The impact of active faulting on the geotechnical properties of the Upper Gotvand Dam, southwest Iran

Fault movement in dam foundations is an earthquake hazard feature that may lead to severe structural distortions. Such movements may manifest as rock failures, crushing and displacements of rock masses. Faults with surface breaking and potential block movements are also of considerable interest in the issue of dam safety. The Upper Gotvand Dam is the highest rock fill dam in Iran which was constructed over the Karun River in the Khuzestan Province, southwest Iran. Its location in the Zagros active belt, the most active seismotectonic zone of the country, makes it prone to abutment instability and potential seepage through the foundation and abutments. The dam foundation is on the sandstones of the Agha-Jari Formation while the abutments are composed of rock mass of the Bakhtyari Formation. The right abutment of the dam is highly fractured and displaced due to action of the Pir-Ahmad active fault.. Geotechnical sections along the dam axis and across the right abutment indicate four strike faults near the right bank as well as sharp changes in the dip of bedding along the dam axis. The Agha-Jari layers are tightly folded beneath the dam axis with a 30° inter-limb angle forming a chevron fold. The geomechanical classification of the outcropped rocks is in the range of weak to very weak. The whole region is affected by an N-S basement tectonic lineament introduced in this research as the Lali-Ahvaz trend. Variations in the geotechnical properties of the dam site are mostly due to the movement of the Pir-Ahmad Fault. The activity of the fault should be re-assessed for future operations and producing dam maintenance plans as fault activity could diminish future dam safety.