Suction Distribution Research Articles

Formulation of ultimate bearing capacity for strip foundations based on the Meyerhof theory and unsaturated soil mechanics

This study presents an ultimate bearing capacity formulation of strip foundations in unsaturated soils within the framework of the Meyerhof theory. The flowchart of calculation steps for this formulation is provided with a trial method according to suction distributions. The validity of the formulation obtained is demonstrated by two available theoretical solutions and experimental data. Comparisons are also performed between the results from the Meyerhof and the Terzaghi theories. It is found that the effects of matric suction and its distribution are significant; strength nonlinearity generates three types of the ultimate bearing capacity variation with respect to matric suction.

The Role of Cover Thickness in the Rainfall-Induced Landslides of Nocera Inferiore 2005

In the context of rainfall-induced landslides involving pyroclastic soils, the present work analyzes the influence of cover thickness on slope stability conditions. To this aim, the slope failure that occurred in Nocera Inferiore (4th March 2005) is selected as a reference test case, providing the actual weather forcing history that preceded the event, the hydraulic characterization of the soil involved, and the lowermost boundary condition (variously fractured calcareous bedrock underlying the cover). By maintaining unchanged soil hydraulic properties, the relationship between domain thickness, initial soil suction distribution, and slope instability induced by critical rainfall is investigated by numerical analyses. These refer to a rigid unsaturated domain subject to one dimensional flow conditions under the effects of incoming (precipitation) and outcoming (evaporation) fluxes applied at the uppermost boundary. The main outcomes indicate that critical event duration increases significantly with increasing the domain thickness. This relationship is strongly influenced by initial suction distribution. A linear relationship results for soil suction that is assumed to be constant at the beginning of the critical event. However, this relationship is quadratic if, by simulating the actual antecedent meteorological conditions, suction at the beginning of the critical event is the main function of the domain thickness. Additional numerical analyses were carried out to characterize the influence of a different lowermost boundary condition. Outcomes indicate that, for the same thickness, critical duration is substantially longer if the cover contact is with the same material as that of the cover.

Reliability analysis of riverbank stability accounting for the intrinsic variability of unsaturated soil parameters

The paper presents a probabilistic study aimed at investigating the role of the soil hydraulic response on the stability of existing river embankments, for which the uncertainty and the inherent variability of geotechnical and hydraulic properties are typically greater when compared to new flood defence structures. The study has been carried out with reference to a specific, thoroughly investigated 20 m-long segment of the river Secchia banks (northern Italy), which experienced a catastrophic sudden failure after a period of intense rainfall, in January 2014. By taking such well-documented case as a base, the proposed probabilistic analyses consider three key aspects, typically disregarded in routine risk assessment procedures: i) transient seepage flow through earth structures due to time dependent hydraulic loads, ii) unsaturated conditions of soils forming the river embankment, iii) uncertainty of the soil model parameters, with special emphasis placed on the impact of intrinsic variability of unsaturated soil parameters. The numerical results, obtained from the application of the Point Estimate Method, allow identifying the crucial role of suction distribution on the probability of failure of the riverbank slopes and clearly show that such probability can be significantly underestimated when the variability of hydraulic parameters is neglected.

Laboratory investigation of suction distribution in a modified capillary barrier system

This paper investigated the effect of transport layer in the diversion capacity of natural capillary barrier system using laboratory slope model. The slope model was constructed with stainless steel and 5 mm thick acrylic sheets. Grade VI and grade V soils classified as sandy silt of very high plasticity (MVS) and silty gravel of high plasticity (MHG), respectively were arranged in the slope model with sand and gravel transport layers sandwiched at their interface. The model was subjected to different rainfall intensities using rainfall simulator to determine the behaviour of water flow and suction distribution in the slope model. The results obtained show a modification in the suction distribution behaviour and the natural capillary barrier effect was sustained. It was also observed that a transport layer formed with gravel material was more effective in diverting the infiltrating water compared to that of gravelly sand. This occurred because the upper grade VI layer possessed capillary forces due to its finer pore structures and relatively large air entry value, thus, it retained the infiltrating water, and the gravel transport layer possessed relatively larger pore structures compared to the grade VI layer and hence it possessed higher hydraulic conductivity values and small water entry value. This arrangement, provide a capillary break and allowed the infiltrating water to flow above the interface. Therefore, the inclusion of transport layer provides a definite path through which the infiltrating water flows and diverted laterally. Thus, improving the performance of natural capillary barrier effect.

