Soil Suction Changes Research Articles

Seismic Response of Pile Foundations in Clayey Soil Deposits Considering Soil Suction Changes Caused by Soil–Atmospheric Interactions

Seismic Response of Pile Foundations in Clayey Soil Deposits Considering Soil Suction Changes Caused by Soil–Atmospheric Interactions

Numerical simulation of replacement method to improve unsaturated expansive soil

AbstractIn this paper, a parametric study is done with various removal and replacement materials to study the effectiveness of the removal and replacement method on the wetting depth in the expansive soil and the amount of differential heave caused by climate conditions and common irrigation scenarios for the southern region of Syria. Soil suction changes and associated soil deformations are analyzed using finite element codes, VADOSE/W and SIGMA/W. The paper concludes that the optimum thickness for replacement with high permeability soil should be at least 1 m. In addition, it concludes that replacing soil with a permeability coefficient lower than the permeability coefficient of the site soil contributes to a 56% and 79% reduction in total and differential heave, respectively.

Numerical Simulation on the Effect of Infiltration and Evapotranspiration on the Residual Slope

Soil suction plays an important role in governing the stability of slopes. Environmental sustainability could be jeopardized by hazards, such as slope failures (forest destruction, landscape alteration, etc.). However, the quantification of the suction effect on slope stability is a challenging task as the soil suction is usually affected by the precipitation and evapotranspiration. Numerical simulation plays an important role in the estimation of contour in soil suction due to rainfall and evapotranspiration as long-term and widespread monitoring is rarely conducted. The result of numerical simulation is highly dependent on the accuracy of the input parameters. Hence, suction monitoring plays an important role in verifying the result of numerical simulation. However, as a conventional tensiometer is limited to 100 kPa soil suction, it is hard to verify the performance of numerical simulation where suction is higher than 100 kPa. The osmotic tensiometer developed by Nanyang Technological University (NTU) can overcome this problem. It is now possible to monitor changes in soil suction higher than 100 kPa (up to 2500 kPa) for an extended period in the field. In this study, a procedure was proposed to estimate suction changes in residual soil based on rainfall and evapotranspiration data. Numerical simulation was carried out based on the soil properties and geometry of a residual soil slope from Jurong Formation Singapore. Changes in soil suction due to rainfall and evaporation were simulated and compared with the readings from the NTU osmotic tensiometers installed at 0.15 m and 0.50 m from the slope surface in the field. It was observed that numerical simulation was able to capture the variations of suctions accurately at greater depths. However, at shallow depths, erratic suction changes due to difficulties in capturing transpiration.

Probability Distribution of Soil Suction of Engineered Turf Cover and Compacted Clay Cover

It is unlikely to predict the distribution of soil suction in the field deterministically. It is well established that there are various sources of uncertainty in the measurement of matric suction, and the suction measurements in the field are even more critical because of the heterogeneities in the field conditions. Hence it becomes necessary to probabilistically characterize the suction in the field for enhanced reliability. The objective of this study was to conduct a probabilistic analysis of measured soil suction of two different test landfill covers, compacted clay cover (CC) and engineered turf cover (ETC), under similar meteorological events. The size of the two test landfill covers was 3 m × 3 m (10 ft. × 10 ft.) and 1.2 m (4ft.) in depth. The covers were constructed by excavating the existing subgrade, placing 6-mil plastic sheets, and backfilling the excavated soil, followed by layered compaction. Then the covers were instrumented identically with soil water potential sensors up to specified depths. One of the covers acted as the CC, and the other cover was ETC. In ETC, engineered turf was laid over the compacted soil. The engineered turf consisted of a structured LLDPE geomembrane overlain by synthetic turf (polyethylene fibers tufted through a double layer of woven polypropylene geotextiles). The sensors were connected to an automated data logging system and the collected data were probabilistically analyzed using the R program. There were significant inconsistencies in the descriptive statistical parameters of the measured soil suction at both covers under the same climatic conditions. Soil suction measured in the field ranged between almost 12 to 44 kPa in ETC, while it was in the range of almost 1 to 2020 kPa in the CC. The histogram and quantile-quantile (Q-Q) plot showed the data to be non-normally distributed in the field. A heavy-tailed leptokurtic (Kurtosis=13) distribution of suction was observed in the ETC with substantial outliers. In contrast, the suction distribution in CC was observed skewed to the right containing a thinner tail indicating an almost platykurtic distribution. The distribution of suction in the field under engineered turf was observed to be reasonably consistent with time compared to bare soil under the same meteorological events. The results obtained from this study revealed the engineered turf system to be an effective barrier to inducing changes in soil suction against climatic events.

