Prediction of Infiltration of Water into Aggregated Clay Soil Samples1
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Prediction of Infiltration of Water into Aggregated Clay Soil Samples1
F. A. GUMBS AND B. P. WARKENTIN2
ABSTRACTThe physical propertiesstability, water retention, diffusivity
and conductivityrelevant to the study of infiltration intoaggregated media were measured for four aggregate sizes:2.38-2.00 mm; 1.168-0.840 mm; 0.417-0.295 mm; 0.105-0.053mm referred to in the study as sizes 1, 2, 3, and 4. There were< 15, 5, and 4% aggregate breakdown of sizes 1, 2, and 3during infiltration. These media were considered stable to infil-tration. The large volume changes (swelling and shrinking)which result from wetting and drying aggregates of sizes 4 andthe nature of the hysteresis curves suggest that aggregate rear-rangement rather than breakdown may be dominant. Therelease of water in two distinct steps and the several thousand-fold decrease in conductivity at the moisture content of thesetwo steps, particularly for sizes 1 and 2 suggest that these mediacan be described as well-aggregated, i.e. having a discontinuityof pore sizes.
The hysteresis in moisture retention, equilibrium moistureretention curves, and the changes in moisture retention withtime were measured for confined and unconfined samples of theaggregates. Wetting and drying diffusivities and conductivitiesof confined and unconfined samples were also measured andused in the prediction of horizontal and vertical infiltrationunder zero and small negative pressures into columns of eachof the four aggregate sizes using the diffusion equation. Diffu-sivities and conductivities were larger on wetting than on dry-ing and generally larger in unconfined than in confined samples.
Horizontal and vertical infiltration were reasonably well pre-dicted when water infiltrated under negative pressure and thediffusivities and conductivities used were calculated from infil-tration profiles developed under the same water tension. Pre-dictions of infiltration under zero pressure were generally notas successful.
For these media, the values of water tension, diffusivity andconductivity at any water content depend on the rate of wetting.The values to be used in the prediction of infiltration musttherefore be measured for times of wetting which correspondto the duration of infiltration.
The classical diffusion equation can be used to predict infil-tration into aggregated clay soils if the correct diffusivities andconductivities are used.
Additional Index Words: water retention, aggregate stability,diffusivity, conductivity, hysteresis.
THE PREDICTION of infiltration in laboratory soil columnsis considered an essential step in the successful predic-tion of infiltration into field soils. Several workers have suc-cessfully predicted the rate of infiltration and the infiltrationprofile in columns of nonsoil media (Youngs, 1957) andin columns of soil (Davidson et al., 1963; Gupta and Staple,1964; Rawlins and Gardner, 1963; Nielsen et al., 1962;Miller and Klute, 1967; Staple and Gupta, 1966) with theuse of the diffusion equation. The infiltration of water intostable well-aggregated soils, where a large difference existsbetween the inter- and intra-aggregate pores, has not re-ceived as much attention. Philip (1968) proposed a theoryfor infiltration into well-aggregated soils, and concludedthat the size of the soil aggregates must be large and thepermeability of the aggregates must be low for this theoryto be required. There is therefore a need to test the applica-bility of the diffusion equation to well-aggregated soils. Thiswill have practical applications to infiltration into crackingand plowed aggregated soils.
The accuracy with which infiltration is predicted dependsboth on the models used and on the accuracy with whichthe required parameters are measured. Gupta and Staple(1964) showed that the calculated depth of infiltration issignificantly influenced by small changes in the conductivityand diffusivity values from the maximum soil-water contentregion.
256 SOIL SCI. SOC. AMER. PROC., VOL. 39, 1975
The amount of water retained in a soil at a given suctiondepends on a number of factors, e.g., method and rate ofwetting or drying through its effects on entrapped air, his-tory of wetting and drying, duration of wetting or drying,applied external load, and sample size. Confinement of aswelling sample is equivalent to an overburden pressure.On the other hand, if the sample is allowed to swell, bulkdensity and pore size distribution will change and waterretention cannot be related to the initial conditions of themedium. A water retention curve is, therefore, character-istic only for a set of defined conditions.
A number of methods (Childs and Collis-George, 1950;Marshall, 1958; Millington and Quirk, 1959) is availablefor computing hydraulic conductivity from water retention.The study of Kunze et al. (1968) has shown that each ofthe methods has its limitations and that best agreement be-tween computed and measured values was obtained byusing a modified Millington and Quirk equation. They alsofound that best results were obtained when the number ofpore classes is kept between 5 and 10. Nielsen et al. (1960)found that Marshall's method gave computed values thatwere higher than measured values.
