# Application Ranges of EPB Shield TBM in Weathered Granite Soil: A Laboratory Scale Study
###### Abstract  
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. soil conditioning; earth pressure balance shield; weathered granite soil; foam; polymer; excavation +
Footnote †: journal: Applied Sciences  
chamber from the tunnel face when tunneling below the groundwater level. [PERSON] [3] proposed a compressibility value of 1.9%/0.5 bar or above for conditioned soil so that the face support pressure will be applied uniformly to the tunnel face in the working chamber and excavated materials will be efficiently extracted from the chamber through screw conveyors. In addition, many other studies were conducted recently, either to find optimal soil conditioning parameters or to assess the effectiveness of soil conditioning and/or EPB TBM performance [7; 8; 9]. [PERSON] and [PERSON] [1] proposed choosing appropriate soil conditioning agents based on the particle-size gradation curves in the coarse ground shown in Figure 1; however, we would like to emphasize that the appropriate mixing ratio of the soil conditioning agents might also depend on the characteristics of the soil itself. The characteristics of weathered granite soil with rock origin of granite or granite graiss are different from those of sand and clay. The generally understood characteristics of this soil are as follows [10]. First, the particle-size gradation of the weathered granite soil has a wide spectrum of coarse to fine particles depending on the rock origin and weathering process; the characteristics of these soils are unique, neither sand nor clay. Second, the particle-crushing characteristics of the weathered granite soil appear to be more dominant than those of sand [11; 12; 13]. As such, the particle-size gradation curve of weathered granite soil changes after soil conditioning; in particular, the soil particle percentage that passes through a #200 sieve increases after soil conditioning. Third, weathered granite soil has a complicated relationship with water that depends on the water content. The natural water content of the saturated granite soils (below the groundwater table) on the Korean peninsula are within the range of 11-21% in most cases [14]. Particle-crushing capacity increases if water is added to granite soils, increasing the compressibility and decreasing the shear strength [12]. [PERSON] et al.