Environmental Engineering Reference
In-Depth Information
Table 20.15.
Checklist of conditions to be noted on visual inspection (adapted from ANCOLD, 1976).
(a) Vegetation on dam and within 15 metres beyond toe of dam:
- Overgrowth: Requiring cutting for dam surveillance, requiring weed control for dam surveillance,
indicating seepage or excessive seepage or moisture;
-Wet terrain vegetation: Watch for boils, sand cones, deltas, etc., changes with the season, and pond
level changes;
- Incomplete - Requiring repair: Poor growth, destroyed by erosion.
(b) Foundations
- Drainage ditches clogged with vegetation;
- Dam areas, moisture on dry days;
- Flowing water: quantity, location, clarity;
- Boils;
- Silt accumulations, deltas, cones.
(c) Embankment
-Crest: Cracking, subsidence;
- Upstream face: Cracking bulging, surface erosion, gullying, wave erosion;
- Downstream face: Cracking, subsidence, bulging, erosion gullies; moisture on dry days;
damp areas, seeps;
- Berms and within 15 metres beyond toe of dam: Erosion, gullies, damp areas, seeps.
(d) Spillways and outlet works
- Reservoir level;
- Discharge tunnel or conduit condition, seeps, cracks;
- Seepage or damp areas around conduits;
-Erosion around or walls below conduit;
- Boils in the vicinity of conduit;
- Spillway slabs for uplift, subsidence, cracking.
(e) Areas of previous repair.
-Effectiveness of repair;
-Progression of trouble into new area.
20.4
HOW IS THE MONITORING DONE?
20.4.1
General principles
Some good general principles on dam monitoring are given by the French National
Committee in ICOLD (1989):
(a) Dams are structures with a long life span.
Reliable
instruments with a similar life span
are necessary, and they must be accessible, verifiable and replaceable;
(b) They must be
sensitive
to give warning of sudden changes or trends that are very small
but may be significant indicators of distress;
(c) They should be
simple
so that frequent readings can be made quickly by site or oper-
ating staff without the need for specialists; in this way, the dam is more or less perma-
nently monitored, and the records are available for statistical analysis;
(d) In recent decades far more attention has been focused on deformations and hydraulic
behaviour in the
foundations
;
(e) V
isual inspection
and surroundings, by people who know the dam, is very important
because even the best instruments will not find fissures, leakages, dam spots or their
growth, etc.;
(f) The choice of parameters to be measured and the positioning of instruments cannot
always be optimal at the design stage. The number of instruments sometimes amazes
but is justified by the need for effective monitoring of a so far unfamiliar structure.
Optimisation of the system can be made by abandoning some instruments or installing
others as the real behaviour becomes better understood;
