1. Introduction
Expansive soils are widely distributed across more than 60 countries and regions worldwide [
1]. Owing to their unique mineral composition, which includes hydrophilic minerals such as illite and montmorillonite, these soils are highly sensitive to moisture changes. Unsaturated expansive soils exhibit significant volume expansion when wetted, followed by contraction upon drying. This characteristic poses a serious threat to the stability of structures built on them [
2,
3]. In areas with hot, humid climates and specific geological conditions, disasters in expansive soil regions are frequent [
4,
5,
6]. Rainfall infiltration, a primary environmental factor, activates the expansion and contraction of expansive soils by altering their hydrological mechanisms. It increases the soil’s self-weight and water content, and alters its internal pore structure, rendering the soil unstable [
7,
8]. Therefore, a thorough understanding of the quantitative relationship between rainfall conditions and the response of the soil’s pore structure volume change is crucial to prevent and address related engineering issues.
Climate change is altering precipitation patterns and is expected to increase the frequency and intensity of extreme rainfall events in many regions. This change exacerbates the harmful effects of expansive soils [
9,
10]. However, the quantitative relationship between rainfall parameters and the behavior of expansive soils remains inadequately defined. Van Genuchten [
11] pioneered the field by proposing a foundational framework for correlating soil–water retention curves with pore size distributions. On this basis, some researchers have used traditional geotechnical testing methods to explore the effects of different water contents on expansive soils [
12,
13,
14]. Despite these advances, most studies have concentrated on the macro-scale swelling properties, with a limited understanding of the microscopic changes in pore structure under rainfall infiltration. Consequently, this area of study necessitates further in-depth research and exploration.
Changes in pore structure are considered a fundamental factor influencing the macroscopic mechanical properties of soils [
15,
16]. Skvortsova et al. [
17] contend that under external environmental influences, soil structure is complex and dynamic, making the geometry of the pore space a crucial indicator of the soil’s structural state. Over decades, several traditional methods for testing pore structure, such as mercury intrusion porosimetry, gas adsorption method, and imaging techniques, have been developed. However, it is apparent that while these traditional pore testing methods are theoretically robust and widely used, they have stringent requirements for specimens and are prone to disturbances during soil sample preparation, limiting their ability to continuously monitor the evolution of the internal structure [
18,
19,
20,
21]. In recent years, advanced nuclear magnetic resonance (NMR) relaxation measurement techniques have enabled non-invasive and continuous monitoring of changes in soil pore size distribution. The NMR technique offers the advantages of being rapid, non-destructive, contact-free, and efficient in time usage. Furthermore, the information on pore size distribution, represented through T2 distribution, provides a more comprehensive understanding.
Within this technical framework, Zhiqiang Mao [
22] used core nuclear magnetic resonance (NMR) T2 distribution and capillary pressure analysis data, and the pore structure of the rock was revealed more comprehensively. Similarly, Kuan Liu [
23] integrated triaxial tests, NMR experiments, and scanning electron microscope (SEM) experiments to observe the effects of wet–dry cycling on the macroscopic mechanical parameters and microstructure of remodeled expansive soil samples. He discovered that as the number of wet–dry cycles increased, the soil’s shear strength decreased, the stress–strain curve became more ductile, and the pore structure shifted from microporous to macroporous. Additionally, G.F. Wei [
19] applied NMR techniques to analyze the stage characteristics and mechanisms influencing the swelling curve from the perspective of changes in pore space, noting a decrease in swelling rate with prolonged water absorption. Fugang Shi [
24], meanwhile, investigated the alterations in the pore size distribution of expansive soil during dewatering and shrinkage using the NMR relaxation method. These studies demonstrate the effectiveness of NMR techniques in examining the microscopic changes in the pore structure of expansive soils. However, existing research has not yet integrated NMR data with rainfall conditions to establish a quantitative connection between these conditions and microscopic changes in pore volume. Therefore, we have combined NMR measurements with laboratory-simulated rainfall experiments to gain a deeper understanding of the microscopic mechanisms through which water-induced changes in pore structure trigger swelling strains.
