inside the sample through the soaking liquid to connect the clay particles. Moreover, the FA remaining in the sample's pores can reduce the pore size of some pores inside the sample and enhance the structural connection of the sample. In this test, to control the soaking solution's pH value to be constant, the soaking solution is continuously supplemented with an FA reagent. Te interaction between the FA and the clay particles inside the sample continued and gradually became stable with the increase in soaking time. Terefore, with the increase in soaking time, the FA makes the sample UCS increase rapidly and then increase slightly. (2) Te content of HA reagent in the sample is less than 5%, and the efect of HA on the recovery of the cohesive soil's interparticle bonding structure and the FA's electrostatic repulsion in the pore water is insignifcant. In addition, less HA is adsorbed on the surface of the clay particles, and the FA in the soaking solution can continuously penetrate the sample's interior and combine with the clay particles. Te FA gradually exerts the bonding efect between the clay particles. [34] Terefore, the sample UCS continues to increase with immersion time and gradually tends to be stable. (3) When the content of HA reagent in the sample is more than 5%, in the early stage of soaking (Stage 1), FA continuously invades the inside of the sample through the soaking liquid, causing colloidal bonding between the clay particles inside the sample. Compared with the efect of HA on the dispersion of clay particles, the bonding efect of FA on the clay particles inside the sample is more signifcant, and the efect of FA on the sample UCS is dominant. However, as the soaking time increases (Stage 2), the adsorption phenomenon between the FA in the pore water and the clay particles inside the sample gradually tends to be saturated. HA can continuously inhibit the recovery of the connection structure between soil particles, resulting in the enhancement of the dispersibility of the soil skeleton of the sample [35][36][37]. Finally, the UCS curve of the sample with the HA reagent content greater than 5% showed two stages of apparent increase frst and then decrease. (4) By increasing the content of the HA reagent, the clay particles will be adsorbed and combined with more HA. On the one hand, it occupies a binding site where FA can bind to clay particles. On the other hand, the electrostatic repulsion between HA and FA is enhanced. As a result, the FA in the pore water is challenging to adsorb on the surface (containing HA particles). Moreover, the enhancement efect of the FA on the sample UCS is weakened. Ultimately, the samples with higher incorporation of HA reagent began to show a decreasing trend at earlier soaking age. By increasing the concentration of FA in the soaking solution, the content of FA in the pore water of the sample can be increased so that the clay particles inside the sample can adsorb more free FA. Te bound FA encapsulated on the soil skeleton increases, and the colloidal bonding efect between clay particles (containing HA particles) produced by FA is enhanced [26]. Finally, with the increased concentration of FA (the decrease of the pH value of the soaking solution), the sample UCS's tendency to decrease caused by HA shows a signifcant hysteresis efect.

Analysis of the Efect of Humic
Acid on the Microstructure of Organic Soil. Figures 11(a)-11(f ) show the scanning electron microscope (SEM) images of samples with diferent HA contents immersed in distilled water for 90 days at a magnifcation of 2000 times (test group 1). By observing Figure 11(a), it can be concluded that in this test, the internal structure of the alluvial cohesive soil sample is well connected, the pore size of the pores is small, and the connectivity between the pores is poor. Comparing  Figures 11(a)-11(f ), it can be found that when the content of HA reagent in the sample increases in the range of 5% to 25%, the internal pore size of the sample gradually increases. Te pores are gradually connected, and the soil particles in the sample are gradually overhead, forming an apparent loose overhead skeleton, and the connection of the sample structure is weakened. Te reason is that HA can react with high-valent cations on the surface of clay particles, reducing the cation valence. Te electrostatic attraction between the cations and the negative charges on the surface of the clay particles weakens, the thickness of the difusion layer increases, and the connection between the soil particles weakens [26]. On the other hand, the HA particles will be adsorbed on the clay particles' surface, increasing the clay's dispersibility [8]. It hinders the coagulation between clay particles and causes the deterioration of the physical properties and structure of the soil. Figures 12(a)   Advances in Materials Science and Engineering the pH value of the soaking solution decreases, the pore size of the pores inside the sample gradually decreases, and the connectivity of the pores gradually weakens. Te coupling of the specimen structure is strengthened. Te reason is that FA can penetrate the sample's pores with the soaking liquid and gradually wrap on the soil skeleton through adsorption.  Small pore size Poor pore connectivity (a) No obvious connection of pores (b) Pore size starts to increase and start connecting (c) Te pore size gradually increases and clearly connected Te pore is clearly connected Connected pores Moreover, due to its colloidal properties [32], colloidal connections are generated between clay particles (containing HA particles) [26]. On the other hand, the soluble FA particles will be flled into pores inside the sample, and the pore size inside the sample will be reduced.

Conclusion
Tis paper uses mixing and infltration methods to simulate the peat soil environment, and humic acid (HA)'s long-term infuence trend on organic soil's strength is analyzed through the unconfned compressive strength (UCS) test. Te efect of the humic group (HG) on the microstructure of organic soil is investigated by scanning electron microscopy (SEM) test. Te infuence mechanism of HG on the engineering properties of organic soil is preliminarily revealed, and the following conclusions are obtained: (1) HA can signifcantly reduce the UCS of the sample, it continues to decrease with the increase of the content of HA reagent added, and the UCS of the sample decreases slightly with the increase in soaking time. Tis test phenomenon is because the HA adsorbs with the clay particles, reducing the valence of highvalent cations on the surface. Moreover, the electrostatic attraction between the cations and the negative charges on the surface of the clay particles gradually weakened. Te thickness of the electric double layer of the clay particles increases, the coagulation between them is weakened, the dispersibility is enhanced, and the recovery of the interparticle connection structure is inhibited. (2) Fulvic acid (FA) can signifcantly enhance the UCS of the sample, it continues to increase with the concentration of FA, and the UCS of the sample increases rapidly at frst and then slightly increases with the gradual increase of immersion time. Tis test phenomenon is because the FA in the soaking solution invades the sample's interior and can remain as a fller in some pores inside the sample. Te pore size of the sample is reduced, and the structure's connection is strengthened. Moreover, the decrease in pH value will weaken the protonation of acidic functional groups in the FA monomer, resulting in the enhancement of the hydrophobicity of some nonpolar components in the FA, weakening the water solubility and the electrostatic repulsion in the organic matter-mineral system will decrease accordingly. Te results favor the adsorption of FA. (3) Under each HG environment, the UCS of the sample decreased rapidly when the HA reagent increased from 0% to 10%. Moreover, the decrease rate of the UCS decreased when it increased from 10% to 25%.
Larger pore size Pore connectivity is obvious (a) Pore size reduction Weakened pore connectivity (b) Pore size is signifcantly reduced Pore connectivity is not obvious When the content of the HA reagent is less than 10%, the infltration of FA can signifcantly increase the UCS of the sample, and when it is more than 10%, the growth rate of the sample UCS decreases. With the increase in immersion time, the UCS of the sample with the HA reagent content of not more than 5% continued to increase and gradually became stable. Te intensity curve of the samples with the HA reagent content of more than 5% showed two stages of frst increase and then decreased. (4) Te addition of HA can signifcantly increase the pore size of the sample, improve the connectivity between the pores, and weaken the connection of the sample structure. Te infltration of FA can reduce the pore size of the sample, weaken the connectivity between the pores, and strengthen the connection of the sample structure.

Data Availability
Te data used to support the fnding of this study are included in the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest.