clay seal is critical. The type is readily installed in existing dams and is suit-
able where the phreatic surface is sensibly static; its merits lie in simplicity
of operation, reliability and low cost.
The closed-circuit hydraulic piezometer, best exemplified by the
Bishop type instrument illustrated in Fig. 7.2, offers more rapid response
to porewater pressure change. It is suited to lower-permeability soils and
to non-saturated soils, and therefore to determining negative as well as
positive porewater pressures in compacted earthfill. The twin 3 mm bore
hydraulic leads are permanently filled with de-aired de-ionized water and
can be laid for considerable distances (200 m) to a suitably located
instrument house, where measurement is effected by a pressure transducer
or, in older installations, a mercury manometer. De-airing is required at
intervals to flush out occlusions of air or water vapour entering the
hydraulic leads from the fill or from the pore fluid. The use of a fine-pored
(1μ nominal pore size) ‘high air-entry value’ ceramic element (Fig. 7.2)
considerably reduces the frequency with which de-airing is required.
Consideration must be given to the maximum elevation of the hydraulic
leads relative to piezometer tip, readout point and anticipated porewater
pressure range to avoid problems associated with negative pressures in the
leads. The Bishop piezometer has proved effective, durable, and reliable,
and has been widely installed in embankments during construction. It may
also be employed to estimate in-situpermeability.
INSTRUMENTATION 297
Fig. 7.2 Bishop-type twin-tube hydraulic piezometer