Soil Response Research Articles

Mechanism of the post-suffusion mechanical response of gap-graded soils from the perspective of force-chain evolution

The discrete element method (DEM) coupled with the pore-scale finite volume (PFV) method was used to simulate the suffusion and post-suffusion behavior of gap-graded soil with different initial fines content (fc). A series of drained triaxial tests were performed on the non-eroded, eroded and reconstituted samples. The results indicate that the erosion ratio (Er) increases as the fc of the sample increases. The mechanical response of the sample with an initial fc of 15 % is almost unaffected by suffusion. The dilatancy and peak deviatoric stress ratios of sample with initial fc of 25 % and 35 % are significantly lower with increasing erosion ratio. When the Er is greater than 8.2, eroded samples with an initial fc of 35 % collapse during shearing, and the eroded and reconstituted samples behave as dilation and contraction, respectively. As the initial fc increases, the mechanical response of the reconstituted samples differs more and more from that of the corresponding eroded samples. The force chain analysis indicates more force chains in the eroded sample than in the reconstituted sample, resulting in higher dilatancy of the eroded sample with fc of 25 % and 35 % during shearing.

Effects of different vegetation components on soil erosion and response to rainfall intensity under simulated rainfall

Vegetation effectively prevents soil erosion, however, the reductions in runoff and erosion caused by the different components of vegetation are unclear. Under simulated rainfall intensities (60, 90, and 120 mm h−1) and litter amount (5, 15, and 25 g m−2), five different treatments were established: intact grass (IG), litter and roots (LR), canopy and roots (CR), only litter (OL), only roots (OR), and a control bare plot (CK). The effects of the canopy, roots, and litter on erosion were quantitatively analysed. (1) The runoff rate and erosion rate were reduced by different treatments, and the IG treatment most effectively reduced runoff and sediment, followed by the LR, OL, CR, and OR treatments. (2) The velocity, runoff, and sediment reductions by litter were optimal. The velocity and runoff reduction effects of the roots were stronger than those of the canopy, reducing the sediment reduction more than the litter at 5 g m−2. The velocity, runoff, and sediment reductions produced by the canopy were weak, providing average contributions of 11.40%, 8.01%, and 7.34%, respectively. The vegetation more strongly affected sediment than runoff reduction. (3) Different treatments most effectively reduced runoff and sediment under a 60 mm h−1 rainfall. The contributions of the canopy, roots, and litter to reducing runoff and sediment decreased with increasing rainfall intensity. (4) Erosion rate and shear stress showed a linear relationship. The soil erodibility Kr and critical runoff shear stress τc of the IG plot were lowest and highest, respectively. Litter and roots decreased Kr and increased τc, respectively. Compared with the bare plot, Kr decreased by 44.44%–61.54% and 15.38%–40.74% and τc increased by 3.85%–7.44% and -1.65% to 7.69% in litter and roots, respectively. This study provides insights regarding the mechanism through which vegetation controls erosion and effective scientific basis for rational vegetation allocation.

An effective stress-based approach to modeling the chemo-mechanical behavior of saturated active clay

Pore water chemistry can exert significant controls over the chemo-mechanical behavior of chemically active soils, which has not been characterized by using the traditional Terzaghi’s effective stress to a satisfactory extent. In this paper, based on the concept of intergranular stress, a generalized effective stress for saturated active soils is reconceptualized and incorporated into the framework of the modified Cam–Clay (MCC) model to describe the mechanical response of soils upon complex chemical and mechanical loadings. The proposed model is capable of capturing very well the diverse features of the chemo-behavior of saturated active soils, while inheriting all the advantages of the MCC model, by introducing only one additional parameter. Within this context, a simple equation is derived to predict the development of swelling pressure with the variation of pore water chemistry. It is shown that the proposed swelling-pressure equation predicts very well the development of swelling pressure for expansive soils with a dilute pore solution, though discrepancy appears at high concentration. Because the double layer of clay particles is severely suppressed at high salt concentration, the microfabric of soil can be significantly changed, pointing to the importance of taking into account the effect of soil fabric in the constitutive modeling of active soils with high pore water concentration.

