Soil Suction Research Articles

Vegetation changes through recurrent fire affect soil water behavior and enhance landslides in the mountainous region of Rio de Janeiro state, southeast Brazil

The role of fire in vegetation-soil–water interactions and its implications for soil stability and slope failures is poorly documented in humid tropical regions. This paper focuses on the relationships between vegetation change through recurrent fire and its effects on hydraulic conductivity and soil suction. Two monitoring sites were chosen on steep slopes in the mountainous domain of the Atlantic rainforest: the first is representative of degraded secondary rainforest (DRf) 25–30 years after slash and burn agriculture; the other represents herbaceous-shrubby vegetation (HS) resulting from wildfires in short-time intervals (<5 years). Two fires had previously occurred on the HS slope, one in December 2014 and the other in September 2019. Continuous records of rainfall and soil suction at depths of 10, 20, 50, 100, 150, and 220 cm provided a 6-year data collection from January 2015 to December 2020. Hydraulic conductivity (Ksat) field tests were conducted around the two monitoring sites before and after the September 2019 fire. The main results include: a) soil suction in the topsoil of both vegetation covers follow the rainfall inputs with high values of Ksat (DRf = 195 mm/h; HS = 203 mm/h); b) while quick rainfall-suction responses prevail within the DRf soil profile, soil suction at HS maintains a mean suction value around -27 kPa below 100 cm deep; c) field observations attest to the occurrence of hydrophobicity after the 2019 fire; d) Ksat tests just after this fire indicated a decreasing mean value from 203 mm/h to 46 mm/h; and e) by the end of the dry season after fire, there is a delayed increase in soil suction reaching -143 kPa at a depth of 100 cm and in smaller proportions at 220 cm deep. These results indicate that post-fire soil changes tend to favor the sudden loss of soil stability and slope failure during extreme rainfall events.

Pseudo-static analysis of 3D unsaturated bench slopes stabilized by multiple rows of piles

Bench slopes incorporated with multiple rows of anti-slide piles remain an effective technique in landslide disaster preventions and are widely employed in the design of new subgrades and embankments. To provide more realistic results for engineering guidance, the three-dimensional (3D) effect and suction in soils require explicit considerations. Employing the upper bound limit analysis method, a 3D limit analysis of bench slopes stabilized by multiple rows of piles is performed in this paper. The upper bound solutions of several illustrative slopes are optimized and compared with the published ones, indicating the validity of the optimization program and the proposed method. A series of parameter analyses are conducted regarding the roles of suction and anti-slide piles in slope safety assessments. The results show that the suction effect and pile reinforcement increase continually as the pile location increases and the influence of bench width can be ignored. The stability of bench slopes stabilized by multiple rows of piles differ with that of slopes stabilized by single row of piles dramatically. The maximum pile reinforcement reaches 70% for sandy soil slopes but only 30% for clayey soil slopes. The influences of surcharge load and seismic load on the pile reinforcement can nearly be neglected.

Soil suction effects on CPT data interpretation

The Cone Penetration Test (CPT) is one of the most widely used in situ test for site characterization and estimation of geotechnical parameters estimative. Its benefits include speed, reliability, repeatability, and the ability to provide continuous data. Although the interpretation is well-established for saturated and dry soils, it remains limited for unsaturated soils. This paper presents and discusses the influence of the unsaturated condition in CPTs performed on tropical soil site. Four CPT campaigns and gravimetric water content profiles have been determined in different periods over two years. Soil-water retention curves (SWRC) were used to estimate in situ soil suction. It was observed that the CPT data were influenced by soil suction up to 6.0 m depth. Two semi-empirical approaches based on bearing capacity theory and effective stress principle were used for data interpretation. These approaches allowed to assess the soil suction influence on CPT and to define the typical test profile with no suction effects. The importance of considering soil suction in the CPT interpretation on unsaturated soils is highlighted, and the effective stress approach is suggested as a starting point.

