In recent years, there has been increasing demand to reduce costs for public works in Japan. Correspondingly, great efforts have been directed toward using and upgrading existing infrastructures efficiently. In this context, the authors have been studying methods for enhancement and improvement of existing port facilities.
　Because the caisson-type quay wall is one of the major types used in Japan, it is desirable that this type quay wall have more front-water depth to better accommodate the larger and larger vessels coming alongside. A method often employed for this improvement is to construct a new pier front onto the caisson quay wall. It is impossible, however, to adopt this method for a caisson quay wall when there is not enough frontal space: thus, a new improvement method applicable to such situations is being developed in the collaborative work between the Port and Airport Research Institute and the Japan Dredging and Reclamation Engineering Association. In the new method, a rubble mound beneath a caisson is solidified then cut to increase the front-water depth of the caisson.
　Several series of model tests and numerical analyses related to the design and construction of the new method were conducted. The results declared the feasibility of the new method. This paper presents the results of the studies and suggests a guide lines for the new construction method in accordance with the results.