ABSTRACT
In principle, either test can be performed on all types of deposits; however, in practice variable head tests are most commonly used for low permeability units (e.g. silts, clays, tills) and constant head tests are most commonly used for highly permeable deposits (e.g. sands, gravels). The reason for this is related to the ease of measurement. For low permeability deposits it is relatively easy to monitor the gradual rise or fall in water level with time after some perturbation in water level, but it is somewhat more difficult and/or expensive to monitor accurately the flow corresponding to a constant head difference between the piezometer and the equilibrium water level for these soils. For permeable deposits (k > 10-6 m/s) the change in head in the piezometer resulting from some perturbation may be so quick that it is difficult and/or expensive to record accurately. In these cases, however, it is usually quite easy to measure the flow corresponding to a constant head difference.
