Vertical Soil Column Research Articles

Air and water flow in a sealed, ponded vertical soil column: experiment and model : Touma, J; Vachaud, G; Partnage, J Y Soil Sci V137, N3, March 1984, P181–187

Air and water flow in a sealed, ponded vertical soil column: experiment and model : Touma, J; Vachaud, G; Partnage, J Y Soil Sci V137, N3, March 1984, P181–187

Nonsteady three‐phase immiscible fluid distribution in porous media

An experimental investigation was conducted to evaluate the three‐phase fluid distribution caused by a petroleum spill in an unconfined aquifer. The experiment consisted of introducing oil at the top of a vertical column of soil containing water and measuring the oil and water contents at various elevations and times. The static fluid distribution was then compared to the calculated distribution based on two‐phase capillary pressure versus saturation data obtained by a separate laboratory experiment. It was concluded that the water saturation at static equilibrium can be estimated satisfactorily by assuming it is a function of the capillary pressure between oil and water.

Early stage infiltration of water into horizontal and vertical soil columns

Water content distributions during horizontal, vertical-up, and vertical-down infiltration into air-dry soil, were measured experimentally using gamma radiation attenuation equipment. A sufficient number of soil columns were analyzed to ascertain fiducial limits of both measured and calculated soil water properties and parameters. The first three coefficient functions of Philip’s solution of the Richards’ flow equation applied to infiltration as well as to the soil hydraulic conductivity and soil water diffusivity as functions of soil water content were ascertained from the measured water content distributions.

Observed variations of earthquake motion across a sediment-filled valley

Abstract The response to earthquake motion of a site in one sediment-filled valley was observed to depend strongly on frequency and the position of the site within the valley and weakly on the input signal's azimuth and incidence angles. The observed input motion was weak, varying between 10−5 and 10−3g. Measurements of ground velocity were made on profiles across and along the valley and simultaneously on two nearby rock sites. The valley was 400 m wide and 700 m long, with a maximum sediment thickness of 60 m. The average seismic impedance contrast between the sediments and underlying basement rock was about 6:1. Ratios of Fourier spectra from soil sites to spectra from nearby rock sites showed apparent site amplifications of as much as a factor of ten, depending strongly on frequency and the distance of the site from the valley edge. The scatter in spectral ratios for earthquakes with different azimuths and angles of incidence was about a factor of two; thus local site effects that changed the amplitude of incident motion by a factor of two or more could be identified in the records of most earthquakes. Ground motion at valley-edge and mid-valley sites, separated by less than 100 m, differed by as much as a factor of five. This difference could cause large differential motion in structures spanning these sites. A theoretical, flat layer model of sediment response predicts the average behavior of the middle of the valley but not that of the valley edge. The lowest frequency responance apparently involves the sediments across the width of the valley, not just the vertical column of soil beneath each site.

Factors affecting the performance of drainage envelope materials in structurally unstable soils

Rather subjective criteria are applied for the need and kind of drainage envelope material to use. To gain more information on the performance of envelope materials in structurally unstable soils, flow tests were carried out in vertical soil columns filled with an unstably aggregated silt loam soil. To discover the most unfavourable situation in evaluating envelope materials, experiments with both downward and upward flow were performed. Also tests were carried out to find out the effect of soil sample height. Further, factors affecting soil aggregate stability, such as the initial moisture content and the hydraulic gradient, were considered. Two soil aggregate conditions, sensitive to degradation, were used in the investigation of the performance of both coarse and fine-textured envelopes.

Sensitivity of a frost heave model to the method of numerical simulation

A unifying numerical method is developed for solution of frost heave in a vertical freezing column of soil. Within one general computer code a single unifying parameter can be preselected to employ the commonly used Galerkin finite element, subdomain weighted residual, or finite difference methods as well as several other methods developed from the Alternation Theorem. Comparing results from the various numerical techniques in the computation of frost heave to measured frost heave in a laboratory column indicates there is little advantage of one numerical technique over another. One numerical technique, the subdomain method, was used to investigate discretization errors. The model is relatively insensitive to spatial discretization but is significantly sensitive to temporal discretization. The primary reason for this is that an updating procedure, rather than a more accurate iterative procedure, is used to evaluate nonlinear parameters that arise in the moisture transport and heat transport equations.

