Due to these uncertainties on the data necessary to design an underground construction, the decision-making process during the design phase is generally based on the designer's experience and sensitivity; moreover the decisions are usually taken from the designer without having a complete knowledge of the possible response of the excavated ground as the values of the parameters that describe the ground behaviour are varied.

Nowadays some decision-making support tools are available to help the designer in the task of designing underground structures that are technically feasible, inexpensive and relatively fast to construct [1]. One of the most popular tools that deals with these tasks is the DAT (Decision Aided Tunnelling) tool that allows the evaluation of the effects of the uncertainties of the data related to the ground conditions [2]. While the uncertainties affect the values of cost and time of construction of the designed underground work, the DAT allows an iterative calculation for the cost and the time associated with the construction of a tunnel while the ground conditions vary in accordance with the level of uncertainty of the input data given by the DAT user. Finally, the DAT tool shows the results in a graphical way and the designer can make decisions after knowing the effect of the uncertainties on the tunnel cost and time of construction.

In the paper presented together with this summary, a methodology based on a statistical approach is described. All the parameters involved in the design of a tunnel are considered as statistical variables and the calculations are implemented by means of methods based on the Monte-Carlo analysis technique [3]. The results of the calculations are statistical variables as well, characterized by average values and probability density functions. The statistic nature of the variables, allows the risk associated with the construction of the tunnel to be defined; in this way, it is possible to refer to a design solution in terms of its cost, time of construction and risk.

Having three different values associated with each possible design solution, using the modeFRONTIER software [4], a multi-objective optimization tool [5], it is possible to formulate an optimization process with the aim of minimizing the cost and the time related to the construction of the tunnel and the risk associated with the excavation procedures.

1

Y.W. Kim, "Decision support framework in adaptable tunnelling", Department of Civil & Environment Engineering, Massachusetts Institute of Technology, 1984.

2

H.H. Einstein, "The Decision Aids for Tunnelling (DAT) - a brief review", Department of Civil & Environment Engineering, Massachusetts Institute of Technology.

3

M.R. Spiegel, "Statistica", Collana Schaum, McGraw-Hill Italia, 1994.

4

ESTECO, "Tutorials", modeFRONTIER 3.0 user's manual.

5

S. Rao, "Engineering Optimisation, theory and practice", John Wiley and Sons, 1996.

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