Saturated permeability and water retention capacity in biochar-methanotrophs-clay for new landfill cover system

A new landfill cover system, biochar-methanotrophs-clay (BMC) cover is recommended for reducing methane emissions at landfills. It also contributes to decreasing soil permeability and improving soil water retention in a long time, due to highly porous structure of biochar and the growth metabolism of methanotrophs. To determine the effects of biochar content, oxidation aging times and methane-filled days on hydraulic properties, a total of 60 groups of experiments were conducted. The saturated hydraulic conductivity (ksat) was obtained by flexible wall permeameter with controllable hydraulic head pressure. The results showed that the ksat of BMC increased with increasing biochar content and oxidation aging times, while decreased with adding methane-filled days. The soil-water characteristic curves (SWCCs) were obtained with soil suction measured by the filter paper method. The results indicated the water retention capacity of MBC reduced with increasing oxidation aging times but increased with adding methane-filled days. Detected by mercury intrusion porosimetry (MIP), fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM), the differences displayed the changes of pore structures and extracellular polymeric substances (EPS). The oxidation aging of biochar increased the volume of pores, resulting in the increased ksat and the decreased water retention capacity. However, the growing of methanotrophs decreased the volume of pores, resulting in the ksat decreased and the water retention capacity increased due to EPS. No matter how many times the oxidation aging process was experienced, the BMC with longer methane-filled days exhibited relatively lower ksat and better water retention capacity. This implied a more stable barrier capacity to reduce water infiltration in the long term. By combing a series of macro and micro experiments, this paper provides theoretical guidance for the application of biochar-methanotroph-clay mixture to landfill covers.

Analysis of Water Migration and Spoil Slope Stability under the Coupled Effects of Rainfall and Root Reinforcement Based on the Unsaturated Soil Theory

Root reinforcement is an effective slope protection measure due to root water absorption and soil suction. However, the coupled effect of rainfall and root reinforcement remains unclear, resulting in a challenge to evaluate slope stability in complex environments. This paper regards the root–soil composite as a natural fiber composite and quantifies its reinforcement effect using direct shear tests. The unsaturated soil seepage–stress theory was employed to simulate the effect of rainfall on water migration and the stability of spoil, overburden, and vegetated slopes. Field measurements and pore water pressure tests verified the simulation results. Furthermore, the influences of the slope angle, rainfall parameters, and vegetation cover thickness on slope stability were analyzed. The results showed the following: (1) The root reinforcement enhanced the soil’s ability to resist shear deformation, substantially improving soil shear strength. The cohesion of the root–soil composite (crs = 33.25 kPa) was 177% higher than that of the engineering spoil (ces = 12 kPa) and 32.21% higher than that of the overburden soil (cos = 25.15 kPa). (2) The overburden and vegetated slopes had lower permeability coefficients and a higher shear strength than the spoil slope, and the effect was more pronounced for the latter, resulting in lower landslide risks. The water migration trend of the vegetated slope was characterized by substantial runoff and a low sediment yield. The safety factors of the spoil slope, overburden slope, and vegetated slope were 1.741, 1.763, and 1.784 before rainfall and 1.687, 1.720, and 1.763 after rainfall, respectively, indicating a significantly higher safety factor of the vegetated slope after rainfall. (3) The slope angle significantly affected slope stability, with lower safety factors observed for higher rainfall intensities and durations. Under these conditions, the slope angle should be less than 30°, and the soil thickness should be 0.5 m for herbaceous vegetation and shrubs and 1.0 m for trees.

