Permeability and Flow of Water in Soils - Elements of Soil Mechanics - page 58

There are several types of equipment available which can be used to measure soil suction values.
Amongst them are psychrometers, porous blocks, filter papers, suction plates, pressure plates and tensi-
ometers, the last being the most popular for in situ measurements. A useful survey was prepared by Ridley
and Wray ( 1995 ).
The psychrometer method
A psychrometer is used to measure humidity and is therefore suitable to measure total soil suction, i.e.
the summation of the matric and the osmotic components. The equipment and its operation have been
described by Fredlund, Rahardo and Fredlund ( 2012) .
A sample of the soil to be tested is placed in a plastic container. A hole is then drilled to the centre of
the specimen, a calibrated psychrometer inserted and the drilled hole backfilled with extra soil material.
The whole unit is finally sealed with plastic sheeting and placed in an air tight container, where it is left
for three days with its temperature maintained at 25°C. After this time the soil sample is deemed to have
achieved both thermal and vapour pressure equilibrium and relative humidity measurements can be taken.
The filter paper method
With this technique, described by Campbell and Gee ( 1986 ), both total and matric suctions can be meas-
ured. In a typical test the soil specimen is prepared in a cylindrical plastic container and a dry filter paper
disc is placed over its upper surface. (Fig. 2.16) . This filter will measure the matric suction.
A perforated glass disc is placed over the filter paper and a further filter paper is then placed over the
glass. As this top filter paper is not in actual contact with the soil sample it can only measure the total
suction.
The assembled specimen/filter paper is left for at least a month, at a temperature of 25°C, in order to
obtain thermal and vapour equilibrium. At the end of this time the assembly is dismantled and the water
contents of the specimen and the two filter papers determined. The water contents obtained for the filter
papers can be converted into the required suction values by using a suction/water content curve for the
filter paper material.
The tensiometer
Stannard ( 1992 ) presented a review of the standard tensiometer and covered the relevant theory, its
construction and possible uses. The apparatus is mainly used for in situ measurements and consists of a
porous ceramic cup placed in contact with the soil to be tested.
A borehole is put down to the required depth and the ceramic filter lowered into position. Water is
then allowed to exit from a water reservoir within the tensiometer and to enter the soil. The operation
continues until the tensile stress holding the water in the tensiometer equals the stress holding the water
in the soil (i.e. the total soil suction).
The tensile stress in the water in the tensiometer is measured by a pressure measuring transducer and
is taken to be the value of the total soil suction.
Fig. 2.16 Soil suction measurement - an arrangement for the filter paper method.