POINT LOAD STRENGTH OF ROCK LUMPS

Abstract

Recently developed 'Point-load strength index tests' are quite simple, quick and economical. The apparatus developed by Roorkee University, is portable and can easily be used in the field. This apparatus has been adopted by I.S.I. (IS':8764- 1978). It is common to conduct point-load strength index tests on rock cores. Uniaxial compressive strength tests on rock lumps have also been performed by many investigators. Review of literature showed that the point-load lump strength index tests were not tried on rock lumps. The proposed study was aimed to fill this gap to some extent. Lleven rock pieces were taken from the Shiwaliks and tested for point-load lump, strength index (1.0, uniaxial compressive strength (q0) and uniaxial (Brazilian) tensile strength (qt). Mohs hardness (H) and unit weight (y ) were also determined. Two sizes of .ock lumps were tested. Bigger lumps ranged-between 500 to 1000 gm in weight and smaller lumps ranged between 50 to 100 gm in weight. The point-load lump strength index is defined as the ratio of peak load to the square of the distance between the conical point loading platens which is measured at the time of fracture of the lump. Further, uniaxial compressive strength was corrected for unit slanderness ratio and point-load lump strength index for bigger lumps was corrected for the standard size of 25 mm. Results showed that hardness increases with the increase in unit weight and qc/qt. It indicates that Mohs hardness is a measure of brittleness of rock material. The ratio of uniaxial compressive strength to the point-load lump strength index (qc/Ii:) was found to increase with hardness (H). However,qc/Iip decreases with the increase in unit weight. Correlations after combining the data of both smaller and bigger lumps were also found to have similar trends, It was concluded that this corre-latidn is quite good for predicting the uniaxial.: strengths of rocks from the point load lump strength index even for weak and weathered rocks. A draft Indian Standard Code on the 'Determination of Point-load Lump Strength Index' has been given in Appendix -