1. Introduction
With the increasing prevalence of infrastructure construction, clay with significantly varying organic matter content is frequently encountered. In particular, the organic matter content of peat-rich soils can exceed 10%, and, in some cases, the organic matter content of peat may reach levels close to 30% to 98% [
1]. Such organic-rich clays often lead to issues in engineering, including excessive foundation settlement and failures in cement–soil mixing processes [
2]. Concurrently, the large-scale extraction of groundwater, coupled with environmental factors such as extreme droughts and floods, has caused dramatic fluctuations in groundwater levels in certain regions, placing organic clay in an unsaturated state. Additionally, the organic matter within these organic soils generates biogases, such as methane and carbon dioxide, during degradation [
3], further contributing to the unsaturated condition of the organic matter layers. Therefore, studying the water retention characteristics of organic clay will significantly advance the treatment of organic clay layers.
In engineering practice, soil–water characteristic curve tests are commonly employed to investigate the water retention properties of soil [
4,
5,
6,
7]. By using a soil–water characteristic instrument to determine the relationship between the matrix suction and moisture content of the soil, the soil–water characteristic curve can be plotted. This curve serves as a fundamental tool for analyzing the relationships between the physical and mechanical parameters of unsaturated soils, and it is of significant importance for studying the water retention, deformation, strength, and permeability of unsaturated soils [
8]. W.P.M. Black [
9] conducted a comparative study on the mineral composition’s influence on the soil–water characteristic curves of Columbia sand and Palontina clay. The characteristic curve of the sand exhibited a steep decline, indicating that water is rapidly expelled at lower moisture contents. In contrast, the characteristic curve of the clay was relatively gentle, demonstrating a stronger water retention capacity and slower drainage rates at higher moisture contents. J.M. Fleureau et al. [
10] focused on the soil–water characteristics of Jos-signy silt and FoCa red clay, deriving their respective characteristic curves. Their research highlighted the behavior of compacted clay along drying and wetting paths, demonstrating the reversibility of these paths, which is crucial for understanding soil behavior under varying moisture conditions. Sun et al. [
11] successfully obtained the soil–water characteristic curve for compacted red clay from Guilin across the entire suction range by employing a combination of techniques, including the pressure plate method, filter paper method, and saturated salt solution vapor equilibrium method. The results from these three methods exhibited good consistency, providing a practical approach for measuring soil–water characteristic curves throughout the full suction range Gui et al. [
12] utilized the filter paper method to measure the soil–water characteristic curve of highly decomposed peat soil, demonstrating its strong water retention capabilities, which are essential for applications in wetland management and agricultural practices. Liu et al. [
13] investigated the soil–water characteristic curve of saline soils, exploring the effects of compaction and salt content on the characteristics of the soil–water curve. Their findings contribute to a better understanding of how salinity and compaction influence soil behavior, particularly in arid and semi-arid regions. Cai et al. [
14] conducted an in-depth study on the soil–water characteristics of sandy loess, utilizing scanning electron microscopy to reveal the mechanisms by which microstructural characteristics of pores affect soil–water properties. This research underscores the importance of soil microstructures in determining water retention. Sreedeep et al. [
15] measured the permeability suction of marine soils using the pressure plate method and chilled mirror dew point technology, emphasizing the significant impact of permeability suction on soil properties. Their work highlights the complexities of soil behavior in marine environments, which is critical for coastal engineering applications. Collectively, these studies provide valuable insights into the water retention properties of various soil types, enhancing our understanding of soil behavior under different environmental conditions and informing practical applications in geotechnical engineering [
16,
17].
