LNEC support to the design adaptation of Alto Tâmega dam due to the rockslide on the right bank slope
3.2. Monitoring plan and safety control procedures adaptations Numerical modelling for structural calculations of the dam, incorporating the new geometry of the insertion surface, demonstrated marginal changes in stress fields within the dam body. Consequently, the following potential risk scenarios were identified:
• detachment and failure of rock mass blocks on the downstream slope of the dam foundation due to dam loads;
• global failure caused by discontinuities in the dam foundation; • modifications to the dam thrust on the right bank foundation;
Comprehensive studies conducted by the designer, reviewed by the owner with LNEC’s support, confirmed that general safety requirements remained met. In accordance with the revised design, the monitoring plan for the right bank was enhanced with the following equipment installations:
• levelling marks on the right bank foundation gallery, to monitor vertical displacements; • prismatic targets for geodetic observation of displacements in the new concrete structure; • piezometers for measuring water pressure in the rock mass under the new concrete structure;
• weirs for measuring seepage and leakage flows within the new concrete structure; • load cells in the anchors of the new concrete structure; • rod extensometers and inclinometers for measuring displacements in the rock mass;
• three-dimensional joint meters for measuring relative displacements in fault traces (within the rock mass foundation gallery, GRMD);
• thermometers to monitor temperature variations in the new concrete structure during construction.
The locations of the rod extensometers installed in the dam body and immediately downstream on the right side are shown in Figure 14. Acknowledging that the landslide and subsequent measures altered the characteristics of the rock mass near the most stressed area of the right bank, actions were implemented to ensure the adaptations achieved the intended effects for dam and foundation behaviour. These actions included:
• construction of dam blocks on the right bank commenced only after completing the new concrete structure to an elevation higher than the blocks’ foundations;
• installation and operational readiness of essential monitoring equipment before concreting block B6;
• intense monitoring during activities involving rock mass drilling on the right bank foundation (e.g. for rock mass consolidation, installation of bolts and anchors on fault F13, and waterproofing and drainage curtains);
• immediate availability of monitoring results on a digital platform for all entities involved in safety control;
• interpretation of monitoring results using a numerical model of the dam-foundation system, including the downstream concrete structure and its connection to the rock mass foundation;
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