Standing precariously on the sill, you swing your hammer and the whole trestle shook, especially nerve wracking doing it in the dark with only fire light to see with.
[A.E. Field and L.J. Robertson, 'The Gap is Bridged', Australian War Memorial MSS0956]
The Burma-Thailand travelled through a region containing many rivers, streams, and gullies. Some 688 bridges were constructed along its length, amounting to about fourteen kilometres of its overall length. Only eight bridges were constructed from steel, one being the famous 'Bridge on the River Kwai' that spans the Kwae Yai at Kanchanaburi. The majority of bridges were constructed from local wood.
Bridges could range from small spans crossing a stream or gully to large multi-levelled structures. The Wampo viaduct hugging the cliffs along the Kwai Noi, for instance, was 200-metres long and 8- to 9-metres high in places.
Wooden 'trestle' bridges were composed of a number of short spans supported by a frame, which was termed the trestle. These were particularly suited to the Burma-Thailand railway as they were simple to construct. Nevertheless, the terrain, the lack of resources and the speed at which the railway was built meant that bridge construction was often a precarious and dangerous task. One bridge, about 3.5 kilometres along the track from Hellfire Pass (Konyu Cutting), fell down three times during construction, prompting Australian prisoners to call it 'Pack of Cards Bridge'.
Bridge construction used local wood, which was vital given the difficulty in transporting materials to construction sites. Ideally, the wood used for bridge construction was teak, as it rotted more slowly compared to other softer woods. However, this was not always possible and parts of bridges were often built with softer, less durable wood.
Prisoners formed work gangs in the jungle cutting down wood and hauling it to worksites. Another group of prisoners would cut this wood into standard sizes. This was dangerous work, and a number of prisoners were killed or injured by falling trees, logs rolling down the hills or by splinters.
Once wood had been readied, the bridge was put together by yet another group of prisoners under the direction of Japanese engineers. Their first task was to lay the foundations. Where the ground was stable, these footings were constructed out of concrete into which the bridge trestles were inserted. Many of these footings can still be seen along the railway.
In places where the soil was soft the bridge foundations were constructed using pile driving. Long timber piles were driven into the ground using a heavy weight suspended by a rope from a timber scaffold. A team of prisoners below pulled the weight up to the desired height, after which it was dropped onto the pile. The weight would then be lifted up again and the process continued until the pile was driven far enough into the ground.
The whole scaffold would then be moved to the next pile in the bridge. Prisoners could spend their whole day lifting the weight, often while standing in the river the bridge was intended to cross.
Once the bridge foundations were laid, the pre-cut timber pieces were assembled at the site. Bamboo scaffolding was created and the heavy beams lifted up using ropes and pulleys. As with many tasks on the railway, muscle power was central to this process.
Work on bridges was dangerous. Workers ran the risk of falling off the tall structures, especially when carrying heavy loads or when the timber was wet and slippery.
Reportedly prisoners attempted to sabotage the bridges they were building, placing termite nests on the bridge timbers, or substituting poor quality wood which would make the bridge unstable.
Almost all the wooden bridges on the railway have disappeared, the notable exception being the Wampo viaduct.