Potential geotechnical hazards make it difficult to select an appropriate TBM in adverse and difficult ground conditions. So, use of an efficient decision making technique which can take into account several criteria in such conditions is necessary. Analytic hierarchy process (AHP) is one of the most commonly used MCDM methods, which integrates subjective and personal preferences in performing analyses. However, AHP involves human subjectivity, which introduces vagueness type uncertainty and necessitates the use of decision-making under uncertainty. In this paper, vagueness type uncertainty is handled using fuzzybased techniques. The traditional AHP is modified to fuzzy AHP using fuzzy arithmetic operations. The efficacy of the proposed F-AHP was demonstrated through dealing with the selection of the most appropriate rock TBM in adverse geology. Based on the analysis results, both the single and double shield TBMs have the same preference for selection in difficult ground conditions.
Generally when mechanized tunneling is selected as construction method, the most important problem is often related to selection of the most appropriate TBM and its performance prediction. Many investigators have studied risk and TBM selection issues separately. Among them, the efforts of Nord and Stille  introducing effective parameters in tunneling techniques selection, Einstein et al.  introducing the decision aids in tunneling, AITES-ITA Working Group No. 14  introducing standards for TBM selection, Bieniawski et al.  presenting rock mass excavability (RME) indicator when selecting excavation technique should be noted. Only few studies on TBM selection focusing risk assessment have been reported. This is undoubtedly due to the complexity and frequency of effective decision variables. More recently, Shahriar et al.  proposed a methodology for rock TBM selection in difficult ground conditions by assessing geotechnical risk. This approach is now extended to fuzzy AHP methodology as a decision support aid. The primary objective of this approach is to present a methodology to guide decision-making under vagueness type uncertainty. This task is achieved by proposing a risk-based fuzzy analytic hierarchy process (F-AHP). The efficacy of integrating fuzzy-based technique to AHP is illustrated using an application of selecting three rock TBMs in difficult ground conditions.
AHP introduced itself as an approach for evaluating complex multiple criteria alternatives involving subjective judgment. A decision-maker determines his/her weights by conducting pairwise comparisons between criteria. Though the aim of AHP is to capture a decision maker's knowledge, the conventional AHP cannot fully reflect the human thinking style. Linguistic and vague descriptions could not be solved easily by AHP until the recent development in fuzzy decision-making . Fuzzy set theory, first proposed by Zadeh , and the subsequent fuzzy decision-making methods offer a more natural way of dealing with these preferences instead of exact values. The traditional AHP does not take into account the uncertainty associated with the mapping of one's perception to a number .