Soil Mechanics Research Articles

Environmental Driving Mechanism and Response of Soil’s Fungal Functional Structure to Near-Naturalization in a Warm Temperate Plantation

In this study, the near-naturalization process of Pinus tabulaeformis plantations in Baxianshan National Nature Reserve was divided into three stages depending on the proportion of P. tabulaeformis present, resulting in the following categories: the P. tabulaeformis forest stage, the mixed forest stage, and the near-natural forest stage. Natural secondary forests were selected as a control. We assessed alterations in the soil’s fungal functional structures from three aspects: functional mode, vegetative mode, and growth mode, and their responses to vegetation and soil factors were also explored. The results showed that ectomycorrhizal, saprophytic, and plant pathogen types were dominant in the functional mode, and plant pathogens were most abundant in the P. tabulaeformis forest stage. Meanwhile, ectomycorrhizal fungi were the least abundant in the near-natural forest stage. In the vegetative mode, saprophytic, pathophysiological, and symbiotic types were dominant, and pathophysiological types were the most abundant in the P. tabulaeformis forest stage. In the growth mode, microfungi dominated, and the abundance of clavarioid decreased with near-naturalization. The degree of variation in functional structure in the three dimensions increased with near-naturalization, but the structure of natural secondary forests converged. The species composition of tree layer obviously affected the abundance and functional structure of fungi in the three modes, among which Quercus mongolia and Carpinus hornbeam were the most significant. The soil’s pH and nitrate content significantly affected the structure of the functional mode, and the soil’s dry matter content and C/N ratio significantly affected the structure of the vegetative mode. In this study, we explored the interaction between the plant community and soil ecological system during the near-naturalization process of plantations in terms of soil fungi functions, further clarifying the role of soil functions in the succession of plant communities and providing a new perspective on the in-depth exploration of ecosystem interactions during the succession of plantations.

Effect of Calcium Carbide Residue on Shrinkage Mechanism of Clayey Subgrade Soil

ABSTRACT Calcium carbide residue (CCR) has been identified as an effective replacement for traditional lime for the stabilization of expansive soils. The shrinkage of expansive clay severely threatens the foundation layers’ long-term stability. The efficacy of CCR stabilization in reducing compressibility and enhancing the strength and stiffness of clay has been widely reported; however, the calcium-based additive stabilized soil tends to lose its engineered integrity under field-exposed seasonal moisture variation, specifically, shrinkage induced by prolonged drying. In this study, the shrinkage characteristic of CCR-treated clay (6%, 9%, and 12%) slurry was investigated by digital image analysis and mercury displacement method to quantify volumetric shrinkage strain. The results were compared with those of 4% and 6% lime-treated clay, which revealed the higher efficacy of CCR in reducing shrinkage strain by 47–57%. A characteristic shrinkage curve that depicted the significant variation of shrinkage phases in CCR-treated clay samples is developed. The dominance of the proportional shrinkage phase is observed in both untreated and treated clay samples. The shrinkage caused by evaporation, hydration, and decalcification is postulated in treated clays. This study is expected to contribute to the understanding of shrinkage mechanisms of CCR-treated clay and assist in the future design of stabilized embankment sections or subgrade for unpaved roads.

Soil microbial respiration does not respond to nitrogen deposition but increases with latitude

AbstractFacing global changes, substantial modifications in soil microbes and their functions have been widely evidenced and connected. However, the response of soil microbial respiration (MR) to increasing nitrogen (N) deposition and the role of microbial characteristics in controlling this response remain elusive. In this study, we quantified the intensity of the soil MR in terrestrial ecosystems that suffered elevated N deposition. High‐throughput quantitative sequencing and phospholipid fatty acids were employed to analyse microbial community properties and biomass, whilst microbial necromass was quantified using biomarker amino sugars. Our results revealed that soil MR kept stable under N deposition. Microorganisms maintained their respiration rates by modifying the characteristics of enzymes rather than altering microbial community properties or biomass. Notably, soil MR increased with latitude across study sites, which was attributed to the restriction of microbial activity by bacterial necromass. Supporting this observation, the recalcitrance of the soil carbon (C) pool to microbial degradation was evidenced to be the stability mechanism underlying the spatial variations in MR. Overall, we propose that MR is resistant to short‐term N deposition, whilst it exhibits a pronounced latitude dependence as shaped by the recalcitrant C pool. Our findings provide crucial insights into the microbial mechanisms of soil C dynamics under global change, contributing to the advancement of soil C models.

