Deformation monitoring of earth dams using laser scanners and digital imagery

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.