Search

Spatial variability of soils can have a profound effect on seismic site amplification, which cannot be reasonably captured using conventional one-dimensional analyses. In this study, the shear-wave velocity of soils is modelled as a 2D spatially correlated random field. Extensive simulations are conducted to quantify the influence of spatial variability of soils on site amplification using 2D Spectral Element Method. Key influential factors include variation and spatial correlation of shear wave velocity in both horizontal and vertical directions. Numerical simulations show that site amplification at resonance frequencies is subdued, and its variation depends on these influential factors. Spatial correlations of site amplification are also investigated, showing that the correlation range of site amplification is frequency dependent. The correlation range is much longer at low frequencies than that at high frequencies. The analyses imply that length scales of both soil heterogeneity and wavelength should be considered simultaneously when quantifying the spatial variability of ground motion amplification. [ABSTRACT FROM AUTHOR]

Object and purpose of the study: Underground aquiferous point and linear structures operated in difficult ground conditions lose their structural safety as a result of the influence of external forces and the environment. It is important to establish the features of the operation of underground aquifers and communications in an inhomogeneous enclosing soil massif, taking into account the interaction of structures with the ground in case of non-stationary force impacts and environmental impacts with an assessment of their structural safety. Methods: The use of analytical and experimental research methods to evaluate the work of structures in the soil mass, taking into account force influences and environmental influences. Results: The features of interaction of the housing of a collapsible pipe with a soil mass with a multidirectional horizontal component of a non-stationary load are established. The negative impact of the combination of the multidirectional impact of the power load and internal hydrostatic pressure on the structural safety of the housing of a prefabricated underground structure is shown. Practical significance and conclusions: The results obtained on the picture of the stress-strain state of an inhomogeneous enclosing soil massif allowed us to establish the nature and boundaries of the safe operation of structures and to make suggestions on the choice of ways to strengthen and protect underground structures, taking into account their operating conditions.

Abstract Buildings along the flood plain of River Osun, southwest Nigeria, are usually thought of as been a safe haven for residence over the years. But in recent times with vast (increasing) population and growing urbanization, some of the buildings currently experience structural damages not related to constructional designs but rather ground conditions around building foundations. The ground conditions (seepages in this case) and how it influences properties of soils (water moisture, permeability etc.) to support the foundation of buildings. This paper attempts to map out seepages in heterogeneous soils around buildings in Erinle, southwest Nigeria where building cracks probably as a result of water seepages from subsurface through fissures and fractures where noticed. These cracks are a probable manifestation of a weakened foundation that could overtime result to a collapse, hence the need to investigate seepage prone zones. To achieve this, electrical resistivity (ER) and Electromagnetic Ground Penetrating Radar (EM-GPR) surveys were made along affected buildings to map out possible causes of deterioration. Geo-sections (analyzed ER data) shows high resistivity layer (topsoil) underlain by low resistivity layer (weathered basement) confirmed by planer reflections (topsoil) and fairly smooth to smooth reflections (weathered basement) in Radargrams (analyzed GPR data). An integration of the geo-section and radargram produced from ER and GPR data shows that buildings along traverses 1, 2, 3 and 5 is most prone to seepages especially as it exhibits very low anomalously resistivity values (

In situ observations of soil water state variables under natural boundary conditions are often used to estimate the soil hydraulic properties. However, many contributions to the soil hydrological literature have demonstrated that the information content of such data is insufficient to accurately and precisely estimate all the soil hydraulic parameters. In this case study, we explored to which degree prior information about the soil hydraulic parameters can help improve parameter identifiability in inverse modelling of in situ soil water dynamics under natural boundary conditions. We used percentages of sand, silt, and clay as input variables to the ROSETTA pedotransfer function that predicts the parameters in the van Genuchten-Mualem (VGM) model of the soil hydraulic functions. To derive additional information about the correlation structure of the predicted parameters, which is not readily provided by ROSETTA, we employed a Monte Carlo approach. We formulated three prior distributions that incorporate to different extents the prior information about the VGM parameters derived with ROSETTA. The inverse problem was posed in a formal Bayesian framework and solved using Markov chain Monte Carlo (MCMC) simulation with the DiffeRential Evolution Adaptive Metropolis (DREAM) algorithm. Synthetic and real-world soil water content data were used to illustrate the approach. The results of this study demonstrated that prior information about the soil hydraulic parameters significantly improved parameter identifiability and that this approach was effective and robust, even in case of biased prior information. To be effective and robust, however, it was essential to use a prior distribution that incorporates information about parameter correlation.