If the porous medium is unstable, the wetting or dryinghydraulic conductivity, diffusivity, and water retention willbe a function of water content and time; time embodieschanges in structure in the medium. Many of the methodsavailable, and which have been used for measuring, dependon the system coming to equilibrium. Infiltration into well-aggregated soils is a rapid process and if the structure ofthe medium changes slowly with time then the method ofmeasurement of the parameters used to predict infiltrationmay be very important.
In this study the stability of the medium, and hysteresisin water retention, hydraulic conductivity, and diffusivityare examined. The definition of the medium in terms ofmacropores (interaggregate pores) and micropores (intra-aggregate pores) is examined. The diffusion equation is usedto predict the rate of infiltration and the moisture profilesfor water infiltrating under zero and negative pressure. Theerror made in predicting infiltration using the parametersmeasured under different conditions is studied.
MATERIALS AND METHODSSoil
The soil used was the Ormstown silty clay loam belonging toGrey Gleysolic Group and classified as an Orthic Humic Gley-sol. The soil was air-dried, crushed lightly with a wooden ham-mer and sieved with a 2.38-mm sieve. Aggregates larger thanthis size were then more severely crushed in an electricallydriven rotary crusher. The following aggregates were thenobtained by dry sieving: 2.38-2.00 mm; 1.168-0.840 mm;0.417-0.295 mm; 0.105-0.053 mm. These are referred to assizes 1, 2, 3, and 4.
Water Retention MeasurementsWater retention measurements at low suctions (0-120 cm
water suction) were carried out with a Haines-type apparatus(Haines, 1930). The water retained at higher suctions (up to15 bars) during drying was determined on pressure plate appa-ratus. No measurements were made at these suctions for thewetting process.
/) Wetting and Drying Equilibrium CurvesThe water con-tent at a given suction was determined with a separate sample,3.2 cm diameter and 1 cm high. Duplicate samples showed verysmall differences for sizes 1, 2 and 3 (about = 0.05% watercontent); for size 4, if duplicates differed by more than 0.35%water content a third sample was used. The mean of the twosamples that did not differ by more than 0.35% was taken. Thismethod of measurement meant that the samples were rapidlywetted or dried to a given water content. The water contents areplotted on a weight basis, which will be the same on a volumebasis for sizes 1, 2, and 3 when they are confined since the bulkdensity is 1.0 g/cm3. The bulk densities of unconfined samplesvary with water content and the curves are not functions of theinitial bulk densities.
A sample was confined in its 1-cm high ring on the porousplate of the Haines apparatus by loading the top of the ring witha weight. The load on the sample during wetting would bemainly reaction of the load to the swelling pressure of the sam-ple. When samples of aggregate sizes 1, 2 and 3 were confinedduring saturation then no shrinkage was observed on subse-quent drying to 100 to 120 cm of water suction.
2) Wetting and Drying Time CurvesA confined sample(4.5 cm diameter and 1 cm height) was used for one completewetting and drying cycle on the Haines-type apparatus. Thesample was maintained at each suction for the specified timeand a new sample used at each of the succeeding times.
3) Scanning Hysteresis CurvesThese were determined ona sample (4.5 cm diameter, 1 cm in height) of each of the ag-gregate sizes. The samples were wetted rapidly to zero suction,so that the first curve of the hysteresis loop was drying. Theprocess was reversed at 100 cm of water suction, except for size4 which was reversed at 120 cm of water suction. Equilibriumwas considered to be achieved if no uptake or release tookplace over a period of several hours.
DiffusivitiesDiffusivities were calculated from measured water content/
distance profiles developed in horizontal columns according tothe method of Bruce and Klute (1956). For water infiltratingat zero suction the porous plate separating the soil column fromthe water source was an aluminium disc perforated with holesabout 1 mm in diam. At higher suctions a sintered glass platewith air entry value about 10 to 15 cm of water higher thanthe suction of the infiltrating water was used to minimize plateimpedance.
Unsaturated Hydraulic ConductivityFour methods were used to measure or to estimate the un-
saturated hydraulic conductivity (USC).1) Unsaturated hydraulic conductivity was calculated from
diffusivity and the slope of the w