Probabilistic Analysis of Soil-Water Characteristic Curves

Direct measurement of the soil-water characteristic curve (SWCC) is costly and time consuming. A first-order estimate from statistical generalization of experimental data belonging to soils with similar textural and structural properties is useful. A simple approach is to fit the data with a nonlinear function and to construct an appropriate probability model of the curve-fitting parameters. This approach is illustrated using sandy clay loam, loam, loamy sand, clay, and silty clay data in Unsaturated Soil Database. This paper demonstrates that a lognormal random vector is suitable to model the curve-fitting parameters of the SWCC. Other probability models using normal, gamma, Johnson, and other distributions do not provide better fit than the proposed lognormal model. The engineering impact of adopting a probabilistic SWCC is briefly discussed by studying the uncertainty of unsaturated shear strength due to the uncertainty of SWCC.     

Process Modeling of Storm-Water Flow in a Bioretention Cell

A two-dimensional variable saturated flow model was developed to simulate subsurface flow in bioretention facilities employing the Richards’ equation. Variable hydrologic performances of bioretention are evaluated using the underdrain outflow hydrographs, outflow volumes for 10 storms with various duration and depth, and flow duration curves for 25 different storms. The effects of some important design parameters and elements are tested, including media type, surrounding soils, initial water content, ratio of drainage area to bioretention surface area, and ratio of cell length to width. Model results indicate that the outflow volume via underdrain is less than the inflow; the flow peak is significantly reduced and delayed. Underdrain outflow volume from loamy sand media (with larger Ks) is larger than that from sandy clay loam media. The saturated hydraulic conductivity, storage capacity, and exfiltration into surrounding soils contribute to the hydrologic performance of a bioretention cell. Initial media storage capacity is affected by the hydraulic properties of media soils, initial water content, and bioretention surface area. The exfiltration volume is determined by the surrounding soil type and exfiltration area, dominated by flow through the bottom of the media.     

Sorbent Wicking Device for Sampling Hydrophobic Organic Compounds in Unsaturated Soil Pore Water. I: Design and Hydraulic Characteristics

The hydraulic characteristics of horizontally installed sorbent wick sampling devices were evaluated through wick tracer studies and laboratory soil column experiments to assess the influence of horizontal wick length and sampler interface design on sampling pore water in unsaturated soils. The nominal sampler design consisted of a cylindrical porous metal interface packed with granular-activated carbon encapsulating the end of a fiberglass wick that extended 100 cm horizontally from the interface before dropping 100 cm vertically to a collection vessel. The maximum sampling rate of horizontally installed wick systems declines exponentially with increasing horizontal wick length, while the vertical length influences the range of soil–water pressures that may be sampled. The nominal design sampled pore water from clay loam laboratory columns at 8 to 14 mL?h?1 under steady-state infiltration conditions and 2 to 5 mL?h?1 under draining conditions across a ?10 to ?45 cm H2O soil–water pressure range. Sampling rates in medium-grained sand under similar flow conditions were less than that of the clay loam due to reduced water content and reduced interface/soil contact area. An analysis of observed sampling velocities versus calculated soil water contents and hydraulic conductivities indicated that the design performs best when the soil water content is greater than 0.15 and unsaturated hydraulic conductivity is greater than 0.2 cm?h?1. A hydraulic model was developed that estimates the sampling velocity of the nominal design based on sampler interface pressure, which was linearly correlated with soil pressure.     

Laboratory Evaluation of Sand Underdrains

Constant-head hydraulic conductivity tests are performed on layered heterogeneous porous media to evaluate the use of underdrains to calculate the hydraulic conductivity of an overlying, less permeable medium. The layered profiles consist of a barrier layer comprising sand mixed with 10% kaolin, overlying a foundation layer comprising sand mixed with only 5% kaolin. Underdrains are evaluated by replacing excavated portions of the foundation layer with only sand. The results indicate that preferential flow of water occurs around, rather than through, the sand underdrains resulting in an underestimate of the measured hydraulic conductivity of the barrier layer assuming 1D, saturated flow in accordance with standard practice. The observed preferential flow effect is consistent with previously published numerical simulations of unsaturated flow through similarly layered heterogeneous soil profiles that indicate lateral flow around underdrains due to the contrast in unsaturated properties of the soils. The results of this study have important ramifications with respect to the use of underdrains to measure in situ hydraulic conductivity of compacted clay liners for waste containment.     

