To increase the passive resistance of earth retaining structure, such as a quay wall, the quantitative evaluation of the improvement effect is important in the practical design. In particular, when the front of the steel pipe sheet pile is improved in the normally consolidated clayey ground to increase the passive resistance to earth pressure from the ground behind the quay wall, there are many unclear points about the mechanism of passive resistance and the relationship between the improvement range and the improvement effect. A simple design method using a beam-spring model can be used. However, this method is designed for sandy soil, and its applicability to viscous soil is not yet confirmed. In this study, the applicability of the design method using the beam-spring model to the solidification and improvement of an existing gravity-type quay wall using steel pipe sheet piles is examined using centrifuge model experiments.