The Effect of Bulk Density and Initial Water Content on Infiltration in Clay Soil Samples1
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The Effect of Bulk Density and Initial Water Content onInfiltration in Clay Soil Samples1
F. A. GUMBS AND B. P. WARKENTIN2
ABSTRACTInfiltration measurements were made on swelling clay soil
samples packed into columns. Small increases in bulk densityover the range 1.10 to 1.25 g/cm3 markedly decreased the rateof water movement. The magnitude of the effect was greaterfor confined samples than unconfined samples at all initialwater contents. A 1-cm compact layer in the profile retardedwater movement if the soil was confined. In partially confinedsamples the soil in the compact layer would swell on wetting,and water movement was retarded only when the bulk densityafter swelling still exceeded the bulk density of the remainderof the column. Bulk densities below 1.05 g/cm3, and heat ofwetting in partially confined samples with 0% initial watercontent produced nonlinear distance to wet front vs. squareroot of time relationships. Comparison of horizontal and verti-cal infiltration showed that under these experimental conditionsgravity contributed significantly to water movement at highinitial water content.
Additional Index Words: diffusivity, compacted soil, swellingclay, aggregates.
THE RATE OF ADVANCE of the wet front depends on perme-ability, changes in bulk density, layering in the profile,and on initial water content of the soil. Olsen (1960) mea-sured an exponential increase in permeability with increasein porosity. Millington and Quirk (1959) and Philip(1957a) found that permeability increases exponentiallywith increase in initial water content. Phillip (1957b) hasshown that the rate of advance of the wet front increases,and rate of infiltration decreases, with increasing initialwater content for both short and long time infiltration.Hanks and Bowers (1962) and Miller and Gardner(1962), found that when there is textural layering in aprofile, infiltration is controlled by the less permeable layer.
There is little information available on the effect of bulkdensity changes and initial water content on water infiltra-tion into swelling clay soils. This study reports such mea-surements on swelling clay soil samples. The rate of ad-vance of the wet front was measured for vertical and hori-zontal infiltration into confined or partially confined soilcolumns with the water entering at small positive pressures.The rates of advance of the wet front, taken from the slopesof the distance to the wet front against the square root oftime, are used to evaluate the effect of bulk density onthe infiltration. Diffusivity-water content relationships arecalculated to evaluate the effect of initial water content.
1 Contribution from the Dep. of Soil Science, Macdonald
College of McGill University. Part of the work submitted asan M.S. thesis by the senior author. This study was supportedby a Grant in Aid of Research from the National ResearchCouncil, Canada. Received Jan. 10, 1972. Approved June 27,1972.
2 Graduate Research Student and Professor, respectively, De-
partment of Soil Science. The senior author is now Lecturer inSoil Science, University of the West Indies, Trinidad.
No attempt is made in this paper to predict infiltrationfrom the theories proposed recently to describe water infil-tration into swelling soils (Philip and Smiles, 1969; Za-slavsky, 1964). Such predictions require more measure-ments of soil parameters, and more information on theinteraction between volume change and soil water poten-tials, than were available in this study.
Results on infiltration into columns of sieved and packedsoil samples may not be valid for clay soils in the field,because of differences in void size distribtuion. The cracksand natural peds in field soils result in larger units thanused in the laboratory, and in greater nonhomogeneity.Despite these differences, the effects observed for unsatu-rated flow would be expected to occur in the field. Thebulk densities around 1.1 g/cm3 used in the experimentsare slightly lower than the values of 1.2 to 1.3 g/cm3measured for the plow layer of this soil.
The soil used in this study was the Ste-Rosalie clay, a humicgleysol. A sample was taken from the C-horizon, which has60-70% clay, 25-30% silt, 0-10% sand, and 2-5% organicmatter (Lajoie and Baril, 1954). The minerals present are micaand chlorite, with lesser amounts of quartz, feldspar, and am-phibole, and small amounts of montmorillonite or vermiculite.
