Construction
Many areas of construction faced difficulties, which made for slow going. In order to avoid existing subway lines and other underground structures, nearly the entire line had to be constructed at least 10 meters below street level; in particular, where the line crosses the Neya River and No. 2 Neya River, it reaches over 30 meters below ground (between Shigino and Midoribashi, where it crosses the No. 2 Neya River, it reaches 37 m, the greatest depth of any line in the Osaka subway network). Because of this, it also features a greater number of steep grades compared to existing lines.
The ground stratum the Imazatosuji Line runs through is a "super-soft clay deposit", which is soft and has a high water content; in many instances, bulldozers were useless in excavating the soil. Absorbent cylindrical piles made of chalk were driven in an effort to stabilize the ground, but had relatively little effect.
The northernmost part of the line runs under narrow streets, less than 8 meters across in the case of Daidō-Toyosato – Zuikō Yonchōme. Because of this, the tunnels in this area are bored one atop the other, a method of construction without any other examples in the Osaka subway system. Further south, there is an extremely tight curve between Shimizu and Shimmori-Furuichi (at the Midori 1-chōme intersection), with a radius of 83 meters; this is the tightest curve on any Osaka subway line (excluding inter-line connections and depots). Trains must reduce speed to 30 km/h in this area.
Because of these engineering difficulties, the expected 1/3 cost savings of employing smaller cross-section tunnels and linear motor trains did not emerge; instead, construction costs ballooned to 271.8 billion yen.
Read more about this topic: Imazatosuji Line
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