Geotechnical characterization of tropical soil via laboratory testing

This manuscript presents the evaluation of relevant geotechnical properties of the soil profile in the northern region of Campinas, which are important for geotechnical engineering practice and development in São Paulo State. Disturbed and undisturbed samples from the subsoil were collected through the excavation of inspection shafts at the experimental site for Soil Mechanics and Foundations research at the State University of Campinas, located at the Faculty of Civil Engineering, Architecture and Urbanism (FEC). Laboratory tests were conducted, including soil characterization, soil shear strength, soil compressibility, soil permeability, and soil suction tests. The characterization tests indicated that the upper soil layer is subject to surface effects and more direct weathering action, with noticeable variations in the liquid and plasticity limit values. The permeability test results showed no significant variation between the values obtained in the vertical and horizontal directions. The soil-water characteristic curves for the studied profile were bimodal, typical of tropical soils with macro and microaggregated structures.

Effects of chemical solution components on the contact angle of typical minerals in soil: quartz, orthoclase and plagioclase

Different chemical solutions can significantly change the contact angle (CA) of soil, but few studies have studied the change rule and action mechanism of the CA from the mineral composition of soil essence. In unsaturated soil mechanics, the CA is an important parameter to calculate the wet suction between soil particles in unsaturated soil. When the chemical composition of the soil pore liquid changes, the CA will also change, which will affect the wet suction and other parameters, thus changing the macroscopic mechanical properties of the soil. In this study, the CA of air-solution-mineral phases with different solution components (pH, type and concentration of salt solution) of different minerals (quartz, orthoclase and plagioclase) were measured. The results show that the CAs of quartz, orthoclase and plagioclase all rise first and then drop with the rise of pH. The peak CAs are pH = 5, pH = 4 and pH = 3, respectively. Quartz, orthoclase and plagioclase all have valley values in different concentrations of NaCl and KCl solutions. In CaCl2 solution, only quartz has valley value, while orthoclase and plagioclase increase monotonously. Quartz in soil plays a main role in the influence of soil CA, followed by orthoclase and plagioclase. The CA of different minerals in different chemical solutions is mainly controlled by electric double layer interaction and functional groups interaction. In different salt solution environment, in addition to the above two effects, the mineral CA is also affected by the surface tension.

Development of the triaxial equipment: a state of art

Triaxial shear testing is one of the most crucial investigations in geotechnical laboratories. It is essential for designing specific projects and understanding soil behavior in geotechnical engineering. Since Casagrande developed the modern shear apparatus at Harvard University, it has been refined to provide a more accurate characterization of shear strength behavior. This research paper aims to offer a concise overview of triaxial equipment development, focusing on apparatus that is easy to set up, prepare, and operate reliably, while remaining cost-effective. The primary objective is to contribute to the accurate determination of suitable techniques and devices that meet specific testing objectives. We review the theoretical background of both the triaxial apparatus and the direct shear box and illustrate the basic working principles of various volume change measurement techniques using triaxial devices. We also present different operational principles employed in designing volume change devices for soil testing. Our analysis reveals that some of these devices require complex operational techniques and equipment, making them challenging to use for repetitive testing. Consequently, we conclude that the ideal volume change device for soil mechanics testing should prioritize robustness and ease of operation.

Use of Lignin, Waste Tire Rubber, and Waste Glass for Soil Stabilization

The complex interactions between soil and additives such as lignin, glass powder, and rubber tires were investigated using principles of material and soil mechanics. Previous research has mainly focused on individual additives in clay soils. In contrast, this study investigates soil improvement with two different types of waste materials simultaneously. The improvement of soil properties by hybrid waste materials was evaluated using several laboratory tests, including the standard Proctor test, the unconfined compressive strength test, the California Bearing Ratio (CBR) test, and cyclic triaxial tests. The aim of this research is to identify key parameters for the design and construction of road pavements and to demonstrate that improving the subgrade with hybrid waste materials contributes significantly to the sustainability of road construction. The mechanical and physical properties were evaluated in detail to determine the optimal mixtures. The results show that the most effective mixture for the combination of waste glass powder and rubber tires contains 20% glass powder and 3% rubber tires, based on the dry weight of the soil. For the combination of waste glass powder and lignin, the optimum mixture consists of 15% glass powder and 15% lignin, based on the dry weight of the soil. These results provide valuable insights into the sustainable use of waste materials for soil stabilization in road construction projects.

