29 - Safety Concepts Applied to Rock Masses

Abstract

This chapter focuses on safety concepts applied to rock masses. The conventional assessment of safety in civil engineering works is obtained through a deterministic approach. Rock mechanics follow this rule. To take account of the many uncertainties and of the scatter in the data and also to cover the fact that models are necessarily approximate, a factor of safety is introduced. The margin between the real state and the minimum limiting state, which can be adequate in the absence of all uncertainties, is measured by the factor of safety—a scalar number supposed to lump together all imperfections in the data and the model. The numerical value of the factor of safety F is determined empirically for different types of materials. It is convenient when reasoning in the probability analysis context to discard the factor of safety concept F, which is a ratio, in favor of the margin of safety M, defined as the difference between resistance R and load L. It is vital for safety that instruments should monitor physical quantities that are significant in the failure scenario mechanisms. Monitoring of the works by appropriate instruments, during their construction and operation, is now considered an essential component in safety enhancement.