## 1 Introduction  
During an earthquake, the characteristics of the seafloor exhibit a significant reduction in strength and stiffness, accompanied by a substantial increase in pore water pressure. The reduced mutual support among solid soil particles typically results in damage to underwater foundations, such as collapse and significant deformation. This often comes with significant economic losses and casualties. For instance, in the 2011 Great East Japan Earthquake, the foundations of thousands of structures were damaged [1]. In earthquakes such as the 1983 Central Japan Sea Earthquake [2], the 1964 Niigata Earthquake [3], and the 1995 Southern Hyogo Prefecture Earthquake [4], among others, many foundations experienced significant deformation and collapse. Over the past few decades, extensive research has been conducted on seismic-related damage to soil and structures [5; 6; 7; 8]. Piles are widely used to mitigate the impact of dynamic loads on structures [9; 10; 11; 12; 13]. Since shear seismic waves carry the most seismic energy, most existing studies on pile-soil interaction assume that it is induced by vertically propagating S-waves [14; 15; 16; 17]. Under seismic loads, the deformation of most structures is caused by lateral soil movement beneath the structure [18]. Therefore, lateral dynamic load tests on piles have been extensively researched. Various methods, including empirical and analytical approaches [19; 20; 21], numerical analyses [22; 23; 24], and physical experiments [25; 26], have been widely employed in the dynamic load analysis of piles. These methods effectively analyze pile-soil interface interactions, non-uniform settlement of piles, the influence of soil liquefaction, and the impact of changes in saturation pore water pressure on pile strength. Most of the foundation above strengthening studies involve lateral cyclic loads or seismic tests simulated on shaking tables, effectively addressing pile-soil movement issues under seismic conditions [27]. However, the mechanical behavior of piles essentially represents a process of energy changes. Further research is needed to understand the relationship between energy dissipation and plastic deformation during the seismic response of piles. Underwater piled foundations are often reinforced using sand compaction piles (SCP). The construction of sand compaction piles involves the insertion of sand into the ground through casings and compaction by means of vibration and impact techniques [28; 29; 30; 31]. SCP, combined with the surrounding soil, forms a composite stratum, creating drainage pathways that reduce pore water pressure, thereby enhancing densification effects [32; 33]. In seismic safety assessments, most research on sand compaction piles (SCP) primarily focuses on significant lateral displacements caused by earthquakes, which can lead to structural damage. Alternatively, researchers aim to enhance seismic performance by considering factors such as the replacement rate of the piled foundation, slenderness ratio, and arrangement methods [34; 35; 36]. In order to explore the seismic response of piled foundation structures, a significant number of seismic model tests have been conducted, including shake table tests [37; 38] and centrifuge tests [39; 40]. Furthermore, the finite element analysis method is widely employed for the investigation of the seismic behavior of the pile-structure-soil [41; 42; 43]. Most such studies primarily emphasize the bending moment and acceleration responses of piles. However, they often do not take into account the impact of pile group arrangements on seismic performance assessments. There is a lack of research examining the relationship between pile arrangement and the extent of pile plastic regions. This paper investigates the mechanical behavior of the pile-soil interface in the sand compaction pile (SCP)-immersed tunnel-water pressure system (SIS system) using physical experiments. Numerical analysis is employed to validate the physical experiments.