###### Abstract  
Vital underground structures such as sewers, power transmission lines, subways, and underpasses are potentially vulnerable to adverse effects from aboveground construction. In this study, the influence of pile installation on nearby existing tunnels was investigated. Both a laboratory model test and finite-element numerical analysis were conducted. Twelve different combinations of horizontal and vertical offsets between the pile and the tunnel were investigated. Different surcharge loads (allowable and ultimate) were also considered. In this way, the appropriate separation distance between the existing tunnel and the piles was established for sandy, medium-compaction soil. Although this study considers simple ground conditions, it facilitates safe construction by confirming the appropriate separation distance and comparing the areas that cannot be penetrated by the structures of each standard.  
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[PERSON] et al. [4] numerically analyzed the deformation of a tunnel with a vertical pile-tunnel offset of 3.0 Dt (where Dt is the outer diameter of the tunnel) at the top of the tunnel, varying the horizontal offset from 0.5 to 4.5 m under soft-clay and stiff-clay soil conditions. They found that the positions of the pile tips and the stiffness of the ground layer are the main factors determining the deformation of the tunnel: when the horizontal pile-tunnel offset was 0.5 m, the deformation of the tunnel increased to \(-\)2.0 Dt in stiff clay and \(-\)4.0 Dt in soft clay. Several other studies on the mutual behavior of pile, tunnel, and ground have been reported, as research continues into safer construction in complex urban areas [5; 6; 7; 8].  
Numerical analysis is widely used in the research of tunnels and piles, because it can apply various conditions that cannot be otherwise applied in the field. [PERSON] et al. [9] studied the effect of competent caliche layers on the capacity measurement of axially loaded drilled shafts using the Osterberg test. In this study, Osterberg test data were used to calibrate the numerical model using Plaxis, a finite element analysis software. [PERSON] et al. [10] studied the interactive behavior of a shielded TBM tunnel and a single pile by 3D finite element numerical analysis and observed that the behavior of the pile was significantly affected by the relative position of the tunnel and the pile.  
Various countries and institutions have established standard safety zones for construction around tunnels (Table 1). The London Underground Limited (LUL) in the UK proposed a clearance of 3 m for bored piles and 15 m for driven piles installed around a tunnel. The Japanese Society of Soil Mechanics and Foundation Engineering has set a horizontal protection width of 0.5 m around tunnels, as well as a 5 m vertical safety zone [11]. In Korea, the Seoul Metropolitan Rapid Transit Corporation has designated 4-6 m from the outside of the tunnel as a non-construction zone and a distance of up to twice the tunnel diameter (2.0 D) as a zone where partial construction is possible, depending on adjacent structures, surcharge loads, and soil conditions [12]. As shown in Table 1, Busan City, Korea, established five zones during the construction of Busan Subway Line 1 (A: area that cannot be penetrated by structures; B: area in which only temporary equipment can be installed during construction; C: area in which soil-arresting work can be performed; D: area where three ground floors and one underground floor, or five or less underground floors, are allowed; E: no restriction). Table 1 summarizes the areas that cannot be penetrated by structures in each country.  
In this study, the effect of pile installation, according to the offset, was studied using a model test and a numerical analysis. The results obtained were compared with domestic and foreign standards to determine whether the current standards are appropriate.  
Figure 1: South Korean infrastructure sectors, with percentage of each sector constructed over 50 years ago [1].