Soil conditioning is a key factor in increasing tunnel face stability and extraction efficiency of excavated soil when excavating tunnels using an earth pressure balance (EPB) shield tunnel boring machine (TBM). Weathered granite soil, which is abundant in the Korean Peninsula (also in Japan, Hong Kong, and Singapore), has different characteristics than sand and clay; it also has particle-crushing characteristics. Conditioning agents were mixed with weathered granite soils of different individual particle-size gradations, and three characteristics (workability, permeability, and compressibility) were evaluated to find an optimal conditioning method. The lower and upper bounds of the water content that are needed for a well-functioning EPB shield TBM were also proposed. Through a trial-and-error experimental analysis, it was confirmed that soil conditioning using foam only was possible when the water content was controlled within the allowable range, that is, between the upper and lower bounds; when water content exceeded the upper bound, soil conditioning with solidification agents was needed along with foam. By taking advantage of the particle-crushing characteristics of the weathered granite soil, it was feasible to adopt the EPB shield TBM even when the soil was extremely coarse and cohesionless by conditioning with polymer slurries along with foam. Finally, the application ranges of EPB shield TBM in weathered granite soil were proposed; the newly proposed ranges are wider and expanded to coarser zones compared with those proposed so far.

Prestressed soil-nail system has two reinforcing components: steel bar and PC strands. The steel bar with relatively less elongation yields earlier than PC strands. Thus, yield displacements of these two components should be matched to maximize the design load (capacity) of prestressed soil-nail. To achieve this, PC strands need to be prestressed before applying pullout load. In this study, load transfer mechanisms of soil-nail and prestressed soil-nail were determined based on skin friction theory and load transfer theory. The load transfer was derived analytically based on the assumption that skin friction at the interface was fully mobilized. It was then compared with results from field pullout tests performed to identify in-situ load transfer mechanism. Additionally, optimum prestress level required to maximize the pullout loading capacity was evaluated and compared with those obtained from field tests.