Double walled underground storage tanks (USTs) have become the industry standard for safe, reliable, and environmentally responsible storage of fuels and hazardous liquids. As regulatory expectations increase and land values rise, businesses can no longer risk leaks, soil contamination, or costly downtime. In this guide, we present a comprehensive, technically detailed overview of double wall underground tanks, their construction, performance advantages, and compliance requirements, enabling informed decisions for long-term storage solutions.
What Is a Double Walled Underground Storage Tank?
A double walled underground storage tank is a closed vessel installed below ground level, consisting of:
- An inner primary tank that contains the stored product (fuel, chemicals, water, etc.).
- An outer secondary containment (second wall or jacket) that surrounds the primary tank.
- An interstitial space between the two walls that is monitored for leaks.
This secondary containment dramatically reduces the risk of environmental contamination by ensuring any leak from the inner tank is contained before reaching the surrounding soil or groundwater. Continuous or periodic monitoring of the interstice allows for early detection, minimising product loss and remediation costs.
Key Components and Construction of Double Wall USTs
A high-quality double wall underground storage tank is defined by its engineering and materials. Proper selection and specification are critical to safety, durability, and regulatory compliance.
Primary Tank (Inner Vessel)
The primary tank is designed to hold the stored liquid under operating conditions, typically at atmospheric pressure. Common materials include:
- Steel tanks
- Made from carbon steel plates.
- Often internally coated with epoxy or other tank linings for corrosion resistance.
- Suitable for petroleum products, diesel, jet fuel, and some chemicals.
- Fibreglass reinforced plastic (FRP) tanks
- Corrosion-resistant by design.
- Ideal for aggressive liquids such as ethanol blends, biodiesel, or certain chemicals.
- Lightweight, reducing transportation and installation costs.
Secondary Containment (Outer Wall or Jacket)
The outer wall provides structural protection and environmental safety:
- Can be steel-over-steel, FRP-over-steel, or FRP-over-FRP depending on application.
- Designed to withstand soil loads, groundwater pressure, and traffic loads above the installation area.
- Dimensioned so that the interstitial space is continuous and monitorable, with no blind spots.
Interstitial Space
The interstice between the inner and outer walls is a critical safety feature:
- May be dry, vacuum-monitored, or liquid-filled (brine or other compatible fluid).
- Equipped with leak sensors, vacuum gauges, or float switches.
- Connected to a monitoring system that triggers alarms for any loss of vacuum or unexpected fluid presence.
This configuration ensures early leak detection from either the primary or secondary wall, allowing proactive maintenance before a release reaches the environment.
Materials Options: Steel vs. FRP Double Wall Tanks
Selecting double walled underground tanks requires understanding the strengths of the two dominant materials.
Double Wall Steel Underground Tanks
Advantages:
- High structural strength and impact resistance.
- Proven performance in fuel storage across decades.
- Suitable for installations under heavy traffic, such as filling stations and truck stops.
- Can be custom-fabricated to complex shapes and large volumes.
Considerations:
- Requires effective external corrosion protection, such as:
- Dielectric coatings
- Cathodic protection systems (sacrificial anodes or impressed current)
- Internal coatings must be compatible with the stored product.
- Requires routine inspection and maintenance to ensure coating integrity.
Double Wall FRP Underground Tanks
Advantages:
- Inherent corrosion resistance; no rusting, no need for external coatings.
- Suitable for ethanol blends, biodiesel, and many aggressive chemicals.
- Lightweight for transport and installation.
- Non-conductive, reducing stray current corrosion issues.
Considerations:
- Requires specialised design for high-temperature products.
- Must confirm chemical compatibility with all stored fluids.
- Structural design and backfill must prevent external damage or deformation.
Many facilities opt for hybrid configurations, such as steel primary with FRP outer jacket, combining structural strength with corrosion resistance.
Design Considerations for Double Walled Underground Storage Tanks
An optimally performing double wall UST system begins at the design stage. Key engineering parameters include:
Capacity and Configuration
- Tank volumes commonly range from 5,000 to 200,000 litres and above.
- Horizontal cylindrical tanks are standard for fuel stations and industrial use.
- Compartmental tanks can store multiple products in a single shell (e.g., diesel and gasoline), each compartment double-walled and independently monitored.
Operating Temperatures and Products
- Design must account for product type, temperature range, and density.
- Material selection should ensure compatibility with:
- Gasoline, diesel, kerosene, jet fuel
- Ethanol blends (E10, E15, E85)
- Biodiesel blends
- Industrial solvents and chemicals
Structural Loads and Burial Depth
- Tanks must withstand:
- Earth loads from backfill.
- Hydrostatic pressure from groundwater.
- Live loads from vehicles, if installed beneath driveways or forecourts.
- Calculations should follow recognised standards (e.g., API, UL, EN norms where applicable).
Internal and External Corrosion Protection
- Steel tanks require:
- External coating systems.
- Cathodic protection design (anode quantity, placement, monitoring).
- Internal linings for fuels containing water or corrosive components.
- FRP tanks focus on:
- Resin type and reinforcement selection.
- Proper curing and quality control.
Leak Detection and Monitoring Systems for Double Walled USTs
One of the most significant advantages of double walled underground storage tanks is their ability to integrate advanced leak detection systems.
Interstitial Monitoring
- Vacuum monitoring
- Interstice is evacuated to a set vacuum level.
- A drop in vacuum suggests a breach in the inner or outer wall.
