Soil Matric Suction Research Articles

Mechanical response of the real tree root architecture under lateral load

The impact of climate change on tree stability is often associated with a higher risk of windthrow due to higher frequency and greater magnitude of extreme climatic conditions. Higher lateral loads due to an increase in maximum wind and rainfall reduce tree anchorage because of a decrease in soil matric suction and consequently the overall strength in the system of trunk, root, and soil. This study compared the mechanical response of trees with different root architectures using static loading tests conducted in the field and numerical analysis of laser-scanned root systems. For this case, mature trees of Khaya senegalensis (Desr.) A. Juss., Samanea saman (Jacq.) Merr., and Syzygium grande (Wight) Wight ex Walp. were tested and analyzed. The root system models consisted of root system architectures obtained using 3-D laser scanning. A parametric analysis was conducted by varying the modulus of elasticity of the soil (Es) from 2.5 to 25 MPa, and the results were compared with those of the static loading tests to obtain the overall mechanical responses of the root–soil systems. The results showed important dependencies of the mechanical responses of the root–soil system on the root architecture in withstanding the lateral load. The numerical models also allowed estimation of the effective leeward and windward anchorage zones with different soil elastic moduli and rooting architectures to define the extent of the tree root protection zones.

Untangling the influence of soil moisture on root pullout property of alfafa plant

Root pullout property of plants was of key importance to the soil reinforcement and the improvement of slope stability. To investigate the influence of soil moisture on root pullout resistance and failure modes in soil reinforcement process, we conducted pullout tests on alfalfa (Medicago sativa L.) roots at five levels (40, 30, 20, 10 and 6 kPa) of soil matric suction, corresponding to respectively 7.84%, 9.66%, 13.02%, 19.35% and 27.06% gravimetric soil moisture contents. Results showed that the maximal root pullout force of M. sativa decreased in a power function with increasing soil moisture content from 7.84% to 27.06%. Root slippage rate increased and breakage rate decreased with increasing soil moisture content. At 9.66% soil moisture content, root slippage rate and breakage rate was 56.41% and 43.58%, respectively. The threshold value of soil moisture content was about 9.00% for alfalfa roots in the loess soil. The maximal pullout force of M. sativa increased with root diameter in a power function. The threshold value of root diameter was 1.15 mm, because root slipping force was greater than root breaking force when diameter >1.15 mm, while diameter ≤1.15 mm, root slipping force tended to be less than root breaking force. No significant difference in pullout forces was observed between slipping roots and breaking roots when they had similar diameters. More easily obtained root tensile force (strength) is suggested to be used in root reinforcement models under the condition that the effect of root diameter is excluded as the pullout force of breaking roots measured in pullout tests is similar to the root tensile force obtained by tensile tests.

Case Study of the Effect of Rainfall Infiltration on a Tunnel Underlying the Roadbed Slope with Weak Inter-Layer

It is a common type of tunnel under-cross the roadbed slope with weak inter-layer. Rainfall infiltration will cause soil softening, matric suction and strength reduction, which seriously threaten the safety of slope and tunnel. This paper presents a case where a tunnel underlying the roadbed slope with weak inter-layer in Guiyang 1# line, China. The seepage calculation model in MIDAS GTS/NX software, considering hydraulic permeability coefficient characteristic curve and soil-water characteristic curve, was used to analyze the stress and deformation of the roadbed slope and tunnel. The results reveal that rainfall infiltration has great influence on the deformation of roadbed slope with weak inter-layer and the force of internal tunnel. Slope displacement increases rapidly in the early stage and decreases in the late stage, but the absolute value of displacement increases continuously and the slope safety factor decreases gradually, which makes the slope in a dangerous state. The force of tunnel lining increases in the early stage and decreases with the recovery of soil strength after rainfall. The control effects of anchor cable and anti-slide pile reinforcement on slope sliding and tunnel stress were studied. And the anchor cable reinforcement measure is more effective on reducing slope slip and tunnel lining structure internal force.