Evaluation of Water Retention Functions and Computer Program “Rosetta” in Predicting Soil Water Characteristics of Seasonally Impounded Shrink–Swell Soils

Soil water retention is a critical factor influencing irrigation decisions and hence agricultural crop yields. However, information on soil water retention characteristics (SWRC) is seldom available for irrigation planning, crop yield modeling, or hydrological simulations, especially for problematic soils, such as seasonally impounded shrink-swell soils. As large scale direct measurement of SWRC is not viable due to a number of reasons, researchers have developed pedotransfer functions (PTFs) to estimate SWRC from easily measured soil properties, such as texture, organic matter content, bulk density, etc. However, PTF applicability in locations other than those of data collection has been rarely reported. One of the most recent PTFs that has shown overall reasonable predictions in evaluation studies is Rosetta, a numerical code for estimating soil hydraulic parameters with hierarchical pedotransfer functions. Relatively, the development of large databases makes it one of the widely used PTFs. If validated for spatial application, it has immense use potential in countries like India, where data on soil hydraulic properties are seldom available, a deficiency that hampers better simulations in processes, like partitioning runoff and infiltration, assessing evapotranspiration, irrigation scheduling, etc. Rosetta is also relatively flexible allowing estimation of hydraulic properties from easily available minimum input of textural fractions. This study was conducted to evaluate (1) an applicability of four widely used soil water retention functions to describe SWRC; and (2) the computer program Rosetta for its validity. Statistical indices, i.e., root mean square error (RMSE), mean absolute error, maximum absolute error, and degree of agreement (d) were computed to evaluate “goodness-of-fit” of the four functions to the measured SWRC data. These indices were also used to compare measured SWRC with estimates of SWRC by Rosetta. For soil samples collected from 41 profiles, 175 SWRC were measured in the laboratory. The van Genuchten function fitted relatively better (RMSE = 0.052?m3?m?3) to SWRC of clay soils, whereas the Brooks–Corey (BC) function was better in expressing SWRC of clay loam and sandy clay loam soils with RMSE = 0.06 and 0.07?m3?m?3, respectively. Campbell and Cass–Hutson (CH) functions were of intermediate value. Worst performing functions were BC (clay soils), Campbell (clay loam), and CH (sandy clay loam) with corresponding RMSE = 0.059, 0.065, and 0.077?m3?m?3. Estimates of two important points on the SWRC curve, i.e., field capacity and permanent wilting point were predicted with relatively better accuracy for clay and sandy clay loam soils by all the four functions. RMSE and d ranged from 0.027?to?0.043?m3?m?3 and from 0.73 to 0.88 for clay soils. Corresponding values for sandy clay loam soils were 0.008?–0.019?m3?m?3, and 0.92–0.98. However, in clay loam soils, only two functions were found suitable. Estimates of SWRC obtained by applying hierarchical rules in Rosetta were reliable (RMSE<0.05?m3?m?3). Magnitude of average RMSE increased progressively in clay loam, clay and sandy clay loam soils (0.028<0.035<0.042?m3?m?3). The study established that SWRC of the “Haveli” soils could be estimated using generic PTF and thus information that is prerequisite in simulating hydrological processes occurring in seasonally impounded soils could be acquired.     

