Abstract:Groundwater flow is one of the most important factors for the design of a ground heat exchanger (GHEX) since the thermal environment of the ground around the buried GHEX is significantly affected by heat convection due to the groundwater flow. Several preceding studies have been conducted to develop analytical solutions to the heat transfer model of GHEX with consideration of groundwater flow. One of these solutions is the combined heat transfer model of conduction and convection. However, the developed combined analytical models are inapplicable to all of the configurations of ordinary GHEXs because these solutions assume that the inner part of the borehole is thermally inert or consists of the same material as that of the surrounding ground. In this paper, the applicability of the combined solid cylindrical heat source model, which is the most suitable to energy piles until now, was evaluated by performing a series of numerical analyses. In the numerical analysis, the inner part of the borehole was modeled as two different materials (i.e., permeable ground formation and impermeable fill such as concrete) to evaluate applicability of the analytical solution along with different diameter-length (D/L) ratios of borehole. In a small value of the D/L ratio, the analytical solution to the combined heat transfer model is in good agreement with the result of numerical analysis. On the other hand, when increasing the D/L ratio, the analytical solution significantly overestimates the effect of groundwater flow on the heat transfer of GHEXs because the analytical solution disregards the existence of the impermeable region in the borehole. Consequently, such tendency is more critical in the GHEX with a large D/L ratio such as large-diameter energy piles.

An expanded steel pipe pile increases the cross-sectional area of conventional micropile by expanding the steel pipe to exhibit a higher bearing capacity owing to increased frictional resistance. However, construction cases of the expanded steel pipe pile are insufficient due to the absence of equipment for expanding steel pipes inside the ground. In this study, hydraulic expansion equipment was developed to verify the reinforcing impact on the bearing capacity and field applicability of the expanded steel pipe pile. A series of laboratory and test bed experiments was conducted to measure the expansion time and deformation of carbon steel pipes by using the developed equipment. The results of these experiments demonstrated that the developed equipment has sufficient ability and constructability to be used in the field for constructing expanded steel pipe piles. Then, field load tests were performed by constructing expanded and conventional steel pipe piles to confirm the improved bearing capacity of the expanded steel pipe pile compared to that of the conventional micropile. As a result, the expanded steel pipe pile exhibited a 20.88% increase in bearing capacity compared to that of the conventional steel pipe pile.

The development of underground spaces in urban areas plays a crucial role in the regeneration and sustainability of cities. However, the conventional underground excavation works in metropolises limit the use of the ground facilities owing to stability, noise, and vibration problems, which may cause huge economic damage. In this study, a method of pre-constructing slabs of underground facilities was analyzed to improve the stability of the floating and underground extension method, even during the use of the target building. First, a numerical simulation was conducted to compare the stability of the top-down method with slab pre-construction with that of the conventional top-down method. Then, the stability of the test bed was checked by applying this construction method to the field. As a result, the top-down method with slab pre-construction significantly improved the stability of the target building by preventing the buckling of the columns and supporting members. The increase in the displacement of existing columns and supporting members was controlled after the pre-construction of the slab. In addition, the crack width and ground settlement were stable within the management standards at the field. Thus, this construction method is expected to be crucial in pursuing urban regeneration and sustainability through the efficient development of underground spaces.

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Blast loading varies based on the location of the explosion. Furthermore, blast loading can be classified into unconfined explosions and confined explosions. Many studies have evaluated blast resistance performance based on unconfined explosions, focusing on military applications. However, there is a paucity of studies considering confined explosions. Given that confined explosions are significantly different from unconfined explosions, full-scale field experiments are necessary for the development of numerical models. Therefore, in this study, the performance of blast resistance panels was evaluated as a method for reducing explosion pressure in facilities such as underground ammunition storage. Two structures were manufactured using normal-strength and high-strength concrete, and 5.9 kg of TNT was blasted internally. The experimental results confirmed that the maximum acceleration could be reduced by 28.87% and 61.65% in the normal-strength and high-strength concrete structures, respectively, when using a blast resistance panel.