Prediction of Soil Moisture Content Profiles under Impermeable SurfacesAuthor(s): Donald A. DavidsonSource: Area, Vol. 8, No. 2, 23rd International Geographical Congress (Moscow) (1976), pp. 153-156Published by: The Royal Geographical Society (with the Institute of British Geographers)Stable URL: http://www.jstor.org/stable/20001098 .

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Prediction of soil moisture content profiles under impermeable surfaces Donald A. Davidson, Saint David's University College, Lampeter

Summary. A method is describedfor predicting moisture content at various depths below a highway using soil moisture retention curves and is shown to produce satisfactory results.

The monitoring of soil moisture content is necessary for a large number of problems and a variety of measurement techniques are in use (Hillel, 1971). A study in Ontario concerned with evaluating the influence of heat stored from the summer in restricting the depth of post penetration under highway surfaces during the winter required detailed soil moisture records. For each month a soil moisture value was required at a large number of depths below the highway surface in order to compute heat capacities. The method which was adopted involved the laboratory determination of suction-moisture content relationships and the field monitoring of water table levels below the highway. The soil moisture characteristic curves were used to predict moisture contents, for times as required, according to the difference in elevation between the water table and the sampling positions. The assumption in this approach is that essentially hydrostatic conditions exist below the highway surface because of its imper meable nature. The height above the water table is therefore a measure of the suction. The aim of this note is to outline the necessary laboratory procedures as well as to comment briefly on the nature of some results.

The maximum sampling depth was 4 3 m. If the water table was at that depth, the soil there would have a suction of zero whilst the surface soil wouldhave been subject to a negative pressure of 430 cm of water assuming hydrostatic conditions. Thus concern was restricted to producing soil moisture characteristic curves only in the range up to -430 cm of water corresponding to pF 2 63. This range was covered in two stages, first with the samples in suction plate cells with columns of water dangling from the cells (at height varying from 25 to 240 cm above a reservoir of water) and second, with the samples still in the same cells, but applying air pressure (ranging from 0-5 kg cm-2 to 1P5 kg cm-2).

Method An important modification was made to the cells1 used for the tests in view of the need for accuracy. The first part of the experiment involved dangling tubes of water from the cells and samples were extracted from cells at various heights and the moisture content determined. Thus suctions were applied to the sam ples of up to -240 cm of water whilst for higher values, air pressure was

applied. This necessitates that all the water from under the porous plate is removed before opening the cells in order that samples are not moistened by uptake of water on release of the air pressure. For this purpose a second outlet was added to the base of the cell and then the inner base of the cell was machined such that drainage could take place from one outlet to the other. Such drainage is assisted by applying a slight air pressure to one outlet. This modification has

153

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154 Soil moisture content profiles

a further advantage in that bubbles of air in the tubes, when the cells have

columns of water attached to one outlet, can be easily removed by opening the

other outlet if the reservoir of water is held above the cell. It is most desirable to insert undisturbed samples into the cells for the tests

which can begin immediately once the samples are saturated if the material is sandy. If the material is clay-rich (and compressible), and has been sampled by a Shelby tube, then it is necessary to extrude the sample about two weeks before the suction tests are commenced. During this time thin slices (about 60 mm) of the clay should be kept in contact with free water, in order that suctions

produced by dilation on removal from the confinement of the tube be dissipated. If this precaution is not taken, then moisture contents may increase during the tests in the lower suction range, giving faulty soil moisture characteristic curves.

1600 -

800 l

400 -

g 200

E

c: loo 0

ur -3

(f)

50 -

25

10- I I I

0 5 10 15 20

Moisture content % dry weight

Figure 1. Soil moisture retention curve for an undisturbad sandy sample.

The cells are left for four days at particular suctions or pressures before

extracting samples for determination of the moisture content. The results can

be plotted as shown on Figure 1 for an undisturbed sandy sample; in this

instance the test was repeated with two different samples of the same material. It should be noted that the result is uniformly a drying curve.

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Soil moisture content profiles 1 55

Prediction of moisture content At one highway site it was possible to compare actual field moisture contents

with predicted moisture content values from the soil moisture characteristic curves. When the samples were collected, the height of the water table was noted

and by knowing the depths of the samples, the suctions were determined assum ing hydrostatic conditions. For example, by reference to Figure 1, the vertical difference between the sampling position and the water table was 46 cm; the suction values were converted to the pF scale and then a best-fit line was com

puted for the four lowest points in order to predict the moisture content at a

pF of 1 66, that is a 46 cm water column of suction. The result was a predicted

moisture content of 1344% by weight to compare with an actual moisture content of 13 4 %, obtained by averaging results from three determinations. This procedure was repeated for two other samples which were disturbed and the

results are shown in Table 1. For all three samples the soil moisture retention curves were obtained twice for each sample and the predicted moisture content

is based on the composite curves.

