It is a characteristic of underground works that technical and economic decisions have to be considered under conditions of uncertainty. This uncertainty arises from the inability to form a complete structure of the medium, the rock mass, in terms of geological and geomechanical properties before construction, limitations of the existing methods of rock-excavation interaction analysis as well as from the variability in human and equipment performance and political and economical factors. All these factors result in a highly risky environment. 

Practitioners in the field of ground engineering have been dealing with this “risky” environment using their experience, often resulting in expensive and conservative design solutions. Probably, up until recently, this was acceptable, but under the current global conditions of budget restraint, highly competitive
markets, and society’s awareness of risk, an evaluation of risk is often explicitly required.
In other fields of science and engineering, decisions under uncertainty have been routinely addressed using Bayesian updating (1763; Harr, 1987) but only occasionally dealt with in tunneling due to the complexity of the medium and a lack of suitable and effective tools for performing such an analysis. Decision Aids in Tunneling, DAT, is a tool that was especially developed to respond to this challenge.