Page 232 - Geotechnical Engineering Soil and Foundation Principles and Practice

Pore Water Pressure, Capillary Water, and Frost Action
Pore Water Pressure, Capillary Water, and Frost Action 227

11.7 MEASURING MATRIC POTENTIAL

11.7.1 Filter Paper Method

A simple procedure that is used in soil science and demonstrates the princi-
ples might be called the ‘‘limp potato chip’’ method, except that instead of
potato chips it uses filter paper. A standardized filter paper is selected and must
be calibrated by wetting and then re-weighed after equilibrating at different values
of suction. These are provided by glass desiccators containing selected salt
solutions.

A test is performed by placing three layers of oven-dried filter paper in contact
with about 200 g of soil in a container that is sealed and allowed to equilibrate.
The outer layers of paper protect the middle layer from becoming soiled, and it is
the middle layer that is used in the determination. The filter paper is removed
and weighed, then oven-dried and re-weighed. Then by reference to the filter
paper calibration curve the matric potential is obtained at that particular soil
moisture content. The soil also is weighed, oven-dried, and re-weighed to obtain
its moisture content.

If the filter paper contacts the soil, the result represents combined matric suction
and osmotic suction, the latter related to the dissolved salts and clay mineral
surface activity.

11.7.2 Principle of a Tensiometer
A more rapid method for measuring matric potential of a soil either in
the laboratory or in the field is with a tensiometer. The essential features are
shown in Fig. 11.12. A porous ceramic cup is sealed to a glass or plastic tube
that is connected to a vacuum gauge. At the top of the tube is a cap with an
O-ring seal.

The tensiometer must first be prepared by soaking the ceramic cup in water that
has been recently de-aired by boiling. Then a vacuum is pulled at the top to draw
water up into the tube and de-air the gauge. The top is sealed off and the system
tested by allowing water to evaporate from the surface of the ceramic, which
should create a negative pressure that can be read on the gauge. The cup then is
re-immersed in de-aired water and the gauge pressure should return to zero in one
or two minutes. If it does not, the de-airing procedure is repeated and the gauge
may have to be re-zeroed.

The saturated tensiometer cup then is imbedded into close contact with the soil
that is to be measured. Unsaturated soil sucks water through the porous wall
of the cup to create a negative pressure reading on the gauge. The end point,

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