A bund wall is a barrier built around a storage tank or container to catch and hold any liquid that leaks, spills, or escapes. Typically made of concrete or compacted earth, it acts as a second line of defense, preventing hazardous materials from spreading into the surrounding environment, contaminating waterways, or creating fire hazards. You’ll find bund walls at oil depots, chemical plants, fuel stations, farms, and anywhere bulk liquids are stored.
The concept is simple: if the tank fails, everything it holds stays inside the walled enclosure instead of flowing across the ground. This type of protection is known as secondary containment, and in most countries it’s a legal requirement for facilities storing oil, fuel, or hazardous chemicals.
How a Bund Wall Works
A bund wall surrounds a tank on all sides, creating a shallow basin underneath and around it. The floor of this basin is sealed so liquid can’t soak into the ground. If the tank cracks, a pipe joint fails, or someone overfills it, the spilled liquid drops into the bunded area and stays there until it can be safely pumped out or cleaned up.
This prevents several serious problems at once. Leaked fuel or chemicals can’t reach storm drains, rivers, or groundwater. Flammable liquids can’t flow toward ignition sources or buildings. And the spill stays in one known location rather than spreading across a site, which makes cleanup far faster and cheaper. For a catastrophic failure where an entire tank empties at once, the bund is sized to hold all of that liquid.
Sizing and Capacity Rules
The most important design question for any bund is: how much liquid can it hold? Regulations vary by country, but the most common standard is that a bund must hold at least 110% of the capacity of the largest single tank inside it. That extra 10% accounts for the volume of firefighting foam that might be sprayed in during an emergency. In the UK, the Health and Safety Executive recommends this 110% figure, with bund walls at least 0.5 meters (about 20 inches) tall.
In the United States, the EPA’s Spill Prevention, Control, and Countermeasure (SPCC) rules require secondary containment sized for the entire capacity of the largest single container, plus enough extra height (called freeboard) to hold rainfall without overtopping. When multiple tanks sit inside one bund, the calculation is based on the single largest tank, not the combined total of all tanks, though some jurisdictions require additional capacity.
Large tanks generally get their own individual bund. Smaller containers, like drums or intermediate bulk containers (IBCs), can share a bunded area or sit on spill pallets, which are essentially miniature portable bunds.
Construction Materials
Most permanent bund walls are built from reinforced concrete, masonry block, or compacted earth. Concrete is the most common choice for industrial sites because it’s durable, impermeable, and resistant to most chemicals. Earth bunds (sometimes called berms) work well for very large outdoor tank farms where concrete would be impractical, though they need a sealed liner to prevent liquid from soaking through the soil.
The bund floor matters just as much as the walls. It needs to be impermeable, properly sloped, and chemically compatible with whatever is being stored. If a facility stores acids or other corrosive substances, the concrete may need a specialist coating or liner to prevent the spilled chemical from eating through the floor before it can be cleaned up.
For temporary or mobile operations, like oilfield sites, modular bund systems made from heavy-duty PVC fabric or steel panels can be assembled quickly and reconfigured as needs change. Steel spill pallets are preferred for flammable liquids, since polyethylene pallets can melt under extreme heat. Flexible fabric bunds are more economical and work well as short-term solutions for drum or tank storage.
Managing Rainwater Inside the Bund
One of the most overlooked challenges with bund walls is dealing with rain. Because the bund is designed to be a sealed basin, rainwater collects inside it and has nowhere to go. If that water isn’t removed, it gradually fills the available containment volume, which means the bund can’t hold as much in an actual spill. A bund half-full of rainwater is a bund that’s half as effective.
The standard approach is to install a collection sump, a low point in the bund floor where liquids gather so they can be pumped out. The floor is graded to slope toward this sump. Critically, the sump should never be connected directly to stormwater drains or sewers, because any water sitting inside a bund may be contaminated with traces of whatever the tank holds. Pump controls are placed outside the bunded area for safety.
Accumulated rainwater needs to be tested or visually inspected before disposal. If it’s clean, it can sometimes be reused on site or discharged to a sewer with the appropriate trade waste approval. If it’s contaminated, it has to be collected by a licensed waste contractor. Installing a drain valve in the bund wall is generally prohibited because it creates a potential failure point: someone could leave it open, or it could leak, defeating the entire purpose of the containment.
Inspection and Maintenance
A bund wall only works if it’s structurally sound and leak-free. New bund installations are typically tested by filling the entire enclosure with water (with the storage tank still in place) and checking for any seepage. Any leak in a new bund is considered unacceptable and must be repaired before the system goes into service.
Over time, bunds can develop problems. Concrete cracks from thermal expansion, settling, or the weight of tank support structures pressing on the base. Masonry walls can lose mortar or develop cracks at the corners where stress concentrates. Earth bunds are vulnerable to plant roots, which create pathways for liquid to seep through the wall. Even small vegetation growth on or near a bund wall is a warning sign worth investigating.
Regular visual inspections should check for:
- Cracks in walls, floors, and especially corners
- Missing or damaged blocks in masonry construction
- Vegetation growing on or through earth bunds
- Tank support damage where saddles or cradles press on the bund floor
- Sealant deterioration around pipe penetrations through the bund wall
Older bunds that haven’t been tested in years should be assessed by a qualified inspector before being hydrostatically tested, since filling a weakened structure with water could cause it to fail. For double-skinned tanks, which are a form of built-in secondary containment, leak detection between the inner and outer walls can use vacuum monitoring, pressure sensors, or liquid-level monitors.
Where Bund Walls Are Required
Any facility that stores oil, fuel, or hazardous chemicals in bulk typically needs bunded containment. In the U.S., the SPCC regulation applies to facilities that store more than 1,320 gallons of oil aboveground in containers of 55 gallons or larger. In the UK and EU, similar requirements fall under environmental permitting and the COMAH (Control of Major Accident Hazards) regulations for larger sites.
Common settings include fuel storage depots, chemical manufacturing plants, oil refineries, agricultural fuel tanks, transformer stations (which contain insulating oil), and warehouses storing drums of hazardous liquids. Even a single heating oil tank at a commercial building may require bunding depending on its size and proximity to drains or watercourses. The principle is always the same: if a leak could cause environmental damage, fire, or contamination of water systems, secondary containment is needed.