The construction of rock tunnels is inherently associated with large geotechnical uncertainty, which not only affects the structural safety and other design requirements but also causes challenges in meeting time plans and budgets. Time and cost estimations of tunnel projects are however still mainly deterministic. The introduction of the contractual document called Geotechnical Baseline Report highlights, however, a need for clients and contractors to address how geotechnical uncertainty affects time and cost estimations. Using probabilistic time and cost estimation methods, a key parameter is the proportion of different construction classes along the tunnel. This paper introduces the Dirichlet distribution (a.k.a. multivariate beta distribution) as a suitable probabilistic model for this epistemic uncertainty and discusses its potential for Bayesian updating and risk assessments of Geotechnical Baseline Reports.