Evaluation of a Dielectric Sensor for Measurement of Soil-Water Electrical Conductivity

The salinity status of soil can be obtained through the measurement of the pore-water electrical conductivity σp. In the present study the WET sensor’s capability in predicting σp through the simultaneous measurements of the soil bulk electrical conductivity (σb) and the soil dielectric constant (K) is investigated. The estimation of σp is based on a model by Hilhorst, which relates σp to the two other quantities K and σb and an extra fitting parameter K0 which is incorporated in the software of the device. The study involved experimental measurements in the laboratory using four different soils with a large range in soil texture and volumetric water content θ. In each soil type four different electrical conductivity aqueous solutions were used. The results exhibited a rather strong linear relationship between K and σb. The slope of this linear relationship appeared to depend on both the soil type and the electrical conductivity of the pore water. The value of K0 seems to be soil specific and increases when the salinity level increases. The σp prediction according to the linear model of Hilhorst deviates for almost all soils tested, except for the case of the sandy soil. Besides the linear model of Hilhorst, the Malicki and Walczak linear model was also tested for all soils under investigation and the model of Munoz-Carpena et al. was tested for sand. Malicki and Walczak model performance at predicting σp values was approximately the same as the Hilhorst model for sand and slightly better for sandy loam. In general, one could argue that the linear models could predict σp with some accuracy for the cases of coarse porous media.     

Validation of a Predictive Form of Horton Infiltration for Simulating Furrow Irrigation

Due to spatially varying conditions the improvement of furrow irrigation efficiency should be sought not just for a limited number of furrows or for one specific irrigation event. A simplified predictive modeling approach of the averaged advance-infiltration process is proposed in this paper. Horton’s equation, derived from the asymptotic form of the Talsma-Parlange infiltration equation, allows us to use a predictive approach for the advance infiltration process by means of the exact solution of the Lewis and Milne water balance equation. The references to the works of White and Sully, for a surface point source, result in the use of parameters which characterize the hydraulic properties of the soil: Δθ (saturated water content minus initial water content); Ks (saturated conductivity); and λc (macroscopic capillary length). The physical meaning of parameters involved in the proposed modeling is attested using field experiments carried out in a loamy soil plot context. Assuming a same Δθ measured value before irrigation for the whole of a 30 furrow sample, the averaged values of λc and Ks obtained from calibration on the advance trajectory are comparable to those derived from local infiltration tests (disk permeameter and double ring methods). The applicability of the model is then extended to heavy clay soil where the parameters λc and Ks still agree with the values proposed in the literature. This paper can be considered as a contribution to the development of a tool for evaluating the impact of irrigation practices on the efficiency at the plot and cropping season scale.     

Wastewater Reuse Effects on Soil Hydraulic Conductivity

The wastewater total suspended solids (TSS) concentration effects on the saturated hydraulic conductivity, Ks, of a clay and a loam soil were investigated on laboratory repacked soil cores by a constant head permeameter. Both municipal wastewater (MW) and artificial wastewater (AW) with different TSS concentrations were used, with the aim to evaluate, by comparison, the effects of biological activity. The development of a surface sealed layer was investigated in loam soil columns supplied with AW and equipped with water manometers at different depths to detect the hydraulic head gradient changes. In the loam soil, Ks reduced to about 80% of the initial value after infiltration of 175?mm of MW with TSS = 57–68?mg?L?1. Reductions in Ks were more remarkable in the clay soil. An empirical relationship was proposed to predict the relative hydraulic conductivity, Kr, i.e., the ratio between actual and initial hydraulic conductivity versus the cumulative density loading of TSS. Hydraulic head gradients in the top layer (0–20?mm) of the soil columns increased during application of AW, as a consequence of the formation of a sealed layer, denoting that the surface pore sealing was the main mechanism responsible for the observed Ks reductions. Laboratory data were gathered in a numerical simulation code specifically created to assess the consequences of Ks reduction on water movement through the soil profile. Simulation of both ponded and sprinkler irrigation with MW resulted in reduced infiltration and increased surface ponding condition compared to the application of fresh water (FW).     

How Reliable is the Plasticity Index for Estimating the Liquefaction Potential of Clayey Sands?