Upper bound seismic limit analysis of geosynthetic-reinforced unsaturated soil walls

The assessment of the internal stability of geosynthetic-reinforced earth retaining walls has historically been investigated in previous studies assuming dry backfills. However, the majority of the failures of these structures are caused by the water presence. The studies including the water presence in the backfill are scarce and often consider saturated backfills. In reality, most soils are unsaturated in nature and the matric suction plays an important role in the wall's stability. This paper investigates the internal seismic stability of geosynthetic-reinforced unsaturated earth retaining walls. The groundwater level can be located at any reinforced backfill depth. Several nonlinear equations relating the unsaturated soil shear strength to the matric suction and different backfill type of soils are considered in this study. The log-spiral failure mechanism generated by the point-to-point method is considered. The upper-bound theorem of the limit analysis is used to evaluate the strength required to maintain the reinforced soil walls stability and the seismic loading are represented by the pseudo-dynamic approach. A parametric study showed that the required reinforcement strength is influenced by several parameters such as the soil friction angle, the horizontal seismic coefficient, the water table level, the matric suction distribution as well as the soil types and the unsaturated soils shear strength.

Temperature Effects on the Hydraulic Properties of Unsaturated Sand and Their Influences on Water-Vapor Heat Transport

AbstractThermally driven moisture and vapor transport in unsaturated soil under a thermal loading applied by underground structures affects the magnitude and distribution of matric suction. There i...

Determination of soil-water retention curve: an artificial intelligence-based approach

Soil Water Retention Curve (SWRC) is a fundamental relationship in unsaturated soil mechanics, knowledge of which is essential for determining major mechanical and hydraulic properties of unsaturated soils. There are several empirical, semi-empirical and physically-based models which have been proposed to date for estimating SWRC. While the physically-based models which employ the basic soil characteristics such as grain-size and pore-size distributions are regarded superior to purely empirical models, their Achilles’ heel is the several simplifying assumptions based on which these models are constructed, thereby, restricting their applications and influencing their accuracy. Given the complexity of the soil porous structure, one may resort to the new inference techniques rather than mechanistic modelling to find the relationship among soil physical characteristics and the retention properties. Therefore, an alternative approach to purely empirical relationships as well as physically-based and conceptual models for determining SWRC is the use of Artificial Intelligence (AI) based techniques to acquire a relationship for SWRC based on the soil basic properties, especially grain size distribution and porosity. Among AI-based methods, Multi-Gene Genetic Programming (MGGP), often used to establish a close form equation for a complex physical system, offers a suitable alternative to the current approaches. In this study, a database compromising of 437 soils (containing various soil types, namely, sand, clay, silt, loam, silt loam, clay loam, sandy loam, sandy clay loam, silty clay loam, silty clay, and loamy sand soils) was used along with MGGP to establish a relationship among suction, saturation, porosity and grain size distribution. The proposed equation had a reasonable agreement with the experimental data compared to the other grain-based and physically-based models.

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.

Estimating the Stand-Up Time of Unsupported Vertical Trenches in Vadose Zone

Stability of trenches in vadose zone primarily depends on matric suction distribution with depth. The depth of ground water table, type of soils, and local climate conditions such as rainfall are major contributing factors to the matric suction distribution, and thus the stability of unsupported trenches. Trench failures are labelled as the cause of many work-related injuries and deaths in the construction industry. Hence, the design of unsupported trenches should be done with utmost caution. The focus of this study is directed towards investigating the influence of rainfall infiltration on the stand-up time of unsupported vertical trenches excavated in unsaturated coarse- and fine-grained soils. For this, a series of numerical analyses were carried out to estimate stand-up time of unsupported vertical trenches considering possible practical scenarios such as various rainfall intensities and groundwater table depths, impermeable membranes on the ground surface, and tension cracks. The commercial geotechnical modelling software, GeoStudio was used in this study to simulate excavation, redistribution of pore-water pressure due to excavation, rainfall infiltration, and slope stability analysis.