Numerical Modelling of Matric Suction in Unsaturated Soil under Shallow Foundation Under Varying Soil and Hydrological Conditions

In recent times, extreme hydrological events have disrupted the performance of various structures, particularly the structural foundations responsible for transferring the superstructure's weight to the natural ground. This disruption underscores the significance of matric suction and soil saturation, which are influenced by hydrological conditions like precipitation, soil shear strength, and foundation settlement. These factors are essential when designing structures in specific locations with distinct geotechnical parameters. To address these challenges, our research employs Plaxis 2D numerical modeling to investigate the dynamic changes in matric suction within soil beneath shallow foundations under varying rainfall conditions. Our approach involves a fully-coupled flow method, incorporating the Van Genuchten hydraulic model. In recognition of practical constraints, we utilize fundamental soil classification parameters for model configuration. Our findings reveal the substantial impact of matric suction fluctuations during rainfall on soil deformation, as indicated by displacement patterns. This highlights the critical importance of matric suction in comprehending soil behavior. Furthermore, we observe that higher initial water table levels correlate with reduced variations in matric suction and soil deformation during rainfall, emphasizing the regulatory role of water table depth. In conclusion, this study emphasizes the necessity of considering matric suction and water table depth in structural design and geotechnical analysis, particularly when faced with extreme hydrological events. By comprehending these factors, we can enhance our understanding of soil behavior, improve foundation stability, and develop more effective design strategies for structures in various environmental conditions.

Stability Analysis of Three-Dimensional Tunnel Face Considering Linear and Nonlinear Strength in Unsaturated Soil

The shear strength of unsaturated soils exhibits significant nonlinearity, while previous studies often simplified it with linear strength models. The objective of this paper is to investigate the distinctions in the stability of three-dimensional (3D) tunnel faces when using linear and nonlinear strength models. A new 3D rotational failure mechanism and an extended form of the Mohr–Coulomb (M-C) failure criterion were integrated into the kinematically limited analysis (KLA) framework to describe the failure characteristics of tunnel faces. Subsequently, the factor of safety (FS) of the 3D tunnel faces was calculated using the strength reduction method (SRM). In the discussion section, the impacts of nonlinear shear strength, matric suction in the unsaturated soils, and the 3D geometric parameters of the tunnel on the stability of the tunnel face were analyzed. The outcomes indicate that, in unsaturated soil conditions, diverse nonlinear strength calculation models and soil types exert disparate influences on the FS of 3D tunnel faces. The main novelty of this study lies in establishing an effective method for assessing the stability of tunnel faces in unsaturated soils.

Elastoplastic solution for cylindrical cavity contraction in unsaturated soils

This paper develops an elastoplastic solution for cylindrical cavity contraction in unsaturated soils under constant suction conditions. The elastoplastic cavity contraction problem is formulated into a set of first-order ordinary differential equations by introducing a new auxiliary variable, which is solved as an initial value problem. The new solution is validated by comparison with numerical simulation results. Finally, parametric studies show that, as soil suction increases, the internal support pressure decreases faster with cavity contraction, the unloading-induced plastic zone becomes narrower, and the changes in effective stresses are smaller for a given tunnel convergence.

DETERMINATION OF THE MATRIX SUCTION OF LOW-PLASTICITY SILTS AND ITS CORRELATION WITH THEIR COLLAPSE POTENTIAL

Soils are usually in a partially saturated state, meaning that both air and water occupy their voids. This condition can affect the stability of structures because it is closely related to wetting deformation (collapse). A low-plasticity silt (ML) composed of 45% kaolin with a liquid limit of 36% and a plasticity index of 10% was used in this study. In this study, the soil suction matrix was determined using the filter paper method established in ASTM D5298, correlating it with the moisture content and percentage of deformation induced by wetting. It was concluded that, as the moisture content decreases, the suction and the percentage of deformation by wetting increase, since, for a moisture content of 9% the suction is 14901.6 kPa and the deformation is 0.58%, while for a moisture content of 21% the suction is 1028.1 kPa and the deformation is 0.07%. Thus, a change in the slope of the SWRC was evident starting at 9%, which indicates a greater effect of suction and deformation below this moisture content.

