Political construction project: Russia deliberately lowered the official seismic hazard rating for the area to justify building the bridge on highly unstable soil with its own fault system.

That’s not necessarily a bad move, if you’re Russia and seeking to annex Crimea. That is, even if the bridge will ultimately go down, having the transport capacity may be critical in the short term. Right now, Ukraine has knocked out the ferries. If Russia didn’t have the Kerch Strait rail and road bridge — even a flawed, decaying bridge — it’d have a lot less access to Crimea.

Before the Trans-Siberian Railroad was complete in its present form, for a period of time, Imperial Russia built temporary tracks in winter over waterways. Impermanent, but if it was critical enough to get the transport that it provided now, being disposable was acceptable.

https://en.wikipedia.org/wiki/Circum–Baikal_railway

The Circum–Baikal railway (Russian: Кругобайка́льская желе́зная доро́га or Кругобайка́лка, abbreviated “КБЖД”) is a historical railway in the Irkutsk region of Russia. It runs along the Northern shore of the Southern extremity of Lake Baikal from the town of Slyudyanka to the Baikal settlement. Until the middle of the 20th century the Circum–Baikal railway was part of the main line of Trans–Siberian Railway; later on, however, a duplicate section of the railway was built.

With the purpose of establishing a through railway connection, before the Circum-Baikal was finished, it was decided to link the shores of the lake with a train ferry. Trains were carried on the special ice breaker-ferry SS Baikal which had three parallel tracks on its train deck. Another, smaller icebreaker-ferry, the “Angara”, was also built which carried passengers and goods, but not trains. In the cold winter of 1903/04 when the icebreakers were not strong enough to break the ice, a railway line was laid on the ice, and railway wagons were pulled by draft animals.