Several significant design and construction issues related to geotechnical engineering of the Trans-Tokyo Bay Highway project are described. They are a) large scale improvement of existing soft clay deposits by in-situ cement mixing, controlling the strength of cement-mixed soft clay~ b) construction of large offshore embankments by using a slurry of cement-mixed sand at the ramp sections, also controlling the strength, and dry cement-mixed sand at a flat place of the embankment~ c) construction of shield tunnels with the world''s largest diameter in successive stiff cementmixed soil and very soft clay at a very shallow depth and underground connection of shield machines with the help of the ground freezing technique~ and d) construction of a huge offshore diaphragm wall of 98 m in internal diameter and 119 m in height. A new method to determine the design strength of cement-mixed soil, which was adopted for the fust time in this project, and a serious groundwater seepage accident in the ground inside the diaphragm wall during the construction of the Kawasaki man-made island are also described.


The Trans-Tokyo Bay (TTB) Highway is a 15.1 kIn long toll highway, which links Kawasaki city with Kisarazu city across the centre of Tokyo Bay (Fig. la) (Uchida et al., 1996~ JSCE et al., 1996~ Tatsuoka et al., 1997). The structure comprises two 9.5 kIn-long shield tunnels which start from an embankment at the Ukishima Access on the Kawasaki side, pass through the Kawasaki man-made island and end at the Kisarazu man-made island, followed by a 4.4 kIn-long bridge extending to Kisarazu city (Fig. lb). Along the route, the largest seabed depth is about 30 m and the ground condition is generally poor (Fig. 2). In August 1976, the Japan Highway Public Corporation took over the survey work.

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