## 1 Introduction  
Rapid population growth necessitates more urban building. The limited land in cities can be more quickly and less expensively returned to productive use if existing buildings and foundations could be expanded and reused rather than completely reconstructed or replaced [1; 2; 3]. The vertical extension of existing buildings is one way of reusing existing foundations to improve and increase the utilization of urban space. The South Korean government published guidelines that older existing buildings taller than 12 stories can be extended vertically with an additional two or three stories [4]. However, extensions are likely to impose additional loads on the existing foundations which will exceed the allowable bearing capacity, resulting in excessive overall settlement and differential settlement. Therefore, underpinning with new piles is required to secure the safety and stability of the structure. Various underpinning technologies were described by researchers [5; 6; 7; 8; 9; 10], among which the micropilling technique can be considered one of the most common methods for underpinning. Micropiles are drilled and grouted steel reinforced piles with a diameter of less than 300 mm. Micropiles can be installed in arbitrary directions and the installation causes minimal disturbance to adjacent structures. Therefore, they are widely used in underpinning [11; 12; 13; 14]. [PERSON] and [PERSON] conducted a full-scale field test to investigate the bearing behavior of micropiles under compressive and tensile conditions [15]. [PERSON] et al. reported on micropile behavior for the stabilization of a high railway embankment by model tests and numerical analysis [16]. [PERSON] et al. investigated the reinforcing effect of micropiles on the bearing capacity's mechanism of spread footings [17]. One of the most important issues in existing pile foundation underpinning by micropiles is the load sharing between the micropiles and existing piles. The Korea Highway Bridge Design Standardoffers guidance for foundation underpinning applicable to the vertical extension for high-rise buildings, stating that the load of the existing superstructure is taken only by the existing piles and that the additional loads are shared equally by the existing and underpinning piles [18]. The Chinese Technical Code for Improvement of Soil and Foundation of Existing Buildings states that the additional loads from a vertical extension should be taken only by the underpinning piles [19]. However, there is a lack of clear guidelines and publications about load sharing between existing and underpinning piles in the foundations of vertically extended buildings. For shallow foundations underpinning with piles, researchers have only investigated pile-soil interaction. [PERSON] and [PERSON] proposed a simplified method of the underpinning piles' design for calculating the settlement reduction of the foundation and load sharing of the underpinning piles and subsoil [20]. [PERSON] and [PERSON] provided a simplified design procedure for the shallow foundation underpinning with micropile based on field tests [21]. Furthermore, because of limited field data, [PERSON] and [PERSON] conducted a parametric study on the investigation of load transfer mechanism between micropiles and soil during initial and additional loading based on the field results by [PERSON] and [PERSON] [22]. However, the different axial stiffness of the existing and underpinning micropiles complicates the mechanism for sharing loads among the existing piles, soil, and underpinning micropiles. The influence of micropiles on the underpinning of piled foundations has not been previously reported. Therefore, this study aims to evaluate the effect of the underpinning micropiles' axial stiffness on the load sharing between them and the existing piles when subjected to additional loads during vertical building extension.