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Groundwater investigation techniques 153

Fig. 5.12 Single-value characteristic curves showing the relationship of (a) volumetric moisture content, θ, and (b) hydraulic conductivity,
K, against pressure head, ψ, for three hypothetical soils. Values of pressure head less than zero indicate unsaturated soil conditions. After
Freeze and Cherry (1979).

tensiometers can be used to monitor the position of conductivity, K, of the soil, characteristic curves can
the water table surface ( Jones 1997). To measure suc- also be constructed to show hydraulic conductivity
tions above the limit of a tensiometer (about 0.85 as a function of pressure head. Single-value character-
bar), an electrical resistance block is used. A porous istic curves for hypothetical uniform sand, silty sand
block of gypsum, nylon or fibreglass mesh contain- and silty clay soils are shown in Fig. 5.12 and demon-
ing two electrodes is situated in the ground in close strate the different behaviour of the soil types with
contact with the soil and hydraulic equilibrium decreasing pressure head. When the soil is com-
established between the block and soil. The water pletely saturated, the maximum volumetric moisture
content of the block is calibrated against the electrical content, θ, is equal to the soil porosity such that equa-
resistance between the two electrodes to provide a tion 5.11 equates to equation 2.1. As the soil dries,
means of measurement of the soil moisture content. the uniform sand with its larger diameter pore space
Disadvantages of the electrical resistance block is that is quickly drained and both the water content and
it has a long response time for water to seep through hydraulic conductivity decrease rapidly. On the other
the block and reach hydraulic equilibrium and mea- hand, the larger clay content of the silty sand and silty
surements can be affected by dissolved salts in the clay creates a larger porosity as a result of the smaller
soil water. The technique is less reliable at suctions grain size distribution. Because of the smaller pore
of <4 m but does have the advantage of operating at diameter, the water is held in the silty soils for longer
high suctions. until a greater suction creating an increasingly negat-
The relationship between soil moisture content ive pressure head is able to drain the pore water. As a
and pressure head is conveniently displayed by the result of the finer soil texture, the silty clay retains
presentation of soil characteristic curves or retention both a higher moisture content and hydraulic con-
curves. Also, given that the degree of interconnection ductivity compared with the uniform sand under dry
between saturated pore spaces will affect the hydraulic soil conditions. The slope of the line representing