## 6 Conclusions  
This study explored the impacts of different influencing factors on the seismic performance of underwater concrete piers strengthened using the IPCSAM through a quasi-static test and extended parameter analysis. A restoring force model for strengthened piers was ultimately established, and the following conclusions were drawn. 1. As an undrained strengthening technique, the proposed IPCSAM exhibits good seismic strengthening effects on underwater concrete piers. Compared with the unreinforced pier columns, the peak load, ductility coefficient, and initial stiffness of the specimens strengthened using the IPCSAM increased by approximately 83.5-106.4%, 16.3-50.2%, and 83.9-177.3%, respectively, while significantly improving the secant stiffness, energy dissipation per cycle, and equivalent viscous damping. In addition, the bearing capacity, stiffness, and energy dissipation capacity of the strengthened specimen with initial damage could be improved to a near-undamaged state, excluding the ductility. 2. As a filling material, SSAWC can improve the seismic performance of strengthened specimens. The initial self-stress produced by the SSAWC can provide a circumferential preloading effect on the core pier column to improve its bearing, stiffness, energy dissipation, and deformation capacities. Compared with the specimen using AWC, the peak load, ductility coefficient, and initial stiffness of the specimen with a self-stress value of 1.6 MPa increased by 17.3%, 43.5%, and 8.3%, respectively. Moreover, the secant stiffness, energy dissipation per cycle, and equivalent viscous damping were improved. In addition, under the premise of satisfying the bearing capacity design requirements, the self-stress value of the SSAWC should not exceed 2.2 MPa, the strength grade of the SSAWC should be 1-2 grades higher than the concrete strength grade of the core pier column, and the SSAWC should adopt the constructional reinforcement ratio. 3. The LCSS exhibited reliable connection performance and strength in the quasi-static test and remained intact and confined when the specimen was damaged. An increase in the reinforcement ratio of the LCSS from 0.25% to 0.5% increased the peak load, initial stiffness, and ductility coefficient of the strengthened specimen by 9.76%, 16.18%, and 22.24%, respectively. The secant stiffness, energy dissipation per cycle, and equivalent viscous damping were also improved. An appropriate LCSS thickness can effectively improve the seismic performance of strengthened specimens, and the recommended thickness-diameter ratio is 1/10. 4. An increase in the length-diameter ratio of a strengthened pier column can significantly reduce its seismic performance. An appropriate axial compression ratio can improve the bearing capacity, stiffness, ductility, and energy dissipation capacity of strengthened pier columns, and the axial compression ratio should not exceed 0.3. 5. The proposed restoring force model of pier columns strengthened using the IPCSAM has good applicability and can accurately reflect the hysteresis characteristics of the strengthened specimens. For the skeleton curve model, the relative errors between the measured and calculated values of the characteristic points for each strengthened specimen were within 10%, and only 5 of 30 groups of contrast data had an error exceeding 5%. A certain error existed in the initial stiffness between the measured and calculated hysteresis curve models, and the deviation after the yield stage was small. The IPCSAM adopted in this study is an undrained strengthening method with economical, efficient, and shipping traffic-friendly benefits. Moreover, applying SSAWC self-stress and the LCSS constraint can effectively improve the seismic performance of strengthened piers.