Relationship between reactive soil movement and footing deflection: A coupled hydro-mechanical finite element modelling perspective

Supporting footings of lightweight buildings on reactive soils are susceptible to damage induced by the cyclic shrink-swell movement of the ground, causing extensive financial damage. Due to this issue and limitations of existing analytical methods, the main aim of this study is to investigate the interaction between shrink-swell soils and raft footings (i.e., stiffened rafts and waffle pod rafts), by performing parametric simulations using a simplified three-dimensional hydro-mechanical finite element model. The developed model considers the link between soil saturation, soil suction and volumetric soil strain. Stiffened rafts and waffle pod rafts were observed to interact differently with reactive soils. Stiffened rafts formed complex soil profiles induced by the obstruction of soil moisture flow provided by in-ground beams. This had a significant effect to changes in soil suction and formation of soil profiles in the covered ground. Waffle pod rafts performed better on swelling soils, while stiffened rafts were more robust on shrinking soils due to the additional support provided by the in-ground beams counteracting applied line loads. A developed improved design guideline based on the performed parametric simulations, considering the influence of geometry, suggests that raft footings require stiffer sections than a traditional method.

Numerical simulation of a geotextile soil wall considering soil-atmosphere interaction

This paper presents a numerical method for considering soil-atmosphere interaction applied to infiltration into an unsaturated geosynthetic reinforced soil wall and its effect on wall stability. A hypothetical nonwoven geotextile reinforced soil wall was subjected to simulated conditions of evaporation and precipitation over 2 years considering local climate variation in São Paulo city, Brazil. Net infiltration and actual evaporation were quantified inside and outside the reinforced zone, allowing for the assessment of changes in soil suction and factors of safety. The study discusses the implications of using in-plane draining reinforcements (e.g. nonwoven geotextiles) and soil-atmosphere effects. Results show that soil suction and factors of safety variation are more dependent on consecutive days of precipitation than on isolated heavy rainfalls. For the climate conditions considered in this study, results show that approximately 50% of the precipitation and potential evaporation transformed into net infiltration and actual evaporation, respectively. Additionally, the numerical results indicate that after the first wetting of the soil inside the reinforced soil zone, the evaporation to the atmosphere did not remove water from the inside of the geosynthetic reinforced wall because of the capillary break.

Time-dependent collapse potential of unsaturated collapsible gypseous soils

PurposeThis study aims to predict the volume changes and collapse potential (CP) associated with the changes in soil suction by using the pressure cell and the effect of initial load on soil suction. Three types of gypseous soils have been experimented in this study, sandy gypseous soil from different parts of Iraq. A series of collapse tests were carried out using the oedometer device [single oedometer test (SOT) and double oedometer test (DOT)]. In addition, large-scale model with soil dimensions 700 × 700 × 600 mm was used to show the effect of water content changes in different relations (collapse with time, stress with time, suction with time, etc.).Design/methodology/approachA series of collapse tests were carried out using the oedometer device (SOT and DOT). In addition, a large-scale model with soil dimensions 700 × 700 × 600 mm was used to show the effect of water content changes in different relations (collapse with time, stress with time, suction with time, etc.).FindingsThe CP increases with the increasing of the void ratio for each soil. For each soil, the CP decreased when the initial degree of saturation increased. Kerbala soil with gypsum content (30%) revealed collapse value higher than Tikrit soil with gypsum content (55%) under the same initial conditions of water content and density, this is because the higher the Cu value of Kerbala soil is, the more well-graded the soil will be. Upon wetting, the smaller particles or fractions of the well-graded soil tend to fill in the existing voids, resulting in a lower void ratio as compared to the poorly graded one. Consequently, soils with high Cu value tend to collapse more than poorly graded ones. The compressibility of the soil is low when loaded under unsaturated condition, the CP for samples tested in the DOTs under stress level 800 kPa are greater than those obtained from collapse test at a stress level of 200 kPa.Originality/valueThe initial value of suction for all soils increases with initial water content decreases.