Seven soil profiles developed on calcium carbonate–rich slope deposits in the Polish Carpathians were studied in order to: i) determine the micromorphological features of heterogeneous soils formed in a carbonate depositional environment, and ii) track primary and secondary calcium carbonate forms and their distribution in such stratified soils. Three cases of soils with different arrangements of calcium carbonate were distinguished, controlled mostly by slope processes. For instance, the increasing content and random distribution of angular and subangular rock fragments found in the overall soil matrix and the irregular coarse: fine size limit suggested different intensities of accumulation and mixing of soil material transported along the slope. Slope processes, together with the calcium carbonate content, mineralogical characteristics and texture influenced the type and arrangement of the bfabric pattern. The calcium carbonate distribution within the soils, besides the obvious inheritance from parent material, was governed by the translocation and mixing of deposits on slopes. The climatic conditions prevailing in the area favour the development of secondary forms of calcium carbonate. However, only three of the seven studied profiles contained pedogenic forms of calcium carbonate, yet they were distributed randomly. The occurrence, distribution and preservation of secondary carbonates depended on the content of primary calcium carbonate and soil features such as texture. The transported material down the slope may indicate a very low content of primary calcium or lack thereof, hence its pedogenic forms could not be created. [ABSTRACT FROM AUTHOR]

Simulation of variably saturated soil water flow requires the use of pressure head, or soil moisture, or a switching between the two, as the primary variable for solving Richards’ equation. Under unfavorable conditions, such as heterogeneity, rapidly changing atmospheric boundary, or sudden infiltration into dry soils, the traditional non-switching method suffers from numerical difficulties. Solving this problem with a primary variable switching method is less preferred due to the mathematical complexity. While the Picard method is more popular for solving the non-switching models due to its simplicity and stability, two different forms of Richards’ equation are combined into one numerical scheme for switching under specific hydraulic conditions. The method is successfully implemented in a one-dimensional model solved by a Picard iteration scheme. A threshold saturation based on the soil moisture retention relation is used for switching between either form of the Richards’ equation. The method developed here is applicable for simulating variably saturated subsurface flow in heterogeneous soils. Compared with traditional methods, the proposed model conserves mass well and is numerically more stable and efficient. [ABSTRACT FROM AUTHOR]

Adoption of fertiliser trees in Zambia remains very low while efforts to understand the farmers’ decision-making process in embracing sustainable agricultural practices still eludes research. One area not vigorously pursued in understanding the farmers’ adoption process is the role that farmer classification of on-farm soils plays in adoption and impact of sustainable technologies such as the fertiliser trees. Therefore, the objective of this study was to estimate adoption rates and impact of fertiliser trees on heterogeneous soil types according to farmers’ classifications. Using data randomly collected from 324 households in the 2011 farming season in Chongwe district of Zambia, it was found that farmers were able to classify the dominant soils on their farms into sandy, clay, sand loamy and loamy soils. Most of the farmers (60%) who perceived their soils to be sandy also indicated facing soil fertility challenges. Perception of being on sandy soils was significantly associated with adoption of the fertiliser trees. Propensity score estimates showed that the technology significantly increased maize productivity on soils perceived to be sandy and sand loamy. The non-significant impact results of the technology on relatively perceived high fertile loamy and clay soils could be indicative of diminishing marginal effect of the technology on already fertile soils. Although adoption of the technology on relatively fertile soils is important for fertility sustainability, its promotion on farms with degraded soils could ensure full expression of its potential, and hence, increase its adoptability chances. Targeting of farmers who receive fertiliser tree seedlings should embrace this condition. [ABSTRACT FROM AUTHOR]

