Embankments were the most common type of engineering task along the railway. They could range from small earthworks levelling out undulating terrain to massive artificial hills rising out of the jungle. These structures were necessary to maintain a gentle gradient along the railway, as steam trains could only climb or descend gentle slopes. Embankments were used where the level of the ground had to be raised to accommodate the railway but bridges were impractical or unnecessary.
As with all work on the railway, the first step in the construction of an embankment was to clear the jungle along the path the railway was to take. The width of the land cleared depended on the size of the embankment to be constructed. The larger the embankment, the wider its base would be. One embankment, around two kilometres from Hellfire Pass, was seven metres tall and thus required a huge base to be cleared.
To speed progress on the railway, Japanese surveyors erected timber guide frames along the route of each embankment. The guards could then direct construction without engineers or surveyors, freeing these men for more complex tasks.
The embankments themselves were constructed from rock and soil. As with the construction of bridges, the Japanese used local materials in the embankments. In some places, the excavation of cuttings provided material for the construction of nearby embankments.
However, in other places rock and soil had to be laboriously carried to the construction site by prisoners and rōmusha. In these cases, one group of labourers would be engaged in digging using basic hand tools such as shovels, picks and hoes (chunkels). These tools were basic, poor in quality and often broke.
Another group of labourers would carry the material to the site of the embankment. These men would use baskets or a sack spread between two bamboo poles, called a tanka. The journey from the quarry to the embankment differed from site to site, but could be extremely arduous, especially as the embankment grew in height.
During the wet season, the steep sides of the embankments became slippery and unstable making the trek to the top gruelling for the tired labourers. Once at the top they had to stamp the earth down and return for another trip.
Initially, the daily quota for each man working on embankments was to move one cubic metre of earth. As pressure increased to complete the railway, so too did the daily quota. Eventually the quota reached three cubic metres a day, which would have been a challenging task even for fit men.
It is a tribute to their construction that many embankments remain relatively intact along the Thai–Burma railway today. Unlike wooden bridges, many of which collapsed or rotted away after the railway fell into disuse after the war, embankments even when hidden now form part of the physical and cultural landscape.