Abstract
The 844 M long double-track Hikawadai tunnel section of the No. 8 line of the Teito Rapid Transit Authority is being driven by a 10 M o/d shield and lined with 9.8 M o/d concrete segments. The line incorporates curves of 800 M and 1500 M radius. The shield passes under the Shakujii river bed with an overburden of 8 M and then passes beneath residential areas where the overburden is 12-18 M. The upper portion of the tunnel lies in a sand and gravel layer known as Tokyo gravel stratum. The sand layer at the crown is loose and liable to collapse. The large-scale slurry shield was chosen for the double-track tunnel excavation to overcome problems of sand instability and drainage. The success of the largest shield used in Japan is said to be due to exhaustive soil investigation, careful design and manufacture of equipment, and close slurry control.
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