Seismic dynamic response by approximate methods

AbstractCurrent seismic safety evaluation for earth dams relies on approximate methods of analysis for prediction of non‐linear, transient, dynamic response. One of these approximate methods uses a Strain Reduction Factor (SRF), and has been widely applied in a variety of one‐dimensional and two‐dimensional soil structural analyses. A second method considered, Equivalent Temporal Damping (ETD), has been previously applied to several seismic dynamic analyses of earth dams. The relative accuracy of the two methods is assessed by comparing them with incremental plasticity, nearly exact numerical solutions. For a simple shear element subjected to deterministic and non‐stationary random input accelerations, a serious overprediction of the maximum peak shear response occurs by the SRF method, whereas the ETD results agree very closely with the incremental plasticity solutions. The SRF and ETD methods are also applied to seismic dynamic response analysis of a Vertical Soil Column system, and the same trends as established in the previous case are observed. It is concluded that, in lieu of combined incremental plasticity and finite difference or finite element numerical solutions, the ETD approach is the more accurate of the methods tested for seismic dynamic response analysis of earth dams.

Delayed aquifer yield as a phenomenon of delayed air entry

Delayed release of pore water from a pumped, unconfined aquifer is treated as a situation of restricted air movement in the vadose zone due to layers of high water content. This restricted movement produces below‐atmospheric pressures in the pore air above the water table, which cause the water table initially to drop faster than the dewatered zone. The initial yield thus is less than the full specific yield, which develops after a certain water table drop. Equations are derived that relate delayed yield to air entry value and height above water table of saturated layers in the vadose zone. These equations agreed with results from a delayed yield experiment on a vertical soil column and produced ratios between initial and final yield that resembled those calculated by others from actual pumping tests. An axisymmetric flow system, simulated by interconnected jars with narrow standpipes, produced delayed yield drawdown curves that were amenable to Boulton's analysis.

The application of the overburden potential theory to swelling soils

From a consideration of alternative loading paths in the shrinkage diagram of swelling soils it is shown that hysteresis in the relationship between the voids ratio and the moisture ratio may occur even though no actual reversals of loading have been allowed nor any irreversible structural changes have taken place. It is therefore argued that the application of the overburden potential theory to the analysis of situations such as water profiles above water tables in vertical soil columns is questionable. The need for experimental studies is stressed.

Étude du mécanisme des transferts d'eau et de soluté en zone non saturée avec prise en compte d'une fraction liquide immobile

Click to increase image sizeClick to decrease image sizeAssuming that when a soil volume is unsaturated part of the water can be trapped by air between particles and in dead end pores, the classical scheme of hydrodynamic dispersion, represented by eq. (1), is modified in order to be more representative with the criterion that during the movement of solutes in soil lateral diffusion of chemicals in or out of the stagnant water is possible. The change of salt concentration during a steady flow is now represented by the set of equations (2) and (3) where subscripts m and im refers to the mobile and immobile fraction respectively. Three parameters must be known in order to solve this set of equations: the apparent dispersion coefficient D(cm2/hr) ; the diffusion mass transfer coefficient α: (hr-1) and the mobile fraction ƒ = θm/θ. A series of experiments where done in a vertical soil column of soil for steady state flow of water and a pulse or step change of salt concentration at the inlet. The salt concentration where measured with time at several depths inside and at the exit of the column. Values for Dap,α ,ƒ were obtained by curve fitting observed and calculated concentration, at one depth (22 cm) using an explicit discretization of eq. (2). The same values, first determined during a step change of salt concentration, corresponding to the experiment on fig. 3, were first used to calculate salt distribution at other depths and at the exit for this experiment, and then to compute salt distribution during an other experiment, done with the same water flux, but for a pulse change of salt concentration (fig. 4). Excellent agreement was obtained in bath case ; the model presented a good description of the extensive tailing observed inside and at the outside of the column, which could not be simulated with the classical "dispersion- convective" equation (fig. 3.a). Finally a sensitivity study of the influence of each parameters was done (fig. 5 to 7). It appears that the immobile fraction of water has the strongest influence on the relative concentration curves. Further experiments should now be done in order to determine the variations of each parameter with the water content and to solve the transient infiltration problems.