Development of A New Model on PC Segmental Retaining Wall Using Solid Bar Anchors and Fin Fish

The development of transportation infrastructure is very important for economic growth. In the context of the construction of precast concrete (PC) segmental retaining walls, the use of innovative technologies such as solid bar anchors and fish fins is the main focus of improving the efficiency and reliability of the structure. This research aims to develop a new model that optimizes the use of solid bar anchors and fin fish in PC segmental retaining walls, with the hope of improving structural performance and extending the service life of the wall. The research methodology includes literature studies, development of mathematical models based on soil mechanics and finite element theory, and numerical simulations using PLAXIS software. The results show that the MSE Wall system with solid bar anchors and fin fish meets the SNI Geotechnical No. 8460 standard of 2017, with safety factors of high bolling, shear, and ground carrying stability. The use of a crossbar necklace configuration with a center to center (ctc) distance of 30 cm provides greater tensile strength than a ctc of 50 cm. This research is expected to make a significant contribution to the development of construction technology that is more efficient, economical, and sustainable.

Explicit consideration of geomaterial uncertainties in the load resistance factor design of piles driven into rock-based intermediate geomaterials

The study presents a method for estimating the end bearing of piles driven into rock-based intermediate geomaterials (IGMs). Existing design methods, rooted in soil mechanics, often fall short when applied to IGMs. Moreover, the geomaterial (geological and ground properties) uncertainties are neglected and the piles are designed based on subsurface information from distant boreholes. To address this gap, the study introduces an approach that explicitly considers geomaterial uncertainties by combining empirical models and geostatistical simulation to predict end bearing. Using data from 87 piles driven into various rock-based IGMs with dynamic load testing as the construction control technique, this study introduces the Proposed Design Method (PDM) for the prediction of end bearing. PDM is compared to the Current Design Method (CDM), which is based on traditional soil-mechanics methods. Predictions from these methods are subsequently evaluated against the measured end bearings. The results demonstrate that while the PDM yields comparable predictions to the CDM, it offers a more robust consideration of geomaterial uncertainties. The PDM leads to an approximate 13.60% to 19.35% increase in uncertainties compared to CDM across various IGMs. These findings advance the understanding of pile behavior in the presence of geomaterial uncertainties and inherent variability.

Near-surface soil hydrothermal response feedbacks landslide activity and mechanism

Surface moisture has recently been reported to be used in regional-scale landslide early warning. Nevertheless, near-surface multi-depth hydrothermal measurements as a hillslope scale are often less concerned and rarely linked to landslide kinematics. In this paper, we selected two neighboring landslides with different deformation mechanisms as case studies. Using in-situ multi-source sensors, we monitored real-time soil temperature and moisture at specific depths within approximately 1.5 m. The measurements span two complete monsoon seasons, representing concurrent dry and wet hydrological extremes. Statistical Pearson correlation analysis was employed to quantify the relationships between landslide activity and environmental variables such as soil temperature and moisture content. The results indicate that the near-surface soil temperatures and moisture contents contribute to a better understanding of the factors controlling landslide activity, in which variations synergistically reflect hydrothermal interaction and potential deformation mechanisms. These soil temperatures and moisture contents at certain depths (specifically at 20, 50, and even 100 cm) show moderate to strong correlations (with Pearson correlation coefficient values ranging from 0.4 to 0.8) with landslide deformation. In cases where discrete daily rainfall data exhibited unsatisfactory correlations due to their data attributes, soil temperature and moisture effectively served as alternative indicators for rainfall inputs, aiding in the analysis. Overall, this work emphasizes the critical influence of soil moisture and temperature on landslide dynamics. This study also highlights the need for comprehensive monitoring and forecasting strategies that consider a wide range of environmental factors to mitigate landslide risks associated with climate change, particularly in the context of intensified extreme weather events.

Constitutive modelling of a dry sand–structure interface under high-frequency vibration with a constant normal load

Sand–structure interfaces undergo strength loss and volume contraction under high-frequency vibration, potentially compromising the performance of underground structures in high-speed railway engineering. However, an appropriate method for quantitatively characterizing the behaviour of the sand–structure interface under high-frequency vibration is currently lacking, impeding the advancement of related design methods. In this work, the primary deformation and strength characteristics, such as the evolution of shear stress and volumetric strain, of the dry sand–structure interface under high-frequency vibration are summarized via a modified direct shear apparatus under a constant normal load. The concept of “vibro-induced virtual pore pressure” is subsequently introduced to modify the expressions of the critical state, yield, and bounding surfaces. The relationships between vibro-induced volume contraction and the vibration stress amplitude and state parameters are also established. Ultimately, a constitutive model describing the behaviour of the dry sand–structure interface under high-frequency vibration is formulated within the frameworks of critical state soil mechanics and bounding surface plasticity. The model comprises 14 parameters, all of which can be calibrated via laboratory experiments. Through comparison with experimental observations, the model is found to accurately simulate the key behaviours of the dry sand–structure interface under both static and high-frequency vibration conditions with a constant normal load, effectively reflecting the influence of the density, normal stress, and vibration stress amplitude of the sample.

