supplemented by tilt data from pendula installed in internal shafts, and
from inverted floating pendula anchored at depth within boreholes in the
foundation rock. The relative movement and tilt of adjacent monoliths
may also be determined from simple mechanical or optical joint-meters. In
the case of embankments, the longitudinal crest settlement profile is of
prime importance. Embankment deformation data may also be derived
from the use of sensitive borehole inclinometers to determine profiles of
internal horizontal displacement incrementally through the height of a
dam in both planes.
(b) Porewater pressures
Hydraulic piezometers are of two types: the simple Casagrande standpipe
and the ‘closed-circuit’ twin-tube type. The Casagrande piezometer, Fig.
7.1, uses an electrical dipmeter to record the phreatic level; integrity of the
296 DAM SAFETY: INSTRUMENTATION AND SURVEILLANCE
Fig. 7.1 Casagrande-type standpipe piezometer in borehole
