In deep geological disposal system designs, it is important to minimize the installation area for cost effectiveness while satisfying the thermal requirements of the systems. An effective method to reduce the installation area for the systems is to employ an enhanced buffer material, as this can decrease the spacing between the disposal tunnels and deposition holes. Therefore, this study aims to evaluate the effect of an enhanced buffer material on the thermal behavior of the systems and their spacing. First, the discrete element method (DEM) was adopted to validate the thermal conductivity of the enhanced buffer material used, which was a mixture of bentonite and graphite. Then, a 3D finite element method (FEM) was conducted to analyze the proper disposal tunnel and hole spacing considering three cases with thermal conductivities values of the buffer as 0.8 W/(m K), 1.0 W/(m K), and 1.2 W/(m K). The results showed that the disposal tunnel and hole spacing could be reduced to 30 m and 6 m, respectively, when the temperature of the buffer surface facing the canister was 100 °C with a thermal conductivity value of approximately 1.2 W/(m K) or if more than 3% of graphite is added.

Construction of urban tunnels using a tunnel boring machine (TBM) is often favored over drilling-and-blasting based excavation method. Often, TBMs are customized and re-used in fields. However, little effort has been made to verify the status quo of the reused TBM prior to its placement, and no apparatus or method is available to examine the excavation performance of whole, assembled TBMs. Therefore, we present a newly developed apparatus which can examine the excavation performance of small cross-section TBMs with the cutterhead diameter of ∼3.5 m at full scale. The equipment was verified with a rock-like concrete specimen, in which the excavation of TBM was examined while varying the thrust and cutterhead rotational speed. The test results reveal the positive impact of thrust on the penetration rate (PR) and penetration depth (Pe), as well as a linear relationship between PR and the cutterhead speed. This proves that the presented full-scale TBM excavation testing method is capable to produce reliable and remarkably consistent results. The presented full-scale excavation testing method can be used to examine the functionality of a TBM of interest and further explore the effect of different ground conditions, such as rock strength, joints, and weathering, and the effect of different cutterhead design, including the number of cutters, cutter shape, and cutter location.