DEM simulation for shear behavior in unsaturated granular materials at low-stress state

Abstract In this study, the three-dimensional discrete element method (DEM) is used to model the unsaturated triaxial test at a low-stress state. The contact model, which considers the matric suction of soils as well as the contact area among particles, and the flexible boundary condition are proposed. Prior to the simulation for the unsaturated triaxial test, we identify the effect of them on the numerical results to get an optimal condition. The deformability of the lateral boundary highly affects the post-peck shear behavior. The specimens with the contact area become brittle like unsaturated soils because it makes interparticle bonding more fragile. According to the comparison of the numerical and experimental results, all the simulations can predict the pre-peck shear behavior as well as the shear strength of unsaturated soils, but some of them underestimate the post-peck shear behavior. In the simulation for the unsaturated soil at a relatively large stress state, the regenerated menisci have a key role in the residual state. In the simulation for the unsaturated soil with higher water content, not only the menisci at contact points but also the bulk water around particles should be taken into account. The particle-scale matric suction in the contact model can exhibit a linear relation with the matric suction of soils, while there is a nonlinear relation between the interparticle stiffness and the matric suction of soils. These relations can be criteria for choosing the input parameters related to the matric suction in further studies. DEM simulation can also provide the microscopic response such as the propagation of the particle-scale fractures and the distribution of particle’s angular velocities.

A Novel Experimental Technique to Investigate Soil–Pipeline Interaction under Axial Loading in Saturated and Unsaturated Sands

Abstract Comprehensive experimental and numerical models are required for reliable investigation of unsaturated soil-structure interaction problems. In this study, a reliable testing technique and a numerical modeling methodology were developed to study the performance of a buried pipeline system in sand subjected to relative soil movement in a direction parallel to its axis under both saturated and unsaturated conditions. A novel testing system was developed using a specially designed test box of 1.5-m length, 1.2-m width, and 1.1-m height. The system utilizes the hanging column technique to achieve different soil matric suction profiles. The experimental results on the tested sand suggest that the measured axial force exerted on a pipe in unsaturated sand is significantly higher compared with the saturated condition. The measured behavior of the prototype pipe was numerically modeled using the commercial software SIGMA/W extending the principles of saturated and unsaturated soil mechanics. The predicted axial force, following the suggested finite element analysis (FEA) methodology, is in good agreement with the measured axial force on the tested prototype pipe. Both the experimental technique and numerical method proposed in this article are useful for the practicing engineers in the rational design of pipelines in sandy soils, taking account of the influence of saturated and unsaturated conditions in sandy soils.

A physical model for the interaction between unsaturated soils and retaining structures

Temporary and permanent retaining structures interact with soils that are usually in unsaturated conditions. In this work, a 1g-scale physical model is presented to investigate the interactions between retaining walls and unsaturated soils. The physical model is equipped with a water-filled hydraulic cylinder connected to a pressure-volume controller to measure the horizontal component of the later earth thrust and high capacity tensiometers to measure soil matric suction. A system of low-friction linear guideways has been installed at the base of the wall-model. The failure surface is observed through a 3 cm thick glass wall on one side of the container. A series of images are acquired during the tests, and Particle Image Velocimetry (PIV) technique has been used to identify the displacement field. Selected test results on a fine sandy soil are presented, emphasizing the differences in the lateral thrust between dry and partially saturated conditions. The presented results show the impact of the partially saturated condition on both the magnitude of the horizontal component of the lateral earth thrust and the failure mechanism at active state.

A novel framework to study the effect of tree architectural traits on stemflow yield and its consequences for soil-water dynamics

A novel experimental approach and numerical framework are proposed to study the effect of tree architectural traits on stemflow yield and its effects on soil-water dynamics. The framework includes a data mining workflow employing information from two experimental steps: (i) evaluation of the effect of tree aboveground architecture on stemflow yield and (ii) quantification of specific parameters for soil-water dynamics with and without stemflow. We studied double-funnelling (stemflow and root-induced preferential flow) under three sycamore (Acer pseudoplatanus L.) trees growing on a slope in Scotland during the summer season and measured architectural traits. Stemflow yield ranged from 1.3 to 3.8% of the incident rainfall, with funnelling ratios of between 2.2 ± 2.1 and 5.2 ± 3.9. Double-funnelling to a depth of up to 400 mm beneath the soil surface occurred as matrix flow and was significantly and positively correlated with the vertical root distribution. Soil-water dynamics were distinctly different with and without stemflow. Our framework revealed that the number of tree branches, their insertion angle, leaf number, and stem basal diameter influenced stemflow yield within rainfall thresholds of 1.1 and 3.5 mm d-1. The framework also showed that stemflow yield had a negative impact on soil matric suction, while air temperature was the most influential covariate affecting soil-water dynamics, likely due to its strong correlation to evapotranspiration during the summer season. In spite of the study limitations, such as small sample size and differences between individuals, we show that the proposed framework and experimental approach can contribute to our knowledge of how stemflow generated aboveground triggers major responses in soil-water dynamics belowground.