The present work describes the program Simulation of Consolidation with Vertical Drains (SICOMED_2018), a tool for the solution of consolidation processes in heterogeneous soils, with totally or partially penetrating prefabricated vertical drains (PVD) and considering both the effects of the smear zone, generated when introducing the drain into the ground, and the limitation in the discharge capacity of the drain. In order to provide a completely free program, the code Next-Generation Simulation Program with Integrated Circuit Emphasis (Ngspice) has been used as a numerical tool while the Matrix Laboratory (MATLAB) code was used to program and create an interface with the user through interactive screens. In this way, SICOMED_2018 is presented as an easy-to-use and intuitive program, with a simple graphical interface that allows the user to enter all the soil properties and geometry of the problem without having to resort to a complex software package that requires programming. Illustrative applications describe both the versatility of the program and the reliability of its numerical solutions. [ABSTRACT FROM AUTHOR]

This study focused on the estimation of groundwater recharge rates and travel time of conservative contaminants between ground surface and aquifer. Numerical simulations of transient water flow and solute transport were performed using the SWAP computer program for 10 layered soil profiles, composed of materials ranging from gravel to clay. In particular, sensitivity of the results to the thickness and position of weakly permeable soil layers was carried out. Daily weather data set from Gdańsk (northern Poland) was used as the boundary condition. Two types of cover were considered, bare soil and grass, simulated with dynamic growth model. The results obtained with unsteady flow and transport model were compared with simpler methods for travel time estimation, based on the assumptions of steady flow and purely advective transport. The simplified methods were in reasonably good agreement with the transient modelling approach for coarse textured soils but tended to overestimate the travel time if a layer of fine textured soil was present near the surface. Thus, care should be taken when using the simplified methods to estimate vadose zone travel time and vulnerability of the underlying aquifers.

An isogeometric analysis (IGA) is introduced to obtain a head-based solution to Richards equation for unsaturated flow in porous media. IGA uses Non-Uniform Rational B-Spline (NURBS) as shape functions, which provide a higher level of inter-element continuity in comparison with Lagrange shape functions. The semi-discrete nonlinear algebraic equations are solved using a combination of implicit backward-Euler time-integration and Newton-Raphson scheme. The time-step size is adaptively controlled based on the rate of changes in the pore pressure. The results from the proposed formulation are compared and verified against an analytical solution for one-dimensional transient unsaturated flow in a homogenous soil column. The proposed method is then applied to four more complex problems including two-dimensional unsaturated flow in a two-layered soil and a semi-circular furrow. The test cases in two-layered soil system involve sharp variations in the pressure gradient at the intersection of the two media, where the pore water pressure abruptly changes. It is shown that the proposed head-based IGA is able to properly simulate changes in pore pressure at the soils interface using fewer degrees of freedom and higher orders of approximation in comparison with the conventional finite element method. [ABSTRACT FROM AUTHOR]

The paper presents a 2D upward infiltration experiment performed on a model porous medium consisting of fine sand background with two inclusions made of coarser sands. The purpose of the experiment was to investigate the effects of structural air trapping, which occurs during infiltration as a result of heterogeneous material structure. The experiment shows that a significant amount of air becomes trapped in each of the inclusions. Numerical simulations were carried out using the two-phase water-air flow model and the Richards equation. The experimental results can be reproduced with good accuracy only using a two-phase flow model, which accounts for both structural and pore-scale trapping. On the other hand, the Richards equation was not able to represent the structural trapping caused by material heterogeneity. [ABSTRACT FROM AUTHOR]

The present work describes the program Simulation of Consolidation with Vertical Drains (SICOMED_2018), a tool for the solution of consolidation processes in heterogeneous soils, with totally or partially penetrating prefabricated vertical drains (PVD) and considering both the effects of the smear zone, generated when introducing the drain into the ground, and the limitation in the discharge capacity of the drain. In order to provide a completely free program, the code Next-Generation Simulation Program with Integrated Circuit Emphasis (Ngspice) has been used as a numerical tool while the Matrix Laboratory (MATLAB) code was used to program and create an interface with the user through interactive screens. In this way, SICOMED_2018 is presented as an easy-to-use and intuitive program, with a simple graphical interface that allows the user to enter all the soil properties and geometry of the problem without having to resort to a complex software package that requires programming. Illustrative applications describe both the versatility of the program and the reliability of its numerical solutions.

