Abstract:
Monitoring earth dams plays an essential role in evaluating the structural safety condition
of this type of dams. Monitoring activities are related mainly with safety but also with the
collection of valuable data to enhance the unders tanding of the behaviour of these
structures. These purposes are not mutually independent but rather complementary (ICOLD,
1982). Monitoring a dam involves several activities, namely: design of the monitoring plan,
installation of monitoring devices, reading those devices at some pre- established
frequencies, conversion of measurement s to meaningful engineering quantities,
interpretation of these quantities, comparison with models, dam (visual) inspection and
issuing a safety repor t. A monitoring plan should define which observable quantities are
needed and which devices should be installed to measure those quantities. Furthermore, the
plan should establish the location of the devices, their measuring procedures and
monitoring frequencies depending on the age of the dam, extreme events, height, external
actions, conservation conditions, available know- how and technology, human and economic
losses in the case of failure, etc.. The quantities involved in dam monitoring are those
related both with the actions in the dam and with the dam response. Surface displacement s
are impor tant quantities to be determined, especially in what concerns safety and long term
behaviour (Tedd et. al, 1997). Surface displacement can be related to internal deterioration
processes, such as internal erosion or slope failure or to less impor tant phenomena as
secondary consolidation or creep. In order to evaluate these displacement s, surface marks
should be located at regular space intervals, usually in the dam crest, at the upstream and
downst ream sides, in the berm or berms, and less commonly in other places in the
downst ream slope. Several combined terrest rial imaging systems (CTIS) have been made
commercially available during the last decade and are now a reliable and proven technology.
The laser component of these CTIS provide a dense set of instrument al spherical polar
coordinates of any unknown point that reflects the laser radiation; the CCD photo sensors
component provide the RGB intensities for those points. The polar coordinates can be easily
transformed into a meaningful object Cartesian reference system. Given their high sampling
frequency CTIS offer an unprecedented density of spatial information concerning an object
under study. This technology has been widely used by the architectural and archaeological
communities but there is enormous potential in monitoring applications where such dense
data sets could provide great insight into the nature of structural deformations (Lichti et al,
2000). This paper tries to evaluate this new technology and points to a methodology for
measuring surface deformations on earth dams.
