Discovering a sudden, jagged crack across your garage floor or noticing that your driveway has started to dip can immediately trigger concerns about massive structural repairs and drained savings. For many property owners, the root cause of these issues is not a problem with the concrete itself, but rather what has disappeared beneath it. When the soil supporting a heavy concrete surface washes away or compacts over time, it leaves behind a hidden, empty pocket of air. If you are dealing with uneven floors, strange hollow sounds, or widening cracks and are researching what happens under concrete slab foundations, understanding how these unseen voids form and how to detect them is the first step toward saving the slab before it breaks apart completely.
Table of Contents
Warning Signs There May Be a Void Under Your Concrete Slab
Because you cannot see through the floor, empty spaces can grow for months or even years before causing obvious physical destruction. Most of the time, the concrete gives small hints first, and many of them can be spotted during a simple walk around the property:
- Sinking or uneven sections: one slab sits lower than its neighbour, or a driveway or walkway slopes in a way it didn’t before.
- Cracks that keep growing: a stable hairline crack is usually less concerning than one that widens or spreads over time.
- A hollow sound when tapped: knocking on the surface with a hammer or walking across it may produce a dull, drum-like sound instead of a solid one.
- Gaps along the slab edges: a visible space where the slab meets the ground, a wall, or a neighbouring section suggests the soil underneath has pulled away.
- Water pooling in new places: puddles that now collect on driveways, sidewalks, or around the foundation may mean the surface has settled into a low spot.
What Causes Voids to Form Under Concrete Slab Foundations
Voids form when the soil holding up the slab moves, washes away, or packs down. Water is the most common cause. Poor drainage, leaking pipes, or runoff that gets under the concrete can carry away fine soil and leave empty pockets behind. In Calgary, freezing and thawing makes this worse. Moisture in the ground freezes and expands, then shrinks as it thaws, and over a few seasons this loosens the soil and opens gaps.
Calgary’s clay-heavy soil adds to the problem. Clay swells when wet and shrinks when dry, so in dry spells it can pull away from the underside of the slab. Poor compaction during construction can also leave the ground under concrete slab foundations too loose to carry the load, so it keeps settling long after the concrete was poured.
How Professionals Detect and Confirm Voids
Visible cracks and uneven concrete can indicate a possible problem, but they cannot confirm what is happening beneath the surface. A professional assessment helps determine whether a void is present, how extensive it may be, and whether the slab has already started to lose stability.
The process usually begins with a visual inspection. Contractors examine the affected concrete, look for cracks and signs of settlement, and check surrounding areas for possible causes such as poor drainage, water leaks, or changes in ground conditions. They may also compare floor levels to identify sections that have dropped or moved.
For a closer look beneath the slab, professionals may use specialized detection methods, depending on the site and the conditions.
- Ground-penetrating radar (GPR): This method uses radar signals to investigate what is beneath the concrete. It can help identify changes in underground conditions and potential gaps, although results depend on soil conditions, moisture, depth, and the equipment used. It does not guarantee that every void will be detected.
- Sounding and surface checks: Contractors may tap or test sections of concrete to identify differences in sound that could indicate a lack of support. This can help narrow down areas for further investigation, but it is not enough on its own to confirm a void.
- Targeted test openings or drilling: In some cases, small openings are made through the slab to check the conditions below. These can help confirm whether an empty space exists and provide access for further assessment or repair.
Once the findings are reviewed, the contractor can estimate the location and extent of the affected area and determine whether the slab needs additional support. They should also consider what caused the void in the first place. For example, a water leak or drainage problem may need to be corrected before any repair work begins.
The exact inspection method will depend on the size of the slab, the suspected depth of the void, site access, and the type of ground beneath it. The goal is to make an informed repair decision rather than relying on surface symptoms alone.
Why Ignoring a Void Gets More Expensive
A small void beneath concrete may not seem urgent when the surface still looks mostly intact. However, the longer the slab remains without proper support, the greater the chance of further movement and damage. What starts as a minor crack or slight dip can develop into a more extensive repair if the underlying problem continues.
One of the main risks is progressive cracking and slab damage. Concrete is strong under compression, but it can crack when sections are left unsupported and subjected to weight. Vehicles, machinery, stored materials, and regular foot traffic can place additional stress on these areas. As the unsupported section bends or shifts, existing cracks may widen and new ones may appear.
There can also be safety and access concerns. Uneven concrete creates trip hazards for pedestrians and can make it harder for vehicles or equipment to move safely across a property. In commercial facilities, damaged floors, loading areas, and walkways may interfere with daily operations or require certain areas to be restricted until repairs are completed.
Another concern is the spread of the underlying problem. If water continues washing away soil or a damaged utility line remains unrepaired, the affected area may become larger. This can increase the amount of work needed to restore proper support beneath the slab.
Early investigation can help property owners understand the extent of the problem before more serious damage develops. Repairing a localized void may be more manageable than dealing with a larger area of settled or cracked concrete. More importantly, identifying and addressing the cause can help prevent the same problem from returning.
If you notice changes in your concrete surface, it is worth having the area assessed rather than waiting for the damage to become more obvious. A timely inspection can help clarify what is happening under concrete slab foundations and what steps are needed to protect the concrete over the long term.
Conclusion
Watching a crack spread across your floor or feeling a slab sink beneath your feet is stressful, especially when you are worried about the high cost and disruption of tearing out and replacing the entire surface. But as we have covered, catching the warning signs early changes the entire situation. You do not have to wait for the concrete to break apart completely to do something about it. By understanding what is happening under concrete slab foundations and acting quickly when you notice a hollow sound or a growing gap, you can save the existing slab, avoid a messy excavation, and keep your repair budget under control.
If you have noticed any of these signs on your property, our team at Ener-Spray can help you understand what is happening and what your options are.