To fix a settling slab, the voids under the slab must be filled. The best solution is to use the Polyurethane Foam. This process is similar to raising concrete, without actually lifting the slab (unless wanted). This process involves the injections of the dual liquid material underneath the slab through 5/8 inch holes drilled into the concrete. Once injected under the slab, the Polyurethane Foam expands to any and every void and to compact the soil around the void. This helps to create a more stable base for the entire foundation.
Some of the most common issues that result in void formation
Soil Shrinkage: When the weather is extremely hot and dry, the soil under the slab will shrink, forming voids and gaps underneath the slab.
Erosion: These voids form due to erosion from excessive rainfall, which washes away the soil under and around the concrete slab.
Hydro-static Pressure: Heavy precipitation can lead to an increase in hydro-static pressure in the soil. This pressure pushes on the concrete causing it to shift, settle, and even develop cracks in the concrete slab.
If the intentions of the job are to only fill the void without creating any lift of the slab, Polyurethane Foam is the way to go. The foam, once pumped, will never wash away, never lose its density or ever be disrupted by water. It is an amazing material.