• Hydraulic properties of soils with rock fragments were indirectly determined. • Measurements from field drainage experiments and undisturbed soil samples were used. • Parameter optimization was accomplished with the Hydrus-1D "Inverse solution". • Bimodal hydraulic functions showed more suitable for soils with rock fragments. • Using a sequential optimization, hydraulic functions could be parameterized. The application of inverse modeling to determine the hydraulic properties of layered soils with rock fragments (RF > 2 mm) is associated with complex challenges such as selecting suitable hydraulic functions and defining a strategy to optimize many parameters. These two issues were addressed in this study by performing field drainage experiments in layered soil profiles (three layers) with different particle size distributions and different percentages of RF. Water contents (θ) and pressure heads (h) were monitored at different soil depths during drainage experiments (h ≥ −200 cm). Data of θ and h for a drier range (h < −5000 cm) were determined with a dew point potentiometer on disturbed soil samples. The unimodal and bimodal van Genuchten-Mualem functions were evaluated, and their parameters were optimized using the Hydrus-1D "Inverse solution". The RETC curve fitting software was used to estimate the second set of parameters of the bimodal function. The parameters for the uni- and bimodal models (15 and 27 parameters, respectively) were repeatedly optimized by fixing some parameters while estimating others. The unimodal van Genuchten-Mualem function provided a satisfactory fit of θ and h measurements only when either drainage or dew point potentiometer measurements were used. On the other hand, the bimodal van Genuchten-Mualem function provided a satisfactory fit of θ and h measurements when both sets of data (drainage and dew point potentiometer measurements) were used simultaneously. Furthermore, the optimized parameters were physically consistent except for a few high saturated hydraulic conductivity values. Therefore, bimodal functions should be considered to represent the hydraulic properties of soils with RF. The problem of many calibrated parameters when using bimodal functions can be managed using sequential optimization, which allowed us to estimate 27 parameters for layered soils with RF successfully. [ABSTRACT FROM AUTHOR]

The lookup table option, as an alternative to analytical calculation for evaluating the nonlinear heterogeneous soil characteristics, is introduced and compared for both the Picard and Newton iterative schemes in the numerical solution of Richards’ equation. The lookup table method can be a cost-effective alternative to analytical evaluation in the case of heterogeneous soils, but it has not been examined in detail in the hydrological modeling literature. Three layered soil test problems are considered, and the robustness and accuracy of the lookup table approach are assessed for uniform and non-uniform distributions of lookup points in the soil moisture retention curves. Results from the three one-dimensional test simulations show that the uniform distributed option gives improved convergence and robustness for the drainage problem compared to the non-uniform strategy. On the other hand, the non-uniform technique can be chosen for test problems involving flow into initially dry layered soils.

Xie and Gough (2013) developed an algorithm to expand the application of the well-known Stefan equation in order to predict the freeze or thaw depth in multi-layered soils. This discussion paper demonstrates that the development of this algorithm is premised on mathematical expressions that are not physically tenable. A previously proposed equation for applying the Stefan approach to multi-layered soils is herein derived from first principles using the appropriate governing equations, boundary conditions and initial conditions. Numerical methods are employed to demonstrate the accuracy of this equation derived from first principles. This approach can be implemented in existing cold regions hydrology models to estimate the active layer thickness in permafrost regions underlain by layered soils. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