In this study, we employed NMR relaxation measurements, complemented by simulated rainfall experiments conducted in the laboratory, to systematically investigate the specific effects of rainfall intensity and duration on the pore structure of expansive soils. This methodology enabled us to delve into the microscopic interactions between wetting events and changes in soil volume, thereby offering new insights into this complex phenomenon. By integrating the relationship between rainfall parameters and pore structure characteristics into a modeling framework grounded in physical principles, our research not only improves the accuracy of predicting the field behavior of expansive soils but also aids in understanding their damage mechanisms. This study holds critical significance in developing disaster response strategies for regions with expansive soils, offering essential guidance for related engineering design and soil management practices.
2. Materials and Methods
2.1. Expansive Soil Characterization
The soil samples used in this study were collected from an expansive soil slope in Ye County, Pingdingshan, alongside the South-to-North Water Diversion Project channel. The sampling location coordinates were 113.15° E and 33.55° N, with a sampling depth of 1–1.5 m. Soil samples are representative of the region. The soil was brownish-red clay, stiff plastic, mostly filled with grayish-green clay.
The retrieved soil samples were divided into ten groups, according to GBT 50123-2019 [
25] (China Standard), and a series of basic physical tests were performed on the collected soil samples, taking the average of the results of the test. The test results of the basic physical properties of soil samples and test standard deviations are shown in
Table 1, and the particle distribution curve is shown in
Figure 1, in which the free expansion rate of soil samples is 43.7%, according to ASTM D 2487-93 [
26], and it can be judged that the soil samples taken are low plasticity clay.
2.2. Sample Preparation Protocol
This study used remolded expansive soils, prepared through the following procedure:
(1) Pre-treatment of soil samples: the collected Pingdingshan expansive soil was naturally air-dried and crushed using a crushing hammer (see
Figure 2a).
(2) Sieving and drying: according to GBT 50123-2019 [
25] (Chinese standard), the processed soil samples were sieved using a 2 mm aperture sieve mesh, and then the soil samples were put into a 100 °C oven for 24 h.
(3) Adjusting the moisture content of soil samples: after drying, the moisture content of the soil samples was adjusted to 14%, and the soil samples were frequently turned to prevent clumping during the addition of water.
(4) Sealing and resting of soil samples: after adjusting the water content, we sealed the soil samples with plastic film and left them for 24 h to ensure that the soil and water were evenly mixed, as shown in
Figure 2b.
(5) Review of water content: after completion of the resting, we re-tested the water content of the soil sample to ensure that it was within the range of 14 ± 1%.
(6) Preparation of specimen: since steel and iron materials will interfere with the magnetic field of the NMR test, it was necessary to apply petroleum jelly between the specimen and the ring cutter for subsequent separation.
(7) Compacting and shaping: We put the soil sample into the ring knife mold with a height of 20 mm and a diameter of 61.8 mm and compacted it 25 times with a compactor, then took out the specimen and flattened the two ends with a chipping knife.
Figure 2c shows the compacting instrument of the soil sample.
(8) Separation and weighing: We removed the specimen from the ring knife and weighed the specimen mass. The prepared specimen block is shown in
Figure 2d.
(9) Repeat preparation: We repeated the preparation of several sets of test blocks according to the above steps to meet the needs of subsequent tests.
Figure 2.
Test block preparation diagram. (a) Crushing. (b) Sealing and resting. (c) Compacting. (d) Molding.
Figure 2.
Test block preparation diagram. (a) Crushing. (b) Sealing and resting. (c) Compacting. (d) Molding.
2.3. Simulated Rainfall Equipment
To simulate real rainfall conditions, the concept of rainfall intensity must be introduced. According to the rainfall intensity grading standards (see
Table 2), based on GB/T 28592-2012 [
27] (Chinese standard), the rainfall intensity is divided into 4 levels—light rain, moderate rain, heavy rain, and rainstorm—for which experiments were carried out separately.
