Bunding, also called a bund wall, is a constructed retaining wall around storage "where potentially polluting substances are handled, processed or stored, for the purposes of containing any unintended escape of material from that area until such time as remedial action can be taken."
The term can also refer 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 these tanks and pipes are toxic, and bunding is used to prevent the liquid from causing damage (either by force or its chemistry). If a large tank has a catastrophic failure, the liquid alone can cause extensive 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 passed.
Bunding is a legal requirement in many countries particularly around tanks, storage vessels and other plant that contain liquids which may be dangerous or hazardous to the environment.
Particular examples which receive specific attention in the UK, the rest of Europe and the USA are oil and fuel storage tanks and transformers at electricity sub-stations which are filled with oil for cooling and insulation purposes.
It is reasonably easy to construct a "water-tight" bund around the base of a tank or vessel. A concrete base and a sealed wall of masonry, brickwork, concrete or even prefabricated steel provides the holding capacity.
In the UK commercial installations exceeding 200 litres and domestic installations exceeding 2500 litres require a bunded tank to comply with Environment Agency 'control of pollution regulations'.
Almost all regulations require a holding capacity of 110% of the largest tank within the bund or 25% of the total capacity of all the tanks within the bund whichever is the greatest. In addition further guidelines in some countries (e.g. the UK) recommend additional measures such as providing sufficient "freeboard" or height of wall above the maximum holding capacity to accommodate dynamic factors such as surge in situations of major tank failure or storm driven waves in larger bunds. As a rule (and unless specific local laws prevail) most operators work to the 110% capacity guide.
As noted above, electricity sub-station transformers contain significant amounts of oil. A 110kVA transformer may have up to 40,000 litres of cooling/insulating oil contained within the body of the transformer and its associated coiling radiators and storage tanks. Unlike ordinary fuel storage tanks, these are complex structures, with a higher propensity for leakage of the oil. Using the UK as an example, as electricity industry privatisation took place in the 1980s and 1990s the new electricity companies were made aware of their environmental responsibilities. Most area and national companies realised that they had several thousand transformers, many of which had been leaking into the ground below them for many years. The companies embarked on an upgrading programme involving the construction of "water-tight" bunds to retain any oil leakage and to prevent further pollution and contamination.
They immediately encountered the problem of water build-up from rain being retained by the now "water-tight" bund; the unwanted rain-water reduces the holding capacity of the bund. Once the water level reaches more than 10% of the holding capacity of the bund, it is no longer fit for purpose and the water must be removed. The water is also likely to be, at best, moderately contaminated with a small film of oil on the top of it or, at worst, substantially contaminated by a thick layer of oil. This is worse on older, leakier transformers. This also can apply to any oil storage tank
Oil floats on water and, if still clean enough to see through, has a different refractive index than the water below, making the oil/water interface difficult to judge. This makes manual pumping difficult and unsafe. Removing the entire contents for disposal as hazardous waste is expensive and environmentally unacceptable. However, in the UK at least, the latest regulations require some formal method to be put in place for the removal of the rainwater. One of the systems recommended is an automatic pump system which is able to discriminate between oil and water. A good system should work continuously and automatically and must fail to safety (e.g. not pumping). It should also provide alarms for conditions such as high water (indicative of pump or system failure) and high oil to warn that action to skim off the waste oil should now be done. These automatic pump systems are usually referred to as "BundGuards".
In the UK, manmade earthwork structures are sometimes constructed around housing developments, especially near industrial sites. The mounds of earth with vegetation (normally shrubs and trees) diminish the noise from the industrial site. The bunds then create a more natural landscape instead of thick or high barriers. They can be built in curved or sinuous forms, depending on the landscape. Bunds are also used to protect housing from noisy transport routes such as motorways and railways.
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. Bund wall failures have occurred in the UK, such as at Warrington, Cheshire in 1994. A polypropylene tank holding about 30 tonnes of 40% aqueous caustic soda sprang a leak about halfway down the main wall. The corrosive fluid jetted out over the bund wall, causing great damage to the surrounding factory and adjacent premises. The bunding at Buncefield also failed to contain petrol and other fluids.