The short answer: A hollow sound or exposed edge can suggest a support problem, but it does not measure the void. Filling a space, raising a slab and correcting water or soil movement are separate decisions made after inspection.
Why a slab can sit over nothing
A void is a pocket of empty space under concrete where soil used to be. Concrete is strong when squeezed and weak when bent. With solid ground underneath, a slab is mostly squeezed, which is why a thin driveway can carry a loaded pickup.
Take the ground away and the slab becomes a bridge. Every load now bends it and stretches the bottom face. Mesh or rebar buys time, but sooner or later it cracks across the span or snaps off a corner, often under a weight it carried fine last year. By then the hollow can reach well past the visible crack.
A ten-minute check you can do yourself
Grab a hammer, a flashlight and a long screwdriver. Tap the middle of a panel that has never moved so you know what solid sounds like. Then work toward the spot that worries you, taking phone photos as you go.
None of this maps the hollow. It tells you whether one is likely and roughly where it starts, and that is the most useful thing you can point out to us on a site visit, so note where the sound changed. Look and listen for these.
- A dull, drummy note where solid concrete clicks sharp.
- A gap under an exposed edge the screwdriver slides into.
- A corner that rocks when you shift your weight on it.
- Fans of sand or silt on the pavement after rain or watering.
What washes the ground out in New Mexico
Much of southern and eastern New Mexico sits on caliche, soil cemented hard by calcium carbonate. Water cannot soak through it quickly, so it spreads sideways along the top and carries fine soil out from under a patio edge, storm by storm. Some ground is loose and collapsible: it holds up while dry, then compacts and drops once soaked.
The water does the rest. Monsoon storms dump heavy rain on hard ground that cannot take it in, and a roof funnels its share out of a few downspouts. Irrigation works slower: a drip line leaking beside a sidewalk, a sprinkler soaking one joint nightly, a swamp cooler bleed line dripping by the patio. Old trench backfill keeps settling too.
Filling, lifting and fixing the water are separate jobs
Void filling restores support under concrete that still sits at the right height. Foam goes into the hollow, expands until it fills the space, and the slab stops acting like a bridge. Nothing moves on the surface, and that is the point.
Lifting is for a slab that has dropped. The same foam packs the hollow first, then keeps expanding and raises the concrete toward a target you agree on, the next panel or the garage threshold. Many slabs need a bit of both. One broken into many pieces may be better replaced, and we will say so.
The third job is the water. Foam takes up the space the soil left but does nothing about the leak, downspout or grade that emptied it. Leave that cause in place and the soil right beside the new foam can wash out next.
| Need | What the work addresses | What it does not establish |
|---|---|---|
| Void filling | Space beneath a slab that may still be at the intended height. | That every hidden area is filled or future movement is impossible. |
| Lifting | A settled slab and an agreed height or slope target. | That cracks, leaks or surrounding grading are repaired. |
| Water-source repair | A leak, runoff or drainage problem affecting support. | That an already settled panel returns to position by itself. |
How a void fill is done and watched
First we walk the slab with you: where it moves, where the water runs, what the edges give away. Together we settle on which area gets treated. Joints, drains and utilities get located before any drilling, so tell us about any line you know is under the concrete.
Access is a few dime-sized holes. Two-part polyurethane is injected in short bursts while someone watches the slab, its joints and edges. Foam takes the easiest route and can surface in a crack or a joint, which is why it goes in slowly. Nobody can see a void’s whole outline from above; we tell you which area got treated, patch the holes and say when it can take vehicles.
When a plumber or landscaper should go first
If water is still moving under the slab, filling the void right now is like patching a pothole in a running ditch. Get the source handled first, then fill, and the foam has a much better chance of staying put.
A plumber finds and repairs leaks, including drain lines a meter test will not catch. A landscaper, or you with a shovel, can extend downspouts, pull irrigation back and shape the soil so it sheds water. Cracks climbing a house wall or doors that start to stick can point to foundation movement and need their own look. These signs point to another trade first.
- The water meter creeps with every fixture in the house shut off.
- A damp patch at the slab edge stays wet through dry weather.
- Downspouts empty right at the slab with no extension.
- The ground slopes toward the concrete instead of away.
In short
- A slab over a void works like a bridge, and concrete cracks when it has to bend.
- Ten minutes of tapping, rocking and looking for washout tells you whether a hollow is likely.
- Caliche sends water sideways and collapsible soil drops when soaked, so watch where your water goes.
- Foam fills space but does not stop water, so get the leak or drainage handled first.