Study on environmental pollution behavior/fate of ammunition soil and microbial remediation of TNT and its intermediates

At present, research is lacking regarding the environmental behavior/fate of trinitrotoluene (TNT) pollutants produced by military activities and technologies for the efficient remediation of toxic byproducts. In this study, fresh explosive (TNT)-contaminated farmland soil was simulated to analyze the environmental behavior and trends of TNT using the enzyme system response, ionomers, microbial diversity, and the secondary nitrogen cycle network. Compound microbial agents were also prepared to repair soil contaminated by TNT, 2-amino-4,6-dinitrotoluene (2-ADNT), 4-amino-2,6-dinitrotoluene (4-ADNT), and 2,4-diamino-6-nitrotoluene (2,4-DANT). TNT significantly inhibited 23.1–63.6% of the soil nitrogen cycle-related enzymes. The soil ion network fluctuated significantly. The abundance of Sphingomonadaceae showing key tolerance/degradation TNT was significantly upregulated. Bryobacter and MND1 were important driving forces of soil community structure change. At the same time, TNT presence increased the risk of spread of soil biological diseases and unbalanced basic nucleotide, carbohydrate, and amino acid metabolism in the soil. The key network of the soil response to TNT was pyrimidine metabolism (biomarkers: uridine and deoxycytidine). Interestingly, soil glycolysis/gluconeogenesis and the tri carboxylic acid cycle (TCA cycle) maintained steady-state levels despite TNT toxicity. TNT was transformed into 17 soil metabolism intermediates. In addition, 12 isolated bacterial strains showed high degradation efficiency for TNT, 2-ADNT, 4-ADNT, and 2,4-DANT, and activated the mineral element defense network to resist the toxic effects of TNT, and Mn, Mg and B species were involved in the process of microbial degradation of pollutants. A compound mixture of these microbial agents efficiently degraded soil TNT and its intermediate toxic byproducts. Our analysis of the environmental behavior and trends for TNT provides a microbial remediation strategy for ammunition-contaminated soil and is of great significance for soil productivity recovery and climate change.

Understanding hydrologic controls of sloping soil response to precipitation through machine learning analysis applied to synthetic data

Abstract. Soil and underground conditions prior to the initiation of rainfall events control the hydrological processes that occur in slopes, affecting the water exchange through their boundaries. The present study aims at identifying suitable variables to be monitored to predict the response of sloping soil to precipitation. The case of a pyroclastic coarse-grained soil mantle overlaying a karstic bedrock in the southern Apennines (Italy) is described. Field monitoring of stream level recordings, meteorological variables, and soil water content and suction has been carried out for a few years. To enrich the field dataset, a synthetic series of 1000 years has been generated with a physically based model coupled to a stochastic rainfall model. Machine learning techniques have been used to unwrap the non-linear cause–effect relationships linking the variables. The k-means clustering technique has been used for the identification of seasonally recurrent slope conditions in terms of soil moisture and groundwater level, and the random forest technique has been used to assess how the conditions at the onset of rainfall controlled the attitude of the soil mantle to retain much of the infiltrating rainwater. The results show that the response in terms of the fraction of rainwater remaining stored in the soil mantle at the end of rainfall events is controlled by soil moisture and groundwater level prior to the rainfall initiation, giving evidence of the activation of effective drainage processes.

Modular polymer stormwater collection structure response to one-week design-truck load

Full-scale physical modelling was used to evaluate the response of modular polymer stormwater collection structures when subjected to design-truck loading that was sustained for 1 week (i.e. a parked vehicle) and when buried with minimum soil cover. Surface loading was applied on a steel load pad that simulates one-half of the AASHTO design-truck single-axle that was held constant for 1 week to assess the stability of the buried system and demand on the platen and column components of the modules to allow independent load rating of the buried structure. No collapse or limiting state occurred. The 1-week creep buckling resistance was found to be nearly four times the factored column demand to 1-week loading for both module types. New insight into the time-dependent behaviour of the soil–structure system was gained by comparing the buried response to creep tests conducted on isolated columns and platens. The response of the buried soil–structure system was less severe than unconstrained creep. Column loads were found to not increase with time. Bending deflections increased by 1.04 to 1.26 times when buried rather than by 3.0 to 3.4 times for unconstrained creep as platen creep when buried was greatly restrained by the soil.

