Geotechnical and geomechanical characteristics of the rocks along tunnel of Kulekhani III Hydro-electric Project

Abstract

 Geotechnical and geomechanical properties are important to understand tunnel behaviour and they differ according to rock types. Therefore, tunnel passing through different rock types is selected for the present study. The tunnel alignment of Kulekhani III hydroelectric project crosses five stratigraphic formations which compriseing eight lithological units.

The rocks of the area have most dominantly three sets of joints in which the foliation plane is prominent. In geotechnical study of intact rock, seven geotechnical properties - viz unit weight (γ), uniaxial compressive strength (σ_ci), tensile strength (σ_ti), young’s modulus (E_i), poison’s ratio (ν), friction angle (c_i) and cohesion ( i) were measured in lab and on the basis of the intact rock properties five geotechnical properties - uniaxial compressive strength (σ_cm), tensile strength (σ_tm), young’s modulus (E_m), friction angle (cm) and cohesion ( m) of rock mass were determined.

RMR, Q and GSI were used for geomechanical classification and the distributions of the geomechanical class values were studied. The relationship between UCS of rock mass and geomechanical classifications RMR and Q were studied for all rock types. Quite similar results were observed with both RMR and Q systems. The trend of correlations of each rock type with both classification systems follows almost analogous order. Power type continuous equation is observed for Q and exponential type relation is obtained for RMR. The general trend of correlation of UCS rock mass with RMR and Q is calculated which has R2 more than 0.9. The observed relations were compared with empirical relations proposed by other researchers and the results of the present study lie between the upper and lower boundaries set by other researchers. Among them, harder rocks have better correlation than softer rocks. It is also observed that higher the poisson’s ratio of the rock shown higher the order of linear correlation of rock mass properties with its geomechanical properties.