Enzymatic Stoichiometry Reveals the Metabolic Limitations of Soil Microbes under Nitrogen and Phosphorus Addition in Chinese Fir Plantations.

Increasing nitrogen (N) deposition alters the availability of soil nutrients and is likely to intensify phosphorus (P) limitations, especially in P-limited tropical and subtropical forests. Soil microorganisms play vital roles in carbon (C) and nutrient cycling, but it is unclear whether and how much N and P imbalances affect the soil's microbial metabolism and mechanisms of nutrient limitations. In this study, a 3-year field experiment of N and P addition (control (CK), 100 kg N ha-1 yr-1 (N), 50 kg P ha-1 yr-1 (P), and NP) was set up to analyze the extracellular enzyme activities and stoichiometry characteristics of the top mineral soils in Chinese fir plantations with different stand ages (7, 20, and 33 years old). The results showed that the enzyme activities associated with the acquisition of C (β-1,4-glucosidase (BG) and β-d-cellobiohydrolase (CBH)) and P (acid phosphatases (APs)) in the N treatment were significantly higher than those in the CK treatment. Moreover, vector analysis revealed that both the vector's length and angle increased in stands of all ages, which indicated that N addition aggravated microbial C and P limitations. The P and NP treatments both significantly decreased the activity of AP and the enzymes' N:P ratio, thereby alleviating microbial P limitations, as revealed by the reduction in the vector's angle. Stand age was found to promote all enzymatic activities but had no obvious effects on the limitation of microbial metabolism with or without added nutrients in the soils under Chinese fir. Available N, Olsen-P, and pH were the main drivers of microbial metabolic limitations related to C nutrients. These results provide useful data for understanding the change in soil microbial activity in response to environmental changes, and suggest that P fertilization should be considered for management to improve productivity and C sequestration in Chinese fir plantation in the context of increased deposition of N.

Research on Solidification Methods and Stabilization Mechanisms of Sulfate Saline Soils

In cold regions, saline soils can cause dissolution, settlement, and salt expansion of the roadbed under the influence of freeze–thaw cycles, so they need to be stabilized during road construction. In this study, lime, fly ash (FA), and polyacrylamide (PAM) were used to stabilize sulfate saline soils, and the stabilized saline soils were subjected to the unconfined compressive strength test (UCS), splitting test, and freeze–thaw cycle tests (FTs). The stabilization mechanism of the three materials on saline soils was also studied via scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), and X-ray photoelectron spectroscopy (XPS). The test results showed that the addition of lime, FA, and PAM to saline soils can improve the mechanical properties and frost resistance of saline soils. After 28 d of curing, the UCS of FA-, PAM-, and lime-stabilized saline soils increased by at least 55%, 23%, and 1068%, respectively, and the splitting strength increased by at least 161%, 75%, and 2720%, respectively. After five freeze–thaw cycles, the residual strength ratios (BDRs) of the UCS of L2 (lime 8%), F2 (FA 11%), and P2 (PAM 1%) stabilized soils and saline soils were 71.78%, 56.42%, 39.05%, and 17.95%, respectively, and the decreasing trend tended to be stable. The saline soils stabilized by lime and FA were chemically stabilized, and their mechanical properties and frost resistance were better than the physical stabilization of PAM.

On-bottom stability of a radial fin integrated surface-laid pipe-section against vertical penetration

This study focuses on the on-bottom stability of surface-laid submarine pipelines during vertical penetration under thermal-induced buckling. Submarine pipelines are susceptible to buckling due to their high slenderness ratio, and the resistance to such displacement relies on pipeline stiffness and soil resistance. To investigate this, laboratory model tests were conducted using a scaled-down pipe section placed on a clay bed. The clay bed was prepared with remoulded Kaolin clay, ensuring uniform moisture content, and the model pipe was scaled down from a typical industry prototype. These tests were conducted in 2D plane strain conditions within a tank, equipped with observation windows to monitor pipe movement and soil behaviour. A significant improvement in the pipe section's penetration resistance and on-bottom stability was observed after incorporating fin in the pipe. The addition of fins altered the earth pressure distribution beneath the vertically loaded pipe, leading to a larger failure mechanism in the adjacent soil. Furthermore, the soil-surface-heaving progressed towards the tank wall with increasing angular distance between the radial fins. Thus, the research demonstrated the effectiveness of radial fins in enhancing the stability and resistance of submarine pipelines against vertical penetration, shedding light on potential improvements in submarine pipeline design and deployment.