Bearing capacity of lunar soil ground under extraterrestrial environmental effects

Lunar environment features a low gravity field and ultra-high vacuum which shows significant effects on the bearing behavior of lunar soil ground for future lunar habitation. Therefore, the 2D Discrete Element Method was employed to simulate the plate load tests under different gravity fields in non-vacuum (i.e., excluding the van der Waals force) and high-vacuum (i.e., including the van der Waals force) environments, analyzing the bearing behavior and soil response under extraterrestrial environmental effects. The results show that a bilinear line bounded by 1g can be employed to describe the gravity effect on the gravity factors for ultimate bearing capacity and coefficient of subgrade reaction. The slopes are different in low and high-gravity fields, indicating that the high-gravity effect in a centrifuge cannot be directly applied to predict the bearing behavior of lunar soil ground in a low-gravity field. The affected area becomes deeper in depth and narrower in width with the decreasing gravity level in non-vacuum environment, while the affected area becomes deeper in depth and wider in width with the decreasing gravity level in high-vacuum environment. Such observations can provide advice for developing analytical models for the bearing capacity of lunar soil ground considering the extraterrestrial environmental effects.

A new seismic input method for layered soil and its application in dynamic response analysis of underground structure

With the acceleration of urbanization, the seismic problem of underground engineering is getting more and more attention. In actual projects, foundation soils often show a complex layered distribution. In terms of layered ground motion input, this study propose a new layered ground motion input method based on infinite element boundary, which avoids the application of spring and damping elements on the boundary nodes of layered soil, simplifies the ground motion input process and has high accuracy at the same time. To reasonably simulate the nonlinear seismic response of layered soil, the equivalent linear model of soil is nested to Mohr-Coulomb (MC) model to establish the viscoelastic-plastic model. The seismic response analysis procedure of the soil-structure monolithic model considering the nonlinearity of the layered soil is given. On this basis, the soil-structure nonlinear seismic response analysis is carried out to study the effect of burial depth on the seismic performance of underground structures, taking a project as an example. The results show that with the increase of the burial depth of the structure, the top–bottom displacement difference shows a change law of first increasing and then decreasing under the action of earthquake; From the internal force response, the most unfavorable burial depth exists due to the joint influence of the burial depth of the structure and the characteristics of the layered soil; For the same burial depth condition, the dynamic bending moment caused by the seismic load action is larger compared to the shear response.

Micromechanics and Ultrasonic Propagation in Consolidated Earthen-Site Soils.

Although nondestructive ultrasonic technologies have been applied in laboratory and field tests in the field of heritage conservation, few studies have quantified the relationship among the real microstructures, micromechanical properties, and macroscopic acoustic responses of earthen-site soils. This paper develops a micromechanics-based multiscale model for quantitatively exploring the ultrasonic propagation characteristics of elastic waves in untreated and consolidated earthen-site soils. Scanning electron microscope images and image processing technology are integrated into the finite-element simulation. The effects of microstructure and wave features on the acoustic characteristics of soils are quantitatively investigated under pulsive loading. The simulation results of untreated and consolidated soils are efficiently compared to ultrasonic test data. It is demonstrated that the integration of microstructure image processing and multiscale modeling can predict the ultrasonic pulse velocity well, which improves the accuracy of laboratory testing and field monitoring and better serves the evaluation and implementation of engineering practice in the field of heritage conservation.

Response of soil–water characteristics to pore structure of granite residual soils

Granite residual soil is a special regional soil with special mineral composition and pore structure characteristics, which is easy to induce serious geological disasters or engineering problems, so it is particularly important to study its mechanical properties of unsaturated soil and its control mechanism. However, the effects of dry density and initial water content on soil–water characteristic curve (SWCC) and their mechanisms are still unclear. Therefore, samples with different dry densities (1.30 g/cm3, 1.50 g/cm3, 1.70 g/cm3) and initial water content (14 %, 20 %, 22 %) were set up in this paper. SWCC test was conducted on the two groups of samples under the humidification path and dehumidification path using filter paper method. Combined with scanning electron microscopy (SEM) test and pore-size distribution (PSD) test, the influence mechanism of different micro-pore structure on SWCC and hysteresis characteristics of granite residual soil was analyzed qualitatively and quantitatively. The results show that the samples with different dry densities basically coincide with each other in the high suction segment. The larger the dry density is, the smaller the range of transition zone in the low suction segment is. As the initial water content of the sample increases from 14 % to 22 %, SWCC changes from a single increase curve to a double increase curve, and the corresponding pore-size distribution curve (PSDC) changes from a trimodal curve to a bimodal curve. The SWCC of granite residual soil has obvious hysteresis effect, and the hysteresis area becomes smaller with the increase of dry density. The inflection point exists in the hysteresis area of SWCCS with higher initial water content. The distribution range of macropore is determined by dry density, and the distribution range of small pore is determined by initial water content. The transformation of SWCC from a single increase curve to a double increase curve is mainly controlled by the distribution range of small pores. The bottleneck effect in the migration process of pore water in the soil and the pore redistribution during water intake and water loss are the main reason for the hysteresis of SWCC. The results of this work provide some guidance for the study of unsaturated soil mechanical properties of granite residual soils.

