Iraqi National Journal of Earth Science

Abstract

This study is conducted along fifteen seismic refraction traverses in ‎the reservoir part of the failed ‎Chaqchaq dam NW of Sulaimani City. The results show that ‎the area consists of three ‎geological layers. ‎The first layer ‎consists of soil with a thickness ‎ranging between 0.05-3.10m, whereas the second layer ‎consists of rock fragments ‎ranging between 1.93-13.11m and the ‎third layer is specified as a ‎consolidated and cohesive ‎limestone of the Kometan Formation that lies at a ‎depth ranging between 2.0-16.2m. The ‎‎Kometan limestone ‎surface‎ is irregular due to weathering and fracture as a result of tectonic movement ‎leading to the collapse of the area and later on filled with sediments ‎of recent deposits, or maybe decamped by ‎water then karstification ‎occurred.‎
‎Based on the estimated geotechnical parameters from ‎Vp and Vs and measured density, the result will ‎‎indicate that the first and second layers is weak and fissured and subjected to ‎sinkholes buried with recent sediments. ‎Whereas‏ ‏the third layer ‎indicates the harder-to-fracture rocks, the research would find the linear‏ ‏‎relationship between Vp and Vs of the first, second, and third layers. The relationships between ‎‎Poisson's ratio and Vs/Vp for the first layer and Vs/Vp for the second layer are pointed as ‎inverse ‎relationships. This means that increasing Poisson's ratios reduces both Vs/Vp ratio and the brittleness ‎of the materials. Also, the elastic modulus and shear (rigidity) modulus as well as bulk modulus values ‎are directly proportional with increasing depth, especially within the hard and cohesive Kometan Fn.‎
‎The results also show the basic design of the dam construction was not done scientifically. The beginning parts of most traverses appear to the presence of sinkhole that was related to the filling of the layers with water during the winter rainy season and then penetrating deeply, which leads to the collapse of the layer and forming this sinkhole, this sinkhole can be considered as the reason for the dam collapsing.