Movement of Two Nonionic Surfactants in Wettable and Water‐Repellent Soils

AbstractThe movement of two nonionic surfactants (Soil Penetrant 3685, Aqua Gro) and their effect on water flow through wettable and water repellent soils was investigated. Surfactant concentrations of 0, 100, 500, 1,000, 1,600, and 3,200 ppm were applied to the top of vertical soil columns and the concentration of surfactant in the column effluent was measured. When the concentration of surfactant in the effluent did not significantly change with time, leaching of the columns with tap water was initiated. Both surfactants affected the hydraulic conductivity of the hydrophobic soil. The conductivity effects appeared to be related to aggregate destabilization, micelle formation, and particle migration, all of which caused a general decrease of flow rates with time. Adsorptive characteristics were found to affect greatly the shape of the effluent concentration versus time curve. Aqua Gro was shown to be more strongly adsorbed and less subject to leaching than was Soil Penetrant. A theoretical model was tested for its ability to qualitatively predict experimental effluent concentrations. Allowing for the spreading effects of dispersion, there was reasonable agreement between most experimental and theoretical values. The behavior and movement of surfactants in soils are a function of adsorption isotherms, mixing or dispersion due to flow velocities, solute concentration, and the physical and chemical characteristics of the porous medium.

Experimental investigation of the deformability of thawed clay bases

1. Soil deformations occur predominantly in the vertical soil column whose boundaries pass through the limits of the plate.

Effects of air pressure on water flow in an unsaturated stratified vertical column of sand

By the use of a vertical stratified column of unsaturated soil the effects of air pressure on constant flux infiltration and gravity drainage were studied experimentally. Water content was measured by γ ray attenuation; water pressures were obtained simultaneously at 10 different levels in the column by using tensiometers connected to pressure transducers. In addition, the differential ports of some transducers were connected by a ‘scanivalve’ either to the external atmosphere or to the soil air phase by using hypodermic needles inserted through the column wall. The soil column consisted of three layers: fine sand over coarse sand over fine sand. The wall of the column was also provided with a series of adjustable air ports to allow experiments either with or without lateral air movement. It was shown that (1) the local soil air pressure can differ significantly from the external atmospheric pressure, e.g., +50 mb for rain with an intensity of 3 cm/hr or −15 mb for gravity drainage, and (2) the water flow depends not only on the boundary conditions in terms of water content and/or water pressure but also on the air pressure conditions. We conclude that the air pressure must be taken into account in determining the soil water suction, and we suggest that the flow equations must be written in terms of two‐phase immiscible fluid flow.

Numerical simulation technique for vertical drainage from a soil column

A numerical method for simulating the time-dependent drainage from a vertical soil column was developed. The method offers the possibility of computing the discharge as well as the hydraulic gradient for any desired time, if the capillary conductivity and the moisture content of the soil are given as functions of suction. The soil column is considered to consist of thin layers, each of them having different capillary conductivity and water content. The number of layers, their individual thickness and their position is time dependent and their determination is inherent in the calculation procedure. For each layer a water balance is made for small time intervals.The required calculation procedure has been programmed in FORTRAN-Subset for IBM 1130.The results obtained with this method have been compared with data obtained from laboratory tests and with data found in literature.

