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Accumulation of Moisture in Soil Under an Impervious Surface J

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Accumulation of Moisture in Soil Under an Impervious Surface J Accumulation of Moisture in Soil Under an Impervious Surface J. L. MICKLE, Associate Professor of Civil Engineering, and M. G. SPANGLER, Professor of Civil Engineering, Iowa State University The objective of this study was to compare the long-time ac­ cumulation of moisture in a soil subgrade beneath an imper­ vious surface with the estimated equilibrium moisture contents based on measurements of the moisture retention characteris­ tics of the soil and the elevation of the groundwater table. Mois­ ture accumulation due to the formation of ice was not considered. A theoretical approach, based on thermodynamics, was used to evaluate the free energy per unit mass of water in a soil water system in terms of component free energies. Thus, the effects of adsorptive and gravitational force fields, surface tension, pressure and dissolved materials were considered. The experimental investigation was conducted in two phases. The first phase involved the routine tasks of periodically de­ termining soil moisture contents, soil temperatures and water table elevations under a 150-foot square impervious surface constructed of alternate layers of asphalt roofing paper and asphalt cement. The second phase was conducted to determine the soil moisture retention characteristics, in the form of desorption curves, and other properties of a series of undis­ turbed soil samples taken from under, and adjacent to, the im­ pervious surface near the close of the field investigations. EXPERIMENTAL INVESTIGATION •THE field laboratory site was on the Iowa State University Experimental Farm at Ankeny, Iowa. The parcel selected for the investigation was on a gentle swell of un­ dulating, glaciated land. Drainage in general was quite satisfactory. The glacial till was interspersed with lenses of sand and gravel probably associated with the ground moraine of the Des Moines lobe of the Wisconsin glacier. Five individual test plots were selected at various positions on the 150-ft sq imper­ vious surface. In addition to the five plots on the surface a sixth (control) plot, sup­ porting normal vegetation, was located approximately 10 ft west of the west edge of the surface (Fig. 1). Each 10-ft sq test plot was marked off with a 1-ft grid system. The intersections of the grid lines were numbered and used as a means of control for routine weekly soil moisture sampling procedures. The depth of the water table was determined weekly in each of 17 water table tubes and continuously in two 16-in. wells (Fig. 2). Official U.S. Weather Bureau precipitation data are given. Soil temperatures were determined continuously under the impervious surface and under normal vegetive cover using thermocouples and a 16-point recording potentiom­ eter. The air temperature was determined 1 ft above ground level (Figs. 3 and 4). As a part of the second phase of this project, a series of undisturbed soil samples in Shelby tubes were taken at the field laboratory. A total of 24 holes were driven, one at each corner of each test plot, using a screw drive mechanism Paper sponsored by Committee on Environmental Factors Except Frost and presented at the 46th Annual Meeting. 91 92 rather than a drop hammer. Continuous Fence samples were taken in every case to a --- depth of 10 ft. In the laboratory, soil physical char­ 15 14 13 acteristics were determined for each 0 0 0 Water table tubes Shelby tube sample, a total of 170 in all. East Desorption curves were determined using well 0 individual pressure plate apparatuses. 160 3J~1 Test plats J~: 4 0 12 The individual apparatus permitted the determination of an entire desorption Impervious sufoce curve using a single undisturbed soil 170 5 · ~:r Water table oll sample that was not dislodged from the • . tubes plate at any time; the entire apparatus was weighed at each pressure. Samples 1-;I l-;-4 were approximately 2% in. in diameter G1 olO 1-2 1:1 by 3 in. high and were saturated prior to Thermocouple Undishirbed the test. Other properties determined samples were built density, Atterberg limits, par­ ticle size distribution, textural classifica­ tion and specific gravity. Composite 911 ~p I 4P 6P 89 iop Scale, feel desorption curves were drawn for all of the 24 test holes (Figs. 5, 6 and 7). The Figure 1. Planimetric layout of the field laboratory. 0 2 3 Test plot 4 ,_... Depth, number I \ feet 5 I :,; ... _\ ~ 6 I· .... ~ .. '~ 4 ..... ~ 7 ' , )'.