A proposed probabilistic analysis methodology for tunnel support cost estimation depending on the construction method

The extent of support in tunneling projects can vary greatly depending on the geological conditions, and in turn may greatly affect project costs. For the purpose of project pre-feasibility assessment, methodologies are proposed according to the tunnel construction method, conventional tunneling (NATM), or mechanized tunneling (TBM). For both cases, statistical analysis tools in conjunction with stress and strain calculations are used in order to assess the volume of tunnel support. It is suggested that for TBM tunnels, the Probability of Failure is adopted as a failure criterion, and the volume of support computed accordingly. For NATM tunnels, a fixed Factor of Safety should be used in order to predict the support distribution and resultant support volume. A practical example is presented, for the comparison of support cost and construction schedule estimation of the two excavation methods, for a tunnel to be excavated through weak rock.