<p>Surcharge loading often induces excessive lateral displacement of soft soil and adjacent piles, necessitating the development of effective deformation control strategies. This study aims to investigate the influence of surcharge loading on adjacent piles and evaluate the efficacy of the capsule expansion technology (CET) as a lateral deformation control measure through field trials. Meanwhile, numerical simulations were also conducted to further elucidate the interaction mechanisms of the soil–piles–capsules, as well as the deformation control mechanisms associated with capsule expansion. The results indicate that the horizontal deformation of the piles displayed an inverted triangular distribution, closely resembling the deformation pattern of the surrounding soil under surcharge loading. During tube-a-manchette grouting, the injection of cement-based slurry into the soil resulted in undirected splitting and expansion, which failed to produce significant stress concentrations on the pile shaft, thus limiting its effectiveness in mitigating pile deformation. Conversely, CET treatment effectively reduced the maximum horizontal displacement to 10.5 mm, achieving a control efficiency of 56.1%. The capsule expansion could effectively squeeze and push the surrounding soil, inducing additional stress on the adjacent pile which is significantly greater than that caused by surcharge loading, ultimately achieving controlled pile deformation. Furthermore, surcharge-induced lateral stresses were found to increase pile deflection and bending moments. Following CET treatment, the maximum bending moment at a depth of -8 m was reduced by 43.9%. These findings confirm that CET is a promising technique for mitigating pile deformation induced by surcharge loading.</p>