Effect of wetting and drying on soil physical properties

Agricultural soils are subject to seasonal wetting and drying cycles. Effect of drying stress, as influenced by one cycle of wetting and drying, on physical properties of a clay–loam soil was investigated in the laboratory. The physical properties studied were soil bulk density, cone penetration resistance, shear strength, adhesion and aggregate size and stability. Three drying stress treatments were made by wetting air-dried soil of initial moisture content of 12% (on dry weight basis) to three different higher moisture contents, namely 27, 33 and 40%, and then drying each of them back to their original moisture content of 12%. Thus, the soil was subjected to three different degrees of drying stress. The results showed that the soil strength indicated by cone penetration resistance and cohesion, and soil aggregate size, increased with the degree of drying stress. However, the soil bulk density did not change significantly with the drying stress.     

Effect of Rainfall-Induced Soil Seals on the Soil Water Regime: Drying Interval and Subsequent Wetting

The effects of rainfall-induced soil seals on drying processes and on infiltration following drying intervals are simulated for two different soils, a loam and a sandy loam. The simulated drying processes include water content redistribution without evaporation and under a constant evaporation rate of 5 mm day^–1. During evaporation, the water content at the seal surface decreases rapidly. A high water content gradient develops within the seal, which increases along the drying interval. It indicates that, at least during the first hours of drying, the seal layer fulfilled all the evaporation demand and therefore dries faster that an unsealed soil where the evaporation is supplied by a much deeper zone of the soil profile. This phenomenon is more accentuated in the loam than in the sandy loam soil. Considering the subsequent infiltration curves during rainfall following different drying intervals, the ponding time and the post-ponding infiltration rates increase when the antecedent drying period is longer, but no significant effect on the final infiltration is found following drying intervals of few days. Also, the water content at the sealed soil surface before rainfall seems to play a major role on infiltration. Very close infiltration curves were obtained after different drying intervals that ended with similar surface water content.     

Effects of tine rake angle and aspect ratio on soil reactions in dry loam soil

Study was conducted in a laboratory glass-sided soil bin with dry compact loam soil with 5.2% (d.b.) moisture content. The specific objective of this study was to determine the effects of flat rigid tine rake angle (forward angle between tine face and horizontal soil surface) and aspect ratio (tine width/tine depth) on soil reactions. The tine was moved in the soil in a quasi-static condition and soil reactions were recorded using L-shaped force transducers. Corresponding soil failure patterns were observed through a glass window. Tine rake angles of 50°, 90° and 130° were used. The effects of aspect ratio were studied by varying both width and depth of the tines. Individual effects of width and depth were investigated by maintaining a constant aspect ratio of 2.0 but varying width and depth. Observations indicated that soil reactions are affected by tine design parameters. For all tine rake angles and aspect ratios, soil reactions were observed to be cyclic in nature and could be matched well with corresponding soil failure patterns. The horizontal and vertical soil reactions were in phase. Investigations into the individual effects of tine width and depth revealed that the aspect ratio alone cannot account for changes in soil reactions. The effects are mainly due to the individual changes in width and depth.     

Enhancing the Bolzon–Schrefler–Zienkiewicz Constitutive Model for Partially Saturated Soil

This paper presents an enhanced version of the elasto-plastic model for partially saturated soil first proposed by Bolzon, Schrefler and Zienkiewicz in 1996, “BSZ” model, which uses the effective stress tensor and suction as independent stress variables. It is recalled that the effective stress tensor proposed by Lewis and Schrefler in 1982 is thermodynamically consistent and, compared with other choices of stress tensors, results particularly suitable for partially saturated soil mechanics. A hydraulic constitutive relationship and a hydraulic hysteresis are introduced in the model, to take into account the irreversible deformation during cyclic drying and wetting until structural collapse. For this reason the plastic rate of strain is split into the sum of two components: one depending on the effective stress tensor and the other one on suction. This is the new feature of the BSZ model. This enhanced model is then cast into a thermodynamical framework at macroscopic level and it is shown that it is possible to derive the constitutive law from the Helmholtz free energy and a dissipation function, both for associative and non- associative plasticity. Finally the model predictions have been compared with experimental data for Sion slime, with particular emphasis on the deviatoric part, and model predictions of hysteretic behaviour have been investigated in case of a wetting and drying cycle on compacted betonite–kaolin.     