This paper examines the validity of the plasticity index (PI) as a criterion for estimating the liquefaction potential of clayey soils under cyclic loading. The results of undrained cyclic stress-controlled ring-shear tests on artificial mixtures of sand with different clays saturated with water indicated that an increase in PI decreased the soil potential to liquefy, and soil with PI>15 seemed to be nonliquefiable, a finding that is in agreement with the results of other researchers. However, in this study some deviations from this relation were found when a bentonite–sand mixture was treated with solutions of different ions, thus bringing into question the effectiveness of PI as a measure of the liquefaction potential of clayey soil having a certain pore water chemistry.     

Scaling Laws for Centrifuge Modeling of Capillary Rise in Sandy Soils

The application of geotechnical centrifuge modeling to environmental problems seems promising. In this paper, one aspect of similitude laws concerning the flow of water through soils is investigated. Within the Network of European Centrifuges for Environmental Geotechnic Research, several tests have been carried out to study similitude laws describing the capillary ascension in porous media at different levels of acceleration. The aim of this paper is to present the results obtained at Ruhr-Universit?t Bochum. A fine sand is used in the experiment. For the visualization of capillary height in the soil sample, image processing is used. Different boundary conditions (constant or variable water level) have been investigated and discussed. All these experiments confirm that capillary rise appears scaled by the factor N and time seems to be scaled with N2. Thus, these results support the possibility of extending the use of accelerated small-scale models to the capillary phenomenon in centrifuge and open the way to more complex investigations of flow and pollutant transport in unsaturated soils.     

Threshold Shear Strain for Cyclic Pore-Water Pressure in Cohesive Soils

Threshold shear strain for cyclic pore-water pressure, γt, is a fundamental property of fully saturated soils subjected to undrained cyclic loading. At cyclic shear strain amplitude, γc, larger than γt residual cyclic pore-water pressure changes rapidly with the number of cycles, N, while at γc<γt such changes are negligible even at large N. To augment limited experimental data base of γt in cohesive soils, five values of γt for two elastic silts and a clay were determined in five special cyclic Norwegian Geotechnical Institute (NGI)-type direct simple shear (NGI-DSS), constant volume equivalent undrained tests. Threshold γt was also tested on one sand, with the results comparing favorably to published data. The test results confirm that γt in cohesive soils is larger than in cohesionless soils and that it generally increases with the soil’s plasticity index (PI). For the silts and clay having PI=14–30, γt = 0.024–0.06% was obtained. Limited data suggest that γt in plastic silts and clays practically does not depend on the confining stress. The concept of evaluating pore water pressures from the NGI-DSS constant volume test and related state of stresses are discussed.     

Water Drainage in Double-Porosity Soils: Experiments and Micro–Macro Modeling

This paper presents the experimental validation of a macroscopic model of unsaturated flow in double-porosity soils, which was developed using the asymptotic homogenization method. The model was implemented into a code which enables micro–macro coupled calculations of macroscopically one-dimensional and microscopically three-dimensional problems. A series of drainage experiments were carried out in a column filled with a double porosity medium. The porous medium is composed of Hostun sand and porous spheres made of sintered clay, periodically distributed in the sand. The characteristic pores sizes of the two media differ by two orders of magnitude. During the experiments the water content evolution inside the column, the capillary pressure, and the flux at the bottom of the column were measured. The numerical simulations results showed a good agreement with the experimental data, confirming the predictive ability of the model. The experimental and numerical evidence of the influence of the microporous inclusions on the flow dynamics (flux retardation, water retention in the microporosity), is clearly shown.     

Assessment of the Axial Load Response of an H Pile Driven in Multilayered Soil

Most of the current design methods for driven piles were developed for closed-ended pipe piles driven in either pure clay or clean sand. These methods are sometimes used for H piles as well, even though the axial load response of H piles is different from that of pipe piles. Furthermore, in reality, soil profiles often consist of multiple layers of soils that may contain sand, clay, silt or a mixture of these three particle sizes. Therefore, accurate prediction of the ultimate bearing capacity of H piles driven in a mixed soil is very challenging. In addition, although results of well documented load tests on pipe piles are available, the literature contains limited information on the design of H piles. Most of the current design methods for driven piles do not provide specific recommendations for H piles. In order to evaluate the static load response of an H pile, fully instrumented axial load tests were performed on an H pile (HP?310×110) driven into a multilayered soil profile consisting of soils composed of various amounts of clay, silt and sand. The base of the H pile was embedded in a very dense nonplastic silt layer overlying a clay layer. This paper presents the results of the laboratory tests performed to characterize the soil profile and of the pile load tests. It also compares the measured pile resistances with those predicted with soil property- and in situ test-based methods.     

