Engineers Australia

Q-slope: An empirical rock slope engineering approach in Australia

Neil Bar and Nick Barton

Abstract

The Q-slope method for rock slope engineering provides an empirical means of assessing the stability of excavated rock slopes in the field. Q-slope allows geotechnical engineers and engineering geologists to make potential adjustments to slope angles as rock mass conditions become apparent during the construction of reinforcement-free road or railway cuttings and in open pit mines. Through case studies across Australia, the Americas, Asia and Europe, a simple correlation between Q-slope and long-term stable slopes was established. The Q-slope method is designed such that it suggests stable, maintenance-free, bench face slope angles of, for instance, 40-45°, 60-65° and 80-85° with respective Q-slope values of approximately 0.1, 1.0 and 10.

Q-slope was developed by supplementing the Q-system which has been extensively used for characterizing rock exposures, drill core and underground mines and tunnels under construction for the last 40 years. The Q’ parameters (RQD, Jn, Jr and Ja) have remained unchanged in Q-slope, although a new method for applying Jr/Ja ratios to both sides of a potential wedge is used, with relative orientation weightings for each side. The term Jw has been replaced with the more comprehensive term Jwice, which takes into account long-term exposures to various climates and environments. SRF categories have been developed for slope surface conditions, stress-strength ratios and major discontinuities such as faults, weakness zones or joint swarms.

This paper discusses civil and mining engineering applications of the Q-slope method in Australia for a variety of ground conditions from very weak to strong rocks, blocky to massive, isotropic rock masses to laminated, heterogeneous, highly anisotropic rock masses. A case study is also presented to illustrate the compatibility of Q-slope with P-wave velocity and acoustic and optical televiewer data obtained from borehole geophysical surveys to determine appropriate rock slope angles.