Why are most high-speed railways built on bridges to save land?

Not only to save land, but also to ensure the smoothness of high-speed rail tracks, especially in areas with a lot of rainfall.

For high-speed railways, which generally exceed 200 kilometers per hour, the unevenness of the track can lead to bumps in train operation at best, and even major accidents such as derailments. For the subgrade of piling soil, the tilting of high-speed trains will lead to partial settlement of the subgrade, especially after heavy rain, the subgrade will become soft and more prone to settlement. This will cause slight deformation of the rails laid on it, forming a safety hazard. The bridge subgrade of the high-speed railway will be very deep, and the settlement is smaller than that of the pile of soil.

For example, the world's first high-speed railway, Japan's Tokaido Shinkansen, is deeply troubled by this. Of the 515 km from Tokyo to Shin-Osaka, 280 km (53%) of the Tokaido Shinkansen, which opened in 1964, is made of mud subgrade, and only 22% is viaduct. Moreover, due to the difficulty of land acquisition, the route between stations deliberately bypassed densely populated areas when planning, and many sections were built on relatively soft foundations. However, there was not a lot of precipitation on the Tokaido Shinkansen route, which led to the fact that the Tokaido Shinkansen continued to carry out roadbed reinforcement construction from its opening in 1964 to 2003.

However, even so, the Tokaido Shinkansen is still vulnerable to heavy rain, and JR Central has to install a large number of rain observation equipment and roadbed monitoring equipment along the line, and suspend the operation of the Tokaido Shinkansen in case of heavy rain. According to the standards released in 2022, JR Tokai will restrict operations as long as the observed rainfall reaches one of the following four indicators: hourly rainfall of 60mm or more, hourly rainfall of 40mm or more + continuous rainfall of 150mm or more within 24 hours, rainfall of 300mm or more within 24 hours + precipitation of 2mm or more within 10 minutes, and soil rainfall index exceeding the set value. Therefore, during the rainy summer season, the Tokaido Shinkansen can often be delayed or even suspended, and even now there is no way to completely solve this problem.

At that time, the Tokaido Shinkansen used a large number of soil subgrades, which may have cost considerations, but after counting the subsequent maintenance costs and losses caused by delays, it may cost more money. Subsequent constructionTohoku ShinkansenObviously, this lesson has been learned, only 5% of the 717km from Tokyo to Morioka is piled with dirt roadbed, and the proportion of viaducts has reached 56%. Now I haven't heard much about the Tohoku Shinkansen being delayed due to heavy rain.