H. Karimi - Agriculture Faculty, Ilam University, Ilam, ۶۹۳۱۵-۵۱۶, Iran.
K. Taheri - Karst research and study bureau of west region, Kermanshah regional water authority, Iran

Large number of collapsed sinkholes had been developed 15 years (1989-2004) in Famenin and Kabudar Ahang plains, Hamadan province, west of Iran. This paper discuses the Hamadan sinkholes conditions and their mechanism of formation. There is Iimestone bedrock in the base of a thick cohesive alluvial aquifer in the area. High purity of Iimestone, considerable porosity and existence of numerous joints and fractures denote to high karstification capability of the limestone. Overexploitation of groundwater during last decades created a significant drawdown in aquifer water table. Besides, deep wells penetrated to the limestone bedrock and evacuated aquifer materials. Therefore, large cavities resulted from sand productive wells. Increasing effective stresses had caused to produce some sinkholes which they were mainly categorized in the dropout sinkholes. The sinkholes of the plains are roughly coinciding with the maximum drawdown in the aquifer, therefore, one of the main factors in the formation of sinkholes is the water table drawdown in the aquifers and underground karstification finally complete the formation mechanism.

Sinkhole, Hamadan, Kabudar Ahang, Famenin, droput, collapse, overexploitation, groundwater drawdown