ABSTRACT: Results of the studies on the clay mineralogy of flysch deposits, cut by the derivation tunnel T-2 Aslantaş Dam, had shown the dominancy of hydrous mica (illite) in the sliding layer and the layer below, whereas the dominancy of chlorite in the layer above. Texturally the layers were also found to be different from each other. The differences in clay mineralogy are partly affected by the sedimentation and partly the environmental conditions (Geologic and pédologie). The formation of empty spaces due to décalcification processes caused by rainfall and the textural differences givje rise to the. formation of water saturated sliding layer- Under such conditions the amount of dynamite to be used becomes very important. A little more than sufficient amount of dynamite can cause a collapse of such structures. The flysch layers which are sometimes as much as one mUJmeter in thickness, having different clay minerals present in their clay fraction give different swelling pressure. In this respect, the pressure analysis made on the samples taken during the mechanical drilling and Shelby samples can not show satisfactory results. Therefore a study on clay mineralogy is needed on the samples taken from the flysoh layers for such structures