This report reviews the art and practice of excavating and constructing underground facilities in clay/shale media, as part of a multi-year evaluation of the technical feasibility of direct disposal of spent nuclear fuel (SNF) in dual-purpose canisters (DPCs). The purpose is to review worldwide examples of large-scale excavations in clay/shale media, the methods used for excavation and construction, and the costs. It is anticipated that this information will help to show the feasibility of construction for a deep geologic respository for (on the order of) 10,000 large, heavy, heat-generating waste packages. This report refines the clay/shale disposal concept for DPC-based waste packages, in support of future studies that include cost estimation. Earlier studies compared reference concepts for disposal of SNF and high-level waste (HEW) in various media (clay/shale, crystalline and salt) with one result being that construction and maintenance costs for openings in clay/shale were considered to be relatively high (Hardin et al. 2012). A later study adopted in-drift emplacement instead of large-diameter emplacement boreholes, for large, heavy DPC-based waste packages. This change would increase by severalfold the length of emplacement drifts needed to dispose of all the commercial SNF projected to produced in the U.S. (Hardin et al. 2013). This in turn raised questions of engineering feasibility: whether -300 km of emplacement drifts could be efficiently constructed, then remain stable for at least 50 years during which the waste packages would be ventilated, with little or no maintenance. This report affirmatively answers these questions, based on experience with similar tunnels that have been in service for highways, railroads, and water conveyance for durations approaching 50 years or longer, in the U.S. and Europe.
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