Numerical Simulation of Offshore Suction Bucket Foundation Pullout Characteristics under Undrained Conditions

The suction bucket jacket foundation is widely regarded as a crucial solution for constructing offshore wind farms in water depths ranging from 30 m to 50 m. When subjected to complex loads, the bucket primarily relies on vertical movement to withstand the corresponding loads. This paper investigates the undrained pullout characteristics of the bucket foundation through numerical simulation while keeping its weight constant. This study examined how the pullout capacity of the suction bucket jacket foundation is affected by the aspect ratio and soil conditions. It revealed the changing patterns of bucket–soil frictional resistance and suction during the undrained pullout process, along with their contributions to the pullout capacity of the foundation. The results indicate that the peak pullout load of the bucket increases with decreasing L/D, and the response is more pronounced in soft clay. The frictional resistance changes from upward to downward with increasing displacement, with the maximum frictional resistance occurring at the base of the foundation. The lower part of the footing has a faster response. The suction force increases with displacement, and the proportion of suction force to peak pullout tends to increase as L/D decreases. The soil failure displacement corresponding to the occurrence of the peak pullout load of the foundation lags behind the displacement at which the frictional resistance of the bucket wall stabilizes.

Estimating the Climate-Controlled Soil Parameters and the Distorted Mound Shape for Analysis of Stiffened Rafts on Expansive Soils

A stiffened raft is considered one of the efficient foundation systems for lightweight structures resting on expansive soils. Most existing design methods of stiffened rafts require an analysis of the interaction between the raft and the distorted mound shape of the expansive soil. In most design methods, the distorted mound shape is represented in 2D by the edge distance and the maximum differential movement through a nonlinear equation. This study presents a rational method for estimating the climate-controlled soil parameters that are used for estimating the 3D distorted mound shape of the expansive soil from the routine geotechnical tests’ results. These parameters include the equilibrium soil suction, the amplitude of surface suction change, the diffusion coefficient of the soil, the suction compression index, and the active zone depth. The proposed method is explained through its application to calculate the climate-controlled parameters for expansive soils in different locations throughout Saudi Arabia. A parametric study is carried out using a suction diffusion and soil movements program called SUCH to investigate the effect of the climate-controlled soil parameters and raft dimensions on the shape of the distorted mound, maximum differential movement, and edge moisture variation distance. The results of the parametric study are used in a regression analysis to develop an equation for estimating the edge moisture variation distance, which is considered a major barrier to using existing design methods of stiffened rafts as a function of the climate-controlled soil parameters and the aspect ratio of the raft. The findings indicate that the aspect ratio of the raft and the climate-controlled soil parameters have a significant effect on the shape of the distorted mound, its maximum differential movement, and its edge moisture variation distance. Additionally, the proposed equation of the edge moisture variation distance predicts comparable values to that estimated by the program SUCH.

Effects of moisture content and soil suction on the permanent deformation of tropical subgrade soils: experimental investigations and modelling

This study presents the effects of moisture variation and the influence of matric suction on the permanent deformation (PD) of three tropical soils with different geological-geotechnical characteristics used in road subgrades in southern Brazil. The experimental programme consisted in determining the soil-water characteristic curves and in dynamic triaxial tests to obtain the PD in different compaction and post-compaction moisture contents. The variation of compaction moisture content caused microstructural changes, influencing the plastic behaviour of soils: the higher the initial moisture content, the greater their accumulated permanent deformations. As expected, the post-compaction moisture variation (wetting process) tended to increase the plastic deformation of materials, evidencing the influence of the suction variation on the performance of the soils studied. In addition, matric suction proved to be the best variable to represent the effects of moisture variation on the plastic behaviour of soils subjected to cyclic loading. Thus, a PD prediction model for tropical soils with the inclusion of this parameter was proposed. The model proved to be highly predictive and may become an important tool to be incorporated into current mechanistic-empirical design methods.