• Aeolian silt mantles reveal different clay mineralogy than the underlying substrates. • Aeolian silt admixture was an important source of chlorite. • Smectite and mica have partially been inherited from the underlying bedrock. • Clay minerals transformation determined the presence of vermiculite and smectite. Loess may be integrated into slope deposits at different soil depths and may influence related physical, chemical and mineralogical soil properties. Therefore, tracking the border between deposited loess and underlying materials and estimating the depth of loess penetration is a challenge. Five soils from the Lower Silesia province in south-western Poland having different types of geologic substrate and being covered by loess deposits of various thicknesses were chosen to: 1) trace the origin of phyllosilicates in these heterogeneous soils (loess vs. underlying bedrock); 2) determine the influence of the geologic substrate and the loess mantle on clay mineral transformation in the soil; and 3) relate clay mineralogical traits to soil morphology. The loess consisted of long-distance and local aeolian sources as shown by the Ti/Zr ratios and heavy mineral composition. Geochemical and mineralogical traces of loess were easily detected in the mixed zone and in part also in the basal layer. The loess deposits are characterised by mica, chlorite, kaolinite, interstratified mica-vermiculite or hydroxy-interlayered vermiculite, vermiculite and a minor amount of smectite. Aeolian silt admixture was an important source of chlorite at all sites. The vertical distribution of mica indicated two types of sources — loess input and substrate. Smectite, however, has mostly been inherited from the underlying bedrock (serpentinite, glacio-fluvial deposits and basalt slope sediment) or was formed in the soils from mica or chlorite. Where present in the loess, smectite was only found in small amounts. The presence of kaolinite, HIV and mixed layered mica-vermiculite could be derived either from aeolian input or neoformation and thus actively occurring mineral (trans)formation reactions. Similar to mica, vermiculite was derived from both aeolian input and the geological substrate. The widespread loess deposits in this region rejuvenated the soil formation process, as evidenced by their mineralogical and chemical composition. [ABSTRACT FROM AUTHOR]

Aims: This study aimed to compare stepwise multiple linear regression (SMLR), partial least squares regression (PLSR) and support vector machine regression (SVMR) for estimating soil total nitrogen (TN) contents with laboratory visible/near-infrared reflectance (Vis/NIR) of selected coarse and heterogeneous soils. Moreover, the effects of the first (1st) vs. second (2nd) derivative of spectral reflectance and the importance wavelengths were explored. Methods: The TN contents and the Vis/NIR were measured in the laboratory. Several methods were employed for Vis/NIR data pre-processing. The SMLR, PLSR and SVMR models were calibrated and validated using independent datasets. Results: Results showed that the SVMR and the PLSR models had similar performances, and better performances than the SMLR. The spectral bands near 1450, 1850, 2250, 2330 and 2430 nm in the PLSR model were important wavelengths. In addition, the 1st derivative was more appropriate than the 2nd derivative for spectral data pre-processing. Conclusions: PLSR was the most suitable method for estimating TN contents in this study. SVMR may be a promising technique, and its potential needs to be further explored. Moreover, the future studies using outdoor and airborne/satellite hyperspectral data for estimating TN content are necessary for testing the findings. [ABSTRACT FROM AUTHOR]

Guidelines of effective soil hydraulic parameters were developed to be applicable in simulating average infiltration and subsequent moisture redistribution over a large-scale heterogeneous field. Average large-scale infiltration and redistribution in heterogeneous soils were quantified through multiple simulations of local-scale processes. The effective hydraulic parameters were derived to simulate the average amount of infiltrating water, and to capture the subsequent surface soil moisture redistribution averaged over the large heterogeneous landscape. The results demonstrated that the effective hydraulic parameters typically exhibited a step change from infiltration to redistribution, with the size of the step change being related to the degree of hydraulic parameter heterogeneity and the correlations among the hydraulic parameters. However, the effective hydraulic parameters did not change significantly over time for the moisture redistribution. It was further demonstrated that the size of the step change was smallest for effective saturated hydraulic conductivity. Editor Z.W. Kundzewicz; Associate editor Y. Guttman Citation Zhu, J.T. and Sun, D.M., 2012. Soil hydraulic properties for moisture redistribution in a large-scale heterogeneous landscape. Hydrological Sciences Journal, 57 (6), 1196–1206. [ABSTRACT FROM AUTHOR]

Abstract: The Green–Ampt model for infiltration into homogeneous soils predicts a monotonically decreasing infiltration rate and a wetting front that initially advances as the square root of time. Infiltration in heterogeneous soils, however, can exhibit non-monotonic infiltration rates and wetting front advances that differ from the square root of time (“anomalous diffusion”). Here it is postulated, that if the length scales of the heterogeneities can be assumed to be power law distributed, it may be appropriate to model infiltration in heterogeneous soils in terms of fractional derivatives. Then, by expressing the hydraulic flux as a Caputo fractional derivative (order 0< α ⩽1) of the head, a fractional Green–Ampt infiltration model is obtained. It is shown that solutions of this model predict non-monotonic and anomalous diffusion behaviors consistent with observations in field infiltration trials; a finding that indicates that a non-local moisture flux model, based on fractional derivatives, is a plausible model for describing infiltration into heterogeneous soils. [ABSTRACT FROM AUTHOR]