- Constant monitoring with alarms provides early warning.
- Liquid brine monitoring
- Interstice is filled with a non-hazardous brine solution.
- Float sensors detect changes in level or the presence of product.
- Suitable for retrofits or specific regulatory environments.
Automatic Tank Gauging (ATG)
- Measures product level and temperature within the inner tank.
- Detects inventory discrepancies indicating possible leaks.
- Interfaces with interstitial monitoring for integrated alarms and data logging.
Secondary Containment for Piping and Accessories
Complementing the tank itself, double wall piping, sumps, and spill containment are essential:
- Double wall product lines with interstitial monitoring.
- Containment sumps under dispensers and fill points.
- Overfill prevention devices integrated with monitoring systems.
Together, these technologies deliver a complete, monitored system that meets or exceeds stringent environmental standards.
Installation Best Practices for Double Wall Underground Tanks
Proper installation is just as important as the tank design. Poor installation can compromise even the best-designed system.
Site Preparation
- Conduct a geotechnical assessment to understand soil bearing capacity, groundwater level, and potential contaminants.
- Design excavation dimensions to allow a safe working space and proper backfill around the tank.
- Plan for stormwater management and safe access during installation.
Bedding and Backfill
- Use clean, well-graded, non-corrosive granular material (often pea gravel or crushed stone) for bedding.
- Ensure uniform bedding support under the tank to prevent point loading.
- Backfill in compacted layers to avoid voids, settlement, or tank deformation.
Anchoring and Buoyancy Control
- In high groundwater areas, use deadman anchors, concrete hold-down pads, or other engineered anchoring systems.
- Prevent uplift or flotation when the tank is empty or partially filled.
- Anchor design must consider the worst-case groundwater elevation and the minimum product levels.
Piping, Venting, and Connections
- Use compatible, appropriately rated double wall piping to connect tanks to dispensers.
- Install normal vents and emergency vents according to standards.
- Minimise fittings and joints to reduce leak potential.
- Provide flexible connectors to accommodate ground movement.
Testing and Commissioning
Before putting the tank into service:
- Hydrostatic or pneumatic leak testing of the tank and interstice as per local codes.
- Tightness testing of piping and fittings.
- Calibration of automatic tank gauging and leak detection systems.
- Documentation of all tests and certifications for regulatory approval.
Regulatory and Environmental Compliance
Double walled underground storage tanks are central to achieving compliance with environmental protection regulations. The specifics vary by country and region, but common requirements include:
- Secondary containment for tanks and piping.
- Continuous or periodic leak detection with documented alarm management.
- Approved design standards (e.g., UL, EN, ISO, or equivalent national standards).
- Routine inspection and maintenance schedules.
- Record-keeping for:
- Installation and commissioning.
- Modifications and repairs.
- Leak test results and monitoring logs.
- Incident responses and corrective actions.
Investing in compliant double wall UST systems reduces the risk of:
- Soil and groundwater contamination.
- Heavy regulatory fines and penalties.
- Costly remediation projects.
- Operational shutdowns and reputational damage.
Maintenance, Inspection, and Life Cycle Management
To fully benefit from double walled underground storage tanks, a structured life cycle management plan is essential.
Routine Inspections
- Visual inspection of above-ground components: fill points, vents, sumps, manways.
- Verification of interstitial monitoring readings and alarm functionality.
- Regular review of inventory reconciliation and ATG data.
Periodic Testing
- Scheduled tightness tests for tanks and piping where required.
- Performance verification of cathodic protection systems (for steel tanks).
- Calibration and functional checks of sensors, floats, and gauges.
Preventive Maintenance
- Cleaning and inspection of internal surfaces during planned outages.
- Replacement of worn or damaged gaskets, seals, and fittings.
- Firmware or software updates for monitoring controllers.
Service Life and Replacement
- Double walled tanks are typically designed for a service life of 30–40 years or more, depending on regulations and usage.
- Proactive planning for end-of-life replacement avoids emergency shutdowns.
- Proper decommissioning and removal prevent future liabilities.
Advantages of Double Walled Underground Storage Tanks
Adopting double wall underground storage tanks offers significant technical, environmental, and financial benefits:
- Enhanced environmental protection
- Dual containment and monitored interstice minimise leak impact.
- Regulatory compliance
- Meets or exceeds modern UST requirements in most jurisdictions.
- Early leak detection
- Interstitial and ATG systems enable rapid response.
- Reduced remediation costs
- Contained leaks are easier and cheaper to manage.
- Operational reliability
- Robust materials and design improve uptime and asset longevity.
- Property value protection
- Avoids contamination that could devalue land or restrict its use.
For fuel stations, industrial plants, data centres, hospitals, and logistics hubs, double walled UST solutions represent a strategic, long-term investment in safety and risk management.
Choosing the Right Double Walled Underground Storage Tank Supplier
Selecting a reliable tank manufacturer and installer is just as important as choosing the tank type itself. Key criteria include:
- Proven experience in double wall UST design and fabrication.
- Compliance with recognized international standards and certifications.
- Engineering support for site-specific design, including structural and corrosion analysis.
- Capability to provide integrated systems:
- Tanks
- Double wall piping
- Leak detection
- Automation and monitoring
- Strong after-sales service, including maintenance, inspections, and upgrades.
By partnering with a specialised provider, facilities can ensure that their double walled underground storage tanks deliver optimal performance, regulatory compliance, and long-term peace of mind.