Collapse failure in dry clay soils caused by tine implements

The relation between forces applied to the soil and the resultant soil reaction was studied in dry clay soils under a quasi-static condition. As a tine advanced in dry compact clay soils at 5.2% dry basis moisture content, masses of soil collapsed one by one in front of the tine. The horizontal and vertical components of soil resistance measured were cyclic and in phase, with distinct peak and trough values. The peak values and trough values indicated the soil stress conditions before and immediately after each failure occurred. The frequency of failure depended on the size of the tine. The magnitude of the peak values depended on level of compaction and trough values on density of collapsed mass. The paper presents the details of observations.     

Limitations of passive earth pressure theory for cage wheel lug and tine force predictions

Experiments in wet clay soil with cage wheel lug showed that the failure pattern in front of a lug was totally different from that assumed in passive soil pressure theory. Based on the failure pattern, the area of deformation zone and surcharge buildup in front of the lug, it was observed that the existing passive soil pressure theory could not be used to describe the soil movement caused by the action of the cage wheel lug. While working with a tine, four types of soil failure patterns were observed. It was found that these types of soil failures in front of a rigid tine were a strong function of soil moisture content. Passive pressure theory does not accurately predict the forces measured.     

Application of a Soil Water Hysteresis Model to Simple Water Retention Curves

The Parlange hysteresis model is reformulated as a pair of recurrence relations to provide relationships between wetting and drying phases to any order. The model is applied to the classical van Genuchten model for soil water retention used as the main wetting curve. The nonphysical behaviour of these retention curves is related to the existence of a point of inflection in the van Genuchten model when it is used for the wetting boundary. Where the van Genuchten form is used as the main drying curve, the Parlange hysteresis model provides an ordinary differential equation describing the main wetting curve. A number of simple analytical solutions, relating to particular values of the parameters of the van Genuchten model, then provide forms for the main wetting curve. In general, a numerical integration is required to generate the main wetting curve, for general values of the parameters of the van Genuchten model. The recurrence relations for the hysteresis cycling are still applicable, even when the main wetting curve is only known numerically. The new main wetting curves do not have inflection points and there is no nonphysical behaviour. The model is then applied to the experimental data of Viaene et al. (1994)     

Effect of tine rake angle and aspect ratio on soil failure patterns in dry loam soil

Studies were conducted in a laboratory glass-sided soil bin with dry compact loam soil (c = 0.02 kPa, Φ = 20° and cone index 210 kPa) with the specific objective of observing the effect of flat tine rake angle and aspect ratio on soil failure patterns. The tine was moved in the soil in a quasi-static condition and soil failure patterns were observed through a glass window. Tine rake angles of 50°, 90° and 130° were used while aspect ratio effects were studied by varying both width and depth of the tine. Individual effects of width and depth were investigated by maintaining a constant aspect ratio of 2.0 but varying width and depth. Results obtained indicated that soil failure patterns are affected by tine design parameters. Soil failure patterns were observed to be of progressive shear type in all cases. For 50° rake angle tines, the patterns consisted of inclined shear lines starting from the tine tip and gradually moving upwards towards the horizontal soil surface, intersecting it at an average failure angle of 32°. In the case of 90° rake angle tines, the inclined shear surface was at a distance from the tine tip whereas, for 130° rake angle tines, prismatic-shaped stationary soil wedges were formed adjacent to the tine. Failure angles for the 90° and 130° rake angle tines were almost the same as those for 50° rake angle tines. The results of this study also indicated that aspect ratio alone cannot account for changes in soil failure patterns, their corresponding soil reactions, forward rupture or surcharge profiles. The effects are mainly due to the individual changes in width and depth. There were no distinct zones as described in the passive soil pressure theory. Soil failures were in regular cycles resulting in corresponding variations in the soil reactions on the tines.     