Geomechanics and Geology of Marine Terraces of the Crotone Basin, Calabria (Italy)

This study investigates the geomechanical behavior of five terrace orders in the Crotone Basin. The purpose is to understand the physical–mechanical parameters of these terraces to determine whether rock or soil mechanics principles should be applied for stability analysis. Samples were collected from each terrace following an extensive field survey. Laboratory analyses were conducted to measure pulse velocities, uniaxial unconfined compressive strength, and compressive strength with truncated conical platens. The findings revealed key physical–mechanical parameters of the rocks, which are crucial for stability assessments. The Crotone Basin, known for its mineral resources such as hydrocarbons and rock salts, has been studied geologically since before the 1950s, but there is a lack of geomechanical data in existing literature. Therefore, the results presented here are novel and provide a basis for future studies on the instability of rocky slopes composed of similar soft rock types. These results will aid in accurate geological–geotechnical model reconstructions. While the findings can be applied to similar cases, it is important to note that each analysis site, despite showing similar phenomena, is unique and requires individual investigation.

Disintegration characteristics and mechanism of red clay improved by steel slag powder

Red clay is prone to softening and disintegration when exposed to water, often triggering severe geological disasters. To enhance its resistance to disintegration, this paper employs a homemade wet-dry cycling disintegration apparatus, combined with XRD, SEM, MIP, and fractal theory, to analyze the effects of different activators on the disintegration characteristics and microscopic mechanisms of SSP-improved soil. Under wet-dry cycling, the disintegration process of SSP-improved soil can be divided into five stages: rapid water absorption, saturation wetting, rapid disintegration, softening and dissolution, and stable non-disintegration. As the activator and SSP content increase, the resistance to disintegration of SSP-improved soil gradually enhances. After the addition of activators, the porosity, pore area, pore length and width, and probability entropy of SSP-improved soil decrease (i.e., 7 % NS < 7 % CSW < 7 % NH < undisturbed soil), while the average shape factor, regional probability distribution index, and fractal dimension increase. This is because activators promote ion exchange, cementation, crystallization, and carbonation reactions in SSP-improved soil, generating a substantial amount of hydration products such as C-S(A)-H, Ca(OH)2 and CaCO3, which fill and encapsulate inter-particle pores, thereby reducing water flow channels within the pores. In contrast, the pore structure between particles in undisturbed soil is simple and well-connected, providing favorable conditions for the migration and dissolution of fine particles and the loss of cementing materials, which accelerates the destruction and disintegration of the soil structure.

Enhanced Soil Fertility and Carbon Sequestration in Urban Green Spaces through the Application of Fe-Modified Biochar Combined with Plant Growth-Promoting Bacteria.

The soil of urban green spaces is severely degraded due to human activities during urbanization, and it is crucial to investigate effective measures that can restore the ecological functions of the soil. This study investigated the effects of plant growth promoting bacteria (Bacillus clausii) and Fe-modified biochar on soil fertility increases and mechanisms of carbon sequestration. Additionally, the effects on C-cycling-related enzyme activity and the bacterial community were also explored. Six treatments included no biochar or Bacillus clausii suspension added (CK), only Bacillus clausii suspension (BC), only biochar (B), only Fe-modified biochar (FeB), biochar combined with Bacillus clausii (BBC), and Fe-modified biochar combined with Bacillus clausii (FeBBC). Compared with other treatments, the FeBBC treatment significantly decreased soil pH, alleviated soil alkalization, and increased the alkali-hydro nitrogen content in the soil. Compared to the individual application of FeB and BC, the FeBBC treatment significantly improved aggregates' stability and positively improved soil fertility and ecological function. Additionally, compared to the individual application of FeB and BC, the soil organic carbon (SOC), particulate organic carbon (POC), and soil inorganic carbon (SIC) contents for the FeBBC-treated soil increased by 28.46~113.52%, 66.99~434.72%, and 7.34~10.04%, respectively. In the FeBBC treatment, FeB can improve soil physicochemical properties and provide bacterial attachment sites, increase the abundance and diversity of bacterial communities, and promote the uniform distribution of carbon-related bacteria in the soil. Compared to a single ecological restoration method, FeBBC treatment can improve soil fertility and carbon sequestration, providing important reference values for urban green space soil ecological restoration.