Impacts of wave-induced seabed response on local scour around a pipeline: PORO–FSSI–SCOUR–FOAM

The problem of local scour around a submarine pipeline is vital for oil and gas projects. Conventional scour models around pipeline practically include the flow and sediment transport characteristics, but they ignore the interactions between flow and seabed soil in the scouring process. In this study, a new two-dimensional model for simulating the scouring process is established while considering the impact of the wave-induced soil response and the associated seepage. With the present model, the influence of seepage and soil response on the modified Shields number, repose angle and prediction of the local scour depth and profile are examined. The numerical results indicate that the influence of repose angle when modified by seepage in the whole scour process is small. Meanwhile, the wave-induced dynamic soil response promotes the development of the scour profile and eventually increases the maximum scour depth to some extent, regardless of the change in the soil characteristics. However, at smaller KC numbers, the variations in the maximum scour depth with seabed properties become more significant.

The influence of soil fabric on the monotonic and cyclic shear behaviour of consolidated and compacted specimens

While the fabric of soil can significantly influence its behaviour, the effect of varying fabric parameters on the subgrade shear response is still not well understood. This study creates soil specimens with different fabrics which are then captured through X-ray microscopic-computed tomography scanning and quantified by image processing techniques. A comprehensive laboratory investigation is conducted to understand how the soil fabric affects its monotonic and cyclic shear behaviour. The results indicate that the consolidation method creates a more homogeneous fabric with mainly small-to-medium interconnected pores, whereas the compaction technique creates significantly large and mostly inter-aggregate pores with lower connectivity. In this regard, the consolidated specimens exhibit an elastic-perfectly plastic behaviour, while the compacted specimens show strain-hardening transformation during isotropic monotonic shearing. Under anisotropic conditions, the compacted specimens exhibit a greater strain softening response and excess pore pressure than the consolidated specimens because they have a weaker fabric. Furthermore, the compacted specimens show a smaller threshold strain at a lower critical number of cycles due to the collapse of large pores. These current findings prove the decisive role that soil fabric plays in determining the shear response and failure of subgrade soils.

Editorial: Special Issue on Experimental Investigations from Very Small Strains to Beyond Failure

Abstract This Geotechnical Testing Journal Special Issue on Experimental Investigations from Very Small Strains to Beyond Failure presents selected papers originally submitted to the Eighth International Symposium on Deformation Characteristics of Geomaterials held in Porto, Portugal, from September 3–6, 2023 (IS-Porto 2023). This Symposium aimed at exploring and sharing ideas about the complex stress–strain response of geomaterials, including laboratory methods from very small to large strains; anisotropy and localization; time-dependent response of soils; characteristics of treated, unsaturated, and natural geomaterials; applications in field methods; evaluation of field performance in geotechnical structures; and physical and numerical modeling in geomechanics. The Symposium was a three-day event organized by the Faculty of Engineering of the University of Porto, under the auspices of the Technical Committee TC101 Laboratory Stress Strain Strength Testing of Geomaterials of the International Society for Soil Mechanics and Geotechnical Engineering (ISSMGE). This symposium was endorsed by the Portuguese Geotechnical Society (SPG), the Faculty of Engineering of the University of Porto (FEUP), under the activities of the Research Unit Institute of R&D in Structures and Construction (CONSTRUCT) of the Portuguese Foundation for Science and Technology (FCT). Building on the tradition of previous Symposia from Sapporo in 1994 to IS-Glasgow 2019, IS-Porto 2023 brought together more than 240 researchers and practitioners from 37 countries to share their experiences and explore new directions in testing, and in constitutive and numerical modelling of geomaterials. This Special Issue of ASTM International’s Geotechnical Testing Journal assembles eleven papers that illustrate some most recent, innovative, and significant advancements in laboratory equipment and experimental methodologies and procedures for the experimental investigations of soils from very small strains to beyond failure. Although most soil research laboratories are now equipped with the latest technology, in terms of transducers or testing apparatus, the inherent complex nature of soils makes studying them an ongoing challenge. This issue reflects the variety of topics that are still being considered, from difficult terrestrial soils, such as high plasticity stiff sedimentary clays (Sorensen et al. 2023), pumice soils (Hashimoto et al. 2023), gravelly (Nikoonejad and Mohsen Haeri 2023) and residual soils (Cheng and Leong 2023), and mine tailings (Narainsamy et al. 2023), to sampling and testing soils from Mars (Chaparro-López et al. 2023). Characterizing liquefiable soils, particularly partially saturated or gassy soils, and its post-liquefaction behavior continues to remain a subject of investigation, particularly in seismic regions, and is discussed in four papers, with emphasis on the applicability of partial saturation for liquefaction mitigation (Ahmad et al. 2023; Kaminski and Grabe 2023; Mele et al. 2023; Molina-Gómez et al. 2023). Greener methods of soil improvement are also explored, for example, biocementation under chemical damage (Ribeiro et al. 2023). The guest editors of this special issue would like to gratefully acknowledge the dedication and effort of all the authors and peer-reviewers of these papers, and the collaboration of the ASTM Editorial Team in the production of this special issue.