Heat and mass transfer in unsaturated porous media at a hot boundary: I. One-dimensional analytical model

We present a model of heat and mass transfer in an unsaturated zone of sand and silty clay soils, taking into account the effects of temperature gradients on the advective flux, and of the enhancement of thermal conduction by the process of latent heat transfer through vapor flow. The motivation for this study is to supply information for the planned storage of thermal energy in unsaturated soils and for hot waste storage. Information is required on the possibility of significant drying at a hot boundary, as this would reduce the thermal conductivity of a layer adjacent to the boundary and, thus, prevent effective heat transfer to the soil. This study indicates the possibility that the considered system may be unstable, with respect to the drying conditions, with the occurrence of drying depending on the initial and the boundary conditions. An analysis performed for certain boundary conditions of heat transfer and for given soil properties, disregarding the advective flux of energy, indicated that there are initial conditions of water content for which heating will not cause significant drying. Under these conditions, fine soils may be better suited for heat transfer at the hot boundary, due to their higher field capacity, although their heat conduction coefficients at saturation are lower than those of sandy soils. At present, these conclusions are limited to the range of 50–80°C. Potential effects of solute concentration at the hot boundary are indicated.     

非饱和土土水特征曲线(SWCC)测试与预测

非饱和土土水特征曲线(SWCC)表示了土中含水量与吸力之间的关系。文章介绍了6种常用方法,各有其适用范围。体积压力板仪可量测最大基质吸力值为1500kPa的干燥曲线和浸湿曲线;超过1500kPa时,可用盐溶液法进行量测;Tem-ple仪可量测基质吸力达100kPa的干燥曲线;滤纸法可用于测量土体的基质吸力与总吸力;Dew-point电位计可用于量测土样总吸力变化,尤其适合渗透吸力的量测;TDR探头适合于量测小于300kPa的基质吸力。用GDS非饱和土三轴仪可以进行SWCC测试,测试范围主要取决于陶土板的进气值。用准确的数学模型对测得的含水量、吸力数据进行拟合,对于预测非饱和土力学性质、渗透系数、抗剪强度及分析边坡稳定性有重要意义。由于准确测试SWCC难度较大,并且测试影响因素较多,所以根据土体孔隙大小分布和颗粒大小分布情况预测SWCC,也是一种较好的方法。     

Experimental and numerical analysis of two-phase infiltration in a partially saturated soil

The purpose of this study is to analyze the effects of the soil air flow on the process of water infiltration in a 93.5 cm deep vertical column for varied boundary conditions at the surface - positive time constant head; time constant fluxes smaller and greater than saturated soil hydraulic conductivity.Several experiments conducted on a sandy soil column with and without a possible air flow through the wall are presented. Continuous and simultaneous measurements of water content and air and water pressure heads at different depths allow the analysis of the air and water movements within the soil and the determination of the capillary pressure and relative permeability for each phase as functions of the volumetric water content.A numerical solution of the equations describing the simultaneous flow of air and water is compared with the experimental data and with the traditional one-phase flow modeling. The results show that the air movement may significantly affect water flow variables such as infiltration rates, water content profiles, and ponding times.Furthermore, some basic assumptions used in two-phase flow modeling, such as the hydrodynamic stability of the wetting fronts and the pertinence of the relative permeability concept, are discussed in the light of the experimental data.     

A Numerical Approach for Non-Linear Moisture Flow in Porous Materials with Account to Sorption Hysteresis

A numerical approach for moisture transport in porous materials like concrete is presented. The model considers mass balance equations for the vapour phase and the water phase in the material together with constitutive equations for the mass flows and for the exchange of mass between the two phases. History-dependent sorption behaviour is introduced by considering scanning curves between the bounding desorption and absorption curves. The method, therefore, makes it possible to calculate equilibrium water contents for arbitrary relative humidity variations at every material point considered. The scanning curves for different wetting and drying conditions are constructed by using third degree polynomial expressions. The three coefficients describing the scanning curves is determined for each wetting and drying case by assuming a relation between the slope of boundary sorption curve and the scanning curve at the point where the moisture response enters the scanning domain. Furthermore, assuming that the slope of the scanning curve is the same as the boundary curve at the junction point, that is, at the point where the scanning curve hits the boundary curve once leaving the scanning domain, a complete cyclic behaviour can be considered. A finite element approach is described, which is capable of solving the non-linear coupled equation system. The numerical calculation is based on a Taylor expansion of the residual of the stated problem together with the establishment of a Newton–Raphson equilibrium iteration scheme within the time steps. Examples are presented illustrating the performance and potential of the model. Two different types of measurements on moisture content profiles in concrete are used to verify the relevance of the novel proposed model for moisture transport and sorption. It is shown that a good match between experimental results and model predictions can be obtained by fitting the included material constants and parameters.     