Evaporation-Induced Water and Solute Coupled Transport in Saline Loess Columns in Closed and Open Systems

Evaporation-induced water and solute coupled transport is a significant cause for soil salinization that leads to a series of engineering and environmental problems. In the artificial evaporation environment, including relative humidity, atmospheric temperature, atmospheric pressure, radiation intensity, and wind speed, evaporation of loess columns with sodium sulfate, temperature profile, and water and solute transport in closed and open systems were investigated. In the former case, a range of stability was noted in the lower part of the column where the heat, water, and solute coupled transport only exhibited a weak response to a specified evaporation environment. As the environment was more favorable for evaporation, or at a higher gradient of temperature, this range extended downwards, while above this range the heat, water, and salt profiles in the evaporation-affected domain changed dramatically, characterized by a basic law that solutes moved with water and were then retained with water desalinated. Evidences were found from the profiles that the water contents in the evaporation-affected range decreased but the salt contents increased, especially in the surface. In an open system, by contrast, there was little difference in temperature but greater in water and salt profiles. Three stages were found from the supply of external water during evaporation. In the initial stage, the higher gradient formed between the initial moisture state and the preset external water level led to a rapid supply of external water. As the evaporation proceeded, a relatively stable water profile was reached as the intensity of water supply approached to that of evaporation, accompanied by a continual migration of solutes towards the surface. Due to the accumulation of precipitated salts, water transport was slowed down, and the intensity of water supply decreased. The changes in soil suction may account for the above behavior.

Prediction of soil suction using measured climatic data

This paper presents calibration and validation of a one-dimensional soil–atmosphere model created using the SVFlux software package for predicting soil suction changes based on the measured climate data at two Oklahoma Mesonet sites. The parameters for the soil water characteristic curves and the hydraulic conductivity function were obtained from published calibration studies conducted using laboratory test results on soil samples extracted from these sites. The plant cover quality, the root depth and the root uptake distribution function were first calibrated using 1-year data at one site. While a rectangular root uptake distribution function overpredicted suction at deeper layers, the triangular uptake distribution function overpredicted suction at shallower depths. In addition, reducing the root depth to 1 m improved the predictions at surface layers and led to overestimation of the suction at deeper layers. Based on the root mean squares of the differences between the simulated and the measured values and weighting the shallower depths more, a rectangular root uptake distribution function and a root depth of 1 m were selected to predict suction values at the calibration site and the second site for another year. Reasonable predictions of soil suction values at shallower depths were obtained at both sites.

Modelling soil suction changes due to mixed species planting

Vegetation-induced changes of soil hydrology (i.e., infiltration rate and soil matric suction) has important implication to the stability of earthen infrastructure. Predicting the plant hydrological effects is challenging when multiple species are present. Most existing models consider only single-species due to lack of reliable and relevant experimental data and difficulty to partition solar radiant energy into individual species in a mixed-species condition. This paper aims to develop an improved hydrological model that can determine the water infiltration rate and suction responses under both single- and mixed-species (one tree and one grass species) condition. The model is verified by double-ring infiltration tests and field monitoring conducted in bare, grass-only, tree-only and mixed tree-grass plots. The model was subsequently used for parametric analyses. The mixed species plot had the lowest infiltration rate and preserved the greatest suctions because the tree-grass competition for water created high soil moisture deficit. Parametric study identifies a threshold ratio of tree and grass leaf area index of 2.0, beyond which the effects of grass root-water uptake on water competition with the adjacent tree were negligible. Induced suction in the mixed tree-grass plot was close to that in a tree-only plot. Contribution of tree root-water uptake to induced suction in a mixed species plot did not increase further when the leaf area index ratio exceeds 5.0.

