Most people assume a swimming pool is simply too heavy to move. It seems impossible that a structure weighing several tonnes could ever lift out of the ground.
Surprisingly, it can.
While hydrostatic events are uncommon, they are a recognised engineering consideration for every in-ground swimming pool. Understanding why they occur helps explain why proper installation, drainage and hydrostatic protection are so important.
Why can a swimming pool float?
The answer lies in a simple engineering principle known as Archimedes’ principle.
When groundwater surrounds an underground structure, it creates an upward buoyant force. It’s the same principle that allows ships to float, even though they weigh thousands of tonnes.
A swimming pool behaves in exactly the same way.
If the upward force created by groundwater becomes greater than the forces holding the pool in place, the structure will try to move upwards. Engineers refer to this as hydrostatic uplift.
Normally, the weight of the pool, the water inside it and the support provided by the surrounding ground all work together to resist this buoyant force. However, if groundwater levels rise significantly, or the pool is emptied at the wrong time, those forces can become unbalanced.
What causes hydrostatic pressure?
Hydrostatic pressure is created whenever groundwater builds beneath or around the pool. This may occur after:
- Prolonged heavy rainfall
- Flooding
- Naturally high groundwater
- Poor site drainage
- Changes to surrounding landscaping
- Leaking underground services
Although many people associate groundwater with properties near rivers or lakes, temporary groundwater can develop on almost any site after significant rainfall.
What happens underneath the pool?
Once a fibreglass pool is installed, it is surrounded by engineered backfill, which supports the shell and transfers loads into the surrounding ground.
During a hydrostatic event, groundwater pressure typically builds beneath the floor of the pool. As that pressure increases, it applies an upward force to the underside of the shell. If the upward force becomes greater than the resistance provided by the pool, the water inside it and the surrounding installation, the structure may begin to move.
This movement doesn’t simply affect the shell. It can also disturb the engineered backfill supporting the pool, creating voids and reducing the structural support around the installation.
What is hydrostatic damage?
Hydrostatic damage occurs when groundwater pressure causes structural movement within the pool installation. The forces involved can be substantial.
Depending on where the pressure is concentrated, damage may include distortion of the pool floor, cracking around the floor-to-wall radius, movement of coping, plumbing damage and loss of support within the surrounding backfill.
Unlike minor cosmetic issues, hydrostatic damage often affects the structural integrity of the installation and can be difficult and expensive to rectify.
Fortunately, hydrostatic events are relatively rare when pools are correctly engineered and installed.
Why prevention is the key
The best way to deal with hydrostatic pressure is to prevent damaging forces from developing in the first place.
Good pool engineering considers the entire installation — not just the shell itself. Ground conditions, drainage, engineered backfill and hydrostatic protection all work together to manage groundwater safely throughout the life of the pool.
Why HydroPro™ is different
Most traditional fibreglass pools rely on a 50 mm hydrostatic relief valve located in the deepest part of the pool. The principle is simple: if groundwater pressure becomes high enough, the valve opens and allows groundwater to enter the pool, helping equalise the pressure.
While this approach has been used throughout the industry for many years, it relies on groundwater reaching a single relief point before pressure can be reduced.
Compass developed HydroPro™ as a different engineering solution. Rather than relying solely on a single 50 mm relief valve, HydroPro is designed as an integrated hydrostatic management system that rapidly relieves groundwater pressure beneath the pool before damaging uplift forces can develop.
By treating hydrostatic protection as part of the overall installation system — working together with engineered backfill, drainage and correct installation practices — HydroPro provides a more comprehensive approach to managing groundwater throughout the life of the pool.
The bottom line
Hydrostatic events are uncommon, but they demonstrate an important principle of pool engineering.
A fibreglass pool is far more than a shell placed in the ground. It is part of an engineered system where the pool, the surrounding backfill, drainage and groundwater management all work together.
When these elements are properly designed and installed, the risk of hydrostatic damage is significantly reduced, allowing homeowners to enjoy their pool with confidence for many years to come.
Frequently asked questions
Should I be worried about draining my pool for maintenance?
Draining is the highest-risk time for hydrostatic damage because the water inside the pool is what usually holds it down against groundwater pressure below. Before draining any fibreglass pool — even partially — always speak to your pool builder about whether it’s safe on your site and what precautions need to be taken.
Does insurance cover hydrostatic damage?
It depends entirely on your policy. Many home insurance policies exclude hydrostatic events, others require specific pool cover, and some cover damage but not the underlying cause. Ask your insurer directly and don’t assume — better to know before something happens than after.
How can I tell if my pool is at high risk?
Sites with high groundwater, low-lying land, heavy clay, poor drainage or nearby water features carry higher risk. Any site near a river, creek or floodplain also warrants extra attention at installation. If you’re unsure about your own site, a proper geotechnical assessment can tell you what you’re working with.
What should I do if I suspect my pool has been affected by hydrostatic pressure?
Don’t drain the pool. Look for visible signs like movement of coping, unexpected water loss, or unusual sounds from the shell. Call a qualified pool builder for an on-site inspection — the first step is always to identify what’s actually happening before deciding on any repair.