Some engineering geological properties of quartz porphyry, Genting Sempah area, Selangor-Pahang

Quartz porphyry or rhyolite porphyry is a general name for one of the rock types which forms part of the roof pendant in the Main Range in the Genting Sempah area of Selangor-Pahang. This is a volcanic rock with a porphyritic texture and grey to greenish grey as its main colour. The phenocrysts are of medium grain size (2 mm-5 mm) with a fine grained groundmass « 2 mm). Determination of the strength of the quartz porphyry enabled its classification as having very high strength, with an average value of the uniaxial compressive strength of 222 MPa, and a maximum value of 235 MPa. The average point load index has a value of 9.7 MPa with an average tensile strength (Brazilian test) of 16.5 MPa. The presence of discontinuities leads to a reduction in strength. For uniaxial compressive strength, a reduction of 40% and 70%, that is in two distinctly different groups was obtained. The respective average values are 136 MPa and 66 MPa. For the point load index, a reduction of60% to an average value of3.8 MPa was obtained and the tensile strength showed a reduction of 65% to an average value of 5.7 MPa.