Effects of shrub on one-dimensional suction distribution and water infiltration in a three-layer landfill cover system

目的:研究灌木对干燥和湿润过程中三层土质覆盖系统的吸力分布的影响。 创新点:发现对于三层土质覆盖系统,干燥过程中植物蒸腾作用不仅提高了土的吸力,同时也增加了土层之间的吸力梯度(图3)。 方法: 通过土柱试验模拟干燥和湿润过程,并测量吸力的变化。 结论:1. 植物蒸腾作用导致植有灌木的土在干燥过程中所产生的吸力较没有植物的土高60%~200%。2. 植物蒸腾作用提高了植有灌木的土的持水能力;在湿润过程中水分穿透植有灌木的土所需时间比没有植物的土长50%(图3~4)。

Development of allowable bearing capacity for strip foundations in unsaturated soils

An allowable bearing capacity formulation of strip foundations is developed for unsaturated soils under the non–hydrostatic in–situ stress. Four cases are analyzed according to both in–situ stress states and suction distributions. The proposed closed–form formulation is easy to use along with non–negative factors as an application condition. Furthermore, the obtained results are compared against three published results reported in the literature, and good agreements between these results are observed. Finally, a parametric study is performed to present how unsaturated strength characteristics and in–situ stress states affect the allowable bearing capacity.

An Approach to Assess the Stability of Unsaturated Multilayered Coastal-Embankment Slope during Rainfall Infiltration

This study aims to develop a simple but effective approach to investigate the stability of an unsaturated and multilayered coastal-embankment slope during the rainfall, in which a Random Search Algorithm (RSA) based on the random sampling idea of the Monte Carlo method was employed to obtain the most dangerous circular sliding surface, whereas the safety factor of the unsaturated slope was calculated by the modified Morgenstern–Price method. Firstly, two typical distributions of matric suction were illustrated and the associated methods for determining the strength parameters of unsaturated soil were developed. Based on this, the Morgenstern–Price method was further modified to calculate the safety factor, and RSA was adopted to locate the most dangerous sliding surface of the unsaturated multilayered coastal-embankment slope. Finally, the slope breaking process under rainfall infiltration was simulated through continuously searching the critical slip surfaces under different groundwater levels by RSA. The results indicated that the stability of the unsaturated embankment slope was gradually deteriorated with the increase of rainfall infiltration. It was also found that both of the distributions of the matrix suction (ua-uw) and the suction angle (φb) had significant effects on the safety factor of the embankment slope. Basically, linear distribution of (ua-uw) along the depth and linear relationship between φb and (ua-uw) should be adopted in assessing the stability of the unsaturated multilayered coastal-embankment slope.

Field monitoring of suction in the vicinity of an urban tree: exploring termite infestation and the shading effects of tree canopy

The objective of this short communication is to explore spatial distribution of soil suction in the vicinity of an urban tree considering effects of termite infestation and also radiant energy interception by canopy. A site in an urban landscape containing a mix of species cover (grass cover in the vicinity of trees) was selected for monitoring. A field monitoring program was designed to monitor soil (soil suction, moisture), vegetation parameters (tree height, grass leaf area), and meteorological parameters during drying/wetting cycles. As expected, before termite infestation, suction magnitude and rate of change were highest near the tree stem and at shallower depths, reflecting the dominance of transpiration over evaporation. However, after termite infestation, soil suction diminished significantly near the tree stem and increased farther from the tree stem, a phenomenon accompanied by loss of canopy area, as captured by a color analysis technique (i.e., an image processing method). These findings reflected a reduction in stomatal conductance, itself an indicator of transpiration loss through the stomata—likely because reduced canopy shading effects caused increased evaporation. Soil suction at deeper depths seemed not to be significantly affected, generally due to canopy and termite infestation. In summary, the study sheds new light on issues of tree maintenance by showing how termite infestation can significantly affect the performance of green infrastructure.

Investigation of Soil Active Wedge Angle with Linear Matric Suction Distribution Below the Footing

In this paper, a theoretical approach is established to determine the active wedge angle of unsaturated soils, based on the limit equilibrium method, while matric suction varies linearly with respect to depth. In the previous researches, constant suction distribution profile within the soil has been pointed out, however, generally linear and nonlinear suction profiles develop above the ground water table. The range of suction value covers transition zone and residual zone of unsaturation. Sensitivity of the proposed model, with respect to different parameters, shows that the value of active wedge angle is more dependent on the internal friction angle, in comparison to other properties. Therefore, instead of a complex relation, simple equation can be used. The results show that the active wedge angle for unsaturated soils is equal to $$\frac{\pi }{6} + \frac{7}{8}\phi$$. Comparisons between the presented model and experimental data show that, for some values of internal friction angle, the active wedge angle in unsaturated state is close to saturated state.

