1 First National Dam Safety Conference CWC, TNWRD and IITM A Brief Study of Arch Dams Behaviour Dr. B. R. K. Pillai Zika Smiljkovic Dr. A. K. Dhawan Project Director, DRIP Dam Design Engineer Dam Instrumentation Specialist, Central Water Commission, New EGIS Eau EGIS India Delhi Montpellier, France Email: dir‐drip‐
[email protected] 2 Causes for Potential Unusual (Irrecoverable) Behavior of Arch Dams 3 Alkali Silica Slow Reaction (ASSR) Flow chart of stages of alkali silica formation: Mechanism:Alkali solution in cement paste pores attacks reactive minerals of concrete aggregate producing thus alkali silica gel which in presence of water swells. The gel usually builds up within the aggregate fragments which then swells causing expansion of concrete and cracking of surrounding cement paste. Affects initially the aggregate. 4 ASSR Displays • Long lasting irreversible chemical process causing: irreversible upstream deflection of dams crest, upheaval of dam crest and mostly continuous cracking of upper galleries. • Expansive reaction from a few to approx. 30 years. Initiation, development and, slow down phase following exhaustion of alkalis. • Impairing the concrete strength and integrity, and state of equilibrium of a dam. 5 • Cases reported: Chambon dam, upheaval of 3.6mm/yr; Arch dams in Alpine region, U/S drift up to 1mm/yr. • Cracking of inspection galleries reported: Pian Telesio Dam(Italy), Portodemouros dam (Spain). • Karun Dam, Iran, 205m height and 14 yr old, exposed to ASSR. • ASSR limited with service compressive field > 6MPa, as reported. Tensile stresses are less constrainable to ASSR development. • Upstream drift mechanism: lower RWL with higher concrete temperature →upstream deflection of upper arches → tensile stress on U/S dam face → pronounced U/S dam face swelling→ irreversible upstream dri.