Abstract: This work deals with a comparison of different numerical schemes for the simulation of contaminant transport in heterogeneous porous media. The numerical methods under consideration are Galerkin finite element (GFE), finite volume (FV), and mixed hybrid finite element (MHFE). Concerning the GFE we use linear and quadratic finite elements with and without upwind stabilization. Besides the classical MHFE a new and an upwind scheme are tested. We consider higher order finite volume schemes as well as two time discretization methods: backward Euler (BE) and the second order backward differentiation formula BDF (2). It is well known that numerical (or artificial) diffusion may cause large errors. Moreover, when the Péclet number is large, a numerical code without some stabilising techniques produces oscillating solutions. Upwind schemes increase the stability but show more numerical diffusion. In this paper we quantify the numerical diffusion for the different discretization schemes and its dependency on the Péclet number. We consider an academic example and a realistic simulation of solute transport in heterogeneous aquifer. In the latter case, the stochastic estimates used as reference were obtained with global random walk (GRW) simulations, free of numerical diffusion. The results presented can be used by researchers to test their numerical schemes and stabilization techniques for simulation of contaminant transport in groundwater. [Copyright &y& Elsevier]

Abstract: This article utilizes a Monte Carlo stochastic framework to investigate the influence on the mean and variance of the predicted mean pressure head profile of statistical assumptions regarding the parameters that enter the mathematical description of the problem of infiltration in unsaturated, heterogeneous layers. The parameters are treated as random functions with an exponential auto-covariance function expressing their spatial continuity. Four different truncated distributions are taken to describe the parameters according to field observations and various phases of site characterization campaigns. The exponential distribution is seen to produce the largest (in absolute value) mean and variance in the pressure head profile. For all distributions the variance in pressure head increases with increasing mean pressure. A second topic of this article is to investigate, the relative importance of each parameter, in terms of the mean and the variance of the predicted pressure. For uniformly or triangularly distributed parameters the saturated hydraulic conductivity appears to dominate the mean-behavior and the uncertainty in the system''s solution. For lognormally or exponentially distributed parameters another parameter, the van Genuchten pore-size distribution index, is the dominant factor. [Copyright &y& Elsevier]

The spatial distribution of fine roots of two deciduous tree species was investigated in contrasting growing conditions in southern France. Hybrid walnut trees ( Juglans regia× nigra cv. NG23) and hybrid poplars ( Populus euramericana cv. I214) were both cultivated with or without annual winter intercrops for 10 years on deep alluvial soils. Soil samples for measuring the fine root distribution of both trees and crops were obtained by soil coring down to 3-m depth at several distances and orientations from the tree trunk. The distribution of live fine roots from walnut and poplar trees was patchy and sometimes unexpected. In the tree-only stands, fine root profiles followed the expected pattern, as fine root density decreased with increasing depth and distance from the tree trunk. However, many fine root profiles under intercropped trees were uniform with depth, and some inverse profiles were observed. These distributions may result from a high degree of plasticity of tree root systems to sense and adapt to fluctuating and heterogeneous soil conditions. The distortion of the tree root system was more pronounced for the walnut trees that only partially explored the soil volume: in the tree-only stand, the walnut rooting pattern was very superficial, but in the intercropped stand walnut trees developed a deep and dense fine root network below the crop rooting zone. The larger poplars explored the whole available soil volume, but the intercrop significantly displaced the root density from the topsoil to layers below 1 m depth. Most tree root growth models assume a decreasing fine root density with depth and distance from the tree stem. These models would not predict correctly tree–tree and tree–understorey competition for water and nutrients in 3D heterogeneous soil conditions that prevail under low-density tree stands. To account for the integrated response of tree root systems to such transient gradients in soils, we need a dynamic model that would allow for both genotypic plasticity and transient environmental local soil conditions. [ABSTRACT FROM AUTHOR]