Soil–environment interactions modelling for expansive soils

Water content and matric suction changes in unsaturated expansive soils due to environmental variations contribute to the overall volume changes (shrinkage or swelling). The soil movements cause tilt in trees, highway surfaces, building foundations and pipelines and pose problems to the functionality of the infrastructure. In this paper, a modulus of elasticity-based method approach (MEBM) proposed by Adem and Vanapalli (2013a) is tested for modelling the soil–environment interactions for a test site with an expansive soil deposit that was modelled by Ito and Hu (2011) for 1 year. In the MEBM, the soil suction changes in the active zone and the associated modulus of elasticity were estimated and used as key parameters in a volume change constitutive relationship to model the soil–environment interactions over time. The MEBM approach reasonably models the water content and matric suction changes over time and the swelling–shrinkage movements. The study results are encouraging for extending the simple MEBM in practice for rational design purposes of both the sub- and superstructures constructed in or on expansive soils.

Water Permeability Function for Soils that Undergo Volume Change as Suction Changes

The saturated/unsaturated coefficient of permeability is required when modeling engineering problems involving transient seepage and contaminant transport. Inaccuracies in the estimation of the coefficient of permeability function can cause errors in numerical modeling results and as a result, affect subsequent engineering decisions. Void ratio changes and degree of saturation changes influence the coefficient of permeability function of a soil. Existing methodologies for estimating the unsaturated permeability function give consideration to degree of saturation changes and may produce reasonable results for soils such as sands and silts which exhibit little volume change with suction change. However, inaccurate results are generated when the soils involved change volume as soil suction changes (e.g., initially slurry Regina clay). Revisions to previously proposed methodologies are needed for the estimation of a saturated/unsaturated coefficient of permeability function that accommodates both void ratio change and degree of saturation change when simulating the drying process. A modification of the Fredlund–Xing–Huang (Can Geotech J 31(4):533–546, 1994) estimation procedure for the permeability function is presented in this paper. Consideration is given to both volume change and desaturation of the soil as soil suction is increased. A laboratory data set for Regina clay is presented and interpreted using the revised methodology. The results illustrate a procedure that can be used to estimate the saturated/unsaturated coefficient of permeability function for a soil that undergoes volume change and desaturation as soil suction increases.

A site assessment of pavement cracking in a drought environment: a case history

This paper builds upon the findings of a site investigation into the causes of longitudinal cracks that developed soon after the re-construction of Himes and Johnson Streets in the City of Norman, Oklahoma between May to September 2012. The re‒construction of the city street pavement and base course were completed during a protracted drought period, and shortly after completion of the pavement section longitudinal cracks occurred predominantly along the curb line. The case history concluded that the drought conditions resulted in significant subgrade soil shrinkage. The shrinkage caused changes in soil suction with depth resulting in tensile stresses in the asphalt pavement and stabilized subbase that lead to the pavement cracking. A simple practical moisture/suction variation model with depth (Hamberg) was used in conjunction with the soil water characteristic curve (SWCC) to make predictions of the soil suction changes that are resulting in the pavement cracking. The paper documents the comparison of the Hamberg model used in the case history with the Mitchell and finite element (FEM) models. The purpose of the comparison was to see if improved accuracy of the Mitchell model and a finite element model resulted in significant changes in the prediction of soil suction.

Unsaturated Soil Mechanics Principles to Remove and Replace Mitigation for Expansive Clays

The efficacy of removal and replacement as a mitigation alternative for expansive soil is discussed in view of numerical unsaturated flow/deformation results. Observations with respect to selection of depth and replacement soil type (similar, more, or less permeable than underlying expansive clay) are presented in the context of unsaturated flow and stress/deformation principles. Comparisons are made between various remove and replace options on the basis of extent and degree of wetting (changes in soil suction) and on ground surface total and differential movements. Selection of optimal removal and replacement depth and replacement material type requires consideration of unsaturated flow principles and understanding of the role of both net normal stress and matric suction in unsaturated soil response. Complexities in the unsaturated flow process which impact the efficacy of the remove and replace method are primarily because of the highly nonlinear nature of the unsaturated soil storage function (SWCC) and hydraulic conductivity function. Further, because the unsaturated hydraulic conductivity of soils depends on the value of soil suction, placement water content of the replacement layer has significant effect on performance. The net result is that the impact of various remove and replace options is not always intuitive. The findings presented, although not encompassing of all possible removal and replacement configurations or surface flux conditions, set the stage for application of remove and replace mitigation methods based on fundamental unsaturated soil mechanics principles.