Variability in the measured soil‒water characteristic curve with respect to the numbers of specimens

The soil‒water characteristic curve (SWCC) defines the relationship between the amount of water in soil and soil suction. The SWCC is commonly used to estimate the hydraulic conductivity function (HCF) and the shear strength function (SSF). Therefore, an accurate determination of the SWCC is crucial for implementing the principles of unsaturated soil mechanics. The SWCC is commonly determined from a limited number of experimental data because SWCC measurements are time-consuming and costly. As a result, the minimum number of required soil specimens is crucial for a SWCC test when considering the accuracy of the determined SWCC and the experimental expenses. In this study, both engineered from sand and kaolin mixtures and residual soils from Bukit Timah Formation in Singapore are selected to prepare soil specimens for SWCC measurements. The SWCCs obtained from the specimens with engineered soil are consistent, while those from specimens with residual soil are slightly scattered. This indicates that one specimen is sufficient to determine the SWCC for engineered soil samples, while a minimum of two specimens should be prepared for the determination of SWCC for residual soil samples from Bukit Timah Formation.

Study on the hydro-thermal-salt-mechanical coupling characteristics of sulfate saline soil under freeze-thaw cycles

The theoretical and experimental studies have been carried out on the water and salt migration and deformation characteristics of sulfate saline soil during freeze-thaw cycles. Based on the theory of unsaturated soil mechanics and the thermoelastic continuum and considering the influence of phase change within the pore on thermodynamic and hydrodynamic parameters, the multi-physical fields coupled model of hydro-thermal-salt-mechanical in unsaturated sulfate saline soil has been established. The variation processes of the temperature field, water field, salt field, and stress field of the soil during freeze-thaw cycles were analyzed, and the validity of the theoretical model was verified by indoor experiments. The results show that there are significant attenuation and hysteresis effects when heat is transferred in the soil during freeze-thaw cycles. The water content of migration in the soil increases with the height of the soil column, while the increment of migration water content decreases with the number of freeze-thaw cycles. The formation and dissolution of salt crystals from top to bottom and the sudden increase in the salt crystallization rate are mainly caused by variations in the solubility of the salt solutions due to temperature changes. The formation and dissolution of ice and salt crystals in the soil induce expansion and contraction, and the freeze-thaw cycle conditions have a significant effect on the expansion and residual deformation of the soil.

Study on the dry shrinkage characteristics and size effect of swell-shrink characteristic soil.

Swell-shrink characteristic soils exhibit a high susceptibility to cracking during the drying process, which poses a significant risk of various geological disasters. Among these, the occurrence of drying shrinkage acts as a prerequisite for the cracking phenomenon. Therefore, it is of utmost importance to comprehend the specific characteristics associated with the drying shrinkage mechanism. To investigate the drying shrinkage behavior of swell-shrink characteristic soils, a series of drying shrinkage experiments were conducted on long strip samples of red clay and expansive soil. Utilizing three-dimensional digital image correlation (DIC) technology, the surface displacement, strain, and anisotropic shrinkage rates of the soil samples during the drying process were obtained, and the size effect on the drying shrinkage of swell-shrink characteristic soil were analyzed. The research findings are as follows: The displacement development of the soil samples in the X and Y directions can be divided into two stages: a linear growth stage and a stable displacement stage. In the Z direction, the soil surface deformation can be divided into three stages: soil surface arching, vertical shrinkage, and shrinkage stabilization. The drying shrinkage of swell-shrink characteristic soil exhibits anisotropy, with the vertical shrinkage rate being the largest, followed by the longitudinal and then the transverse directions. Additionally, the soil sample shrinkage exhibits a size effect, whereby the shrinkage rates in all directions increase with increasing sample width and thickness. During the drying shrinkage process, the stress state on the soil surface evolves from initial tensile strain to subsequent compressive strain. The strain at different positions and times within the soil sample is not uniform, resulting in the non-uniformity and anisotropy of the sample shrinkage. This study provides important insights into the cracking mechanism of swell-shrink characteristic soils and serves as a valuable reference for related laboratory experiments, which will contribute to better prediction and control the geological hazards caused by the drying shrinkage of swell-shrink characteristic soils.