Method of Measurement for Soil Moisture Characteristics Using Centrifuge

Abstract

A centrifuge is a quick means for obtaining the relation between soil wetness (W) and suction (h), and is widely used for this purpose in Japan.
This method encounters problems such as the existance of suction profiles in soil samples, movement of water in continuous system following centrifuging and the influence of soil compaction due to soil weight in the centrifugal force field. In this paper an examination is made of the influence these problems have on the W-h relation measured, and the most ideal method for using a centrifuge is proposed.
The average wetness (W) of a soil sample measured can be regarded as an approximation of the proper functional wetness corresponding to average suction (h) determined as follows.
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where, r₀, r₁, r₂ are the distance from the axis of rotation to the boundary h=0, and the lower and upper plains of the sample, respectively. d₀ is sample radius and ω and g are the angular velocity and acceleration of gravity, respectively.
If the function W (h) is linear, W is always equal to W (h), but generaly, non-linearity of W (h) results in a difference between W and W (h). However, such a difference is lessened by placing the soil sample on a ceramic plate, since the suction range in soil becomes narrow and W (h) can be approximated linearly within such a range.
Two experiments were carried out on three types of soil using a centrifuge; one with a ceramic plate and the other without a plate. The results were compared with data obtained by the other methods using a suction plate, pressure plate, and a vapor pressure method. The data obtained using the ceramic plate agreed with those from other method.
Moisture flow from a ceramic to soil sample following centrifuging could be estimated by an additional check of centrifuging ceramic alone, and soil moisture content which were in equilibrium could be accurately determined.