ENGINEERING CLASSIFICATION AND PROPERTIES OF PHYLLITE

Abstract

Laboratory investigations on phyllite, obtained from Tehri Dam Project site have been reported herein. In the present study, an attempt has been made to determine the strength indices, physical properties, mineralogical composition and then to classify the phyllite on the basis of Deere and Miller (1968) chart. Also, the effect of saturation on strength indices in unconfined and confined conditions has been made. Thin section studies revealed that phyllite mainly contains biotite, quartz and felspar. Based on the unixial compressive strength and modulus ratio values, this phyllite falls in the category DL, on average both in dry and saturated conditions. Point load strength test results are used to define anisotropy in terms of anisotropy index. Anisotropy index is found to be 0.2. Anisotropy curve is found to be U-type. Due to saturation, uniaxial strength is reduced by 20%. Triaxial tests at different confining pressures upto 50 MPa revealed that ultimate strength (0-1) varied non-linearly with confining pressure (0-3). This non-linearity decreased with saturation. Transition from brittle to ductile failure has been observed at lower confining pressure (about 22.5 MPa) than that (about 45 MPa) in dry condition). It has been observed that Hoek-Brown criterion is very much suited to phyllite in dry condition. Whereas Ramamurthy et al. criterion compares well with the observed results in saturated condition. Also the strength parameters of the anisotropic (jointed) rock mass have been determined.