Dynamic response of unsaturated full-space caused by a circular tunnel subjected to a vertical harmonic point load

This study proposes a model to analyze a deep tunnel embedded in unsaturated soil. Key feature is the consideration of the coupled soil water and air. The analysis emphasizes on the dynamic responses of wave field induced by a unit vertical harmonic point load on the invert level of tunnel. The tunnel is modelled as an infinitely long shell with a circular section. The soil is simulated as an unsaturated poroelastic full-space in which the solid matrix is filled with the coupled soil water and air. Stress compatibility and displacement continuity are used to describe the interaction between the tunnel and the soil. A good agreement regarding the results of soil displacements between the present model and existing models for saturated soil and elastic media confirms the validity of the proposed model. The numerical analysis investigates the effects of water saturation on soil displacements and matric suction under different load frequencies. The results reveal that the dynamic responses of unsaturated soil are different from those in saturated soil. Increased water saturation can vary the value of displacements and matric suction of the soil, but not their distribution pattern around the tunnel. Displacements and matric suction distribution curves along the circumferences of the tunnel have more peak points in the case of higher load frequencies.

Determining the bearing capacity factor due to nonlinear matric suction distribution in the soil

Bearing capacity is often calculated in dry or saturated conditions, leading to overconservative designs, for a wide range of climates in the world. Extensive researches show that bearing capacity is significantly affected by the soil matric suction. However, in most of the presented models, uniform (and sometimes linear) suction distributions are taken into account for computing the bearing capacity. Also, there is no exact solution in the residual zone of unsaturation. In the present study, a simple method is proposed to predict the bearing capacity of footings placed on unsaturated soil, using the limit equilibrium concept. Linear and uniform variations of matric suction are considered in computations, as well as the nonlinear suction distribution. The framework of the proposed model is analogous to Terzaghi’s equation, and a novel factor is developed, during calculations, as the suction bearing capacity factor. In the case of full saturation, the proposed model is simplified to the Terzaghi’s equation. Estimated results are compared with the experimental and theoretical data available in the literature. Predicted values are in a good agreement with the measured data in the transition zone and residual zone of unsaturation.

Effects of fractal dimension and water content on the shear strength of red soil in the hilly granitic region of southern China

Deep residual soil, formed by the long-term weathering of large granite under the impact of high temperature and abundant rainfall in Southern China, is prone to instability and deformation, which has caused a large number of collapsing gullies. In this study, five different soil grading ratios and five water contents were employed to systematically investigate the effects of the soil fractal dimension and water content on the red soil shear strength. The soil fractal dimension increased with soil fine particle content (clay and silt); the soil saturated hydraulic conductivity (Ks) decreased with the soil fractal dimension; whereas the soil air entry value and residual water content (θr) showed a positive relationship with the fractal dimension. Shearing tests showed that the soil shear strength increased with normal stress but decreased with the soil water content. This reduction in the soil shear strength caused by the increasing water content was more notable for the soil with a lower fractal dimension. Analyses of the main effect tests showed that the water content presented greater effects on the soil cohesion and friction angle than the soil fractal dimension. A stepwise regression analysis showed that a linear relationship occurred between soil matric suction and soil cohesion (average R = 0.981 and P = .004) or friction angle (average R = 0.951 and P = .016). For the same matric suction, the soil cohesion and friction angles decreased with the soil fractal dimension, and the relationship between the soil fractal dimension with the soil cohesion (R = 0.951 and P = .013) and friction angles (R = 0.922 and P = .026) could be described by logarithmic functions. Soil fractal dimensions influenced both the soil hydraulic properties and shear strength parameters, and the overall effects of fine particles loss would decrease the shear strength of gully red soil.

