Characteristic triaxial strength of intact rock for LSD

Abstract

Determination of characteristic values may be a fundamental step in the design process when reliability-based design (RBD), limit states design (LSD) and load and resistance factor design (LRFD) approaches are applied. However, there seems to be no recognised approach for obtaining characteristic values of triaxial rock strength. This paper compares the use of non-linear regression and non-linear quantile regression models for obtaining estimates of characteristic triaxial strength of intact rock by fitting the non-linear Hoek–Brown empirical strength criterion to published datasets of triaxial rock strength. It is shown that the form of results obtained from a quantile regression model (i.e. a ‘characteristic criterion’) may be more useful to practising engineers than those produced by a non-linear regression model. For an extensive dataset, the methods give similar results. However, with small datasets of a size generally encountered in geotechnical engineering, both methods may be unreliable. This suggests that objective techniques that augment the limited test data should be developed.