Belowground C sequestrations response to grazing exclusion in global grasslands: Dynamics and mechanisms

Globally, grazing exclusion is a widely implemented management strategy for restoring degraded grassland ecosystems and sequestering carbon (C). However, there is limited understanding regarding the temporal responses and underlying factors influencing ecosystem C stocks following grazing exclusion. In this study, we conducted a comprehensive synthesis of data from 199 independent experiments (454 pairwise observations) to analyze responses of plant and soil C stocks to grazing exclusion across four distinct grassland ecosystems (desert, typical, meadow, and alpine) in the globe. We found that rates of change in plant biomass C stocks and soil organic C stocks exponentially or rationally decreased with years since enclosure. Grazing exclusion generally enhanced aboveground biomass C in plants, while its effects on C stocks of belowground biomass and soil were more contingent upon various factors, such as climate, initial levels of C stocks, and grazing exclusion duration. Furthermore, the responses of C stocks of plant biomass and soil to livestock grazing cessation tend to stabilize over time, with equilibrium typically reaching after approximately 40 years, while soil C sequestration responses exhibited a lagged pattern compared to plant biomass C. Our results underscored the effectiveness of grazing exclusion as an effective strategy to enhance C stocks in regions characterized by low C content and non-water limited conditions. We propose that grazing exclusion for 1–5 years was the best restoration time for typical, meadow and alpine grasslands. Given the limited effects of grazing exclusion on soil organic C stocks of desert types, grazing exclusion might not be an effective measure to increase the soil organic C stocks in water-limited areas like desert grasslands.

Effects of Three Long-Term Land Use Patterns on Soil Degradation in the Yellow River Delta: Evidence from Ecological Stoichiometry

The irrational land use patterns in the Yellow River Delta (YRD) have resulted in an imbalance in ecological stoichiometry, leading to secondary salinization and soil degradation. However, there is limited knowledge about the long-term response of soil and enzyme stoichiometry to land use. This hampers our ability to optimize land use in the YRD to alleviate nutrient limitation and thus promote ecological stoichiometric balance. We investigated the stoichiometry of soil and enzyme carbon (C), nitrogen (N), and phosphorus (P) in three land use patterns (Alfalfa artificial grassland, AG; wheat–maize rotation field, WM; native grassland, PC) established for 19 years in the YRD. The results showed that the soil stoichiometry of the three land uses in the YRD was lower than the world and Chinese averages, indicating lower C and N levels. Nutrient limitations of soil microorganisms were C and P due to an enzyme C:N ratio greater than 1:1 and vector angle greater than 45°. The three land use patterns have different advantages in alleviating nutrient limitations in the YRD. AG promotes soil macroaggregate formation, reduces soil salt content, improves nutrient availability, and mitigates N limitation. This makes AG more conducive to improving the poor soil structure, high soil salinity, and stoichiometric imbalance in the YRD to mitigate local soil degradation and be suitable for long-term continuous cultivation. WM is beneficial for increasing soil total C content due to straw return. However, WM does not reduce soil salinity. WM is more suitable for intercropping or crop rotation to improve soil C content in the YRD. Although PC can alleviate soil microbial C limitation due to its significantly lower vector length than AG and WM, the low nutrient levels hindered its ability to alleviate local soil nutrient limitation. In conclusion, our study provides a theoretical basis for rational land use in the YRD to mitigate soil degradation.

