The foundation of the Higashi Shinagawa pier section had to be built on an extremely narrow area of sea located in parallel to and in the proximity of the detour, the existing Route1 (Haneda Line), and Tokyo Monorail. Hence, considering the restrictions on the size of the foundation that will be newly built as part of this renewal project and its impact on the surrounding structures, we had to choose from a limited number of construction methods, thus posing many challenges in the construction.

Steel pipe sheet piles were selected as the material for constructing the foundation of the renewed line as they have a proven success record in the marine environment and help keep the foundation area small. The Higashi Shinagawa Pier section is built on a site where the uppermost layer of the ground comprises of sludge and the thick middle layer comprises of weak cohesive soil, because of which it is difficult to secure sufficient shaft friction when laying the foundation. In order to achieve greater shaft friction, we decided to make the foundation rectangular in shape, which helped to secure a larger surface area even when the available space was small. The plan is to proceed gradually with renewal of phases I and II of the renewal line to finally build a Ramen bridge integrating the inbound and outbound lines. Hence, at the completion of Phase I, the structure was looking poorly balanced compared to how it should look at the end. To tackle this issue, we got rid of the eccentric loads by changing the position of the pillars to support the midpoint of the superstructure. Later, by the completion of phase II, we had successfully minimized the impact of differential settlement by integrating the foundations. As the structure to be built in this construction project is expected to have a useful life of 100 years, the thickness of the steel pipe sheet pile has been increased to 11 to 25 mm in order to allow a 3-mm margin for corrosion.

<Progress of Construction of Steel Pipe Sheet Pile Foundation (Jetting-assisted Sheet Pile Driving)>

Jetting-Assisted Sheet Pile Driving using a hydraulic pile press-in/extraction machine was the method chosen to install the steel pipe sheet pile in a narrow space. As the impact of horizontal displacement of the steel pipe sheet pile is high when the ground material inside the pile is being excavated, a decision was taken to install a 1.5-m thick underground beam at the bottom of the pile cap to add a point of support before excavation and control the displacement. The underground beam inside the steel pipe sheet pile was installed by Jet Mixing Method that can reduce the cost of and number of steps involved in installation by improving a larger area (with larger blade diameters).