Drainage of Soil Profiles

The scaled approximations of Brooks and Corey for water content versus capillary pressure and capillary conductivity versus water content were used to obtain a scalded diffusion function that was dependent only upon the pore-size distribution index, λ. A predictor-corrector finite difference scheme was used to obtain numerical solutions for the scaled differential equation for drainage from vertical soil columns. Results of several numerical solutions were presented for two soil profiles and for two greatly different pore-size distributions. The effects of soil hydraulic properties upon drainage were discussed and used to interpret the term field capacity. Experimental data obtained by several independent investigators were compared with theoretical solutions of discharge as a function of time using the Brooks-Corey approximations. The comparisons between experiment and theory were reasonable and some explanations for lack of agreement were presented.

Hysteresis During Infiltration and Redistribution in a Soil Column at Different Initial Water Contents

Experimental results dealing with flow processes involving hysteresis effects in the suction‐water content relationship were obtained on a vertical column of soil submitted to the following successive controlled flow conditions: (1) redistribution of water following a constant head infiltration in an initially air‐dried column. (2) constant head infiltration followed by redistribution, and (3) constant flux infiltration followed by controlled evaporation and redistribution.Water contents were measured by using gamma ray attenuation, and water suctions were obtained simultaneously from 10 pressure transducers distributed along the soil column, each one being connected to a tensiometer. A data acquisition system permitted fully automated measuring sequences. All the data were punched on tape and treated with a computer.Examination of the changes of water content and water suction at a given depth permitted us to obtain precise information on the hysteretic behavior of Ψ (θ). This analysis showed that the scanning curves for a single reversal can be defined uniquely with reference to the transition water content θ*, that the independent domain theory appears to be inadequate for describing soil water hysteresis completely, and that k(θ) can be considered practically unique.

Capillary Conductivity and Soil Water Diffusivity Values from Vertical Soil Columns1

AbstractWater evaporated from vertical columns of Yolo loam, 100 cm long, for 200 days at ambient temperatures of 10, 20, or 30C ± 1C. Distribution of soil water was determined by gamma‐ray attenuation at 5‐cm intervals, concomitantly with determinations of soil water pressure. Values of capillary conductivity based upon measured fluxes and hydraulic gradients are presented, along with values of diffusivity based upon fluxes and water content gradients.

Analysis of Steady‐State Evapotranspiration from a Soil Column

AbstractNumerical analysis is applied to the steady‐state flow equation for evapotranspiration from a vertical soil column. The water uptake by the plant roots is incorporated into the flow equation as a (negative) source term. The evapotranspiration rate and its partitioning between plant transpiration and soil evaporation are specified as the top boundary condition. The lower boundary condition is a water table. The results of the analysis for a Pachappa sandy loam give pressure head, water content, source strength and soil water flux profiles. This study has (i) demonstrated that solutions to the flow equation with a source term can be obtained by numerical means, (ii) shown something of the theoretical influence of the parameters in the source term model on the flow in the soil, (iii) qualitatively indicated a transfer of water from wet soil via the root system, and (iv) indicated that the source term has little effect on the pressure head profile unless the evapotranspiration rate is about the same order of magnitude as the theoretical limiting value.

Rainfall Infiltration into a Vertical Soil Column

Rainfall Infiltration into a Vertical Soil Column

Numerical Method for Analyzing Hysteresis‐Affected, Post‐Infiltration Redistribution of Soil Moisture

AbstractAn especially devised numerical procedure applied to the Darcian model of soil moisture flow makes it possible to analyze the post‐infiltration redistribution of water in semi‐infinite, vertical soil columns. The method takes into account the effects of hysteresis by making it possible to find, within the hysteretic system of curves, the unique curve which characterizes moisture transformations at each soil depth. The method can be applied to cases in which the moisture flux at soil surface is zero as well as to those in which the redistribut on is accompanied by constantrate evaporation.The results obtained by applying the new method to data on moisture retention and conductivity of a sandy soil demonstrate the importance of hysteresis in the redistribution processes.