Nutrient Retention in Vegetated and Nonvegetated Bioretention Mesocosms

Thirty well-established 240L bioretention mesocosms were used to investigate retention of dissolved nutrients by bioretention systems. Ten mesocosms were comprised of 80?cm sandy loam, ten of 80?cm loamy sand, and ten of pea gravel with 20?cm of loamy sand. Half were vegetated with shrubs/grasses, while the other half had no vegetation (barren). In the first part of our study, the loam and sand mesocosms were dosed with synthetic storm water comprising 0.8?mg?L?1 total phosphorus (TP) and 4.8?mg?L?1 total nitrogen (TN). TP retention in the vegetated loam was 91% compared to 73% in the barren, and TN retention was 81% compared to 41% in the barren loam. TP retention was 86–88% in the sand treatments, while TN retention in the vegetated sand was 64%, compared to 30% in the barren. In the second part of our study, all 30 mesocosms were loaded weekly with 45?cm of tertiary effluent with high nutrient loads (22.3?m?year?1 hydraulic load at a flow-weighted average of 4.5?mg?L?1 TP and 4.8?mg?L?1 TN, or 1,012?kg?ha?1?year?1 TP and 1,073?kg?ha?1?year?1 TN). After 50 weeks of loading, cumulative TP retention was 92% in the vegetated loam, 67% in the sand, and 44% in the vegetated gravel. However, TP retention by barren media was 56% in the loam, 39% in the sand, and 14% in the gravel. Cumulative TN retention was 76% in the vegetated loam, 51% in the sand, and 40% in the vegetated gravel. In contrast, maximum TN removal by barren media was 18% in the loam. The increase in TP retention by vegetated systems substantially exceeds phosphorus uptake rates for plants, suggesting that other processes are involved. The increase in TN retention by vegetated systems also exceeds nitrogen uptake rates for plants, suggesting that denitrification is involved.     

Shear Strength and Shear-Induced Volume Change Behavior of Unsaturated Soils from a Triaxial Test Program

A series of unsaturated soil triaxial tests were performed on four soils including sand, silt, and a low plasticity clay. Attempts were made to correlate unsaturated soil properties from these tests and data from the literature with soil-water characteristics curve (SWCC), soil gradation, and saturated soil properties. The feasibility of estimating unsaturated soil property functions from saturated soil properties, SWCCs and gradation data, is demonstrated. A hyperbolic model for estimation of the unsaturated soil parameter, ?b, versus matric suction is presented. Shear induced volume change behavior was also studied, and results are included in this paper. Although not correlated with soil index properties, these shear-induced volume change data are important to complete stress-deformation analyses, and represent a significant addition to the existing data base of unsaturated soil properties.     

Impact of Soil Type and Compaction Conditions on Soil Water Characteristic

Tests were conducted to determine the variation of water content and pore water suction for compacted clayey soils. The soils had varying amounts of clay fraction with plasticities ranging from low to high plasticity. The unsaturated soil behavior was investigated for six conditions, covering a range of compactive efforts and water contents. The experimental data were fit to four commonly used models for the water content-pore water suction relationship. Each model provided a satisfactory fit to the experimental data. However, the individual parameters obtained from the curve fits varied significantly between models. The soil water characteristic curves (SWCCs) were more sensitive to changes in compaction effort than changes in compaction water content. At similar water contents, the pore water suction increased with increasing compaction effort for each compaction condition and soil type. For all compaction conditions, the lowest plasticity soils retained the smallest water content and the highest plasticity soils retained the highest water content at a specified suction. In addition, SWCCs for soils compacted in the laboratory and in the field were similar.     

