When installing underground water tanks, one of the most overlooked site conditions is groundwater. If ignored, it can cause structural damage or even complete tank failure. The mechanism behind that failure is hydrostatic crush pressure, and it is worth understanding before a tank is specified rather than after it is in the ground.
What hydrostatic crush pressure is
Hydrostatic crush pressure occurs when groundwater surrounds an empty underground tank. With no water inside the tank to counteract the external force, the pressure from the groundwater can compress the tank walls, causing them to collapse inward. The condition that makes it dangerous is emptiness. A full tank pushes back from the inside; an empty one has nothing to push back with. That is why the case to design for is an empty tank sitting in wet ground, not the full tank you picture in normal use. The force is invisible from the surface and it acts constantly.
When groundwater is and is not a concern
If your site has no groundwater risk, this is not a concern. But if groundwater is present, or may be seasonally present, it is essential to plan accordingly. The word seasonally does a lot of work in that sentence. A site walked in a dry month can read as dry ground and still sit above water for part of the year, so the question is not only what the ground is doing on the day you look at it. That is why the assessment belongs with a geotechnical report rather than with an impression formed on site.
What designers and specifiers should put on the plans
If groundwater is expected at the installation site, it is recommended to use an underground tank certified to AS/NZS 4766:2020, and to include an engineered uplift restraint system. These requirements should be clearly outlined on your consented plans to ensure compliance and correct installation. Getting them onto the plans matters as much as choosing them, because the plans are what the installer builds from and what the inspection is checked against. A requirement that lives only in a conversation is a requirement that can quietly disappear somewhere between specification and backfill. Our underground specification tool is built for this stage of the job.
Prevention one: a tank certified to AS/NZS 4766:2020
The AS/NZS 4766:2020 standard confirms a tank can withstand constant external pressure, even when completely empty and surrounded by groundwater. Think of it like a submarine: it is built to handle full submersion without collapsing. The Promax ENDURO Super Duty Underground Tank is New Zealand's only tank certified to this standard. It is engineered to withstand full-time hydrostatic pressure and submersion. What the certification buys you is the removal of guesswork. Rather than reasoning about whether a given tank will cope with a given water table, the standard settles the question for the worst case. The ENDURO vs XPRESS comparison covers the wider range.
Prevention two: a socked subsoil drain
A site-specific option is to install a socked subsoil drain at low level around the tank. This system redirects groundwater away from the tank, preventing hydrostatic pressure from building up in the first place. The two approaches solve the same problem from opposite directions. A certified tank is built to take the pressure. A subsoil drain stops the pressure arriving at all. Which one suits depends on the site, and the drain is the more site-dependent of the two, because it relies on there being somewhere for the water to go and on the system continuing to work over the life of the installation.
Hydrostatic crush is not the same as flotation
Both are caused by rising groundwater, but they have very different effects. Hydrostatic crush is groundwater squeezing the tank inward. Flotation is groundwater pushing the tank upward, lifting it out of the ground. They are worth keeping separate because different things solve them. Crush is answered by the construction of the tank itself, or by draining the water away. Flotation is usually managed using restraint systems. A single site can present both at once, so answering one does not automatically answer the other. And not all restraints are created equal, as the next section explains. We cover the flotation side in how to keep underground tanks in place.
Uplift restraints versus installation anchors
Two different products get spoken about as though they were interchangeable. They are not.
Uplift restraints
- Fully engineered to prevent flotation during full groundwater rise
- Covered by Promax PS1 certification
- Recommended as standard unless a geotechnical report confirms no uplift risk
Installation anchors
- Designed to hold the tank in place during installation only
- Not suitable for long-term flotation protection
- Not covered by PS1, and not recommended unless specifically engineered for flotation control
Important: you cannot use both systems. If you are using uplift restraints, installation anchors are not required, and vice versa.
Tanks under driveways
Tanks placed beneath driveways may benefit from the added weight and structural support above. In practice, these tanks have not shown flotation issues. However, as installation anchors are not PS1-certified, Promax cannot formally guarantee flotation performance under these conditions, even though they often prove sufficient in the field. That gap between field experience and formal guarantee is worth stating rather than reading past. What works in practice and what is certified are two different statements, and a consent, a producer statement or an insurer will be interested in the second one.
How Promax approaches the risk
Groundwater is an invisible but powerful force. If there is any chance it could impact your site, taking the right precautions can make the difference between a safe installation and a costly failure. At Promax we take a safety-first approach: unless a geotechnical report states otherwise, we assume groundwater may rise to ground level. That is the conservative reading, and it is the one that leaves the fewest surprises. Choose a tank engineered for the job, or manage the risk by draining it away. If you are weighing the location itself, when underground tanks make more sense than above-ground is a useful companion piece.
Need help specifying the right underground tank? Call us on 0800 77 66 29 or email sales@promax.co.nz to speak with our technical team.