the Possibility of Axis Site of Atasu (Trabzon/Turkey) Dam)

In this study, determination of alteration degree of the basalt and the possibility of axis site of Atasu  dam was aimed. According to this aim engineering geological map of axis site scale of 1/2000 was prepared. The cores obtained from the boreholes along the axis of dam site were classified according to ISRM(1981) weathering classification  on this cores samples geomechanics experiments and measurements were done for joint frequently and RQD.

            The orientations of discontinuity planes in the units cropping out at the axis of the dam site were measured and evaluated by stereographic projection technique and the effects of these  discontinuities on permeability and slope stability were investigated. The joint system labeled as (1) involved by the basalts at the axis of dam site leads some problems in permeability.

            Based on the results of the kinematical stability analysis carried out for the basalts at the axis of dam site it was determined that joint system and flow surfaces will not causes any slope instability. Since the pyroclastics medium and highly weathered, any possible mass movement may occur in the form of circular failure.

            The basalt and pyroclastic rocks at the axis of the site were classified after examining  borehole cores based on the result of  a geomechanical laboratory testing program and data obtained from in- situ permeability tests. These classifications indicated that the basalts are in the classes of rocks with the fine-medium quality according to the RQD value, fractured, medium void ratio  low permeable rocks.

            The pyroclastic rocks are in classes of rocks with medium- very fine quality according to the RQD value, fractured, extremely-high void and impermeable – slightly  permeable and high permeable rocks. On the basis of the uniaxial compressive strength and P wave velocity, the basalts have medium to high strength and medium- low  weathered, the pyroclastic rocks have medium strength, medium-high weathered.

            At the axis of the dam site zoning was done according to permeability, RQD, joint frequency, weathering degree, according ISRM weathering is low at the levels where RQD values are high, and it is high where RQD values are low, it was seen that RQD values were decreased at the levels where joint frequency values are high it was determined, Lugeon permeability values are low at the levels  where joint frequency values are high.

The basalt have been subjected to the physical disintegration  together with the chemical weathering and hydrothermal alteration. Establishing their effects on the rock are quite difficult since, there has been an interaction between these different weathering processes. The physical disintegration caused the loosening the mass, enlarging of the previous fractures, increasing the amount of the discontinuities, crumbling and breaking down of the rock into small pieces. As the result of these phenomena, while the volumeand surface area of the rock have increased, the grain size of the rock has decreased. The abundance of the unstable minerals in the basalt reduces the rocks strength. In these rocks the amounts of the SiO₂, Na₂O, and K₂O have diminished while the amount of  MgO, Al₂O₃, Fe₂O₃ and CaO rose with chemical weathering. To give a quantitative description to the weathering degrees in the basalt rocks, petrographical weathering index was distinguished for the each sample, which was effected in different rates by the weathering processes. Apart from this index, micro fracture density and chemical indexes were used.

The relationships between the weathering classifications, geomechanics properties petrographic indexes and chemical weathering indexes of the basalts at the axis of the dam site were examined by regression analyses and it was aimed to determine the properties achieved by expensive and difficult laboratory tests by assistence  of easier and inexpensive ones. The regression analyses for the geomechanical properties examined revealed that linear, strong-very strong relationships exist between these properties.

Key Words: Geomechanical properties, permeability, weathering degree, Chemical weathering index, Rock mass classification