Contact Erosion at the Interface between Granular Coarse Soil and Various Base Soils under Tangential Flow Condition

Under embankment dams and dykes, horizontal groundwater seepage prevails. If the subsoil is layered, and if some coarse layers are not appropriate filters for finer layers, there can be contact erosion at the interface between fine and coarse soils. In order to study contact erosion threshold, some base-soil and coarse-soil combinations were submitted to a flow parallel to the interface between the coarse soil and the base soil. Critical velocities and critical hydraulic gradients were measured for various base soils. Using effective base-soil grain diameter, an empirical expression for critical velocity was proposed that is well adapted for silts or sand/clay mixtures as well as for sands. The mass of eroded soil was measured relative to the flow velocity for each base-soil/coarse-soil setup. The shear stress applied to the interface between base soil and coarse soil was derived from the hydraulic gradient. Using an empirical relationship between applied shear stress and measured eroded mass, erosion rate was estimated for each base-soil/coarse-soil setup.     

Membrane Pervaporation for Wastewater Reuse in Microirrigation

A novel wastewater microirrigation technology for plants to extract reclaimed water from hydrophilic, homogenous dense membrane modules placed directly in the soil was evaluated. A series of tests were conducted in the laboratory to examine the effects of membrane configuration (hollow fiber (HF) and corrugated sheet (CS) membranes), soil texture (a loam and loamy sand soil), soil water content, feed pressure, and contaminant concentration on water permeate flux. The performance was evaluated in terms of soil water content, soil electroconductivity, water permeate flux and enrichment factor using borate, selenate, sodium chloride and glucose as model compounds. The results showed that the water permeate fluxes ranged from 0.21 to 1.04?L/m2/d for CS modules and from 0.10 to 1.00?L/m2/d for HF modules, respectively. Soil water content and feed pressure were identified as the main controlling factors for water flux. The enrichment factors were found to be less than 0.25 for all the tested contaminants. Thus, it was concluded that this membrane technology holds promise either to treat brackish ground water or to reuse wastewater for agricultural micro-irrigation.     

Numerical Solution for Laterally Loaded Piles in a Two-Layer Soil Profile

Piles are often embedded in a layered soil profile, such as sand or clay layer underlain by rock. Several existing solutions are available for laterally loaded piles in a layered soil system. However, these solutions are only applicable to constant soil stiffness for each layer. In this paper, a variational approach is employed to numerically solve the problem of laterally loaded piles in layered soils using beam on an elastic foundation model. The soil stiffness can be either constant with depth or linearly varying with depth. The numerical solution is validated against an existing solution for linearly varying soil stiffness in a single soil layer system and an existing solution for a two-layer soil system with constant soil stiffness. Case studies using the proposed solution for field lateral load tests on full size drilled shafts embedded in weak rock with an overlying sand layer are presented. The simplicity and the relative ease of using the solution make it a good alternative approach for estimating the deflection and moment responses of a laterally loaded pile in a two-layer soil profile.     

Theory of Plastic Sand Flow with Fluid Pressure Effect

Fundamental principles of elastic–plastic mechanics of soils and rocks are given on the base of the original publications. The solid friction and dilatancy effects are included in the nonstandard form of nonassociative rule of plastic flow. The resulting hyperbolic system of equations is represented for a plane case. The slip surfaces are assumed to be jump tangential discontinuities of a velocity field. The possibility of limit equilibrium at slip surfaces is accounted for. The attempts to account for grain rotations, permitting study of slip surface structure, are discussed. The Biot–Frenkel model of interpenetrating continua is developed for plastic flow of porous saturated matrix. In this case the solid matrix state is determined by the effective stresses and pore pressure diffusion happens in plastically flowing matrix. To illustrate the theory possibilities, solutions for failure and mass sand flow, driven by the pore pressure gradient, are selected. They are important especially for oil/gas reservoirs with a weak matrix, typical for offshore geology.