Enzyme Activity and Dissolved Organic Carbon Content in Soils Amended with Different Types of Biochar and Exogenous Organic Matter

Biochars are proposed as a strategy for long-term carbon sequestration. High resistance for decomposition, low decay rate and long estimated lifetime allow for stable forms of carbon to be retained in the environment. Nevertheless, the application of pyrolyzed feedstock, particularly along with exogenous organic matter, may affect carbon dynamics in soil through the introduction of labile compounds and the stimulation of extracellular enzymes. The aim of this research was to evaluate the influence of biochars and unprocessed organic amendments in two agricultural soils on the dissolved organic carbon (DOC) content and activity of three enzymes involved in carbon turnover. In the incubation experiment, the activity of dehydrogenase, β-glucosidase, and cellulase and the DOC content were measured on days 30, 60, 90, 180, and 360. The addition of biochars stimulated dehydrogenase and β-glucosidase, while cellulase was suppressed. Fresh biomass enhanced the activity of the enzymes through a priming effect. DOC content was the highest in treatments with high enzyme activity, suggesting that it acted as a source of energy for microbes. The findings suggest that the biochar properties and the presence of exogenous organic matter affect microbial response in soil, which might be crucial for carbon sequestration. However, long-term studies are recommended to fully understand the mechanisms that determine the response of soil biota to biochar.

Application of compost assisted by Fe3O4 nanoparticles in di (2-ethylhexyl) phthalate-contaminated soil remediation: Biostimulation strategy, Soil responses, and RSM/CCD Optimization

Globally, contamination of agricultural soils by phthalate esters (PAEs) caused by direct consumption of plastic mulch films has been confirmed. The most widely used plasticizer is di (2-ethylhexyl) phthalate (DEHP), which is a more recalcitrant endocrine-disrupting chemical (EDC). Because of its low solubility and hydrophobicity, it remains in the soil longer, causes bioaccumulation in agricultural products, and has negative repercussions for food safety. In this study, the performance of kitchen organic waste compost assisted by Fe3O4 nanoparticles in DEHP removal efficiency (%) and soil C:N ratio (two responses) was optimized using Response Surface Methodology (RSM) based on Central Composite Design (CCD) in Design-Expert software (11.0.3.0). Under optimum conditions, a DEHP concentration of 10 mg·kg−1 (dw soil), a retention time of 35 days, an NPs dose of 0.99 g·kg−1 (media), a removal efficiency of 91.6 %, and a soil C:N ratio of 10.5 with a desirability of 0.963 were determined. A quadratic model (P-value <0.0001, adjusted R2 = 0.974 (Y1), 0.943 (Y2)) was used to predict the variables and their interactions. The agricultural soil responses in the treatments amended by compost and Fe3O4 NPs (SCN) showed a significant increase in SOM, TC, TN, AP, K, and Fe nutrients when compared to the control (P < 0.05). After 35 days, in the SC1N3 treatment (DEHP concentration = 10 mg·kg−1, NPs dose =1.2 g·kg−1), with higher DEHP removal efficiency (89.57 %), the C:N:P ratio was equal to 100: 9.75:0.69, and the total microbial colony count was 3.6 × 109 CFU/ml at pH 7.45. The study found that compost nutrients and Fe-based nanoparticle micronutrients can enhance DEHP degradation by stimulating the soil's native microflora. As a result, the synergistic potential of compost and Fe3O4 nanoparticles can be considered a promising, cost-effective, and agri-environmentally friendly approach in the “assisted bioremediation” strategy of DEHP-contaminated soils.

FEM-BEM modeling dynamic responses of saturated soil around a fluid-filled lined tunnel subjected to water hammer

Dynamic stress responses of saturated soil around a fluid-filled lined tunnel caused by a water hammer are investigated by a frequency-domain FEM-BEM coupled model. The fluid is modeled as an inviscid and compressible fluid, the lining is modeled by elastic medium and conceptualized as a hollow cylinder of finite length, and the saturated poroelastic medium is adopted to model the soil. Initially, governing equations of fluid and those of lining are solved by FEM in the frequency domain, while those of soil are solved by BEM in the same domain. In the following, fluid, lining, and soil are coupled based on the conditions of deformation compatibility and force balance on their interfaces. Water pressure (inside the tunnel), the distribution of lining displacement and dynamic stress responses of saturated soil generated by the water hammer are presented. It is concluded that the dynamic stresses and the pore pressure change periodically in saturated soil under a water hammer. Modeling soil as an elastic medium inaccurately evaluates the distribution of lining displacement. The soil permeability has a significant influence on the normal stresses of soil and pore pressure but has a slight effect on the shear stresses of soil.