Numerical analysis of buried pipes under field geo-environmental conditions

Buried pipes are vital infrastructures and are mostly used to transport energy and other essential commodities. These pipes are generally buried within the top layer of soil deposits, and therefore, are highly affected by different geo-environmental conditions. In this paper, a modeling approach is introduced for the analysis of buried pipelines through real-life scenarios. The model provided reasonable estimates for the elastic deformations of a soil-pipe system under different soil and loading conditions. The model was then used to address the performance of a hypothetical pipeline buried in an unsaturated clay soil. The modeling analysis captured the pipe displacements that occurred due to the change in soil suction associated with changes in the soil moisture content. The soil suction was estimated based on field measurements, and was, then used as an input to the model. The change in volumetric water content in the area studied was found to be as low as 5 %, as high as 20 %, and corresponded well with the seasonal variation in climate conditions. Direct correlations between the change in soil moisture content and the resulting pipe displacements were developed. The results indicated that normalized pipe displacements up to 6 % occurred due to the relative increase in volumetric water content of 20 % representing the change from the field condition to full saturation. The magnitude of pipe displacements increased significantly with the decrease of the pipe depth within the active soil zone.

Evaluation of expansivity of Neogene clays and glacial tills from central Poland (Warsaw area) on the basis of suction tests

This paper presents the characteristics of the expansivity and quantitative prediction of heave of clay soils from Poland based on suction testing. 11 Neogene clays and 6 glacial tills, differing in genesis and plasticity, were analyzed to identify the expansive potential using eight empirical methods. The laboratory studies included measurements of soil-water characteristic curves and soil index properties . Data from water content, volume and suction measurements served to determine the suction compression index C h and the suction-water content index Dh/Dw of the soils examined. The values of these indices are clearly dependent of the clay fraction content in tested soils. Compared with glacial tills, Neogene clays are expected to exhibit larger changes in volume due to changes in soil suction. Comparative results of the evaluation of expansivity suggest that the chosen classification methodologies provided consistent expansion ratings for glacial tills, while values for Neogene clays vary from low to very high. The lowest expansivity classes have been obtained from evaluation based on soil suction. The McKeen (1992) method has been adopted for calculations of the potential field heave of Neogene clays and glacial tills. In order to provide a rational estimation of expansive soil behavior, specialists should take into consideration the specific site and design features.

Experimental Investigation of Soil-Atmosphere Interaction in an Instrumented Embankment Constructed with Two Treated Clays

This paper discusses the field monitoring of an experimental embankment divided into two symmetrical instrumented sections constructed with two clays treated with lime and/or cement in the northeast of France. The soil-atmosphere interaction is investigated in the monitored embankment. The field instrumentation included spatial and temporal changes of the soil suction, moisture, and temperature at predefined locations within the embankment, as well as measurements of meteorological data, collected from April to November 2011. The data show similarities in the suction daily variations trend in the two treated clays. Maximum changes in suction occur near ground surface. Even at the location of -0.75 m from the slope face the interchange of water between the atmosphere and the ground was observed in the daily soil suction measurements. No significant hydrologic response of the soils to rainfall was observed during the period of water deficit. The rainfall events showed a significant effect on the soil suction changes in the initial period of water surplus after a long period of water deficit. The comparison of the period of water deficit observed in the responses of the mean monthly suction and moisture measurements in the treated soils and the one estimated by simple water balance models based on standard meteorological observations in the region indicate that there is an overall qualitative agreement between the modelling and observational results.