The soil sample was dried and ground, first in a mechanicalgrinder to break down the large clods and then by glass-on-glass grinding. In the sample as used, the aggregate sizes 0.60-0.84 mm, 0.42-0.60 mm, 0.30-0.42 mm, 0.19-0.30 mm, andless than 0.18 mm were present in the approximate ratio of2:1.6:2:2:1. These aggregates are relatively stable on wetting,so this ratio of aggregate sizes also represents the sample afterwetting. Compaction to bulk density values up to 1.2 g/cm3 didnot crush the aggregates.
Infiltration was studied with soils packed in columns madeup of Lucite rings 1 cm in height. The first measurements weremade using columns 4.5 cm inside diameter. Most of the meas-urements were with columns 3.2 cm inside diameter, becausethe smaller samples were more convenient. The work of Lalet al. (1970) indicates that the rate of advance of the wet frontin horizontally confined columns increases as column size in-creases. This is due to the different degree of swelling whichtakes place in the top layers of the soil column. In the experi-ments reported below, comparisons are therefore made onlybetween soil columns of the same size.
The columns were packed by compressing known weightsof soil to predetermined heights. For bulk densities below 1.15g/cm3 the compression was done by hand in the already assem-bled columns; for the higher bulk densities the soil was com-pressed in the 1-cm sections with a hydraulic press and thesections assembled and squeezed together. Care was taken toachieve the best possible contact between sections by disturbingthe surface of the soil to a depth of about 1 mm with a manu-ally rotated wire brush before the next increment of soil or thenext 1-cm section was applied.
Infiltration measurements were made on duplicate samples.If the difference in the slopes of the distance to the wet front(x) vs. square root of time (f%) of successive infiltration runswas greater than about 5%, a third replicate was used and theaverage of the three taken (provided no difference was greaterthan 10%). Abnormal results (i.e. greater than a 10% differ-
GUMES & WARKENTIN: EFFECT OF BULK DENSITY AND WATER CONTENT ON INFILTRATION IN CLAY 721Table 1Summary of slopes of x vs. f& lines (\) under different conditions of infiltration
Partially confinedBulk density
Bulk density1st cm
0.87 ,0.64 2'61
0.74 . _,0.54 1>M
ConfinedBulk density X AX/ADp
1st cm Average g/cms
1.197 1.1.275 1.
AD,, = 0.1 g/cm'
ence), which occurred in about 10% of the trials, were dis-carded.
The "confined" samples had a porous stone fixed at thewater entry end of the column. The "partially confined" sam-ples were free to swell in the vertical direction at the top ofthe column. A 1.5 cm layer of water was ponded on the surface,protected by a loose disc of filter paper. The base of the col-umn had a fixed porous plate.
Samples with different initial water contents, Wi, of 0%,3%-4%, 7%-9%, and 19% by weight were prepared asfollows:
1) 0% Water ContentSamples were oven dried at 105-110C for 24 hours.
2) 3%-4% Water ContentThis was the water content ofthe air-dry soil.
3) 7%-9% Water ContentSamples in thin layers werekept for periods of up to 10 days in an atmosphere of 100%relative humidity.
4) 19% Water ContentThe soil samples were spread thinlyin a narrow band on a plastic sheet and sprayed with the calcu-lated weight of water. The samples were then mixed thor-oughly and stored in double walled plastic bags for 1 week.
The start of any infiltration experiment was taken at / 0when the first drops of water contacted the soil surface. There-after, readings were taken of the time, the distance to the wetfront, and the volume of water infiltrated.
At the end of the experiment the water supply was stopped,the head of water drained away, and the 1 cm sections sepa-rated by pushing a thin (0.25 mm) rigid shim between eachpair of rings. The entire sectioning took 5-10 min, dependingon the length of the column. The soil slices were then quicklyremoved from the rings and the water content determinedgravimetrically.
RESULTS AND DISCUSSIONRate of Wetting
Infiltration theory summarised by Philip (1969) pre-dicts a linear x vs. f*- relation for horizontal infiltration,and a linear x vs. t^ relation for the early phases of infil-tration in the vertical direction with a gradual shift to anonlinear x vs. f% at longer times. These relationships holdif the boundary and initial conditions can be transformedby the use of a variable X = x/i*. In t