Rainfall induced residual soil slope instability: building cracked and slope failure

Rainfall is one of the factors of slope failures due to the intensity of rainfall that will seep and increase the moisture content of geo-materials. One of the important role in slope stability is matric suction. This study present the relationship between rainfall and suction of soil which induced the slope instability and the slope stability analysis involving a building which may cause by instability due to suction variation. The slope had separated into three parts which are top of slope, middle of slope and toe of slope. Every part has been inserted with three tensiometer with 0.3 m, 0.45 m and 0.9 m depth. Suction of soil on slope was carried out by using tensiometer. Intensity of rainfall was collected by using rain gauge which placed at open area which to compare with matric suction of soil. Several site and laboratory tests were carried out to identify the soil properties. Stability of slope was analyzed using Slope/w and Manual Calculation via input parameters collected from laboratory test and field test. Fellenius’ method was used to interpret the factor of safety for slope study. This includes the analysis of the factor of safety of slices at the selected area with its matric suction value. It was found that when the rainfall increased, the suction of soil decreased and affected to decrease of factor of safety. Hence, the constructed slope was suggested to be redesign to improve its safety factor.

Potential of thermal imaging in soil bioengineering to assess plant ability for soil water removal and air cooling

Soil bioengineering uses living vegetation to improve the ecological value and engineering performance of infrastructure slopes. Vegetation can stabilise slopes and provide co-benefits such as air cooling for potential mitigation of the urban heat island effect. This study represents the first attempt to apply the thermal imaging technique to simultaneously evaluate the abilities of plants to induce suction via transpiration (hydrological reinforcement) and cooling in the context of soil bioengineering. A pilot glasshouse experiment was performed to evaluate the use of thermal imaging to determine plant transpiration of Corylus avellana and Ilex aquifolium. Field thermal imaging was also carried out on a 20 m-long vegetated embankment where soil matric suction was measured. In addition to C. avellana and I. aquifolium, Ulex europaeus was also selected for testing in the field. U. europaeus had the greatest cooling effect, largest value of the index of stomatal conductance and the largest induced soil suction. Thermal imaging can be a useful preliminary tool to evaluate the cooling and water removal effects induced by different plant species. Whilst thermal imaging has a great potential to remotely estimate the transpiration of multiple individual plants at one time, plant surface temperature was subject to variabilities of environmental factors and above-ground plant properties that require further investigation before the technique can be widely applied.

Effects of different irrigation management parameters on cumulative water supply under negative pressure irrigation

The pressure of the water supply is controlled to a negative value relying on soil matric suction and is called negative‐pressure irrigation (NPI), a subsurface irrigation method used to improve water use efficiency; however, the impacts of initial soil water conditions and emitter hydraulic conductivity on water movement under NPI remain unknown. To study the effects of irrigation management parameters on water movement under NPI, 300 scenarios using different soil textures, initial soil matric potentials, emitter hydraulic conductivities, negative pressure inside emitter and time were simulated using the Hydrus-2D software package. The effects of these variables on water movement under NPI were analyzed following the simulations, and a empirical model was created to quantify the cumulative water supply based on initial soil matric potential, emitter hydraulic conductivity, negative pressure inside emitter, and time. The results showed that the cumulative water supply increased as the emitter hydraulic conductivity increased or initial soil matric potential decreased. The relationships between the cumulative water supply and both the hydraulic conductivity of the emitter wall and the absolute value of the initial soil matric potential were logarithmic. Cumulative water supply had exponential and linear relationships with the absolute value of the negative pressure inside emitter and time, respectively. The empirical model created to quantify the cumulative water supply under NPI yielded estimations that were in good agreement with the measured values. Therefore, this model can be applied to calculate water supply under NPI, providing a smiple and reliable tool for future NPI applications and management.