Microscopic mechanism of the combined effects of confining pressure and fines content on suffusion in gap-graded underfilled soils

Suffusion is a typical form of internal erosion, which is the primary cause of seepage failures in hydraulic geo-structures. Revealing the underlying mechanism of suffusion is crucial for designing hydraulic geo-structures and preventing associated failures. Extensive studies have investigated the combined effects of fines content and confining pressure on suffusion in gap-graded soils. However, few of them have focused on underfilled soil, which may be largely different from overfilled soil. This study aims to study microscopic mechanism of suffusion in underfilled soil using the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM). Three fines content (15 %, 20 % and 25 %) and three confining pressures (50, 100 and 200 kPa) are considered for investigating their combined effects on suffusion. The results show two distinct patterns of macroscopic response for the underfilled soils. That is, the cumulative eroded fines mass decreases with increasing confining pressure, especially for samples with high fines content. Additionally, samples with low fines content exhibit negligible suffusion-induced volumetric deformation, while those with high fines content display significant deformation. Particle migration analysis shows that the samples are subjected to non-uniform erosion along the seepage direction, with minimal changes in the number of particles in the middle section, but a notable reduction near the inlet and outlet sections. Moreover, greater confining pressure results in fewer migrating fine particles, particularly in samples with high fine content. Furthermore, particle contact analysis indicates that in samples with low fines content, effective stress is mainly transmitted through coarse particles, resulting in a reduced impact of confining pressure on the internal stability of fine particles. Conversely, in samples with high fines content, a greater proportion of fine particles and coarse particles jointly form the load-bearing skeleton, making them more prone to confining pressure. Hence, two distinct microscopic mechanisms are proposed to well explain the suffusion behavior of underfilled soil from particle-scale perspectives.

Understory vegetation had important impact on soil microbial characteristics than canopy tree under N addition in a Pinus tabuliformis plantation

Forest ecosystem is a complex community, and its soil microbes play a vital role in global matter cycling. However, the patterns of rhizosphere soil microbial characteristics among tree-shrub-grass and their responses to nitrogen (N) deposition are still unclear. This study evaluated the microbial biomasses, processes (gas fluxes), and enzyme activities in the rhizosphere soils of tree (Pinus tabuliformis), shrub (Rosa xanthina), and grass (Carex lanceolata) and their responses to N addition (0, 3, 6, 9 g N m−2 y−1 corresponding to N0, N3, N6, N9) in a P. tabuliformis plantation. (1) Microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), microbial biomass phosphorus (MBP) contents, as well as CO2, CH4 flux rates in the rhizosphere soil of C. lanceolata were significantly higher than those in the rhizosphere soils of P. tabuliformis and R. xanthina (P < 0.05). The activities of alkaline phosphatase (ALT), leucine aminopeptidase (LAP), β-glucosidase (BG), D-cellulosidase (CBH), N-acetyl-glucosaminidase (NAG), and xylosidase (XYL) in the rhizosphere soils of C. lanceolata and R. xanthina were significantly higher than those in the rhizosphere soil of P. tabuliformis. (2) Soil MBC content, N2O, and CH4 flux rates increased after N addition, while the CO2 flux rate decreased. MBP content initially increased and then reduced with N additions, and the maximum values observed in N3 or N9 treatments. (3) In the rhizosphere soil of P. tabuliformis, N addition enhanced the NAG, BG, CBH, and XYL activities. While low N increased and high N decreased these enzyme activities in the rhizosphere soils of C. lanceolata and R. xanthina, with the maximum values obtained in N3 treatment. (4) N addition directly reduced the CO2 flux rate and indirectly enhanced the gas flux rate by increasing soil MBC and MBP contents. Species change indirectly affected the microbial biomass and enzyme activity by altering soil chemical properties, which eventually affected gas flux rates. Our study emphasizes the effects of tree-shrub-grass on soil microbial characteristics, which can improve the evaluation of soil ecological processes and responses to N deposition in forest ecosystems.