Effect of Unsaturated Soil Properties on the Intensity-Duration Threshold for Rainfall Triggered Landslides

Rainfall triggered landslides are one of the most common natural hazards in many countries all around the world, as well as in Turkey. This study investigates the effect of unsaturated soil properties on landslide-triggering rainfall intensity-duration threshold. In addition to the time to failure, the suction (negative pore water pressure) distribution in the slope, the shape and depth of the failure surface are also evaluated. Properties of soil-water characteristic curve which affect the distribution of suction in the soil in response to changes in water content are varied in a parametric study. Effects of air entry value (relates to particle size), de-saturation rate (relates to uniformity of particle size distribution), saturated volumetric water content (relates to void ratio) and residual volumetric water content (relates to fines content and characteristics) are evaluated by determining the intensity-duration threshold numerically by carrying out infiltration and slope stability calculations. The variation of unsaturated soil properties is found to significantly alter the landslide mechanism (ranging from mere surface erosion to groundwater level rise in response to complete infiltration) and consequent intensity-duration thresholds for the same slope geometry. Among the parameters considered, air entry value appears to be the most influential parameter. The effects of the slope angle and initial moisture condition on threshold rainfall intensity-duration are also investigated. This study could be useful for creating the basis and mechanical understanding for future research on early warning systems.

The effect of irrigation on slope stability in the Heifangtai Platform, Gansu Province, China

Agricultural irrigation in the Loess Plateau induces many loess landslides. Long-term flood irrigation is frequent on the top platform of loess slopes, where many landslides occur. To confirm the relationship between irrigation and landslides, we monitored the soil moisture content and groundwater level in Heifangtai Platform, and performed tests on unsaturated soil, including TRIM (Transient Release and Imbibition Method) and triaxial shear test. Then, we simulated the changes of the seepage field on slopes for flood irrigation at different locations and analyzed the effect of flood irrigation on slope stability. Monitoring results show that flood irrigation affects soil moisture content far more than rainfall. With the increasing depth, the response time of the change of soil moisture content increases gradually. The lag effect is obvious. During irrigation, at a depth of 0.1–0.2 m, the soil moisture content increases sharply; at a depth of 0.5–3 m, it increases gradually under different degrees; and at a depth of 5 m, it increases little. After irrigation, the soil moisture content decreased rapidly at a depth of 0.1 m and 0.2 m and returned to the average level nearly a month later. However, the soil moisture content continued to increase slowly over a period at a depth of 0.5–3 m. During the monitoring period, the cumulative rainfall was <110 mm, and the depth affected by rainfall is not >0.5 m. These data show the change of groundwater level is out of sync with irrigation activities and has a visible lag effect. According to the results of seepage and stability analysis, short-term irrigation has little impact on the groundwater level. However, it affects the flow field and causes the matric suction of the superficial loess to decrease. Long-term irrigation causes the groundwater level to increase locally. Compared with single irrigation of high intensity, the reduction of slope stability is more evident under long-term irrigation of low intensity. Changing the irrigation methods to reduce deep infiltration might be one useful precaution to minimize sliding. The closer the distance of the irrigation location to the slope edges is, the higher the effect of flood irrigation on the distribution of pore pressure and matric suction will be. Irrigation near potential slide sites has an apparent cumulative effect and lag effect on slope stability. The longer the irrigation time and the closer to the edge of the slope the irrigation is, the higher the effect on slope stability will be. Avoiding flood irrigation near or at the potential sliding region may reduce the frequency of landslides.

Stochastic modelling of relative water permeability in vegetative soils with implications on stability of bioengineered slope