Predicting resilient modulus of fine-grained subgrade soils considering relative compaction and matric suction

The resilient modulus has been used to characterise the stress-strain non-linear behaviour of subgrade soils and is an important parameter for pavement design and analysis. This study aims at developing an accurate and efficient methodology to estimate the resilient modulus of subgrade soils. Therefore, repeated load triaxial tests were carried out in this study. The soil matric suction was measured by the pressure plate test and the soil-water characteristic curve (SWCC) was described using the Fredlund & Xing model. Then, the influence of stress state, matric suction and relative compaction on the resilient modulus were analysed. Subsequently, a new resilient modulus model was proposed to incorporate the relative compaction and matric suction in addition to the stress state. The new model matched 7 sets of experimental data well and the coefficients of determination were high, which indicates that this new model is reasonable and widely applicable. Finally, the correlations between the basic properties variables of soil samples and the regression coefficients of the new model were established.

The influence of vegetation cover on evapotranspiration atop waste rock piles, Elk Valley, British Columbia

AbstractSurface mining in the Elk Valley, British Columbia, involves removing vegetation, soil, and rock to access underlying metallurgical coal. Subsequent waste rock is placed into adjacent valleys, frequently burying headwater streams. Due to their coarse texture, waste rock piles increase infiltration and percolation, increasing solute transport and concentration of geochemicals in downstream surface waters. Previous research suggests that weathering solutes are transport limited, and it is hypothesized that revegetation will enhance evapotranspiration (ET) and reduce percolation through the waste rock, potentially reducing loading. This study examined the surface‐atmosphere water and energy exchanges using the eddy covariance technique for three waste rock surfaces with different levels of reclamation: (a) an ~25‐year‐old mixed coniferous forest, (b) a grass site, and (c) bare waste rock. Measurements were taken from May to October in 2013 and 2014. Soil moisture and matric suction were measured to 1‐m depth. Sap flow at the forested site was measured to partition transpiration from total ET. In all years, ET rates were greatest at the forested site, followed by the grass cover and lowest at the bare waste rock site. Growing season ET rates at the forest were 56% higher than grass in 2013 and 35% higher in 2014. At the vegetated sites, climate was the main driver of ET, with high radiation, and warm and dry conditions enhancing fluxes. Maximum ET at these sites corresponded with peak growing season, with vegetation increasing both transpiration and rainfall interception. At the bare rock site, ET was weakly related to atmospheric conditions, and ET rates briefly increased during periods following rainfall when near‐surface soil moisture was enhanced. Transpiration comprised 29% of overall ET at the forest site from late July to early October. Results suggest that vegetation establishment can be incorporated into mine reclamation plans to enhance ET rates and limit percolation, potentially reducing downstream geochemical loads.

Effects of thermal boundary condition on methane oxidation in landfill cover soil at different ambient temperatures.

Microbial aerobic methane oxidation (MAMO) has been considered as an environmental-friendly method for mitigating methane emission from municipal landfill sites. Soil column has in a landfill cover under one-dimensional (1-D) condition. However, most of the published soil column tests failed to simulate 1-D heat transfer due to the use of thermal conductive boundary at the sidewall. In the present study, a heavily instrumented soil column was developed to quantify the effects of thermal boundary condition on the methane oxidation efficiency under different ambient temperatures in landfill cover soil. The sidewall of the soil column was thermally insulated to ensure 1-D heat transport as would have been typically expected in the field condition. Two soil column tests with and without thermal insulation were conducted at a range of controlled ambient temperatures from 15 to 30°C, for studying how soil moisture, matric suction, gas pressure, soil temperature and gas concentration evolve with MAMO. The test results reveal that ignoring thermal insulation in a soil column test would result in a greater loss of soil heat generation by MAMO and hence oxidation efficiency by up to 100% for the range of temperature considered. When the ambient temperature increased to 30°C (but less than the optimum temperature for MAMO), the MAMO efficiency increased abruptly at first but then decreased substantially with time, and this is likely due to the accumulation of biomass generated by MAMO.

A Semi-empirical Approach-Based Model for Swell Characterization of Expansive Clays

Expansive soils often pose as a potential geo-hazard for structures built on them due to the volumetric changes associated with moisture variation. Soil water characteristic curve (SWCC) is a direct representation of the pore and grain distribution of clays while the variation of in situ soil matric suction governs the hydration processes. In the present research, five natural expansive clays were studied at their 95% of maximum dry unit weight. Swell behavioral evaluations are conducted using standard test procedures. Both desorption and adsorption SWCC’s were determined using pressure cell apparatus and filter paper method. Adsorption paths are utilized for analysis study as swelling behavior is the main scope of the research. A novel path traversed by the soil specimen on the void ratio-moisture content-log (soil suction) plot, and the corresponding slope, namely the modified specific moisture capacity, ‘α’, is identified as the main influential parameter. This modified specific moisture capacity exhibited direct relationship to swelling strains and swelling pressure of all five natural clays. A simple model for predicting one dimensional swell strain and swell pressure was established based on reliable predictions obtained using the modified specific moisture capacity of clays.