However, there has yet to be a comprehensive analysis of how organic matter affects water retention. Research has found a pattern for water retention curves with organic matter; for instance, Yang et al. [
18] demonstrated that organic matter in silt soils enhances soil water retention, while Ebel et al. [
19] indicated that organic matter is a major factor influencing soil water retention. Organic clay is a multiphase porous medium, and its engineering properties are characterized by complexity and variability. This complexity and variability are primarily controlled by its micro- or mesostructure, making the study of its microstructure significantly important [
20]. Due to current limitations in testing instruments and technologies, research on the microstructure of unsaturated soils is restricted, focusing mainly on the testing of micro-pore size and distribution. The primary methods for testing the micro-pores of geotechnical porous media include CT scanning, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) [
21,
22,
23,
24]. The CT testing method can non-destructively obtain the pore distribution of geotechnical media in three-dimensional space; however, the resolution of CT images is relatively low, leading to lower accuracy. The nuclear magnetic resonance method utilizes the hydrogen nuclei of water in different states within unsaturated soils, which produce varying transverse relaxation times under different radiofrequency magnetic fields. However, this method is highly random and costly. In contrast, scanning electron microscopy and mercury intrusion porosimetry are mature techniques with lower costs, allowing for the analysis of the distribution of pores and pore size distribution patterns in soils.
In the literature, research on the water retention properties of organic clay is relatively scarce, and the mechanisms by which organic matter influences its water retention are not well understood. Therefore, this study employed three different methods—the pressure plate method, filter paper method, and vapor equilibrium method—to measure the soil–water characteristic curves of organic clay across the entire suction range. The aim is to gain a deeper understanding of the relationships between suction, moisture content, saturation, and organic matter content in organic clay. Additionally, SEM and MIP tests were conducted to analyze the microstructural characteristics of soils with varying organic matter content, such as the distribution of organic matter within soil pores and the patterns of pore size distribution, thereby investigating the mechanisms by which organic matter affects the water retention properties of clay. Simultaneously, organic clay plays a crucial role in various engineering applications, including geotechnical engineering and environmental engineering. Understanding its water retention properties is essential for predicting its behavior under different conditions and optimizing its utilization in these applications.
2. Materials and Methods
2.1. Experimental Materials
The composition of natural soils is complex and variable, with significant randomness and variability in mineral and organic matter content, making it quite challenging to study the effects of organic matter in isolation. To address this issue, this study prepared soil samples using an artificial mixing method. Red clay was used as the matrix, and natural peat was added to effectively control the organic matter content in the samples. The red clay was obtained from the Chenggong campus of Kunming University of Science and Technology. The peat samples were collected from a wetland park on the northern shore of Dianchi Lake in Kunming, Yunnan Province, at a depth of approximately 0.3 m. This peat has a high degree of decomposition, with a residual fiber content of about 4.5% and organic matter content exceeding 97%, classifying it as highly decomposed peat [
25]. Following the relevant standards outlined in JTG 3430—2020 “Test Methods of Soils for Highway Engineering” [
26], the basic physical properties of the soil samples were tested, focusing primarily on the determination of the liquid limit, plastic limit, particle-specific gravity, and specific surface area. The liquid limit represents the upper moisture content at which fine-grained soil remains in a plastic state, while the plastic limit indicates the lower moisture content for the same state. These parameters provide essential criteria for classifying the engineering properties of soil and determining its condition. The particle-specific gravity is defined as the ratio of the weight of soil particles dried to a constant weight at temperatures between 105 °C and 110 °C to the weight of an equal volume of water at 4 °C. The specific surface area refers to the total surface area per unit mass of the material. The liquid and plastic limits were measured using a cone penetrometer, the specific gravity was determined using the specific gravity bottle method, and the specific surface area was assessed using the methylene blue method. Additionally, the mineral composition of the test soil samples was determined by the Testing and Analysis Center of Yunnan Province. The experimental soil samples are shown in
Figure 1, and the various indicators are presented in
Table 1.
Through X-ray diffraction (XRD) mineral analysis (
Figure 2a), the main mineral components of red clay were found to be kaolinite, gibbsite, hematite, and quartz. The total content of kaolinite, gibbsite, and hematite reaches as high as 82.1%. The primary chemical compositions of these three minerals are Al
2Si
2O
5(OH)
4, Al(OH)
3, and Fe
2O
3, respectively.