Vegetation is known to influence the hydrological state variables, suction $$ \left( \psi \right) $$ and volumetric water content ( $$ \theta_{w} $$ ) of soil. In addition, vegetation induces heterogeneity in the soil porous structure and consequently the relative permeability ( $$ k_{r} $$ ) of water under unsaturated conditions. The indirect method of utilising the soil water characteristic curve (SWCC) is commonly adopted for the determination of $$ k_{r} $$ . In such cases, it is essential to address the stochastic behaviour of SWCC, in order to conduct a robust analysis on the $$ k_{r} $$ of vegetative cover. The main aim of this study is to address the uncertainties associated with $$ k_{r} $$ , using probabilistic analysis, for vegetative covers (i.e., grass and tree species) with bare cover as control treatment. We propose two approaches to accomplish the aforesaid objective. The univariate suction approach predicts the probability distribution functions of $$ {\text{k}}_{\text{r}} $$ , on the basis of identified best probability distribution of suction. The bivariate suction and water content approach deals with the bivariate modelling of the water content and suction (SWCC), in order to capture the randomness in the permeability curves, due to presence of vegetation. For this purpose, the dependence structure of $$ \psi $$ and $$ \theta_{w} $$ is established via copula theory, and the $$ k_{r} $$ curves are predicted with respect to varying levels of $$ \psi - \theta_{w} $$ correlation. The results showed that the $$ k_{r} $$ of vegetative covers is substantially lower than that in bare covers. The reduction in $$ k_{r} $$ with drying is more in tree cover than grassed cover, since tree roots induce higher levels of suction. Moreover, the air entry value of the soil depends on the magnitude of $$ \psi - \theta_{w} $$ correlation, which in turn, is influenced by the type of vegetation in the soil. $$ k_{r} $$ is found to be highly uncertain in the desaturation zone of the relative permeability curve. The stochastic behaviour of $$ k_{r} $$ is found to be most significant in tree covers. Finally, a simplified case study is also presented in order to demonstrate the impact of the uncertainty in $$ k_{r} $$ , on the stability of vegetates slopes. With an increment in the parameter $$ \alpha $$ , factor of safety (FS) is found to decrease. The trend of FS is reverse of this with parameter $$ n $$ . Overall FS is found to vary around 4–5%, for both bare and vegetative slopes.

From flapping to heaving: A numerical study of wings in forward flight

Direct Numerical Simulations of the flow around a pair of flapping wings are presented. The wings are flying in forward flight at a Reynolds number Re=500, flapping at a reduced frequency k=1. Several values of the radius of flapping motion are considered, resulting in a database that shows a smooth transition from the wing rotating with respect to its inboard wingtip (flapping), to a vertical oscillation of the wing (heaving). In this transition from flapping to heaving, the spanwise-averaged effective angle of attack of the wing increases while the effect of the Coriolis and centripetal accelerations becomes weaker. The present database is analyzed in terms of the value and surface distribution of the aerodynamic forces, and in terms of 2D and 3D flow visualizations. While the former allows a decomposition of the force in pressure (i.e., the component of the force normal to the surface of the wing) and skin friction (i.e., tangential to the surface of the wing), the latter allows the identification of specific flow structures with the corresponding forces on the wing. It is found that the aerodynamic forces in the vertical direction (lift) tend to increase for wings moving with larger radius of flapping motion, becoming maximum for the heaving configuration. This is mostly due to the increase of the spanwise-averaged effective angle of attack of the wing with the radius of the flapping motion. Also, the local changes in the effective angle of attack have a strong effect on the structure of the leading edge vortex, resulting in changes in the distribution of suction along the span near the leading edge of the wing. The effect of the apparent accelerations is mostly felt on the spanwise position where the separation of the LEV occurs. On the other hand, the differences in the force in the streamwise direction (thrust/drag) between the configurations with different radius of flapping motion seems to be linked to the position of the stagnation point dividing the suction and pressure side boundary layers, which seems to be controlled by the local effective angle of attack. Finally, the results of the DNS are used to evaluate the performance of an unsteady panel method, and to explain its deficiencies.

Suction control of flow separation of a low-aspect-ratio wing at a low Reynolds number

Suction-based flow control is explored as one of means of enhancing the performance of a low aspect ratio (AR) wing. Large-eddy simulation (LES) was performed to investigate the flow past a three-dimensional (3D) NACA0012 profile wing tip at a chord-based Reynolds number of 35966, such configurations and flow conditions are typical of those utilized in a small unmanned air system (UAS). The flow separation appears at an angle of attack a = 15°, which is used as the example for our investigation. After validating of our numerical results, the suction distribution was designed and mounted near the wing root, and two different control parameters, suction coefficients and angles, were investigated to assess the benefits of flow control and to determine their effectiveness. Our results show that suction control can improve the lift, and the suction coefficient has larger effect on the aerodynamic performances of the wing than that of the suction angle. The lift enhancement is attributed to the standing vortex near the leading edge and the trajectory of wing tip vortex is also affected during the suction control.