Bunding
Bunding, also called a bund wall, is a structure designed to prevent inundation. The area inside the structure is often called the bund. It can be used in reference to dikes, but it is frequently used to describe liquid containment facilities that prevent leaks and spillage from tanks and pipes, though sometimes any barrier is referred to as bunding. Frequently, the liquids in tanks and pipes are toxic, but bunding is used to prevent the liquid from causing damage by force as well as chemically. If a large tank has a catastrophic failure, the liquid alone can cause extensive damage. In 1919, a 15-meter high molasses tank in Boston burst, killing 21 and injuring 150. A few other tanks have failed in a similar manner in the United States, but they have usually resulted in relatively few deaths. However, bunding's main purpose in unoccupied areas is to prevent environmental damage.
If built properly, bunding is large enough and strong enough to contain the contents of an entire tank, though regulations may require it to be up to a third larger. When multiple tanks share a bund, the capacity is based on the largest tank. One of the most common designs for large tanks is a concrete or masonry wall around the tank with a concrete floor. The outside of the wall may be reinforced with an earth berm. Concrete works very well for many liquids, but it is unsuitable for some applications like containing strong acids. Using earth berms for bunding is not recommended for most situations, though liners can be used to decrease permeability. Smaller tanks often use containers made of steel or plastic. The material used depends on cost, the chemical properties of the liquid and its density. Plastic tanks cannot hold very dense liquids at high wall levels. Large, exposed bunding will need a sump pump or some other system to remove precipitation, though it may also be used to transfer spilled liquid into another container. Rainwater must be treated if the liquid being stored is toxic because there may be small amounts of it surrounding the tank. The bund may have a roof to prevent precipitation from getting in, but steps must be taken to provide adequate ventilation when storing flammable liquids. If the wall is over a meter high, it may require a ladder or steps to allow people to escape quickly. Another design uses a channel that drains the liquid to a secondary container.
When the risk of tank failure is not as likely or when it would not cause extensive damage, the bunding may be designed to merely contain small leaks from hoses and valves. This bunding may not be able to contain the entire volume of the tank. Plastic and steel are used, but another common method is making a hump or lip around the perimeter of a concrete floor. Some bunding is temporary, such as short-term chemical storage in the field. A hump or slope type bunding is helpful when vehicles need access to the area. There is also a type of bunding that compresses when a vehicle passes over and expands once it has past.