Environmental Engineering Reference
In-Depth Information
temperature. However, the instruments have the advantage of being easy to read, easily
auto-data logged and independent of the relative location of the piezometer tip and read-
out unit. As with vibrating wire instruments, they are susceptible to damage by lightning
strike and overvoltage protection is needed.
20.4.4.5
Should piezometers be installed in the cores of earth and earth and
rockfill dams?
Many large dams built from the 1960s to 1990s have piezometers installed in the dam
core. These were usually installed as the dam was constructed and the piezometer tubes
(or wires for vibrating wire piezometers) were taken to the downstream side in trenches
as described above. In some dams, the wires or pneumatic piezometer tubes were taken to
the top of the dam in groups in vertical riser pipes.
Either method leaves a potential weakness in the dam and some senior dam engineers,
including the authors, are reluctant to put such potential weaknesses into a dam core,
because of the chance that internal erosion and piping may initiate along the trenches/ris-
ers. It is arguable whether, for a well-designed and constructed dam with filters down-
stream of the core, it matters what the pore pressures are.
Having said that, the authors have on a number of occasions found the long term trends
in pore pressures valuable in assessing slope instability and potential piping problems. If
piezometers must be installed, it is suggested the tubes/wires are laid in a winding fashion
in a trench backfilled as recommended in Bureau of Reclamation (1987) with well com-
pacted dry mixture of bentonite and filter sand (1 part bentonite, 3 parts filter sand by dry
weight). The degree of compaction should match that of the adjacent fill. Seepage cutoff
trenches should be provided, extending at least 0.6 m into the adjacent fill, and 0.3 m
wide, where zone contacts are crossed between each piezometer in the earthfill core and
at not more than 15 metre intervals.
The bentonite sand backfill should be compacted because otherwise a gap may form at
the top of the trench as the sand densifies under compaction equipment and on saturation.
This may make it more likely for tubes/wires to be broken but the potential for a gap must
be avoided.
The sides of the trench are likely to desiccate and crack, and cracked material should be
removed before backfilling.
Piezometers should not be installed too close to the upstream face of the core - say no
closer than a quarter of the core width and preferably not in the upstream half for narrow
cores.
Where existing piezometers have ceased to function, it is usually better to go
without the information, than to run the risk of damaging the core during drilling and
backfill grouting of the holes needed to install new piezometers. If there are signs of dete-
rioration necessitating installation of piezometers in the core, these should be installed in
boreholes using dry drilling techniques, such as hollow flight augers so as to avoid
hydraulic fracture of the core. The backfill cement/bentonite grout should be installed in
short lengths, using a tremie system, with time between stages in the backfill to allow
the grout to develop strength, so hydraulic fracture will not be caused by the grout. This
work should be supervised by experienced geotechnical professionals familiar with dam
engineering.
20.4.5
Displacements and deformation
Internal displacements and deformations were often monitored on larger earth, earth and
rockfill and concrete face rockfill dams. This allowed confirmation of design assump-
tions, particularly relating to long-term settlement of earthfill and foundations, and to
concrete face designs.