Rigid Plastic FE Slope Stability Analysis Combined With Rain Fall Water Infiltration

This paper presents a new slope stability analysis regarding rainfall-induced landslides by coupling a saturatedunsaturated seepage analysis and a rigid plastic finite element method (RP-FEM). Currently, a more valid and reliable disaster prevention system detecting the risk of slope instability due to sudden intense rainfall is required in Japan. However, conventional slope stability methods often fail to predict this new type of landslides. Therefore, the aim of this study is to propose the new slope stability analysis method in the context of rainfall infiltration. This method introduces the effect of seepage force, an increase of unit weight and a reduction of apparent cohesion due to the change in soil suction to obtain the slope stability load factors and collapse mechanisms. Consequently, this method can provide the relatively accurate and valid analysis results, which can be well compared with experimental data. Moreover, it is ascertained that this method can evaluate the different type of slope failure mechanisms: an initial small failure at the toe of the slope caused by the seepage forces and a large-scale failure due to the degradation of the soil apparent cohesion.

Physical and numerical study of lateral diversion by three-layer inclined capillary barrier covers under humid climatic conditions

Southern China has a humid climate and often receives rainfalls that are of high intensity and (or) long duration. This paper investigates the performance of an inclined three-layer cover with capillary barrier effect (CCBE) comprising silt, sand, and gravel, for usage under humid climatic conditions. Physical modeling tests were carried out to observe the response of the three-layer CCBE system to a continuous heavy rainfall of about 70 mm/h. The layered cover model, housed in a 2 m long and 1 m wide instrumented box, is made up of 0.2 m thick silt, 0.1 m thick sand, and 0.1 m thick gravel, and the inclination of the model is 1V:3H. The movement of wetting front, changes in soil suction, and the primary components of water balance were measured during the operation of the physical models. The experimental data was used to calibrate the hydraulic parameters of the numerical model using the unsaturated flow software, SVFlux. Numerical modeling was subsequently carried out on a 60 m long inclined CCBE system to investigate the effective length of lateral diversion under prolonged rainfall. The main findings of the experimental and numerical studies are as follows: (i) the physical model tests showed that the response of the three-layer CCBE system to a heavy rainfall of 70 mm/h was different from the previous observations on experiments where the rainfall was less than 1.6 mm/h; (ii) correlation between the physical modeling and the numerical modeling indicated anisotropic behavior with respect to the hydraulic conductivity in the unsaturated sand layer; (iii) the long inclined, three-layer CCBE system (i.e., 0.6 m thick silt, 0.2 m thick sand, and 0.2 m thick gravel) had an effective length of lateral diversion over 10 m for 30 days of prolonged rainfall (i.e., 1080 mm in total).

Evaluating suction profile in a vegetated slope considering uncertainty in transpiration

Understanding root-water uptake in soil slopes is essential because any changes in soil moisture would lead to changes in soil suction and in turn changes in soil shear strength and slope stability. Uncertainties associated with plant characteristics are present in the root-water uptaking process. The objective of this paper is to evaluate the suction profile in a vegetated slope with a grass cover and the stability of the slope during rainfall infiltration. Random field theory is applied to model the spatial variation of the maximum transpiration rate of Bermuda Grass, which is a native grass species in Hong Kong. The slope is subjected to 12h, 24h, and 48h of drying processes before rainfall. A modified Richards equation governing water flow in unsaturated–saturated media is employed to incorporate a sink term of Bermuda Grass. The effects of initial conditions induced by the drying processes of different periods prior to rainfall on the suctions retained in the vegetated slope after rainfall are investigated. Uniform and triangular root distributions are considered to simulate likely root architectures and their influence on the suctions retained. Slope stability analysis is further carried out to investigate how plant transpiration contributes to slope stability. The results demonstrate that, given the same maximum transpiration rate, the root distribution has little influence on inducing or retaining suctions. At a low infiltration flux, the higher the coefficient of variation of the maximum transpiration rate and the longer the pre-drying time, the wider the range of the suctions that a vegetated slope can retain. A pre-drying process of 48h results in a significant increase in the slope factor of safety.