The investigation of the Geomechanical Properties and Alteration Degrees

of the Rock Units at the Boztepe (Malatya/Turkey) Dam Site

In this study, it was aimed to determine the geomechanical properties and alteration degree of the basalts and tuffites cropping out at the axis of Boztepe dam site. For this purpose, geological map of the reservoir area with a scale of 1/20 000 and engineering geological map of the axis of the dam site with a scale of 1/2500 were prepared. The cores obtained from the boreholes along the axis of dam site were examined and core specimens were collected to use in the geomechanical laboratory tests.

The orientations of discontinuity planes in the units cropping out at the axis of the dam site and the reservoir area were measured and evaluated by stereographic projection technique and the effects of these discontinuities on permeability and slope stability were investigated. The joints labeled as (1) and (4) and flow surfaces involved by the basalts at the axis of dam site lead some problems in permeability. Since the tuffites are impermeable, it is considered that there is not any effect of discontinuities on permeability.

 Based on the results of the kinematical stability analyses carried out for the basalts at the axis of the dam site and reservoir area, it was determined that joint systems and flow surfaces will not cause any slope instability. Since the tuffites are highly weathered, any possible mass movement may occur in the form of circular failure.

The basalts and tuffites at the axis of the dam site were classified after examining the borehole cores, based on the results of a geomechanical laboratory testing program and data obtained from in-situ permeability tests. These classifications indicated that the basalts are in classes of rocks with fair-poor quality according to the RQD value, low to medium joint frequency, low to medium void ratio and impermeable-slightly-high permeable rocks. The tuffites are in classes of rocks with very poor-poor-fair quality according to the RQD value, medium-low joint frequency, very low void and impermeable-slightly permeable rocks. On the basis of the uniaxial compressive strength and modulus ratio, the basalts have medium to-low strength and high modulus ratio, the tuffites have very low strength and low to medium modulus ratio.

At the axis of the dam site and the derivation tunnel zoning was done according to permeability, RQD, weathering degree according to ISRM and Unified Alteration Index. In the basalts slightly altered, moderately altered, highly altered and extremely altered degrees according to Unified Alteration Index are equivalent to slight, moderate, high and complete weathering degrees respectively in ISRM weathering degree classification. In the tuffites extremely alteration degree according to Unified Alteration Index is equivalent to moderately and highly weathering degrees. In zoning done according to RQD value and permeability; permeability is low at the levels where RQD values are low or high; and is high at the levels where RQD values are moderate.

In derivation tunnel by RMR system classification; type of excavation, stand-up time of rocks, type of support are determined. According to RMR rock classification the tuffites between 0.00-105.00 m are in poor rock, the basalts between 105.00 - 240.00 m are in good rock and the basalts between 240.00-565.00 m are in fair rock classes.

The relationships between the properties of the basalts and tuffites 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 assistance of easier and inexpensive ones. The regression analyses for the geomechanical properties examined revealed that, linear, polynominal, logarithmic and exponential relationships exist between these properties. For the tuffites the relationships are also linear, polynomial and exponential.

Key Words: Engineering classification, Geomechanical properties, permeability, regression analysis, slope stability, unified alteration index, weathering degree