Experimental study on unfrozen water and soil matric suction of the aeolian sand sampled from Tibet Plateau

Aeolian sand is sometimes used as geomaterial in cold regions of China. This paper tries to investigate the unfrozen water, matric suction of aeolian sand, which is related to its thermal and mechanical properties. The pF-meter matric suction sensor coupled with 5TM moisture content sensor that can operate in the subfreezing environment was used to measure the moisture content and matric suction of aeolian sand under different conditions. Combined with thermal resistor temperature probe, the relationship between unfrozen water and temperature is analyzed. The results indicate a significant phenomenon of latent heat absorption and release at freezing and thawing points. In addition, the phenomenon of the supercooling was observed with a value of −1.96 °C during stepwise temperature change. Most water in the sand freezes at freezing point 0 °C under rapid and stepwise temperature change. There is approximately 3% of unfrozen water content retained when the sand temperature drops to −15 °C. The soil matric suction increases very slowly at the stable freezing stage. These results provide the theoretical support for future research on the moisture migration mechanism and engineering application of aeolian sand in cold regions.

Prediction of shrinkage behavior of soft soil using ramp loading consolidation theory

Soft soil is a highly saturated soil and its low shear strength undermines the reliability for construction. Hence, to reclaim soft soil for construction, more research is required to improve our understanding on the changes in soft soil properties with ongoing dewatering. Shrinkage behavior of soft soil is studied in this paper. Some of the previous studies focused on the simulation of soil volume change and matric suction change while neglecting the soft soil settlement during fully saturated stage of shrinkage. The aim of this research is to predict the settlement of soft soil at the stage where soil is fully saturated and moisture content is high. During this stage, suction of soil is assumed to be zero and the decreased pore water pressure induced by evaporation is taken as the main reason for settlement. With the soil shrinkage happening, the pore water pressure decreases due to water loss and effective stress increases, being the mechanical reason for soil settlement. To predict the settlement during fully saturated stage of soil shrinkage, the ramp loading consolidation theory is applied. The shrinkage test of saturated kaolin with initial moisture content of 150% has been conducted in laboratory and vertical settlement was recorded. Ramp loading consolidation simulation method is applied to soft soil settlement estimation and the prediction fits well with the experiment.

Coupled effects of atmospheric CO2 concentration and nutrients on plant-induced soil suction

Some studies have shown that an increasing atmospheric CO2 concentration reduces plant transpiration while others have demonstrated that it interacts with nutrients in soil to enhance plant transpiration and plant growth including an increase in leaf area. However, none of these studies has focused on plant-induced suction in vegetated soils. The objective of this study is to quantify transpiration induced soil suction by Schefflera heptaphylla in NPK (Nitrogen, Phosphorus, Potassium) nutrient supplied, heavily compacted silty sand under two different atmospheric CO2 concentrations (400 ppm and 1000 ppm). Three replicates of the plant were grown in NPK nutrient supplied silty sand and their plant characteristics and soil matric suction were measured under 400 and 1000 ppm atmospheric CO2 for three months. Due to the supply of nitrogen-rich nutrient in silty sand, leaf area index (LAI) of plant increased by 22% under 1000 ppm CO2 compared to 400 ppm CO2. Thus, the larger LAI induced higher soil suction substantially since no significant difference in soil suction was found between current and elevated atmospheric CO2. LAI of Schefflera heptaphylla could be a reliable parameter to understand and predict soil suction as verified by strong correlation coefficient (R2 = 0.85–0.98; P value < 0.1) of peak induced suction and atmospheric CO2 concentration. This study reveals that the use of vegetation in soil structures needs proper management with additional supply of nutrients to thrive under future atmospheric condition and to induce suction hence improve shear strength in vegetated soil structures.