Decision Plots for preliminary design of single-shield TBMs

In this note a procedure for a back-of-the-envelope estimation of the correct length of a single-shielded TBM is reported in the context of a TBM preliminary design phase. It is based on four Decision Plots (DPs) and some associated equations, whose input data are the isotropic in-situ stress, the rock mechanical parameters, the maximum tolerated over-excavation and the shield skin friction coefficient. The essential output is the friction force the machine must overcome (the required thrust force), that can be compared with the available thrust force. The derivation of the DPs is illustrated and their practical use shown. Although the procedure involves some simplifying approximations, its performance is confirmed through two application examples and a comparison with the results obtained using design monograms of other authors.