Figure 2b presents the XRD mineral analysis spectrum of the peat. Analysis reveals that the mineral composition of the soil sample is primarily illite and quartz, with illite constituting 76.5% and quartz accounting for 23.5%. The main chemical components of these two minerals are SiO
2 and Kal
2(Si
3Al)O
10(OH)
2, respectively.
2.2. Sample Preparation
Six groups of soil samples with different organic matter contents (4.15%, 8.89%, 20.57%, 32.27%, 41.65%, and 51.14%) were prepared, corresponding to the labels S1, S2, S3, S4, S5, and S6. Additionally, a standard sample of red clay was prepared, labeled Y1. The preparation process is as follows:
- (1)
Treatment of red clay: ① The red clay was air-dried under sunlight, with the moisture content measured every 24 h until a stable moisture level was achieved. ② Following air drying, the red clay was pulverized using a hammer, and subsequently processed through a soil grinder to obtain a particle size that passed through a 0.5 mm sieve. ③ The ground red clay was then placed into a large crucible, where it was immersed in a hydrogen peroxide solution and heated to boiling using a laboratory electric furnace. This step was conducted to eliminate organic matter and other impurities present in the red clay. ④ After boiling the red clay solution, the electric furnace was turned off, and upon cooling of the crucible, the waste liquid was decanted into a waste container. Deionized water was added repeatedly for washing and filtering to remove any residual hydrogen peroxide from the red clay. ⑤ The filtered red clay was subjected to the same treatment as described in Steps 1 and 2. ⑥ Finally, the moisture content of the red clay was measured.
- (2)
Treatment of peat: ① A quantity of peat soil was collected and fragmented using a soil knife, then evenly spread in a rectangular tray for subsequent processing. ② Using tweezers, visible remnants such as leaves and stems were meticulously removed from the peat. ③ The processed peat was then thoroughly mixed using a soil mixer to ensure homogeneity. ④ Following the mixing process, the moisture content of the peat was measured.
- (3)
Preparation of experimental soil samples: ① Based on the target organic matter content and moisture level of the experimental soil samples, the required masses of red clay and peat, as well as the volume of water, were calculated accordingly. ② The necessary quantities of red clay and peat were accurately weighed using an electronic balance and placed into separate soil mixing containers for later use. ③ The red clay and peat were then combined and thoroughly mixed using a high-speed blade mixer. During the mixing process, a calibrated spray bottle was employed to gradually apply deionized water until the total volume of water added matched the calculated requirement. ④ The prepared soil samples were sealed in plastic bags and stored in a cool, shaded environment for no less than 90 days to facilitate the thorough integration of mineral soil and organic matter. It is noteworthy that drying (air-drying) may lead to irreversible damage to the “active” substances present in the peat.
The samples were prepared using the single-layer compaction method, controlling the dry density to ensure that the void ratio for each group of samples was consistent at e
0 = 1.5. The specific preparation process is as follows: ① Adjust the initial moisture content of all soil samples to
w0 = 45%. ② Measure the particle-specific gravity (Gₛ) of the soil samples with different organic matter contents. ③ Substitute the initial moisture content, void ratio, specific gravity, and other parameters of the soil samples into Equation (1) [
27] to calculate the sample density, and then use Equation (2) to determine the mass of soil required for sample preparation. ④ Select the appropriate ring knife compaction apparatus and ring knife, assemble them in order, and then use an electronic balance to weigh the corresponding mass of soil to be placed into the compaction mold, followed by vacuum saturation.
In the equations, ρ is the density of the soil (g/cm3), w is the moisture content (%), e is the void ratio, Gₛ is the specific gravity of the soil particles, ρw is the density of water (g/cm3), m is the mass of the soil sample (g), and v is the volume of the ring knife (cm3).
