Hazards and mechanism of sinkholes on Kabudar Ahang and Famenin plains of Hamadan, Iran

Abstract

A large number of collapse sinkholes have been developed in the period [1989][1990][1991][1992][1993][1994][1995][1996][1997][1998][1999][2000][2001][2002][2003][2004] in Famenin and Kabudar Ahang plains, Hamadan province, west of Iran. This paper discusses the Hamadan sinkhole conditions and their mechanism of formation. There is limestone bedrock at the base of a thick cohesive alluvial aquifer in the area. High purity of limestone, considerable porosity and existence of numerous joints and fractures favors high karstification of the limestone. Overexploitation of groundwater during the last decades created a significant drawdown in aquifer water table. Besides, deep wells penetrated to the limestone bedrock and evacuated fine-grained materials of overlain alluvial aquifer. Therefore, large cavities resulted from sand productive wells. Increasing effective stresses have mainly caused the formation of dropout sinkholes or cover-collapse sinkholes. The sinkholes of the plains roughly coincide with the maximum drawdown in the aquifer. Therefore, one of the main factors that has controlled the formation of sinkholes is the water table drawdown in the aquifers. Karstification of limestone bedrock and thick cohesive soil of overlain aquifer enhance the formation mechanism of sinkholes.

Hydrogeology of Karstic Area

“…Rapid failure of the ground surface occurs when the soil collapses into a void that has been slowly enlarging and stoping upwards while soil was washed into the limestone fissures beneath (Drumm et al, 1990;Tharp, 1999;Karimi and Taheri, 2010). They are also known as cover collapse sinkholes.…”

“…Frequent triggering factors in the development of land subsidence and sinkholes are overexploitation from groundwater aquifers and groundwater level fluctuations. Moreover, existence of fine-grained alluvium, tectonic features especially faults and dissolution of karstic formations were reported as effective factors in the formation of sinkholes (Karimi and Taheri, 2010).…”

“…As an example from Karimi and Taheri (2010), in order to understand the geotechnical conditions of the soil column in Famenin plain, Iran, a specific exploration borehole having 4 inches diameter, 115 m depth, rotary drilling method without drilling mud and undisturbed sampling in each 1.5 m was drilled in the distance of 15 m from Jahan Abad collapse sinkhole (6F-1). The thickness of overburden alluvium was 94 m, and the soil was mainly fine grained.…”

“…The following factors favor the formation of sinkholes in the Famenin and Kabudar Ahang plains, Iran (Karimi and Taheri, 2010 The effective normal stress at any depth in the aquifer can be calculated with the following equation (Terzaghi et al 1996):…”

“…Hundreds of thousands of new subsidences have been reported worldwide, ranging from 1 × 1 × 1m holes to features >100m in length or diameter and tens of meters deep. Relationship between effective and neutral stresses in the soil mass model (U, neutral stress; Ef, effective stress) (Karimi and Taheri, 2010).…”