// '- ,,,,r~ .. ~~ ~L ~ ~ '-.... -- ---r ~ 8 ~ ~ ,__"'!I '" '\ ~ .__ ~ ......._ y ~ .- 9 - . ~l '-.... I 5~ l_.......-' 10 "' r---. ONDJ FMAMJ JAS 1957 1958 Precip­ itation, Inches 11 U I t1§ ~lliJONO J FMAMJJAS 1957 1958 Figure 2. Water tab le depths and precipitation data. 8 I Mar 2 6 10 .~TTI1W· Mor .... .... o: 141 .Bi I 1 12 Air t "'(o 1214\ ,6\ \ ~ ~ ,, I t I I CX> Feb Air " I I I I a> Feb I IO Depth,fHt IO I I ~,':\}\ \\~ ' I I \ '! \ en I Depth, feet \ I I en Jan ' I \ \ Jon ' ' Dec \~ \r, ~\\ \ \ \ Dec \1 \\'' \~ i\>L~~ ; ,, ' \ ' \ Nov . ,, I\.\', Nov " ~\ ~ ' ,,.........·""" Oct .. ~ Oct ~ ',~ ,~, Sep Septr ~ '1l!Ff~ l '4~ ", Aug J II ' \''' li9J Fe.lj-:rq\'­ Auo .. ' .. , 16,,', )Aii' ''7/J l/ / 1 ~ t>.Air ~o I~ \ ' July ''tiI/ ,' 'I \ I July '}' '4 ,, , rllf/17(/ I ,,' ,,' ,.... June I ,A ,,,,. ~JuneI IO IO v ,v,o en May ~tit/ en t I ~ - MCly J~ ~# ~v. , , ~ , Apr w ) _,, Apr If' IC Mor ~t· Al·I~ Mor l ~ l ~\\\~4 I " Feb 8 Ak/~,l '.'4 ~~ H 0 10 20 30 40 50 60 70 80 90 100 Feb " 0 10 20 30 40 !SO 60 70 80 90 100 Temperature • F Temperature ° F Figure 3. Soil temperatures, covered area. Figure 4. Soil temperatures, control area. <C> ~ 94 0 I I I Clay loam 0 - Field A P• 1.23 LL 35.5 - - data PL255 Pl 100 - -- - ,_,0 ,__ I I I ,__ 2 - >-- Clay loam B ,__ - >-- P• 1.35 LL 35.7 ~ 0 PL 24.6 Pl II.I 3 Clay loam ~ Depth , c P• 1.77 LL 32.7- feet 4 PL 14.7 Pl 18.4 - Clay loam 5 ~ - D P•l.73 LL24.5 PLl'l'.4 Pl 7.1 - 0 6 ~ Sandy loam P•i.71 I I - -\ ~ LL24B PL 16.0 7 Pl 88 LJ_j_ P• 1.89 LL 25.0 "tDF .IWater I Itable Loam PL 14.6 Pl 10.4 - 8 6 I() 14 18 Z2 26 30 34 Percent moisture 100 -:;..- .- -- BO I~~ ,/ A ~ / )~ .l'"F 60 ~~ Percent ,, pass inc~ ,/ / 40 ~ B ~ ..1"c ~ ~ KE 20 .... "" -;.., ~~... D 0 0.001 0.005 0.01 0.05 0.1 0.5 5 IO Particle size, millimeters Figure 5. Test hole 1-1. Above: composite desorption curve; below: particle size distribution curves. · (In Figs. 5- 7 the following symbols are used: P =dry bulk density, Pl= plasticity index, LL = liquid limit, NP= non-plastic, PL= plastic limit.) field data points shown were moisture contents determined when the undisturbed samples were taken. CORRELATION OF DATA All soil moisture contents determined during the period October 1957 to September 1958 were tabulated (3). The tabular values were in chronological order and each test plot is listed in numerical order. Originally, direct comparisons of the field data were to be made with the appropriate desorption curves determined in the laboratory. This plan presupposed a somewhat uniform status of the soil types and environmental conditions at the test site. It was later found that such a correlation involved the simultaneous treatment of several salient variables: soil moisture content, soil char­ acteristics, variation of soil characteristics within a given test plot, soil sample depth, water table fluctuation, time and soil temperatures. Since such a comparison was 95 0 \ Sandy clay loam 0 Field A P•l.36 LL31.0 - data PLl9.6 Pl 11 .4 - - 0 t-f-1- Gravelly sand 2 1 B P=l.58 N.I? 0 3 c Gravelly Depth, sandy loom 4 P= l. 58 feet ~ N.P. Sand P•l.43 - 0 N.P. o 6 I I I E I Silty clay P•L5B LL 35.I Silty clay loom PL• 24.7 Pl 10.4 7 P=l.50 LL 31.0 0 PL 22.B Pl B.2 B Water table !\ IF I 6 10 14 18 22 26 30 34 Percent moisture 100 .,~ ,..· C.._, ,.,.. v ~ ./ ~ J J BO .... V 1 - / v ' ~........... / v I F I 60 ~" ! ' I 0-...., I I Pwclrll ~ posslng .I .I 'I II A' j 40 J .... ·" ,' 'l J ., v ... 20 / l'i' "L! ~r t;::: ,/' 'a :ii!- -- ... " - ....... 0 - 0.001 0.005 0.0I 0.05 0.1 0.5 5 10 Particle size. mll limeter1 Figure 6. Test hole 2-1. Above: composite'desorption curve; be low: particle size distribution curves. virtually impossible, it was necessary to make some assumptions and adjustments in the plan. The period October 1957 to September 1958 was selected to eliminate water table fluctuation as a variable. During this period water table fluctuations were at a mini­ mum and the individual water table tubes were in full operation. Time was eliminated as a variable by always assuming an equilibrium condition. Obviously an equilibrium condition was never reached, but the assumption was neces­ sary for simplication. As a first trial the moisture contents of the undisturbed samples determined at sampling were compared with the desorption curves determined from these same samples. In so doing, a direct comparison was possible because the effect of changing soil characteristics within the test plots was eliminated and because during the sam­ pling period (October 1958) the soil temperatures at all depths were approximately the same.
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