Table 1. Comparison of measured and predicted soil moisture contents for 3 samples

Measured moisture content (% dry weight) Predicted

Sample Sample description field samples as moisture content number collected (% dry weight)

1 Sands (undisturbed sample) 13-4 13 4 2 Sands (disturbed sample) 11-1 12 5 3 Sands (disturbed sample) 16 6 14 6

It was not possible to collect sufficient samples for results to be subjected to statistical analysis, but it can be seen that the predicted values are of the

correct general magnitude. The results suggest that undisturbed samples give better results than disturbed ones, a conclusion to be expected. The difficulty with sandy material is being able to insert it into the cells without the sample disintegrating. As already stated the method assumes hydrostatic conditions in the ground, and also the moisture contents are predicted from a ' drying' curve. Clearly because a water table rises and falls, the suction-moisture content relationship should have a ' wetting ' as well as a ' drying ' curve. The prediction of water content from a drying curve would be in error if the water table was rising. However, for the very artificial situation below a highway it is hoped that hydrostatic conditions do apply and also that uncertainties as to ' wetting '

and ' drying' are relatively minor. Also hysteretic effects decrease with depth because amplitudes of change in water content are reduced. From the three test results it seems that these assumptions are reasonable and hence the method yields moisture content values of reasonable proximity to field conditions.

Clearly this technique for monitoring moisture content is limited to rather

specialized circumstances; usually the hysteretic domain would have to be determined in order to predict soil water content with accuracy (Royer and Vachaud, 1975). The great advantage of the method is that once the soil moisture characteristic curves have been obtained, only one measurement (depth to water table) is necessary in order to predict moisture contents at a large

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156 Soil moisture content profiles

number of depths and such a procedure could be automated. The results from

the test samples indicate the validity of the method and thus it can be suggested

that the technique could be applied to other geotechnical problems when there is need to predict moisture contents below impermeable surfaces.

Acknowledgements This work was carried out in the Geotechnical Science Laboratories, Carleton University, Ottawa. It was financed by the Ministry of Transportation and Communication, Ontario under the Joint Highway Research Programme. The author is very grateful to Prof. P. J.

Williams, the director of the project for all his help.

Note 1. Tempe pressure cell, manufactured by Soiltest Inc.

References Hillel, D. (1971) Soil and water: physical principles and processes Royer, J. M. and Vachaud, G. (1975) ' Field determination of hysteresis in soil-water charac

teristics ', Soil Sci. Soc. Amer. Proc. 39, 221-3

IBG Study Groups: reports for 1975 (part 1)* Biogeography Study Group The Group has continued to prosper and has maintained a growing and active member ship. It has now become formally affiliated with the IBG.

Two meetings were held during the year: a field symposium in the North York Moors in September, 1975 and a symposium at the Lanchester meeting of the IBG on the subject of ' Protected landscapes and ecological change '. The former attracted 27 members. The theme was the relationships between ecology and land use. Ian Simmons and Margaret Atherden outlined the palaeoecology of the Moors, indicating types of habitats deemed worthy of some form of conservation management. Three invited local speakers, a land agent, a forester, and a planner, specified their own priorities in land management and the conflicts that were produced. Len Curtis led the party through the deep, narrow griffs on Levisham Moors where unusual and deep soil profiles had been exposed. Finally, the Hole of Horcum was duly inspected accompanied by a spectacular display of hang-gliding which, contrary to rumour, was not specially arranged by the meeting's organizers, Ian Simmons and Margaret Atherden, to whom, in any event, the warmest thanks are due for providing a most stimulating and informative weekend.

The Lanchester Symposium (reported in detail in Area 8 (1976), 1) consisted of six papers which ranged in reference from Exmoor to Lapland and from the Lake

District to New Guinea. The relationship between the particular and the general was a key theme throughout. Ian Simmons introduced the concept of scale in protected environments ranging from the preservation of individual species to the management of distinctive or rare habitats and regions. Subsequent papers developed this theme in widely contrasting environments and much useful discussion was generated. The

*A complete list of the officers and committee members of study groups for 1976 appears on the

inside back cover of Area 8 (1976), 1. Reports of the Medical, Population, Rural, Transport, and

Urban study groups will appear in the next issue-Editor

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