The most important thing to know is that, by law the drum bund needs to be able to hold at least 25% of the total amount on the bund or 110% of the largest drum on there. This is to ensure a safe work environment and of course to keep the ground unpolluted.
Bunds are there to protect the workplace but also the environment in general. If you have oil product or dangerous goods such as acids or chemicals, you have to protect the workplace but also the ground you work on for the environment. This is of course for you own safety but also required by EU law.