DHARMSOIL MOISTURE–PERMEABILITY AND CAPILLARITY 121
various types of piezometers have been developed. (Lambe, 1948; Bishop 1961; Whitman
et al. 1961) Piezometers such as ‘Casagrande Piezometer’ have been developed for use in field
installations.
When instantaneous stress release occurs, even a negative value of pore pressure can
develop, with values which might go below even absolute zero (minus 1 Atmosphere).
In general, it is more convenient to determine first the elevation and total heads and
then compute the pressure head by subtracting the elevation head from the total head.
Thus, the following are the interesting points in energy heads :
(a) The velocity head in soils is negligible.
(b) Negative pore pressure can exist.
(c) Direction of flow is determined by the difference in total head.
(d) Elevation and total heads are determined first, and then the pressure head by
difference.
(e) Absolute magnitude of elevation head, which depends upon the location of the datum,
is not important.
5.5 THE DETERMINATION OF PERMEABILITY
The permeability of a soil can be measured in either the laboratory or the field; laboratory
methods are much easier than field methods. Field determinations of permeability are often
required because permeability depends very much both on the microstructure—the arrange-
ment of soil-grains—and on the macrostructure—such as stratification, and also because of
the difficulty of getting representative soil samples. Laboratory methods permit the relation-
ship of permeability to the void ratio to be studied and are thus usually run whether or not
field determinations are made.
The following are some of the methods used in the laboratory to determine permeability.- Constant head permeameter
- Falling or variable head permeameter
- Direct or indirect measurement during an Oedometer test
- Horizontal capillarity test.
The following are the methods used in the field to determine permeability. - Pumping out of wells
- Pumping into wells
In both these cases, the aquifer or the water-bearing stratum, can be ‘confined’ or
‘unconfined’.
Permeability may also be computed from the grain-size or specific surface of the soil,
which constitutes an indirect approach.
The various methods will be studied in the following sub-sections.5.5.1 Constant-Head Permeameter
A simple set-up of the constant-head permeameter is shown in Fig. 5.4.