2.3. Water Retention Characteristics Testing
To obtain the soil–water characteristic curve (SWCC) of organic matter clay across the entire suction range, three testing methods were employed. The pressure plate method and filter paper method were used to test the soil–water characteristic curve in the low-suction stage, while the saturated salt solution vapor equilibrium method was used to test the soil–water characteristic curve in the higher-suction stage.
2.3.1. Pressure Plate Method Testing
Prepare compacted samples with a diameter of 61.8 mm and a height of 20 mm, and then perform vacuum saturation. The prepared soil samples were placed in the pressure membrane apparatus, and the desired suction values were adjusted. Each suction value was maintained for 48 h. The specific suction path is shown in
Table 2. To reduce errors, multiple tests (
n = 4) were conducted for each suction value.
2.3.2. Filter Paper Method Testing
Three pieces of filter paper were placed at the interface of the prepared soil sample. The top and bottom filter papers served as protective layers, while the middle filter paper was used for testing. The soil sample was placed in a sealed container, which was wrapped with cling film to prevent moisture loss. The specific experimental procedure follows the methodology outlined in reference [
28]. In this study, Whatman No. 42 filter paper was selected as the key material. To reduce errors, multiple tests (
n = 4) were conducted for each suction value. For the determination of the calibration curve, the bilinear calibration curve equation proposed by Leong [
29], based on their experimental results, was referenced, namely
In the equation, Sm represents the matric suction and Wf indicates the water content of the filter paper.
2.3.3. Vapor Equilibrium Method Test
During the experiment, 11 types of supersaturated salt solutions listed in
Table 3 were first prepared and poured into the bottom of the desiccator. The prepared soil samples were then cut into blocks and placed on the partition within the desiccator. The desiccator was placed in a constant temperature water bath, maintaining a constant temperature of 20 °C with an accuracy of 0.1 °C. The mass of each sample was measured every 24 h until it stabilized. To reduce errors, multiple tests (
n = 4) were conducted for each relative humidity. For each relative humidity, the final measurement result was the average of the test values. The Archimedes’ buoyancy method was used to determine the volume of the equilibrated samples [
30], and based on this, the saturation of the samples was calculated, allowing for the construction of a relationship curve between suction, water content, and saturation.
2.4. Microscopic Measurement
2.4.1. SEM Test
The scanning electron microscopy (SEM) experiment was conducted using a Sigma 300 model scanning electron microscope produced by Carl Zeiss AG, Oberkochen, Germany. The acceleration voltage used for testing was 20 kV, with a beam diameter of approximately 3.0 nm and a working distance of about 8 mm. The target material was high-purity platinum, and the sputtering current was set to 30 mA [
31].
2.4.2. MIP Test
The mercury intrusion test was conducted using an AutoPore V9600 fully automatic mercury porosimeter produced by Micromeritics, Norcross, GA, USA. The specific performance parameters include low pressure (3.3 to 200 kPa), high pressure (0.2 to 230 MPa), and pore diameter (0.005 μm to 370 μm). To ensure the dryness of the mercury intrusion samples, this experiment employed a freeze-drying method, as described in references [
32,
33], to prevent soil shrinkage, which could affect the reliability of the test results. The specific process [
34] is as follows: a cubic soil block with a volume of approximately 2 cm
3 was cut out using a knife, frozen in liquid nitrogen (−196 °C) for 15 min, and then placed in a freeze dryer under vacuum for 24 h to allow for the moisture in the soil block to sublimate and dry. Finally, the dried soil samples underwent mercury intrusion tests at both low and high pressures to obtain the pore size distribution.
Table 4 lists the basic information of the samples used in the microscopic measurements. Four soil samples with different organic matter contents were selected: Y1, S1, S2, and S3. Additionally, three suction values for the S3 soil sample were chosen, specifically 3.20, 13.10, and 21.82 MPa. In total, seven test samples were prepared for the SEM and MIP experiments.