Shear strength model of unsaturated frozen soil based on soil freezing characteristic and soil water characteristic

The strength characteristics of unsaturated soil after freezing significantly influence the safety of engineering construction in frozen soil regions. In this study, the soil freezing characteristic curve (SFCC) and soil water characteristic curve (SWCC) of unsaturated lean clay were tested. Based on the similarity between SFCC and SWCC, the unfrozen water content of SFCC was used to replace the water content of the SWCC to obtain the expression for soil suction under different negative temperatures. A shear strength model of unsaturated frozen soil was established. According to unsaturated triaxial tests under different conditions, the effectiveness of the established unsaturated soil shear strength model was verified. The test results show that in the rapid freezing stage of the SFCC, the volumetric unfrozen water content with an initial matrix suction of 57 kPa exhibited the largest rate of change. The stress-strain curve of soil at positive temperature is strain hardening type, and that at negative temperature presents different types under the influence of temperature, initial matrix suction and confining pressure. The effective cohesion and internal friction angle of soil varied significantly under the influence of temperature and initial matrix suction. When the initial matrix suction was constant, the unsaturated shear strength of the soil increased nonlinearly with a decrease in temperature. When the temperature was ≥−5 °C, the shear strength increased with an increase in initial matrix suction, but decreased when the temperature was ≤−5 °C. The model fitting results were consistent with the measured results of the unsaturated triaxial test, which verifies the validity of the model. This research can provide a reference for studying the mechanical characteristics of unsaturated frozen soil under negative temperature.

Water Evaporation and Soil Suction Measurements of Different Soil Types in Jordan

Water Evaporation and Soil Suction Measurements of Different Soil Types in Jordan

Load-Settlement Analysis of Axially Loaded Piles in Unsaturated Soils

Unsaturated soil covers a significant part of the world, and studying the behavior of deep foundations in this medium is an important step in increasing accuracy and economic efficiency in geotechnical studies. This paper presents an analytical solution to investigate the load-carrying characteristics of single piles embedded in unsaturated soils, accounting for the effect of groundwater level on the pile’s response. For this purpose, relationships for shear modulus and Poisson’s ratio for unsaturated soils were collected from the literature to consider their effects as key parameters on pile performance. A parametric study was conducted to evaluate the effect of soil moisture content on the behavior of the pile-soil system for different soil types, and the effect of pile slenderness on its load-settlement behavior was studied for varying soil moisture contents. The results indicate that the pile stiffness increases as the soil suction increases while below a critical slenderness value, hence increasing the pile load capacity. However, this improvement occurs within a limited range of soil suction that is narrower for coarse-grained soils. The pile settlement corresponding to soil failure was also evaluated by modifying the existing solutions for unsaturated soils. The developed solutions were verified against the predictions of published solutions as well as the results of finite element analysis and pile load tests. It was found that the system stiffness decreases by 50% when the water table rises from the pile toe level to the ground surface in the studied soil.

A new mathematical model for the estimation of shear modulus for unsaturated compacted soils

Small-strain shear modulus ( G) is an essential parameter for many geotechnical analyses. Most of shallow foundations are constructed in an unsaturated soil and the shear modulus of the unsaturated soil fluctuates because of the precipitation, evaporation, and rising of ground water table. In this paper, a new mathematical model is proposed for the estimation of the shear modulus function, Gunsat, which defines the relationship between small-strain modulus of unsaturated soil and matric suction. In the proposed model, the soil-water characteristic curve in the form of the degree of saturation is used as the input information. There are additional two parameters named n and C, which can be calibrated with the experimental data, are adopted in the proposed model. The estimated results show good agreement with the experimental data from literature. The proposed method can be used to track the tendency of Gunsat and minimize the data points from the laboratory tests.

Experimental and modeling study of shear strength of unsaturated fine-grained soils with bi-modal water-retention characteristics

