Here is the movie bridge under construction in Sri Lanka (Ceylon), during 1957. This photo also reveals that the river water level is lower than shown in the movie. I am assuming that this is the drier part of the year, for they have not done anything to divert the river flow.
You may notice that the right-hand cantilever piers are inserted into the river bed. To do this, they used a cofferdam.
For those not familiar to what a cofferdam is. This photo ought to be self explanatory.
However, there was no such advanced equipment for the POWs to build the bridge. They had to pile-drive the piers into the ground, and river beds, by hand.
Teams of POWs, at both sides of the trestle, hoisted the heavy weight up and then dropped onto the pier. They did this repeatedly, day and night, for months while building the various bridges along the 415 km railway.
First real bridge (circled), on the river Kwai, is made from timber. The second bridge got constructed and then bombed by RAF. Soon afterwards the timber bridge got completely destroyed too. Afterwards, the second bridge got repaired. They used a different, simpler, design structures (not curved), to replaced the destroyed sections. Bottom photo is how it looks today.
Edited by Tecko, 08 November 2018 - 11:51 PM.