Quantile value method versus design value method for calibration of reliability-based geotechnical codes

• A new reliability-based code calibration method, namely QVM, is studied geometrically in the standard normal space.
• FORM-based methods are also studied and compared to QVM.
• Code calibration methods based on constant partial factors are found to be not robust for cases with wide ranges of COVs.
• QVM outperforms FORM-based methods for cases with wide ranges of COVs.

This paper compares two methods for geotechnical reliability code calibration, namely the well known design value method (DVM) based on first-order reliability method and a recently developed method based on quantile, called the quantile value method (QVM). The feasibility of calibrating a single partial factor to cover the wide range of coefficients of variation (COVs) commonly encountered in geotechnical designs is studied. For analytical tractability, a simple design example consisting of one resistance random variable and one load random variable is first examined. A resistance factor is first calibrated using a single calibration case associated with a typical COV. The objective is to evaluate the departure from the target reliability index analytically when this calibrated resistance factor is applied to validation cases associated with a range of COVs. The results show that QVM is more robust than DVM in terms of achieving a more uniform reliability level over a range of COVs. Two realistic geotechnical design examples are studied to demonstrate that the theoretical insights garnered in the simple analytical example are applicable.