An uneven concrete slab is certainly an eyesore. However, it can also be dangerous. If someone trips and gets injured, you could find yourself facing a lawsuit. Therefore, this isn’t a repair you want to put on the back burner.
There were two “old school” methods for fixing uneven slabs: digging up and replacing the slab, or via a repair method called “mudjacking”. Both were less than perfect solutions though.
Mudjacking involves drilling small holes in the slab, and then using them to inject a slurry (soil, water, cement) under it. The problem with this method is the weight of the slurry. Anything that contains cement is going to be heavy, and if the slab sank because the soil wasn’t able to support it, adding even more weight in the form of a cement slurry certainly isn’t going to do the trick. In fact, the slab could sink even deeper into the soil.
Today, the vast majority of sunken slabs are lifted using a technique called polyurethane foam injection. This fast, minimally-invasive technique involves drilling small holes in the slab (they’ll be covered up later), and then using them to inject the special polyurethane foam. Once the foam is injected under the slab, it immediately starts expanding, and this is what lifts the slab. Most jobs take no more than a few hours and then you can immediately start using the slab again. It’s also a green solution because the foam will not leach harmful chemicals into the soil.