Based on Equation (1) and the rainfall intensity classification criteria, the flow ranges corresponding to different rainfall intensities can be deduced.
where
Q is the flow rate required to simulate rainfall, mL/h;
A is the cross-section area of the ring cutter,
;
p is the amount of precipitation,
;
t is the time of rainfall, h.
Based on the calculated flow rate ranges and rainfall intensity level classifications, the flow controllers in the simulated rainfall device (see
Figure 3) were adjusted to simulate rainfall of various intensities (flow rates adjusted to median values). The device consists of three parts from top to bottom: a water storage tank, a flow controller for simulating different rainfall conditions, and a container for holding the soil samples, with small holes at the bottom to provide downward seepage channels for the samples.
2.4. NMR Measurement Parameters
Nuclear magnetic resonance (NMR) techniques detect pore structures in samples by receiving resonance signals from H atoms in the pore spaces; thus, the samples must be saturated with H atom-containing liquid before NMR testing. Since water is an important variable in the simulated rainfall experiments, the samples cannot be saturated with water, based on the relevant literature [
28,
29]. In this paper, each group of specimens was put into kerosene for saturation treatment after rainwater treatment, and the saturation time was 24 h. After saturation, nuclear magnetic porosity testing was carried out, and the instrument used was MesoMR12-060H-I NMR Permanent Magnet Benchtop MRI Analyzer (Neumay Analytical Instruments Co., Ltd., Suzhou, China), with a magnet frequency of 12.84 MHz and a magnetic field strength of 0.3 ± 0.05 T.
Figure 4 shows the specimens that are undergoing the kerosene saturation treatment.
The NMR pore measurement principle is based on the relaxation mechanism of H-atom transitions. Surface relaxation occurs at solid–liquid interfaces, i.e., the particle surfaces of rocks. Its relaxation rate depends on pore size, with smaller pores corresponding to faster relaxation, generally occurring within smaller-sized pores. Under ideal conditions, relaxation satisfies the following equation:
where
is the transverse relaxation time;
is the transverse surface relaxation rate;
is the pore surface area to fluid volume ratio. For simple-shaped pores,
is related to pore size. A cylindrical pore model was used in this study. Assuming a pore radius of r and length
L, the
value is 2/r.
5. Conclusions
In this study, the pore size distribution of expansive soils in the Pingdingshan area, China, under different rainfall conditions was analyzed in depth by advanced nuclear magnetic resonance (NMR) techniques, revealing the effects of rainfall intensity and rainfall duration on the microstructure of expansive soils. The experimental design covered different rainfall intensities, ranging from light to heavy rainfall, and different durations ranging from 0.5 to 3 days, and comprehensively analyzed the dynamic process of soil micropore to macroporosity changes during rainfall.
The main findings of this study include the following:
- (1)
With the increase in rainfall duration, the proportion of micropores in the soil decreased, while the proportion of meso- and macropores increased, indicating that rainfall promoted the interconnectivity of pores and enhanced the permeability and drainage capacity of the soil. Especially under heavy rainfall and storm conditions, this change is more significant and the pore structure rapidly adjusts to reach a new stable state, which provides important information for understanding and predicting the behavior of expansive soils under extreme rainfall events.
- (2)
Under different rainfall conditions, the quantitative relationships between dominant pore size, porosity, and rainfall conditions follow the LogisticCum model, which exhibits a highly accurate fit, where the correlation coefficients, R2, reach 0.87 and 0.91, respectively, which suggests that the model is capable of predicting the field performance of expansive soils incorporating rainfall factors, and provides an important predictive tool for engineering design and disaster prevention.
These results emphasize the value of the application of NMR techniques for non-destructive, accurate measurement of the pore structure of expansive soils and the importance of systematic analysis of the effects of rainfall intensity and duration in improving the prediction of engineering behavior of expansive soils. This study not only provides new perspectives for understanding the behavior of expansive soils under different rainfall conditions but also provides a theoretical basis for risk-based engineering analysis and design, especially in preventing and responding to rainfall-induced destabilization of expansive soils.