5. Discussion
5.1. Influence of Organic Matter on the Pore Characteristics of Clay
In pure mineral soils and soils with low organic matter content (such as sample S1), the predominant microstructural form is skeletal, with aggregate shapes being ellipsoidal. The soil particles primarily contact each other at edge–face or edge–edge interfaces. The pore types are mainly inter-particle pores, with uneven diameters (
Figure 8). It is noteworthy that, under typical conditions, natural clayey soils tend to form aggregates, resulting in two types of pores: those between aggregates and those within aggregates, leading to a bimodal PSD curve. In contrast, for remolded soils, if the moisture content is low, they can also form aggregates, resulting in a bimodal PSD curve. However, if the moisture content is high and the matrix is uniform, aggregates do not form, and the PSD curve becomes unimodal [
37]. Some soil samples in
Figure 9 exhibit bimodal characteristics related to this phenomenon.
In soils with higher organic matter content (such as sample S3), the predominant microstructural form shifts to a laminated structure, with aggregate shapes being plate-like. The soil particles contact each other primarily at face–face interfaces, resulting in more complex pore characteristics. This complexity arises because, in addition to inter-particle pores, organic matter aggregates contain a significant number of disconnected micropores. This phenomenon has been confirmed through various testing methods in studies on peat soils, leading to a broad consensus [
38,
39]. Such changes not only increase the specific surface area of the soil, but also enhance its air entry value, thereby improving moisture retention capacity at low suction levels. Furthermore, the transformation in pore size distribution indicates the formation of more effective pathways for moisture and gas exchange.
5.2. Influence of Organic Matter on the Water Retention Characteristics of Clay
The soil–water interaction mechanism of organic matter-containing clays under different suction stages needs to be further analyzed, as shown in
Figure 10. The water holding capacity of the soil is mainly controlled by the capillary action between the particles in the low-suction phase. Due to the presence of organic matter, the inter-particle connections are tighter, which improves the ability of the soil to hold water. Simultaneously, the hydrophilic nature of the organic material helps increase the water retention capacity of the parent material.
As the suction value increases, the soil body in unsaturated state is affected by more adsorption. At this stage, the soil’s water retention capacity is primarily determined by the interactions between mineral soil particles and organic molecules. Due to the complex structure of the organic matter, a more stable bond may be formed with the soil particles, thus slowing down the water loss rate of the soil body to some extent. Upon entering the high-suction phase, the water-holding properties of the soil are more influenced by the water bound within the soil particles and organic matter molecules. At this stage, the soil–water curves of both pure clay and organic matter-containing clay tend to coincide, indicating that under high-suction conditions, the water retention performance of the soil is mainly governed by the nature of the soil particles and organic matter molecules themselves, and has little to do with their relative contents.
This study investigates the water retention characteristics of organic clay matter by adding peat to red soil. The results are similar to conclusions drawn from other organic materials, such as biochar and lignin [
40,
41,
42,
43], which also enhance the water retention properties of the soil as the organic matter content increases.
5.3. Organic Clay Water Retention and Its Impact on Engineering
The water retention capacity of organic clay significantly influences engineering safety and stability, particularly in infrastructure construction. Research indicates that as the organic matter content increases, the water retention capacity of the soil is notably enhanced. Under low-suction conditions, soil samples with higher organic matter content can maintain a greater amount of moisture even under high-suction conditions. This characteristic is crucial for the stability of engineering structures.
Additionally, the increase in organic matter content alters the microstructure of the clay, transforming the mineral–organic aggregates from ellipsoidal to plate-like shapes. This change in particle morphology affects the pore characteristics and moisture retention mechanisms within the soil matrix. Organic clay typically exhibits a higher proportion of closed-type pores, which may result in reduced permeability and affect water movement within the soil. This characteristic must be carefully considered when designing drainage systems to ensure effective water management following rainfall, preventing waterlogging or soil erosion. At high suction levels, the water retention performance of organic clay is primarily influenced by the inherent properties of the soil particles and organic matter molecules, rather than their relative contents. Overall, the water retention capacity of organic clay not only affects the physical properties of the soil, but also has profound implications for engineering design, construction, and maintenance.