Abstract: This study examines the effect of random heterogeneity of soil properties on bearing capacity. The stochastic soil property considered is the undrained shear strength and two major sources of uncertainty are identified with it: inherent spatial variability (modeled as a non-Gaussian, homogeneous stochastic field) and uncertainty in the estimation of its expected value (modeled as a random variable). The two sources of uncertainty are treated separately, before being eventually combined. A Monte Carlo simulation approach is followed in combination with non-linear finite element analysis. It is demonstrated that the inherent spatial variability of soil shear strength can drastically modify the basic form of the failure mechanism in this bearing capacity problem. Consequently, there is no ‘average’ failure mechanism (surface) in this problem, leading to the conclusion that Monte Carlo simulation is the only methodology capable of providing a solution to this geomechanics problem. It is further demonstrated that this behavior of the failure mechanism translates into a substantial reduction in the ultimate bearing capacity (in an average sense), compared to the corresponding deterministic (homogeneous soil) case. In addition, differential settlements are computed in the stochastic analysis, something impossible in a deterministic analysis of a symmetric problem. A parametric study is performed using fragility curves to investigate the effects of various probabilistic parameters involved in the problem. It is found that the coefficient of variation and the marginal probability distribution of the soil''s shear strength (both controlling the amount of loose pockets in the soil mass) are the two most important parameters in reducing the bearing capacity (in an average sense) and producing substantial differential settlements in heterogeneous soils (compared to homogeneous soils). A technique is finally introduced for determining ‘overall’ fragility curves that account for both inherent soil spatial variability and uncertainty in the expected value of soil strength. Based on such ‘overall’ fragility curves obtained at failure (ultimate bearing capacity), nominal values of the bearing capacity of a heterogeneous soil deposit corresponding to an exceedance probability of 5% are established for a range of probabilistic characteristics. [Copyright &y& Elsevier]

Abstract: The hyperbolic method has proved applicable for settlement prediction in complex soil formations. However, when an appreciable portion of the settlement is due to secondary compression, settlement prediction based on the slope of the initial linear portion of the hyperbolic plot requires a correction factor that would be different for different amounts of secondary compression. Although inverse of the slope of the final linear portion of the hyperbolic curve can provide reasonable estimates of total settlements, including secondary compression, establishing the curve slope requires data beyond 90% consolidation, and this renders the hyperbolic method less useful for practical applications. This paper examined the feasibility of predicting total settlements of heterogeneous soils, which also exhibit prominent secondary compression behaviour, by utilizing the hyperbolic method for estimating ultimate primary consolidation settlement and the time for its completion, and the C α /C c concept of compressibility for predicting secondary compression. The applicability of the proposed procedure was examined using the results of laboratory tests and an instrumented test embankment constructed on a typical sabkha formation. The combined use of the hyperbolic method and the C α /C c concept has been found to provide reasonable estimates of total settlements where the hyperbolic curve was initially concave downward. Contrarily, for hyperbolic plots that do not show convex curvature toward the origin, the hyperbolic method predicted a significantly erroneous time for the completion of primary consolidation, hence leading to unreasonable estimates of secondary compression. Thus, where settlement versus time fully obeys the rectangular hyperbolic relationship, total settlement, inclusive of secondary compression, can simply be estimated as the reciprocal of the slope of the fitted straight line. [Copyright &y& Elsevier]

This study analyzes the wetting front migration in layered unsaturated soils which have uncertain hydraulic properties. A Monte Carlo scheme was used to propagate the uncertainty of hydraulic parameters. RANUF, a computer program, was developed to solve the one-dimensional, pressure-based form of Richards' equation and to implement the Monte Carlo scheme. Uncertainty propagation was investigated for two-layered soils of various alternating fine over coarse or coarse over fine layer configurations and of various nonrandomized and/or randomized layer arrangements. The effects of changing initial and boundary conditions were also investigated. Randomness was introduced via the saturated hydraulic conductivity, Ks, which was assumed to be distributed lognormally with a coefficient of variation of about 10 percent. It was found that in layered soils the mean profiles (i.e., water content and pressure head) remained essentially unchanged regardless of which layer (or layers) was (or were) randomized; however, the variance profiles were affected. Also, higher uniform initial water content tended to inhibit uncertainty, but higher supply rates did not show any characteristic trend for uncertainty behavior. [ABSTRACT FROM AUTHOR]

