###### Abstract
In contrast, the PCSAM uses several precast concrete segments reinforced with basalt FRP grids to form a sleeve ring wrapping the pier via piecewise assembly; the sleeve is then tightened by prestressing high-strength wire ropes wound on the surface of the pier through customized anchorages; finally anti-washout underwater concrete (AWC) is poured into the interspace between the sleeve and pier to achieve undrained and deep-water strengthening [23; 25]. As a prestressed strengthening technique, the PCSAM eliminates stress-strain hysteresis through active strengthening and enables the pier and strengthened aspect to work together, which can significantly improve the seismic performance of components by constraining the core concrete of piers with circumferential pressure. Nevertheless, the PCSAM has certain defects, including a large prestress loss and poor durability of the wire ropes, poor accuracy and connection performance of the sleeves, large strength loss of the filled concrete, and uncertain bonding properties between the filled concrete and segment sleeves [2]. To address these limitations, our research team has proposed an improved PCSAM (IPCSAM) [2], depicted in Figure 1. On the one hand, regarding the design theory and prefabrication technology of the shield tunnel lining segments, the connection of the reinforced segment sleeve is improved by a lining concrete segment sleeve (LCSS), owing to its high precision in prefabrication [26], good joint connection strength and connection effects [27], and good durability [28]. On the other hand, self-stressed anti-washout underwater concrete (SSAWC) is applied as a filling material between the pier column and LCSS to generate accurate initial self-stress inside the filling layer. That is, an additional appropriate expansion agent is used in the SSAWC instead of applying prestress on the outer surface of the pier using wire ropes. This has the following advantages: (a) Circumferential confining pressure is applied to improve the mechanical properties of concrete in the core pier column [2]. (b) The prestress and strength losses of filled concrete are reduced [1; 29]. (c) The bonding strength between the filling layer and adjacent members is increased by reducing the shrinkage effect of the filled concrete [30]. [PERSON] et al. [2] found that the axial compression performance of strengthened piers is effectively improved using the IPCSAM. However, the seismic performance of underwater concrete piers strengthened using the IPCSAM remains to be evaluated in depth. To achieve this, several factors influencing the seismic performance of the strengthened piers must be considered: (a) The thickness, concrete strength, and reinforcement ratio of the LCSS. [PERSON] [23] proposed that increasing the reinforcement ratio of FRP bars embedded in concrete segments improves the seismic performance of strengthened bridge columns, indicating that a high LCSS reinforcement ratio might improve the seismic performance of strengthened piers. (b) The self-stress value, thickness, strength, and reinforcement ratio of the SSAWC. [PERSON] et al. [31; 32; 33; 34] found that setting an appropriate self-stress value can enhance the seismic performances of the concrete-filled steel tubular columns. Moreover, [PERSON] [35] reported that an excessive prestress level could reduce the columns' ultimate displacement and ductility. (c) Length-diameter ratio, axial compression ratio, and initial damage of the pier column. [PERSON] et al. [36; 37; 38; 39; 40; 41] reported that a high axial compression ratio of the pier column could reduce ductility and rapid stiffness degradation. Meanwhile, [PERSON] et al. [18; 42; 43] strengthened pier columns with varying degrees of initial damage using prestressed steel jackets and CFRP, restoring the seismic performance of damaged piers to intact conditions or better.