Weathered rock characterization using drilling parameters

Abstract

The characterization of weathered rocks by the use of drilling parameters is presented. Drilling parameters obtained from the drilling process monitoring system during the installation of soil nails into weathered rocks are used in this investigation. A new drilling index Pi, simplified from the concept of the specific energy, is used in an attempt to differentiate weathered rocks. With Pi or the penetration rate, boundaries of weathered rocks between (1) soil and rock ranges, and (2) grade II and grade III rocks in accordance with the six-fold rock material decomposition grade are identified. For weathered rocks in the soil range, quantitative characterization has been established in this study by using the drilling parameters. An empirical equation has been established for estimating the standard penetration resistance N value from the drilling parameter Pi, which is used in turn to determine the Mohr–Coulomb friction angle based on the work of Schmertmann. Although this approach tends to yield a slightly larger Mohr–Coulomb friction angle than that from laboratory tests, results obtained in this attempt reveal that estimation of the Mohr–Coulomb friction angle from drilling parameters is attainable with reasonable accuracy.Key words : drilling, standard penetration, percussion index, penetration rate.