Bunding is a form of secondary containment designed to catch and hold liquids if a storage tank, drum, or container leaks, overfills, or spills. Think of it as a walled enclosure, usually with an impervious floor, built around a primary storage vessel so that any escaped liquid stays contained rather than spreading into the surrounding environment. It is one of the most common methods used to protect soil, groundwater, and waterways from contamination by oil, fuel, and hazardous chemicals.
How Bunding Works
Every liquid storage setup has a primary container: the tank, drum, or intermediate bulk container (IBC) that holds the substance. Bunding acts as the backup. If the primary container fails for any reason, the bund catches what comes out. The concept is simple, but the engineering matters. A bund must be built from materials that are impervious to whatever liquid it might need to hold, and it needs enough structural strength that it won’t crack or leak under pressure.
In practice, a bund looks like a low-walled compound surrounding one or more tanks. The walls and floor form a sealed basin. For oil tanks, IBCs, and mobile fuel bowsers, a bund is the standard form of secondary containment.
Sizing Requirements
Bunds aren’t arbitrarily sized. The widely applied rule 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, rainwater, or the liquid displaced by other equipment sitting within the bund. If a bund surrounds multiple tanks, the total enclosed capacity is typically capped at 60,000 cubic meters, and individual bunds are recommended for very large tanks.
Wall height also has limits. Guidelines from the UK’s Health and Safety Executive recommend bund walls be at least 0.5 meters tall but no higher than 1.5 meters. Taller walls create safety risks because workers may not be able to escape quickly if the bund fills with liquid or catches fire.
Construction Materials
The most common materials for permanent bunds are reinforced concrete, brick, and stone. Whatever the choice, the material must be completely impervious to the stored substance and strong enough to withstand daily wear, weather, and the hydrostatic pressure of a full spill.
Not all materials hold up equally. Concrete bunds work well for many liquids, but acidic substances can eat through concrete over time if spills are left sitting. Hollow concrete blocks (sometimes called besser blocks) are a poor choice because their porosity and numerous mortar joints create too many potential leak points. Earthen bunds, essentially mounded soil walls, are generally not recommended unless no other option exists, since soil is rarely impervious enough to contain hazardous liquids reliably. The bund floor is just as important as the walls: it needs the same impermeability and chemical resistance, with no damp course that could wick liquid through.
Temporary vs. Permanent Bunding
Permanent bunds, built from concrete, steel, or polyurethane, are the standard for industrial facilities, fuel depots, and warehouses that handle hazardous materials on an ongoing basis. They require minimal replacement, meet strict regulatory standards, and provide the highest level of containment. The trade-off is cost and inflexibility: once poured or welded in place, they don’t move.
Temporary (portable) bunding fills the gap where permanent structures aren’t practical. These include:
- Collapsible bunds: Flexible-walled enclosures that fold flat for transport, commonly used around drum storage or during vehicle maintenance
- Portable floor bunding: Roll-out or snap-together floor sections that create an instant contained area
- Drive-over bunds: Low-profile ramps that vehicles can cross while still maintaining a sealed perimeter for spill control
Construction sites, fuel transfer zones, and mobile workshops are typical settings for temporary bunding. Setup is fast, costs are lower, and the system can be relocated as work shifts. The downside is durability: portable bunds wear out faster and may not satisfy the strictest regulatory requirements for long-term chemical storage.
Regulatory Requirements
Bunding isn’t optional in most jurisdictions where hazardous liquids are stored. In the United States, federal regulations under 40 CFR 267.195 require secondary containment for all new and existing tank systems holding hazardous waste. The rules are specific: the containment system must prevent any migration of waste into soil, groundwater, or surface water at any time. It must be built from materials compatible with the stored waste, placed on a foundation that resists settlement and uplift, and equipped with a leak-detection system capable of identifying a failure within 24 hours.
Any spilled or leaked material, along with accumulated rainwater, must be removed from the bund within 24 hours or as soon as practicable. The UK, Australia, and the EU have comparable requirements, though the exact capacity percentages and inspection schedules vary by region. The 110% rule for bund sizing appears across multiple national standards.
Maintenance and Inspection
A bund that looks intact on the surface can still have hidden cracks, degraded sealant, or chemical damage to its lining. Regular inspection is essential, and the frequency depends on the stored substance, the bund material, and local regulations. Industry standards call for visual inspections on a routine schedule and additional checks whenever the bund undergoes material repairs or alterations.
Beyond visual checks, several testing methods can assess bund integrity more thoroughly: hydrostatic testing (filling the bund with water to check for leaks), ultrasonic testing, radiographic testing, and acoustic emissions testing. Hydrostatic testing is most commonly used on new bunds and after major repairs. Between formal tests, you should look for visible signs of deterioration, pooling liquid inside the bund when no spill has occurred, staining on the exterior walls, and any accumulation of oil or chemicals on the bund floor.
Rainwater management is a practical challenge. Outdoor bunds collect precipitation, and that water has to go somewhere. Leaving it indefinitely reduces available containment capacity, meaning a real spill could overflow the bund. But draining it carelessly risks releasing contaminated water into the environment. Most bund designs include a valve or pump system that allows operators to inspect accumulated water for contamination before releasing it, keeping the bund ready for its actual purpose.
Common Applications
Bunding shows up wherever bulk liquids pose an environmental or safety risk. The most familiar examples are fuel storage depots and heating oil tanks, but the same principle applies to chemical manufacturing plants, paint and solvent warehouses, agricultural chemical storage, and transformer stations that contain insulating oil. Even relatively small operations, like a farm diesel tank or a workshop oil drum, often require some form of bunding under local environmental rules.
In fire safety planning, bunds serve a dual role. They contain not just the stored liquid in a tank failure but also the contaminated water generated when firefighters suppress a blaze. Without bunding, thousands of liters of fire-suppression water mixed with burning fuel or chemicals can flow directly into drains, rivers, and soil, sometimes causing more environmental damage than the fire itself.