A dynamic model for soil cutting by blade and tine

A dynamic model for soil cutting resistance prediction by blade and tine was developed, taking account of shear rate effects both on soil shear strength and soil-metal friction, besides the conventional soil slice inertia, for both brittle and flow failure of soil. The model was verified with a series of tests in a soil bin with a blade and a tine, and the results were acceptable.     

The motion of a stone embedded in non-cohesive soil disturbed by a moving tine

As part of an investigation into impact damage on soil-working implements, a glass-sided model box has been used to study the motion of 10, 50 and 100 mm diameter hemispheres in sand as a 38 mm wide tine inclined at 45° approaches. The observations were made using high-speed photography. It was found that the sand did not always cause the hemisphere to move before contact with the tine, and that motion was determined by the position of the centre of the hemisphere relative to the boundary of soil disturbance ahead of the tine. This effect was independent of velocity. A minimum size of hemisphere was found below which motion always began before contact was made with the tine. In the particular arrangement used this was about 20 mm. Movement of the hemisphere before contact reduced the contact stresses and the practical implications of this are discussed.     

Influence of Numerical Cementation on Multiphase Displacement in Rough Fractures

We present an application of 3D X-ray computed microtomography for studying the influence of numerical cementation on flow in a cement-lined rough-walled fracture. The imaged fracture geometry serves as input for flow modeling using a combination of the level set and the lattice Boltzmann methods to characterize the capillary-dominated fluid displacement properties and the relative permeability of the naturally cemented fracture. We further numerically add cement to the naturally cement-lined fracture to quantify the effect of increasing cement thickness and diminishing aperture on flow properties. Pore space geometric tortuosity and capillary pressure as a function of water saturation both increase with the numerically increased fracture cement thickness. The creation of unevenly distributed apertures and cement contact points during numerical cement growth causes the wetting and non-wetting fluids to impede each other, with no consistent trends in relative permeability with increasing saturation. Tortuosity of wetting and non-wetting fluid phases exhibits none to poor correlation with relative permeability and thus cannot be used to predict it, contrary to previous findings in smoother fractures.     

A novel implicit method for coastal hydrodynamics modeling: application to the Arcachon lagoon

The present Note reports on numerical modeling of shallow flows in coastal areas. Successful numerical simulations of such flows should be able to cope with strong irregularities of the bathymetry and to reproduce the covering/uncovering (wetting/drying) of tidal flats due to the tidal oscillations of the free surface. Also, adoption of large time steps is necessary to simulate phenomena which last actually several days or months. In the present study, a new numerical model based on an implicit resolution of the shallow water equations is proposed. A penalty method has been employed for numerical treatment of dry zones emerging during the wetting and drying processes. The capability of the present model has been verified by comparison with standard test cases. Further applications and comparisons have been also carried out to simulate the tidal propagation in the Arcachon lagoon. To cite this article: A. Le Dissez et al., C. R. Mecanique 333 (2005).     

Zero mass error using unsteady wetting–drying conditions in shallow flows over dry irregular topography

A wetting–drying condition (WDC) for unsteady shallow water flow in two dimensions leading to zero numerical error in mass conservation is presented in this work. Some applications are shown which demonstrate the effectiveness of the WDC in flood propagation and dam break flows over real geometries. The WDC has been incorporated into a cell centred finite volume method based on Roe's approximate Riemann solver across the edges of both structured and unstructured meshes. Previous wetting–drying condition based on steady‐state conditions lead to numerical errors in unsteady cases over configurations with strong variations on bed slope. A modification of the wetting–drying condition including the normal velocity to the cell edge enables to achieve zero numerical errors. The complete numerical technique is described in this work including source terms discretization as a complete and efficient 2D river flow simulation tool. Comparisons of experimental and numerical results are shown for some of the applications. Copyright © 2004 John Wiley & Sons, Ltd.     

Hysteretic Θ(S) Curve Prediction: Comparison of Two Models

The prediction of the first-order wetting and drying scanning curves is attempted by two different methods. These are: Mualem model II and Poulovassilis and Kargas (P–K). Model II by Mualem was chosen deliberately as the most appropriate. Experimental (S) data obtained in the laboratory for a sand mixture and a real soil were used for the comparison. Moreover, data presented originally by Poulovassilis (1970) were also used for the same purpose. It is shown that the P–K method gives better results than Mualems model II. Some remarks on Mualems model are also included.