| Summary: | In the design of shield tunnels, it is expected that a design can be more economical as more site investigation data are available. Nevertheless, the cost of site investigation will also increase as more site investigation data are required. It is thus important to assess the potential benefit of a site investigation program before it is conducted considering the uncertainty in the soil properties and the site investigation outcomes. In this paper, a probabilistic framework is suggested to assess the effectiveness of a site investigation program for the design of shield tunnels through the random field theory. An efficient method based on the generalized extreme value distribution is used to calculate the failure probability of the tunnel, through which the expected benefit from a site investigation program can be estimated conveniently. The result shows that a site investigation program is more valuable when a greater target design reliability index is needed. The expected benefit from a site investigation program increases as the borehole intensity increases, and it also increases as the scale of fluctuation of the soil properties increases. The method suggested in this paper provides a useful tool for planning borehole layout for the design of shield tunnels, which is promising for the optimization of site investigation.
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