A simple method for predicting surface irrigation advance trajectories using infiltration parameters and inflow rate as inputs was developed. The difference between the inflow rate and the sum of infiltration rates over the wetted portion of the field equals the flow rate available for advance. An average (characteristic) infiltration rate ahead of the wet portion is computed using a fixed time step. An advance step (for a fixed time step) is calculated from the ratio of the flow rate available for advance and characteristic infiltration rate. Predictions of advance by the proposed method were compared with field observations, with the kinematic wave model, and with analytical solutions of Philip and Farrell (1964). In all cases, the method provided predictions that were in good agreement with field observations, and performed similarly to the kinematic wave model. The method offers a simple and efficient tool for prediction and evaluation of surface irrigation systems under various soil types and variable inflow rates. The method is particularly useful for predictions in fields with spatially and temporally variable intake properties. [ABSTRACT FROM AUTHOR]

• The streamtube model (STM) ability to model flow in unsaturated soils is analyzed. • A 3D numerical model is developed for unsaturated soils under a rainfall event. • Mean soil moisture and Darcy's velocity of STM and 3D numerical model are compared. • Spatial variability in both saturated hydraulic conductivity and air entry suction. • Regression equations are proposed to correct STM estimates for sandy soils. The streamtube approximation has been commonly used for upscaling of flow and transport in spatially heterogeneous unsaturated soils. This study investigates the prediction ability of streamtube approach to model flow in heterogeneous unsaturated sandy soils under a rainfall event at a plot scale. Towards this aim, a numerical model for solving 3-D Richards's equation was developed and verified for 1-D and 3-D cases. Turning band method was used to generate the random field of soil hydraulic parameters. The 3-D soil field heterogeneity was represented by considering variation in both van Genuchten parameter (α) and K s. For streamtube models, two cases of heterogeneity were investigated: i) variability in K s only, and ii) variability in both K s and α. Both the parameters were assumed to follow a log-normal distribution. For different boundary conditions at the soil surface and varying soil heterogeneity, the average Darcy's flux and mean field-scale soil moisture obtained using streamtube approach were compared with 3-D numerical solutions. The results identify that for high recharge rate at the soil surface, streamtube model is a good approximation of the 3-D heterogeneous soil field, and the average difference in mean soil moisture obtained from 3-D model and streamtube based 1-D model was below 10% when variability in both K s and α was considered for all timesteps. The inclusion of variability in α leads to improvement in prediction capability of streamtube model. Further, regression equations were developed that can be utilized to predict the mean field soil moisture for heterogeneous 3-D sandy soil fields given the mean field soil moisture obtained from streamtube approach for both the cases. The regressions equations follow exponential model for case-1 and linear model for case-2 with R 2 values 0.80 and 0.99, respectively. The developed equations can be reliably used to predict mean field soil moisture for heterogeneous sandy soil fields. [ABSTRACT FROM AUTHOR]

This study focused on the estimation of groundwater recharge rates and travel time of conservative contaminants between ground surface and aquifer. Numerical simulations of transient water flow and solute transport were performed using the SWAP computer program for 10 layered soil profiles, composed of materials ranging from gravel to clay. In particular, sensitivity of the results to the thickness and position of weakly permeable soil layers was carried out. Daily weather data set from Gdańsk (northern Poland) was used as the boundary condition. Two types of cover were considered, bare soil and grass, simulated with dynamic growth model. The results obtained with unsteady flow and transport model were compared with simpler methods for travel time estimation, based on the assumptions of steady flow and purely advective transport. The simplified methods were in reasonably good agreement with the transient modelling approach for coarse textured soils but tended to overestimate the travel time if a layer of fine textured soil was present near the surface. Thus, care should be taken when using the simplified methods to estimate vadose zone travel time and vulnerability of the underlying aquifers. [ABSTRACT FROM AUTHOR]