The mechanical behavior and strength characteristics of unsaturated fine-grained soils with dual-porosity are of crucial importance in geotechnical designs. Nanyang fine-grained soils have been selected as typical dual-porosity structure soils to perform experimental tests under a wide range of suction and different initial densities while studying its stress-strain-strength properties constitute the main scope of this study. Axial translation and vapor equilibrium techniques are jointly employed to apply a wide suction range. Our data suggest that soil behavior transits from strain-hardening with shear-induced contraction to strain-softening with shear-induced dilation as suction and density increase. By exploiting a bi-modal soil–water retention curve (SWRC) explicitly separating capillarity and adsorption mechanisms, the shear strength is allowed to be analyzed in the capillary suction stress-shear stress space. The strength envelop exhibits bi-linear characteristics. Building upon these findings, we propose a bi-linear shear strength criterion specifically for dual-porosity fine-grained soils. We utilize the obtained test data to evaluate existing strength criteria based on effective stress and dual stress variables that consider the bi-modal SWRC characteristics. The comparison indicates that the proposed bi-linear shear strength criterion can more reasonably represent the variation of shear strength under a wide range of suction for unsaturated dual-porosity fine-grained soils.

Effect of compaction and moisture content on the deformation characteristics of unsaturated sandy soils varying fines content

ABSTRACT This research explores how the dynamic behaviour under cyclic compression loading affects compaction characteristics. Three types of sand, varying in fines content, are compacted under different saturation ( S r ) and compaction ( D c ) levels. Suction-controlled drained cyclic loading tests are conducted to predict axial strain accumulation, with initial suction measured for each compacted state. The study seeks to understand the correlation between axial strain accumulation and initial suction across soil types. Findings indicate that initial soil suction, influenced by saturation, significantly impacts axial strain under cyclic loading. Lower saturation enhances particle bonding, increasing initial suction and reducing strain accumulation. Moreover, an increase in fines content results in greater strain accumulation. Despite that, sands with elevated fines (18.8%) can achieve similar soil performance through effective compaction with precise saturation control. This underscores the role of saturation in moisture regulation during compaction, particularly for sands with higher non-plastic fines content, in achieving effective compaction.

Comparative study on suction obtained using the membrane filter method from triaxial apparatus and pressure plate apparatus

To investigate the stability of the earth embankments and slope failures during natural disasters, it is necessary to consider that soil is naturally unsaturated. Hence, it is essential to understand the variations of pore air pressure and pore water pressure, in terms of suction. The relationship between soil suction and soil moisture content is generally attained by soil water characteristic curve (SWCC), which is critically important in characterizing the mechanical behaviour of unsaturated soils. The SWCC is obtained from pressure plate apparatus by imposing the suction continuously by draining water from the saturated specimen and/or supplying water to unsaturated soil, which differs from the actual soil compaction process at the site. Therefore, the current study employs a methodology of suction measurement directly at a certain degree of compaction and degree of saturation using the membrane filter method in the triaxial apparatus, to meet the realistic conditions in the field. A comparative study has been done using SWCC with the imposed suction and the measured suction (Natural suction) at each compacted state. The results from both the methods exhibited an increase in the suction and air entry value with the increase in the degree of compaction, proving that an increasingly positive effect of suction contributed while increasing the density. Whereas the suction measured immediately after the compaction (Natural suction) in triaxial apparatus is more consistent with the suction imposed in the wetting process of SWCC. Further deep understanding of the microstructure behaviour of each compacted state and the SWCC is necessary.

Reliability of Soil Water Characteristics Curve under Different Normal Stresses for Unsaturated Sand

The soil water characteristics curve (SWCC) demonstrates a link between soil moisture content and suction. The SWCC is almost treated as an index parameter in unsaturated soil. The soil permeability and shear strength can be linked to SWCC. SWCC is established using filter paper for the wetting path. This paper compares SWCC for both cases stress-dependent and no stress (reference) in the wetting path. The samples of soil are brought from Al-Najaf City with a gypsum content of 29%, Iraq. The stress-SWCC is studied using a modified Oedometer cell with controlled water and air entrance to apply a specific matric suction. While the reference SWCC is estimated using the filter paper method. Matric suction is conducted at a range of pressures from 90 kPa to 0 kPa. The tests use three net normal stresses of 100, 200, and 400 kPa. The results show that there are decreasing values of the SWCC with increasing normal stresses. An interesting result is that this decrease is high and has zero matric suction. The water entry value (WEV) with the most significant value of the water entry change is represented by the matric suction of 50 kPa.