By its nature, rock engineering is subject to a great deal of uncertainty and variability, which needs to be taken into account in the design process and in the management of rock-related risks. Variability is a property of nature and is a measure of the change in rock mass characteristics over time and space. The variability can be described with statistics and the probabilities related to variability can be interpreted as a frequency of occurrence. Uncertainty is a state of mind and is a product of our lack of knowledge, which can be reduced with more measurement and improved understanding. Probabilities related to uncertainty are best interpreted as a degree-of-belief.
Uncertainties include measurement errors, insufficient data, sampling bias, stress state uncertainty, and model uncertainty. The understanding of variability can be improved by collecting more data and improving the quality of data through training and quality control. Stress and model uncertainty remain a challenge in rock engineering. Some degree of subjective engineering judgement will therefore always be required in geotechnical design.
