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

Fault movement in dam foundations is one of the earthquake hazard features 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 Karun River in the Khuzestan Province, southwest Iran. Its location in the Zagros active belt as the most active seismotectonic zone of the country makes it prone to instability of the abutments and seepage potential through the foundation and abutments. The dam foundation is on the sandstones of Agha-Jari Formation while the abutments are composed of dislocated rock mass of Bakhtiari Formation. The right abutment of the dam is highly fractured and displaced due to the action of Pir-Ahmad active fault. Geotechnical sections along the dam axis and across the right abutment indicate four along strike faults near the right bank as well as sharp changes in 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 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 operation and maintenance plans of the dam as could be harmful for future dam safety.

Active faulting, Geotechnical properties, Pir-Ahmad fault, Gotvand Dam