For a plaza located in a new sub-center of the Greater Tokyo Area, OHTORI, PWWJ (Peter Walker William Johnson Partners), and NTT Urban Development Company created a design that metaphorically borrows a square of forest from the nearby Hikawa Shrine and places it on top of a shopping center to create a green roof that connects people to adjacent buildings, an arena, and the train station.
The absolutely flat plaza sits above a continuous 1.5 meters of specially designed soil and a custom-designed structural-support system in which 220 matched Zelkova trees are planted. The trees align with the structural grid of the building below. Rainwater infiltrates the plaza through metal grating, down to the earth layer, where it is collected and drained. The expansive soil volume encourages root growth, which make the trees more stable, although the necessarily sandy nature of the constructed soil provided less support than soil on the ground would have. To alleviate this problem OHTORI Consultants developed a subsurface mechanical system that allows the trees to withstand typhoon winds.
The plaza is designed as a multi-functional space: During the day, skylights on the plaza deliver sunlight to the spaces below, which at night illuminate the forest with their artificial light. The plaza is used for music and dance performances and is flooded in winter for skating.